Assessment of Solar Energy Potential Limits within Solids on Heating-Melting Interval

Abstract

The solar furnace works by using the electric energy produced by a photovoltaic system, which converts solar energy, solar radiation, into electric energy. The performances of the solar furnace used in various applications from industry are influenced by various factors. One of these factors imposes the acquisitions of certain large densities of the radiant power, and it requires a geometric form of the concentrator. The research is based on the behavior of some metallic alloys at elevated temperatures, for purifying some materials and for the achievement of some chemical synthesis. An important technological condition is a temperature which is achieved by concentrating solar radiation. This temperature is necessary to produce metallic material in the crucible, without other complementary energy for the thermal process. Steel or aluminum production requires very high quantities of thermal energy. Usually, this energy is given by electric power, natural gases, or conventional fuels. The solar furnace uses the energy given by the sun. For the manufacturing of the electrothermal furnaces, a series of specific materials are used, which are necessary for the obtaining of the furnace chamber, for the heating elements, as well as for the measurement systems of the temperature.

1. Introduction

Many of the photovoltaic systems function independent of the wiring system (off-grid). These systems are made of solar panels matrixes, control systems, storage systems, and DC or AC consumption devices. The scheme of a PV system is shown in Figure 1.

Figure 1.

Scheme of an off-grid PV system.

Panel matrixes are made of modules that consist of serially or parallelly connected panels depending on requirements. The panels are generally made of 36 serially connected cells (the number of cells commonly used are 32, 36, 48, 60, 72, and 96). For example, a solar panel comprising 32 cells typically can produce 14.72 volts output. Specific batteries are used for PV systems for storage and to stabilize tension. Control systems have charging regulators and converters/inverters which can be either DC-DC or DC-AC and blocking diodes. Control systems assure the interface between all the components of the PV system for protecting and controlling the system [1, 2, 3, 4, 5, 6].

A solar furnace will use solar energy in the processes for technological purposes. This installation belongs to the thermal installations category. The advantages of thermal installations based on solar energy are as follows:

• In terms of conversion system, a powerful thermal source can be concentrated into a tight space with a direct consequence on getting some high temperatures (over 2.000°C);

• As in any heating installation based on electric power, installations based on solar cells also have the possibility of accurate and rapid temperature adjustment;

• Electric system used for the conversion of solar power into thermal power allows simple automation with high reliability;

• Transfer from classical energetic sources (solid or liquid fuels or electric power achieved through their burning) to alternative sources can be the future alternative required by the endearment of classical sources and their powerful pollution.

As a result of these electrothermal installations advantages, installations that rely on solar energy sources can be used in different industrial processes [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17] such as:

• Fabrication technologies of ferrous or nonferrous materials;

• Metals heating for thermal processing;

• Manufacturing of various abrasive materials;

• Welding of metallic or nonmetallic alloys;

• Glass and ceramics processing;

• Drying, preparation, and sterilization of food products.

The documentary research visits made at Instituto de Soldadura e Qualidade, Lisabona, and at Ecole Européenne d’Ingénieurs en Génie des Matériaux, Nancy, opened new collaboration perspectives with renowned teams from abroad for achieving new results due to their experience in the domain. Another documentary research visit is made at the Dipartimento di Ingegneria Aerospaziale e Meccanica, Secunda Universitadegli Napoli, Italia.

To achieve the necessary elevated temperatures, an important condition is to use quality materials. The best option is the use of electric resistor furnaces.

2. Materials and methods

2.1 Documentary study regarding solar furnaces and the importance of climatic conditions

In order to design a PV system, we start from the consuming requisite power (load resistance) which in this case is an electric furnace with a capacity of 1 liter and a 0.55-kWh power, a lab furnace, the working time and climatic conditions of the zone where the system is installed (Figure 2).

Figure 2.

The necessary climatic conditions.

As observed from the column which gives average horizontal solar radiation per day, we have the sufficient radiation quantity in order to produce consuming requisite power of 0.55 kWh. By adjusting the system to an optimum bend degree, a significant radiation increase can be obtained. If a tracking system is used, a higher quantity with almost 40% can be obtained, as shown in Figure 3, where considered system losses are also considered.

