POWER GENERATION FROM RENEWABLE RESOURCES

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1 6 POWER GENERATION FROM RENEWABLE RESOURCES

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3 6.1 Solar energy conversion Concentration systems Overview Although world energy consumption has grown at an average yearly rate of about 2.3% in the last 150 years, one quarter of the world population today still has no access to electricity and more than one third of the population, mainly concentrated in the developing countries, resorts almost exclusively to biomass as their primary energy source. On the other hand, many of these countries are in areas where there is considerable solar radiation, and if it were possible to exploit it using simple and economic technologies, this could be a decisive contribution to these countries increasing energy demand. In the electricity sector alone, it is foreseen that world consumption will double over the next 30 years, mainly as a result of the high increase in demand which will come from developing and emerging countries; at present, this sector represents about one third of the total world energy requirements, whose growth is predicted to be of about 75% over the same period. Therefore, to satisfy such a remarkable increase in demand over the next few decades, it is clear that it will no longer be possible to rely only on traditional primary energy sources (mainly coal, oil and natural gas). Thus, all available energy sources will be used in the most efficient way, giving special attention to the renewable energies which, due to their nature, do not have the problem of the progressive depletion of exploited reservoirs. Fig. 1 shows the potential theoretical energy contributions with those that can technically be exploited, which the main renewable sources could supply worldwide. The above considerations, together with a greater awareness of the consequences of climate changes on a planetary scale, induced by the emissions of an increasing number of industrial plants, have created a renewed interest for solar thermoelectric power plants in the more industrialized countries as well as in the international institutions that must promote and sustain development in underdeveloped countries. Solar radiation must be converted to high temperature thermal energy in order to be adequately exploited in these power plants. To make this conversion possible, radiation reaching the ground must first be concentrated. The concentrating and conversion of solar energy means the inclusion of all the technologies, systems and plants that exploit such energy as a source of high temperature thermal energy by concentrating solar radiation onto special receivers. This entails only the use of the direct component and the loss of the diffused component. Therefore, the regions of the Earth that are suitable for the exploitation of solar energy in thermoelectric power plants are those where direct radiation reaching the ground has an average technical usable potential (10 18 J) ,500, theoretical potential energy (10 18 J) 100, solar radiation biomass water power wind power Fig. 1. Potential of the main renewable energy sources (Solar Millennium AG, 2003) VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 531

4 POWER GENERATION FROM RENEWABLE RESOURCES yearly power equal to at least 200 W/m 2, equivalent to annual energy of 1,750 kwh/m 2 ; in the best sites it is possible to reach an average power of 320 W/m 2, equivalent to yearly energy of 2,800 kwh/m 2. As shown in Fig. 2, the areas where it is possible to exploit solar radiation by means of concentrating plants are mainly found in the developing or the emerging countries. In these regions, by using solar concentrating technologies that are available today, each square kilometre of receiving surface could allow the supply of energy of approximately 300 GWh/yr on average to the electricity grid, which is equivalent to the yearly production of a traditional 50 MW thermoelectric power plant working about 6,000 h/yr. Thus, it would be possible to make a fuel saving of almost 500,000 barrels of oil a year and also decrease the CO 2 emissions by 200,000 t/yr on average. The exploitation of less than 1% of the energetic potential, made available by solar concentrating technology, would be sufficient to comply with the recommendations of the Intergovernmental Panel on Climate Change (IPCC) for the long-term stabilization of the planetary climate. At the same time, the exploitation of solar energy would become economically competitive compared to the exploitation of fossil fuels. Historical outline The history of solar concentration began thousands of years ago. The properties of concave reflective surfaces exposed to the Sun s rays to cause the combustion of a variety of materials were known to the most ancient populations in the Orient and in the Mediterranean area, where they became known as burning mirrors. It is said that in the Second century B.C. Archimedes, the famous mathematician from Syracuse, used mirrors to set fire to the Roman fleet from a distance when its commander, the consul Marcellus, kept the city under siege. The first documented uses of mirrors include lighting fires to cooking food, to heating water and dwellings. Later, lens systems were manufactured, for instance by the French chemist A.-L. Lavoisier (1772), with which it became possible to reach temperatures (in excess of 1,000 C) high enough to melt metals. The diffusion of the steam engine, using coal as fuel, which at that time was widely available, hindered the use of solar energy applications. However, the problem of the depletion of coal reserves was brought to attention about one hundred years later. On this basis, A. Mouchot introduced the first solar engine at the Universal Exhibition of Paris of It consisted of a 20 m 2 parabolic dish reflector which, by concentrating the Sun rays on a recipient containing 70 litres of water, produced sufficient thermal energy in 30 minutes to generate enough steam to operate a machine. In Paris, during the same period, A. Pifre, one of Mouchot s assistants, introduced a printing press powered by a parabolic dish collector which, on suitability for solar thermal power plants excellent good suitable unsuitable Fig. 2. Map of direct solar irradiation (Solar Millennium AG, 2003). 532 ENCYCLOPAEDIA OF HYDROCARBONS

5 SOLAR ENERGY CONVERSION a typical September day, was capable of printing copies of Le Journal Soleil. From then on, the first applications for pumping stations, desalinization plants and cooking food were developed in the regions most exposed to the sun, such as North Africa. In Bombay, India, W. Adams, representative of the British Crown, after critically considering Mouchot s project, decided it was better to erect an array of smaller mirrors, adequately aligned and set in a semicircle on the boiler, moving them so as to track the apparent path of the Sun, in order to obtain higher temperatures with lower costs and more simple maintenance. At the end of 1878, he started to erect an installation, later called the tower solar plant, by gradually adding mirrors until a temperature of 800 C was reached, thus producing steam which had enough pressure to operate a medium-power engine. In 1887, the Swedish-American inventor J. Ericsson experimented with irrigation plants for the sunny Pacific coasts, using a small hot air engine powered by a linear trough collector; a more simple structure than the dish collector, with a pipe-shaped boiler situated longitudinally along the reflector, in the focal line of the parabola. However, in 1901, on a farm in Pasadena, California, A. Eneas, an English engineer erected for a demonstration, a solar engine similar to Mouchot s design and capable of pumping about 7 m 3 per minute of water, to irrigate the arid Californian ground. In 1910, F. Shuman, an engineer from Pennsylvania, built a solar boiler powered by parabolic trough collectors and capable of operating a large 30 kw engine to irrigate a farm in the desert with 25 m 3 per minute of water. Consequently, he erected five collectors 60 m long with openings of 4 m on a surface of about 4,000 m 2 in Meadi, south of Cairo (Egypt). This was the first solar installation on an industrial scale. However, the First World War broke out around that time and the large-scale drilling in the great crude oil basins in the Middle East and in the American Continent also began; once again, the abundant availability of fossil fuel hindered the use of concentrated solar energy. This technology which, as Shuman stated at the beginning of the Twentieth century, uses the most rational energy source had to wait until the 1980s to be revived, when the threat of depletion of the oil reserves and the threat of a permanent state of conflict in the regions of crude oil extraction arose. Therefore, industrial experimentation, based on the experiences of Ericsson and Shuman, were oriented towards the parabolic trough collectors that represent the best compromise in the ratio cost/produced energy in most of the exploitable sites. In the middle of the 1980s, the company Luz erected a solar plant in the Californian Mojave Desert (United States) with parabolic trough collectors to produce steam used in a thermodynamic cycle, which supplied 14 MW of electrical power. Other installations of the same type were erected in that area giving a total of 354 MW of electrical power, all of which are still in operation. Simultaneously, another solar power plant was erected, once again in the Mojave Desert, based on central tower technology (Solar One). This pilot plant, with 10 MW of electrical power, used an area of about 160,000 m 2 and was connected to the Southern Californian electricity grid. It remained in operation from 1981 to Subsequently, a second tower installation (Solar Two) was built, operational from 1996 to 1999, which used a mixture of molten salts instead of water as the heat transfer fluid. In Europe, Italy hosted the first significant demonstrative European plant in the field of high temperature solar power plants at Adrano in Sicily. The tower-type Eurelios plant, whose construction was begun in 1979 by an Italian-French-German consortium as part of a European Community research program, had a design power of 1 MW and was in operation until In Spain, the most important European research centre for solar concentrating technologies, known as Solar Platform of Almeria (SPA), near the town of that name, has been in operation since the beginning of the 1980s. Numerous experimental plants, mainly financed by the European Community, have been erected at this centre over the years to study various technological lines, especially central tower systems and parabolic trough collector systems. Basic concepts of solar concentration As previously mentioned, the concentration of solar radiation becomes indispensable when there is a requirement for thermal energy at a temperature higher than the temperature that can be reached employing a flat surface for its collection and conversion (flat collector). To obtain a higher temperature, a suitable optical system (the concentrator) is used to collect and send the radiation onto a component (the receiver) where the energy is converted into high temperature thermal energy. Moreover, collecting only direct radiation requires the concentrator to be moved during the day to track the path of the Sun in the sky. In order to reach high temperatures, solar thermal flow on the receiver must be increased. Therefore, the receiver must have a surface smaller than that of the concentrator, corresponding to the flat cross section of its reflecting surface. The characterizing parameter of a concentration system is the concentration factor C, which is defined as the ratio between the area A A of VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 533

