Open access peer-reviewed chapter

# Fuel Production Using Concentrated Solar Energy

By Onur Taylan and Halil Berberoglu

Submitted: May 23rd 2012Reviewed: October 3rd 2012Published: February 6th 2013

DOI: 10.5772/54057

## 1. Introduction

Limited reserves of fossil fuels and their negative environmental effects impose significant problems in our energy security and sustainability. Consequently, researchers are looking for renewable energy sources, for instance solar energy, to meet the energy demands of a growing world population. However, terrestrial solar energy is a dilute resource per footprint area and is intermittent showing substantial variability depending on the season, time of the day, and location.

One strategy to overcome these drawbacks of solar energy is to concentrate and use it for cleaning and upgrading dirty fuels such as coal and other hydrocarbons or converting renewable feedstocks such as biomass into carbon-neutral solar fuels. In this way, the intermittent and dilute solar energy can be concentrated and stored as a chemical fuel which can be easily integrated to our existing energy infrastructure. These advantages of solar fuels produced with concentrated solar radiation make them an attractive solution in our quest for renewable and clean fuels. Figure 1 shows the energy potential and carbon emissions by most commonly used fuels along with solar hydrogen.

Most common and available methods for solar fuel production are thermolysis, cracking, reforming, gasification and through thermochemical cycles. All these methods require high temperatures to produce solar fuel. Therefore, in these methods, there are some qualities of the feedstock or the reactor that need to be satisfied to attain high temperatures and efficient solar fuel production. For instance, the physical size and porosity of the feedstock play an important role. As the surface area-to-volume ratio of the feedstock increases, more reaction sites will be available for the reaction to occur, which increases the process efficiency. The feedstock should also have a narrow bad gap to lower the energy requirement for chemical process. Additionally, the material on the reactor walls should have high optical absorption to increase the temperature of the reactor and withstand high temperatures, and the window material should have high transmissivity to let the solar energy in to the reactor. More detailed property requirements are given by Nowotny et al.[1].

This review chapter consists of four sections. Following the introduction, the second section “Concentrated Solar Fuel Production Methods” reviews the different routes of producing solar fuels according to the feedstock material used in the processes. These include (i) thermolysis of water, (ii) thermochemical cycles, (iii) cracking of gaseous hydrocarbons, and (iv) gasification and reforming of coal and biomass. These methods are compared with each other based on their temperature, pressure, thermodynamic efficiencies, and by-products. The third section “Concentrated Solar Reactors” provides a comprehensive review of different concentrated solar reactor designs reported in the literature. This section first reviews the current solar concentration methods and describes in detail the effects of concentrating factors on the heat flux and temperatures that can be achieved. Then, the section describes the design and basic principles of operation of different solar reactors, their applicability for the different methods described in the preceding section, and their temperature and pressure capabilities. Moreover, the section summarizes the reported solar to fuel conversion efficiencies of each design. Finally, the chapter ends with the conclusions and outlook of fuel production with concentrated solar energy outlining the challenges, new research directions and novel applications.

## 2. Concentrated solar fuel production methods

This section describes different methods of producing solar fuels according to the feedstock material used in the respective processes.

### 2.1. Thermolysis of water

The term “thermolysis of water” refers to the thermal decomposition of water molecules into hydrogen and oxygen gases. Historically, due to high availability and simple molecular form of water, researches on solar fuel production started with direct hydrogen production by thermolysis of water using solar energy as,

H2OH2+12O2ΔH300K=286 kJmolE1

The reaction given in Equation (1) is an endothermic process, i.e., it requires energy to break the bonds. However, breaking all the bonds in water molecules requires temperatures as high as 2500 K [2]. At lower temperatures, partial decomposition occurs. Although it is possible to reach 2500 K with concentrated solar energy, the reactor where this process takes place shows material issues related to high temperatures. Additionally, after the dissociation of water molecules, hydrogen and oxygen gases require separation at high temperatures in order to prevent back-bonding, i.e., reproduction of water molecules with an exothermic process. Some solutions include cooling the reactor down by injecting a gas or expanding these gases through nozzle at the end of the reactor [2, 3]. Other solutions include using double or tubular membranes or using multi-stage steam ejectors to lower the exit pressure [4]. However, these solutions further reduce the efficiency of the process, and thus no commercial plant using this technology exists.