Figure 3.

Comparison between a fixed system with a 37^o optimum angle and a tracking system.

In order for a PV system to obtain a standard power of 1 kW, eight-module Mitsubishi PV-MF 130 EA2LF type, polycrystalline silicon, 8 x 130Wp = 1.04 kW should be used.

For this system, we use 3 batteries of 130Ah, 12 V with 2 days functioning reserve even without the sun (Figure 4 and 5).

The calibration of the storage system is made based on the use period and the days when the system provides power from the batteries. The charging regulator for batteries has a basic function, that is, the charging stops when the batteries are completely charged.

There are some other functions such as consuming disconnection when the tension is small and temperature compensation [3, 4, 5].

Blockage diodes are used to avoid batteries discharge, on cells, during the night period or during cloudy days. For some systems, maximum power point trackers (MPPT) can be used.

The purpose of this device is to maintain the operating tension of the system to a maximum tension which is independent of the load resistance changes.

2.2 The determination of the geometric arrangement and the components of a solar furnace used for processing metallic and nonmetallic materials

Figure 4.

Scheme of an electric system with PV for direct current.

Figure 5.

Scheme of an electric system with PV for alternating current.

2.3 The analysis of heating process within solar furnaces

2.3.1 Mathematical model of the heating regime within the working chamber of a solar furnace

The factors that influence the performances of the solar furnace used in industrial applications that impose the procurement of some large densities of the radiant power and the need of a geometric perfection of the concentrator are classified in three categories as follows:

• The first type is pertinent to the basic geometry of the parabolic concentrator defined by focal distance, f, and its opening, D. Once these were chosen, it results in the dimensions of the solar image, focusing factor and the ideal maximum values of temperature and radiant power density into the image, regardless of the concentrator’s construction and its installation place.

• The second type consists of the factors that reduce the performance of the solar furnace due to its construction and installation place. These factors are the transmission of the energetic factor of the atmosphere, the directional reflection energetic factor of the mirrors, and the index of geometric perfection of the parabolic concentrator.

• The third type includes the factors according to the receiver’s properties: its adsorption and emission energetic factors and heat losses that take place through conduction and convection.

The available potential power $P_f$ in sun’s image from the focal plane of a solar furnace is given by the relation:

$$P_f=\pi \bullet R_d\bullet D_a\bullet E_0\bullet f^2\bullet {sin}^2{\theta }_{\mathit{max}}$$E1

where:

$R_d$ – directional reflection energetic factor of the parabolic mirror (including heliostats, if they exist);

$D_a$ – transmission energetic factor of the atmosphere where the furnace is installed;

$E_0=1353$W/m² – (constant);

$f$ – focal distance;

${\theta }_{\mathit{max}}$ – the opening angle of the parabolic concentrator.

If a solid corp is disposed in the solar image, the available fraction of potential power effectively absorbed by the corps would be determined by the absorption factor and the form of this corps-receiver. As such, the maximum temperature that could be obtained into a solar furnace depends on the properties of the receiver disposed of in the focal zone of the furnace:

$$T_{\mathit{max}}=T_s\bullet {\left(R_d\bullet D_a\right)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$4$}\right.}\bullet {\left(sin{\theta }_{\mathit{max}}\right)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}$$E2

where $T_s=5800K$ – temperature at sun’s surface.

2.3.2 The determination of the technological parameters involved in the heating process of a solar furnace

The most important research is made on the behavior of metals and refractory materials at elevated temperatures, for purifying nonmetallic materials and for the achievement of some thermochemical synthesis.

One of the technological parameters is the temperature that is obtained by focusing on the sun’s rays. The furnace uses the temperature in order to melt the metallic material in the crucible, without other complementary energy for the thermal process (Figure 6) [5, 18].

Figure 6.

The schematic representation of the installation that is used to melt a metallic material using solar energy.

Steel or aluminum production needs very high quantities of energy. This is usually given by electric power, natural gases, or conventional fuels. A solar furnace uses the energy given by solar radiation.

