SOLAR THERMAL

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Proceedings of a Course held at the JOINT RESEARCH CENTRE of the COMMISSION OF THE EUROPEAN COMMUNITIES Ispra (Varese) - Italy

in the framework of

COURSES September 3 - 7, 1979

LEGAL NOTICE Neither the Commission of the European Communities nor any person acting on behalf of the Commission is responsible for the use which might be made of the following information.

SOLAR THERMAL

Proceedings of a course held at the Joint Research Centre of the Commission of the European Communities, Ispra, Italy. September 3 - 7,1979 Edited by J. GRETZ Commission of the European Communities Joint Research Centre, Ispra, Italy

Published for the COMMISSION OF THE EUROPEAN COMMUNITIES by ELSEVIER SEQUOIA

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Publication arranged by tha Directorate General Information Market and Innovation of the Commission of the European Communities, Luxembourg.

ECSC, EEC, EAEC, Brussels and Luxembourg, 1980 EUR 6670 EN

CONTENTS SOLAR THERMAL POWER GENERATION Editorial 1 Guest editorial 2 Thermomechanical solar power generation 3

J. Gretz (Ispra, Italy) The 1 MW(el) experimental solar power plant of the European Community 13

J. Hofmann (Munich, F.R.G.) and J. Gretz (Ispra, Italy) Layout, application and economic efficiency of solar farm systems 25

J. E. Feustel, O. Mayrhofer and U. Wiedmann (Munich, F.R.G.) 100 - 1000 kW(el) medium-power distributed-collector solar system 41

J. L. Boy-Marcotte (Plaisir, France) Considerations on a combined and hybrid solar/fossil fuel cycle 53

C. Micheli (Milan, Italy) A solar farm with parabolic dishes (Kuwaiti-German project) 65

G. Schmidt and H. Zewen (Munich, F.R.G.) and S. Moustafa (Safat, Kuwait) The heliostat field layout of the EEC experimental solar power plant 77

V. Hartung, J. Hofmann and Chr. Kindermann (Munich, F.R.G.) Mirror fields for tower-type solar power plants 91

The late G. Francia (Genoa, Italy) The influence of the heat transfer fluid on the receiver design 99

M. J. Bignon (Neuilly, France) The heat pipe and its application to solar receivers I l l

W. B. Bienert (Cockeysville, MD, U.S.A.) Selective absorbant surfaces for high temperature solar collectors 125

P. Beucherie (Ispra, Italy) Comparison of heat transfer fluids for use in solar thermal power stations 139

M. Becker (Cologne, F.R.G.) System and components design of a sodium heat transfer circuit for solar power plants 151

D. Stahl, F. . Boese and S. Kostrzewa (Bergisch Gladbach, F.R.G.) Layout of gas cycles for solar power generation 163

K. Bammert (Hannover, F.R.G.) Long-term storage of solar energy in industrial process heat and electricity production: an analysis with reference to Mediterranean weather 185

R. Visentin (Rome, Italy) The Spilling steam motor 199

G. Spilling (Wohlen, Switzerland) Integration problems of an intermittent power generating plant

Reprinted from Electric Power Systems Research, Volume 3

Editor-in-Chief, M. E. Council.

EDITORIAL Included in the concept and design of the Electric Power Systems Re-search Journal by the editorial staff was the need to provide special issues devoted solely to a specific topic relevant to electric power systems research. Volume 3 is devoted to research on large-scale central station thermomechan-ical solar power generation.

The editorial staff appreciates the hard work and dedication of the many authors whose papers appear in these special issues and especially that of J. Gretz who has so graciously accepted the responsibility of Guest Editor for these issues.

As sure as the sun shines, our need for electrical energy will continue to grow. Predictions of a levelling off of our requirements have not materialized and in many areas just the opposite has been the case. Since the embargo of 1973 more attention has been given to energy sources and the conservation of certain types commonly used by the public. Spot shortages coupled with ever increasing cost per unit of energy has stimulated a conscientiousness of conservation in the living, buying and driving habits of the public. Much more needs to be and can be done, but the masses of energy users must have an improved educational vehicle to enable them to practice energy conservation now that conscientiousness has been achieved. Conservation alone will not satisfy the needs for electrical energy in the future.

Oil will continue to be an expensive source of electrical energy. Coal is abundant in many areas of the world but its use involves high environmental protection costs. Proponents of nuclear energy find it more and more diffi-cult to see plants 'come on the line', especially in the United States.

