6 power generation from renewable resources · 6 power generation from renewable resources. 6.1.1...

6

POWER GENERATIONFROM RENEWABLE RESOURCES

6.1.1 Concentration systems

OverviewAlthough world energy consumption has grown at

an average yearly rate of about 2.3% in the last 150years, one quarter of the world population today stillhas no access to electricity and more than one third ofthe population, mainly concentrated in the developingcountries, resorts almost exclusively to biomass astheir primary energy source.

On the other hand, many of these countries are inareas where there is considerable solar radiation, and ifit were possible to exploit it using simple andeconomic technologies, this could be a decisivecontribution to these countries’ increasing energydemand. In the electricity sector alone, it is foreseenthat world consumption will double over the next 30years, mainly as a result of the high increase indemand which will come from developing andemerging countries; at present, this sector representsabout one third of the total world energy requirements,whose growth is predicted to be of about 75% over thesame period. Therefore, to satisfy such a remarkableincrease in demand over the next few decades, it isclear that it will no longer be possible to rely only ontraditional primary energy sources (mainly coal, oiland natural gas). Thus, all available energy sourceswill be used in the most efficient way, giving specialattention to the renewable energies which, due to theirnature, do not have the problem of the progressivedepletion of exploited reservoirs. Fig. 1 shows thepotential theoretical energy contributions with thosethat can technically be exploited, which the mainrenewable sources could supply worldwide.

The above considerations, together with a greaterawareness of the consequences of climate changes ona planetary scale, induced by the emissions of anincreasing number of industrial plants, have created a

renewed interest for solar thermoelectric power plantsin the more industrialized countries as well as in theinternational institutions that must promote andsustain development in underdeveloped countries.Solar radiation must be converted to high temperaturethermal energy in order to be adequately exploited inthese power plants. To make this conversion possible,radiation reaching the ground must first beconcentrated.

The concentrating and conversion of solar energymeans the inclusion of all the technologies, systemsand plants that exploit such energy as a source of hightemperature thermal energy by concentrating solarradiation onto special receivers. This entails only theuse of the direct component and the loss of thediffused component. Therefore, the regions of theEarth that are suitable for the exploitation of solarenergy in thermoelectric power plants are those wheredirect radiation reaching the ground has an average

531VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY

6.1

Solar energy conversion

tech

nica

l usa

ble

pote

ntia

l (10

18 J

)

theoretical potential energy (1018 J)

0

100

200

300

400

500

600600

2,500,000

100 100

100,000 158

30

100

700

solar radiation biomass water power wind power

Fig. 1. Potential of the main renewable energy sources (Solar Millennium AG, 2003).

yearly power equal to at least 200 W/m2, equivalent toannual energy of 1,750 kWh/m2; in the best sites it ispossible to reach an average power of 320 W/m2,equivalent to yearly energy of 2,800 kWh/m2.

As shown in Fig. 2, the areas where it is possible toexploit solar radiation by means of concentratingplants are mainly found in the developing or theemerging countries. In these regions, by using solarconcentrating technologies that are available today,each square kilometre of receiving surface could allowthe supply of energy of approximately 300 GWh/yr onaverage to the electricity grid, which is equivalent tothe yearly production of a traditional 50 MWthermoelectric power plant working about 6,000 h/yr.Thus, it would be possible to make a fuel saving ofalmost 500,000 barrels of oil a year and also decreasethe CO2 emissions by 200,000 t/yr on average.

The exploitation of less than 1% of the energeticpotential, made available by solar concentratingtechnology, would be sufficient to comply with therecommendations of the Intergovernmental Panel onClimate Change (IPCC) for the long-term stabilizationof the planetary climate. At the same time, theexploitation of solar energy would becomeeconomically competitive compared to the exploitationof fossil fuels.

Historical outlineThe history of solar concentration began

thousands of years ago. The properties of concave

reflective surfaces exposed to the Sun’s rays tocause the combustion of a variety of materials wereknown to the most ancient populations in theOrient and in the Mediterranean area, where theybecame known as burning mirrors. It is said that inthe Second century B.C. Archimedes, the famousmathematician from Syracuse, used mirrors to setfire to the Roman fleet from a distance when itscommander, the consul Marcellus, kept the cityunder siege. The first documented uses of mirrorsinclude lighting fires to cooking food, to heatingwater and dwellings.

Later, lens systems were manufactured, forinstance 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 theuse of solar energy applications. However, the problemof the depletion of coal reserves was brought toattention about one hundred years later. On this basis,A. Mouchot introduced the first solar engine at theUniversal Exhibition of Paris of 1878. It consisted of a20 m2 parabolic dish reflector which, by concentratingthe Sun rays on a recipient containing 70 litres ofwater, produced sufficient thermal energy in 30minutes to generate enough steam to operate amachine. In Paris, during the same period, A. Pifre,one of Mouchot’s assistants, introduced a printingpress powered by a parabolic dish collector which, on

532 ENCYCLOPAEDIA OF HYDROCARBONS

POWER GENERATION FROM RENEWABLE RESOURCES

excellentsuitability for solar thermal power plants

good suitable unsuitable

Fig. 2. Map of direct solar irradiation (Solar Millennium AG, 2003).

a typical September day, was capable of printingcopies of Le Journal Soleil.

From then on, the first applications for pumpingstations, desalinization plants and cooking food weredeveloped in the regions most exposed to the sun, suchas North Africa. In Bombay, India, W. Adams,representative of the British Crown, after criticallyconsidering Mouchot’s project, decided it was better toerect an array of smaller mirrors, adequately alignedand set in a semicircle on the boiler, moving them soas to track the apparent path of the Sun, in order toobtain higher temperatures with lower costs and moresimple maintenance. At the end of 1878, he started toerect an installation, later called the tower solar plant,by gradually adding mirrors until a temperature of800°C was reached, thus producing steam which hadenough pressure to operate a medium-power engine.

In 1887, the Swedish-American inventor J. Ericsson experimented with irrigation plants for thesunny Pacific coasts, using a small hot air enginepowered by a linear trough collector; a more simplestructure than the dish collector, with a pipe-shapedboiler situated longitudinally along the reflector, in thefocal line of the parabola. However, in 1901, on a farmin Pasadena, California, A. Eneas, an English engineererected for a demonstration, a solar engine similar toMouchot’s design and capable of pumping about 7 m3

per minute of water, to irrigate the arid Californianground.

In 1910, F. Shuman, an engineer fromPennsylvania, built a solar boiler powered by parabolictrough collectors and capable of operating a large 30 kW engine to irrigate a farm in the desert with 25 m3 per minute of water. Consequently, he erectedfive collectors 60 m long with openings of 4 m on asurface of about 4,000 m2 in Meadi, south of Cairo(Egypt). This was the first solar installation on anindustrial scale. However, the First World War brokeout around that time and the large-scale drilling in thegreat crude oil basins in the Middle East and in theAmerican Continent also began; once again, theabundant availability of fossil fuel hindered the use ofconcentrated solar energy. This technology which, asShuman stated at the beginning of the Twentiethcentury, uses “the most rational energy source” had towait until the 1980s to be revived, when the threat ofdepletion of the oil reserves and the threat of apermanent state of conflict in the regions of crude oilextraction arose. Therefore, industrial experimentation,based on the experiences of Ericsson and Shuman,were oriented towards the parabolic trough collectorsthat represent the best compromise in the ratiocost/produced energy in most of the exploitable sites.

In the middle of the 1980s, the company Luzerected a solar plant in the Californian Mojave Desert

(United States) with parabolic trough collectors toproduce steam used in a thermodynamic cycle, whichsupplied 14 MW of electrical power. Otherinstallations of the same type were erected in that areagiving a total of 354 MW of electrical power, all ofwhich are still in operation. Simultaneously, anothersolar power plant was erected, once again in theMojave Desert, based on central tower technology(Solar One). This pilot plant, with 10 MW of electricalpower, used an area of about 160,000 m2 and wasconnected to the Southern Californian electricity grid.It remained in operation from 1981 to 1988.Subsequently, a second tower installation (Solar Two)was built, operational from 1996 to 1999, which useda mixture of molten salts instead of water as the heattransfer fluid.

In Europe, Italy hosted the first significantdemonstrative European plant in the field of hightemperature solar power plants at Adrano in Sicily. Thetower-type Eurelios plant, whose construction wasbegun in 1979 by an Italian-French-Germanconsortium as part of a European Community researchprogram, had a design power of 1 MW and was inoperation until 1986.

In Spain, the most important European researchcentre for solar concentrating technologies, known asSolar Platform of Almeria (SPA), near the town of thatname, has been in operation since the beginning of the1980s. Numerous experimental plants, mainlyfinanced by the European Community, have beenerected at this centre over the years to study varioustechnological lines, especially central tower systemsand parabolic trough collector systems.

Basic concepts of solar concentration As previously mentioned, the concentration of

solar radiation becomes indispensable when there is arequirement for thermal energy at a temperaturehigher than the temperature that can be reachedemploying a flat surface for its collection andconversion (flat collector). To obtain a highertemperature, a suitable optical system (theconcentrator) is used to collect and send the radiationonto a component (the receiver) where the energy isconverted into high temperature thermal energy.Moreover, collecting only direct radiation requires theconcentrator to be moved during the day to track thepath of the Sun in the sky.

In order to reach high temperatures, solar thermalflow on the receiver must be increased. Therefore, thereceiver must have a surface smaller than that of theconcentrator, corresponding to the flat cross section ofits reflecting surface. The characterizing parameter ofa concentration system is the concentration factor C,which is defined as the ratio between the area AA of


SOLAR ENERGY CONVERSION

the collection surface of the collector, also called theintercepting surface, and the area AR of the surface ofthe receiver:

AAC �12AR

A concept closely related to the concentrationfactor is the acceptance angle (2qc), or the angularinterval in which all, or almost all, the rays areintercepted by the receiver. The maximumconcentration factor, for a two-dimensional systemwith a linear-type receiver (such as the parabolictrough collector) is equal to:

1C2D,theoretical �

123244

sinqc

while for a three-dimensional system with a receiver atthe focal point (such as the parabolic dish collector orthe tower system) is:

1C3D,theoretical �1233244

sin2qc

The minimum acceptance angle that allows all ofthe rays coming from the solar dish to be sent to thereceiver can be calculated on the basis of geometricalconsiderations. The Sun has a diameter of about 1.4�106

kilometres while the average distance between the Sunand the Earth is about 150�106 kilometres. Therefore,solar rays reach the Earth with a divergence of 0.25°.According to the above relations, the maximumconcentration factor for a two-dimensional system isabout 215, while for a three-dimensional system itreaches a value of over 45,000. In practice, however,the concentration factors of real systems are muchlower, due to a series of technological limits.Acceptance angles which are notably larger than thesolar divergence must be used because of errors intracking the Sun and inaccuracies in the shape of theconcentrator and in the positioning of the receiver.Moreover, the choice of the manufacturing solution forthe receiver and concentrator can further reduce thefactor to a half or a quarter of its theoretical value.

Therefore, the actual concentration factor desiredin a solar plant, after establishing its typology, involvesa compromise between optical and thermalperformances. A receiver which is as small as possiblemust then be chosen so as to limit thermal losses,while an increase in its dimensions allows thecollection of all solar rays even if there areimperfections in the concentrator.

Let us consider, for instance, the case of theparabolic trough collector. Fig. 3 shows its crosssection with a plane perpendicular to the focal axis.The reflective surface of the concentrator has theshape of a parabola with the equation: y�x2/4 f and the

radiation is focussed onto a cylindrical receiver withradius r, placed on the focal line at distance f from thevertex of the parabola. If the ray with the maximumdivergence accepted by the system (the dotted line inthe figure) must reach the receiver, the concentrationfactor obtained in this configuration is:

2xA sinaC2D,parab�123�113C2D,theoretical2pr p

where a is the half-angle of sight of the parabola fromits focus and 2xA is the opening of the collector. Fromthe formula it can be seen that in this simple systemthe maximum concentration factor, occurring ata�90°, cannot have a value higher than approximately70, without even considering acceptance angles higherthan solar divergence and further sources of error.Bearing in mind the actual acceptance angles, thetracking errors, the tolerances in the manufacturing ofthe reflective surfaces and other inaccuracies, theconcentration factors in real 2D systems do not exceeda value of 30. Very often, the plane cross section of thecylindrical receiver in 2D systems is considered to bethe surface of the receiver. In this case, theconcentration factor is calculated by using the diameterof the receiver rather than its circumference; thus, thenumerical values are multiplied by a factor of p.

