Performance of commercial PV device panels in 2016a.
\r\n\tWe accept scientific papers which can be presented as original research papers and review papers. The required length of the full chapters is 10-20 pages and the chapters should be original works (not republished).
\r\n\tAs a self-contained collection of scholarly papers, the book will target an audience of practicing researchers, academics, Ph.D. students and other scientists. Since it will be published as an Open Access publication, it will allow unrestricted online access to chapters with no reading or subscription fees.
There is a need in the modern world for sustainable means of producing clean energy economically, on a very large scale. The planet\'s human population is inexorably increasing toward the 10 billion mark [1–3]. The rapid growth in population presents both opportunities for companies seeking to broaden markets for their products, and challenges for governments, as the growing populations demand their share of prosperity. It is well known that prosperity generating economic growth requires energy [4–6]. To meet the demands for prosperity, carbon based fossil fuel consumption has increased accordingly, resulting in unacceptable levels of air pollution in major conurbations in both advanced and developing countries [7–10]. Much of the pollution comes from burning fossil fuels inside internal combustion engines (ICEs) of motor vehicles and ships. Coal burning thermal power plants used for electricity generation also contribute substantially to the rise in air pollution [11, 12].
Scientific research has hitherto yielded various solutions to the clean energy challenge using innovative approaches ranging from development of hybrid gasoline‐electric motor vehicles (HEVs), plug‐in hybrid gasoline‐electric motor vehicles (PHEVs), pure battery electric vehicles (BEVs), fuel cell electric vehicles (FCEVs), advanced catalysts for reducing exhaust emissions as well as carbon capture technologies applicable to coal burning thermal power plants [13–17]. These solutions however, work only to mitigate the problem of carbon based fossil fuel emissions and do not address the fundamental problem, namely, how to circumvent carbon based fossil fuels in energy generation and ground transport applications. Although HEV, PHEV, BEV and FCEV technologies offer promise in reducing pollution at least locally, they represent at best, an incomplete remedy to a major problem. HEVs and PHEVs still require combustion of gasoline inside an ICE while BEVs require copious quantities of electrical energy for battery charging, generated by power plants connected to the electric grid. The power plants supplying the electric grid can be hydroelectric or nuclear, but far more often, are fossil fuel burning thermal power plants that are only incrementally more efficient than internal combustion engines in motor vehicles [18]. Since hydroelectric generating capacity in the United States of America (U.S.A.) has already been reached and construction of new nuclear power stations is fraught due to well substantiated fears of radiological leaks, it becomes apparent that the only way to meet the increased demand for electricity from electric vehicle proliferation is by constructing more fossil fuel burning thermal power plants [19, 20].
The FCEVs exist at present in small numbers as vehicle prototypes that function primarily as technology demonstrators [21, 22]. FCEVs are unique however, because they represent the only motor vehicle technology that uses hydrogen (H2) fuel to generate electric energy to power a motor driving the wheels of the vehicle. Although in existence in various forms since the 1960s, FCEVs have not proliferated for manifold reasons, the principal ones being the absence of means for safely storing hydrogen fuel on board, coupled with a lack of means to economically generate sufficiently pure hydrogen (H2(g)) fuel to prevent poisoning sensitive catalysts that might be present in the fuel cells [23, 24]. The existing methods of storing hydrogen on board motor vehicles utilize cryogenic storage of liquid hydrogen (H2(l)), storage of hydrogen (H2(g)) gas at pressures as high as 70 MPa (10,153 psi) in cylinders made from composite material, and storage as a metal hydride (MHX) in tanks filled with porous metal sponge or powder comprised of light group 1 and 2 metals and/or transition metal elements, namely, Titanium (Ti) or Nickel (Ni) [25–30]. Such direct hydrogen storage methods however, are impractical due to the high cost of suitable transition metals Ti and Ni, and moreover, because an infrastructure is needed to supply hydrogen directly in large volume to fill liquid or gas tanks or to saturate or replenish the metal sponge within the storage reservoir inside a motor vehicle, a procedure fraught with all of the well known safety risks associated with handling large volumes of elemental hydrogen [31, 32]. Furthermore, the existing industrial method of generating hydrogen (H2(g)) gas using steam reforming of natural gas, the latter containing mostly methane (CH4), produces significant quantities of carbon monoxide (CO) even after application of the shift reaction, the latter meant to transform the CO into carbon dioxide (CO2) [33, 34]. The presence of even minute quantities of CO on the parts per million (ppm) order of magnitude in H2(g) fuel, results in rapid poisoning of sensitive platinum (Pt) catalysts present in the latest generation of low operating temperature, proton exchange membrane (PEM) fuel cells [35]. Catalysts based on a mixture of platinum and ruthenium (Pt‐Ru) developed to overcome the sensitivity of pure Pt to carbon monoxide poisoning are not cost effective for large scale application in motor vehicle transport applications due to the dearth of ruthenium [36]. Since hydrogen production by conventional steam reforming methods generates significant quantities of CO and CO2, it becomes difficult to justify using the approach to generate hydrogen (H2) fuel for FCEVs given that the purpose of advancing such technology is to eliminate carbon based fossil fuel emissions.
Despite challenges, hydrogen (H2) which is stored in near limitless quantity in seawater is the only alternative fuel that is more abundant and environmentally cleaner with the potential of having a lower cost than nonrenewable carbon based fossil fuels. We have shown in previous published work that a novel apparatus and method for safely generating hydrogen fuel at the time and point of use from ordinary salinated (sea) or desalinated (fresh) water (H2O) will enable a vehicle range exceeding 300 miles per fueling using direct combustion of the H2 fuel in appropriately configured internal combustion engines of the Otto or Diesel types, which is comparable to the vehicle ranges presently achieved with gasoline or Diesel fuels, while providing a sustainable, closed clean energy cycle [37]. The novel hydrogen generation apparatus enables hydrogen (H2(g)) fuel to be produced on demand in the motor vehicle using a controlled chemical reaction where liquid water (H2O(l)) is made to react with solid sodium (Na(s)) metal reactant to produce hydrogen (H2(g)) gas and sodium hydroxide (NaOH(s)) byproduct according to Eq. (1).
\n\nThe high purity hydrogen (H2(g)) fuel produced on demand by the novel hydrogen generation apparatus can be used to safely power FCEVs without contaminating the sensitive Pt catalysts present in PEM fuel cells or any other types of catalysts in fuel cells, because the hydrogen is not derived from carbon based fossil fuels, and therefore does not contain even trace amounts of carbon monoxide or sulfur compounds. The seawater reactant can be concentrated to as much as 252.18 grams of sea salt solute per kilogram of seawater solution to provide a fusion temperature TEu = –21.2 °C (251.95 K), that is equivalent to the eutectic temperature of a 23.18% by weight NaCl in NaCl‐H2O solution [38, 39]. The concentrated sea salt in seawater solution allows the hydrogen generator to operate reliably over a wide ambient temperature range from –21.2 °C (251.95 K) to 56.7 °C (329.85 K) prevailing in the 48 conterminous states of the U.S.A. [37]. The sodium hydroxide (NaOH) byproduct of the hydrogen generating chemical reaction is stored temporarily within the hydrogen generation apparatus and is recovered during motor vehicle refueling. The NaOH is subsequently reprocessed by electrolysis to recover the sodium (Na) metal for reuse in generating hydrogen fuel.
In this chapter, we describe in detail our company\'s design approach for constructing a novel, scalable, self‐contained electrolytic sodium (Na) metal production plant that uses electric power sourced from the sun. The solar powered electrolytic production plant is meant to form an integral part of a hydrogen fuel, sustainable, closed clean energy cycle in conjunction with the novel, hydrogen generation apparatus, enabling Na metal to be produced cost effectively without negative impact to the environment [37].
For its successful implementation, the hydrogen fuel, sustainable, closed clean energy cycle requires a means of producing quantities of sodium (Na) metal cost effectively on a large scale by electrolysis of sodium hydroxide (NaOH), the latter created as a byproduct of hydrogen (H2(g)) fuel generation inside motor vehicles according to Eq. (1). The electrolysis is performed either on pure sodium hydroxide (NaOH) or on a mixture of NaOH and sea salt, the latter consisting primarily of sodium chloride (NaCl), according to Eqs. (2) and (3) [40–42].
\n\nThe electrical cost of electrolysis can be estimated from the standard reduction potentials of the oxidation and reduction half reactions that occur at the anode and cathode, respectively of the electrolysis cell when implementing Eqs. (2) and (3) [43].
\n\nFrom Eqs. (4)–(6), the minimum potentials of Eov° = –4.07 V and Eov° = –3.11 V are needed to electrolyze NaCl and NaOH, respectively. These voltages are significantly higher than the potential of Eov° = –1.23 V needed to electrolyze ordinary H2O(l) to produce H2(g) at the cathode and O2(g) at the anode, however, the benefit from not having to store volatile H2(g) in very large industrial quantities and to transport it between the production plants and refueling stations, outweighs the added electrical cost of producing the solid Na(s) metal.
In the United States of America, the only clean renewable source of energy available in sufficient abundance to implement Eqs. (2) and (3) on a large scale is the radiant energy from the sun that illuminates vast tracts of flat, arid, desert land in West Texas, New Mexico, Arizona and Southern California. The weather in the southwestern U.S.A. is mostly warm and arid with high solar irradiance all year and therefore, constitutes the ideal location for constructing scalable, self‐contained solar powered electrolytic sodium (Na) metal production plant units by the thousands [44–48]. Each sodium (Na) metal production plant has to be capable of operating autonomously as a self‐contained factory, requiring minimal maintenance and resources. The diagram showing all of the material and energy inputs and outputs of the self‐contained sodium (Na) metal production plant is presented in Figure 1.
Self‐contained sodium (Na) metal production plant operating resources diagram.
In Figure 1, electric power for the Na metal production plant is produced using photovoltaic (PV) device panels spatially arrayed and electrically interconnected on a vertical tower structure that maximizes the use of scarce real estate or land area. Up to NP = 30,000 PV panels, each having an active area AP = 1 m2 are mechanically assembled onto the tower, yielding a total PV device panel array active area given as APA = NP × AP = (30,000 PV panels) × (1 m2) = 30,000 m2. Sodium hydroxide (NaOH) or a mixture of NaOH and NaCl recovered from motor vehicle hydrogen generators during refueling, must be supplied to the electrolytic cells to replenish the consumed reactants. When operating the hydrogen generation apparatus in warm tropical climates, NaOH exclusively can be recovered from motor vehicle hydrogen generators during refueling because desalinated (fresh) water (H2O(l)) can safely be used as a reactant without risk for it to freeze. Sodium (Na) metal is produced at the electrolytic cell cathode, and steam (H2O(g)), oxygen (O2(g)) gas and some chlorine (Cl2(g)) gas are produced at the cell anode, the latter resulting from electrolysis of the NaCl in sea salt. The H2O(g) and O2(g) can be released directly to the atmosphere while the Cl2(g) must be collected, condensed to a liquid and stored in bottles, for subsequent sale to customers that consume chlorine including the paper and polymer (plastic) manufacturing industries [49]. It is also possible to collect and condense the steam generated at the anode and use the liquid water (H2O(l)) for crop irrigation in arid, desert environments where water resources are limited. The layout of the self‐contained sodium (Na) metal production plant is shown in Figure 2.
Layout of the self‐contained sodium (Na) metal production plant (NOT TO SCALE).
The self‐contained sodium (Na) metal production plant shown in Figure 2, consists of a solar tower that comprises a photovoltaic (PV) device panel array active area given as APA = 30,000 m2. It also consists of a prefabricated Quonset or Q‐type metal building having a semicircular cross section, assembled onto a concrete pad foundation that houses electrical switch gear, voltage step down DC‐DC converters, the sodium hydroxide (NaOH) electrolytic cells, sodium (Na) metal packaging unit and chlorine (Cl2) gas separation and bottling unit. A control room permits monitoring the operation of the plant. Two above ground storage tanks are shown located outside of the Q‐type metal building with one on either side, for storing aqueous sodium hydroxide (NaOH(aq)) solution. Each above ground storage reservoir has a liquid volume capacity of VNaOH(aq) = 37,850 L (10,000 Gal), and is used to replenish the NaOH in the electrolytic cells after the reactant in the cells has been consumed by electrolysis.
Many factors influence the production yield of sodium (Na) metal during a normal day of plant operation. The two most important factors include the power conversion efficiency of the PV device panels and the magnitude and duration of solar irradiance incident on the PV panels. Other factors affecting the Na metal yield include the power conversion efficiency of the voltage step down DC‐DC converter and the efficiency of the electrolytic cell in recovering the Na metal from fused NaOH(l) or from a mixture of fused NaOH(l) and NaCl(l). The efficiency of the voltage step down DC‐DC converter depends mainly on how much power is dissipated or lost in the solid state transistors as a result of high frequency on‐off switching. The efficiency of the electrolytic cell in this work is viewed in terms of the number of electrons from the electrolytic cell current ICELL, flowing through an electrolytic cell actually needed to produce an atom of Na metal. An alternate definition of the electrolytic cell efficiency might consider the theoretical electric power required to be supplied to the electrolytic cell to produce one mole of Na metal as calculated from the known Gibbs free energy of NaOH(l), divided by the actual power required to be supplied to the electrolytic cell to produce one mole of the Na metal, a definition that we do not consider here. Ideally, each electron flowing through the electric circuit comprising the electrolytic cell should produce one atom of Na metal. In the analysis that follows, it will be assumed that the voltage step down DC‐DC converters and the electrolytic cells have efficiencies ηDC‐DC = 100% and ηCELL = 100%, respectively.
The solar tower comprising the photovoltaic (PV) device panel array supplies electric power to the sodium (Na) metal producing electrolytic cells. The amount of electric energy supplied by the solar tower is a key determinant of the quantity of Na metal that can be electrochemically separated from the NaOH reactant. The energy conversion efficiency of the photovoltaic (PV) device panels is therefore a critical parameter for determining the amount of Na metal that can be produced by the self‐contained sodium (Na) metal production plant. Ideally, the photovoltaic (PV) device panels of the solar tower should have maximum optical to electric energy conversion efficiency approaching the thermodynamic limit ηPVmax = 93% [50, 51]. According to the Shockley‐Queisser theory, such a high conversion efficiency requires PV devices comprising manifold semiconductor junctions [52, 50]. Contemporary, commercially available single junction, monocrystalline silicon photovoltaic (PV) devices attain energy conversion efficiencies ranging between ηPV = 15 –18% for front‐illuminated silicon devices and up to ηPV = 21.5% for back‐illuminated silicon devices as summarized in Table 1.
\n\nPV panel technology | PV panel area (m2) | Module efficiency (%) | VOC (V) | VMPP (V) | IMPP (A) | ISC (A) | |
---|---|---|---|---|---|---|---|
Sunpower SPR‐X21‐345 | Monocrystalline Si | 1.63 | 21.5 | 68.2 | 57.3 | 6.02 | 6.39 |
Suntech STP290S‐20 | Monocrystalline Si | 1.63 | 17.8 | 39.8 | 31.7 | 9.15 | 9.55 |
LG LG280S1C‐B3 | Monocrystalline Si | 1.64 | 17.1 | 38.8 | 31.9 | 8.78 | 9.33 |
First Solar FS‐4117‐2 | Thin film CdTe | 0.72 | 16.3 | 88.2 | 71.2 | 1.65 | 1.79 |
Sharp NU‐U240F2 | Monocrystalline Si | 1.63 | 14.7 | 37.4 | 30.1 | 7.98 | 8.65 |
Performance of commercial PV device panels in 2016a.
aPV panel performance data sourced from respective product datasheets. ASTM AM 1.5G solar irradiance of 1000 W/m2.
In Table 1, the PV device panels from the Sunpower manufacturing company stand out as having the highest efficiency, due to the use of back‐illumination of the silicon device layer. Multijunction devices that offer higher efficiencies remain at a research and development stage and have not reached a level of maturity or cost effectiveness to be ready for release as commercial PV panel products [53]. Our company, AG STERN, LLC is researching the development of advanced, high efficiency PV devices based on novel, very high transmittance, back‐illuminated, silicon‐on‐sapphire semiconductor substrates expected capable of transmitting 93.7% of the total solar irradiance into the semiconductor device layer and therefore capable of achieving an energy conversion efficiency ηPV = 90% with proper engineering of the semiconductor photon absorbing layers and PV device structure [54–57].