The electric power industry throughout the world is therefore faced with a limited menu of boiler fuels, all at increasingly high cost. The economics of these fuels has motivated scientists and engineers, as well as ratepayers, to take a second look at solar energy. Solar energy is not free as many propo-nents would lead us to believe, but incurs very high conversion and storage costs.

Solar energy research is presently being conducted by numerous groups, either government or privately sponsored. A broad classification of solar en-ergy might include such non-depletable sources as wind, wave, ocean currents and tides, as well as those associated directly with solar radiation.

The papers in these issues deal with research on concentrating solar radi-ation on collectors and the conversion of solar energy to other forms, one of which is electrical.

GUEST EDITORIAL

The articles of this volume of Electric Power Systems Research are lec-tures which have been given within the frame of the ISPRA Courses, held at the Joint Research Centre of the European Communities, Ispra, Italy. They are organised in order to disseminate scientific knowledge in areas within the research programme of the Joint Research Centre and to exchange views with scientists of other organisations on an international basis.

After the flat-plate solar collectors for warm water production and house heating, solar power plants for electricity and process heat production are on the verge of entering the industrial stage. Experimental, demonstration and commercial power plants are now being built in the kilowatt to mega-watt range by several industries. The time is ripe, therefore, to exchange ex-perimental results and scientific and technical points of view on the matter.

Direct conversion by means of photovoltaic cells and thermomechanical conversion are the main candidates for solar power/electricity generation. In order to keep the solar collector field of the latter (mirror field) size reason-ably small, high efficiency conversion processes must be applied, i.e. use of high or very high working temperatures, which in turn requires concentration of the solar radiation.

Most solar power plants today are steam cycle systems. In order to im-prove on the potential and cost benefits, different technology/cycles should be investigated. At this moment, gas and hybrid gas/steam cycles seem to be very promising. Open cycle gas turbine plants need no refrigeration, which is an extra benefit in remote and sunny areas where the provision of cooling water may be a problem.

Solar thermal power generation is feasible today; no major scientific breakthroughs are required, but rather the development and cost reduction of more or less available components, as well as optimisation of the systems linking together those components: receiver, heliostat, prime mover, storage, heat cycle.

Gratitude is expressed to the authors of the course for their intellectual contribution as well as for preparing the material to be printed. Thanks are given also to the Head and all members of the Division Training and Educa-tion of the Joint Research Centre at ISPRA for the organisation of the course.

Joachim Gretz Joint Research Centre

of the European Communities Ispra (Varese), Italy

Electric Power Systems Research, 3 (1980) 3 1 1 Elsevier Sequoia S.A., Lausanne Printed in the Netherlands

Thermomechanical Solar Power Generation*

JOACHIM GRETZ Solar Energy Program/Project Directorate, CCR Euratom, 2102 Ispra, Varese (Italy)

SUMMARY

After considerations on the thermodynamics and technicalities of solar energy conversion into power, indicating the advantage of using high temperatures, the 1 MW(el) helioelectric power plant EURE LIOS of the European Communities is described.

The conversion of solar energy into hydrogen, and transportation of solar energy by means of hydrogen and/or hydrogenated fuels are discussed. The solar power plant is, at about 80% of the overall costs, the most costintensive component in the system comprising solar energy conversion into mechanical power, electrolysis, chemical reactor and transportation of chemicals over long distances (several 1000 km) over land and sea to the user's site. Liquid and gaseous hydrogen, methanol and ammonia are considered.

Industrial solar hydrogen production is discussed, especially by electrolysis, because of its present readiness for application.

Some qualitative considerations on the influence of largescale solar energy conversion on local climate indicate that there should be no heating up of the atmosphere above the ground but a slight cooling of the ground underneath the mirrors.

CONTENTS

1. Introduction 2. Thermomechanical power generation 2.1. Thermodynamic considerations 2.2. EURELIOS, the 1 MW(el) power plant

of the European Communities

Paper presented during a Course on Solar Thermal Power Generation held at the Joint Research Centre of the Commission of the European Communities, Ispra (Varese), Italy, in the framework of ISPRA Courses, September 3 7, 1979.

3. Solar energy transportation 3.1. Solar hydrogen 3.1.1. Solar hydrogen production 3.2. Storage 3.3. Transport 4. Costs and learning 4.1. Learning 4.2. Costs 5. Influence of largescale power plants on

local climate 6. Summary and outlook

1. INTRODUCTION

Solar energy seems to be the best candidate for a renewable energy source because its high thermodynamic potential allows its conversion into heat, electricity and synthetic fuels, i.e., all forms of energy which are convenient for man's use.