6 POWER GENERATION FROM RENEWABLE RESOURCES the collection surface of the collector, also called the intercepting surface, and the area A R of the surface of the receiver: r y A C 12 A A R A concept closely related to the concentration factor is the acceptance angle (2q c ), or the angular interval in which all, or almost all, the rays are intercepted by the receiver. The maximum concentration factor, for a two-dimensional system with a linear-type receiver (such as the parabolic trough collector) is equal to: 1 C 2D,theoretical sinq c while for a three-dimensional system with a receiver at the focal point (such as the parabolic dish collector or the tower system) is: 1 C 3D,theoretical sin 2 q c The minimum acceptance angle that allows all of the rays coming from the solar dish to be sent to the receiver can be calculated on the basis of geometrical considerations. The Sun has a diameter of about kilometres while the average distance between the Sun and the Earth is about kilometres. Therefore, solar rays reach the Earth with a divergence of According to the above relations, the maximum concentration factor for a two-dimensional system is about 215, while for a three-dimensional system it reaches a value of over 45,000. In practice, however, the concentration factors of real systems are much lower, due to a series of technological limits. Acceptance angles which are notably larger than the solar divergence must be used because of errors in tracking the Sun and inaccuracies in the shape of the concentrator and in the positioning of the receiver. Moreover, the choice of the manufacturing solution for the receiver and concentrator can further reduce the factor to a half or a quarter of its theoretical value. Therefore, the actual concentration factor desired in a solar plant, after establishing its typology, involves a compromise between optical and thermal performances. A receiver which is as small as possible must then be chosen so as to limit thermal losses, while an increase in its dimensions allows the collection of all solar rays even if there are imperfections in the concentrator. Let us consider, for instance, the case of the parabolic trough collector. Fig. 3 shows its cross section with a plane perpendicular to the focal axis. The reflective surface of the concentrator has the shape of a parabola with the equation: y x 2 /4f and the y x 2 4f Fig. 3. Concentration factor for parabolic troughs. radiation is focussed onto a cylindrical receiver with radius r, placed on the focal line at distance f from the vertex of the parabola. If the ray with the maximum divergence accepted by the system (the dotted line in the figure) must reach the receiver, the concentration factor obtained in this configuration is: 2x A sina C 2D,parab C 2pr p 2D,theoretical f where a is the half-angle of sight of the parabola from its focus and 2x A is the opening of the collector. From the formula it can be seen that in this simple system the maximum concentration factor, occurring at a 90, cannot have a value higher than approximately 70, without even considering acceptance angles higher than solar divergence and further sources of error. Bearing in mind the actual acceptance angles, the tracking errors, the tolerances in the manufacturing of the reflective surfaces and other inaccuracies, the concentration factors in real 2D systems do not exceed a value of 30. Very often, the plane cross section of the cylindrical receiver in 2D systems is considered to be the surface of the receiver. In this case, the concentration factor is calculated by using the diameter of the receiver rather than its circumference; thus, the numerical values are multiplied by a factor of p. However, there is a class of concentrating systems that almost reaches the theoretical limit; these are called non-imaging systems as they do not faithfully reproduce the image of the solar disk because they do not maintain the reciprocal direction of the individual rays. The combination of a conventional system, such as the parabolic trough system of the previous example, with a non-imaging system, used in a second stage, allows the concentration factor to approach the theoretical limit value. Inside the receiver, the concentrated solar radiation is converted into thermal energy at a temperature which is proportional to the actual concentration a x A q c q c x 534 ENCYCLOPAEDIA OF HYDROCARBONS

7 SOLAR ENERGY CONVERSION factor. The energy balance of a concentration system can be considered to formulate the law relating the temperature to this factor. According to the Stefan-Boltzmann law, the radiant power from the Sun is proportional to the fourth power of its thermodynamic temperature. Only a fraction of this power, proportional to the square of the sine of the solar divergence angle (q S ), reaches the ground on Earth. Therefore, the incident radiant power (f S ) on the collecting area (A A ) is proportional to: f S A A sin 2 q S TS 4 where T S is the apparent temperature of the Sun, equal to about 6,000 K. The power loss of the receiver (f R ), when considering only the radiative-type losses in a first approximation, is proportional to: f R A R TR 4 having indicated the thermodynamic temperature and the area of the receiver by T R and A R respectively. In the hypothesis that the available power (f U ) is a fraction h of the incident power, the thermal balance of the receiver can be written as: f S f U f R hf S f R From the previous equations, remembering that A A A R C, the operational temperature of the receiver proves to be proportional to: 24 T R T S [(1 h)c] 1 4 The graph in Fig. 4 shows the maximum operational temperature of the receiver obtained from the above equation, with the usual values for the parameters that appear in the constant of proportionality as well as in the constant of efficiency of each concentration system. Solar technologies The objective of solar concentrating plants is to use solar energy instead of traditional fossil fuels to produce high temperature thermal energy. The thermal energy thus obtained can be used in a variety of industrial processes (such as, for instance, the desalination of sea water and the production of hydrogen from thermochemical processes) or in the production of electricity, thereby contributing to limiting the world consumption of fossil fuels and emissions into the atmosphere. At present, the main objective of concentrating solar plants is the generation of electricity. In this case, solar thermal energy is used in conventional thermodynamic cycles, such as those with steam turbines, gas turbines or Stirling engines. Fig. 5 schematically shows the differences between traditional and solar thermoelectric plants. When the solar source is used for the production of thermal energy, the concentration system does not create risks or annoyance to the nearby population. In regions with high solar radiation (average yearly power over 300 W/m 2 ) it is possible to generate energy equivalent to that of the combustion of a barrel of oil from a square meter of collection surface, thereby avoiding the emission of approximately 500 kg of CO 2 into the atmosphere. Thermal solar energy can be stored during the day to avoid the effects of the variations of the solar source, thereby making the system more flexible and meeting the needs of productive processes. Alternatively, it is possible integrate it with fossil fuels flat-plate collector T max 395 K parabolic trough T max 900 K theoretical 2D system T max 1,500 K real 3D system T max 2,600 K theoretical 3D system T max 5,600 K 6,000 receiver temperature (K) 5,000 4,000 3,000 2,000 1, ,000 10, ,000 concentration factor Fig. 4. Relation between operating temperature and concentration factor. VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 535

8 POWER GENERATION FROM RENEWABLE RESOURCES Fig. 5. Comparison between a traditional thermoelectric plant and a solar source plant. fossil fuel tank CO 2, NO x, SO 2... steam generator steam electric power generation steam turbine generator water condenser and cooling system fossil fuel heat production pump electric substation solar heat production electric power generation hot pump steam steam turbine generator solar field pump thermal storage cold water steam generator condenser and cooling system pump electric substation or renewable fuels, such as oil, natural gas and biomasses. Solar plants can use various technologies for the concentration of solar radiation although it is possible to identify the following phases of the process for each case: a) collection and concentration of solar radiation; b) conversion of solar radiation into thermal energy; c) transfer and possible storage of the thermal energy; d) utilization of the thermal energy. Some of the main issues of solar plants are the collection and concentration of solar radiation, which, by its nature, has a low power density. Collection and concentration are performed, as stated above, by means of a concentrator, formed by panels of a suitable geometric shape with reflective surfaces, normally common glass mirrors. All of the concentrators in a solar plant, arranged in order on the ground and suitably spaced so as not to interfere with each other in the radiation collection, make up the solar field. The receiver can have various shapes. It may be the only one for the entire solar field or there may be one connected to each concentrator. It converts solar energy into thermal energy, which is then transferred to a fluid which circulates through the receiver itself. The thermal energy transferred by the heat transfer fluid can be stored in various ways before its utilization in the productive process: by using the high heat of the fluid itself placed in insulating tanks, or by transferring its heat to inert materials with high thermal capacity, or to phase change systems. In this way, solar energy, highly variable by nature, can become a thermal energy source which is continuously available for use. An important parameter characterizing solar concentrating plants is the solar multiple, defined as the ratio between the peak thermal power of the receiver and the nominal thermal power used by the productive process. Without thermal storage, this parameter is equal to 1, and all of the collected thermal power is used immediately. Higher values mean that the plant can store the excess thermal energy. The use of solar multiples higher than 2.5 permits the continuous operation of the productive process throughout the day. However, this advantage means an increase in the cost of construction of the plant proportional to the capacity of the thermal storage system. Therefore, the optimum size of this system is chosen by means of an economic analysis; for instance, according to current values, the optimum capacity for storage systems in a thermoelectric plant is one that guarantees continuous production for 6 to 10 hours depending on the nominal electric power, in the absence of solar irradiation. 536 ENCYCLOPAEDIA OF HYDROCARBONS

9 SOLAR ENERGY CONVERSION As mentioned above, concentration systems exploit only direct radiation because they are unable to concentrate diffused solar radiation; they can be linear or focussed on single point systems. The linear concentration systems are more simple but have a lower concentration factor and therefore reach lower operating temperatures than the systems focussed on a single point. There are three main types of plants, depending on the geometry and the positioning of the concentrator with respect to the receiver: the parabolic dish collector, the central tower system and the parabolic trough collector. Parabolic dish collector This system uses reflective parabolic-shaped panels which track the Sun, rotating around two orthogonal axes and concentrating the solar radiation on a receiver mounted on the focal point (Fig. 6). The high temperature thermal energy is normally transferred to a fluid and utilized in an engine positioned above the receiver, where mechanical energy or electricity is produced directly. The ideal shape of the concentrator is a paraboloid of revolution. Some concentrators approximate such a geometric shape by using an array of spherically profiled mirrors mounted on a support structure. The optical design of this component and the accuracy of its manufacture determine the solar radiation receiver/engine Fig. 6. Typical parabolic dish layout. concentrator interception and concentration factors. The interception factor is defined as the fraction of reflected solar radiation which goes through the inlet opening of the receiver and is generally higher than 95%, while the concentration factor has already been defined above. The receiver, which is the most technologically advanced item, absorbs the energy of the radiation reflected by the concentrator and transfers it to the working fluid. The absorbing surface is generally positioned behind the focus of the concentrator to limit the intensity of the incident thermal solar flux to values in the range of 75 W/cm 2. Industrial applications of this system obtain concentration factor values higher than 2,000. With such values, it is possible to achieve very high operating temperatures and elevated efficiency of the conversion from solar energy into electricity at even higher than 30%, the highest in all currently existing solar technologies. For example, a 10 m diameter concentrator is capable of supplying approximately 25 kw e under a direct solar flux of 1,000 W/m 2. For economic reasons, the dimension of the concentrator does not exceed 15 m in diameter, thereby limiting its power to about kw e. However, this is a modular type of technology which allows the construction of low-power power plants for isolated users. The engine used in the above systems converts solar energy into work, as in conventional internal or external combustion engines. The working fluid is compressed, heated and expanded through a turbine or a piston to produce mechanical energy, which can be utilized directly by the consumer or be converted into electricity by means of an alternator. Different thermodynamic cycles and various working fluids have been studied; today, current industrial applications use Stirling and Bryton cycle engines. Either hydrogen or helium is used as the working fluid in the Stirling engines. In turn, the working fluid is cooled, compressed to pressures up to 20 MPa, heated to temperatures even higher than 700 C and then expanded. In order to transfer solar energy to the working fluid at a constant temperature, an intermediate fluid is used for the thermal exchange during a change of phase. Normally, a liquid metal (sodium) is used, which evaporates at the surface of the receiver absorber and condenses on the tube nest of the engine. The sodium vapours, once condensed, reach the zone of the absorber under the effect of gravity and spread over its entire surface by capillarity. On the other hand, the Bryton engine uses air as the working fluid with a maximum pressure of 0.25 MPa (compression ratio equal to 2.5) and a temperature at the inlet of the turbine even higher than 850 C. Due to the high temperatures reached VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 537