We can see in the image how the sun’s rays can be focused toward the crucible where the ore is. This is heated to a very high temperature until it melts.

Pollution is basically inexistent because solar energy is a pure form of energy. The melting materials with very high melting points are one of the main applications of solar furnaces.

The material melting on a portion whose area is approximately equal to the area of the sun’s image can take place in case the exterior of solid material is exposed to very intense radiation from the focal zone of a solar furnace.

As heat enters the solid, the melted material quantity increases and forms a liquid cavity. Through such a process, it is possible to melt the material in the crucible; this happens because of the existence of a high-temperature gradient between melted material and the crucible’s exterior.

In regular furnaces, the crucible is warmed from the exterior, and it has a high temperature continuously than the melted raw material. As a result, the crucible in such furnaces must be made of a material that is more refractory than the substance to be melted, as well as chemically inert toward the melted material.

As the melting point of the examined material rises above 2000°C, the difficulty of achieving these two conditions increases, as there are fewer options to avoid chemical reactions.

Solar furnaces overcome these significant constraints of conventional furnaces when melting materials have high refractivity. As a result, melting can take place in furnaces with a horizontal axis.

The furnace is rotated around its horizontal axis and has an inner diameter several times greater than the diameter of the solar image. When the rotation speed is modest, the melted material stays in the lower part of the furnace, and the turning aids in heat distribution uniformity.

The melted material is centrifuged, generating a cavity that prevents it from flowing out of the furnace at higher rotation rates. The furnace’s external walls, which are typically composed of steel, can be water-cooled to maintain (if necessary) a high-temperature gradient through the walls.

When melting into a specific protective environment, a suitable gas current is passed, as shown in Figure 7. Quartz, zirconium dioxide, corundum, ceramic oxides, and materials like carbides, nitrides, and boron are among the materials that can be examined. Conventional melting processes have many drawbacks for these materials.

Figure 7.

Details of melting installation of metallic alloys using solar energy.

It is also possible to investigate the feasibility of employing solar furnaces for steel melting. Technically, the crucible can be readily made by inserting a refractory powder into the furnace’s cavity and sintering or even melting it through the furnace’s top that is exposed to solar radiation. After that, scrap iron is inserted, melted, and then molted in forms if necessary [6, 7].

The performance of such a solar furnace does not need to be exceptional because there is sufficient temperature of 2000–2500°C.

Other metals that are more expensive than steel, such as titanium, zirconium, and molybdenum, are expected to generate increased attention in the future.

In this instance, an inert protective atmosphere must be ensured, and the challenges and costs associated with this must be considered.

Impurities evaporation, zonal melting, fractioned crystallization, the separation of zirconium oxide from zirconium (zirconium silicate), and material investigation under thermal shock conditions are some of the other applications of sun furnaces.

2.4 The analysis of melting/burning/purifying process within solar furnaces

These objectives consist in developing some mathematical formalism, which allows a better capitalization of the advantages given by the evolution of melting/burning/purifying within solar furnaces [14, 16].

Working out of this mathematical formalism was very useful for the documentary stage from Universidad de Las Palmas de Gran Canaria, Spain, as well as for the discussions on this theme with Prof. Agustin Santana Lopez.

2.4.1 Mathematical model of the melting/burning/purifying process within the working chamber of a solar furnace

Thermal efficiency η_t of an electrothermal installation based on solar energy is given by the ratio:

$${\eta }_t=\frac{Q_u}{Q_u+Q_p+Q_a}$$E3

• Q_u is the quantity of absorbed heat necessary for heating the material;

• Q_p represents heat loses due to heating installation;

• Q_a is the heat quantity necessary for heating the auxiliary components of the installation;

The furnace will be in these working temperature classes that are considered as classification criteria in heating technology:

• Low-temperature furnace (between 600 and 700°C);

• High-temperature furnaces (until 1600°C).