However, there is a class of concentrating systemsthat almost reaches the theoretical limit; these arecalled non-imaging systems as they do not faithfullyreproduce the image of the solar disk because they donot maintain the reciprocal direction of the individualrays. The combination of a conventional system, suchas the parabolic trough system of the previousexample, with a non-imaging system, used in a secondstage, allows the concentration factor to approach thetheoretical limit value.

Inside the receiver, the concentrated solar radiationis converted into thermal energy at a temperaturewhich is proportional to the actual concentration



y

xxA

x2

4f

r

fy�

a

qc

qc

Fig. 3. Concentration factor for parabolic troughs.

factor. The energy balance of a concentration systemcan be considered to formulate the law relating thetemperature to this factor. According to theStefan-Boltzmann law, the radiant power from the Sun is proportional to the fourth power of itsthermodynamic temperature. Only a fraction of thispower, proportional to the square of the sine of thesolar divergence angle (qS), reaches the ground onEarth. Therefore, the incident radiant power (fS) onthe collecting area (AA) is proportional to:

fS�AAsin2qST4S

where TS is the apparent temperature of the Sun, equalto about 6,000 K.

The power loss of the receiver (fR), whenconsidering only the radiative-type losses in a firstapproximation, is proportional to:

fR�ART4R

having indicated the thermodynamic temperature andthe area of the receiver by TR and AR respectively. Inthe hypothesis that the available power (fU) is afraction h of the incident power, the thermal balance ofthe receiver can be written as:

fS�fU�fR�hfS�fR

From the previous equations, remembering thatAA�AR�C, the operational temperature of the receiverproves to be proportional to:

TR�TS[(1�h)C]124

4

The graph in Fig. 4 shows the maximumoperational temperature of the receiver obtained fromthe above equation, with the usual values for theparameters that appear in the constant of

proportionality as well as in the constant of efficiencyof each concentration system.

Solar technologies The objective of solar concentrating plants is to use

solar energy instead of traditional fossil fuels toproduce high temperature thermal energy. The thermalenergy thus obtained can be used in a variety ofindustrial processes (such as, for instance, thedesalination of sea water and the production ofhydrogen from thermochemical processes) or in theproduction of electricity, thereby contributing tolimiting the world consumption of fossil fuels andemissions into the atmosphere.

At present, the main objective of concentratingsolar plants is the generation of electricity. In this case,solar thermal energy is used in conventionalthermodynamic cycles, such as those with steamturbines, gas turbines or Stirling engines. Fig. 5schematically shows the differences betweentraditional and solar thermoelectric plants.

When the solar source is used for the production ofthermal energy, the concentration system does notcreate risks or annoyance to the nearby population. Inregions with high solar radiation (average yearly powerover 300 W/m2) it is possible to generate energyequivalent to that of the combustion of a barrel of oilfrom a square meter of collection surface, therebyavoiding the emission of approximately 500 kg of CO2

into the atmosphere.Thermal solar energy can be stored during the day

to avoid the effects of the variations of the solarsource, thereby making the system more flexible andmeeting the needs of productive processes.Alternatively, it is possible integrate it with fossil fuels



rece

iver

tem

pera

ture

(K

)

0

6,000

5,000

4,000

3,000

2,000

1,000

concentration factor

flat-plate collectorTmax�395 K

parabolic troughTmax�900 K

theoretical2D system

Tmax�1,500 Kreal 3D systemTmax�2,600 K

theoretical 3D systemTmax�5,600 K

1 10 100 1,000 10,000 100,000

Fig. 4. Relation between operating temperature and concentration factor.

or renewable fuels, such as oil, natural gas andbiomasses.

Solar plants can use various technologies for theconcentration of solar radiation although it ispossible to identify the following phases of theprocess for each case: a) collection andconcentration of solar radiation; b) conversion ofsolar radiation into thermal energy; c) transfer andpossible storage of the thermal energy; d ) utilizationof the thermal energy.

Some of the main issues of solar plants are thecollection and concentration of solar radiation, which,by its nature, has a low power density. Collection andconcentration are performed, as stated above, bymeans of a concentrator, formed by panels of asuitable geometric shape with reflective surfaces,normally common glass mirrors. All of theconcentrators in a solar plant, arranged in order on theground and suitably spaced so as not to interfere witheach other in the radiation collection, make up thesolar field. The receiver can have various shapes. Itmay be the only one for the entire solar field or theremay be one connected to each concentrator. It convertssolar energy into thermal energy, which is thentransferred to a fluid which circulates through thereceiver itself. The thermal energy transferred by theheat transfer fluid can be stored in various ways before

its utilization in the productive process: by using thehigh heat of the fluid itself placed in insulating tanks,or by transferring its heat to inert materials with highthermal capacity, or to phase change systems. In thisway, solar energy, highly variable by nature, canbecome a thermal energy source which is continuouslyavailable for use.

An important parameter characterizing solarconcentrating plants is the solar multiple, defined asthe ratio between the peak thermal power of thereceiver and the nominal thermal power used by theproductive process. Without thermal storage, thisparameter is equal to 1, and all of the collectedthermal power is used immediately. Higher valuesmean that the plant can store the excess thermalenergy. The use of solar multiples higher than 2.5permits the continuous operation of the productiveprocess throughout the day. However, this advantagemeans an increase in the cost of construction of theplant proportional to the capacity of the thermalstorage system. Therefore, the optimum size of thissystem is chosen by means of an economic analysis;for instance, according to current values, the optimumcapacity for storage systems in a thermoelectric plantis one that guarantees continuous production for 6 to10 hours depending on the nominal electric power, inthe absence of solar irradiation.



steam turbine generator

CO2, NOx, SO2....

pump

steam

steam

water

water

electric substation

fossil fueltank

electric power generation

fossil fuel heat production

solar heat production

condenser andcooling system

steam turbine generator

pump

pump

solarfield

pump

electric substation

electric power generation

condenser andcooling system

thermalstorage

hot

cold

steamgenerator

steamgenerator

Fig. 5. Comparison between a traditionalthermoelectric plant and a solar source plant.

As mentioned above, concentration systems exploitonly direct radiation because they are unable toconcentrate diffused solar radiation; they can be linearor focussed on single point systems. The linearconcentration systems are more simple but have alower concentration factor and therefore reach loweroperating temperatures than the systems focussed on asingle point. There are three main types of plants,depending on the geometry and the positioning of theconcentrator with respect to the receiver: the parabolicdish collector, the central tower system and theparabolic trough collector.

Parabolic dish collector This system uses reflective parabolic-shaped

panels which track the Sun, rotating around twoorthogonal axes and concentrating the solar radiationon a receiver mounted on the focal point (Fig. 6). Thehigh temperature thermal energy is normallytransferred to a fluid and utilized in an enginepositioned above the receiver, where mechanicalenergy or electricity is produced directly.

The ideal shape of the concentrator is a paraboloidof revolution. Some concentrators approximate such ageometric shape by using an array of sphericallyprofiled mirrors mounted on a support structure. Theoptical design of this component and the accuracy ofits manufacture determine the solar radiation

interception and concentration factors. Theinterception factor is defined as the fraction ofreflected solar radiation which goes through the inletopening of the receiver and is generally higher than 95%, while the concentration factor has already beendefined above.

The receiver, which is the most technologicallyadvanced item, absorbs the energy of the radiationreflected by the concentrator and transfers it to theworking fluid. The absorbing surface is generallypositioned behind the focus of the concentrator to limitthe intensity of the incident thermal solar flux tovalues in the range of 75 W/cm2.

Industrial applications of this system obtainconcentration factor values higher than 2,000. Withsuch values, it is possible to achieve very highoperating temperatures and elevated efficiency of theconversion from solar energy into electricity at evenhigher than 30%, the highest in all currently existingsolar technologies. For example, a 10 m diameterconcentrator is capable of supplying approximately 25 kWe under a direct solar flux of 1,000 W/m2. Foreconomic reasons, the dimension of the concentratordoes not exceed 15 m in diameter, thereby limiting itspower to about 25-30 kWe. However, this is a modulartype of technology which allows the construction oflow-power power plants for isolated users.

The engine used in the above systems convertssolar energy into work, as in conventional internal orexternal combustion engines. The working fluid iscompressed, heated and expanded through a turbine ora piston to produce mechanical energy, which can beutilized directly by the consumer or be converted intoelectricity by means of an alternator. Differentthermodynamic cycles and various working fluidshave been studied; today, current industrialapplications use Stirling and Bryton cycle engines.

Either hydrogen or helium is used as the workingfluid in the Stirling engines. In turn, the working fluidis cooled, compressed to pressures up to 20 MPa,heated to temperatures even higher than 700°C andthen expanded. In order to transfer solar energy to theworking fluid at a constant temperature, anintermediate fluid is used for the thermal exchangeduring a change of phase. Normally, a liquid metal(sodium) is used, which evaporates at the surface ofthe receiver absorber and condenses on the tube nestof the engine. The sodium vapours, once condensed,reach the zone of the absorber under the effect ofgravity and spread over its entire surface by capillarity.

On the other hand, the Bryton engine uses air asthe 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 higherthan 850°C. Due to the high temperatures reached



receiver/engine

concentrator

Fig. 6. Typical parabolic dish layout.

by the working fluid, its efficiency in convertingsolar energy into electricity is higher than that of theStirling engine and can exceed 30%. The residualthermal energy of the fluid at the outlet of theturbine is used to preheat the air coming from thecompressor. In this application the receiver is of thevolumetric absorption type, similar to those used intower plants. The concentrated solar radiation flowsthrough a quartz window and is then absorbed by aporous matrix system (honeycombs and reticularcells of ceramic material). Such a receiver offerslarge thermal exchange surfaces with efficiencies ofconversion from solar energy into thermal energyhigher than 80%. The thermal energy can besupplied to the fluid by means of a methanecombustion chamber so that the engine can operatein 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 thesun by rotation around two axes and concentrate solarlight towards a single receiver. The receiver is mountedon top of a tower and a fluid to remove the solarenergy is circulated within it. The thermal energy thusmade available can be exploited in various processes,especially in the production of electricity.

The operating principle is similar to that of theparabolic dish system, only that the concentratorconsists of a high number of heliostats forming a

collecting surface of hundreds of thousands of m2.Solar rays hitting each heliostat are reflected onto asingle point, fixed in time, which acts as a focal point.The height of the focal point from the groundincreases in proportion to the size of the solar fieldand can even exceed a hundred metres. The heliostatsare positioned so as to completely circle the tower, orare positioned in a hemicycle pointing north. They arespaced apart at a distance sufficient to avoid shadingeffects and this distance increases the further awaythey are from the tower.

Various types of heliostats have been studied toimprove optical efficiency and the control of the Suntracking systems as well as to optimize the supportstructure by making it more simple and lighter. Thishas been done to increase plant efficiency and reducecosts. The collecting surface of each heliostat variesfrom about 40 to 170 m2; normally, glass mirrors areused as reflective material, although alternativematerials such as reflective membranes or metallicsheets have also been tested.

The concentration factor of these plants is higherthan 700. The high concentration factor allows the heattransfer fluid to reach high operating temperatures(higher than 500°C) thus allowing high efficiencyconversion from thermal energy into electricity.Normally, conversion occurs by using the thermalenergy in a traditional water-steam thermodynamiccycle. The characteristics of the produced steam(temperature and pressure) also allow the integrationof tower systems in fossil fuel thermoelectric powerplants. Moreover, these concentrating plants cansupply a thermal storage system which can respond tothe users’ energy requirements in a more satisfactorymanner.

The tower system has proven its technologicalfeasibility for electricity production through therealization and the operation of numerous low-power(between 0.5 and 10 MW) experimental plants invarious countries around the world (Spain, Italy, Japan,France, United States); although its application on alarge scale still requires further testing.