10 POWER GENERATION FROM RENEWABLE RESOURCES by the working fluid, its efficiency in converting solar energy into electricity is higher than that of the Stirling engine and can exceed 30%. The residual thermal energy of the fluid at the outlet of the turbine is used to preheat the air coming from the compressor. In this application the receiver is of the volumetric absorption type, similar to those used in tower plants. The concentrated solar radiation flows through a quartz window and is then absorbed by a porous matrix system (honeycombs and reticular cells of ceramic material). Such a receiver offers large thermal exchange surfaces with efficiencies of conversion from solar energy into thermal energy higher than 80%. The thermal energy can be supplied to the fluid by means of a methane combustion chamber so that the engine can operate in the absence of solar radiation or at night. Central tower systems The tower system with a central receiver (Fig. 7) uses flat reflective panels (heliostats) that follow the sun by rotation around two axes and concentrate solar light towards a single receiver. The receiver is mounted on top of a tower and a fluid to remove the solar energy is circulated within it. The thermal energy thus made available can be exploited in various processes, especially in the production of electricity. The operating principle is similar to that of the parabolic dish system, only that the concentrator consists of a high number of heliostats forming a receiver heliostats Fig. 7. Typical outline of a central tower power plant. collecting surface of hundreds of thousands of m 2. Solar rays hitting each heliostat are reflected onto a single point, fixed in time, which acts as a focal point. The height of the focal point from the ground increases in proportion to the size of the solar field and can even exceed a hundred metres. The heliostats are positioned so as to completely circle the tower, or are positioned in a hemicycle pointing north. They are spaced apart at a distance sufficient to avoid shading effects and this distance increases the further away they are from the tower. Various types of heliostats have been studied to improve optical efficiency and the control of the Sun tracking systems as well as to optimize the support structure by making it more simple and lighter. This has been done to increase plant efficiency and reduce costs. The collecting surface of each heliostat varies from about 40 to 170 m 2 ; normally, glass mirrors are used as reflective material, although alternative materials such as reflective membranes or metallic sheets have also been tested. The concentration factor of these plants is higher than 700. The high concentration factor allows the heat transfer fluid to reach high operating temperatures (higher than 500 C) thus allowing high efficiency conversion from thermal energy into electricity. Normally, conversion occurs by using the thermal energy in a traditional water-steam thermodynamic cycle. The characteristics of the produced steam (temperature and pressure) also allow the integration of tower systems in fossil fuel thermoelectric power plants. Moreover, these concentrating plants can supply a thermal storage system which can respond to the users energy requirements in a more satisfactory manner. The tower system has proven its technological feasibility for electricity production through the realization and the operation of numerous low-power (between 0.5 and 10 MW) experimental plants in various countries around the world (Spain, Italy, Japan, France, United States); although its application on a large scale still requires further testing. The most recent application of this technology is the American Solar Two plant, which was operational until April The plant, having a power of 10 MW, had a solar field consisting of 1,026 heliostats for a total collecting surface of about 81,500 m 2 and a tower 85 m high. The heat transfer fluid used was a mixture of molten salts (sodium and potassium nitrate) at a maximum operating temperature of 565 C. There was a storage system consisting of two cylindrical tanks (hot and cold) approximately 11 m in diameter and 8 m high to store enough energy for a maximum of three hours of operation at full power in the absence of solar radiation. 538 ENCYCLOPAEDIA OF HYDROCARBONS

11 SOLAR ENERGY CONVERSION Various fluids have been tested for the thermal exchange inside the receiver and for the storage of thermal energy: water, air, sodium and molten salts. To the present date, the most suitable fluid for this technology has proven to be a mixture of molten salts consisting of sodium and potassium nitrates (which are the basis of common fertilizers used in agriculture). The choice of molten salts is mainly due to their good thermal exchange coefficient, high thermal capacity, low vapour pressure, good chemical stability and low cost. The salts allow high operating temperatures (up to 600 C); moreover, they can be used directly for storing thermal energy in compact tanks at atmospheric pressure, without using an additional heat exchanger. The typical functional diagram of a tower plant using molten salts as the heat transfer fluid and for thermal storage is shown in Fig. 8. The salts, taken from the low temperature (290 C) tank, are sent to the top of the tower and circulated through the receiver, which consists of a set of steel pipe coils assembled on flat absorbing panels. The salts are heated to about 565 C and then are sent to progressively fill up the high temperature storage tank. Their flow is controlled, according to solar radiation intensity, so as to maintain the receiver outlet temperature constant. When the production of electricity is required, the salts from the hot tank are sent to a heat exchanger (steam generator), where steam at high pressure and a high temperature is produced (12 MPa, 540 C). The steam is then utilized in a conventional thermoelectric cycle; it is expanded in a turbine-alternator group to produce electricity and then condensed, preheated and sent to the steam generator again. The sizing of a solar plant (number of heliostats, thermal power of the receiver and capacity of the thermal storage) depends on the electrical power of the power plant and on its yearly utilization factor, or load factor. This is the ratio between the energy produced and the energy it is possible to produce in a year if the plant always works at its nominal electrical power. Without a thermal storage system, the power plant can only operate when there is solar radiation and can have a maximum load factor of about 25%. To obtain higher values, it is necessary to have thermal storage. In this case, the plant can operate continuously throughout the day, except for the initial phase when the system is loading. For instance, to obtain a load factor of 70%, a thermal capacity equivalent to 15 hours of operation is required in nominal conditions and in the absence of solar radiation. This corresponds to a solar multiplier of 3, which means a solar field three times larger than one without a storage system. Obviously, as previously stated, it is necessary to erect higher towers when the dimensions of the solar field increase. Due to the high concentration factor, this technology can reach even higher operating temperatures when a gas (generally air) is used as the heat transfer fluid to transfer the solar energy. In this case, a pressurized cavity volumetric receiver is used, capable of heating up to a limit temperature of 1,200 C. The receiver consists of a succession of numerous modules, each of which increases the temperature of the gas flowing through by about 150 C. At present, each module can supply a thermal power of about 500 kw e. The operational diagram of a module of the receiver is shown in Fig. 9. Solar radiation, concentrated by the heliostats, reaches each module of the receiver where, by means of a secondary concentrator, it is further concentrated until it reaches an overall concentration factor of approximately 2,000. Then, it goes through a hemispherical quartz window and reaches the absorber positioned inside a pressurized container. The absorber is a metallic or ceramic porous structure and reaches Fig. 8. Operation diagram of a central tower power plant with thermal storage. steam generator hot 565 C thermal storage tanks cold 290 C molten salt steam turbine and electric generator condenser cooling tower VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 539

12 POWER GENERATION FROM RENEWABLE RESOURCES Fig. 9. Operation diagram of a receiver module. secondary concentrator pressure vessel insulation air inlet concentrated solar radiation air outlet quartz window absorber working temperatures from 800 to 1,200 C when radiation is present. The gas, pressurized at about 1.5 MPa, flows through the multi-modular absorber and progressively heats up to 800 C, when metallic absorbers are used, or up to 1,200 C when ceramic-type absorbers are used. In a solar thermoelectric plant, the hot gas can be used for the production of steam or, more efficiently, can be used directly in a gas-steam combined cycle. The operating diagram of the latter, in a tower plant using air as the heat transfer fluid, is shown in Fig. 10. The air at the outlet of the compressor is sent to the receiver, where it is heated and then expanded in the gas turbine. Its temperature at the turbine inlet can be controlled, in case of reduced solar radiation, by burning methane in the supplementary combustion chamber. The gases, still hot at the outlet of the turbine, are sent to a recovery boiler for the production of steam and used in the relative cycle. When a gas is used as a heat transfer fluid, the storage of the thermal energy can be obtained with high thermal capacity ceramic materials placed inside special containers. A further evolution of this concentration system consists in positioning the volumetric receiver at the foot of the tower (Fig. 11). In this case, it is necessary to use a hyperboloid-shaped reflector, installed on the tower, to send direct solar radiation to the receiver. This solution offers, especially for large solar fields, better optical efficiency (optical aberrations are reduced and the concentration factor is increased), a more stable distribution of the thermal flow and a simplification of the plant (all the equipment is at ground level). Parabolic trough collector Among the thermal solar technologies for the production of electricity on a large scale, the parabolic trough collector (Fig. 12) is the one that has reached the highest commercial maturity as is clearly demonstrated by the operation of the Solar Electric Generating Systems (SEGS) plants. As previously mentioned, nine plants of this type, giving a total power of 354 MW, have been in operation in the Mojave Desert in California since the mid 1980s. This technology uses a parabolic-profile linear concentrator, whose reflective surface tracks the Sun by rotating around a single axis; the radiation is focussed onto a receiver pipe placed along the focus of the parabola. Solar energy absorbed by the receiver pipe is transferred to a working fluid, which circulates Fig. 10. Operation diagram of a tower plant coupled with a combined cycle. pressurized volumetric receiver burner (optional) steam generator compresssor steam turbine gas heliostats turbine air inlet stack condenser cooling tower 540 ENCYCLOPAEDIA OF HYDROCARBONS