In order to define the calculus model of the furnace based on solar energy, it is necessary to define the following input data:

• Material that will be heated with all its thermal and electric data;

• Charge shape and dimensions;

• Technological regime that consists in:

  • Heating time until reaching solidus temperature;

  • Heating temperature;

  • Overheating time for generalizing liquid state in the entire mass of the charge;

  • Overheating temperature;

  • Holding time at casting temperature necessary for eventual alloying in the liquid state;

  • Special technological conditions (protection atmosphere, vacuum, etc.);

  • Furnace efficiency.

Heat consumption is calculated from functioning diagram of the furnace, that is, in fact, the variation diagram of load temperature–time function θ_p; taking into consideration the fact that this is a furnace with intermittent functioning, the diagrams presented in Figure 8 are the possible ones.

Figure 8.

Diagrams of possible functioning of the solar furnace: a) melting without holding a constant temperature; b) melting by holding at constant temperature (t_c – A complete cycle; t_î – Heating time; t_m – Holding time at constant temperature; t₀ – Loading – Unloading time).

2.4.2 Determination of technological parameters implied in melting/burning/purifying process within a solar furnace

1. Temperature control

   Within the furnace (Figure 9a), the thermometric transducer T is installed that transmits information of the temperature to the AB adjusting block.

   In comparator C, a tension proportional to the desired value of the temperature θ_d, determined on the basis of the program imposed by technological process and controlled by the block of desired values BVD, is compared with a tension proportional to the real value of the temperature within furnace θ_r.

   If θ_r < θ_d, on–off regulator RBP is transmitting the closing command to adjusting block (connection switch to the power supply), and the furnace is absorbing power P. If θ_r > θ_d, cutting-out command of the switch is transmitted.

   The adjustment made with a dead zone ∆θ is given by the regulator’s characteristic (Figure 9b).

2. Orientation of photovoltaic system

Figure 9.

On–off adjustment of furnace temperature.

Adjusting the system to an optimum angle of inclination, a significant increase of radiation can be achieved, which can be used, instead of positioning the panels on horizontal or at a random angle in general, to place latitude.

If a tracking system is used, an increased quantity with approximately 40% will be achieved, as shown in Figure 10, where system losses are also considered.

Figure 10.

Comparison between a fixed system at an optimum angle of 37°and tracking system.

The use of sensors for orientation can lead to delicate situations in case of sun-clouds alternations, if the system is not properly calibrated and has high energy consumptions.

Taking into account of these considerations, the variant that uses a mathematical algorithm is chosen for solar panel positioning. The orientation makes after the two directions, namely E-V and S-N.

2.5 Characterization of metallic materials heating process within solar furnaces

2.5.1 Materials heating within a furnace

Equation of energetic balance for the furnace can be written as:

$$d{Q}_2=d{Q}_u+d{Q}_a+d{Q}_{\mathit{pd}}+d{Q}_z$$E4

where:

dQ₂ is the elementary heat quantity transmitted toward furnace interior by the heating element:

$$d{Q}_2=P_2·\mathit{dt}=\alpha ·A_1·\left(\theta -{\theta }_0\right)·\mathit{dt}$$E5

dQ_u is the elementary heat quantity that leads to the heating of the useful material within the furnace (absorbed heat):

$$d{Q}_u=c_u·m_u·\mathit{d\theta }$$E6

dQ_a is theheat quantity that leads to heating the attached pieces (stands, supports, etc):

$$d{Q}_a=c_a·m_a·\mathit{d\theta }$$E7

dQ_pd is the elementary thermal losses through furnace walls, opening, leakiness, etc.;

dQ_z is the elementary heat quantity that gathers in furnaces walls:

$$d{Q}_u=c_z·m_z·\mathit{d\theta }$$E8

where:

P₂ – thermal energy (thermal flow) transmitted by photovoltaic systems;

α – heat exchange superficial coefficient;

A_l – area of the total lateral surface of heating elements;

θ –the temperature of heating elements;

θ₀ − the temperature inside the furnace;

dt – interval of elementary time;

c_u, c_a, c_z − mass heats (temperature-dependant) of the heated materials, attached elements, and crucible;

m_u, m_a, m_z − pieces weight, attached elements, and crucible;

dθ − elementary interval of temperature.