The most recent application of this technology isthe American Solar Two plant, which was operationaluntil April 1999. The plant, having a power of 10 MW,had a solar field consisting of 1,026 heliostats for atotal collecting surface of about 81,500 m2 and a tower85 m high. The heat transfer fluid used was a mixtureof molten salts (sodium and potassium nitrate) at amaximum operating temperature of 565°C. There wasa 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 maximumof three hours of operation at full power in the absence of solar radiation.



receiver

heliostats

Fig. 7. Typical outline of a central tower power plant.

Various fluids have been tested for the thermalexchange inside the receiver and for the storage ofthermal energy: water, air, sodium and molten salts. Tothe present date, the most suitable fluid for thistechnology has proven to be a mixture of molten saltsconsisting of sodium and potassium nitrates (which arethe basis of common fertilizers used in agriculture).The choice of molten salts is mainly due to their goodthermal exchange coefficient, high thermal capacity,low vapour pressure, good chemical stability and lowcost. The salts allow high operating temperatures (upto 600°C); moreover, they can be used directly forstoring thermal energy in compact tanks atatmospheric pressure, without using an additional heatexchanger.

The typical functional diagram of a tower plantusing molten salts as the heat transfer fluid and forthermal storage is shown in Fig. 8. The salts, takenfrom the low temperature (290°C) tank, are sent to thetop of the tower and circulated through the receiver,which consists of a set of steel pipe coils assembled onflat absorbing panels. The salts are heated to about565°C and then are sent to progressively fill up thehigh temperature storage tank. Their flow is controlled,according to solar radiation intensity, so as to maintainthe receiver outlet temperature constant. When theproduction of electricity is required, the salts from thehot tank are sent to a heat exchanger (steamgenerator), where steam at high pressure and a hightemperature is produced (12 MPa, 540°C). The steamis then utilized in a conventional thermoelectric cycle;it is expanded in a turbine-alternator group to produceelectricity and then condensed, preheated and sent tothe steam generator again.

The sizing of a solar plant (number of heliostats,thermal power of the receiver and capacity of thethermal storage) depends on the electrical power ofthe power plant and on its yearly utilization factor, or

load factor. This is the ratio between the energyproduced and the energy it is possible to produce in ayear if the plant always works at its nominal electricalpower. Without a thermal storage system, the powerplant can only operate when there is solar radiationand can have a maximum load factor of about 25%.To obtain higher values, it is necessary to havethermal storage. In this case, the plant can operatecontinuously throughout the day, except for the initialphase when the system is loading. For instance, toobtain a load factor of 70%, a thermal capacityequivalent to 15 hours of operation is required innominal conditions and in the absence of solarradiation. This corresponds to a solar multiplier of 3, which means a solar field three times larger thanone without a storage system. Obviously, aspreviously stated, it is necessary to erect higher towerswhen the dimensions of the solar field increase.

Due to the high concentration factor, thistechnology can reach even higher operatingtemperatures when a gas (generally air) is used as theheat transfer fluid to transfer the solar energy. In thiscase, a pressurized cavity volumetric receiver is used,capable of heating up to a limit temperature of1,200°C. The receiver consists of a succession ofnumerous modules, each of which increases thetemperature of the gas flowing through by about150°C. At present, each module can supply a thermalpower of about 500 kWe. The operational diagram of amodule of the receiver is shown in Fig. 9.

Solar radiation, concentrated by the heliostats,reaches each module of the receiver where, by meansof a secondary concentrator, it is further concentrateduntil it reaches an overall concentration factor ofapproximately 2,000. Then, it goes through ahemispherical quartz window and reaches the absorberpositioned inside a pressurized container. The absorberis a metallic or ceramic porous structure and reaches



290°C

565°C

moltensalt

thermalstorage tanks

steamgenerator

condensercooling towersteam turbine and

electric generator

hot

cold

Fig. 8. Operation diagram of a central tower powerplant with thermal storage.

working temperatures from 800 to 1,200°C whenradiation is present.

The gas, pressurized at about 1.5 MPa, flowsthrough the multi-modular absorber and progressivelyheats up to 800°C, when metallic absorbers are used,or up to 1,200°C when ceramic-type absorbers areused. In a solar thermoelectric plant, the hot gas can beused 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 plantusing air as the heat transfer fluid, is shown in Fig. 10.The air at the outlet of the compressor is sent to thereceiver, where it is heated and then expanded in thegas turbine. Its temperature at the turbine inlet can becontrolled, in case of reduced solar radiation, byburning methane in the supplementary combustionchamber. The gases, still hot at the outlet of theturbine, are sent to a recovery boiler for the productionof steam and used in the relative cycle. When a gas isused as a heat transfer fluid, the storage of the thermalenergy can be obtained with high thermal capacityceramic materials placed inside special containers.

A further evolution of this concentration systemconsists in positioning the volumetric receiver at thefoot of the tower (Fig. 11). In this case, it is necessary

to use a hyperboloid-shaped reflector, installed on thetower, to send direct solar radiation to the receiver.This solution offers, especially for large solar fields,better optical efficiency (optical aberrations arereduced and the concentration factor is increased), amore stable distribution of the thermal flow and asimplification of the plant (all the equipment is atground level).

Parabolic trough collector Among the thermal solar technologies for the

production of electricity on a large scale, the parabolictrough collector (Fig. 12) is the one that has reachedthe highest commercial maturity as is clearlydemonstrated by the operation of the Solar ElectricGenerating Systems (SEGS) plants. As previouslymentioned, nine plants of this type, giving a totalpower of 354 MW, have been in operation in theMojave Desert in California since the mid 1980s.

This technology uses a parabolic-profile linearconcentrator, whose reflective surface tracks the Sunby rotating around a single axis; the radiation isfocussed onto a receiver pipe placed along the focus ofthe parabola. Solar energy absorbed by the receiverpipe is transferred to a working fluid, which circulates



insulationpressurevessel

concentratedsolar

radiation

quartz window absorber

secondary concentrator

airinlet

airoutlet

Fig. 9. Operation diagram of a receivermodule.

heliostats air inlet stack condensercooling tower

pressurizedvolumetric

receiver

gasturbine

steamgenerator

burner(optional)

steamturbine

com

pres

ssor

Fig. 10. Operationdiagram of a tower plantcoupled with a combined cycle.

inside the pipe. The collected thermal energy isnormally used to produce electricity by means oftraditional water-steam thermodynamic cycles. Themaximum operating temperature in the collectoressentially depends on the heat transfer fluid beingused; in the plants currently in operation, thetemperature reaches 390°C.

The concentrator has a steel support structure,with a central beam and a series of supports toanchor the reflective panels, thereby guaranteeing itscorrect functioning in windy conditions and otherweather phenomena. The reflective panel normallyconsists of a common glass mirror of a suitablethickness. Alternatively, a panel of compositematerial (honeycomb) can be used that has a thinglass mirror or a reflective film glued onto itsexternal surface.

The parabolic collector has an opening of about6 m and a focal distance of slightly less than 2 m.The concentration factor, referred to the diameter ofthe receiver, is about 80. Initially, this was 50 m inlength, which was successively increased to 100 mand structures of 150 m are now being tested. At the centre of the collector there is the mechanismwhich controls the rotation for tracking the path ofthe Sun.

The heat transfer fluid, travelling inside thereceiver pipe, progressively heats up. Therefore, inorder for the fluid to reach the required operatingtemperature at the outlet, a number of collectors mustbe connected in series. Normally, these are placed intwo parallel rows with a total length of about 600 m,making up a string which creates the unit module ofthe plant. By adding more modules in parallel, the



heliostats

towerreflector

receiver

Fig. 11. Layout of a tower plant with receiver on the ground.

concentrator

receiver

Fig. 12. Typical layout of a parabolic troughconcentration system.

thermal power produced can be increased at will. Therows of collectors must be spaced apart to avoid anyreciprocal shading effect. Usually, the space betweencontiguous rows is 2 to 3 times the opening of thecollector. Their layout on the ground dependsessentially on the conformation of the site. The classicarrangement is a north-south orientation for the axis ofthe collectors, therefore tracking the Sun in aneast-west direction. This permits better collection ofsolar energy, especially during summer months. Theelectricity production plant is at the centre of the solarfield.

The effectiveness of this technology depends onthe optical capability of the concentrator (accuracyof the structure and characteristics of the reflectivepanels) but, above all, on the conversion efficiencyof the receiver pipe that must absorb a maximum ofconcentrated solar energy and have the minimumthermal dispersion. The receiver, which ismaintained in its position along the focal line of theconcentrators, rotates rigidly with the concentratorswhile tracking the Sun. It is formed by elements ofabout 4 m in length connected in series. Eachelement consists of two concentric cylinders: anexternal glass tube, about 12 cm in diameter, and aninternal steel tube of about 7 cm in diameter,connected to each other by folded metallic jointingto compensate for the different thermal dilatations ofthe two materials. A suitable selective coating isdeposited on the external surface of the steelcylinder. This coating must be capable ofmaximizing solar radiation absorption in the visiblespectrum and of minimizing the radiation emissionsin the infrared that are generated by the hightemperature reached in the tube during operation. Avacuum is created in the gap between tube and glassto reduce thermal dispersion by convection.

The operation diagram of a SEGS plant is shownin Fig. 13.

The heat transfer fluid pumped through thestrings of collectors is heated by solar radiation andreaches the maximum operating temperature. Thethermal energy acquired in this way is then used in aRankine (steam) cycle to produce electricity. Theplant can also have an auxiliary supplementaryboiler which uses fossil fuels and is able to supplysteam, even when solar radiation is absent.Consequently, electricity production is made torespond to the demand of the users. An alternativesolution to the supplementary boiler is a system thatpermits the storage of solar thermal energy, makingit available when necessary, by converting thenaturally highly variable solar source into a sourceof continuous energy, which can then be modulatedthroughout the whole day.

The plants now in operation use a synthetic oil(Therminol VP-1) as the heat transfer fluid for theextraction of solar heat. Unfortunately, this has a highcost and, due to the risks of environmental impact incase of leaks, it is not suitable for use in a storagesystem. Consequently, there is always a methanesupplementary boiler in each plant capable ofsupplying up to 25% of the thermal energy used by the power plant.

In various research centres alternative fluids suchas water, with the direct production of steam, andmolten salts are being tested to solve the problemsconnected with the heat transfer fluid and to improvethe competitiveness of this technology. Molten saltspermit a considerable increase in the maximumoperating temperature (from 390 to 550°C) and canbe used directly for thermal storage, as has alreadybeen tested in the tower plants. This is why moltensalts have been chosen as the heat transfer fluid in



solar collectors

290°C390°C

auxiliary naturalgas boiler (optional)

heat transfer fluid (oil)

condensercoolingwater

superheated steam

steam generator

waterflow

steamturbine

electricgenerator

coolingtower

Fig. 13. Operation diagram of a SEGS plant.

the Italian project for a concentrating plantdeveloped by the Ente per le Nuove tecnologie,l’Energia e l’Ambiente (ENEA). A diagram of theplant is shown in Fig. 14.

The molten salts, consisting of a mixture of sodiumand potassium nitrates, are extracted from the lowertemperature tank (290°C) and circulated in thereceiver pipes of solar collector strings, heating up toabout 550°C. They are then sent to the hightemperature tank for thermal storage. The molten saltscoming from the hot tank are then sent to a heatexchanger to produce steam used by the electric powerplant, and then reintroduced into the cooler tank. Theoperating temperature of the plant is controlled bysuitably modulating the flow of salts in the strings ofcollectors according to the intensity of solar radiation.Due to the fact that the mixture of salts starts tosolidify at a temperature of about 240°C, the minimumoperating temperature must be maintained above thisvalue, with an adequate margin, to avoid obstructionsin the circuits.

The high temperatures reached by the heat transferfluid, a quality peculiar to the ENEA project, permiteasy integration of this solar power plant into fossilfuel thermoelectric plants, including the most moderncombined cycle plants, thus obtaining higher finalefficiencies of conversion.