It is useful to recall some figures on the potential of solar energy, land use and material requirements. If Europe's present electricity consumption of about 1.2 X1012 kWh/y were to be met by helioelectric power plants ( = 20%) with European insolation (3 kWh/ m2 per day), about 0.5 1% of Europe's land would be required. Interestingly enough, about the same amount of land is used for our roads (2.8 X 106 km). Of course, it would not make much sense to ruin our culturally unique and beautiful Europe for energy production; there are many deserts in Africa from which it would be preferable to import energy to Europe to the advantage of both continents.

As for the frequently mentioned extremely high material requirements of helioelectric power plants, I quote some U.S. (MacDonnelDouglas) figures. The material requirements to construct 10 GW(el) solar power tower plants per year which corresponds in energy production to 2 3 large

1 GW nuclear power plants per year would be about 1.4 X 106 tonnes of steel and 0.8 X 106 tonnes of glass, i.e., about 0.5% of Europe's yearly steel and mild steel production and probably 6% of its glass production. For comparison, European automobile production requires about three times the abovementioned steel production.

The cost of a solar power plant today is 10 ECU/W(el) (e.g., EURELIOS, the 1 MW(el) helioelectric power plant of the European Communities). U.S. estimates forecast costs of about 1.2 U.S. $/W(el), corresponding to an electricity cost of 80 milis/kWh (at an annuity of 0.15) by 1990.

I. personally, shall make no projections because I would not know on what grounds, by when, or whether at all, these energy costs will be competitive with other energy resources. Besides, I have my doubts about those low investment costs of 1.2 $/W(el). But we shall have a look at cost reduction due to learning later on.

2. THERMOMECHANICAL POWER GENERATION

2.1. Thermodynamic considerations A useful solar energy conversion efficiency

should be the product of the exergetic, collector and cycle/mechanical efficiencies: ^tot l e x ^coll^cycle/mech

The energy, i.e., the work fraction of the heat energy,is

T Tt dEx = dQ 1 u For 1 J of energy and for a coolingwater

temperature of 30 C, as a function of temperature : 5760 (solar surface temp.)

823 K(~550C) 333 K ( ~ 60 C)

Ex = 0.947 J Ex = 0.63 J Ex = 0.09 J

Then, with a typical flatplate collector efficiency (?coU = 0.5), mirror/receiver efficiency (? = 0.75) and cycle/mechanical efficiency ( = 0.35), the conversion efficiencies and land use for power production with a flatplate collector system and a concentrating system are shown in Table 1. This Table is selfexplanatory. I hesitate to set down these

TABLE 1 Efficiencies and land use of power production systems

System T 7 C X T}coU 7cycle/mech ^tot mcoll/ "mcch

Flatplate 0.09 0.5 0.35 1.57% 73 Concen 0.63 0.75 0.35 16.5% 7

trating

obvious Carnot considerations. The reason I do it is that there are still people and industries thinking of and even doing mechanical power conversion with flatplate collector systems.

2.2. EURELIOS, the 1 MW(el) solar power plant of the European Communities [1]

Within the framework of its solar energy R & D programme, in 1975 the Commission of the European Communities decided to build a helioelectric demonstration plant of rather large rating. The size of 1 MW was the result of reasoning that for useful monetary outlay the smallest apparatus should be built, the technology of which would be intrinsically capable of being extrapolated to increasingly higher ratings. At that time, the first evaluative studies indicated the crossover point of investment cost and energy price between distributed systems and power towers to be at about 500 700 kW. To be sure of the correct choice and in order to have a representative rating, 1 MW(el) seemed to be a sound size.

A European Industrial Consortium was set up for the layout and construction of the plant, consisting of:

ANSALDO SpA and Ente Nazionale per l'Energia Elettrica (ENEL), Italy,

CETHEL (combining Renault, FiveCailBabcock, SaintGobainPontMousson and Heurtey S.A.), France,

Messerschmitt Boi ko wBlohm (MBB), Germany.

The special role of ENEL within the project must be mentioned: ENEL not only acts as a member of the Industrial Consortium, but also as 'host partner'. As such, it shall make available, free of charge to the contracting parties and to the EEC, the site and site preparation, the infrastructure and the connection to the electricity grid, as well as a certain number of other related services.

The site of the power plant is at Adrano, a village 40 km west of Catania, Sicily. It has an average elevation of 220 m, a north-south inclination of 5%, and lies near a small river.

The project costs have been evaluated at about 10 million ECU. According to the rules of the Commission, half of the costs are borne by the Commission, the other half by the contractors themselves, or their respective governments.