2.5.2 The achievement for a design algorithm for a solar furnace used in metallic material heating

Solar panel positioning by implementing the mathematical model needs the astronomic considerations.

In order to determine the real position of the sun on the sky, the following angles are important; θ_z – Zenith angle and γ_s Azimuth angle, as shown in Figure 11.

Figure 11.

Sun’s trajectory on the sky – Important angles.

The calculus of these angles is made with mathematical formulas. Calculus formula for Zenith angle can be calculated by the relation:

$$cos{\theta }_z=sin\phi ·sin\delta +cos\phi ·cos\delta ·cos\omega$$

where ϕ is the latitude and is constant for the place where the solar tracker is positioned, for example, for Brasov, it is 45°39′, and δ is a declination and ω is an hour angle.

2.5.3 Virtual design of a heating solar furnace

In order to determine the minimum dimensions of the solar panel, a concave mirror is used as shown in Figure 12. The calculi are made on shading intervals as well as on illumination optimum of the mirror in order to create maximum thermal flow toward the furnace’s crucible (Figure 13).

Figure 12.

The minimum dimensions of the solar panel.

Figure 13.

The mirror position to create maximum thermal flow toward the furnace’s crucible.

Determination of minimum dimensions of the solar panel.

AC = Φ concave mirror.

$\hat{C}$ = 90–21.34 = 68.26.

AC = sin $\hat{B}$ ∙ BC → $\mathrm{BC}=\frac{\mathrm{AC}}{sin\hat{B}}\to \mathrm{BC}=\frac{\mathrm{AC}}{0.363}$

$\mathrm{AB}=sin\hat{C}·\mathrm{BC}\to \mathrm{AB}=0.928·\frac{\mathrm{AC}}{0.363}\to \mathrm{AB}=2.558·\mathrm{AC}\left(\mathrm{AB}\text{minim}\right)$

BD = AC

$\mathrm{BD}=\mathrm{BE}·sin\hat{E}$

$\mathrm{BE}=\frac{\mathrm{BD}}{sin\hat{E}}=\frac{\mathrm{AC}}{0.98}=\mathrm{AC}·1.0183$

$\mathrm{B}{\mathrm{E}}_{max\left(45^{\circ }\right)}=\mathrm{AC}·\sqrt{2}·1.15;\left(1.15=\text{const}.\right)$

$\mathrm{B}{\mathrm{E}}_{\min }=\mathrm{AC}·1.0183$

2.6 The characterization of the melting/burning/purifying process in solar furnaces

These objectives consisted in the development of some mathematical formalism that can allow better exploitation of the advantages accomplished during melting/burning/purifying processes in solar furnaces. The documentary stages made up until now are useful in the elaboration of these formalisms.

2.6.1 The achievement of a design algorithm for a solar furnace used in melting/burning/purifying of the metallic and nonmetallic materials

The solar furnace is a laboratory experimental furnace. During the experiments, we propose to process small quantities of material until 1 kg.

1. Crucible dimensioning

   In order to make the crucible, graphite material is chosen due to its high temperatures characteristic until 2000°C and the relative high thermal conductivity. For accomplishing the calculation, aluminum is chosen as the test material due to its density up to 1 kg more volume than other materials that will be processed.

   The volume of the crucible is calculated using the relation:

   $$v=\frac{m}{{\rho }^{\prime }}=\frac{\pi ·d^2}{4}·h$$E9

   where ρ′ is the material density at the ambient temperature of 20°C, for example, aluminum has ${\rho }_{\mathit{met}\_\mathit{\text{topit}}}^{\prime }=2.72$ kg/dm³.

2. Dimensioning of the furnace masonry and thermal calculus

   The furnace has a cylindrical shape. Its disposal is vertical, and on its walls it chose a configuration with multiple layers, considering the temperature, as shown in Figure 14:

   Let it be the exterior temperature of the refractory coat of 1200°C.

3. Resistors

   The choice of the material from which the resistors are made should be considered so that the maximum working temperature exceeds almost 2–10% of the maximum temperature of the furnace. Knowing that the temperature within the furnace can reach 1600°C, Kanthal is chosen for resistors. Kanthal resistors are like spiral wires.