The Fresnel linear collector (Fig. 15), still in atest phase, is an evolution of the parabolic troughcollector. In this collector, the concentrator issubstituted by segments of parabolic mirrors, placedaccording to the principle of the Fresnel lens. In thiscase, the receiver pipe is placed at the focal pointand 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 useflexible pipes for connections between singlecollectors, or between the collectors and thedistribution network piping, in order to circulate theheat transfer fluid. Moreover, as there is no shadingeffect between adjacent concentrators, the collectorrows do not have to be widely spaced, thus obtaining



290°C

550°C

thermalstorage tanks

steamgenerator

condensercooling towersteam turbine and

electric generator molten salt

hot

cold

Fig. 14. Operation diagramof the ENEA plant.

secondary reflector

solar radiation

receiver

receiver

primaryFresnelreflector

secondaryreflector

reflected solarradiation

water/steam

glass window

Fig. 15. The Fresnel system.

better exploitation of the radiation reaching theground. Normally, this type of plant uses water asthe heat transfer fluid, thereby producing steamdirectly inside the receiver pipe.

Table 1 shows the main technical parameters of thetechnological lines described above. The data havebeen collected from the operation of existing plants(parabolic trough collector and parabolic dish) or from projections on the basis of the performancesof small-size demonstrative plants.

Production of hydrogen from solar sourceOther than in the production of electricity, the high

temperature thermal energy obtained in concentratingsolar plants can also be used in various industrialprocesses, and in particular in the production ofhydrogen by thermo-chemical processes.

Hydrogen is now produced on an industrial scaleusing fossil fuels. Electrolysis is the most maturemethod in the production of hydrogen from water. Itis characterized by a global thermal efficiency ofabout 36%, taking into account the conversionefficiency 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 thethermal energy conversion occurs in a direct wayeither by using renewable or non-renewable sources.At present, among such methods, the thermalsplitting of water is not practical due to the hightemperature needed (2,500-5,000°C) and because ofthe technical difficulties encountered with theseparation of oxygen from hydrogen, once theseelements have been formed.

The thermo-chemical cycles, consisting of a seriesof red-ox reactions that involve intermediatesubstances of a different nature, represent a validalternative to the direct splitting of water; they permitthe energetic barrier and the temperature at which thethermal energy must be supplied (800-1,500°C) to belowered considerably and also make it possible tocarry out the separation of the hydrogen and theoxygen in different phases of the cycle. This type ofprocess has been known since the 1970s, althoughonly in the last few years has interest been renewed asa result of the stimulus of more and more pressingenvironmental issues.

The possibility of thermally feeding these cycleswith solar energy makes such production processesfully renewable and therefore perfectly compatiblewith a strategy of sustainable development. Asimplified diagram of the production of hydrogenusing the Sun as the thermal energy source is shown inFig. 16. The parabolic dish and the central tower are themost suitable concentration systems because of thehigh temperatures required by the thermo-chemicalprocess. The thermal energy absorbed in the receiver isused to feed a chemical reactor where reactions forsplitting the water occur.

The iodine-sulphur cycle is one of the mostpromising options among the various thermo-chemicalprocesses proposed in the 1970s by General Atomicsand is presently being studied by various researchcentres. The cycle is mainly composed of threereactions, two of them exothermic and oneendothermic, whose overall balance is the dissociationof water into hydrogen and oxygen, as shown in thediagram of Fig. 17.



Table 1. Main parameters of concentration solar plants

Power (MWe)

Concentrationfactor

Peak solarefficiency2

(%)

Averageyearly solarefficiency2

(%)

Thermodynamiccycle efficiency

(%)

Loadfactor3

(%)

Usedsurface,

m2/(MWh/yr)

Parabolictrough

10-200 70-804 2110-1517-181 30-40 ST

2425-701 6-8

Fresnel 10-200 25-1004 201 9-111 30-40 ST 25-701 4-6

Solar tower 10-150 300-1,00020351

8-1015-251

30-40 ST45-55 CC 25-701 8-12

Parabolic dish 0.01-0.04 1,000-3,000 2916-1818-231

30-40 SE20-30 GT 251 8-12

1 Estimated data2 Solar efficiency � net electricity production/direct normal solar radiation 3 Load factor � hours of operation of the solar plant/8,760 h/yr4 Concentration factor related to the receiver diameter ST, Steam Turbine; CC, Combined Cycle; SE, Stirling Engine; GT, Gas Turbine

Market perspectives Having examined potential solar source

contributions, to help solve the future energetic andenvironmental problems, as well as considered thetechnologies that have been developed or are beingdeveloped to exploit this source, the drawbacks that,up to now, have hindered the launching of thisrenewable source commercially must be highlightedand its prospect of penetration into the world energeticmarket analysed. The main drawback is linked to thehigh cost per unit of solar thermoelectric plants, sinceit is from 2.5 to 4 times higher than that of fossil fuelplants. The cost of a kilowatt-hour produced byconcentrating power plants, in spite of the low impactof the fuel cost, has been, up to now, at least doublethat of a traditional fossil fuel power plant due to thehigher impact of the operating and maintenance costsas well as to the lower load factor.

Another drawback is connected to the technicalrisk associated with this technology. Although it cannow be considered proven and industrially mature, thistechnology is still perceived as new and not veryreliable in performance. Solar source variability alsoworks 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 theenvironment by the various types of power plantscould prove decisive for the diffusion of concentratingsolar plants, as their emissions are negligible.

In the next twenty years, the potential worldproduction of energy by solar thermoelectric plants isestimated as equivalent to an installed electric powerof 600 GW. According to forecasts, many of theseplants will be erected in developing countries. Sincethese plants, at the moment, have an erection cost perunit much higher than traditional thermoelectricplants, in the short term, their market niche will belimited to regions where the unit cost of fossil fuelsis very high. In the medium term, a growingpenetration of this technology is expected, at anannual rate, in proportion to the progressivereduction of the cost of the kilowatt-hour, supportand incentive policies and the future trend of theinternational prices of fossil fuels.

In order for solar thermoelectric plants to becomereally competitive in the market, they will have to becapable of supplying energy when requested by the



high temperatureheat

concentratingsolar radiation

H2O

H2

1/2O2

chemicalreactor

Fig. 16. Simplified diagram of hydrogen production fromsolar source.

H2SO4

850°C ∆H�371 kJ/mol

I2�SO2�2H2O

SO2

I2

H2O

H2

O2

H2O�SO2�0.5O2

2HI H2�I2

200-700°C ∆H�173 kJ/mol

100°C ∆H��165 kJ/mol

2HI�H2SO4

H2SO4

HI

Fig. 17. Diagram of theiodine-sulphur cycle.

users. Consequently, they will have to be independentfrom the variability of solar irradiation as much aspossible. Only in this way will these plants be able tosatisfy the network load requirements in a reliablemanner, without obliging the operator to keeptraditional power plants in reserve in case of anunforeseeable, sudden reduction or absence of powergenerated by a solar source. As has been previouslymentioned, this is only possible if the concentratingsolar plants are equipped with a suitable energystorage system which provides a power supply whichresponds to variations in demand and compensates forthe fluctuation of direct solar radiation during the dayand its absence at night. The introduction of a storagesystem will also permit a substantial improvement inthe load factor of the power plant, as it determines anincrease in its annual number of hours of operation.

The possibility of integrating the concentratingsolar plants into traditional thermoelectric powerplants already in operation, to increase their totalpower, is a characteristic which might encourage thediffusion of these plants. This will result in a costreduction of the investment per unit of thethermodynamic solar plants and permit them to greatlymodulate their power, even throughout the day,eliminating the drastic reduction of efficiency of theelectricity generation steam cycle, typical of anexclusively solar power plant.

Another aspect which could help the expansion ofthe market is connected to the possibility of erectingthe concentrating solar power plants in areas with highirradiation and then transferring the energy producedin excess of the local requirement to countries withconsiderable or increasing requirements for electricity.As far as this is concerned, it is important to rememberthat transferring electricity for long distances, eventhousands of kilometres, is now already technicallyand economically feasible with high voltage directcurrent underwater lines and cables (High VoltageDirect Current transmission technology, HVDC).

From this point of view, the Mediterranean areacould have a leading role in the exchange betweenEuropean countries – great consumers of electricenergy with low energy resources – and the countriesof North Africa and the Middle East that have manyareas available with high direct irradiation and primaryenergetic sources. At present, there is already analmost complete ring interconnecting the alternatecurrent electricity grids of the Mediterranean countriesand the direct current underwater connection betweenItaly and Greece. Moreover, direct current underwater

connections with a total transport capacity ofthousands of megawatts are being designed to increasethe interconnection between Europe and the NorthAfrican countries. The rapid completion of theseinterconnections could stimulate European enterprisesto invest in the erection and operation of power plantsin North Africa, possibly in partnership with localenterprises. This would also certainly facilitate theerection of solar thermoelectric power plants that,initially, could be integrated with fossil fuels plants.Later, due to the decrease in costs induced by thevolume of the growing market, the erection ofexclusively solar power plants in desert areas wouldalso be facilitated.

Bibliography

Butti K., Perlin J. (1980) A golden thread: 2500 years ofsolar architecture and technology, Palo Alto (CA), CheshireBooks.

Dickinson W.C., Cheremisinoff P.N. (1980) Solar energytechnology 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 impiantosolare termodinamico integrativo presso la centrale ENELdi Priolo Gargallo (SR), ENEA/SOL/RS/2004-15.

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 Departmentof Energy) (1997) Renewable energy technologycharacterizations, Topical Report TR-109496, December.

Kubo S. et al. (2004) A demonstration study on closed-cyclehydrogen production by the thermochemical water-splittingiodine-sulfur process, «Nuclear Engineering and Design»,233, 347-354.

Müller-Steinhagen Freng H., Trieb F. (2004) Concentratingsolar power: a review of the technology, «Ingenia», 18.

Smith C. (1995) Revisiting of solar power’s past, «TechnologyReview», 98, 38-47.

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 SolarMillenium share.

Mauro VignoliniEnte per le Nuove tecnologie, l’Energia e l’Ambiente

Centro Ricerche CasacciaSanta Maria in Galeria, Roma, Italy



6.1.2 Photovoltaic technology

Introduction

The photovoltaic effect is produced by anelectromotive force in an electrically heterogeneousmedium exposed to electromagnetic radiation. Thename derives from the fact that the phenomenon wasdiscovered by Edmond Becquerel in 1839 in anelectrolytic or voltaic cell. The phenomenon istypical of semiconductor-to-metal orsemiconductor-to-semiconductor junctions; if thejunction is illuminated, then electron-hole pairs arecreated within, at the expense of the energy of theincident photons: the potential barrier, located at thejunction, drives the holes towards the lower potentialarea and the electrons in the opposite direction, and socreates an electromotive force (in the order of a fewtenths of a volt); if the junction is part of a closedcircuit, then an electric current is generated. Thiseffect can be applied to the direct conversion (knownas photovoltaic energy, solar electricity or, moresimply, photovoltaic) of solar energy into electricalenergy through suitable devices called solar cells.Individual solar cells are connected to each otherelectrically to form modules, which are sealed to resistthe external environment for several years. Themodules can be used individually or connectedtogether electrically in so-called photovoltaic fields.There are various kinds of photovoltaic systems:

accumulating through groups of batteries, directlyconnected to the electricity grid, or for othersmall-scale use.

Solar electricity has many advantages: it has a lowenvironmental impact, it is renewable, it is modularand can be used directly where it is produced. On theother hand, it is a costly, intermittent and low-densityenergy source. Moreover, the yield or efficiency of theconversion of solar radiation into electricity isrelatively modest, around 15% for industrial solarcells, which makes it necessary to cover large surfaceareas. The photovoltaic market has been expandingrapidly since the end of the Nineties, thanks, above all,to government incentives aimed at encouraging the useof environmentally friendly renewable sources (Fig. 1).However, for this to become a significant energysource worldwide, marked technological progress anda sharp reduction in costs are necessary. The researchand development activities currently underwayinternationally are aimed at these goals.

History of the technology and its applications

The photovoltaic effect was discovered and studiedas part of experiments in various disciplines. Asmentioned, E. Becquerel observed that weak potentialswere created by illuminating one of the electrodes inan electrolytic cell. The first functioning solar cellswere built by W.G. Adams and R.E. Day, using a solid


*

annu

al p

hoto

volt

aic

prod

ucti

on (

MW

)

1

100

1,000

10,000

1978 1982 1986 1990 1994 1998 2002 2006 2010

White Paper target

United StatesJapanEuroperest of the worldtotal

*

10

year

Fig. 1. Forecast for regional and totalmarket growth in 2010.