The solar tower providing power to the electrolytic cells must be scalable in electric power output, and thus capable of allowing PV device panels to be replaced as higher efficiency ones become available, without affecting the overall operation of the self‐contained Na metal production plant in any way, other than increasing the Na metal yield. Since the highest performing PV device panels currently offered commercially are listed in Table 1, it is possible to calculate the expected power output of the solar tower comprising a PV device panel array active area of APA = 30,000 m2, under ASTM direct normal air mass (AM) 1.5D standard terrestrial solar spectral irradiance with a total irradiance IrrAM1.5D = 887 W/m2, as a function of the PV device efficiency, as shown in Figure 3.
Electric power output for a solar tower PV device panel array with area APA = 30,000 m2 under ASTM direct normal AM 1.5D standard terrestrial solar spectral irradiance as a function of the PV device efficiency.
It is clear in Figure 3, that single junction, monocrystalline silicon PV devices having an efficiency ηPV = 21.5% can be arrayed to generate electric power given as PST = 5.72 MW for electrolysis. Once ηPV = 90% efficient PV device panels will become available, the electric power output can be increased to a substantial PST = 23.9 MW, entailing that just 50 of the self‐contained sodium (Na) metal production plants can generate peak power given as PST‐50 = 50 × 23.9 MW = 1195 MW, matching the power generating capacity of a large commercial nuclear power station.
The electric power output of the solar tower comprising photovoltaic (PV) device panels depends on the magnitude and duration of solar irradiance incident on the PV panels, in addition to the PV device energy conversion efficiency described in Section 2.1. It can be assumed that the PV device energy conversion efficiency is constant and might only decrease in value slowly over time [58]. In contrast, the solar irradiance incident on the PV device panels can vary on a daily basis and is determined by two principal factors, namely, the solar geometry and prevailing atmospheric or meteorological conditions.
The sun is effectively a large hydrogen (H2) fusion reactor, spherical in shape with a radius Rsun = 6.96 × 108 m located at a mean distance from the earth given as rsun‐m = 1.496 × 1011 m [59, 60]. The total power output of the sun is given as Psun = 3.8 × 1026 W that radiates isotropically in all directions, resulting in a surface temperature of the solar black body Tsun = 5800 K. Of the total solar power output, earth receives only Pearth = 1.7 × 1017 W which if converted to electric power, vastly exceeds the earth\'s energy needs [61]. The sun therefore constitutes an excellent potential source of clean radiant energy to be harnessed on earth.
The solar geometry has a key role in determining how much radiant energy from the sun will be incident on the PV devices located on earth. To understand how the solar geometry influences the solar irradiance at the earth\'s surface, it will be assumed that earth follows a stable elliptical orbit around the sun described by Eq. (7) [59].
\n\nIn Eq. (7), the distance rsun, represents the distance from the center of the sun to the center of the earth and the angle θ, represents the angle between the present position of the earth in orbit around the sun, and the perihelion position when it is closest to the sun. The eccentricity e = 0.01673, describes the shape of the elliptical orbit of the earth around the sun and the value a, represents the length of the semi‐major axis of the orbit defined as the mean distance from the center of the sun to the center of the earth given as a = {rsun(0°) + rsun(180°)} / 2 = rsun‐m = 1.496 × 1011 m. The period of earth\'s elliptical rotation around the sun is approximately Tes = 365.24 days and the period of rotation around its own axis is approximately Tea = 86,400 seconds or 24 hours, the latter known as a mean solar day [62–66]. The earth\'s angular velocity about its own axis is given as ωea = 7.292115 × 10−5 rad/sec, although the rotation is slowing over time [67]. The obliquity or tilt of the earth\'s axis of rotation with respect to a line perpendicular to the plane of its elliptical orbit around the sun is given as ε = 23.44°, although slight precession and nutation of the axis exists [60]. The plane of the sun passes through the center of the sun and remains parallel to the earth\'s equator during the elliptical orbit. The summer and winter solstices occur approximately on June 21 and December 21, respectively. The spring and fall equinoxes occur approximately on March 21 and September 21, respectively, and are characterized by the length of day being equal to the length of night and the earth\'s equator coinciding with the plane of the sun.
The solar declination angle δ, is the angle made by a ray of the sun (passing through the center of the earth and the center of the sun), and the equatorial plane of the earth. The solar declination angle has a range given as -23.44° ≤ δ ≤ +23.44°. On the summer solstice day when δ = +23.44°, the sun shines most directly on the earth\'s latitude φT‐CAN = +23.44°, known as the Tropic of Cancer. On the winter solstice day when δ = –23.44°, the sun shines most directly on the earth\'s latitude φT‐CAP = –23.44°, known as the Tropic of Capricorn. The longitude at the Greenwich prime meridian λPM = 0° [68]. The solar declination angle can be calculated using a geocentric reference frame with the sun located on a celestial sphere and earth located at the center of the celestial sphere, according to Eq. (8) [60].
\n\nIn Eq. (8), λe represents the ecliptic longitude of the sun, that is the sun\'s angular position along its apparent orbit in the plane of the ecliptic. The plane of the ecliptic is tilted by the obliquity of the ecliptic angle ε = 23.44° representing the angle between the plane of the ecliptic and the plane of the celestial equator, the latter being earth\'s equator projected onto the celestial sphere. The ecliptic intersects the equatorial plane at the points corresponding to the spring and fall equinoxes occurring approximately on March 21 and September 21, respectively with λe = 0° at the March 21 equinox. The Eq. (8) is usually formulated in terms of the day number in a year to account approximately for the elliptic nature of earth\'s orbit around the sun, as given by Eq. (9) [69].
\n\nIn Eq. (9), the solar declination angle δ, is given as a function of the day angle Γ = 2π(Nday − 1) / 365, with day number Nday, in a year where Nday = 1 corresponds to January 1st. The Eq. (9) provides sufficient accuracy due to the relatively small eccentricity value e in Eq. (7). The Eq. (9) shows that the solar illumination of the earth\'s surface throughout the year occurs most directly between tropical latitudes, namely, the Tropic of Cancer (δ ≈ +23.44°) and the Tropic of Capricorn (δ ≈ −23.44°). At the time of the vernal and autumnal equinoxes, the earth\'s equator is located in the plane of the sun and every location on earth receives 12 hours of sunlight.
The PV devices can only generate significant electric power during daylight hours therefore, it is essential to verify that a specific geographic location on earth as defined by the latitude φ, and longitude λ, will receive sufficient hours of high incident solar irradiance during the year, before constructing the self‐contained sodium (Na) metal production plant. To accurately describe the sun\'s position in the sky from sunrise to sunset as well as the resulting number of daylight hours, the apparent solar time (AST) is used to express the time of the day. The sun crosses the meridian of the observer at the local solar noon when AST = 12, however, the latter time does not coincide exactly with the 12:00 noon time of the locality of the observer. A conversion between the local standard time (LST) and the AST can be made by using equation of time (ET) and longitude corrections. The equation of time accounts for the length of the day variation as a result of the slight eccentricity of the earth\'s orbit around the sun as well as the tilt of the earth\'s axis of rotation with respect to a line perpendicular to the plane of its elliptical orbit around the sun. The ET is given by Eq. (10) and has the dimension of minutes [69, 70].
\n\nThe relation between the AST and LST can be made using Eq. (11) that includes the ET and longitude correction which accounts for the fact that the sun traverses 1° of longitude in 4 minutes.
\n\nIn Eq. (11), the local longitude λL, represents the exact longitude value for the location relative to the Greenwich prime meridian of λPM = 0°. The standard longitude λS, is calculated from λL as λS = 15° × (λL / 15°) where the quantity in the parentheses is rounded to an integer value. In Eq. (11), if λL is east of λS then the longitude correction is positive and if λL is west of λS then the longitude correction is negative, while DST = 0 minutes or 60 minutes depending on whether daylight savings time applies. If in effect, DST occurs from March until November.
The hour angle h, represents the angular distance in degrees between the longitude λ, of the observer and the longitude whose plane contains the sun, and is calculated according to Eq. (12) [60].
\n\nIn Eq. (12), the multiplier of 15° arises because the earth rotates around its own axis 15° in 1 hour, and the AST has a value 0 < AST < 24 hours. When the sun reaches its maximum angle of elevation at the longitude of the observer, it corresponds to the local solar noon time where h = 0°. The solar elevation angle αs, above the observer\'s horizon and its complement, the solar zenith angle θsz, can be calculated according to Eq. (13) [60].
\n\nThe solar azimuth angle γs, in the horizontal plane of the observer is given by Eq. (14) [60].
\n\nThe Eq. (14) uses the convention of the solar azimuth angle defined as positive clockwise from north meaning that east corresponds to 90°, south corresponds to 180° and west corresponds to 270°. The solar azimuth angle provided by Eq. (14) should be interpreted as 0° ≤ γs ≤ 180° when h < 0°, meaning the sun is located east of the observer, and interpreted as 180° ≤ γs ≤ 360° when h > 0°, meaning the sun is located west of the observer. The sunrise equation allows to calculate the number of hours of sunlight per day that depends on the solar declination angle δ, and on latitude φ, as shown in Eq. (15), which is derived from Eq. (13) by setting αs = 0° and solving for h.
\n\nIn Eq. (15), the hour angle h, has a value −180° < h < 0° at sunrise and 0° < h < +180° at sunset, and when divided by 15°/hour, yields the number of hours before and after the local solar noon time (AST = 12) when h = 0°, corresponding to sunrise Hsr, and sunset Hss, respectively. Using Eqs. (9) and (15), it is possible to calculate for any geographic location on earth the number of daylight hours on any specific day of the year. The maximum angle of solar elevation αs‐max, above the observer\'s horizon that occurs at the local solar noon time is calculated according to Eqs. (16) and (17) that are derived from Eq. (13) by setting h = 0°.
\n\nThe Eq. (16) is applicable for the northern hemisphere while Eq. (17) is applicable for the southern hemisphere. Formulations for the solar declination, elevation, azimuth and zenith angles which account with greater accuracy for the elliptic nature of earth\'s orbit around the sun exist in the scientific literature often as part of solar position algorithms however, they can be rather complex [71–76]. The air mass can be calculated as a function of the true solar zenith angle θsz, given by Eq. (13), considering the effect of atmospheric refraction, according to Eq. (18) [77].
\n\nThe direct normal total (spectrally integrated) solar irradiance as a function of the air mass (AM) and atmospheric conditions that include the effects of elevation, can be calculated using the Parameterization Model C developed by Iqbal, given in Eq. (19) [78–80].
\n\nIn Eq. (19), Irr0 = 1367 W/m2, represents the solar constant or total irradiance of the AM 0 standard solar spectral irradiance in space prior to attenuation by earth\'s atmosphere and the factor 0.9751 is included because the spectral interval of 0.3–3.0 μm is used by the detailed SOLTRAN model from which the Parameterization Model C is derived. The factor E0, represents the effect of the eccentricity of earth\'s orbit around the sun on the solar constant Irr0, due to the periodically varying distance between the earth and sun, as given by Eq. (20) [69].
\n\nIn Eq. (19), the symbol τr represents transmittance by Rayleigh scattering, τo represents transmittance by ozone, τg represents transmittance by uniformly mixed gases, τw represents transmittance by water vapor and τa represents transmittance by aerosols. The expressions for τr, τo, τg, τw, τa are given in Eqs. (21–25).
\n\nIn Eqs. (21), (23) and (25), AMa represents the air mass at the actual atmospheric pressure P and is given as AMa = AM × (P / P0) where AM, calculated using Eq. (18), corresponds to standard atmospheric pressure P0 = 101325 Pa. The actual atmospheric pressure P, can be calculated using the isothermal atmosphere formula in Eq. (26).
\n\nThe Eq. (26) can be used to calculate the atmospheric pressure P in pascals (Pa) at an elevation hPV, in meters (m) above mean sea level, assuming a constant ambient atmospheric temperature T in Kelvin (K), over the difference in elevations. The molar mass of air Mair = 0.028964 kg/mol, earth\'s gravitational acceleration near the surface g0 = 9.80665 m/sec2, and the universal gas constant Rg = 8.3144621 J/K∙mol [43, 81, 82]. In Eq. (22), U3 = lo × AM, where lo represents the vertical ozone layer thickness in centimeters (cm) at normal temperature and surface pressure (NTP), and AM is given by Eq. (18). The vertical ozone layer thickness is assumed in the present work to have a mean annual value lo = 0.35 cm (NTP). In Eq. (24),
In Table 2, geographic and climate characteristics are specified for four prospective locations of the scalable, self‐contained solar powered electrolytic sodium (Na) metal production plant including El Paso, Texas; Alice Springs, Australia; Bangkok, Thailand and Mbandaka, Democratic Republic of Congo (DRC).
\n\naGeographic coordinates φ, λ & elevation hPV (m) | bMean monthly temperature T (°C) | cMean annual precipitable water w’ (cm) | Mean annual local solar noon air mass (AM) | |
---|---|---|---|---|
El Paso | +31.807°, –106.377°/ 1206 | 17.7 | 1.27 | 1.27 |
Alice Springs | –23.807°, +133.902°/ 545.2 | 20.5 | 1.9 | 1.16 |
Bangkok | +13.693°, +100.75°/ 1.524 | 28.2 | 4.52 | 1.08 |
Mbandaka | +0.0225°, +18.288°/ 316.9 | 25.1 | 3.81 | 1.04 |
In Table 2, the geographic coordinates use the sign convention of +/– latitude φ, for locations north and south of the equator, respectively, and +/– longitude λ, for locations east and west of the prime meridian (λPM = 0°). According to the Köppen‐Geiger climate classification system, El Paso has an arid, desert, cold (BWk) climate, Alice Springs has an arid, desert, hot (BWh) climate, Bangkok has a tropical, savanna (Aw) climate and Mbandaka has a tropical, rainforest (Af) climate [88]. The mean annual local solar noon air mass values in Table 2 are calculated using Eqs. (9) and (16)–(18), and the length of day results provided by Eq. (15), show that proximity to the equator for the self‐contained sodium (Na) metal production plant maximizes the solar irradiance and provides uniform hours of daylight per day throughout the year. Tropical regions however, experience higher mean monthly temperatures and consequently higher mean annual precipitable water vapor levels with a long rainy season and sun obscuring rain clouds. Other factors that can significantly affect incident solar irradiance on the PV device panels include the geographic elevation above mean sea level and aerosols comprising solid and liquid sunlight obscuring particulates in the air that can occur naturally due to dust storms and volcanic activity or from human activity such as slash and burn agriculture [89]. According to the accumulated world meteorological data, the southwestern region of the U.S.A. and the central region of Australia, both constitute nearly ideal locations for the self‐contained sodium (Na) metal production plants due to the existence of an arid, desert climate, vast tracts of flat open land, sparse human population and many clear days with high solar irradiance throughout the year [47, 48, 90, 91].
The architecture of the self‐contained sodium (Na) metal production plant has to provide an optimal balance between high performance, reliability and cost effective operation. The highest performance can be achieved by constructing the solar tower of the plant shown in Figure 2, in a manner that allows the optical k‐vectors from the sun to be normally incident onto the photovoltaic (PV) device panels throughout the entire period of daylight from sunrise to sunset. The Figure 4 shows a solar tower architecture with fixed PV device panels, wherein the panels can rotate and tilt with the solar tower as a single unit to follow the sun\'s overhead trajectory.
Solar tower architecture (NOT TO SCALE).
In Figure 4, the solar tower is fabricated using modular sections comprised of high strength, lightweight aluminum alloy that can be fitted end to end and bolted together until the final slant height of the structure Sh = 300 m is achieved. Up to NB‐L/R = 100 branches that constitute levels, comprised of modular sections each having a final length Bl = 50 m, extend out on each side from the central column of the solar tower resulting in a width for the structure of approximately Sw = 100 m. The PV device panels are mounted along the top and bottom of the branches projecting from the central column of the solar tower. The projecting branches of the solar tower also enable ground glass, light diffusing panels to be placed between the rows of solar panels that allow sunlight to penetrate and effectively illuminate the land area beneath the solar tower to support crop cultivation including rice paddy fields. The central column of the solar tower is fixed at the base to a static pylon and approximately midway up the height of the column, to a boom for elevating the solar tower to a tower elevation angle π/2 - αs, that is the complement of the solar elevation angle αs, the latter given in Eq. (13). The opposite end of the boom is fixed to the center of a railroad flatcar mounted on a linear train track that is capable of being displaced along the track to elevate and lower the solar tower. An electric motor module mounted midway up the height of the column enables the PV device panels to collectively rotate left and right about the axis of the central column to track the solar azimuth angle γs, the latter given in Eq. (14), thereby allowing the optical k‐vectors from the sun to impinge at normal incidence onto the PV device panels throughout most of the day of operation. When the sun has set at the end of the day or when inclement weather of sufficient severity is expected, the solar tower can be lowered to be parallel and nearly flush with the ground as shown in Figure 4, with metal louvers drawn over the PV device panels to safeguard against damage to the panels from storms, strong winds and flying debris that can occur, albeit rarely, in the southwestern U.S.A. [92, 93].