The technical details of the power plant are shown in Table 2, and the project is described elsewhere in this issue (p. 13).

3. SOLAR ENERGY TRANSPORTATION

3.1. Solar hydrogen One answer to the question of what can be

done with solar power, except electricity pro-duction, in order to have it in a convenient and stored form and to transport it over long distances from sunny countries into Europe, is to convert it into hydrogen and other hydrogenated fuels. A detailed discussion of solar energy conversion into hydrogen has been given elsewhere [2, 3 ] . It should be suf-ficient here to recall the unique and universal properties of combustion, energy storage and energy transportation of hydrogen.' It may be worth while, however, to expose some of the reasoning leading to the solar hydrogen concept.

To start with, hydrogen is not a primary, but a secondary energy source and has there-fore to be produced. If water is used as the primary material and solar energy as the primary energy source, then the concept of a universal, clean, inexhaustible fuel for man-kind would be perfect. The most difficult and also the most important problem is the pro-duction of hydrogen.

In a solar energy driven Europe there will be roof and garden space available for decen-tralized energy production, but it is estimated that this will only meet, at most, 5 - 10% of the primary energy consumption. And it would be senseless to sacrifice the precious and culturally unique Europe for additional collector placing. Besides, studies indicate that it would be cheaper to ship solar energy in the form of hydrogen or hydrogen-derivated fuels, like ammonia or methanol, through hydrogen pipelines or by tankers to Europe from the sunny regions of the world,

rather than use Europe's rather poor sun. Gaseous hydrogen may be stored in under-ground porous structures, originally filled with water (aquifers) which would be dis-placed by the pressurized hydrogen, or in depleted oil and gas fields.

3.1.1. Solar hydrogen production [ 2 - 6 ] The splitting of water requires either free

energy or high temperatures [7] . Free energy may be delivered by light quanta (photolysis) or in the form of electricity. The latter requires an electricity conversion device which may be of photovoltaic or thermo-mechanical nature. The situation today is as follows.

Electrolysis. Electrolysis is a proven and convenient way of producing hydrogen. If the development of very high temperature electrolysis (800 - 1000 C) is successful, heat-assisted electrolysis with electric efficiencies of 100% or more may be attractive in connec-tion with thermomechanical helioelectric conversion.

Thermal conversion. Extremely high temp-erature (~3000 C) direct decomposition (thermolysis) is thermodynamically interest-ing, but is, for the time being, technologically not feasible. The use of thermochemical cycles is mainly a question of economics and of adaptation to the high temperatures attain-able with solar concentrating devices.

Quantum conversion. The thermodynamic quality of light makes quantum conversion highly attractive, though much basic research is required.

Bioconversion. Biosystems are already operating in nature but with extremely low efficiencies, bioenergy systems would seem to be an attractive way of fuel production.

Electrolysis being closest to the eventual industrial production of solar hydrogen, the thermomechanical way of electrolytic hydrogen production will be discussed briefly. Electrolysis is an attractive process for the production of hydrogen in that it is a known technique, generates hydrogen separately, and allows discontinuous operation and hence the use of solar energy without storage.

Today's modern electrolysers have electric efficiencies of 75 - 80%, 90% efficiency is in view, and hydrogen production by pure elec-trolysis with solar energy is only a question of component price. If advanced high temper-

TABLE 2 Technical parameters of EURELIOS

General characteristics Experimental plant of central receiver-multiheliostat type Location 37.5 N, 15.25 (Sicily, Italy) Design point: equinox noon and assumed insolation of 1000 W/m2 Power generated: 1 MW(el) into existing grid at design point

Design parameters (power figures at design point) Heliostat field: Heliostats: CETHELtype: MBB type:

Receiver/tower:

Steam cycle:

Thermal storage: Energy storage: Equipment:

Electrical system:

4800 kW(th) to receiver two types, two axiscontrolled, overall inaccuracy 4 mrad (1) ca. 52 m2, 8 focusing modules, 70 heliostats ca. 23 m2 , 16 square elements, 112 heliostats Cavitytype receiver, 4.5 m aperture in 55 m height, 110 inclination. Receiver outlet steam conditions: 512 C, 64 atm, 4860 kg/h (5346 kg/h possible) Turbine connected to receiver (no intermediate heat exchanger). Nominal power: 1200 kW(mec) with steam of 510 C, 60 atm. Feedwater temperature at receiver inlet: 36 C. Cooling water temperature: 25 C max Reduced electrical output for ca. 30 min Vapour 300 kWh; Hitec: 60 kWh Pressurized (19 bar) water reservoir for 4300 kg. Vapour produced from 19 to 7 bar. Two storage tanks, containing 1600 kg Hitec (overall capacity). Heat exchangers for 19 bar, 480 and 410 C steam temperature Power generation: alternator for 1100 kW min for ca. 100 kW/internal power and 1000 kW for external users. Transformers, emergency power supply. Interface to grid: equipment to connect transformers to public grid. Steamcycle control equipment. Command, operation and monitoring centralized in control centre