(selenium) towards the end of 1870. However, theexplanation of the phenomenon only occurredfollowing the quantum interpretation of thephotoelectric effect provided by A. Einstein in 1905. Itwas necessary to wait until 1954 for the firstphotovoltaic devices with a significant conversionyield, when researchers at Bell Laboratories (USA)built the first silicon solar cell with a yield of 6%. Thefirst attempt to market the Bell cells, which wereproduced on a small industrial scale, was greatlyhampered by prohibitive costs. The main applicationsup to the Seventies were electricity supply systems forartificial satellites, given the absence of validalternatives. The American and Soviet space race, andthe need to improve the electricity supply systems ofsatellites led the American government to financephotovoltaic research programmes, enabling thecreation of specialized industrial initiatives at the sametime. Thus, the costs of solar cells fell markedly, albeitremaining prohibitive for applications beyond space ormilitary programmes.

At the start of the Seventies, the use ofphotovoltaic energy for terrestrial applications wasenabled by the development of technologies with lessrigorous specifications than those needed for the cellsused in space applications. Thus, it was possible toreduce costs to around 10-20 dollars/W. However, evenat that level, the cost of energy produced withphotovoltaic modules was approximately 40 times thecost of conventional electricity; the first production forterrestrial use was, therefore, largely aimed atapplications in areas which were either remote or hardto access with the electricity grid. Thus, a market wascreated dedicated to the electrification of oilplatforms, to the supply of energy to anticorrosionsystems for oil wells and pipelines, to the supply ofenergy to marine communication or signallingsystems, as well as the electrification of ruralsettlements in developing countries. This initialterrestrial market favoured the creation of the firstindustrial initiatives in various parts of the world withfairly basic production and very small sizemanufacturers.

The functioning of photovoltaic devices

The functioning of solar cells is connected to thecomplex interaction between light and matter, andinvolves the nature and characteristics of light, thephysical properties of materials and the production ofelectronic devices. Here below is a brief description ofthe properties of semiconductors, aimed at providingan understanding of the main operating mechanisms ofsolar cells. An attempt has been made to reduce andsimplify the explanation as far as possible, although

the phenomena concerned require a quantum handlingof the structure and properties of matter, and itsinteraction with electromagnetic radiation.

Solar radiationThe Sun emits light over a broad wavelength

interval, of which the human eye only sees thevisible fraction. In 1900, M. Planck resolved thediscrepancies between the experimental observationsof the spectrum of electromagnetic radiation inthermal equilibrium and the classical theory of thephenomenon, by introducing the concept ofquantum energy. Subsequently, A. Einstein (1905)highlighted the corpuscular behaviour of radiationand linked the energy E of the single photon to thewavelength l through the formula E�hc/l where h,or 6.626�10�34 J�s, is the Planck constant and c, or2.998�108 m/s, is the speed of light in a vacuum. Inthe quantum description of electromagneticradiation, there are wave and corpuscular aspects(wave-particle duality).

The spectral distribution F(l) of solar radiation,regarded as the emission from a blackbody, isdescribed by Planck’s law (energy density per unittime per unit wavelength):

2phc2F(l) �11111112

hcl5�exp �11��1�klT

where k�1.380�10�23, J/K is the Boltzmann constantand T the thermodynamic temperature of theblackbody (in the case of the Sun, the apparent surfacetemperature is around 6,000 K).

The integral of the spectral distribution over allwavelengths gives the power density HS emitted at thesurface of the Sun:

HS�sT4�6�107 W/m2

where s, or 5.67�10�8 W/m2 K4, is theStefan-Boltzmann constant.

At a distance D from the surface of the Sun: H�HS

R2�D2, where R�6.96�105 km is the Sun’s radius. Theradiation density is 1,353 W/m2 at the edge of theEarth’s atmosphere. Apart from minor variations dueto the Earth’s elliptical orbit around the Sun, this valueis constant. However, on the Earth’s surface, theradiation is affected by alterations due to atmosphericconditions, the latitude and the seasons, as well as theday-night division.

In section 6.1.1, the existence of the sunbelt,whose annual insolation is always significant, ishighlighted. The power density of solar radiation isless than that at the edge of the atmosphere, owingto absorption due to molecules and atmospheric dustand to diffusion (of around 10%) caused by part of



the atmosphere’s molecules. The maximum solarradiation density directed at the Earth’s surface, inthe absence of clouds, is around 950 W/m2 (to whichshould be added the diffused element). In general,reference is made to a standard radiation value at theEarth’s surface in order to compare the performancesof photovoltaic modules and systems against oneanother, while reference is made to local climatedata, if available, to size the actual installationsappropriately. Thus, the so-called standard Sun isdefined in relation to a global air mass of AM 1.5(Air Mass is related to the distance travelled byradiation in the atmosphere and is given by thesecant of the angle q between the normal on theground and the position of the Sun, so thatAM�secq, AM�1 if q�0), equivalent to 1,000 W/m2 (taking account both the direct and diffused components of the radiation). Thestandard Sun corresponds to the radiation level ofthe surface of the Earth at an angle of around 49°.The radiation at the edge of the atmospherecorresponds to zero air mass (AM0).

SemiconductorsThe materials available to make photovoltaic cells

are numerous and often have very differingcharacteristics. For example, there are inorganicsemiconductors in the solid state, of which silicon isby far the most widely used, as is also the case withelectronic technology; among others, we wouldmention germanium and compounds between elementsof the III and V groups (GaAs, InP) or the II and VIgroups (CdTe, CdS) of the periodic table of elements,and also compounds with three or more elements(InGaN, GaInP). Among the materials used forBecquerel type cells is titanium dioxide (TiO2) withsome colorant additives, while for organic cells,nanostructures are used, such as fullerene (C60) orconjugated polymers. Other materials are beingstudied, including silicon nanostructures. For adescription of the operation of solar cells based on themost common technology, that of silicon, see below.

Proprieties of siliconAn element of group IV, silicon has 4 valence

electrons which, in the ideal crystalline form, giverise to 4 covalent bonds with other silicon atoms; inother words, bonds in which each atom shares oneof its own valence electrons with the nearest others,thus achieving the stable electronic configuration(octet). Silicon does not exist in pure form innature, although it is the second most abundantelement on the Earth after oxygen. It is, however,found in the form of various minerals, such as silica(silicon dioxide), and its transformation intocrystals of the desired purity requires particulartreatment (see below).

In a semiconductor such as silicon, in the boundstate and at thermodynamic zero, there are noelectrons available for electrical conduction and thesolid behaves as an insulator. At temperatures otherthan zero, however, thermal agitation allows someelectrons to free themselves, even if their number isvery small. Many more electrons can be freed if thesilicon is illuminated with light whose photons havesufficiently high energy, for example, as occurs withpart of the solar spectrum. The part of the solarspectrum with the highest energy, however, tends tointeract with the inner shells of the atoms, withoutcontributing to the photovoltaic effect. The bindingenergy of silicon electrons is around 1.12 eV, whichcorresponds to photons of radiation with a wavelengthof 1,100 nm (near infra-red).

The photovoltaic effect, and many other propertiesof semiconductors in general, can be explained in fullwith the theory of electronic bands in solids. In anatom, there is only a collection of discrete energylevels that can be occupied by electrons, but whenseveral atoms are brought together to form a solid, thelevels mix to give rise to bands of possible energylevels, separated by empty areas (in the case of idealsolids; Fig. 2). The width of the zone of forbiddenlevels is called the band gap, and corresponds to theminimum energy needed to bring electrons from afully occupied band and, therefore, without any



ener

gy

d

forbidden band(band gap)

conductionband

valenceband

discrete atomicenergy levels

atomic distance

Fig. 2. Simplified diagramof the formation of semiconductor energybands as the distancebetween atoms diminishes.The distance d representsthe semiconductor in equilibrium.

possibility of movement (valence band), to anunoccupied band (conduction band), taking intoaccount that electrons first occupy lower energy states.This representation is equivalent to the passage fromthe covalent bound state to a free state (within thesolid) of one of the outer electrons of the siliconatoms. In reality, the energy bands follow morecomplex patterns connected to the three-dimensionalstructure of the crystals, the temperature and thesymmetry properties of crystalline matrices, and thetype of bonds between the atoms. The width of theband gap is not generally constant and in the case ofsilicon in particular, the minimum of the conductionband does not correspond to the maximum of thevalence band (in such a case, it is said that thesemiconductor has an indirect gap).

The form and the nature of the band gap have amarked influence on the properties of thesemiconductors, particularly in relation to theinteraction with electromagnetic radiation.

Besides the band gap, another importantmeasurement is the Fermi level EF, or the energybelow which all the states are occupied, and abovewhich, they are empty. In the absence of reticularimpurities and imperfections, the Fermi level is at thecentre of the band gap (Fig. 3).

The band model provides us with a relativelysimple explanation of how solar cells work. When anelectron is transferred into the conduction band,following the absorption of a sufficiently energeticphoton, it leaves an electron gap or vacancy in thevalence band known as a hole, which can move withinthe semiconductor and behaves likes a pseudo-particlewith the same charge as the electron, but of theopposite sign.

Electrical conduction in semiconductors, suchas silicon, is due to a flow of electrons in theopposite direction to that of the holes. Moreover,pure silicon has a low density of free carriers, evenin the presence of light. It is common practice toinsert controlled quantities of some elements, inother words to dope the semiconductor in order toimprove its electrical transport properties. Theelements usually used for silicon for photovoltaicapplications are pentavalent phosphorus andtrivalent boron. These elements are inserted inquantities sufficient to raise the number of carriers,without significantly altering the opto-electronicproperties of the silicon. In the case of phosphorus,the impact of these elements is to provide an extrafree electron compared to the tetravalent symmetryof silicon, thus giving it an excess of negativecarriers. In the case of boron, there is an extra holeand the material has an excess of positive carriers.By convention, it is said that silicon doped withboron is type p, while silicon doped withphosphorus is type n.

By using doping techniques, it is possible toincrease the density of electrons (or holes) by up to10,000 times from the level of 1012 cm�3 of theintrinsic silicon to 1016 cm�3 in the typical case ofboron, the most commonly used in the production ofcrystals for photovoltaic applications. In general, thelayers which have been doped with phosphorus haveeven higher densities. This allows the density of theexcess carriers at room temperature to beapproximated by the density of the dopant.

From the viewpoint of the system of bands, thedopants have the effect of introducing energy levelsnear the edges (of the valence band in the case ofboron, and of the conduction band in the case ofphosphorus) and, therefore, of moving the Fermi levelin the direction of the opposing edges, thus making agreater quantity of energy levels available. When thedoped semiconductor is illuminated, a pair of excesscarriers is created: an electron and a hole. One of thesecarriers will be in the majority and the other in theminority, depending on the characteristics of thematerial. For example, in the case of p type silicon,which has an excess of holes, the minority carriers willbe the electrons.

Although the density of photogenerated carriers issmall compared to that of the dopant atoms, theminority carriers have a more important role, in termsof many aspects of the functioning of solar cells, thando majority carriers (in this case holes). When thedoping density is close to that of the silicon atoms(5�1022 atoms cm�3), the semiconductor is termeddegenerate and the description of the material in termsof bands is more complex.



energy ofa free electron

EF, Fermi level

conductionband

qχ

Ec

Ev

E

valenceband

EG

Fig. 3. Simplified band diagram. Ev is the edge of the valence band, Ec the edge of the conduction band, EG the band gap, qc the electric affinity.

Optical propertiesThe capacity of semiconductors to absorb radiation

is not constant over the whole spectrum. For everymaterial, there is an absorption coefficient a, an opticalproperty which also derives from the band structure ofthe semiconductor and is dependent on the wavelength.Finally, each material has a reflection coefficient andwill have transmitted, reflected and absorbedcomponents that differ depending on the wavelength.Not all the incident light can be absorbed by thematerial and not all the absorbed light is equallyinvolved in the creation of carriers, given that theintensity I of the radiation diminishes in the material, inaccordance with the law: I�I0e�ax (where I0 is theintensity of the incident radiation and x the thickness ofthe material crossed). This implies, given the trend withthe variation of the absorption coefficient withwavelength, that the most energetic radiation isabsorbed in the uppermost layers of the solar cell, whilethe less energetic radiation is absorbed deeper down.