The electrical design of the solar tower comprising PV device panels has to accommodate scalability in the power output level, where it is possible to supplant the existing PV device panels with newer and more efficient ones when they become available, without having to modify other components in the solar tower. It is therefore necessary to optimally dimension the electrical conductors embedded within the branches and central column that transmit the electric power generated by the photovoltaic (PV) device panels to the electrolytic cells, according to the magnitude of the current expected to be transmitted once ηPV = 90% efficient PV device panels become available to be installed on the branches of the tower as shown in Figure 4. To develop an accurate design for the aluminum current carrying conductors of the solar tower, it becomes necessary to define the electrical interconnection topology of the photovoltaic (PV) device panels installed on the solar tower. In Figure 5, the equivalent circuit model of the photovoltaic (PV) device panel array installed on the solar tower is shown, with the corresponding straight line approximation of the current versus voltage curve for the PV device panel array.
Circuit model of the photovoltaic (PV) device panel array installed on the solar tower.
In the circuit models shown in Figure 5, IPV represents the PV device current, VOC represents the PV device open circuit voltage, RP represents the parallel resistance of the PV device that ideally should be infinite, RS represents the series resistance of the PV device that ideally should be zero in value, IST represents the output current of the solar tower and VST represents the output voltage of the solar tower. The circuit models shown in Figure 5 are applicable for a single PV cell as well as for an array of PV device panels comprised of PV cells connected in series and/or in parallel [94]. The expressions for the Thevenin equivalent circuit voltage VTH and resistance RTH, are given both for the linear equivalent current source and voltage source circuits. The electrical interconnection topology of the PV device panels on the solar tower must achieve an optimal balance in operating parameters including the solar tower supply system maximum voltage VST‐MAX and maximum current IST‐MAX. It will be assumed in the further analysis and calculations that VST‐MAX corresponds to the maximum power point (MPP) operating voltage of the PV device panel array (VMPP‐PA) rather than to the open circuit voltage VOC, of the array and similarly, that IST‐MAX corresponds to the MPP operating current of the PV device panel array (IMPP‐PA) rather than to the short circuit current ISC, of the array. The assumptions can be considered valid since it is possible to show using Eqs. (27)–(29) that there exists only a small difference in the value between VMPP‐PA and VOC and similarly between IMPP‐PA and ISC if RP is large and RS is small. The Eqs. (27)–(29) describe the solar tower PV array model shown in Figure 5.
\n\nThe Eq. (27), can be derived from the circuit model in Figure 5 by short circuiting the output terminals of the solar tower where IST = ISC. The Eq. (28) is calculated from the solar tower PV array operating as a current source at the maximum power point (MPP), and Eq. (29) is calculated from the solar tower PV array operating as a voltage source at the MPP.
The solar tower must be capable of transmitting PST = 23.9 MW of electric power as indicated in Figure 3, from the photovoltaic (PV) device panels to the electrolytic cells of the scalable, sodium (Na) metal production plant while maintaining reasonable design values for VST‐MAX and IST‐MAX that will not escalate the cost of the system. To achieve an optimal balance between the VST‐MAX and IST‐MAX parameters of the solar tower supply system, it is necessary to identify the most reliable and cost effective approach for maintaining the PV devices at or near their maximum power point (MPP) during operation. Many methods exist for providing a variable or dynamic load to PV devices and performing maximum power point tracking (MPPT), however, for the present application it is necessary to consider that since there are up to NP = 30,000 PV device panels installed on the solar tower, it becomes uneconomical to use any approach that requires dedicated MPPT hardware for so many panels individually or even for small groups of panels and therefore, a collective solution is required for the entire PV device panel array having an area APA = 30,000 m2. A reliable method of operating PV devices very near, if not exactly at the MPP can be implemented by controlling the output voltage of the PV devices [95]. The maximum power point output voltage of a single PV cell VMPP‐C, is always very near in value to a temperature dependent operating voltage of the PV cell and this phenomenon can be used to implement a type of MPPT by controlling the output voltage VST, of the entire PV device panel array with area APA = 30,000 m2. The output voltage VST of the entire PV device panel array can be controlled for example, by using a voltage step down pulse width modulated (PWM) DC‐DC converter with closed loop feedback control that is operated at the proper duty cycle required to maintain the input voltage VIN, to the converter (which is equivalent to the output voltage VST, of the PV device panel array), at the value near to the MPP voltage. In practice, the MPP output voltage of a single junction, monocrystalline silicon PV cell is given as VMPP‐C ≈ 0.4 V which is too low for direct input to a PWM DC‐DC converter [95]. Commercial single junction, front‐illuminated, monocrystalline silicon PV panels of the type shown in Table 1, typically contain 60 PV cell modules connected in series resulting in a maximum power point output voltage for the panel given as VMPP‐P = 30.1 V for the NU‐U240F2 Sharp panel, VMPP‐P = 31.9 V for the LG280S1C‐B3 LG panel and VMPP‐P = 31.7 V for the STP290S‐20 Suntech panel. The maximum power point output voltage of the 96 module, back‐illuminated SPR‐X21‐345 Sunpower panel is given as VMPP‐P = 57.3 V. The series connected PV cell modules use bypass diodes to prevent power loss from an entire chain of series connected cells if one cell becomes shaded, and receives less illumination than other cells causing its resistance to increase, by allowing energy to be collected from PV cells that are not shaded and are outside of the bypassed section of the chain containing the shaded cell(s) [96, 97]. If 15 of the standard 60 cell module single junction, front‐illuminated, monocrystalline silicon PV panel types are connected together in series, then the solar tower supply system maximum voltage can be limited to approximately VST‐MAX ≈ 450 VDC, which corresponds to a low voltage supply according to the International Electrotechnical Commission (IEC) that defines low voltage DC equipment as having a nominal voltage below 750 VDC [98]. A low voltage DC supply system entails a minor risk of electric arcing through the air and an exemption from specialized protection equipment needed for high voltage. If 15 of the 96 cell module, back‐illuminated SPR‐X21‐345 Sunpower PV panels are connected together in series, then the solar tower supply system maximum voltage VST‐MAX ≈ 900 VDC, which exceeds by 150 V the nominal 750 VDC IEC standard of a low voltage supply. Once ηPV = 90% efficient PV device panels will become available, it is expected that they will each have an area AP = 1 m2 and the number of PV cell modules connected together in series will yield a maximum power point voltage for the panel given as VMPP‐P ≈ 30 V that is similar in value to most of the commercial single junction, front‐illuminated, monocrystalline silicon PV panels listed in Table 1, that contain 60 PV cell modules connected in series. Under an ASTM AM 1.5G standard terrestrial solar spectral irradiance with a total irradiance IrrAM1.5G = 1000 W/m2, the corresponding maximum power point current of the ηPV = 90% efficient PV panel can be calculated as IMPP‐P = (ηPV × IrrAM1.5G) / VMPP‐P = 30.0 A, which is a substantially larger current than the IMPP‐P current values listed in Table 1, as might be expected for the more efficient PV panel unit. Therefore, it is necessary to design the current conductors within the scalable solar tower to be capable of transmitting the substantially larger current of more efficient PV panels once they will become available for installation onto the solar tower. The solar tower supply system maximum voltage can be set to VST‐MAX = 450 V, achieved by connecting in series 15 PV device panels with an efficiency ηPV = 90% and VMPP‐P ≈ 30 V, mounted on a single branch section of the solar tower apparatus as shown in Figure 6.
Solar tower branch sections showing groups of 15 series interconnected PV device panels installed.
In Figure 6, each branch section supports the installation of 2 groups of 15 series connected PV panels having an efficiency ηPV = 90% for a total NP‐Bsec = 30 PV panels per branch section. A total of 5 branch sections should be fastened together end to end, to yield NP‐B = 5 × NP‐Bsec = 150 PV panels per branch with 5 groups of 15 series connected PV panels installed along the top of the branch and another 5 groups of 15 series connected PV panels installed along the bottom of the branch. Each branch section has a length Bsecl = 10 m, and contains embedded within the aluminum current conductors, yielding a branch length Bl = 50 m as shown in Figure 4. The groups of 15 series connected PV panels are electrically connected to a pair of aluminum conductors inside the branch, resulting in parallel electrical interconnection for the groups of 15 series connected PV panels that increases the electric current delivered by the solar tower. Each PV panel has a width Pw = 0.66 m and a height Pl = 1.5 m for a total PV panel area given as AP = Pw × Pl = 1 m2. Since the height of each PV panel is given as Pl = 1.5 m, then each branch has an overall height given as Bh = 2 × Pl = 3 m. The solar tower that has a height Sh = 300 m as shown in Figure 4, can therefore accommodate up to 100 branches extending out from the left and right sides of the central column, yielding a total PV panel array area on the solar tower given as APA = 2 × (Sh / Bh) × NP‐B × AP = 30,000 m2.
The electric current from each branch section flows into the electrical conductors installed inside the central column of the solar tower as shown in Figure 6. Therefore, electric current that flows from the outermost branch section toward the central column increases as each branch section contributes additional current generated by the 30 PV panels mounted on it. If an ASTM direct normal AM 1.5D standard terrestrial solar spectral irradiance with a total irradiance IrrAM1.5D = 887 W/m2, is incident on the PV panels having an efficiency ηPV = 90% and VMPP‐P = 30 V, then the maximum power point output current can be calculated as IMPP‐P = (ηPV × IrrAM1.5D) / (VMPP‐P) = 26.6 A. The result for IMPP‐P = 26.6 A, entails that each branch section will contribute IBsec = 2 × IMPP‐P = 53.2 A. Therefore, the current in each branch increases by IBsec = 53.2 A as it flows in from the outermost branch section toward the current conductors of the central column of the solar tower. The total current contributed by each branch of the solar tower is then calculated as IB = 5 × IBsec = 266 A, since a single branch comprises 5 branch sections. Each side of the solar tower has NB‐L/R = 100 branches for a total number of branches on the solar tower given as NB‐ST = 2 × NB‐L/R = 200 branches. It is convenient however, to install four electrical conductors labeled 1, 2, 3 and 4, in the central column of the solar tower as shown in Figure 6. The first pair of current conductors 1 and 2, transmits current from the branches mounted on the left side of the solar tower and the second pair of current conductors 3 and 4, transmits current from the branches mounted on the right side of the solar tower, so that each conductor pair only needs to transmit a maximum current IST‐MAX = 26,600 A.
The four electrical conductors installed within the modular sections of the central column of the solar tower that are located at or near the top of the solar tower, do not have to carry the maximum current IST‐MAX = 26,600 A, rather only the four electrical conductors installed in the bottom most modular section of the central column have to be dimensioned to transmit the maximum current. The diameters of aluminum electrical conductors within the central column sections scale linearly from a diameter DS1 = 2 cm in stage 1 at the top to DS100 = 41.6 cm for stage 100 at the bottom of the solar tower, the latter that transmits the maximum current IST‐MAX = 26,600 A. Figure 6, shows the calculated voltage drop across the aluminum electrical conductors in all 100 modular sections of the central column of the solar tower to be given as VST‐DROP = 3.86 V, corresponding to a low loss considering that the solar tower supply system maximum voltage VST‐MAX = 450 V.
The electrical design of the solar tower described provides manifold advantages including a mostly parallel electrical interconnection architecture for the PV device panels that allows the MPP of the PV device panels to be controlled collectively by controlling the output voltage VST of the PV device panel array using a voltage step down PWM DC‐DC converter designed to have a constant output voltage VOUT, and controllable input voltage VIN, the latter supplied from the PV device panel array and equal to VST, shown in Figure 5. Other advantages include a low voltage DC solar tower supply system with optimal balance between the maximum voltage VST‐MAX ≈ 450 V and maximum current IST‐MAX = 26,600 A, wherein two independent and electrically isolated power supply feeds are provided from the left side branches and right side branches of the solar tower apparatus, respectively. Yet another important advantage of the design includes a low center of gravity for the solar tower apparatus due to the modular sections comprising the central column being heavier near the base and lighter near the top as a result of larger diameter aluminum electrical conductors placed near the base of the tower.
The solar tower apparatus described in Section 3.1 implements a mostly parallel electrical interconnection architecture for the PV device panels that yields two identical, independent and electrically isolated low voltage DC power supplies, each having a maximum voltage VST‐MAX ≈ 450 V and maximum current IST‐MAX = 26,600 A, wherein the two power supply feeds emanate from the left side branches and right side branches of the solar tower apparatus, respectively as shown in Figure 6. The sodium hydroxide (NaOH) electrolysis cell however, requires a substantially lower voltage for operation. The quantity of sodium (Na) metal produced by the NaOH electrolytic cell depends fundamentally on the magnitude of the DC current flowing between the anode and cathode terminals of the electrolysis cell. The Eqs. (4)–(6) provide the standard reduction potentials of the oxidation and reduction half reactions that occur at the anode and cathode, respectively of the electrolytic cell. In practice, the electrolytic cell operating voltage has to be set at approximately VCELL = 4 V for electrolysis of pure NaOH or VCELL = 5 V for electrolysis of a mixture of NaOH and NaCl, the latter derived from sea salt. The higher voltage accounts for the overvoltage effects [99]. The maximum current that can be supplied to an electrolytic cell operating at approximately VCELL ≈ 5 V will be ICELL ≈ 100,000 A and is limited in large measure by the material cost as well as by the mass and physical dimensions of the electrical conductors required to transmit such magnitude of the current.
In a fused or molten state, NaOH(l) is highly corrosive and therefore the only conventional materials capable of withstanding prolonged exposure to its caustic effects at an elevated temperature include graphite, iron and nickel. Graphite however, cannot be used as an anode electrode because it will react with the oxygen (O2) generated to produce carbon dioxide (CO2) and become consumed in the process. Iron can withstand corrosion from fused NaOH(l), and consequently could be used to fabricate the electrolytic vessel however, as an anode electrode, the reaction with steam (H2O(g)) and O2 will quickly oxidize and erode iron. The only material suitable for fabricating both the anode and cathode electrodes remains nickel (Ni) which is significantly more expensive than both graphite and iron. The cost of the Ni electrodes therefore becomes an important factor in limiting the maximum current in the electrolytic cell. The other factor limiting the current in the electrolytic cell becomes the PWM DC‐DC converter that must supply the large currents at the correct output voltage to the electrolytic cell, safely and reliably.
It is necessary to provide two identical voltage step down PWM DC‐DC converters to convert the power PST‐L/R = 11.95 MW delivered by the two independent low voltage DC power supplies of the solar tower, each having a maximum voltage VST‐MAX ≈ 450 V and maximum current IST‐MAX = 26,600 A. The PWM DC‐DC converter must be designed to safely convert the electric power supplied from the solar tower to magnitudes of DC voltage and DC current that are appropriate for supplying the NaOH electrolytic cells. The PWM DC‐DC converters and electrolytic cells are housed together inside the prefabricated Q‐type metal building indicated in Figure 4. If each PWM DC‐DC converter is designed to supply a single NaOH electrolytic cell with a power conversion efficiency ηDC‐DC = 100%, then the maximum current in the single NaOH electrolytic cell would be calculated as IOUT = (VST‐MAX × IST‐MAX) / VCELL = (450 V) × (26,600 A) / 5 V = 2,394,000 A, a value that is clearly beyond the practical realm. Consequently, multiple NaOH electrolytic cells have to be constructed and electrically connected in series to be supplied by a cell current ICELL = 96,500 A that corresponds to approximately one mole of electrons supplied per second [43]. The output voltage of the PWM DC‐DC converter is then calculated as VOUT = (VST‐MAX × IST‐MAX) / ICELL = (450 V) × (26,600 A) / 96,500 A = 124 V. The number of NaOH electrolytic cells that must be connected in series to be supplied by a single PWM DC‐DC converter is calculated as VOUT / VCELL = 124 V / 5 V ≈ 25 cells. Therefore, the self‐contained sodium (Na) metal production plant has a total of 50 NaOH electrolytic cells with 2 sets of 25 cells electrically connected in series and supplied by two identical PWM DC‐DC converters that function as voltage step down converters to transform the output voltage of the solar tower VST, which represents a DC input voltage to the converter unit VIN = VST‐MAX ≈ 450 V, to a constant DC output voltage VOUT = 124 V with high efficiency, while controlling the PWM DC‐DC converter input voltage VIN, and thereby output voltage VST of the solar tower PV device panels to maintain their operation near the MPP.