13 SOLAR ENERGY CONVERSION Fig. 11. Layout of a tower plant with receiver on the ground. tower reflector receiver heliostats inside the pipe. The collected thermal energy is normally used to produce electricity by means of traditional water-steam thermodynamic cycles. The maximum operating temperature in the collector essentially depends on the heat transfer fluid being used; in the plants currently in operation, the temperature reaches 390 C. The concentrator has a steel support structure, with a central beam and a series of supports to anchor the reflective panels, thereby guaranteeing its correct functioning in windy conditions and other weather phenomena. The reflective panel normally consists of a common glass mirror of a suitable thickness. Alternatively, a panel of composite material (honeycomb) can be used that has a thin glass mirror or a reflective film glued onto its external surface. The parabolic collector has an opening of about 6 m and a focal distance of slightly less than 2 m. The concentration factor, referred to the diameter of the receiver, is about 80. Initially, this was 50 m in length, which was successively increased to 100 m and structures of 150 m are now being tested. At the centre of the collector there is the mechanism which controls the rotation for tracking the path of the Sun. The heat transfer fluid, travelling inside the receiver pipe, progressively heats up. Therefore, in order for the fluid to reach the required operating temperature at the outlet, a number of collectors must be connected in series. Normally, these are placed in two parallel rows with a total length of about 600 m, making up a string which creates the unit module of the plant. By adding more modules in parallel, the Fig. 12. Typical layout of a parabolic trough concentration system. receiver concentrator VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 541

14 POWER GENERATION FROM RENEWABLE RESOURCES thermal power produced can be increased at will. The rows of collectors must be spaced apart to avoid any reciprocal shading effect. Usually, the space between contiguous rows is 2 to 3 times the opening of the collector. Their layout on the ground depends essentially on the conformation of the site. The classic arrangement is a north-south orientation for the axis of the collectors, therefore tracking the Sun in an east-west direction. This permits better collection of solar energy, especially during summer months. The electricity production plant is at the centre of the solar field. The effectiveness of this technology depends on the optical capability of the concentrator (accuracy of the structure and characteristics of the reflective panels) but, above all, on the conversion efficiency of the receiver pipe that must absorb a maximum of concentrated solar energy and have the minimum thermal dispersion. The receiver, which is maintained in its position along the focal line of the concentrators, rotates rigidly with the concentrators while tracking the Sun. It is formed by elements of about 4 m in length connected in series. Each element consists of two concentric cylinders: an external glass tube, about 12 cm in diameter, and an internal steel tube of about 7 cm in diameter, connected to each other by folded metallic jointing to compensate for the different thermal dilatations of the two materials. A suitable selective coating is deposited on the external surface of the steel cylinder. This coating must be capable of maximizing solar radiation absorption in the visible spectrum and of minimizing the radiation emissions in the infrared that are generated by the high temperature reached in the tube during operation. A vacuum is created in the gap between tube and glass to reduce thermal dispersion by convection. The operation diagram of a SEGS plant is shown in Fig. 13. The heat transfer fluid pumped through the strings of collectors is heated by solar radiation and reaches the maximum operating temperature. The thermal energy acquired in this way is then used in a Rankine (steam) cycle to produce electricity. The plant can also have an auxiliary supplementary boiler which uses fossil fuels and is able to supply steam, even when solar radiation is absent. Consequently, electricity production is made to respond to the demand of the users. An alternative solution to the supplementary boiler is a system that permits the storage of solar thermal energy, making it available when necessary, by converting the naturally highly variable solar source into a source of continuous energy, which can then be modulated throughout the whole day. The plants now in operation use a synthetic oil (Therminol VP-1) as the heat transfer fluid for the extraction of solar heat. Unfortunately, this has a high cost and, due to the risks of environmental impact in case of leaks, it is not suitable for use in a storage system. Consequently, there is always a methane supplementary boiler in each plant capable of supplying up to 25% of the thermal energy used by the power plant. In various research centres alternative fluids such as water, with the direct production of steam, and molten salts are being tested to solve the problems connected with the heat transfer fluid and to improve the competitiveness of this technology. Molten salts permit a considerable increase in the maximum operating temperature (from 390 to 550 C) and can be used directly for thermal storage, as has already been tested in the tower plants. This is why molten salts have been chosen as the heat transfer fluid in auxiliary natural gas boiler (optional) solar collectors 290 C 390 C superheated steam cooling tower heat transfer fluid (oil) Fig. 13. Operation diagram of a SEGS plant. steam generator water flow steam turbine condenser electric generator cooling water 542 ENCYCLOPAEDIA OF HYDROCARBONS

15 SOLAR ENERGY CONVERSION Fig. 14. Operation diagram of the ENEA plant. hot 550 C thermal storage tanks steam generator cold steam turbine and electric generator condenser cooling tower 290 C molten salt the Italian project for a concentrating plant developed by the Ente per le Nuove tecnologie, l Energia e l Ambiente (ENEA). A diagram of the plant is shown in Fig. 14. The molten salts, consisting of a mixture of sodium and potassium nitrates, are extracted from the lower temperature tank (290 C) and circulated in the receiver pipes of solar collector strings, heating up to about 550 C. They are then sent to the high temperature tank for thermal storage. The molten salts coming from the hot tank are then sent to a heat exchanger to produce steam used by the electric power plant, and then reintroduced into the cooler tank. The operating temperature of the plant is controlled by suitably modulating the flow of salts in the strings of collectors according to the intensity of solar radiation. Due to the fact that the mixture of salts starts to solidify at a temperature of about 240 C, the minimum operating temperature must be maintained above this value, with an adequate margin, to avoid obstructions in the circuits. The high temperatures reached by the heat transfer fluid, a quality peculiar to the ENEA project, permit easy integration of this solar power plant into fossil fuel thermoelectric plants, including the most modern combined cycle plants, thus obtaining higher final efficiencies of conversion. The Fresnel linear collector (Fig. 15), still in a test phase, is an evolution of the parabolic trough collector. In this collector, the concentrator is substituted by segments of parabolic mirrors, placed according to the principle of the Fresnel lens. In this case, the receiver pipe is placed at the focal point and is fixed. Unlike the parabolic trough collector, the movement is applied only to the concentrator. This is an advantage since there is no need to use flexible pipes for connections between single collectors, or between the collectors and the distribution network piping, in order to circulate the heat transfer fluid. Moreover, as there is no shading effect between adjacent concentrators, the collector rows do not have to be widely spaced, thus obtaining Fig. 15. The Fresnel system. secondary reflector receiver water/steam solar radiation reflected solar radiation glass window secondary reflector receiver primary Fresnel reflector VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 543

16 POWER GENERATION FROM RENEWABLE RESOURCES Table 1. Main parameters of concentration solar plants Power (MW e ) Concentration factor Peak solar efficiency 2 (%) Average yearly solar efficiency 2 (%) Thermodynamic cycle efficiency (%) Load factor 3 (%) Used surface, m 2 /(MWh/yr) Parabolic trough ST Fresnel ST Solar tower , ST CC Parabolic dish ,000-3, SE GT Estimated data 2 Solar efficiency net electricity production/direct normal solar radiation 3 Load factor hours of operation of the solar plant/8,760 h/yr 4 Concentration factor related to the receiver diameter ST, Steam Turbine; CC, Combined Cycle; SE, Stirling Engine; GT, Gas Turbine better exploitation of the radiation reaching the ground. Normally, this type of plant uses water as the heat transfer fluid, thereby producing steam directly inside the receiver pipe. Table 1 shows the main technical parameters of the technological lines described above. The data have been collected from the operation of existing plants (parabolic trough collector and parabolic dish) or from projections on the basis of the performances of small-size demonstrative plants. Production of hydrogen from solar source Other than in the production of electricity, the high temperature thermal energy obtained in concentrating solar plants can also be used in various industrial processes, and in particular in the production of hydrogen by thermo-chemical processes. Hydrogen is now produced on an industrial scale using fossil fuels. Electrolysis is the most mature method in the production of hydrogen from water. It is characterized by a global thermal efficiency of about 36%, taking into account the conversion efficiency of thermal energy into electricity (40%) and the intrinsic yield at the electrochemical stage (90%). Therefore, the more advantageous methods, from an energy point of view, are those where the thermal energy conversion occurs in a direct way either by using renewable or non-renewable sources. At present, among such methods, the thermal splitting of water is not practical due to the high temperature needed (2,500-5,000 C) and because of the technical difficulties encountered with the separation of oxygen from hydrogen, once these elements have been formed. The thermo-chemical cycles, consisting of a series of red-ox reactions that involve intermediate substances of a different nature, represent a valid alternative to the direct splitting of water; they permit the energetic barrier and the temperature at which the thermal energy must be supplied (800-1,500 C) to be lowered considerably and also make it possible to carry out the separation of the hydrogen and the oxygen in different phases of the cycle. This type of process has been known since the 1970s, although only in the last few years has interest been renewed as a result of the stimulus of more and more pressing environmental issues. The possibility of thermally feeding these cycles with solar energy makes such production processes fully renewable and therefore perfectly compatible with a strategy of sustainable development. A simplified diagram of the production of hydrogen using the Sun as the thermal energy source is shown in Fig. 16. The parabolic dish and the central tower are the most suitable concentration systems because of the high temperatures required by the thermo-chemical process. The thermal energy absorbed in the receiver is used to feed a chemical reactor where reactions for splitting the water occur. The iodine-sulphur cycle is one of the most promising options among the various thermo-chemical processes proposed in the 1970s by General Atomics and is presently being studied by various research centres. The cycle is mainly composed of three reactions, two of them exothermic and one endothermic, whose overall balance is the dissociation of water into hydrogen and oxygen, as shown in the diagram of Fig ENCYCLOPAEDIA OF HYDROCARBONS