It follows that there are optimal thickness valuesfor each type of semiconductor, based on the opticalproperties of the material. In the case of silicon, suchthickness 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 lightefficiently, i.e. the ability to create an electron-holepair for each incident photon. This ability is measuredby a factor called Spectral Response (SR), given bythe ratio between the current generated and theincident power, or

elSR �13QE

hc

where QE (quantum efficiency) is the ratio betweenincident photons and carriers pairs generated, l is thewavelength and e the absolute value of the electron’scharge.

In particular, in the case of silicon solar cells, QEtakes the form shown in Fig. 4. It can be clearly seenthat the solar cell cannot use all the solar radiation. Inaddition, the cell cannot absorb all the photons withE�EG with the same effectiveness, since the mostenergetic ones create carrier pairs at the surface, wherethere is marked recombination owing to the presenceof energy levels in the band gap due to the material-airdiscontinuity; on the other hand, the photons nearest tothe band gap thresholds are absorbed at a considerabledistance from the illuminated surface and, if thequality of the material is not adequate, the carriersrecombine before being used. In addition, the totalquantity of absorbed photons depends on the fractionof radiation reflected by the surface. The integral ofthe QE over all wavelengths is related to the shortcircuit current. Photons with energy below the widthof the band gap are not absorbed. Therefore, below theenergy threshold, QE is zero. This is also true if theenergy of the photons is markedly higher than that ofthe band gap. Also the absorption due to carrierswhich are already in the conduction band has no effecton the cells’ electricity transport mechanisms, but isactually an obstacle to photovoltaic generation. Thisphenomenon is typically seen in highly dopedmaterials, or can be important at the edges of thebands and is not included in the calculation of a(x).

In the case of devices which are electrochemical orbased on polymers, the absorption of luminousradiation creates electron-hole pairs in an excited state(excitons or excited molecular orbitals), which tend toreturn very rapidly to the original state, owing to thehigh electrostatic attraction (recombination times arein the order of 10�12 s). In this case, the possibility ofgenerating a photo-current is linked to the ability tovery rapidly separate electrons from holes, throughredox solutions or by means of charged materialswhich accept the photogenerated charges and channelthem into an electric circuit.

Photogenerated carriers tend to recombine, and thisprocess is quicker if there are defects in the materialwhich capture the carriers. Since it is inevitable thatthere are defects in the material, caused by impuritiessuch as other types of atoms, distortions in thecrystalline matrix or by surface effects, the ability tomake best use of the photogenerated carriers dependson the properties of the material. The quality of thesemiconductor is generally expressed in terms ofparameters such as the life time t and the diffusionlength Ld of the minority carriers, defined respectivelyas the average time needed for a photogenerated



100

quan

tum

eff

icen

cy (

%)

wavelength

front surfacerecombination

finite valueof diffusionlength

rearsurfacerecombination

reflectionlosses

high energyphotons

absorptionthreshold

ideal cell

Fig. 4. Quantum efficiency of a solar cell and lossmechanisms compared to the ideal transformation of one photon into one electron-hole pair.

carrier to recombine and the average distance travelledby such a carrier before recombining. The twoparameters are linked. In the case of silicon forphotovoltaic use, typical values are in the order ofsome tens of microseconds for t and some hundreds ofmicrons for Ld.

The recombination in semiconductors can comeabout in various ways. Of these, by far the mostimportant for manufactured solar cells is recombinationthrough defects. This mechanism links therecombination properties of the material to itscharacteristics of purity and crystallographic perfection.

DevicesIn order to generate electricity, it is necessary to

build a device which enables the effective separationof the charges and the creation of an electromotiveforce to provoke the flow of the electrical current in anexternal circuit. The essential requisite for thegeneration of electricity is that there is electronicheterogeneity in the structure of the material.

The most common electronic device for theproduction of solar cells is the p-n junction,analogous to that used in solid state diodes.Electrons and holes generated by a photon areseparated by the barrier’s electric field in the p-njunction and channelled to an external circuit. In thecase of silicon, the junction is obtained between theparts that are doped in different ways. In order toexplain the functioning of the device, let us imaginebringing together two parts of silicon, one doped pand the other doped n. Before contact, on the rightside, we will have an excess of electrons, and anexcess of holes on the left side. When the twosemiconductors come into contact, a flow ofcarriers is established by diffusion to rebalance theconcentration gradients. This leaves a doubleelectric layer formed by positive and negativecharges uncovered at the interface between twodifferent materials This double layer, also called thedepletion region, creates an electric field which isopposed to the diffusion which generated it (Fig. 5).In the absence of an external excitation, there is no

net current flow. A junction is effectivelyrepresented by using the band structure, in the caseof a p-n junction, in Fig. 6; the double layer formsan ‘energy step’ for the passage of charges, exceptthe few which manage to cross over, owing tothermal agitation. The step is such that, at the pointof equilibrium, the Fermi levels of the two materialscoincide because a system at equilibrium can onlyhave one Fermi level. Away from the region of thejunction, the bands are unchanged (flat).

When an external excitation is added, such as aphoton with energy higher than that of the band gap,electron-hole pairs are created on both sides of thejunction depicted in Fig. 7. The minority chargesphotogenerated in proximity to the junction leaveuncovered ions which partly neutralize the charge of thedouble layer, thus reducing its height. This mechanismis called minority carrier injection: the carriersphotogenerated on the side of the energy step can morereadily cross it, with the effect of putting the junction indirect conduction. However, the junction is obviouslynot a barrier for the electrons in a conduction band andthe holes in a valence band at the peak of the step.

In the illuminated state, there is no longer a state ofequilibrium, and it is inappropriate to speak aboutFermi levels, since the concentrations of carriers vary,as well as the corresponding statistical distribution.However, since the variation from equilibrium is notlarge, it is possible to precisely define energy levelswhich represent this deviation and which assumedifferent values in different parts of the device. These



depletionregion

holeselectrons np

Fig. 5. Formation of the depletion region.

p n

nv

EF nc

qy

Fig. 6. Band diagram of the p-n junction at equilibrium.

p n

qVEFp EFn

Fig. 7. p-n junction in state of non-equilibrium and generation of electron-hole pairs.

levels are called quasi-Fermi levels or Imrefs byconvention, and correspond to the chemical potentialsof non-equilibrium. The quasi Fermi levels, shown inFig. 7, are very important in the behaviour of the solarcells, since they determine the maximumelectromotive force obtainable, or the size of thephotovoltaic effect.

The mathematical description of the chargetransport in solar cells is given starting from thecontinuity equations, which guarantee theconservation of the total charge, and from the Poissonequation, which relates the electrical potential to thecharge density. In general, some simplifications maybe considered, such as the absence of the electric fieldin active regions (in the example, the p region) and theconstancy of the quasi Fermi levels in the depletedregion. In addition, it is assumed that the concentrationof minority carriers is always much lower than that ofmajority carriers, and that the thickness of the cell ismuch greater than the diffusion length of the minoritycarriers. In ideal conditions, when the generation andrecombination mechanisms do not depend on thephotogenerated currents, the principle of superpositioncomes into play, by which the current of the cell isgiven by the algebraic sum of the current of the diodeand the (negative) photogenerated current.

Superposition has the effect of shifting the typicalcurve of the diode in such a way that it occupies anarea in the fourth quadrant of the current-voltage planeI-V (Fig. 8), or of giving the device a feature of a powergenerator (in that the power absorbed is negative). Thecharacteristic of the illuminated device takes the form

eVI �I0�exp �11��1��ILnkT

where I0 is the inverse saturation current, or dark current,IL is the photogenerated current and n is a ‘ideality’

factor, which measures 1 in the case of an ideal diodeand is greater than 1 in the presence of defects.

Typical parameters of solar cellsIt is possible to extract the important electric

parameters of a solar cell from the characteristiccurve; in other words: a) the short circuit current Isc,generated by the cell at zero potential; b) the opencircuit voltage Voc, which corresponds to the maximumpossible compensation of the electrostatic barrier bythe photogenerated charges; c) the maximum powerpoint Pmax; d ) the so-called FF (Fill Factor) of thetypical curve. The meaning of such parameters isshown in Fig. 9, where the typical curve underillumination for a solar cell is shown conventionallyinverted in the first quadrant, changing the sign of thecurrent. The efficiency is defined as the ratioPmax /Pincident and also FF�Pmax/IscVoc.

Since the short circuit current is directlyproportional to the area of the cell, in general, in orderto compare cells of different areas, the current densityJsc is used; this depends directly on the number andtype of incident photons, i.e. on the intensity of theradiation and its spectrum and the optical properties ofthe material. Finally, it is heavily dependent on therecombination properties, in other words, the degree ofpurity and perfection of the material. For silicon solarcells, typical values of the short circuit current densityare 30-35 mA/cm2.

An expression for the open circuit voltage is foundby setting the term of the current to zero in theequation for the diode:

nkT ILVoc�11 ln �1�1�e I0

This important expression shows that the open circuitvoltage of a solar cell depends, not only on thetemperature and the photogenerated current, but on the



I

VIV

quadrant

diode inthe dark

diode underillumination

Fig. 8. Effect of the superposition principle on the characteristic I-V of a p-n junction solar cell.

I

V

.Isc

Voc

Pmax

Fig. 9. Typical parameters of a solar cell (quadrant IV has been inverted for the sake of convenience).

inverse saturation current, directly linked to the qualityof the material. The ideal solar cell described by thetypical equation set out above cannot be achieved inpractical terms and, in particular, the device suffersfrom the effects of parasitic resistance.

Limits to the efficiency of solar cellsThe efficiency (or yield) of a solar cell cannot be

100%. Studies on what constitutes the theoretical limitfor the yield of a solar cell (or converter) and what theobtainable limits are started when the first cells weremade in the Bell Labs, and continue today.

As far as thermodynamics are concerned, treatingthe Sun-cell system as two sources at differingtemperatures which exchange energy and entropy, themaximum limit for the efficiency of a solar cell withunspecified material and characteristics is 86.4%(thermodynamic approach) in conditions of maximumconcentration; in other words, concentrating the entiresolar radiation flux on a single point. These results canbe reached also by generalizing the fundamental workundertaken in 1961 by W. Shockley and H.J. Queisseron a single junction device. This approach, calleddetailed balance, calculates, through continuityequations, the current produced by a solar cell fromthe difference between generation and recombinationin a two source system, in which the solar cell, orconverter, has chemical potential (equal to theseparation between the quasi Fermi levels); in otherwords, an absorption threshold (approach ofgeneralized or device-based detailed balance). Thegeneration function in the case of Shockley andQueisser was given by Planck’s law for the emissionspectrum from a blackbody at around 6,000 K.

The equivalence between the two approaches canbe shown in mathematical terms since for both it ispossible to express for each photon of energy E, thework in the same form, provided that the chemicalpotential m is expressed in terms of the temperatures ofthe converter.

The work can be expressed in a more general formfor many photons of differing energy, by using ageneralized generation function and multiple orvariable chemical potentials, with some calculationalcomplications which take account of the variousgeneration and recombination mechanisms. Thegeneralizations keep the mathematical equivalencewith the purely thermodynamic treatment if additionalconditions are introduced, which describe thebehaviour of any solar cell in short circuit conditions(the work must be nil under these conditions) and opencircuit conditions (the total flow of carriers must benil, a condition which corresponds to minimumentropy). It is, however, interesting to cite thisgeneralization, which enables a description, for

example, of a device formed by n junctions, each ofwhich absorbs and converts a part of the incidentspectrum. These cells are known as tandems. Othertypes of device have been proposed to reach thetheoretical limits, all of which may be described withthe generalized approach. They are devices thatgenerate multiple electron-hole pairs for each photonabsorbed; or use electrons in states far from the edgesof the bands; or can convert the most energeticphotons into more, but less energetic photons; or havelevels within the band gap that can contribute to thegeneration of pairs.

For a single junction silicon device, the theoreticallimit which can be reached with sunlight is around30% (33% if the AM 1.5 spectrum is considered),considering the material as ideal. Around 30% of theincident solar radiation is lost, since it is notsufficiently energetic, and around another 30% isdissipated as heat because it is too energetic. Theremaining approximate 10% is lost throughrecombination mechanisms (only radiativemechanisms in the ideal case).