The design of voltage step down PWM DC‐DC converters that have a fixed output voltage VOUT and control the input voltage VIN that is variable, has been an active topic of research in the context of photovoltaic power systems applications [94, 95]. The requirements of the present application however, entail a specialized type of large scale direct photovoltaic power conversion for chemical electrolysis not hitherto contemplated or addressed in the scientific/industrial literature. The design for the voltage step down PWM DC‐DC converter with a fixed output voltage VOUT and variable input voltage VIN meant to supply the NaOH electrolytic cells is shown in Figure 7 to consist of a multiphase converter topology, with synchronous voltage step down converter circuits connected in parallel.
Multiphase voltage step down PWM DC‐DC converter power supply for NaOH electrolytic cells.
The synchronous parallel multiphase voltage step down PWM DC‐DC converter power supply shown in Figure 7 offers the inherent advantage of allowing the large load current IOUT = ICELL = 96,500 A to be split among the phases of the converter to match the current carrying capacity of the individual electronic switches, that in practice would consist of power insulated gate bipolar transistors (IGBTs) such as the Model 1MBI3600U4D‐120, manufactured by the Fuji Electric Company with a maximum rated collector‐emitter voltage and collector‐emitter current given as VCE = 1,200 V (at TC = 25 °C) and ICE = 3,600 A (at TC = 80 °C), respectively. It is possible to use 64 such IGBT devices in up to Nφ = 32 phases of the multiphase PWM DC‐DC converter with 2 IGBT devices per phase as shown in Figure 7, to deliver the required current IOUT = ICELL = 96,500 A to the 25 series connected NaOH electrolytic cells indicated as having resistances RC1, RC2, …, RC25, without exceeding the electrical ratings of the solid state switches. Using a multiphase converter topology with synchronous voltage step down converter circuits connected in parallel, also allows the current ICELL = 96,500 A to be split among the 32 inductors L1–L32, present in each of the Nφ = 32 phases, thereby not requiring one single large inductor to transmit the full current IOUT, supplied by the PWM DC‐DC converter to the load comprising the electrolytic cells. Current sharing occurs between the synchronous voltage step down converter circuits connected in parallel according to Eq. (30).
\n\nAssuming the ideal case that current sharing between synchronous parallel voltage step down converter circuits is equal, then IO1 = IO2 = … = ION = IOUT / Nφ where ION = IOUT / Nφ = (96,500 A / 32) = 3,016 A transmitted per phase. Thus, each of the 32 inductors L1–L32, can be designed for a current load ION = 3,016 A, a value well within the capabilities of inductor manufacturers. The output voltage of the multiphase voltage step down PWM DC‐DC converter can be set to a fixed value using a utility scale battery, and thus VBAT = VOUT = 124 V. The input voltage VIN, of the multiphase voltage step down PWM DC‐DC converter power supply for electrolytic cells shown in Figure 7, that also corresponds to the output voltage VST, of the solar tower PV device array has to be controlled reliably to ensure that the PV device panel array operates at or near its maximum power point (MPP).
The synchronous parallel multiphase voltage step down PWM DC‐DC converter power supply in Figure 7 with Nφ = 32 phases, constitutes a complicated nonlinear dynamical system that can be challenging to model, control and analyze accurately to ensure stable operation under wide ranging conditions [100]. The closed loop control system using voltage control and a single feedback loop that can be applied to the present converter is shown in Figure 8.
Closed loop control system for the multiphase voltage step down PWM DC‐DC converter.
The control system for the multiphase voltage step down PWM DC‐DC converter shown in Figure 8 consists of a sensor that senses the process parameter to be controlled, namely, the input voltage VIN to the PWM DC‐DC converter that is also the output voltage VST, of the solar tower PV device panel array. The error amplifier compares the scaled process parameter VIN to the set point value VSET and computes the difference as an error signal VERROR. A proportional, integrator, differentiator (PID) circuit processes the error signal VERROR, and accordingly generates a control signal VCONTROL, that is supplied to a pulse width modulation (PWM) signal generation circuit to produce the control signals that have the correct frequency, duty cycle and phase shift. The PWM signal to the IGBT switches has to be replicated Nφ = 32 times and phase shifted by φShift = 360° / Nφ = 11.25° using a driver circuit to control all the synchronous voltage step down converters connected in parallel in Figure 7. In Figure 9, the electronic circuit schematics for some of the different control blocks in Figure 8 are shown.
Electronic circuit schematics of the control system for the multiphase voltage step down PWM DC‐DC converter.
The electronic circuits shown in Figure 9 are representative of the functional blocks of the PWM DC‐DC converter control system. The sensor circuit can consist of a resistive voltage divider with a unity gain op‐amp buffer that senses the voltage VIN, at the input of the PWM DC‐DC converter according to Eq. (31).
\n\nThe error amplifier subtracts the input voltage VSEN, scaled by the resistive voltage divider of the sensor circuit, from the set point reference voltage VSET, to calculate an error voltage VERROR, according to Eq. (32).
\n\nThe PID unit receives the error voltage VERROR, as an input and produces a control voltage VCONTROL, given as Eq. (33).
\n\nIn Eq. (33), the terms GP, GI and GD represent the gains of the proportional, integrator and differentiator circuits, respectively that can be tuned as needed to achieve optimal control characteristics for the PWM DC‐DC converter. There exist myriad other ways of controlling the PWM DC‐DC converter using only proportional (P) and integrator (I) control functions without the differentiator (D) circuit for example, however, the control system shown in Figures 8 and 9, represents a robust and reliable type of control of the PWM DC‐DC converter. The PWM unit is shown to consist of a fixed frequency square wave voltage oscillator with an op‐amp integrator circuit that converts the square wave into a triangular wave ramp signal which is then compared to the control voltage VCONTROL, from the PID circuit using a comparator, to generate the PWM signal for the IGBT electronic switches of the PWM DC‐DC converter. The driver circuit that generates the Nφ = 32 copies of the PWM signal from Figure 8, with each signal and its complement phase shifted by φShift = 11.25° for all 64 IGBT switches in the converter is not shown in Figure 9. It could be implemented however, using all digital logic. The control system allows the PWM DC‐DC converter to be effectively controlled by setting just one parameter, namely, the reference voltage VSET, to a value that corresponds with the maximum power point (MPP) output of the solar tower PV device panel array to transmit the maximum power available from the PV devices to the NaOH electrolytic cells.
It is possible to gain insight into the operation of the synchronous parallel multiphase voltage step down PWM DC‐DC converter with attached solar tower PV device panel array, from the most common modeling approach using small signal analysis based on state space averaging [101]. The open loop transfer function GV(s), of the voltage step down PWM DC‐DC converter that yields the small signal response of the input voltage process parameter VIN = VST to the duty cycle control variable d, can be calculated straightforwardly by making the simplifying assumption, namely, that the power supply has a single phase (Nφ = 1) rather than Nφ = 32 phases in parallel as depicted in Figure 7. The voltage step down PWM DC‐DC converter from Figure 7 with only a single phase φ1, can be characterized by two pairs of differential equations that describe the current IO1, flowing through the inductor L1, and the voltage VIN, across the input capacitor C1, during the two distinct states of the converter when the switch SH1 is closed and open. The pair of differential equations corresponding to the switch SH1 being closed are given by Eqs. (34) and (35).
\n\nThe pair of differential equations corresponding to the switch SH1 being open are given by Eqs. (36) and (37).
\n\nThe state space form of the above four differential equations describing the PWM DC‐DC converter having just a single phase φ1, is given as Eqs. (38) and (39) for SH1 being closed and open, respectively.
\n\nThe Eq. (38), has the form
When all transients in the PWM DC‐DC converter have stabilized and steady state operation is achieved, then
Calculating out Eq. (41) yields the DC value results given in Eqs. (42) and (43) for IO1 and VIN.
\n\nThe result in Eq. (43) allows to calculate the DC value duty cycle D, needed to maintain the input voltage VIN, of the PWM DC‐DC converter that is equal to the output voltage of the solar tower VST = VIN = 450 V. Thus, the duty cycle D = VOUT / VIN = 124 V / 450 V = 0.28.
The linear model of the open loop transfer function GV(s), for the voltage step down PWM DC‐DC converter at the operating point that can be used to evaluate small signal variations in the duty cycle control variable d used to control the voltage VIN, can be developed by adding a small signal AC perturbation to the DC value D, of the duty cycle. Since a positive increase of the duty cycle d, causes a decrease in VIN as confirmed by Eq. (43), it becomes convenient for modeling purposes to express the duty cycle in terms of a new variable
In Eq. (44), the vector x = X +
In Eq. (45), discarding the DC and nonlinear terms yields Eq. (46).
\n\nTaking the Laplace transform of the averaged state space equation which has the form
In Eq. (47), x(0) represents the initial value of the state vector in the time domain. For calculating the transfer function it is assumed that x(0) = 0. Solving Eq. (47) for X(s) yields the result in Eq. (48).
\n\nApplying the Laplace transform to Eq. (46), yields the result given in Eq. (49).
\n\nSolving Eq. (49) for
The open loop transfer function given as
The analysis to yield the open loop transfer function for the multiphase voltage step down PWM DC‐DC converter with Nφ = 32 phases requires creating a map that contains all the state space equations describing all possible states of the 64 IGBT switches during a period of duration TP, corresponding to a cycle of operation of the PWM DC‐DC converter. For Nφ = 32 phases, there will be in effect 32 switching events occurring per cycle of operation with a phase shift φShift = 11.25°. Thus, there will exist 32 state space equations of the form given in Eq. (52).
\n\nIn Eq. (52), the matrices Ai and Bi describe the PWM DC‐DC converter in the subinterval of duration ti between switchings of the parallel phases where
The solar cycle described in Section 2.2, the electrical design of the solar tower described in Section 3.1, and the electrical design of the sodium hydroxide (NaOH) electrolysis plant described in Section 3.2, entail that the voltage step down pulse width modulated (PWM) DC‐DC converter supplying electricity from the solar tower PV device panel array directly to the NaOH electrolytic cells has to be operated according to a precise protocol.
Prior to sunrise occurring, the sodium hydroxide (NaOH) electrolytic cells are replenished to capacity with concentrated aqueous NaOH(aq) solution from the storage tanks shown in Figure 2, that are located adjacent to the Q‐type metal building that houses electrical switch gear, voltage step down (PWM) DC‐DC power converter units, the sodium hydroxide (NaOH) electrolytic cells, sodium (Na) metal packaging unit and chlorine (Cl2) gas separation and bottling unit. As sunrise commences, the solar tower photovoltaic (PV) device panel array begins generating electric power. The voltage step down PWM DC‐DC converter supplies the electric power from the solar tower PV device panel array to the electrolytic cells at a fixed voltage given as VBAT = VOUT = 124 V. Before NaOH electrolysis and Na metal production can begin, the liquid aqueous NaOH(aq) has to be heated to evaporate all of the water (H2O(l)) content and fuse or melt the remaining anhydrous solid NaOH(s). The presence of water or moisture in the NaOH being electrolyzed can significantly reduce Na metal yield due to side reactions occurring between the Na metal and residual H2O. The sodium hydroxide melts into a liquid state at Tf = 594 ± 2 K [102]. Therefore, electric power delivered by the solar tower initially at sunrise, has to be transmitted to electric heating elements for evaporating the water (H2O(l)) from the aqueous NaOH(aq) and to raise the temperature of the fused anhydrous NaOH(l) to the proper temperature for electrolysis. Once the proper temperature of the fused anhydrous NaOH(l) is reached, current from the solar tower can be transmitted to the cathode and anode electrodes of the 25 electrolytic cells electrically connected in series, to begin production of Na metal according to Eqs. (2) and (3).
The electric current is supplied to the 25 electrolytic cells electrically connected in series, by the synchronous parallel multiphase voltage step down PWM DC‐DC converter at a fixed output voltage set by the utility scale battery given as VBAT = VOUT = 124 V. The current IOUT = ICELL, supplied by the PWM DC‐DC converter from the solar tower to the electrolytic cells is not constant throughout the day, increasing gradually from sunrise until the sun reaches its zenith, and subsequently decreasing gradually as the sun tracks west and eventually sets. To use the electric power generated by the solar tower PV device panel array most efficiently for Na metal production, it is necessary to configure the electrolytic cells as a variable electric load, wherein their electrical resistance can decrease gradually as the sun arcs toward zenith allowing ICELL to increase as more electric power is generated by the solar tower for electrolysis and subsequently, the electrical resistance of the electrolytic cells can begin to increase gradually as the sun passes beyond the zenith toward sunset, when ICELL must decrease as less electric power is generated by the solar tower for electrolysis.
The sodium (Na) metal production plant can effectively be controlled using only two adjustable parameters, including the set point reference voltage VSET, that controls the input voltage VIN, of the PWM DC‐DC converter that is equal to the output voltage of the solar tower VIN = VST, and the electrical resistance of the NaOH electrolytic cells. The set point reference voltage VSET can function either as a coarse or fine adjustment for the current supplied by the solar tower PV device panel array to the electrolytic cells at the fixed voltage VBAT = VOUT = 124 V. The electrical resistance of the electrolytic cells naturally increases slowly over time as the fused NaOH(l) reactant is decomposed by the current flowing between the electrodes of the cell according to Eqs. (2) and (3). The electrolytic cells can be designed for example, with a controllable electrical resistance that can remain relatively constant (or increase or decrease as needed) even as the fused NaOH(l) reactant is consumed, by varying the spacing between the anode and cathode electrodes, where the electrodes can be moved closer together to compensate the loss of reactant volume as it is consumed in the cell.
It is possible to calculate the quantity of Na metal produced throughout the year by the self‐contained sodium (Na) metal production plant sited in the different geographic locations given in Table 2, based on the hours of daylight and the prevailing air mass conditions. It is not necessary to specify a detailed design for the NaOH electrolytic cell to generate an accurate daily estimate of Na metal production yield throughout the year, if it is assumed that maximum electric power available from the solar tower PV device panel array can always be supplied to the electrolytic cells by appropriately controlling the electrical resistance of the NaOH electrolytic cells together with the set point reference voltage VSET of the PWM DC‐DC converter. It is assumed that electrolysis of NaOH, and thereby Na metal production can only occur if the available current supplied by the solar tower PV device panel array has a minimum threshold value of IST = 3,000 A. In Figure 10, the production yield of Na metal is calculated for each day of the hypothetical year 2015, for electrolysis of pure NaOH according to Eq. (2), for the four geographic locations listed in Table 2, using the solar position algorithm (SPA) described in Solar position algorithm for solar radiation applications written by Reda & Andreas in 2004, that is a refined algorithm based on the book, The Astronomical Algorithms written by Meeus in 1998, and is presently regarded as the most accurate [73, 74]. The SPA allows the solar zenith θsz, and azimuth γs, angles to be calculated with uncertainties of ±0.0003° in the range between -2000 to 6000 years.
Calculated sodium (Na) metal daily production yields throughout the hypothetical year 2015, for El Paso, Texas (thick solid), Alice Springs, Australia (thin solid), Bangkok, Thailand (thick dash) and Mbandaka, DRC (thin dash).