17 SOLAR ENERGY CONVERSION Fig. 16. Simplified diagram of hydrogen production from solar source. concentrating solar radiation high temperature heat H 2 O chemical reactor H 2 1/2O 2 Market perspectives Having examined potential solar source contributions, to help solve the future energetic and environmental problems, as well as considered the technologies that have been developed or are being developed to exploit this source, the drawbacks that, up to now, have hindered the launching of this renewable source commercially must be highlighted and its prospect of penetration into the world energetic market analysed. The main drawback is linked to the high cost per unit of solar thermoelectric plants, since it is from 2.5 to 4 times higher than that of fossil fuel plants. The cost of a kilowatt-hour produced by concentrating power plants, in spite of the low impact of the fuel cost, has been, up to now, at least double that of a traditional fossil fuel power plant due to the higher impact of the operating and maintenance costs as well as to the lower load factor. Another drawback is connected to the technical risk associated with this technology. Although it can now be considered proven and industrially mature, this technology is still perceived as new and not very reliable in performance. Solar source variability also works against it, although this can be offset by a reliable and economic energy storage system. In the future, the evaluation of the external costs associated with the emissions released into the environment by the various types of power plants could prove decisive for the diffusion of concentrating solar plants, as their emissions are negligible. In the next twenty years, the potential world production of energy by solar thermoelectric plants is estimated as equivalent to an installed electric power of 600 GW. According to forecasts, many of these plants will be erected in developing countries. Since these plants, at the moment, have an erection cost per unit much higher than traditional thermoelectric plants, in the short term, their market niche will be limited to regions where the unit cost of fossil fuels is very high. In the medium term, a growing penetration of this technology is expected, at an annual rate, in proportion to the progressive reduction of the cost of the kilowatt-hour, support and incentive policies and the future trend of the international prices of fossil fuels. In order for solar thermoelectric plants to become really competitive in the market, they will have to be capable of supplying energy when requested by the Fig. 17. Diagram of the iodine-sulphur cycle. O 2 H 2 SO 4 H 2 O H 2 SO 4 H 2 O SO 2 0.5O C H 371 kj/mol SO 2 I 2 SO 2 2H 2 O 2HI H 2 SO C H 165 kj/mol 2HI H 2 I C H 173 kj/mol I 2 H 2 HI VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 545

18 POWER GENERATION FROM RENEWABLE RESOURCES users. Consequently, they will have to be independent from the variability of solar irradiation as much as possible. Only in this way will these plants be able to satisfy the network load requirements in a reliable manner, without obliging the operator to keep traditional power plants in reserve in case of an unforeseeable, sudden reduction or absence of power generated by a solar source. As has been previously mentioned, this is only possible if the concentrating solar plants are equipped with a suitable energy storage system which provides a power supply which responds to variations in demand and compensates for the fluctuation of direct solar radiation during the day and its absence at night. The introduction of a storage system will also permit a substantial improvement in the load factor of the power plant, as it determines an increase in its annual number of hours of operation. The possibility of integrating the concentrating solar plants into traditional thermoelectric power plants already in operation, to increase their total power, is a characteristic which might encourage the diffusion of these plants. This will result in a cost reduction of the investment per unit of the thermodynamic solar plants and permit them to greatly modulate their power, even throughout the day, eliminating the drastic reduction of efficiency of the electricity generation steam cycle, typical of an exclusively solar power plant. Another aspect which could help the expansion of the market is connected to the possibility of erecting the concentrating solar power plants in areas with high irradiation and then transferring the energy produced in excess of the local requirement to countries with considerable or increasing requirements for electricity. As far as this is concerned, it is important to remember that transferring electricity for long distances, even thousands of kilometres, is now already technically and economically feasible with high voltage direct current underwater lines and cables (High Voltage Direct Current transmission technology, HVDC). From this point of view, the Mediterranean area could have a leading role in the exchange between European countries great consumers of electric energy with low energy resources and the countries of North Africa and the Middle East that have many areas available with high direct irradiation and primary energetic sources. At present, there is already an almost complete ring interconnecting the alternate current electricity grids of the Mediterranean countries and the direct current underwater connection between Italy and Greece. Moreover, direct current underwater connections with a total transport capacity of thousands of megawatts are being designed to increase the interconnection between Europe and the North African countries. The rapid completion of these interconnections could stimulate European enterprises to invest in the erection and operation of power plants in North Africa, possibly in partnership with local enterprises. This would also certainly facilitate the erection of solar thermoelectric power plants that, initially, could be integrated with fossil fuels plants. Later, due to the decrease in costs induced by the volume of the growing market, the erection of exclusively solar power plants in desert areas would also be facilitated. Bibliography Butti K., Perlin J. (1980) A golden thread: 2500 years of solar architecture and technology, Palo Alto (CA), Cheshire Books. Dickinson W.C., Cheremisinoff P.N. (1980) Solar energy technology handbook. Part A: Engineering fundamentals, New York, Marcel Dekker. ENEA (Ente per le Nuove tecnologie, l Energia e l Ambiente) (2004) Progetto Archimede. Realizzazione di un impianto solare termodinamico integrativo presso la centrale ENEL di Priolo Gargallo (SR), ENEA/SOL/RS/ ENEA (Ente per le Nuove tecnologie, l Energia e l Ambiente) (2004) Rapporto energia e ambiente 2003, Roma, ENEA. EPRI (Electric Power Research Institute)/DOE (US Department of Energy) (1997) Renewable energy technology characterizations, Topical Report TR , December. Kubo S. et al. (2004) A demonstration study on closed-cycle hydrogen production by the thermochemical water-splitting iodine-sulfur process, «Nuclear Engineering and Design», 233, Müller-Steinhagen Freng H., Trieb F. (2004) Concentrating solar power: a review of the technology, «Ingenia», 18. Smith C. (1995) Revisiting of solar power s past, «Technology Review», 98, Winter C.J. et al. (1991) Solar power plants. Fundamentals, technology, systems economics, New York, Springer. References Solar Millenium AG (2003) Financing the future. The Solar Millenium share. Mauro Vignolini Ente per le Nuove tecnologie, l Energia e l Ambiente Centro Ricerche Casaccia Santa Maria in Galeria, Roma, Italy 546 ENCYCLOPAEDIA OF HYDROCARBONS

19 6.1.2 Photovoltaic technology Introduction The photovoltaic effect is produced by an electromotive force in an electrically heterogeneous medium exposed to electromagnetic radiation. The name derives from the fact that the phenomenon was discovered by Edmond Becquerel in 1839 in an electrolytic or voltaic cell. The phenomenon is typical of semiconductor-to-metal or semiconductor-to-semiconductor junctions; if the junction is illuminated, then electron-hole pairs are created within, at the expense of the energy of the incident photons: the potential barrier, located at the junction, drives the holes towards the lower potential area and the electrons in the opposite direction, and so creates an electromotive force (in the order of a few tenths of a volt); if the junction is part of a closed circuit, then an electric current is generated. This effect can be applied to the direct conversion (known as photovoltaic energy, solar electricity or, more simply, photovoltaic) of solar energy into electrical energy through suitable devices called solar cells. Individual solar cells are connected to each other electrically to form modules, which are sealed to resist the external environment for several years. The modules can be used individually or connected together electrically in so-called photovoltaic fields. There are various kinds of photovoltaic systems: History of the technology and its applications The photovoltaic effect was discovered and studied as part of experiments in various disciplines. As mentioned, E. Becquerel observed that weak potentials were created by illuminating one of the electrodes in an electrolytic cell. The first functioning solar cells were built by W.G. Adams and R.E. Day, using a solid 10,000 annual photovoltaic production (MW) Fig. 1. Forecast for regional and total market growth in accumulating through groups of batteries, directly connected to the electricity grid, or for other small-scale use. Solar electricity has many advantages: it has a low environmental impact, it is renewable, it is modular and can be used directly where it is produced. On the other hand, it is a costly, intermittent and low-density energy source. Moreover, the yield or efficiency of the conversion of solar radiation into electricity is relatively modest, around 15% for industrial solar cells, which makes it necessary to cover large surface areas. The photovoltaic market has been expanding rapidly since the end of the Nineties, thanks, above all, to government incentives aimed at encouraging the use of environmentally friendly renewable sources (Fig. 1). However, for this to become a significant energy source worldwide, marked technological progress and a sharp reduction in costs are necessary. The research and development activities currently underway internationally are aimed at these goals. 1,000 United States Japan Europe rest of the world total * * White Paper target year VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 547