I'l l o o* kJ >.U J > * Ui

300 kJ

mol 200

WO dS J

mol

ABOVE THIS LIN HYDROGEN AND

^\. BELOW THIS LINE NO HYDROGEN CAN BE PRODUCED

'

1

0

W

ELECTRICITY IS GIVING OUT EX

BETWEEN THE TV AND HEAT A CREATE HYDRi

w-

^

PRODUCING 1 ESS HE

1 ( nntat | H,

iE LINES BOTH ELECTRICI-IRE BEING USED TO

M S =^G

^ s9 ^ ^ d G ,

7000 2000 3000 iOOO

Fig. 1. Thermodynamics of electrolytic water splitting.

ature electrolysers are successfully developed, they may prove to be of interest in connection with thermomechanical helioelectric conversion (see Fig. 1) [8, 9 ] .

High temperature steam produced in the boiler of a power tower would be used partly in turbine expansion for electricity production, and would partly be bypassed and used to supply the TAS heating of the electrolyser.

With increasing temperature the dissociation voltage decreases, according to

dE,'dT = 0 . 2 5 mV/C At a temperature of 1300 K, for instance, the voltage would be 0.88 V, with the corresponding TAS heat requirement of 1.49 0.88 = 0.58 V, since the thermoneutral potential at 1300 is 1.49 V. This heat equivalent of 0.58 V, compensating for the cooling tendency of the cell, can be induced by any external heat source. Since this energy is heat, it is not subjected to Carnot reduction and

the conversion operation becomes thermo-dynamically advantageous.

For an operation temperature of 1300 and with theoretical values of r?m = 1 , over-potential = 0 V, and with Q as thermal heat flow, the turbogenerator heat flow would be

\ I X100 = 88%

and the influence of such a hybrid system on the production cost of hydrogen, as compared to normal electrolysis, for the above case would be

= 3 '(^

where = H2 production cost difference between normal electrolysis and hybrid electrolysis (milis/kWh), a (= 0.16) = annuity (_1 F ir 0.12) = enthalpy fraction, Sy (= 2300) = yearly sunshine hours (h y _ 1 ) , t'i (= 200) = investment cost of solar thermal plants ($/kW(th)), i2 (= 120) = investment cost of the turboalternator ($/kW(el)), j 3 (= 120) = investment cost of the electrolyser ($(kW(el)), (= 0.2) = electricity conversion efficiency.

The numerical values in brackets are estimates for 1985 1990 (note that il is the cost per thermal power) and may illustrate the size of the influence of the hybrid system on the costs. The cost of pure electrolysismade hydrogen would be

a X103

160

+ U + I.

2300 (1000 + 120 + 120) = 87 milis/kWh

Hybrid electrolysismade hydrogen would be cheaper by

160 = [0.12 (1000 200 + 120)] 2300 = 7.7 milis/kWh

Of course, a costbenefit calculation has to decide whether the energy cost gain compensates for the higher investment cost of the sophisticated high temperature electrolyser. In this context the bad amortisation conditions of solar energy components are stressed, indicating the advantage of having low investment costs rather than high performance.

3.2. Storage The delivery of continuous solar energy

may, but must not necessarily, be from storage. For instance, a hybrid solar energy power station, operated in conjunction with a fossil fuel system for 24 h/d plant operation, would ameliorate the bad amortisation conditions of power plants and would be cheaper than storage; this is certainly an attractive concept, especially in these first decades of transition from fossil to renewable fuels.

Any terrestrial solar energy conversion device suffers from the bad amortisation of its investment costs owing to its low annual load factor, which is 25% at best, the average being between 15 and 20%. Storage does not present a solution to this problem since the plant layout is powerproportional rather than energyproportional; in fact, the whole powerproportional solar collection part heliostats, tower, receiver, primary piping, representing, say, 60% of the total plant is badly amortised in either case, with or without storage. Any storage device would only lead to savings on the remaining 40% of the power plant. There are also the investment costs of the storage system to take into consideration. Since any storage device is more expensive than an oil burner, shunting of the latter is a more economic solution the solar energy conversion device can be seen as shunted to the nuclear or fossil plant, rather than the other way round.