The calculation in Figure 10 provides the expected daily sodium (Na) metal production yield under the assumption that the energy conversion efficiency ηPV = 90% for the solar tower PV device panel array and furthermore, current is only transmitted to the electrolytic cells when the solar tower PV device panel array receives sufficient solar irradiance to produce the minimum threshold value of current IST = 3,000 A. The calculation in Figure 10, uses the SPA algorithm to determine the solar zenith angle θsz, throughout the day from sunrise to sunset for each day of the hypothetical year 2015, and calculates the air mass (AM) using Eq. (18). The direct normal total (spectrally integrated) solar irradiance incident onto the solar tower PV device panel array is calculated in turn, using Eq. (19) in a real time manner, as the solar position and air mass change throughout the day for each day of the year. The results from Figure 10, show that consistent quantities of Na metal are produced throughout the year when the self‐contained sodium (Na) metal production plant is located as near as possible to the equator where the length of the day is the most uniform. Further away from the equator, the variability in the length of the day increases however, even at a latitude φ = +31.8°, in El Paso, Texas, the variability in the length of the day is not so significant as to render Na metal production uneconomical during the winter months when the daylight interval becomes reduced. The mean daily sodium (Na) metal production yields for the hypothetical year 2015 are calculated as 41,998 kg/day for El Paso, 41,884 kg/day for Alice Springs, 40,281 kg/day for Bangkok and 40,947 kg/day for Mbandaka, assuming a mean annual aerosol optical thickness value ka = 0.1 reflecting clear days and uninterrupted Na production for all 365 days of the year. Notwithstanding the variability in daily Na metal production throughout the year, El Paso, Texas has the highest elevation above mean sea level when compared with Alice Springs, Bangkok and Mbandaka as shown in Table 2, resulting in an increased solar irradiance incident onto the PV device panel array and thus, the highest mean daily Na metal production. A conservative estimate for the true mean daily sodium (Na) metal production yield for the year might be mNa = 30,000 kg/day of Na metal, as a consequence of nonproductive days due to inclement weather and required plant maintenance. The results in Figure 10, clearly demonstrate that the scalable, self‐contained solar powered electrolytic sodium (Na) metal production plant can be constructed almost anywhere on earth and especially in the southwestern region of the U.S.A., to achieve a hydrogen (H2(g)) fuel, sustainable, closed clean energy cycle.
For the full benefits of the hydrogen (H2(g)) fuel based sustainable clean energy economy to be realized, it is essential to overcome the logistical problems inherent with H2(g) fuel. The H2(g) fuel possesses the unique characteristic that it can be combusted directly inside an internal combustion engine (ICE) to produce useful work without emission of carbon dioxide (CO2) or sulfur oxides (SOX) and with minimal emission of nitrogen oxides (NOX). It can also be converted to electricity directly in a fuel cell to produce useful work with substantially higher efficiency than in an ICE. Regardless of how the versatile H2(g) fuel is applied to produce useful work, it is essential to provide safe, reliable and economical logistics for its use. The major drawback of H2(g) fuel remains the difficulty of direct storage. Fortunately, the element sodium (Na) positioned two rows below hydrogen in Group I of the periodic table of elements, is sufficiently electropositive to be capable of chemically releasing the H2(g) fuel stored in either ordinary salinated (sea) or desalinated (fresh) water (H2O) over a wide temperature range [37]. Sodium (Na) is also sufficiently abundant in nature in the form of sodium chloride (NaCl) in seawater, to make its use economical for H2(g) fuel generation [103]. Therefore, sodium (Na) metal and the sodium hydroxide (NaOH) byproduct resulting from the H2(g) fuel producing chemical reaction in Eq. (1), constitute the ideal intermediate materials needed to render H2(g) into a practical and usable fuel by storing the sun\'s radiated energy as Na metal.
A hydrogen (H2(g)) fuel clean energy economy based on a sustainable, closed clean energy cycle that uses sodium (Na) metal recovered by electrolysis from sodium hydroxide (NaOH) as a means of storing the sun\'s radiant energy collected during daytime hours, provides numerous benefits including safe, reliable and economical logistics. The scalable, self‐contained sodium (Na) metal production plant that stores the sun\'s radiant energy in sodium (Na) metal, can be constructed in almost any geographic location on earth benefitting from ample solar irradiance and clear weather all year. In the U.S.A., the arid, southwestern desert region offers the requisite conditions, including sufficient undeveloped land area to construct scalable, self‐contained solar powered electrolytic sodium (Na) metal production plants by the thousands. Using the southwestern desert region that includes West Texas, New Mexico, Arizona and Southern California as a hub for solar powered sodium (Na) metal production by electrolysis of sodium hydroxide (NaOH), it is possible to develop sufficient Na metal production capacity based on the scalable, self‐contained sodium (Na) metal production plant described, to meet the U.S.A.\'s energy needs for motor vehicle transport and for broader clean electric power applications.
The physical and chemical properties of sodium (Na) metal and sodium hydroxide (NaOH) render these materials ideal from an operational logistical standpoint. The sodium (Na) metal is a solid at room temperature and therefore has negligible vapor pressure. As a result, Na metal can be stored almost indefinitely in hermetically sealed packaging that can be opened much as a sardine can, only when the Na metal must be loaded into a hydrogen generation apparatus to react with either salinated (sea) or desalinated (fresh) water (H2O) according to Eq. (1), to produce hydrogen (H2(g)) fuel on demand [37]. The sodium hydroxide (NaOH) byproduct of the hydrogen producing chemical reaction in Eq. (1), is also a solid at room temperature in its pure form and has negligible vapor pressure. The NaOH is miscible with water in all proportions, enabling the aqueous NaOH(aq) solution to be readily transferred by pumping into and out of sealed tanks for transport by truck, rail car or pipeline to the remotely located self‐contained sodium (Na) metal production plant units for reprocessing by electrolysis according to Eqs. (2) and (3), to recover the Na metal for reuse in generating H2(g) fuel. The NaOH(aq) transportation/storage tanks of the type shown in Figure 2, can be fiberglass or metal with a corrosion resistant internal rubber liner, and must seal hermetically to exclude ambient air that contains carbon dioxide (CO2) which slowly degrades the NaOH(aq), albeit not irreversibly.
To obtain a sense for the magnitude of the logistical effort needed to produce and distribute sufficient sodium (Na) metal to fuel all of the motor vehicles in the U.S.A. while recovering the sodium hydroxide (NaOH) byproduct for reprocessing by electrolysis, it is necessary to consider the total number of vehicles in circulation. According to the Bureau of Transportation Statistics at the United States Department of Transportation (DOT), the total number of registered vehicles in the year 2013 in the U.S.A. numbered 255,876,822 [104]. The figure includes passenger cars, motorcycles, light duty vehicles, other 2‐axle/4‐tire vehicles, trucks with 2‐axles/6‐tires or more and buses. If it is further assumed that each motor vehicle on average consumes the energy equivalent of 16.2 gallons of 100 octane gasoline (2,2,4-Trimethylpentane) per week, then the corresponding quantity of H2(g) fuel having an equivalent heating value is given as 15.8 kg. The generation of 15.8 kg of H2(g) fuel according to Eq. (1), requires that 361.6 kg of Na metal react with approximately 300 kg of water (H2O) [37]. Therefore, the total quantity of Na metal consumed per week in the U.S.A. can be calculated as 255,876,822 vehicles × 361.6 kg/week = 92,525,058,835 kg/week. If each solar tower produces a mass mNa = 30,000 kg/day of Na metal, then in one week the Na metal yield per solar tower will be given as 30,000 kg × 7 days = 210,000 kg/week. The number of solar towers required to meet demand for Na metal will be given as NST = 92,525,058,835 kg/week / 210,000 kg/week = 440,596 solar towers or approximately NST ≈ 450,000 solar towers. While the number of solar towers required might seem very large and the task of constructing them onerous, it is in fact possible to construct the sufficient number of towers to provide Na metal for all the motor vehicles in the U.S.A. The self‐contained sodium (Na) metal production plants can be constructed at a density of approximately ρplant = 30 plant units per square mile to prevent mutual shading when the towers are elevated and rotated to track the sun. The solar tower density and layout necessitate a land area given as Aland = NST / ρplant = 450,000 / 30 = 15,000 square miles, to meet the Na metal demand for all the motor vehicles in the U.S.A. using PV device panels with an efficiency ηPV = 90%, and a land area Aland = 75,000 square miles using PV device panels with an efficiency ηPV = 18% that currently exist commercially. The land area required can be placed into perspective when considering that the area of the state of New Mexico is approximately 121,000 square miles and thus, there exists more than sufficient desert land area for constructing the scalable, self‐contained sodium (Na) metal production plants in the U.S.A.
Our company believes that hydrogen (H2(g)) fuel will earn an important role in motor vehicle transport applications for powering smaller 1–5 kW class secondary power fuel cells for onboard continuous recharging of battery electric vehicles (BEVs), a concept implemented successfully in the 1960s using H2(g) fuel stored in high pressure cylinders [23]. The concept of a smaller hydrogen fuel cell operating at a fixed power output level to continuously recharge an electric storage battery can be extended not only to motor vehicle propulsion systems but also for a broad range of clean electric power applications, including general ground transport that includes commercial trucks, trains, maritime transport as well as powering single family homes, commercial establishments and industrial enterprises. Such an approach will ultimately enable mankind to dispense with use of carbon based fossil fuels for motor vehicle transport applications and most other types of ground based electric power generation.
The technical and economic viability of a novel, scalable, self‐contained solar powered electrolytic sodium (Na) metal production plant has been demonstrated for meeting the hydrogen (H2(g)) fuel clean energy needs of the U.S.A. The scalable, self‐contained sodium (Na) metal production plant uses a solar tower PV device panel array to collect and convert the sun\'s vast radiant energy emission produced by hydrogen fusion, into electric power that is used to recover sodium (Na) metal from sodium hydroxide (NaOH) or from a mixture of NaOH and NaCl by electrolysis. The Na metal can subsequently be reused to generate H2(g) fuel and NaOH byproduct by reacting with either ordinary salinated (sea) or desalinated (fresh) water (H2O). The scalable, self‐contained sodium (Na) metal production plant operation is enabled by a specially designed voltage step down PWM DC‐DC converter consisting of a multiphase converter topology with up to 32 synchronous voltage step down converter circuits connected in parallel. The PWM DC‐DC converter has a fixed output voltage VOUT ≈ 124 V and variable input voltage VIN = VST, that corresponds to the output voltage of the solar tower PV device panel array and can be controlled to maintain the PV device panel array operating near the maximum power point (MPP). Each scalable, self‐contained sodium (Na) metal production plant consists of two voltage step down PWM DC‐DC converters, wherein each unit supplies 25 NaOH electrolytic cells, electrically connected in series, with a current ICELL = 96,500 A, corresponding to approximately one mole of electrons per second. The scalable electrical design of the solar tower allows the PV device panel array to be upgraded with newer and more efficient PV device panels as they become available as a result of progress in scientific research and development. Once PV device panels with an efficiency ηPV = 90% will become available, the power output of the solar tower PV device panel array can reach PST = 23.9 MW that is sufficient for producing a mass quantity of approximately mNa = 30,000 kg of Na metal per day from the electrolysis of NaOH. It therefore becomes possible to meet the hydrogen (H2(g)) fuel clean energy needs of all the motor vehicles in the U.S.A. by constructing approximately 450,000 scalable, self‐contained sodium (Na) metal production plant units of the type described, in the southwestern desert region of the U.S.A. that includes West Texas, New Mexico, Arizona and Southern California. If the land area needed for the scalable, self‐contained sodium (Na) metal production plant units becomes scarce, then purpose built ships equipped with the Na metal production plants can be dispatched into the vast ocean expanses near the equator where high solar irradiance occurs, to convert aqueous NaOH(aq) stored onboard into sodium (Na) metal before returning to port.
a | Length of the semi‐major axis of earth\'s elliptical orbit around the sun | (m) |
A, B | Matrices | |
AP | Photovoltaic (PV) panel area | (m2) |
APA | Photovoltaic (PV) panel array area | (m2) |
Aland | Land area | (mi2) |
AM | Air mass at mean sea level | |
AMa | Air mass at actual atmospheric pressure | |
Bl | Solar tower branch length | (m) |
Bsecl | Solar tower branch section length | (m) |
Bh | Solar tower branch height | (m) |
d | DC‐DC converter duty cycle | |
d′ | DC‐DC converter duty cycle, d′ = 1 - d | |
DC‐DC converter duty cycle small signal AC perturbation | ||
C | Capacitor value | (F) |
D | DC‐DC converter duty cycle DC value | |
D′ | DC‐DC converter duty cycle DC value, D′ = 1 - D | |
DS | Electrical conductor diameter | (cm) |
i | Indices | |
Er°, Eo° | Standard reduction, oxidation half reaction potential | (V) |
Eov° | Standard overall reaction potential | (V) |
E0 | Eccentricity correction for the solar constant | |
GP | Proportional circuit gain | |
GI | Integrator circuit gain | |
GD | Differentiator circuit gain | |
GV | DC‐DC converter open loop voltage transfer function | |
h | Hour angle | (°) |
hPV | Photovoltaic (PV) array elevation above mean sea level | (m) |
Hsr | Hours between sunrise and local solar noon | (hours) |
Hss | Hours between local solar noon and sunset | (hours) |
Irrn | Direct normal solar irradiance | (W/m2) |
IrrAM1.5D | ASTM direct normal AM 1.5D standard terrestrial total solar irradiance | (W/m2) |
IrrAM1.5G | ASTM global AM 1.5G standard terrestrial total solar irradiance | (W/m2) |
IB | Solar tower branch current | (A) |
IBsec | Solar tower branch section current | (A) |
IST | Solar tower output current | (A) |
IST‐MAX | Solar tower maximum output current | (A) |
IO | DC‐DC converter per phase output current | (A) |
IOUT | DC‐DC converter output current | (A) |
ICELL | Electrolytic cell current | (A) |
IPV | Photovoltaic (PV) device current | (A) |
ISC | Photovoltaic (PV) panel short circuit current | (A) |
IMPP‐P | Photovoltaic (PV) panel maximum power point current | (A) |
IMPP‐PA | Photovoltaic (PV) panel array maximum power point current | (A) |
ICE | IGBT collector‐emitter current | (A) |
ka | Aerosol optical depth or thickness | |
lo | Vertical ozone layer depth or thickness | (cm (NTP)) |
L | Inductor value | (H) |
mNa | Sodium mass | (kg) |
NP | Number of photovoltaic (PV) panels | |
NP‐B | Number of photovoltaic (PV) panels per branch | |
NP‐Bsec | Number of photovoltaic (PV) panels per branch section | |
NB‐L/R | Number of branches on the left or right of the solar tower | |
NB‐ST | Number of branches on the solar tower | |
Nφ | Number of phases | |
NST | Number of solar towers | |
Nday | Day number in a year from 1 to 365 | |
P | Absolute pressure | (Pa) |
PST‐L/R | Solar tower left or right half output power | (W) or (MW) |
PST | Solar tower output power | (W) or (MW) |
PST‐50 | Solar tower output power (50 towers) | (W) or (MW) |
Pw | Photovoltaic (PV) panel width | (m) |
Pl | Photovoltaic (PV) panel length | (m) |
rsun | Distance from the center of the sun to the center of the earth | (m) |
R | Resistor value | (Ω) |
RP, RS | Photovoltaic (PV) device parallel resistance, series resistance | (Ω) |
RTH | Thevenin equivalent resistance | (Ω) |
RC | Electrolytic cell resistance | (Ω) |
Sh | Solar tower structure height | (m) |
Sw | Solar tower structure width | (m) |
t | Time duration | |
T | Absolute temperature, ITS‐90 or Celsius temperature | (K) or (°C) |
TC | Surface temperature of IC package | (°C) |
Tf | Fusion temperature | (K) |
TP | Time period for a cycle | |
u, U | Vector, vector DC component | |
VNaOH(aq) | Aqueous sodium hydroxide volume | (Gal) |
VST | Solar tower output voltage | (V) |
VST‐MAX | Solar tower maximum output voltage | (V) |
VST‐DROP | Solar tower central column conductor voltage drop | (V) |
VIN | DC‐DC converter input voltage | (V) |
VOUT | DC‐DC converter output voltage | (V) |
VBAT | Utility scale battery voltage | (V) |
VCELL | Electrolytic cell voltage | (V) |
VOC | Photovoltaic (PV) panel open circuit voltage | (V) |
VMPP‐C | Photovoltaic (PV) single cell maximum power point voltage | (V) |
VMPP‐P | Photovoltaic (PV) panel maximum power point voltage | (V) |
VMPP‐PA | Photovoltaic (PV) panel array maximum power point voltage | (V) |
VTH | Thevenin equivalent voltage | (V) |
VCE | IGBT collector‐emitter voltage | (V) |
VSEN | DC‐DC converter scaled input voltage | (V) |
VSET | DC‐DC converter input voltage, set point reference | (V) |
VERROR | DC‐DC converter control circuit, error amplifier output | (V) |
VCONTROL | DC‐DC converter control circuit, PID circuit output | (V) |
w′ | Precipitable water thickness at actual atmospheric pressure and temperature | (cm) |
x, X | Vector, vector DC component | |
αs | Solar altitude or elevation angle above the observer\'s horizon | (°) |
αs‐max | Maximum solar altitude or elevation angle above the observer\'s horizon | (°) |
φShift | Phase shift | (°) |
γs | Solar azimuth angle | (°) |
Γ | Day angle | (radians) |
ηCELL | Electrolytic cell efficiency | (%) |
ηDC‐DC | DC‐DC converter efficiency | (%) |
ηPV | Photovoltaic (PV) device panel efficiency | (%) |
φ | Geographic latitude | (°) |
λ | Geographic longitude | (°) |
λe | Ecliptic longitude | (°) |
λL | Local longitude | (°) |
λS | Standard longitude | (°) |
θ | Angle between position of earth in orbit around sun and perihelion position | (°) |
θsz | Solar zenith angle | (°) |
ρplant | Photovoltaic (PV) plant density | (mi−2) |
τr | Transmittance by Rayleigh scattering | |
τo | Transmittance by ozone | |
τg | Transmittance by uniformly mixed gases | |
τw | Transmittance by precipitable water vapor | |
τa | Transmittance by aerosol | |
Rg | Molar gas constant | 8.3144621 (J/K·mol) |
Mair | Molar mass, air | 0.028964 (kg/mol) |
Rair | Specific gas constant, air | 287.06194 (J/Kċkg) |
e | Eccentricity of earth\'s elliptical orbit around the sun | 0.01673 |
F | Faraday constant | 96485.3365 (C/mol) |
g0 | Gravitational acceleration near earth\'s surface | 9.80665 (m/s2) |
Irr0 | Solar constant | 1367 (W/m2) |
P0 | Standard atmospheric pressure | 101325 (Pa) |
Psun | Power output of the sun | 3.8 × 1026 (W) |
Pearth | Power output of the sun reaching the earth | 1.7 × 1017 (W) |
Rsun | Radius of the sun | 6.96 × 108 (m) |
rsun‐m | Mean distance from center of sun to center of earth | 1.496 × 1011 (m) |
T0 | Celsius zero point, ITS‐90 | 273.15 (K) |
TEu | Eutectic temperature of NaCl‐H2O solution | −21.2 (°C) |
Tsun | Surface temperature of the solar black body | 5800 (K) |
Tes | Period of earth\'s rotation around the sun | 365.24 (days) |
Tea | Period of earth\'s rotation on its axis (mean solar day) | 86,400 (sec) |
ε | Obliquity or tilt angle of earth\'s rotation axis | 23.44 (°) |
δ | Solar angle of declination | −23.44 ≤ δ ≤ +23.44 (°) |
ηPVmax | Thermodynamic efficiency limit of PV device panels | 93 (%) |
φT‐CAN | Latitude at Tropic of Cancer | +23.44 (°) |
φT‐CAP | Latitude at Tropic of Capricorn | −23.44 (°) |
λPM | Longitude at Greenwich Prime Meridian | 0 (°) |
π | Number, pi | 3.14 |
ωea | Angular velocity of earth\'s rotation on its axis | 7.292115 × 10−5 (rad/sec) |
In this chapter, I reflect on my history pedagogy in a secondary ITE programme, and on ways a critical stance reimagined school history’s curriculum intent, pedagogies and outcomes for informed future-oriented young citizens. The backdrop of Aotearoa New Zealand’s society is introduced to indicate that citizen students and teachers move across a diversity of real and imagined ‘lifeworlds’ ([1], p. 176). Dynamic socio-historical forces are forging educational change in Aotearoa. Intercultural relationships and the centrality of Te Tiriti o Waitangi1 influence ITE and school history’s contextual and practice decision-making. Māori history is being introduced into the schooling curriculum as a foundational continuity of Aotearoa New Zealand histories. Secondary history education is positioned alongside discursive cultures and practices of the academy, public histories and professional, curriculum and assessment policy’ standardisation. Accordingly, as a teacher educator, I am pulled all ways in relation to history education’s identity, purpose, pedagogies and production [2].