20 POWER GENERATION FROM RENEWABLE RESOURCES (selenium) towards the end of However, the explanation of the phenomenon only occurred following the quantum interpretation of the photoelectric effect provided by A. Einstein in It was necessary to wait until 1954 for the first photovoltaic devices with a significant conversion yield, when researchers at Bell Laboratories (USA) built the first silicon solar cell with a yield of 6%. The first attempt to market the Bell cells, which were produced on a small industrial scale, was greatly hampered by prohibitive costs. The main applications up to the Seventies were electricity supply systems for artificial satellites, given the absence of valid alternatives. The American and Soviet space race, and the need to improve the electricity supply systems of satellites led the American government to finance photovoltaic research programmes, enabling the creation of specialized industrial initiatives at the same time. Thus, the costs of solar cells fell markedly, albeit remaining prohibitive for applications beyond space or military programmes. At the start of the Seventies, the use of photovoltaic energy for terrestrial applications was enabled by the development of technologies with less rigorous specifications than those needed for the cells used in space applications. Thus, it was possible to reduce costs to around dollars/w. However, even at that level, the cost of energy produced with photovoltaic modules was approximately 40 times the cost of conventional electricity; the first production for terrestrial use was, therefore, largely aimed at applications in areas which were either remote or hard to access with the electricity grid. Thus, a market was created dedicated to the electrification of oil platforms, to the supply of energy to anticorrosion systems for oil wells and pipelines, to the supply of energy to marine communication or signalling systems, as well as the electrification of rural settlements in developing countries. This initial terrestrial market favoured the creation of the first industrial initiatives in various parts of the world with fairly basic production and very small size manufacturers. The functioning of photovoltaic devices The functioning of solar cells is connected to the complex interaction between light and matter, and involves the nature and characteristics of light, the physical properties of materials and the production of electronic devices. Here below is a brief description of the properties of semiconductors, aimed at providing an understanding of the main operating mechanisms of solar cells. An attempt has been made to reduce and simplify the explanation as far as possible, although the phenomena concerned require a quantum handling of the structure and properties of matter, and its interaction with electromagnetic radiation. Solar radiation The Sun emits light over a broad wavelength interval, of which the human eye only sees the visible fraction. In 1900, M. Planck resolved the discrepancies between the experimental observations of the spectrum of electromagnetic radiation in thermal equilibrium and the classical theory of the phenomenon, by introducing the concept of quantum energy. Subsequently, A. Einstein (1905) highlighted the corpuscular behaviour of radiation and linked the energy E of the single photon to the wavelength l through the formula E hc/l where h, or J s, is the Planck constant and c, or m/s, is the speed of light in a vacuum. In the quantum description of electromagnetic radiation, there are wave and corpuscular aspects (wave-particle duality). The spectral distribution F(l) of solar radiation, regarded as the emission from a blackbody, is described by Planck s law (energy density per unit time per unit wavelength): F(l) phc 2 hc l 5 exp 11 klt 1 where k , J/K is the Boltzmann constant and T the thermodynamic temperature of the blackbody (in the case of the Sun, the apparent surface temperature is around 6,000 K). The integral of the spectral distribution over all wavelengths gives the power density H S emitted at the surface of the Sun: H S st W/m 2 where s, or W/m 2 K 4, is the Stefan-Boltzmann constant. At a distance D from the surface of the Sun: H H S R 2 D 2, where R km is the Sun s radius. The radiation density is 1,353 W/m 2 at the edge of the Earth s atmosphere. Apart from minor variations due to the Earth s elliptical orbit around the Sun, this value is constant. However, on the Earth s surface, the radiation is affected by alterations due to atmospheric conditions, the latitude and the seasons, as well as the day-night division. In section 6.1.1, the existence of the sunbelt, whose annual insolation is always significant, is highlighted. The power density of solar radiation is less than that at the edge of the atmosphere, owing to absorption due to molecules and atmospheric dust and to diffusion (of around 10%) caused by part of 548 ENCYCLOPAEDIA OF HYDROCARBONS

21 SOLAR ENERGY CONVERSION the atmosphere s molecules. The maximum solar radiation density directed at the Earth s surface, in the absence of clouds, is around 950 W/m 2 (to which should be added the diffused element). In general, reference is made to a standard radiation value at the Earth s surface in order to compare the performances of photovoltaic modules and systems against one another, while reference is made to local climate data, if available, to size the actual installations appropriately. Thus, the so-called standard Sun is defined in relation to a global air mass of AM 1.5 (Air Mass is related to the distance travelled by radiation in the atmosphere and is given by the secant of the angle q between the normal on the ground and the position of the Sun, so that AM secq, AM 1 if q 0), equivalent to 1,000 W/m 2 (taking account both the direct and diffused components of the radiation). The standard Sun corresponds to the radiation level of the surface of the Earth at an angle of around 49. The radiation at the edge of the atmosphere corresponds to zero air mass (AM0). Semiconductors The materials available to make photovoltaic cells are numerous and often have very differing characteristics. For example, there are inorganic semiconductors in the solid state, of which silicon is by far the most widely used, as is also the case with electronic technology; among others, we would mention germanium and compounds between elements of the III and V groups (GaAs, InP) or the II and VI groups (CdTe, CdS) of the periodic table of elements, and also compounds with three or more elements (InGaN, GaInP). Among the materials used for Becquerel type cells is titanium dioxide (TiO 2 ) with some colorant additives, while for organic cells, nanostructures are used, such as fullerene (C 60 ) or conjugated polymers. Other materials are being studied, including silicon nanostructures. For a description of the operation of solar cells based on the most common technology, that of silicon, see below. Proprieties of silicon An element of group IV, silicon has 4 valence electrons which, in the ideal crystalline form, give rise to 4 covalent bonds with other silicon atoms; in other words, bonds in which each atom shares one of its own valence electrons with the nearest others, thus achieving the stable electronic configuration (octet). Silicon does not exist in pure form in nature, although it is the second most abundant element on the Earth after oxygen. It is, however, found in the form of various minerals, such as silica (silicon dioxide), and its transformation into crystals of the desired purity requires particular treatment (see below). In a semiconductor such as silicon, in the bound state and at thermodynamic zero, there are no electrons available for electrical conduction and the solid behaves as an insulator. At temperatures other than zero, however, thermal agitation allows some electrons to free themselves, even if their number is very small. Many more electrons can be freed if the silicon is illuminated with light whose photons have sufficiently high energy, for example, as occurs with part of the solar spectrum. The part of the solar spectrum with the highest energy, however, tends to interact with the inner shells of the atoms, without contributing to the photovoltaic effect. The binding energy of silicon electrons is around 1.12 ev, which corresponds to photons of radiation with a wavelength of 1,100 nm (near infra-red). The photovoltaic effect, and many other properties of semiconductors in general, can be explained in full with the theory of electronic bands in solids. In an atom, there is only a collection of discrete energy levels that can be occupied by electrons, but when several atoms are brought together to form a solid, the levels mix to give rise to bands of possible energy levels, separated by empty areas (in the case of ideal solids; Fig. 2). The width of the zone of forbidden levels is called the band gap, and corresponds to the minimum energy needed to bring electrons from a fully occupied band and, therefore, without any Fig. 2. Simplified diagram of the formation of semiconductor energy bands as the distance between atoms diminishes. The distance d represents the semiconductor in equilibrium. energy conduction band valence band forbidden band (band gap) discrete atomic energy levels d atomic distance VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 549

22 POWER GENERATION FROM RENEWABLE RESOURCES possibility of movement (valence band), to an unoccupied band (conduction band), taking into account that electrons first occupy lower energy states. This representation is equivalent to the passage from the covalent bound state to a free state (within the solid) of one of the outer electrons of the silicon atoms. In reality, the energy bands follow more complex patterns connected to the three-dimensional structure of the crystals, the temperature and the symmetry properties of crystalline matrices, and the type of bonds between the atoms. The width of the band gap is not generally constant and in the case of silicon in particular, the minimum of the conduction band does not correspond to the maximum of the valence band (in such a case, it is said that the semiconductor has an indirect gap). The form and the nature of the band gap have a marked influence on the properties of the semiconductors, particularly in relation to the interaction with electromagnetic radiation. Besides the band gap, another important measurement is the Fermi level E F, or the energy below which all the states are occupied, and above which, they are empty. In the absence of reticular impurities and imperfections, the Fermi level is at the centre of the band gap (Fig. 3). The band model provides us with a relatively simple explanation of how solar cells work. When an electron is transferred into the conduction band, following the absorption of a sufficiently energetic photon, it leaves an electron gap or vacancy in the valence band known as a hole, which can move within the semiconductor and behaves likes a pseudo-particle with the same charge as the electron, but of the opposite sign. E q χ E G conduction band valence band Fig. 3. Simplified band diagram. E v is the edge of the valence band, E c the edge of the conduction band, E G the band gap, q c the electric affinity. energy of a free electron E c E F, Fermi level E v Electrical conduction in semiconductors, such as silicon, is due to a flow of electrons in the opposite direction to that of the holes. Moreover, pure silicon has a low density of free carriers, even in the presence of light. It is common practice to insert controlled quantities of some elements, in other words to dope the semiconductor in order to improve its electrical transport properties. The elements usually used for silicon for photovoltaic applications are pentavalent phosphorus and trivalent boron. These elements are inserted in quantities sufficient to raise the number of carriers, without significantly altering the opto-electronic properties of the silicon. In the case of phosphorus, the impact of these elements is to provide an extra free electron compared to the tetravalent symmetry of silicon, thus giving it an excess of negative carriers. In the case of boron, there is an extra hole and the material has an excess of positive carriers. By convention, it is said that silicon doped with boron is type p, while silicon doped with phosphorus is type n. By using doping techniques, it is possible to increase the density of electrons (or holes) by up to 10,000 times from the level of cm 3 of the intrinsic silicon to cm 3 in the typical case of boron, the most commonly used in the production of crystals for photovoltaic applications. In general, the layers which have been doped with phosphorus have even higher densities. This allows the density of the excess carriers at room temperature to be approximated by the density of the dopant. From the viewpoint of the system of bands, the dopants have the effect of introducing energy levels near the edges (of the valence band in the case of boron, and of the conduction band in the case of phosphorus) and, therefore, of moving the Fermi level in the direction of the opposing edges, thus making a greater quantity of energy levels available. When the doped semiconductor is illuminated, a pair of excess carriers is created: an electron and a hole. One of these carriers will be in the majority and the other in the minority, depending on the characteristics of the material. For example, in the case of p type silicon, which has an excess of holes, the minority carriers will be the electrons. Although the density of photogenerated carriers is small compared to that of the dopant atoms, the minority carriers have a more important role, in terms of many aspects of the functioning of solar cells, than do majority carriers (in this case holes). When the doping density is close to that of the silicon atoms ( atoms cm 3 ), the semiconductor is termed degenerate and the description of the material in terms of bands is more complex. 550 ENCYCLOPAEDIA OF HYDROCARBONS