### 2.2. Thermochemical cycles

Some metal oxides are reduced using solar energy since metals provide good storage and transport of energy, such as solar energy. Such metal oxides include, but not limited to ZnO, MgO, SnO2, CaO, Al2O3 and Ce2O3. The reduction step of these metal oxides is generally followed by an oxidation step at lower temperatures than reduction step in order to produces solar fuel, mainly hydrogen. The reduced metal oxides generally react with CO2 or steam. If steam is used in oxidation that step is called hydrolysis. The thermochemical cycles of different metal oxides are generally compared based on their temperature requirements for the reduction step, the reaction or dissociation rates and reaction kinetics.

ZnO is one of the most popular oxides mainly due to its abundance and relatively low temperature requirement for complete dissociation when compared to other metal oxides. Additionally, since ZnO is a simple metal oxide, it does not undergo multiple reactions before its full dissociation. The dissociation of ZnO occurs as according to,

ZnOZn+12O2ΔH2000K=546 kJmolE2

The complete dissociation of ZnO to Zn requires temperatures higher than about 2300 K whereas, for instance, the dissociation of MgO as another simple metal oxide requires about 3700 K at atmospheric pressures [3, 5]. As in water thermolysis, partial dissociations can occur at lower temperatures. Although hydrolysis of zinc is exothermic as given by Equation (3), only 24% of Zn could be oxidized to produce H2 at a reactor temperature of 800 K and an atmospheric pressure [6].

Zn+H2OZnO+H2ΔH300K=-62 kJmolE3

Figure 2 shows the overall process of hydrogen production from zinc-oxide.

As an alternative to ZnO reduction, Abanades et al.[7] proposed SnO2 reduction using solar energy. Once the SnO2 is reduced to SnO in gaseous form using solar energy at temperatures nearly 1600oC, hydrolysis of SnO with steam at about 550oC and ambient pressure takes place in another step to form hydrogen gas as,

SnO2(s)SnO(g)+12O2ΔH1873K=557 kJmolE4
SnO(s)+H2O(g)SnO2(s)+H2ΔH773K=-49 kJmolE5

The advantages of SnO2/SnO reduction when compared to ZnO/Zn reduction are that (i) the SnO2-to-SnO conversion can be increased in Equation (4) by decreasing the pressure of the solar reactor which increases the overall conversion efficiency [7] (ii) SnO has higher melting and boiling points when compared to those of Zn, so that quenching rate of SnO is not as important as of Zn [7] (iii) in ZnO/Zn dissociation, Zn needs to be quenched rapidly below its condensation temperature to prevent recombination, while this is not the case with SnO2/SnO system.

There are some other metals that can be reduced with faster reaction kinetics such as Ce2O3. However, the reduction of Ce2O3 to CeO2 starts at temperatures higher than 2300 K [8, 9]. Full dissociation requires higher temperatures. This requirement of high temperatures creates some material limitations on the material of the reactor and increases the cost of the reactor significantly. Although there are some lab-scale prototypes of Ce2O3/CeO2 solar reactor, it is not preferred due to these limitations and high cost.

Another research was also started with producing hydrogen gas from hydrogen sulfide, H2S, as,

H2SH2+12S2ΔH300K=91.6 kJmolE6

Hydrogen sulfide is a toxic by-product gas of sulfur removing process from natural gas, petroleum and coal. Thermal decomposition of hydrogen sulfide requires about 1800 K [10]. It is advantageous over the other metal oxide reduction processes discussed above since this thermochemical process is only a one-step process that does not require additional oxidation step to produce hydrogen. Additionally, the temperature requirement for dissociation is lower than that for the direct water thermolysis. However, the product gases need to be cooled down after the dissociation as in the water thermolysis or other metal oxide reduction processes [11]. Some studies showed that the temperature of reduction could be reduced to about 1500 K, and they showed that the reproduction of hydrogen sulfide is unimportant below 1500 K [3, 12, 13].