Personal theorising of history and an awareness of curriculum and assessment discourses have disturbed my practice and shaped my critical pedagogy stance. Consequently, I have sought to disrupt pre-service teachers’ conceptual certainties about the nature and purpose of history and the school history curriculum within my teacher education work. I view the notion of ‘certainties’ as the reproduction of history education approaches whereby custom and practice pedagogies, teacher preferences and certitude, act to reproduce familiar contexts, narratives and voices. I reflect on a Problematised History Pedagogy (PHP) intervention designed and implemented within doctoral narrative research. The PHP explored how problematising history curriculum and pedagogy in history teacher education engaged self-fashioning of teaching identities, history conceptions, and reimagining’s of curriculum as discursive practice [3]. The PHP research design and a Dismantling Analysis (DA) method are introduced as a critical pedagogy approach. Aspects of the PHP findings are glimpsed through the pre-service history teachers as research participants’ voices. The PHP research processes and findings continue to inform my teacher education with pre-service history teachers, and I discuss history pedagogy in relation to young people’s lived citizenships.
The 1840 Treaty of Waitangi enabled Britain to establish sovereignty over New Zealand, legalise British subjects, and secure the economic benefits of imperialism. The Treaty of Waitangi and Te Tiriti o Waitangi illuminate language and values-based culturally encoded interpretations of sovereignty, and ways indigenous Māori were merged with British subjects by Treaty article. For Māori, colonising processes brought marginalised political representation, land loss, social and economic neglect and indifference for Treaty rights. Māori have never ceased to resist or to seek redress for breaches of Te Tiriti o Waitangi - viewed as a sacred covenant with the British Crown. The relationship between indigenous Māori and Pākeha settlers is a central feature of subjecthood and citizenship in Aotearoa New Zealand’s history of colonisation and recent decolonising processes. Ongoing migration has been a significant feature of citizenship and identity shaping of the governance of the settler state and of increasing cultural diversity [4]. The Royal Society of New Zealand’s census findings show that Aotearoa New Zealand is increasingly a country with multiple cultural identities, languages and values, and that one in four people living in New Zealand in 2013 were born in a diversity of places elsewhere than New Zealand. The report states:
“The most important example of ‘diversity’ may be in the range of ideas about what is represented and what is valued. A longstanding and deep-seated desire on behalf of the majority community to identify as New Zealanders with a single set of values and practices will be even less apt than in the past” ([5], p. 3).
Citizenship as an ideal will need to reflect this increasing diversity in relation to legal rights and political freedoms and choice, forms of economic and social equality, and identity and belonging. This may manifest as being community or service-minded, participation in clubs and societies, issues-based social action and/or global and digital awareness. Whilst citizenship assumes a body of common political knowledge [6], conceptions of ‘multiple citizenships’ challenge unitary citizenship ideals and politically focused citizen envisioning. Accordingly, identity is perceived in relation to a range of affiliations including national, cultural, religious, indigenous, ethnic and political and globalising processes, and citizenship is re-evaluated as an identity tied to the nation state [7, 8]. The cultural theorist James Banks interrogated liberal, assimilationist, and universal conceptions of citizenship in seeking cultural rights for citizens of diverse cultures, ethnicities, and languages [9]. Feminist and indigenous scholars have challenged assumptions of citizenship as unitary and inclusive sources of identity and belonging.2 Debates about Article 3 of the Treaty of Waitangi and Te Tiriti of Waitangi texts have considered the rights of citizenship, and whether Article 3 guarantees Māori equal opportunities or outcomes. Māori tikanga regulates ‘belonging’ and citizenship in Te Ao Māori3,4.
Citizenship in practice is a powerful cultural construct shaped by dominant groups’ values and beliefs about who might identify and belong as a New Zealander. Pearson [4] has conceived citizenship as double-edged, reflecting norms of inclusion and exclusion, and ideals of belonging voiced by those with the power to express and action ideals. This calls into question nostalgic and prevailing beliefs in Aotearoa New Zealand; that we are good at human rights. Findings of a study about New Zealand’s signing of six major human rights treaties since the 1970s and alignments with issues in contemporary society were reported in ‘Fault Lines: Human Rights in New Zealand’ [10]. It was found that New Zealand was slipping behind in relation to child poverty, gender equality, systemic disadvantage of Māori, and the rights of disabled people to challenge the state. Citizenship is contested, whereby Pākehā5 are one cultural group among many. Citizenship also presents an open space that values diversity and lived citizenships. Researchers Kallio, Wood, and Häkli offer a critical explanation of ‘lived citizenship’:
“…lived citizenship refers to issue-focused, relational and motivated political agency which involves specific orientation, reflexivity or intentionality. These non-essentialist criteria are intended to unsettle dominant notions of the citizen and to recognise the deeply varied experience of being a citizen – especially providing space for the inclusion of those traditionally excluded from the status and esteem of citizenship” ([11], p. 724).
In the context of disrupting certainties in history education, my reference to ‘lived citizenships’ aligns with schools’ history students’ diversity, embodied crossings of lifeworlds, and sense of belonging and agency.
In Aotearoa, history education is filtered through national curriculum and assessment policies and the teaching profession’s code of standards.6 University compliances around digital and online teaching systems, standardised outlines and performance-based research funded outputs also influence the design of teacher education curricula. History papers offered in universities constitute degree specialisms for the teaching of history in the senior years (11–13) of the secondary schooling curriculum. Depending on paper selection and the visions of academic historians, undergraduates engage with a range of historical approaches and discourses including for example, scholar traditional, social reconstructionist, indigenous, interdisciplinary. Pre-service teachers generally enter ITE with history offerings that may not connect with history contexts for study in senior secondary classrooms. Consequently pre-service history teachers fall back on traditional history contexts and approaches they experienced at school. Research findings that focused on teacher perceptions of ‘their’ history curriculum [12] indicated that within five years of teaching, history teachers had assimilated into cultures of school history that maintained certainty about claims to knowledge and perpetuated familiar contexts for assessment purposes. This finding confronted my pedagogy in ITE to activate critique.
The national curriculum policy [13] and qualifications frameworks [14] position history in the Social Sciences Learning Area. Whilst historical contexts, ideas, and skills can be developed in social studies programmes through all primary and secondary school years (1–13), history is taught as an optional subject specialism through Years 11–13 in the senior school. This is dependent on staffing expertise and capacity. In 2020 history education is positioned in competition with social sciences curricula offered in the senior school (E.g. sociology, psychology, education for sustainability, legal studies, business studies, tourism). However, students choose history because of an interest developed through junior secondary social studies, ‘cool teachers’, curiosity about histories that support hobbies, or through a sense of historical consciousness [15] that permeates daily lives.
My professional work in social sciences and history teacher education has involved contractual work for national curriculum and assessment developments. However, during the review and revision of the national curriculum (2007) I recognised my complicity with neo-liberal discourses of curriculum7 and grew increasingly disturbed with my pedagogy that provided little space for questioning why we do the things we do in history curriculum across ITE and schooling sites? Standards-based curriculum objectives could not be left unquestioned, particularly as the revised national curriculum developments [13] represented a shift back to a traditional and neo-conservative envisioning of the history curriculum. A default kind of curriculum was introduced by levels of history achievement standards for national certificates’ qualifications [14]. Despite opportunities for teachers to introduce unfamiliar contexts into history programmes, the history achievement standards’ guidance for teachers indicate ‘sacred’ [16] custom and practice teacher choices of historical content. Commmonly, teachers’ topic preferences focus on histories from Pākeha colonial perspectives, twentieth century theatres of war through discourses of sacrifice and nationhood, exclusive gendered experiences peppered with the ‘odd’ woman worthy. Despite standards-oriented possibilities to engage young people with historical inquiry, perspectives and source interpretation, curriculum objectives, assessment standards and contexts for study reflect a ‘history as progress’ discourse that serves to preserve normalised discourses of historical inquiry. Substantive content-based pedagogy is generally produced at the expense of the ‘hows’ of history. More troubling is the minimisation of the nature, purpose and ‘whys’ of history education. Standards shape historical ‘knowing’ in powerful ways [17]. However, there needs to be more to history in teacher education than reproducing standards’ outcomes from custom and practice uncritiqued pedagogy. In the following section, I recount my conceptions of history that have developed through research as a reflection of teaching selves, and as a response to potentially volatile moments of cultural production ([18], p. 5).
In my search for spaces of professional and academic negotiation in a university environment, discourses of critical social reconstruction and feminist and postmodern assumptions shape the educational selves I choose to embody and voice. I conceive teaching identities as multiple “…found in culture and thereby discursively produced and legitimated” ([19], p. 153). Teacher selves are revealed in power relations, gendered expectations, and learners’ assumptions ([3], p. 72). These selves are based on the “… immense significance of actual people and places, as real, as memory, imagination and desire in the formation of selfhood in teaching and learning”([20], p. 183). Despite the limitations of policy and structural arrangements and established cultures of school history, pedagogy in ITE can open spaces to reflect on why we do the things we do in light of our identities, and the selves we choose to be.
In this time of diversity and uncertainty, a questioning of knowledge claims shapes personal and professional history theorising. The notion of disturbance resonates with my history education work. It is in the observation of lived experiences of the past and the performative moments of disturbance and change, that dialogue can be activated between the past and present. Jenkins [21] argues that history be understood as a post epistemological aesthetic discourse as infinite refigurings and multiple meanings. Deconstructive histories bring into view previously discounted, unseen and unheard voices of the past. Research brings new inquiries and meanings, and historical narrative gets personal to reveal the historian’s voice. Brown offers a helpful explanation of historical narratives that disturb grand narratives and authoritative claims to history:
“A historian may deploy references to historical events in his/her narrative that are verifiably true, but her/his discourse is about selecting and bundling references to events of his/her choice into a periodised and boundaried-off interpretive narrative defined by her/him, that as a whole is invariably untestable. It is this narrative that is the real end product of the History profession, and if its constituent ‘small’ facts may be verifiable, the thing as a whole is fictive in form …” ([22], p. 171).
An exciting dimension of historical inquiry is when the nature, purpose and the ‘doings’ of history are emphasised. Rosenstone, a historian of film genre comments that history matters – that it needs to be meaningful:
“We must tell stories about the past that matter not just to us: we must make them matter to the larger culture. We must paint, write, film, hip hop and rap the past in a way that makes the tragedies and joys of the human voyage meaningful to the contemporary world” ([23], p. 17).
The historian’s motivations and uncovering of voices and silences, opens possibilities for curriculum history. Likewise, representation of lived pasts through a variety of literacies, media, and digital tools brings opportunity for new questions and meaning making to savvy citizen learners. This means pedagogic approaches may include access to sources of evidence of unfamiliar place-based pasts, multiple voices and human agency. Pedagogy involves a relational dialogic [24] that activates knowing and learning, and pedagogies are constructed to take material and social form. Mulcahy suggested pedagogy be viewed as an “emergent property or product of ‘intra-action’ among persons, places, processes and things” ([25], p. 57). I reflect pedagogies as connecting four key dimensions as: the immense significance of people’s identities and situatedness; relationships; embodiment and seeking of authentic selves; knowledge claims related to socio-historical, cultural, structural and material production of meaning. My growing disturbance with un-critiqued school history activated critical pedagogy research that I designed as problematised history pedagogy (PHP) [3]. The desire to mitigate powerful relations in classes and enable pre-service teachers to challenge exclusive historical representations, motivated me. Curriculum and pedagogic disturbance, reflexivity and resistance shaped my critical pedagogy as research within teacher education.
Critical pedagogies involve “understandings and critique of hegemony and power as an organising force in education” ([3], p. 78) to ask questions of the politics of curriculum through reflexive action. Influential cultural theorists have influenced my pedagogic stance. Henry Giroux and the late Joe Kincheloe have viewed teachers as intellectuals who understand power relations and the impacts of their pedagogies for self, society and cultures.8 When applied to history curriculum, critical pedagogy demands that we question what counts as history knowledge; whose interests this knowledge serves, and how curriculum and assessment as discursive production serve to legitimise existing forms of historical knowledge?
The PHP research as a ‘System of Meaning’ [26] was nested and constructed within my doctoral critical pedagogy methodology: ‘Problematised History Pedagogy as Narrative Research: Self Fashioning, Dismantled Voices and Reimaginings in History Education’ [3]. The wider narrative research methodology’s question set the scene: “How does problematising history curriculum and pedagogy in teacher education engage self fashioning of teaching identities, history conceptions, and reimagining of curriculum as discursive practice”? ([3], p. 1). Three further questions emerged to deconstruct this guiding question, and to create an original dismantling analysis method. These questions applied to self-reflexivity, engagement with problematised pedagogy, and evaluation of critical pedagogy and emergent pedagogic spaces ([3], p. 123).