Nonetheless, the yield that can really be obtainedfrom solar cells is lower, given that the material is notideal (the mobility of the carriers is not infinite); otherrecombination mechanisms must be considered (e.g. Auger); and that there are optical losses, throughlateral conduction, surface effects and imperfectmetallic contacts. The best laboratory cell produced sofar on monocrystalline silicon had an efficiency levelof 24.7%, compared to a value of just above 20% for amulticrystalline silicon cell, while the best commercialdevices have values around 20%. The averageefficiency values of the most common commercialcells are, however, even lower, 14-16%. It is believedthat the obtainable limit for silicon solar cells isaround 26%.

The devices which currently have higher yields arethose made with several junctions in a series ofdifferent materials. In particular, theGaInP/InGaAs/Ge device reaches 32% at 1 Sun andaround 39% under concentrated light. The maximumvalues obtained refer to very small surfaces, generallyaround 1 cm2. Commercial cells are currently built onsurfaces between 100 and 400 cm2.

Technological and industrial aspects

The technology for the production of solar cellshas a lot in common with that for electronicsemiconductor devices, especially the use of silicon,the most widely studied and used semiconductor in theworld. Nonetheless, some particular aspects ofphotovoltaic energy, mainly the need to use ratherlarge surfaces compared to those for electronic devices



and the need to limit manufacturing costs as far aspossible, have differentiated the constructiontechniques over time.

MaterialsSilicon used as an active material to produce solar

cells is generally in the form of thin wafers (with athickness of around 250 mm) with a surface areabetween 100 and 400 cm2. The cost of silicon wafersrepresents around 50% of the cost of a photovoltaicmodule, therefore the efficient use of the raw materialis essential for technological progress. The wafers, orlayers, are created by cutting pure silicon ingots incrystalline form, produced with technologies derivedlargely from those used in the electronic industry, andaltered to meet the specifications of the photovoltaicsector. The initial material, called silicon feedstock, iscreated following a complex chain of successivepurifications starting with sand. In simple terms, therefining reaction is: SiO2�2C��Si�2CO, from whicha (solid) metallurgical silicon is obtained, with a purityof 80-99.5%.

This process takes place in a submerged arcfurnace, at a high temperature (around 2,000°C). Thematerial produced (some millions of tonnes/p.a.) ismainly used in the steel and aluminium industry. Atiny fraction is purified for use in the silicon,semiconductor and photovoltaic industry, through thereaction: Si�3HCl��SiHCl3�H2, which occurs in afluidized bed reactor, in the presence of a coppercatalyst. The compound SiHCl3 is liquid and ispurified through multiple fractionated distillations; itis the material used to obtain silicon. Finally, for theelectronic and photovoltaic sector, from the reaction:SiHCl3�H2��Si�3HCl, very pure silicon is obtained,which is deposited by means of heterogeneousnucleation on special filaments in a so-called Siemensreactor; there are also variants with other gases basedon silane and other types of reactor.

This material, together with some recyclable wasteproducts from various processes, represents the initialmaterial for the production of ingots for photovoltaicuse. Currently, annual consumption is around 15,000 t.and is growing. Since the beginning of industrialactivity in the photovoltaic sector, less costlyalternatives have been tested for the production ofsilicon, which are independent from the semiconductorindustry. In particular, significant effort has beenplaced on purifying metallurgical silicon, which costsaround thirty/sixty times less than silicon obtainedfrom the Siemens process. Some tests in this regardare still underway, but an industrial product is notcurrently available. The main difficulties for the directpurification process of metallurgical silicon lay in theelimination of excess boron and the processing cost.

The silicon ingots can be in the form ofmonocrystals, made by developing the material on aninitial crystalline seed, slowly extracted from a quartzrecipient (crucible) that contains pure melted silicon,at a temperature of over 1,400°C (Fig. 10). The processtakes place in an inert atmosphere (usually argon) bycontrolling the quantity of elements such as metal,oxygen and carbon, which can reduce the quality ofthe material. In general, cylindrical ingots areproduced of 50-100 cm in length, with a diameter upto around 20 cm (or 8 inches, due to a conventionlinked to the American industry). The method referredto is commonly called Czochralski, after the name ofinventor. The material thus produced has goodcrystallographic perfection and good purity, with a



pulling direction

seed

crystal

crucible

molten silicon

Fig. 10. Diagram of the Czochralski crystallization method.

hotchamber

heaters

argon

crucible

impuritiesmolten silicon

solid silicon

graphite

cooled pedestaltranslation

thermocouples

coldchamber

Fig. 11. Diagram of the directional solidification method to produce multicrystalline silicon blocks. There are some existing variants on the process shown.

metal content of less than one part in a billion, andoxygen and carbon in the order of hundreds of partsper million.

The second method, currently more common forthe production of silicon wafers, consists of meltingand solidifying silicon contained in large square quartzcrucibles under controlled thermal conditions. Themethod, called directional solidification, is based onthe controlled extraction of the heat of the moltensilicon from the bottom of the crucible, keeping thetemperature of the walls and the top as high aspossible (Fig. 11). This method has been developedspecifically for the photovoltaic sector, since it cansatisfy economies of scale and is a relatively simpleprocess. The material is in a multicrystalline form,with long grains perpendicular to the solidificationfront. In this case, there is no need for crystalline seedsand the ingots can also be very big (typically 250-300 kg, and dimensions of around 70�70�25 cm).It is a material which is of slightly lower quality thanthat obtained with the Czochralski method, but has thesuitable requirements for the production of solar cellswith an efficiency of 14-16% in industrial productionand up to 20% for laboratory devices.

The silicon ingots for photovoltaic use aregenerally p type doped, by mixing controlledquantities of boron to the feedstock to be melted.Typically, the resistance of the ingots is 1W�cm, whichcorresponds to a density of dopant of around 1016 atoms/cm3. The ingots, whether they are in mono- or multi-crystalline form, are thenmechanically processed to be transformed into thinwafers. These squaring and cutting processes are donewith diamond studded blades or steel wires in anabrasive suspension.

DevicesMost commercial solar cells are produced using a

process which is substantially the same as thatdeveloped in research laboratories in the Eighties; it isbased on low cost, high productivity screen technologyin order to print metal contacts with silver andaluminium based inks. The commercial solar cell is ahomojunction diode, produced by bringing togetherdoped p and n zones of the same slice of silicon (Fig. 12). It starts with a wafer containing boron (in theorder of one part per million) which produces theexcess of free positive charges. The dopant n,generally obtained with phosphorus atoms, isproduced with a high temperature thermal process(around 900°C), which occurs by diffusion in a verythin zone near the illuminated surface. The phosphorusdispersed in the silicon occupies a layer of less than amicron under the surface of the wafer which is around250 mm thick.

Other treatments include surface preparation withchemicals to remove any impurities and damage due tothe wafer cutting process, and to reduce the quantity ofreflected radiation.

Before producing the metallic contacts, a thindielectric layer is deposited on the surface exposed toradiation, in order to further reduce the losses byreflection. Industrial production lines use chemicaldeposition systems in silicon nitride plasma. Thevolumes produced (around 1,500 MW in 2005,compared to 10 MW in 1985) has aroused the interestof the machinery and materials industry, thus, finallycreating standards. Further progress is certainly linkedto improvements which will be gradual, but no lesssignificant, such as automated wafer handling – these



Fig. 12. Solar cell: photograph (A) and diagram in cross-section (B).

p-type

metal

p�

n��

A

B

will become increasingly thinner – (from the current240-220 mm to below 200 mm in the next five years),or chemical and thermal treatments which can improvesurface quality and the bulk of the silicon wafers.

It is expected to reach yield values of up to 20% by2010 and to optimize the use of the raw materials. Inreality, there are already some examples ofhigh-efficiency cells with industrial potential atvarious stages of development. Buried contact cells,for example, invented at the University of New SouthWales in Sydney in the Eighties, have been producedsince the start of the Nineties, also on an industrialscale. The main difference in this type of cell from theprinted ones is in the technique of metallization, whichis done by hollowing out thin deep grooves in thewafers by means of a laser, which are then filled withmetal starting from chemical solutions. There is atwofold advantage since it is possible to dope thecontact area more heavily without having to worryabout alignments; in addition, the buried form of thecontacts reduces the shading due to the grid. The yieldof these cells with commercial monocrystalline siliconis around 17% on average in production, with peaks ofover 18%. HIT (Heterojunction with Intrinsic Thinlayer), invented and produced by Sanyo in Japan, isalso highly efficient. Here, the doped frontal region ismade with amorphous silicon: it is, therefore, aheterojunction involving materials of varying electricalcharacteristics.

The maximum efficiency demonstrated in thelaboratory is over 21% and production is around anaverage of 17%. Another high yield device, which iscurrently compatible with industrial production, hasbeen developed by SunPower (United States) as a largearea and large scale development of a cell, created inthe Eighties at Stanford University for spacedeployment or for concentrated modules. TheSunPower cell has both metallic contacts on the back;thus, shading is practically non-existent, losses due torecombination are very limited and, in principle,assembly in modules is simplified. Nonetheless, thematerial must be of a much higher quality than thatused for the commercial cells described previously.The efficiency of SunPower cells on a large area isaround 20%.

As an alternative, various technologies are beingstudied, also for small scale production, which usemuch reduced thicknesses (just a few microns,compared to around 300 for silicon wafers) for theactive material, starting with gas; among these areamorphous silicon (already used in the latter half ofthe Seventies), chalcogenide compounds (CIS, CopperIndium Diselenide) and cadmium telluride (CdTe).Naturally, there are substantial differences betweenone type and another, but the structures used are fairly

similar. A typical amorphous silicon cell has a p-njunction structure, but in order to gather solar radiationmore effectively, two or three superimposed junctionsare built, each ‘specialized’ for a portion of the solarspectrum. The layers are deposited in a vacuum oninert substrates such as glass, with surface areas thatcan even exceed a square metre. The individual cellsare built by making laser grooves and depositingmetallic layers so that the electric cell and moduleconnections are contemporaneous. The front part doesnot have a metallic grid as in conventional cells, but aglass conductor.

In general, thin film technologies are less efficientthan cells on a silicon wafer (6-10%, even though inthe laboratory films in the CIS family have reachedaround 19%) and still suffer from unresolved stabilityproblems. The expected low production costs have stillnot come about, due to the complexities of theprocesses and, not least, the high investment costs.Nonetheless, it is clear that in principle, from theindustrial viewpoint, thin films have some definiteattractions: it is not necessary to produce ingots forcutting, the production of cells and modules iscontemporaneous, the discrete element (for example, aglass sheet) is much larger and can be manoeuvred.For this reason, research and development concentrateon variations in the processes to increase efficiency orto lower costs; among these are mixed devices ofamorphous silicon/microcrystalline silicon or justmicrocrystalline silicon, obtained by recrystallizinglayers of amorphous silicon deposited on glass.

Another category of cells is the one created forspace applications, or for concentrated uses. These areextremely complex devices which use materials otherthan silicon (for example, GaAs or GaAlAs, or theaforementioned GaInP/InGaAs/Ge) in severalsuperimposed layers in order to selectively capture thelight, as partly happens in amorphous tandems. Theefficiency of these very expensive structures can reachup to 39% with high concentration. The high cost isdue to the complex devices and production techniqueswhich use ultra high vacuum and low productivitydeposition systems.

Other types of solar cells have medium-long termprospects. New ideas are being studied to allow thelimit imposed by current technologies to be surpassedand to achieve very high efficiency levels. Work isbeing done on structures such as the aforementionedtandems because it can be demonstrated that a tandemstructure with infinite cells can reach the yield limitimposed by thermodynamics, around 86%. In practice,this will be very difficult since the limit presupposesideal materials. Another very interesting idea is that ofphoton converters, layers of particular materialsdeposited in front of or behind the cells, which



manage to reduce (or increase) the wavelength of theincident photons without losing energy. Thesestructures, too, are theoretically capable of reachingthe thermodynamic limit, although in this case, thereare still no experimental results. The other great areaof interest for the future is linked to the developmentof polymeric or organic based cells. Polymer cells arebased on the creation of electric carriers by the light,which are very quick (in the order of picoseconds),gathered on selective receptors for the type of charge,and then transported towards the electric collectioncircuits. The major attraction of this type of cell is, ofcourse, the potential low cost and the extremesimplicity of production (photovoltaic paints).Nonetheless, in both cases, the conversion yield isfairly low (�5% for polymer cells, up to 11% over asmall area for organic cells) and with serious stabilityproblems. These are, however, very interestingtechnologies, especially because of their very low costpotential.