In that light, hydrogen as a secondary energy source is certainly a potential means of energy storage, but its most attractive feature is that it forms a genuine primary energy source since it originates from water and is split by solar energy.

3.3. Transport As a solar energy vector, hydrogen is not

the only one which should be considered, but also other hydrogenderived fuels such as ammonia and methanol.

A study presented at Ispra [10] on the economics of production and transportation of hydrogenrich fuel from production sites 1000 5000 km distant from the user's site shows a rather interesting result. Whatever the energy carrier is, i.e. gaseous hydrogen, methanol or ammonia, the conversion of solar energy into electricity and then into hydrogen is by far the most costly process in the whole

chain. With assumed solar electricity costs of 80 milis/kWh, the (normalized) methanol at the consumers' site, 5000 km from the production site, would cost 108 milis/kWh.

The storage and transportation costs of the chemicals are a small percentage of the overall costs at the utilization site for methanol and ammonia (0.25 0.55% for methanol, 0.6 1% for ammonia, dependent on the route length). For liquefied hydrogen this percentage rises to 2.2 5% owing to the high cost of the ship and to the low energy density of the cargo. For an electricity generating cost of 80 milis/kWh, there is only a marginal difference in delivered energy cost between the different chemicals considered.

Another study [11] produces similar results, comparing alternative forms of energy transmission from ocean thermal energy conversion (OTEC) plants, i.e. gaseous and liquid hydrogen, ammonia, methanol, gasoline and methane. It defines the overall efficiency of conversion, storage and transportation of OTEC mechanical energy into chemical energy and its conversion back into electricity at the user's site. Interestingly enough, the study assumes an OTEC shaft horsepower cost of 20 milis/kWh. I, personally, have my severest doubts on such optimistic low energy costs from any solar energy conversion process, even in the distant future.

As an appendix almost, a thought should be given to enriched uranium, a 'black sheep' in the solar family, as an energy vector. In the search for a high energy density vector this would be a good candidate. Assuming a burnup of 30 MWd/kg of heavy material and separation work of 140 TI/TWh, the energy density of enriched uranium would be about 1000 times higher than that of hydrogen, or 3000 times higher than that of gasoline.

4. COSTS AND LEARNING

4.1. Learning Except for basic researchintensive R & D ,

like for instance quantum conversion and direct photolysis for hydrogen production, the cost of solar energy conversion is a question of component costs. The constituent components (photocells, mirrors, heliostats, receiver, turboalternator, electrolyser) have a price, a lifetime and an operating efficiency and the energy costs can then be calculated as

a function of insolation and financing conditions.

The cost of any component at an early stage of a new technology enjoys a reduction due to learning. The learning process is usually expressed as the diminution of production cost of an item with increasing production: c = kfn where c = relative cost of item k = constant f = learning factor (0.5 < f < 1) = number of integral production doublings

= 1(/0) In 2"1 (because 2" = a/a0) a0 = number of items produced at time 0, a = number of items produced at the time

under consideration Assuming that at present there are 5 MW(el) installed or nearly completed, and 50 MW(el) by 1990, i.e. a/a0 = 10, then = In 10(ln 2) 1 = 3.3 and c =0.5

The unit cost by 1990 would therefore be down by a factor 2. The learning factor was taken to be an average of 0.8 on the grounds that at the beginning the learning is faster, giving a smaller learning factor ( = 0.7), and gets slower with increasing maturity of the industry (f = 0.9).

The decreased unit cost by a factor 2 due to learning for a tenfold increase in unit pro-duction is small compared with the wide spread of uncertainty of the projected cost for helioelectric power which was almost two orders of magnitude at the beginning of 1976 (Toulouse Conference). That suggests that one should be careful with projections for future solar power plant costs which take into consideration industry's learning, because the learning process discussed above is mainly due to learning in the mere production process of an already industrialized product. At an early stage of a new technique, however, funda-mental inventions may entail major break-throughs in production techniques and there-with cost reduction by orders of magnitude. The cost goals of the U.S. and other countries for photovoltaics illustrate this situation [12]. Cost reduction by a factor 20 within 10 years would correspond to a millionfold

increase in production, or 20 doublings within ten years. However, with U.S. cost projections of 1500 $/kW(el), the peak energy would cost 96 milis/kWh. Without storage, the energy costs would be more like 80 milis/kWh, i.e. five times higher than today's electricity production costs. 4.2. Costs

Two remarks may be made with regard to costs. About half of the costs are for the mirror field, the size of which is inversely proportional to the plant conversion efficiency. Looking at the cost goals of photovoltaics, one is inclined to ask oneself whether there is any chance to improve those costs and, if so, by what measures. The high cost of the mirror field seems to indicate that here is the key to the cost problem, i.e. the reduction of the mirror field by increasing the plant conversion efficiency. Indicative figures show total conversion efficiencies of ~40% for gas turbine/steam cycle hybrid plants, whereas the efficiency of the 1 MW(el) power plant of the EC is still at 16%. Here is room for hope.