The PHP involved my history class’ ten participants in fashioning teaching identities, identifying personal narrative stances, and thinking critically about activating and reflecting on history pedagogy in classrooms. The PHP research was implemented over a year within my secondary GradDipT year’s history programme in ITE [3]. The participants as history graduates from a range of Aotearoa New Zealand universities, brought their conceptions of history, lived experiences, and school to university – back to the classroom contextual preferences and knowledge to the class programme. The PHP research design drew on the late Joe Kincheloe’s thinking about critical pedagogy to shape a coherent ‘system of meaning’ ([26], pp. 224–225).
Figure 1: ‘Problematised History Pedagogy as a System of Meaning and Dismantling Analysis’ indicates the research design’s aim for coherence in relation to questions, processes, analysis, and its reciprocal layering within my wider methodology of critical pedagogy as narrative research. The PHP embedded three processes that I adapted from Joe Kincheloe’s critical action and reflexive research processes ([26], p. 224–225) namely: Phenomenological Empathy; Genealogical Disclosure; Discursive Self-Fashioning.
Problematised history pedagogy as a system of meaning and dismantling analysis.
Phenomenological Empathy elicited evidence of the participant’s values and reflexivity through the history class’ ongoing journal writing, critical discourse analysis of a self-selected history text and post teaching experience conversations.
Genealogical Disclosure initiated participants’ life-storying, socio-historicising of self-texts, and individual’s private and professional theorising of the nature and purpose of history in the senior school curriculum.
Discursive Self-Fashioning involved participants in designing, implementing and critiquing sequences of their own problematised history pedagogy with history classes whilst on their second practicum. This involved teacher identity work, and formative stages of engaging with curriculum and pedagogy as pre-service history teachers.
The research processes constituted both on-campus class (56 hours) and practicum experiences (14 weeks) of activities interspersed throughout my programme of history pedagogy through February–November. The PHP timetabling was indicated within the paper outline, along with detailed guidance for the life-storying, critical discourse analysis (CDA) of a self-selected text, and participant’s planned and implemented sequences of PHP. By mid-April participants had completed their life histories as self-storied accounts; by mid-July the critical discourse analysis (CDA) of self-selected curriculum history texts was complete. Over the second teaching experience (August–September) class members designed, planned, facilitated, and evaluated a sequence of PHP (3 teaching episodes) in senior secondary history classrooms in response to their own pedagogic disturbance. These evidence-based processes were shared through the year in our history class, and are presented as PHP case studies in the wider narrative research [3]. Participants’ journal writing, and their experiences of research processes were regularly shared and discussed during our history class pedagogy. None of the research processes was designed for, or used for assessment purpose. To meet coursework outcomes within the secondary teacher education programme, participants completed three assessment items that were not part of the PHP research. Accordingly, an external examiner assessed this assignment work.
A Dismantling Analysis (DA) resonated with my history and social sciences theorising and practice, and aligned with critical gazes within the wider narrative research methodology. Deborah Britzman’s thinking about ways postmodern thinking expands the range of available interpretive schemes to make possible readings of cultural texts influenced my decision about a method of analysis and interpretation [27]. I sought deconstructive and interpretive purpose to identify contradictions, normalised discourses, disturbances, and resistances in the participants’ PHP research processes. The notion of ‘mantle’ embodies ideas of expertise, identity, validation, knowledge, wisdom, and authority. However, in the DA, I conceptualised the idea of ‘mantle’ in critical ways: Hence ‘mantle’ symbolised a curriculum boundary, was viewed as a layer of hegemony, represented powerful discourse, acknowledged as a cloaked and weighty tradition and as essentialist notions ([3], pp. 133–134). The DA involved recursive interpretive work to unravel participants’ private and professional theorising of history and curriculum representation, pedagogic identities, conceptions of pedagogy, and critique of history as cultural texts. Figure 1: ‘Problematised History Pedagogy as a System of Meaning and Dismantling Analysis’ indicates the three research processes and indicators of participants’ activities that generated evidence collection beyond their reflective journaling. Six themes are identified from the PHP system of meaning, and their indicators assisted data organisation, and the deconstruction and interpretation of participants’ thinking and actions as pre-service history teachers. Interwoven themes were loosely identified as: Private and professional theorising of history; Pedagogic identities; Conceptions of history pedagogy; Conceptions of history curriculum; PHP as cultural texts; Historical representation.
The ethical challenges of embedding research within my history coursework required careful consideration in the research design. Halse and Honey [28] discuss the power politics that are in play with research. A key issue was that the PHP was embedded in my year’s history course, and identity positioning and conflicts of interest needed to be made clear and minimised wherever possible. The research ethics served as a cautionary reminder of the vulnerability of the pre-service history teachers and of my interpretive authority.
The history class comprised ten pre-service teacher participants – eight women and two men mostly aged in their twenties with the exception of two women in their 30s, and a woman aged 51. One participant identified as Māori, and two participants identified biculturally as Māori and Pākeha. Seven class members identified as Pākeha or New Zealand European. All but two class members had experienced school history in their senior secondary years. Each participant brought a variety of history papers combined with social sciences or English papers to their degree qualification/s. Three participants had knowledge of research methods of history, and only five had explored political or cultural aspects of Aotearoa New Zealand histories. The participants’ research processes were produced within planned and identifiable history education contexts, and a collective sense of purpose. However, the PHP did not seek uniform conceptions of self, history thinking, or pedagogy. In my narrative research I attempted to evoke something of the participants’ selves and dispositions as heartbeats pulsed underneath the PHP research processes. This was textured through my professional knowledge of each individual formed through class pedagogy, practicum observations, dialogue, and the relationships we formed. Aspects of emergent PHP findings are discussed as follows in relation to participants’ Private history theorising; Pedagogic identities; Threshold experiences with school history curriculum and pedagogy; Public and accountable discursive practice. Participants’ visibility is included here through glimpses of their voices as evidenced by texts generated by the PHP research processes.
Participants’ genealogical disclosure evidenced in reflective journal writing and autobiographical life-storying generally conceptualised history as living in the past. A discourse of connectedness to the lived experience of the past dominated the class’ historical thinking. Family traditions, heritages of shared values through myths, folklore, stories of heroic deeds, and links to ancestors recollected cultural experiences, values and temporality. A strong discourse of memory and nostalgia permeated participants thinking about connectedness to lived experience. ‘Nostalgia’s’ meaning derives from the Greek nostos (returning home) and algia (pain or distress). This embraces feelings about a disappearing past, temporal dislocation, imagined places and anxiety about change. Max and Jude, the two youngest class members described their families as typical white middle class families. Their self-storying reveals nostalgic discourse. Ƒor Max, childhood and life events were recounted through “the heavy filter of memories …as the mind’s projection unit.” An uncomplicated childhood full of talking animal stories as in Kenneth Grahame’s Wind in the Willows, and Joel Chandler Griffiths’ Uncle Remus stories was evoked. An idealised childhood complete with Grandfathers whose “glory days had come and gone during World War 2” was reimagined ([3], p. 147). Jude’s life-storying was informed by her interest in film representations of history and was accompanied by a DVD slideshow of arresting images. One image stirs memories of a New Zealand summer holiday in the mid-1980s: a campsite by a bay; dinghies beached on the shore; and Combi vans tucked into the shadow of bush-covered hills. A woman [grandmother?] is walking away from the photographer along a track between Pohutukawa trees and water. Her back is straight and strong, and she carries a young child in each arm with comfortable balance. One child looks forward, and the other child looks backwards. Jude chose to place this image at the end of her life history narrative. Whilst keeping the reader wondering, the image suggests Jude’s strong sense of family, her nostalgic view of the past, of moving into the future, and of the landscapes that move her ([3], p. 147).
Participants reflected their phenomenological empathy and values and beliefs about history through reflective journal entries in class, critical discourse analysis of self-selected history text, and conversations. History was commonly conceptualised as a discourse of history as lived outside the past – meaning history’s external representation through the voices of observers and interpreters living outside the past. Marie, who had worked as a museum educator prior to entering secondary teacher education, had a professional and critical awareness of ways historical experiences are represented in the present. Marie wrote about her visit to the United States Holocaust Memorial Museum in Washington DC to indicate her values and beliefs about history.
Marie’s Visit to the Holocaust Museum, Washington.
“I set out one morning for the Holocaust museum. We were lined up outside to go individually through the bag and weapon check, with even our water bottles inspected for potential poisons. Then it was up in an elevator to the top floor to begin the exhibition.”
“As a museum professional I am always looking at the text and labels, checking for display ideas, use of font and graphics etc., so spent considerable time on the first floor in the introductory stage of the exhibition, and was surrounded by classes of secondary school children. As I made my way down the exhibition levels, I grew increasingly depressed at the story being told. I already knew what happened in the Holocaust but to see footage of Jews having lobotomies whilst they were awake, the metal bins filled with parts of bodies—I thought I was going to throw up in the room. The school students were crying, and then we turned a corner and were faced with the hundreds of shoes left by those who had been sent to the gas chambers. It was the most effective and disturbing museum I have ever been to and I still get flashbacks to the film footage and feel a wave of nausea. My uncle who I was staying with in Washington is Polish and had managed to get out of Poland during the War. [He] was very upset that I had gone to the museum. He said it was in the past and it should stay buried, and why do young people want to see such things. I could not really answer him after seeing it.”
Marie’s encounter with Holocaust history brought moral and ethical issues, and history’s representation and purpose in contemporary contexts into sharp relief. The DA uncovered the tensions and difficult moments Marie negotiated between her professional role as a museum educator, and her uncertainties about re-imagining and re-storying history in contemporary society ([3], p. 156). John’s view of history as “uncertain, dependent on interpretation, and individual perceptions” offered a space for a more critical interrogation of historical contexts, and the historian’s motives ([3], p. 152). Ana understood history as socially constructed, and culturally reproduced. She described history as “the multiplicity of the past and present—a fractured multi-faceted discourse” and viewed ideas and concepts that shape the investigative historian’s perceptions as “no less pervasive than the ideas and concepts behind any historical context.” ([3], p. 156). In Jude’s life-storying, she articulated the work of history in this expansive conception:
“History is an area of life that increases understanding of human nature and the world around us. It allows us to know what events, ways of life, people and landscapes there were in the world. It also inspires and creates human emotion and empathy, encourages use of imagination, and interaction with others to express understandings and perspectives.” ([3], p. 158).
All participants expressed certainties about their abilities to interpret historical perspectives as observers living outside the past, and to judge experiences of the past in light of their own values and perspectives. They presented their understandings of interpretive and perspectival thinking with an assuredness about their historical abilities and knowledge to judge others past experiences. However, only three participants had studied any aspect of Aotearoa New Zealand histories and experienced something of history research methods. A discourse of uncertainty in relation to participants’ historical knowledge was evident in their feelings of doubt and discomfort with the affective force of ‘difficult knowledge’.
Participants’ journal reflections prior to their first practicum revealed pedagogic identities and voices. Max’s discourse about shaping a teacher identity was influenced by history teachers he had revered as “people of substance, wisdom, insight, and maturity” ([3], p. 164). Marie invoked her identities and roles as a museum educator, international traveller, observer, employee and student to illustrate her scholarly discourse and high expectations of selves. Val’s sensitivity about body image, and her fears of colleagues’ perceptions were at odds with her outwards confidence and expressive voice. As the May school practicum edged closer, class members revealed pedagogic identities. A powerful discourse of embodiment was expressed as feelings of fear, failure and fraud in relation to becoming history teachers and not meeting colleagues’ professional expectations. Vulnerability and eccentricity were glimpsed in their embodied teaching selves. Maya emphatically reflected: “Then it’s practicum for six weeks! I don’t want to go! I don’t want to go! I DON’T WANT TO GO!!!! I am feeling anxious, nervous, petrified, and generally just scared” ([3], p. 165). A fear of not knowing and feeling like a fraud as a teacher proved a compelling discourse. John experienced panic attacks about not being interesting or effective. Ruth was fearful of not being a successful history teacher. Rosa worried about dealing with disruptive junior students. She feared judgement about “any failure to instill strict discipline over what are problematic classes … my mission is to steel myself to cope with teaching them.” ([3], p. 166). Ways that sense is made of history as teachers and learners have a powerful effect, because our ways of knowing are negotiated through embodied identities and relations. Class members’ thinking about teaching history was shaped by experiences of school history’s discursive production and ways of doing, being, and valuing. School history might be conceptualised as a site of cultural politics in education where hegemonic structures favour desired qualities and material practices over others. The underbelly or often hidden side of education was exposed by the participants’ discourses of embodiment. Uncertainty about pedagogic selves, identities and history knowledge was filtered through participants’ lenses of educational experience.
The participants experienced their first practicum experience after three months of class work and PHP research processes. Curriculum discourses in our class work and practicum preparation presented entry points for understanding school history. Participants’ phenomenological (interpretive) empathy and self-fashioning in relation to history curriculum and pedagogy was elicited through their journal writing and post practicum conversations. Glimpses of threshold experiences of history curriculum and pedagogy are shared as follows. History’s curriculum purpose was not questioned during pedagogic approaches participants experienced whilst on practicum. History pedagogy was experienced as an exclusive citizenship orientation, a kind of unquestioned and unconscious narrative of nationalism and national identity discourse. However the concept of ‘citizenship’ was not referred to as such.
Whilst the discourse of fears, fraud, and failure seemed to go underground as participants settled into practicum, their narratives reveal resilience as they came to grips with colleagues’ expectations and approaches. The wanting to fit in and to be taken seriously meant most participants were reluctant to seek guidance, ask questions about where to find resources and access information, or reveal they had no knowledge of contexts they were teaching. The participants’ shift from a focus on teaching selves to viewing history students as learners, and thinking about history pedagogy as relational pedagogy, was evident in journal reflections and post practicum conversations. Marie reflected on her attempt to bring some meaning to the context of New Zealand’s political leadership by working with skills of historical empathy.
“And so I wanted to use historical imagination. They had to produce a brochure for the 1972 election and imagine themselves as Norman and say where they are going to take this country and why we are going to vote for him. I thought it was an interesting thing to do BUT a lot of the students including the brighter ones would go: I’d rather just write notes Miss, can’t you just put it on the board? Why do we have to do this?” ([3], p. 183).
Participants (Ana, John, Adele, and Marie) questioned learners’ inquiry of specific events-based information of an historical context, when there appeared to be limited engagement with or understanding of human agency, wider social forces and movements, or unpacking of concepts and ideas. John reflected that the Russian Revolution history he taught on practicum was focused on knowledge transmission to pass an NCEA externally assessed examination:
“I wanted to develop their appreciation, their knowledge of history. I don’t even know if it was the NCEA’s fault, or just the way the school did deal with NCEA. But to me it was all driven towards achieving the credits and not about appreciating the subject” ([3], p. 184).
Marie and John evaluated the historical contexts they worked with as difficult for students to make sense of in light of their ages and life experiences. John reflected on the relevance of the Russian Revolution for his students.
“I do suspect they enjoyed it but I think the context would have been most challenging, just trying to work out why things occurred, and how that was particularly relevant. Just, how things have developed today, it would have been a different situation if it happened now: and just trying to link that back to the past and work out why the Tsarist regime behaved the way it did: the whole divine right of Kings and all that kind of stuff, and just the whole different political structure I guess. Trying to understand and get a feeling for the situation at the time would be the most challenging for them” ([3], p. 184).
Most participants supported a need to understand New Zealand histories through bicultural lenses. Whilst this was not their experience of the history curriculum, it was their hope for things to come. Ruth was intensely affected by colleagues’ cursory approaches towards or disinterest in New Zealand’s history.
“I often hear that New Zealand history is boring. I hate it when people say [mimics voices in a dramatic whisper]: ‘Oh why are you doing New Zealand history? It doesn’t have much history, it is only a young country’. Stop patronising! I usually say: “Do you know anything about New Zealand history? It doesn’t sound like you do!” ([3], p. 185).