23 SOLAR ENERGY CONVERSION Optical properties The capacity of semiconductors to absorb radiation is not constant over the whole spectrum. For every material, there is an absorption coefficient a, an optical property which also derives from the band structure of the semiconductor and is dependent on the wavelength. Finally, each material has a reflection coefficient and will have transmitted, reflected and absorbed components that differ depending on the wavelength. Not all the incident light can be absorbed by the material and not all the absorbed light is equally involved in the creation of carriers, given that the intensity I of the radiation diminishes in the material, in accordance with the law: I I 0 e ax (where I 0 is the intensity of the incident radiation and x the thickness of the material crossed). This implies, given the trend with the variation of the absorption coefficient with wavelength, that the most energetic radiation is absorbed in the uppermost layers of the solar cell, while the less energetic radiation is absorbed deeper down. It follows that there are optimal thickness values for each type of semiconductor, based on the optical properties of the material. In the case of silicon, such thickness ranges from a few to around 300 microns (3/10 of a millimetre). Generation-recombination The rate of generation of carrier pairs is linked to the ability of the material to absorb the incident light efficiently, i.e. the ability to create an electron-hole pair for each incident photon. This ability is measured by a factor called Spectral Response (SR), given by the ratio between the current generated and the incident power, or quantum efficency (%) 100 el SR 13 QE hc high energy photons reflection losses front surface recombination wavelength ideal cell absorption threshold Fig. 4. Quantum efficiency of a solar cell and loss mechanisms compared to the ideal transformation of one photon into one electron-hole pair. finite value of diffusion length rear surface recombination where QE (quantum efficiency) is the ratio between incident photons and carriers pairs generated, l is the wavelength and e the absolute value of the electron s charge. In particular, in the case of silicon solar cells, QE takes the form shown in Fig. 4. It can be clearly seen that the solar cell cannot use all the solar radiation. In addition, the cell cannot absorb all the photons with E E G with the same effectiveness, since the most energetic ones create carrier pairs at the surface, where there is marked recombination owing to the presence of energy levels in the band gap due to the material-air discontinuity; on the other hand, the photons nearest to the band gap thresholds are absorbed at a considerable distance from the illuminated surface and, if the quality of the material is not adequate, the carriers recombine before being used. In addition, the total quantity of absorbed photons depends on the fraction of radiation reflected by the surface. The integral of the QE over all wavelengths is related to the short circuit current. Photons with energy below the width of the band gap are not absorbed. Therefore, below the energy threshold, QE is zero. This is also true if the energy of the photons is markedly higher than that of the band gap. Also the absorption due to carriers which are already in the conduction band has no effect on the cells electricity transport mechanisms, but is actually an obstacle to photovoltaic generation. This phenomenon is typically seen in highly doped materials, or can be important at the edges of the bands and is not included in the calculation of a(x). In the case of devices which are electrochemical or based on polymers, the absorption of luminous radiation creates electron-hole pairs in an excited state (excitons or excited molecular orbitals), which tend to return very rapidly to the original state, owing to the high electrostatic attraction (recombination times are in the order of s). In this case, the possibility of generating a photo-current is linked to the ability to very rapidly separate electrons from holes, through redox solutions or by means of charged materials which accept the photogenerated charges and channel them into an electric circuit. Photogenerated carriers tend to recombine, and this process is quicker if there are defects in the material which capture the carriers. Since it is inevitable that there are defects in the material, caused by impurities such as other types of atoms, distortions in the crystalline matrix or by surface effects, the ability to make best use of the photogenerated carriers depends on the properties of the material. The quality of the semiconductor is generally expressed in terms of parameters such as the life time t and the diffusion length L d of the minority carriers, defined respectively as the average time needed for a photogenerated VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 551

24 POWER GENERATION FROM RENEWABLE RESOURCES electrons depletion region Fig. 5. Formation of the depletion region. p holes carrier to recombine and the average distance travelled by such a carrier before recombining. The two parameters are linked. In the case of silicon for photovoltaic use, typical values are in the order of some tens of microseconds for t and some hundreds of microns for L d. The recombination in semiconductors can come about in various ways. Of these, by far the most important for manufactured solar cells is recombination through defects. This mechanism links the recombination properties of the material to its characteristics of purity and crystallographic perfection. Devices In order to generate electricity, it is necessary to build a device which enables the effective separation of the charges and the creation of an electromotive force to provoke the flow of the electrical current in an external circuit. The essential requisite for the generation of electricity is that there is electronic heterogeneity in the structure of the material. The most common electronic device for the production of solar cells is the p-n junction, analogous to that used in solid state diodes. Electrons and holes generated by a photon are separated by the barrier s electric field in the p-n junction and channelled to an external circuit. In the case of silicon, the junction is obtained between the parts that are doped in different ways. In order to explain the functioning of the device, let us imagine bringing together two parts of silicon, one doped p and the other doped n. Before contact, on the right side, we will have an excess of electrons, and an excess of holes on the left side. When the two semiconductors come into contact, a flow of carriers is established by diffusion to rebalance the concentration gradients. This leaves a double electric layer formed by positive and negative charges uncovered at the interface between two different materials This double layer, also called the depletion region, creates an electric field which is opposed to the diffusion which generated it (Fig. 5). In the absence of an external excitation, there is no n net current flow. A junction is effectively represented by using the band structure, in the case of a p-n junction, in Fig. 6; the double layer forms an energy step for the passage of charges, except the few which manage to cross over, owing to thermal agitation. The step is such that, at the point of equilibrium, the Fermi levels of the two materials coincide because a system at equilibrium can only have one Fermi level. Away from the region of the junction, the bands are unchanged (flat). When an external excitation is added, such as a photon with energy higher than that of the band gap, electron-hole pairs are created on both sides of the junction depicted in Fig. 7. The minority charges photogenerated in proximity to the junction leave uncovered ions which partly neutralize the charge of the double layer, thus reducing its height. This mechanism is called minority carrier injection: the carriers photogenerated on the side of the energy step can more readily cross it, with the effect of putting the junction in direct conduction. However, the junction is obviously not a barrier for the electrons in a conduction band and the holes in a valence band at the peak of the step. In the illuminated state, there is no longer a state of equilibrium, and it is inappropriate to speak about Fermi levels, since the concentrations of carriers vary, as well as the corresponding statistical distribution. However, since the variation from equilibrium is not large, it is possible to precisely define energy levels which represent this deviation and which assume different values in different parts of the device. These n v E F p qy Fig. 6. Band diagram of the p-n junction at equilibrium. E Fp p qv Fig. 7. p-n junction in state of non-equilibrium and generation of electron-hole pairs. n n E Fn nc 552 ENCYCLOPAEDIA OF HYDROCARBONS

25 SOLAR ENERGY CONVERSION levels are called quasi-fermi levels or Imrefs by convention, and correspond to the chemical potentials of non-equilibrium. The quasi Fermi levels, shown in Fig. 7, are very important in the behaviour of the solar cells, since they determine the maximum electromotive force obtainable, or the size of the photovoltaic effect. The mathematical description of the charge transport in solar cells is given starting from the continuity equations, which guarantee the conservation of the total charge, and from the Poisson equation, which relates the electrical potential to the charge density. In general, some simplifications may be considered, such as the absence of the electric field in active regions (in the example, the p region) and the constancy of the quasi Fermi levels in the depleted region. In addition, it is assumed that the concentration of minority carriers is always much lower than that of majority carriers, and that the thickness of the cell is much greater than the diffusion length of the minority carriers. In ideal conditions, when the generation and recombination mechanisms do not depend on the photogenerated currents, the principle of superposition comes into play, by which the current of the cell is given by the algebraic sum of the current of the diode and the (negative) photogenerated current. Superposition has the effect of shifting the typical curve of the diode in such a way that it occupies an area in the fourth quadrant of the current-voltage plane I-V (Fig. 8), or of giving the device a feature of a power generator (in that the power absorbed is negative). The characteristic of the illuminated device takes the form ev I I 0 exp 11 1 I L nkt where I 0 is the inverse saturation current, or dark current, I L is the photogenerated current and n is a ideality I IV quadrant diode in the dark Fig. 8. Effect of the superposition principle on the characteristic I-V of a p-n junction solar cell. V diode under illumination I I sc V oc V Fig. 9. Typical parameters of a solar cell (quadrant IV has been inverted for the sake of convenience). factor, which measures 1 in the case of an ideal diode and is greater than 1 in the presence of defects. Typical parameters of solar cells It is possible to extract the important electric parameters of a solar cell from the characteristic curve; in other words: a) the short circuit current I sc, generated by the cell at zero potential; b) the open circuit voltage V oc, which corresponds to the maximum possible compensation of the electrostatic barrier by the photogenerated charges; c) the maximum power point P max ; d) the so-called FF (Fill Factor) of the typical curve. The meaning of such parameters is shown in Fig. 9, where the typical curve under illumination for a solar cell is shown conventionally inverted in the first quadrant, changing the sign of the current. The efficiency is defined as the ratio P max /P incident and also FF P max /I sc V oc. Since the short circuit current is directly proportional to the area of the cell, in general, in order to compare cells of different areas, the current density J sc is used; this depends directly on the number and type of incident photons, i.e. on the intensity of the radiation and its spectrum and the optical properties of the material. Finally, it is heavily dependent on the recombination properties, in other words, the degree of purity and perfection of the material. For silicon solar cells, typical values of the short circuit current density are ma/cm 2. An expression for the open circuit voltage is found by setting the term of the current to zero in the equation for the diode: nkt I V L oc 11 ln 1 e I 0 1 This important expression shows that the open circuit voltage of a solar cell depends, not only on the temperature and the photogenerated current, but on the. P max VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 553