A more general view on the cost issue is interesting here. Automobile costs of, say, 100 $/kW are often quoted as an example of how 60 years of mass production can bring down the costs of such a complicated and complex thing like an automobile (see Table 3). In actual fact, the automobile costs are two orders of magnitude less than today's solar power plants and one order of magnitude less than the projected future ones. An objection would, of course, be that an automobile is built to run, say, 2000 hours in its life and a power plant 20 years, i.e. 100 000 hours. So, I inquired at a wellknown marine diesel factory [13] about the costs of diesel power plants working under power plant conditions. The difference is less striking here, yet one is still led to ask why complicated machines should cost less than simpler ones.

5. INFLUENCE OF LARGESCALE POWER PLANTS ON LOCAL CLIMATE

Some qualitative considerations on the influence of largescale solar energy conversion on the local climate show that there should be no temperature increase of the atmosphere above the ground where the collectors or mirrors are situated; there will be a small temperature reduction of the ground underneath the mirrors (Fig. 2).

Without power plant, albedo 20% With power plant, = 20*/ ( Simplif ied, wi thout atmospheric i n te rac t ion )

1 0 0 / 2 0

/ (A=0,5y)

8 0 = : 80

( = !0) Consumer ' 20 Solar power

plant I I

\ \

( )

Fig. 2. Energy balance at earth surface with and without solar power plant.

Without solar power plants, 10 20% of the incoming radiation is reflected from the earth's surface, according to the earth's albedo. The wavelength of that radiation is around 0.5 and so does not heat up the atmosphere. The other 80 90% of the solar radiation is absorbed by the earth, heats it up and is backradiated as infrared radiation at a wavelength set by the earth's temperature, i.e. Xmax = 10 . Thus, the earth and the atmosphere above it are heated up by 80 90% of the incoming solar radiation. For reasons of illustration, 80% is assumed here.

In the case of solar power plant operation, an amount of free energy proportional to the total conversion efficiency of the plant, say 20% of the incoming radiation, is transported away to the consumer's site, where it will be

TABLE 3 Comparison of specific costs

Solar, actual

k$/kW(el) 10 $/kg 10 (heliostats)

Solar, goal

1.2 3 (heliostats)

Automobile

0.1 3.2

Marine diesel power plant 0.75

10

liberated as heat. This quantity of heat is independent of the type of primary energy source; it will be waste heat in either case. The other 80% will be backradiated from the power plant as losses mirrors, receiver, Carnot in the form of infrared radiation. It heats up the atmosphere above the power plant but the earth underneath the mirrors receives solar radiation only inversely proportional to the ground cover ratio. Owing to the effect of heat conduction from outside the mirror field, there should be only a slight cooling effect on the ground, however. Thus, the sun sees the receiver black in the mirrors, but the optical blackness does not correspond to the thermodynamic blackness. The latter is only black to the degree that the power plant produces free energy, which equals, in our example, the earth's albedo.

6. SUMMARY AND OUTLOOK

To give some sort of outlook, I shall try to answer the tricky question by when solar energy will come seriously into play; and I shall take a rather pessimistic view.

There are inherent laws of marked penetration of different energy sources, worked out by ASA [14], indicating the surprising fact that the different primary energy sources have equal gradients of about 50% market penetration in 100 years, independent of infrastructural, technological or other changes of society (see Fig. 3).

There are at least two comments to be made. Firstly, the logistic model function, describing the market penetration over time, illustrated by the Sshaped curve in Fig. 4, holds for the takeover time period ts, in which the market share increases from 10 to

in'

m

Ili'

>n'!

F U S A

\ WOOD

\ C0*L

NUCLEAR / j.'

GAS / ysoLFus

/ Y ' \ / \ ' v\ v/ A/V\ xx \ 1750 1800 150 1930 1950 2000 2050 2100 2150 2200 2250

YEAR

Fig. 3. U.S. energy consumption from various sources. (From ref. 14, p. 213.)

Fig. 4. Time dependence of the market share f of a technology entering the market (according to the logistic function).