Figure 14.

The diagram of furnace wall: 1 – Refractory coat made of chromium magnesite with a thickness of 15 mm; 2 – Thermal insulation made of 700 diatomites treated with binders with a thickness of 30 mm; 3 – Exterior shell made of steel plate with a thickness of 2 mm.

In each chamber of the furnaces, 27 resistors are placed in the channels of the refractory material. They are disposed symmetrically on vertical and placed over the crucible so that we have a uniform temperature in the entire chamber of the solar furnace.

The length of each Kanthal spiral is 105 mm. The equivalent resistance on each chamber is 51.5 Ω. Figure 2 shows the disposal of all 27 resistors.

The installed power of the furnace is calculated by the relation:

$$P_i=k·P=k·\frac{Q_i}{t_i}\left[W\right]$$E10

where k = 1.1÷1.5 is the safety coefficient that takes into account the possibility of forcing heating regime of the cold furnace, the possibility of decreasing network tension toward its nominal value, the possibility of decreasing time of the thermal insulation properties, the possibility of heating elements aging – that determine a high strength than the one initially calculated and in addition a smaller developed power (Figure 15).

Figure 15.

Chamber disposal of the resistors.

The furnace with small dimensions, two chambers with a crucible, and the volume of 0.5 liters for each active chamber will be the first lab furnace. The reason the furnace is constructed with two working chambers is for the optimization of the working time.

We choose for a chamber furnace with crucible, because of its simple construction, the possibility of using it for different processes (for example, melting, burning, and purifying) as the possibility of realizing some different thermal regimes into the furnace, in essence, is what we propose to accomplish.

The resistor furnace (in the future, we will extend our research on an induction furnace too) has an alternate functioning regime because of the following functioning cycle:

• Loading of the crucible with the metal that will be processed;

• Heating;

• Melting;

• Burning;

• Purifying;

• Unloading.

The furnace will have the possibility of fitting in all working temperatures that are considered to be classifying criteria in heating electric heating technology:

• Low-temperature furnace (between 600 and 700°C the maximum value of the temperature);

• High-temperature furnaces (until 1600°C).

2.6.2 Virtual design of a melting/burning/purifying solar furnace

The scheme of the resistor furnace is presented in Figure 16. The components of the chamber furnace with crucible are as follows:

• chambers made of refractory material (1) and thermal insulation (2);

• the heating elements (3) are placed on the lateral walls of the furnace;

• crucible (4);

• material that will be processed (5);

• the furnace presents the door (6), acted by the lifting device (7) where processed materials are introduced;

• furnace support (8);

• Metallic shell (9).

Figure 16.

Scheme of the furnace supplied from a photovoltaic system with resistors for materials melting/burning/purifying.

For the construction of the electrothermal furnaces, a series of specific materials are used, which are necessary for making furnace chamber, for the heating elements as well as for the measurement systems of the temperature.

3. Results and discussions

3.1 Identification of the optimum geometric arrangement of the components of a solar furnace for heating metallic materials

The photovoltaic system will be composed of (see Figure 17):

• solar panels (it chose panels made of polycrystalline silicon with a very good price/quality ratio and with a guarantee of 20 years) with the required energy;

• batteries system necessary for energy storage and furnace usage under adverse conditions in terms of solar radiation (in the evening or on cloudy days);

  • tension regulator;

  • inverter necessary for transforming continuous current into the alternative current;

  • accessories;

  • solar tracker system (optional – necessary for obtaining a better efficiency of the photovoltaic system, which is permanently oriented so that the solar radiation would drop perpendicular on solar panels).

Figure 17.

The photovoltaic system necessary for supplying the solar furnace with resistors.

The temperature control in resistor furnaces has a special influence on the quality of the final products and on the specific energetic consumptions. With respect to the specific conditions of the technological process, especially the allowed temperature variations in the furnace and in the material, adjustment systems are used with intermittent action or a continuous one. The assembly image of the built solar furnace is given in Figure 18.

Figure 18.

Assembly images of the solar furnace.