Val and Jude were disturbed by the overtly male-centred nature of history programmes they encountered through ‘sacred’ topic preferences and the dated authorship and sexist nature of many history resources. Val reflected on her associate teacher’s approach:
“Being a boys’ school, they really responded to it because they could easily get him off track by asking about guns and tanks and stuff. It was a really positive experience for them and the boys really enjoyed history, because he knew a lot about what they wanted to know about. That’s where I felt I was failing. My weakness as a history teacher is that I have no interest or knowledge in the sorts of history that boys care about” ([3], p. 186)
Jude and Val observed that women’s historical representation was generally addressed as an afterthought. They questioned the conflict-oriented contexts that both young men and women seemed to enjoy. Val reflected on the importance of young men having ‘ownership’ of historical knowledge, but saw this as compromised if historical knowledge was one-sided. Jude reflected: “History cannot be taught effectively if the learners have warped ideas of it and are therefore confused and biased to begin with” ([3] p. 189). They attempted to introduce aspects of women’s historical experience into the topics they taught as purposeful and ‘culturally just’ learning.
Participants recounted their relationships with history colleagues when thoughts were shared about ‘fitting in’ with associates’ pedagogies. Val felt a sense of “guilt and shame about not putting the hours of prep in as her peers.” She perceived her weaknesses and she longed for positive mentoring and constructive feedback rather than ambiguous comment:
“I had a really good lesson with them and I said to my associate “that was a good lesson.” She said, “you reckon!” But she said it in a ‘loving’ (not hostile) way because I had built up too much into being a yelly person and her method of teaching is not that”. ([3], p. 187).
Ana, who saw herself as an advocate for students, found aspects of her history associate’s pedagogic relationships at odds with her vision of pedagogy.
“He’s very passionate about whatever he is doing, and he definitely has a love of it. He has such a huge knowledge base and I think there are certain students that connect with that. But it is very obvious that if you don’t fit his mould of an accepted person, they’re actually wiped quite succinctly, clearly, and labeled” ([3], p. 187).
Inside their threshold experience of school history, participants observed the intended, implemented, and outcomes-based history curriculum largely as substantive reproduction of events-based facts. Few engaged with, or initiated pedagogy that questioned learning outcomes or students’ passivity and disengagement in classrooms. A recurrent discourse articulated their impressions of teachers’ contextual choices as conflict-based and violent. Participants’ reflexivity revealed their rapid socialisation into discourses of teacher professionalism that I am also positioned within. Professional loyalty meant caution in not voicing overt criticism. Loyalty towards their practicum schools and colleagues was evident in the respectful and considered way participants recounted their experiences. Silence might be interpreted in the discourse of teacher professionalism as a shared understanding of what was known, but could not be voiced.
As a follow up to the first practicum experience, participants completed a Critical Discourse Analysis (CDA) of self-selected history textual material commonly used in school history programmes. As a PHP research process, I wanted to engage participants in thinking about how texts construct representations of history and the past, identities and historical relationships, and authority and control. Dismantling analysis of the participants’ CDA revealed that half the class had no prior experience of textual analysis. Whilst participants could identify historical contexts and settings, narrative purpose, and curriculum connections with ease, the identification and analysis of discourses, dominant themes and ideas proved a new and challenging interpretive process for half the class. John’s social reconstructionist curriculum orientation opened a space to reflect on counter-narratives of stories not told, contingency, and human dilemmas. Max’s CDA revealed an awareness of meta-narratives and omissions in historical accounts. Ana’s CDA noted: “The realisation of the ease with which a history can be reinterpreted, and re-constructed through further analysis, exemplifies the interpretive nature of history, the historical process, and the multiplicity inherent in the past.” ([3], p. 203). Max, became absorbed in his CDA, and he wrote with passion about the discursive practice of history as written by the victors, and stated:
“We still give texts such as this to our history students: Where are the ordinary people?”
“According to the text they aren’t important enough to talk about, even collectively! History from the top down – politicians, war-mongers, politics, wars, countries and national desires, conquests and losses. Perhaps the authors are constructing a nice sanitised version of the events leading up to WW2, as if to demonstrate that these “important dates”, places, and people they discuss, are agents somehow able to act in isolation from the peoples they represent. Common people do not make history in other words” ([3], p. 205).
Adele was disappointed with the limited cultural perspectives and misplaced gendered assumptions in her selected text where generic characters were prescribed by the author’s descriptions:
“There is also an assumption in this text that all men were against prohibition and all women for it. Perspectives that should also be examined are those of women who were opposed to suffrage, and men who supported it, as these were important gendered perspectives in the suffrage debate. The author only allows two reasons that men were opposed to suffrage – social status or lack of capacity. This is a Eurocentric exemplar that suggests Maori were not concerned with issues of suffrage. As all Maori men over the age of 21 [could] vote from 1867, it would follow that all Maori would have a stake in the suffrage movement too” ([3], p. 205).
Ana had worked with her selected text with her first practicum class. She perceived that opportunities for stimulating and challenging students’ historical thinking with the ‘quality’ text were not explored:
“[The author] organised and structured this text in a manner that required further active pedagogical engagement than that witnessed. Many students displayed an insightfulness that reflected the qualities of this text and their own level of intellect, rather than the success of the pedagogical style of the teacher” ([3], p. 206).
Few participants felt confident in engaging with historical research methods or analytical skills processes, to uncover and interpret evidence. Interestingly, none of the participants questioned the nature of or the historical purpose of the historical contexts that their texts represented.
Participants’ discursive self-fashioning was revealed through their conceptions of school history curriculum and their pedagogic desire, disturbance and critique. They designed, facilitated and evaluated their own sequences of PHP within a history class in the second practicum. Their PHP cases as a research process drew on their observations and reflections of an aspect of history pedagogy that disturbed them. Participants’ PHP revealed pedagogic voices, identities and relationships in the enacted school history curriculum. Table 1. Participants’ Pedagogic Disturbance and Decisions to Problematise History Pedagogy provides an overview of the PHP decisions designed for implementation within associate teachers’ history classrooms. The problematising contexts mirror my experiences of the history curriculum in its promotion of a Eurocentric male-focused canon of topic contexts. However, these contexts were not the participants’ choice, and they were fortunate to implement teaching experience within schools’ history programmes. Participants’ accounts of their students’ responses to the history curriculum exposed a disturbing picture of student disengagement. Consequently, most of the PHP decisions attempted to mediate this situation by building supportive relationships with students. Whilst the purpose of history programmes did not seem apparent to learners, the authority and perceived threat of the NCEA history assessment hung over them (reflected by all participants). The PHP ‘cases’ as storied into my wider narrative research [3] indicated history students’ disengagement and confusion with their history learning.
Participants | Curriculum Disturbance | Problematised History Pedagogy |
---|---|---|
John | Year 11 students’ perceptions of the actions of/ historical significance of Black Civil Rights leaders USA 1960s | Introduced counter-narratives to engage students in thinking about moral and ethical issues re protest and conflicting positions |
Adele | Year 12 students’ limited contextual and conceptual understandings re. Conflict in Indo-China/Vietnam 1945–1970s | Intensive focus on ideas, e.g. nationalism and identity to support essay writing skills |
Val | Year 12 ‘unwilling’ students’ limited understandings of/organisation/information re the Irish history topic | Established reasons to be learning about history: Essay writing skills and ascertaining students’ conceptual understandings |
Maya | Year 11 students’ ‘disinterest’ in history – World War 2 topic | Focusing students on the relevance of history, and exploring perspectives and viewpoints |
Marie | Year 13 students’ ‘unproductive’ independent learning re. Early Modern English history 1558–1665 | Surveying students’ strengths and weaknesses re history context (knowledge/skills processes/preferred pedagogy). Provision of informed pedagogy |
Ruth | Year 11 students’ limited engagement with human agency/motivations and historical empathy re. Irish republican movement 1916–1919 | Facilitated activities for students to embody the history they were revising – historical imagination and empathy |
Max | Year 11 students discussion sessions re. Black Civil Rights 1950s–1970. Discovery that a group of fearful students was dislocated from the class pedagogy | Activated strategies to observe students’ engagement in pedagogy and elicit students’ responses re historical understandings |
Jude | Year 10 students confusion with connections between random 20th century revolutionary contexts and WW2; Year 11 students boredom with Black Civil Rights 1950s–1970 history | Contextualised Hitler’s leadership and Nazism within a framework of documentary evidence; Focus on womens’ historical experiences and representation |
Ana | Year 12 students’ passive engagement with historical texts re. Vietnamese nationalism 1945–1975 | Facilitated textual analysis and interrogation to stimulate critical thinking. |
Rosa | Did not undertake this research process due to personal circumstances. |
Participants’ pedagogic disturbance and decisions to problematise history pedagogy.
PHP cases exposed history students impressions that history is mostly about note-taking and information about events, cause, effect and consequences, and essay writing – a skill they found demanding and difficult (Adele, Max, Marie, Val, Ana). Concerns about students’ literacy skills were apparent in participants’ decisions to focus on conceptual understandings, revision processes/making sense of information, supporting learning needs, and their rejection of transmissive approaches. The DA of the PHP cases indicates participants’ thinking about their responsibilities as beginning history teachers, and something of their responses to, and reimagining’s of history pedagogy in the school curriculum. The participants’ PHP advanced critique into public spaces of curriculum and assessment policies. Whilst PHP was possible, it was activated within class programmes that embedded teachers’ values and topic preferences, standards’ interpretation, and the use of traditional texts. Despite these constraints, participants acted on their situated disturbance to engage students in pedagogy that was generated as something different. The PHP decisions might be perceived as the practice that teachers and students need to engage with every day, rather than as critical practice. However, the PHP did prompt critique of normalised discourses, exclusive knowledge claims and pedagogic assumptions. Participants reflected on the dominant orientation of school history as a form of inquiry whereby information gathering involved transmission of prevailing knowledge claims. This orientation reflects curriculum and assessment positioning as the public and accountable approach to school history. This was viewed as problematic in terms of reproduction of exclusive and normative thinking. Likewise the orientation of history as shaping and connections was viewed as problematic in relation to nostalgic memory work, unquestioned national narratives, exclusive citizenship and knowledge claims. Glimpses of three reimagined counter orientations were reflected as history as democratic and inclusive (Val, Max, Marie, Ruth) history as social reconstruction (John), and history as a critical project (Ana).
Participants expressed the desire to be ‘switched on’ teachers. This was generally seen as being informed, active, purposeful, observant, dialogic, and inclusive. The participants’ PHP cases are storied in my wider narrative research, and present rich evidence of pre-service teachers’ motivations to engage with history learners. With a sense of being a ‘subversive teacher’ John designed his PHP to go beyond topic constraints and he focused on ways students perceived the historical significance of the civil rights leaders Martin Luther King, and Malcolm X. In evaluating his PHP, John reflected on moral and ethical issues raised in conflict-focused historical contexts:
“Yeah, parts of it worked, parts didn’t. They were still keen on the violence, and I don\'t know if that\'s really because of the two leaders, or just because nowadays kids are into violence and people dying, and war games and that sort of stuff. But when you try and wind it back to the curriculum and the material that you are going to teach, it’s the people dying that gets them going!!! Whereas the whole values and the reasons behind them, the philosophies and all that kind of stuff, it bores them. I found dealing with that was kind of hard. Because my focus was on Martin Luther King and Malcolm X, I wanted the kids to have a broader understanding of the cost of violence” ([3], p. 210-211)
Whilst John enabled his students to form their own opinions and think of different perspectives, he reflected on his PHP motivations to question whether he was being a subversive teacher or a teacher pushing his agenda. Ana’s PHP involved textual analysis as informed by her CDA. She expressed a clear purpose for her PHP that “all text/sources can be open to question and critique, and should therefore not be consumed passively as orthodox and authoritative” and further reflected:
“I believe that only through the active engagement with text, its deconstruction, evaluation, and analysis can students gain the history skills necessary to successfully critique and evaluate the historical information, perspective and bias inherent in any text. This skill is an absolute necessity for the comprehension and understanding of the multiplicity of history in the past, and in essence the diversity of the wider world today. The gaining of this skill therefore becomes a practical and relevant tool for students studying the past, engaging with the past, and goes some way to justifying the relevance of the discipline of history itself” ([3], p. 218).
The PHP research has proved invaluable for my ongoing work in postgraduate teacher education. Emergent findings revealed participants’ reflexivity, and exposed gaps and weaknesses, certainties and uncertainties in conceptions of school history and historical thinking. As a consequence, I deliberately plan for and address the following elements of history education with pre-service teachers, and embed these within my course objectives and assignment work:
Making explicit the constructed narrative nature of history and focusing on historical representation through a range of media including digital sources;
Active deconstruction of historical texts (visual, audio, written, performance …);
Questioning uncritical and normative approaches to perspectives’ thinking and interpretation in pedagogy;
Countering tentative and apologetic approaches to inclusion of gendered and cultural historical agency and experience;
Identifying discursive and disciplinary orientations to history;
Modelling pedagogy that critiques the purpose of history for secondary students;
Identification of exclusive notions of citizenship, and finding ways to understand, confront, and become informed about ‘difficult knowledge’.
Two assignments in my history course work have evolved from the PHP findings and embed critical approaches to pedagogy. One is an E-Portfolio of scholarly research of an unfamiliar Aotearoa historical context relevant to senior history students’ interests. This involves pre-service history teachers in understanding and reflecting engagement with the pedagogic elements listed above. The second assignment involves the research and writing of an article for a history teaching audience that reports on pedagogic disturbance and PHP activated with history students as evidence-based practice.
The PHP findings inform critique of ways history is conceptualised in the national curriculum, and the national history curriculum history achievement objectives’ alignments (since 2007) with the national qualifications framework’s history assessment standards [13, 14]. In September 2019, the Government announced plans to teach New Zealand history in all schools and kura by 2022
The emergent research findings have implications for ways students as young citizens receive and understand history in the schooling curriculum. Students’ interest in and selection of history as a subject requires investment and innovation in approach. Years 11–13 students’ voicing of fears and confusion in relation to their history learning deserves critical attention. Normalised reproduction of topic preferences, often conflict based and centered on mens’ historical experience needs to be questioned in light of perpetuating inequalities and injustice. The PHP highlighted a prevalent view of history teachers that anything different or cultural or social in the history curriculum is an aberration, and likely to be rejected. The PHP findings are situated in my pedagogy and in a particular group of schools. I cannot claim that the findings are representative of all schooling sites.
My motivation for writing this chapter is informed by observations of senior history students who generally experience historical inquiry as disconnected from their embodied lifeworlds, cultural values and experiences. This also increasingly applies to pre-service history teachers as they negotiate the cultural politics of the history curriculum – often without knowledge of Aotearoa New Zealand histories, or conceptions of the nature and purpose of history education. An introduction to the contemporary context of Aotearoa New Zealand society sets the scene for a discussion of young people’s lived citizenships as fluid crossings of identities, and diverse cultures including cyber, popular, social media, and real and imagined spaces of belonging. My positioning as a teacher educator of social sciences and history is introduced in light of complex crossings of professional, academic, public, pedagogic and policy sites of history where discursive production noisily jostles to cast an unstable and contested shape of school history. Curriculum and policy disturbance is recounted because this activated my resistance to history policy decision-making, and moved me to focus research on my history work in teacher education. The conceptualisation of teaching selves, a theorising of history, and description of dimensions of pedagogy present a foundation for my shift to a critical pedagogy stance. PHP research designed as a reciprocal system of meaning layered within wider narrative research is outlined, along with a description of a Dismantling Analysis (DA) that sought to unravel and interpret the symbolic mantle of the cultural politics and power relations of school history. Emergent findings of participants’ PHP are presented within a commentary that brings voice and visibility to the participants’ experiences of school history. A continuity of critical approaches to history pedagogy that has evolved from the PHP research identifies elements in course work for pre-service teachers’ historical thinking.
As identity and belonging is an important element of lived citizenships, then young citizens need to see their pasts as valued and tangible in the histories of this place Aotearoa New Zealand, and its peoples. History education has a responsibility to make visible inclusive representations of the past, to counter normative narratives of certainty, and to expose exclusive notions of being a future-oriented citizen in Aotearoa New Zealand. Historians’ skills and motivations to identify alternative paths and experiences through their narratives, and to be open to critique power(ful) practices and dominant worldviews, deserve attention in history education. Historians might help us see the past as a provocation to view something of our selves in different ways. Dialogue is needed between history researchers and practitioners in teacher education, schools and the academy, to enable young citizens of Aotearoa New Zealand to affirm identities and access lived experiences of the past. They need to be part of the histories of the present, and to see themselves in history.
I was fortunate to work with 10 extraordinary individuals as my students, colleagues and friends to research history pedagogy. As participants, their commitment to the PHP enabled me to see something of their beating hearts, emotions, and embodied selves as teachers. This was a privilege. I am indebted to their disarming honesty that has advanced my knowledge of history education.
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