Albeit with the due differences in terms of the typeof materials and the architecture of the device, ingeneral, the functioning of solar cells can be dealt within the same way as all non-conventional devices basedon silicon with a p-n junction.

ModulesAlmost all the commercially available modules for

terrestrial applications are made with mono- ormulti-crystalline silicon solar cells. For this type of cell,the voltage at the work point is around 0.5 V and thecurrent generated by the cells is typically 30 mA/cm2,hence, connections in series are normally used throughthe welding of tin-coated copper strips. In this way,modules are obtained with output voltages that cantypically charge a six-element accumulator lead battery(nominal 12 V). The area of the individual cells variesbetween 50 and 225 cm2, which implies the possibilityof working also with very high currents, in the order ofa few amperes. For the formation of the modules, it isimportant to use cells with similar electriccharacteristics in order to limit the so-called mismatchlosses. This is necessary because for cells connected inseries, the lower current device dominates, and for cellsin parallel, that with the least voltage.

The strings of interconnected cells are electricallyenclosed in weather-proof materials which allow themto function for several years, even in extremeenvironmental conditions. In particular, the modulemust be capable of resisting atmospheric agents suchas dust, salt, sand, wind, snow, humidity and hail for atleast twenty years, as well as maintaining its electriccharacteristics after prolonged exposure to ultravioletrays. The most widespread technique is that ofenclosing the cells in a transparent polymer (EVA,

Ethyl-Vinyl-Acetate), protecting the front withtempered glass and the back with another glass orplastic sheet, generally in several layers in order tokeep it waterproof. The process with which thisstructure is built is normally hot lamination.

Finally, the module is equipped with a supportstructure, generally an aluminium frame, which allowsit to be mounted on a structure that is wind resistant,and a termination box for the contacts in order to allowthe connection of the modules among themselves inorder to form strings. The box must allow theconnection and guarantee waterproofing and shaperetention throughout the life of the module. Inaddition, by-pass diodes are usually mounted withinthe boxes, which protect the module in case one of thecells or strings malfunctions, for example, due topartial shading.

Certification standards have been developed overthe years by accredited institutes on the basis ofextensive tests and on a number of existing standardsfor the durability of the devices. The modules aresubjected to accelerated ageing tests which simulateextreme conditions of prolonged exposure. The keyEuropean standard for terrestrial flat modules based onsilicon cells is EN/IEC 61215, which envisages, interalia, thermal cycles in conditions of extreme humidity,exposure to high doses of UV radiation and impactwith ice balls shot from specially designed cannons.For the modules which pass these severe tests, it wouldbe reasonable to expect a stable life of 20 to 25 years.This is only hypothetical since the test data relating tomodules this old, albeit reassuring, is not veryextensive. However, it is necessary to bear in mind thatover the last twenty years, materials technology hasgreatly improved, thereby, significantly reducing someof the problems encountered in the past.

The choice of materials is essential in guaranteeingthe duration of the module’s characteristics over time,and is also one of the most costly items, second to thewafers. The front glass, for example, must have goodoptical performance such as high transmittance andlow reflectivity, which are obtained thanks to a low ironcontent. It must also have a chemical or mechanicalstructure to trap light and, finally, must be temperedand capable of resisting hail and shocks. The rearplastic layer, generally polyvinyl fluoride (Tedlar), hasbarrier layers in polyester and/or aluminium in order toincrease its oxygen impermeability. EVA containsadditives which delay yellowing due to UV rays. Theoperating conditions for the lamination process alsohave a significant impact on the modules’ reliability.

SystemsThe photovoltaic modules can be used individually

or connected in various ways (systems) in order to



meet the requirements of the uses to be electrified.These are divided mainly into accumulation systems(stand alone) which charge batteries, and systemsconnected to the main electricity grid (grid connectedsystems) which provide electricity to the grid in thepeak production period, i.e. during the day. Themodular nature of photovoltaic components means thatsystems can be set up from just a few watts (forcalculators, watches, toys) to a few megawatts, as inthe case of the great showpiece stations at thebeginning of the Nineties; the biggest example is the 3 MW Italian plant at Serre (Salerno) and the morerecent ones in Japan and Germany, often on the roofsof buildings (e.g. the 1 MW plant on the CongressHall in Munich). The grid connected market, linked togovernment incentives, is the one growing fastest,while stand alone systems have more specific usessuch as telecommunications or the electrification ofremote sites, road or sea signage, campervans or boats,or the electrification of water pumping systems invillages in developing countries. The main problemsassociated with grid connected systems are efficiencyand safety issues. These concern technical andregulatory aspects linked to interface devices betweenthe photovoltaic systems and the electric grid. In suchsystems, it is also necessary to use converters fromcontinuous current to alternate current. In the case ofaccumulation systems, it is possible to use theelectricity as it is generated, i.e. continuously. Theproblems in the case of accumulation systems aremainly linked to the duration and cost of the batteries,which has an impact on the cost of the initial plant andits maintenance. For both types of configuration, eachelement of the circuit has its own yield (connections,electric devices, modules, etc.); in addition, in thesizing of photovoltaic systems, it is necessary to takeaccount of the site, its position, and the need for apower supply. Having taken this into account,appropriate safety coefficients are used to calculate thetotal energy expected from the plant.

Despite the high cost, the photovoltaic system isalready competitive in some applications: this isgenerally true for stand alone applications, especiallyin remote sites. The cost of electricity produced by thegrid connected photovoltaic system varies, however,between 0.3 and 0.6€/kWh, depending on the quantityof incident radiation (or the location). Although thecost has fallen by 4-5% annually over the last twentyyears, it must compete with the lower cost ofconventional electricity, seeing as incentives are likelyto diminish. The forecasts made by sector analystsindicate that complete competitiveness withconventional sources may be possible in the long term,but only with an increase in the efficiency of thedevices and sharp reductions in material costs.

A separate mention should go to systems based onthe concentration of solar light on very efficient cells(such as, for example, the triple junctionsGaInP/InGaAs/Ge) of small dimensions, in otherwords, up to around 1 cm2. The aim of concentrationsystems is to take advantage of the cells’ ability togenerate electricity, by delegating the surface cover tolens systems. Thus, it is expected to increase cell andsystem efficiency, and to reduce costs by using smallquantities of semiconductor and other materials, suchas lenses and low cost metallic or plastic structures.Given the complex geometry of the lenses,concentration systems have to track the sun in order tokeep the exposed surface constantly perpendicular tothe radiation. In addition, the diffused component doesnot contribute to the generation of electricity, thereforethe applications are favoured in locations where thediffused component is minimal. The concentrationsystems are more complex and sophisticated thanstatic systems based on flat modules, and require acertain quantity of energy in order to move thetracking apparatus. There is no large-scalemanufacturing available, but there are various researchand development initiatives underway to developstable products, with prospects of industrialization. Inrelation to the previous comment, concentrationbecomes an opportunity for solar electricityproduction using high efficiency cells (�30%).

Market development

It was the oil price shock in 1973 that set offworldwide technological development programmesfor energy sources, such as photovoltaic energy, asan alternative to those based on the use of fossilfuels. In particular, large scale plants were built, ofsome hundreds of kW, in deserts and remotelocations to ascertain if conventional electricitydistribution systems could be reproduced usingphotovoltaic energy. The programmes brought aboutdecisive technological progress in terms of thereliability of the photovoltaic modules based onsilicon cells (thanks to the study of their behaviourin extreme conditions), and enabled basicconstruction standards to be set for the industry. Inaddition, alternative technologies were developed,such as thin film technologies with the prospect ofmarkedly cutting costs, thanks to the much reducedthickness of the semiconductor material.Programmes were also launched on semiconductorscompounds, such as gallium arsenide with opto-electronic properties that are superior to those ofsilicon, but also at greater cost. As a result, thephotovoltaic industry has grown since the earlySeventies, driven by the enticing prospect of being



able to represent a substantial percentage of globalenergy needs in the long term.

The market remained largely unchanged in itsgeneral composition until the mid Nineties, withannual growth of around 15% and more markedgovernment support in the United States and Japan. In1990, world production of modules was around 50 MW/yr (the market’s measurement unit isconventionally the total power of the modulesproduced and sold, measured in solar light, simulatedby special illumination systems). In Europe, above allin Germany, considerable environmental pressure ledto promoting the use of low-polluting or renewableenergy sources, and of encouraging the launch ofgovernment support programmes to producephotovoltaic systems with a direct connection to thegrid, to be installed on rooftops. Japan, on the otherhand, promoted similar initiatives even earlier, withmore ambitious aims than the European ones, in orderto diversify the production of electricity and to rely onfossil fuel imports as little as possible.

‘Photovoltaic rooftop’ programmes have decisivelymoved the axis of the market from technicalapplications to grid applications, contributing to thestart of much more significant growth, which in theearly years of the new millennium reached an annualrate of up to 40%. This growth led to a notable drive toindustrialization, technological development, productstandardization, and a marked reduction in costs.Currently, the market is above a GW/yr of modulesproduced and sold, with a prevalence of Japaneseproducers in the world scenario. The leading Europeanmarket remains Germany, with other countries takingsteps to launch incentive programmes. The reasons forsuch programmes are environmental, and aim tocontribute to the diffusion of energy sources with lowemissions of CO2 and other pollutants into theatmosphere, as part of the effort to control climatechange and to stabilize CO2 emissions in theatmosphere (1992, UNFCCC, United NationsFramework Convention on Climate Change). TheEuropean Union, too, has issued directives (EuropeanCommission, 1997, 2000) which set the prospects forcumulative installations of photovoltaic plants inEurope at 3 GW by 2010, and the doubling by thatdate of the share of energy produced from renewablesources, from 6% in 2000 to 12%.

95% of the current market is still based oncrystalline silicon cells, conceptually similar to thefirst Bell cells, but much more advanced in terms ofyield and quite a lot cheaper (just under 3€/watt atthe modular level). Modern production lines producemore than 100 million wafers a year, compared to thefew hundred units of the first standard production.Nonetheless, the impact of photovoltaic energy onglobal production of electricity is very modest, giventhat the cost per kWh produced is still quite highcompared to conventional generation and alsocompared to other forms of renewable energy.

Bibliography

Ashcroft N.W., Mermin N.D. (1976) Solid state physics,Orlando (FL), Harcourt College Publishers.

Green M.A. (1995) Silicon solar cells. Advanced principles& practice, Sydney, University of New South Wales, Centrefor photovoltaic devices and systems.

Green M.A. et al. (2006) Solar cell efficiency tables (Version27), «Progress in Photovoltaics: Research and Applications»,14, 45-51.

Honsberg C., Bowden S. (2000) Photovoltaics. Devices,systems and applications, Sydney, PVCDROM-UNSW.

Markvart T., Castaner L. (2003) Practical handbook ofphotovoltaics, New York, Elsevier.

Perlin J. (2000) Dal sole. L’energia solare dalla ricerca spazialeagli usi sulla terra, Milano, Edizioni Ambiente-ISES Italia.

Shockley W., Queisser H.J. (1961) Detailed balance limitof efficiency of p-n junction solar cells, «Journal of AppliedPhysics», 32, 510-519.

Wenham S. et al. (1992) Applied photovoltaics, Sydney,University of New South Wales, Centre for photovoltaicdevices and systems.

References

European Commission (1997) Energy for the future: renewablesources of energy. White paper for a community strategyand action plan, 26 November, Final Report COM (97) 599.

European Commission (2000) Green paper. Towards aEuropean strategy for the security of energy supply,29 November, Final Report COM (2000) 769.

Francesca FerrazzaEniTecnologie

Nettuno, Roma, Italy



6 power generation from renewable resources · 6 power generation from renewable resources. 6.1.1...

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