In general, the solar chemical process is a clean way to produce hydrogen without any carbon prints. Therefore, the hydrogen as a product of the solar chemical process can be used in fuel cells directly as it is pure. The solar chemical reduction step of the process produce nanoparticles with high surface area to volume ratio, e.g., Zn, SnO which also create additional reaction centers for the hydrolysis to occur [7]. Therefore, the oxidation or hydrolysis occurs fast due to high mass transport of gases in the solid phase [7]. As in the other dissociation processes, the products of the dissociation also need to be cooled in order to prevent re-oxidation. Sandia National Laboratories of US released a comprehensive report on the thermochemical cycle selection with initial selection for further research [14], and Table 1 summarizes the studied thermochemical cycles [15].

### 2.3. Cracking of gaseous hydrocarbons

The term “solar thermal cracking” or “solar cracking” is used for thermal decarbonization of natural gas or other hydrocarbons. As a result of cracking, hydrogen, carbon and other possible products are formed without CO2 emissions. Therefore, this process is another method for clean fuel production. Cracking requires high temperatures of about 1500 K [16] that can be reached using concentrating solar collectors. For example, Maag et al.[17] tested a concentrated solar collector with a concentrating factor of 1720, and obtained a maximum temperature of 1600 K within the solar cavity reactor. In general, the advantages of solar cracking are the increase in value of feedstock using solar energy, pure and uncontaminated products and no CO2 emission [16].

As being the simplest hydrocarbon and the main constituent of natural gas as given in Table 2, methane has been mainly considered for solar cracking. Chemical reaction of evolution of carbon black and methane is given in equation (7) [18, 19]. The kinetic mechanism of methane cracking at 1500 K and atmospheric pressure was proposed as [20, 21],

ZnO1600-1800CoZn+12O2ZnO+H2O400CoZnO+H2E7

### Table 4.

Targets of Department of Energy of US [14] and predictions [78] for cost and efficiency for ZnO/Zn thermochemical cycle.

gge refers to gallon-of-gas-equivalent.

Based on predicted ZnO-to-Zn conversions of 70% in 2015 and 85% in 2025.

In summary, the advantages of solar fuels include:

• Energy content or heating value of feedstock is increased by converting it to another form, solar fuel.

• Producing storable and transportable fuel which is not possible if solar energy is directly used. Thus, eliminates the intermittency problem of solar energy.

• Solar fuels are clean and sustainable. The thermochemical cycles and thermolysis of water that are used to produce solar fuels have no CO or CO2 emissions. However, carbon emission occurs for the gasification or reforming of carbonecous feedstocks. If these feedstocks are biomass cultivated with CO2 from the atmosphere, they are carbon neutral.

On the other hand, these are not mature technologies and still suffer from technical challenges which form the basis for future research including:

• High temperatures needed for solar fuel production processes. High temperatures can be reached with high concentrating ratios. However, high concentrating ratios bring high cost to the system, and high temperatures restrict the material choice.

• Recombination of product gases, especially in thermochemical cycles, is a significant problem. This recombination significantly decreases both the process and overall solar-to-fuel efficiency.

• Quenching is introduced to products in order to reduce the recombination. However, quenching adds additional cost and complexity to the reactor and the process management. For some solar thermochemical processes, membranesare also required to separate product gases.

• Particle accumulation on the window of the reactor is a problem in directly irradiated solar reactors. This problem can be eliminated by introducing an inert gas with high flow rates to the reactor which further complicates the management of reaction in the reactor. Another solution is to heat the reactor indirectly which reduces the solar-to-fuel efficiency.

• Multiple-step chemical reactions are needed to produce hydrogen in most of the thermochemical cycles. More reactions add further components to the system which increase the cost and the management of the overall fuel production process.

• CO and CO2 formation can be noteworthy in case of solar gasification and reforming of carbonecous feedstock, although solar fuels are accepted as clean fuels.

These drawbacks of the solar fuel production prevent the technology to be converted to large scale commercially available power plants. However, solar fuel production processes are thermodynamically efficient, favorable developments to increase the feedstock’s heating values with the unlimited free solar energy. Therefore, in a long-term prospect, solar fuel production is a promising technology that needs significant research efforts for efficiently producing clean and sustainable fuels.

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Onur Taylan and Halil Berberoglu (February 6th 2013). Fuel Production Using Concentrated Solar Energy, Application of Solar Energy, Radu Rugescu, IntechOpen, DOI: 10.5772/54057. Available from:

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