A PID control algorithm will be used for temperature control using PtRh-18 class thermocouple as a sensor. The maximum working temperature of these thermocouples is 1820°C [1, 2].

The PID algorithm is implemented using LabVIEW graphic programming language. Figure 19 presents the panel, program interface, and PID application, and Figure 20 presents the diagram and the proper program.

Figure 19.

The interface of the PID application.

Figure 20.

Diagram of PID application.

3.2 Interpretation of the achieved results

The solar furnace works by using electric energy produced by a photovoltaic system, which converts solar energy, solar radiation, into electric energy.

For a feasibility study for the solar radiation in the Brasov area, an SPN1 pyranometer is purchased. Global and diffuse solar radiation is measured using this device, and direct solar radiation can also be calculated using this device. Solar radiation monitoring starts from February. Global and diffuse solar radiation monitoring is noticed at every 5 minutes.

The conclusions that can be drawn after monitoring the solar radiation, according to Figures 21–23, are as follows:

• the maximum number of cloudy consecutive days was five, but in those days there were time intervals where horizontal global solar radiation exceeded the value of 400 W/m², the value that gives the possibility to the photovoltaic panel’s system to offer enough energy so that the solar furnace with resistor would function under optimum conditions;

• the photovoltaic system is capable of producing almost 6.8 kW in a clear sky day (see Figure 21), thus allowing the furnace usage at maximum capacity and the possibility to store excess energy with the help of the solar batteries;

• During cloudy days, as shown in Figure 22, the quantity of energy generated by the system reduces considerably, so it only produces 1.5 kW, which would be enough for the judicious usage of the furnace;

• Yet, if there are long periods, as shown in Figure 23, then the system will need the energy to be stocked in the solar batteries; batteries system is designed to assure an autonomy of up to 5 days without raising the costs of the system very much;

• For example, for the weakest days energetically speaking, 12 June, the energy quantity a system can generate is under 800 W.

Figure 21.

Distribution of global and diffuse solar radiation during a sunny day (14 June).

Figure 22.

Distribution of global and diffuse solar radiation during a cloudy day (12 June).

Figure 23.

Distribution of horizontal global radiation during the interval 12–17 June.

4. Conclusions

The solar furnace can be used for numerous methods of technological purposes. The benefits that come from this sort of electrothermal installation is primarily based totally on sun electricity sources, and it could be utilized in various unique applications such as:

• For manufacturing metallic alloys;

• For heating operations of thermal processing (hardening and annealing, aging, carburizing, nitro-carburizing) and hot forming processing;

• For manufacturing abrasive materials, calcium carbide, and electrographite;

• For welding the metals and plastic packages;

• For manufacturing glass and ceramics materials;

• For nonmetallic products drying, preparation, or sterilization.

The researchers approach a dynamic thematic of high interest for problems consistency and for the wide area of applications.

The results already achieved within this stage are also noted with the help of the collaboration with researchers from the renowned research institute from Almerian Solar Platform, Spain.

Objectives’ fulfillment required an ample information activity. It was necessary to have a good control of some different mathematical methods as well as good information regarding the experimental research in materials’ melting/burning/purifying process.

The importance and complexity of this thematic make necessary detailed, interdisciplinary research. Thus, the team consists of specialists from the top domain of actual fundamental research such as materials science and physics.

The results already achieved open new work perspectives for the next year within the project in the approached thematic.

5. Perspectives

The materials that can be studied are quartz, zirconium dioxide, corundum, ceramic oxides and materials like carbides, nitrides, and boron for which conventional melting techniques present a series of inconvenient.

The practice possibility for using solar furnaces in steel melting can also be studied. The performances of such a solar furnace must not be special because there is sufficient temperature of 2000–2500°C.

In the future studies, there may be a higher interest for melting some other metals more expensive than steel like titanium, zirconium, and molybdenum. In this case, it must be assured an inert protective atmosphere, and thereby the complications and expenses related to these must be taken into account.

Other applications of solar furnaces are impurities evaporation, zonal melting, fractionates crystallization, zirconium oxide extraction from zircon (zirconium silicate), and the materials studied under thermal shock conditions.