26 POWER GENERATION FROM RENEWABLE RESOURCES inverse saturation current, directly linked to the quality of the material. The ideal solar cell described by the typical equation set out above cannot be achieved in practical terms and, in particular, the device suffers from the effects of parasitic resistance. Limits to the efficiency of solar cells The efficiency (or yield) of a solar cell cannot be 100%. Studies on what constitutes the theoretical limit for the yield of a solar cell (or converter) and what the obtainable limits are started when the first cells were made in the Bell Labs, and continue today. As far as thermodynamics are concerned, treating the Sun-cell system as two sources at differing temperatures which exchange energy and entropy, the maximum limit for the efficiency of a solar cell with unspecified material and characteristics is 86.4% (thermodynamic approach) in conditions of maximum concentration; in other words, concentrating the entire solar radiation flux on a single point. These results can be reached also by generalizing the fundamental work undertaken in 1961 by W. Shockley and H.J. Queisser on a single junction device. This approach, called detailed balance, calculates, through continuity equations, the current produced by a solar cell from the difference between generation and recombination in a two source system, in which the solar cell, or converter, has chemical potential (equal to the separation between the quasi Fermi levels); in other words, an absorption threshold (approach of generalized or device-based detailed balance). The generation function in the case of Shockley and Queisser was given by Planck s law for the emission spectrum from a blackbody at around 6,000 K. The equivalence between the two approaches can be shown in mathematical terms since for both it is possible to express for each photon of energy E, the work in the same form, provided that the chemical potential m is expressed in terms of the temperatures of the converter. The work can be expressed in a more general form for many photons of differing energy, by using a generalized generation function and multiple or variable chemical potentials, with some calculational complications which take account of the various generation and recombination mechanisms. The generalizations keep the mathematical equivalence with the purely thermodynamic treatment if additional conditions are introduced, which describe the behaviour of any solar cell in short circuit conditions (the work must be nil under these conditions) and open circuit conditions (the total flow of carriers must be nil, a condition which corresponds to minimum entropy). It is, however, interesting to cite this generalization, which enables a description, for example, of a device formed by n junctions, each of which absorbs and converts a part of the incident spectrum. These cells are known as tandems. Other types of device have been proposed to reach the theoretical limits, all of which may be described with the generalized approach. They are devices that generate multiple electron-hole pairs for each photon absorbed; or use electrons in states far from the edges of the bands; or can convert the most energetic photons into more, but less energetic photons; or have levels within the band gap that can contribute to the generation of pairs. For a single junction silicon device, the theoretical limit which can be reached with sunlight is around 30% (33% if the AM 1.5 spectrum is considered), considering the material as ideal. Around 30% of the incident solar radiation is lost, since it is not sufficiently energetic, and around another 30% is dissipated as heat because it is too energetic. The remaining approximate 10% is lost through recombination mechanisms (only radiative mechanisms in the ideal case). Nonetheless, the yield that can really be obtained from solar cells is lower, given that the material is not ideal (the mobility of the carriers is not infinite); other recombination mechanisms must be considered (e.g. Auger); and that there are optical losses, through lateral conduction, surface effects and imperfect metallic contacts. The best laboratory cell produced so far on monocrystalline silicon had an efficiency level of 24.7%, compared to a value of just above 20% for a multicrystalline silicon cell, while the best commercial devices have values around 20%. The average efficiency values of the most common commercial cells are, however, even lower, 14-16%. It is believed that the obtainable limit for silicon solar cells is around 26%. The devices which currently have higher yields are those made with several junctions in a series of different materials. In particular, the GaInP/InGaAs/Ge device reaches 32% at 1 Sun and around 39% under concentrated light. The maximum values obtained refer to very small surfaces, generally around 1 cm 2. Commercial cells are currently built on surfaces between 100 and 400 cm 2. Technological and industrial aspects The technology for the production of solar cells has a lot in common with that for electronic semiconductor devices, especially the use of silicon, the most widely studied and used semiconductor in the world. Nonetheless, some particular aspects of photovoltaic energy, mainly the need to use rather large surfaces compared to those for electronic devices 554 ENCYCLOPAEDIA OF HYDROCARBONS

27 SOLAR ENERGY CONVERSION and the need to limit manufacturing costs as far as possible, have differentiated the construction techniques over time. Materials Silicon used as an active material to produce solar cells is generally in the form of thin wafers (with a thickness of around 250 mm) with a surface area between 100 and 400 cm 2. The cost of silicon wafers represents around 50% of the cost of a photovoltaic module, therefore the efficient use of the raw material is essential for technological progress. The wafers, or layers, are created by cutting pure silicon ingots in crystalline form, produced with technologies derived largely from those used in the electronic industry, and altered to meet the specifications of the photovoltaic sector. The initial material, called silicon feedstock, is created following a complex chain of successive purifications starting with sand. In simple terms, the refining reaction is: SiO 2 2C Si 2CO, from which a (solid) metallurgical silicon is obtained, with a purity of %. This process takes place in a submerged arc furnace, at a high temperature (around 2,000 C). The material produced (some millions of tonnes/p.a.) is mainly used in the steel and aluminium industry. A tiny fraction is purified for use in the silicon, semiconductor and photovoltaic industry, through the reaction: Si 3HCl SiHCl 3 H 2, which occurs in a fluidized bed reactor, in the presence of a copper catalyst. The compound SiHCl 3 is liquid and is purified through multiple fractionated distillations; it is the material used to obtain silicon. Finally, for the electronic and photovoltaic sector, from the reaction: SiHCl 3 H 2 Si 3HCl, very pure silicon is obtained, which is deposited by means of heterogeneous nucleation on special filaments in a so-called Siemens reactor; there are also variants with other gases based on silane and other types of reactor. This material, together with some recyclable waste products from various processes, represents the initial material for the production of ingots for photovoltaic use. Currently, annual consumption is around 15,000 t. and is growing. Since the beginning of industrial activity in the photovoltaic sector, less costly alternatives have been tested for the production of silicon, which are independent from the semiconductor industry. In particular, significant effort has been placed on purifying metallurgical silicon, which costs around thirty/sixty times less than silicon obtained from the Siemens process. Some tests in this regard are still underway, but an industrial product is not currently available. The main difficulties for the direct purification process of metallurgical silicon lay in the elimination of excess boron and the processing cost. molten silicon Fig. 10. Diagram of the Czochralski crystallization method. pulling direction seed crystal crucible The silicon ingots can be in the form of monocrystals, made by developing the material on an initial crystalline seed, slowly extracted from a quartz recipient (crucible) that contains pure melted silicon, at a temperature of over 1,400 C (Fig. 10). The process takes place in an inert atmosphere (usually argon) by controlling the quantity of elements such as metal, oxygen and carbon, which can reduce the quality of the material. In general, cylindrical ingots are produced of cm in length, with a diameter up to around 20 cm (or 8 inches, due to a convention linked to the American industry). The method referred to is commonly called Czochralski, after the name of inventor. The material thus produced has good crystallographic perfection and good purity, with a crucible impurities thermocouples translation argon molten silicon solid silicon heaters graphite hot chamber cold chamber cooled pedestal Fig. 11. Diagram of the directional solidification method to produce multicrystalline silicon blocks. There are some existing variants on the process shown. VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY 555

28 POWER GENERATION FROM RENEWABLE RESOURCES metal content of less than one part in a billion, and oxygen and carbon in the order of hundreds of parts per million. The second method, currently more common for the production of silicon wafers, consists of melting and solidifying silicon contained in large square quartz crucibles under controlled thermal conditions. The method, called directional solidification, is based on the controlled extraction of the heat of the molten silicon from the bottom of the crucible, keeping the temperature of the walls and the top as high as possible (Fig. 11). This method has been developed specifically for the photovoltaic sector, since it can satisfy economies of scale and is a relatively simple process. The material is in a multicrystalline form, with long grains perpendicular to the solidification front. In this case, there is no need for crystalline seeds and the ingots can also be very big (typically kg, and dimensions of around cm). It is a material which is of slightly lower quality than that obtained with the Czochralski method, but has the suitable requirements for the production of solar cells with an efficiency of 14-16% in industrial production and up to 20% for laboratory devices. The silicon ingots for photovoltaic use are generally p type doped, by mixing controlled quantities of boron to the feedstock to be melted. Typically, the resistance of the ingots is 1W cm, which corresponds to a density of dopant of around atoms/cm 3. The ingots, whether they are in mono- or multi-crystalline form, are then mechanically processed to be transformed into thin wafers. These squaring and cutting processes are done with diamond studded blades or steel wires in an abrasive suspension. A B p n p-type metal Devices Most commercial solar cells are produced using a process which is substantially the same as that developed in research laboratories in the Eighties; it is based on low cost, high productivity screen technology in order to print metal contacts with silver and aluminium based inks. The commercial solar cell is a homojunction diode, produced by bringing together doped p and n zones of the same slice of silicon (Fig. 12). It starts with a wafer containing boron (in the order of one part per million) which produces the excess of free positive charges. The dopant n, generally obtained with phosphorus atoms, is produced with a high temperature thermal process (around 900 C), which occurs by diffusion in a very thin zone near the illuminated surface. The phosphorus dispersed in the silicon occupies a layer of less than a micron under the surface of the wafer which is around 250 mm thick. Fig. 12. Solar cell: photograph (A) and diagram in cross-section (B). Other treatments include surface preparation with chemicals to remove any impurities and damage due to the wafer cutting process, and to reduce the quantity of reflected radiation. Before producing the metallic contacts, a thin dielectric layer is deposited on the surface exposed to radiation, in order to further reduce the losses by reflection. Industrial production lines use chemical deposition systems in silicon nitride plasma. The volumes produced (around 1,500 MW in 2005, compared to 10 MW in 1985) has aroused the interest of the machinery and materials industry, thus, finally creating standards. Further progress is certainly linked to improvements which will be gradual, but no less significant, such as automated wafer handling these 556 ENCYCLOPAEDIA OF HYDROCARBONS