90%. But to reach the first 10%, it may take another 30 years or so for solar energy since the main parameter dictating the penetration is economic competitiveness, whether we like it or not.

The second remark is on that point. Whenever an energy source has displaced another one, it has been on the grounds of being cheaper. As for solar energy, it has a lot of qualities, but certainly not that of being cheap.

This being said, I do not think that solar energy and even under the assumption that it has not to compete from the year 2000 on with fusion or breeders will cover, say, 50% of our primary energy consumption in less than 100 to 130 years.

Of course, catastrophies may change all that: difficulties with OPEC, a serious reactor or burnt fuel accident, confirmation of C02 phenomena, etc.

In any case, penetration rates may change, but the fact that solar energy is expensive will not. One should have no illusions on that intrinsic fact. No technology will be able to alter the discontinuity and the weak energy density of terrestrial solar radiation. On the other hand, the time of cheap energy is over; the rare petroleum, the dirty coal, the dangerous or paradangerous atom will all require their toll and may make solar energy, one day in the future, a viable energy source.

REFERENCES

1 R. Floris and J. Gretz, La centrale solare da 1 MW(el) della Comunit Europea, Convegno sull' Energia Solare, Genova, Giugno, 1978.

11

2 J. Gretz, On the potential of solar energy conversion into hydrogen and/or other fuels, 2nd World Hydrogen Energy Conference, Zurich, August 1978.

3 J. O. M. Bockris, Energy: The Solar Hydrogen Alternative, Architectural Press, London, 1975.

4 T. Nejet Veziroglu (ed.), Hydrogen Energy, Part A, Plenum Press, New York, 1975.

5 J. Gretz, The conversion of solar energy without concentration, Energ. Nucl. (Milan), 21 (8/9) (1974)504510 .

6 NASAASEE, A HydroEnergy Carrier, Vol. II, 1973.

7 G. Porter and M. D. Archer, In vitro photosynthesis, Interdiscip. Sci. Rev., 1 (2) (1976).

8 H. Matthfer, Die Ntzung der Solaren Strahlungsenergie, Umschau, FrankfurtonMain, 1976.

9 J. E. Funk and R. Reinstrm, Energy requirements in the production of hydrogen from water, Ind. Eng. Chem. Process Des. Dev., 5 (3) (1966).

10 G. Beghi, A. Broggi, G. De Beni, G. Giacomazzi and J. Gretz, Solar heat and synthetic fuels: production and transportation of hydrogenrich chemicals, International Symposium on Solar Energy, Cairo, June 1622, 1978.

11 A. Talib et al., Alternative forms of energy transmission from OTEC plants, International Solar Energy Congress 1977, New Delhi, January 16 21, 1978.

12 L. M. Magid, The current status of the U.S. photovoltaic conversion program, Colloque International sur l'Electricit Solaire, Toulouse, March 1976.

13 MAN, Die Wirtschaftliche Energieerzeugung mit Dampfturbinen, Dieselmotoren, Gasturbinen, Maschinenfabrik AugsburgNrnberg.

14 W. Hfele et al, Second Status Report on the HASA Project on Energy Systems, 19 75, RR761, International Institute for Applied Systems Analysis, Sachsenburg, Austria.

Electric Power Systems Research, 3 (1980) 13 - 24 Elsevier Sequoia S.A., Lausanne Printed in the Netherlands

13

The 1 MW(el) Experimental Solar Power Plant of the European Community*

J. HOFMANN

Messerschmitt-Blkow-Blohm GmbH, Space Division, Postfach 80 11 69, 8000 Munich 80 (F.R.G.) J. GRETZ

Solar Energy Program/Project Directorate, CCR Euratom, 2102 Ispra, Varese (Italy)

1. INTRODUCTION

When in 1976 the EEC placed a contract for a system definition study of a 1 MW(el) solar thermal power station, a great deal of informa-tion was already available, suggesting that the central receiver-multiheliostat concept is right for a power station which shall provide peak power of 900 -1000 kW(el) and more [1 ] .

The basic experimental and theoretical work to reach this conclusion was done in France and Italy, by Trombe and Le Phat Vinh [2] in Odeillo and by Francia [3] in San Ilario near Genoa, long before the dramatic increase of oil prices in 1973 urged serious study of alternative energy sources.

After 1973, national and international au-thorities started comprehensive development programmes to explore and to implement techniques for the utilisation of solar energy. In particular, the programmes in the U.S.A. provided results on the various technical and economic aspects of solar power stations [4] .

With the advice of Francia and on the basis of other work (see ref. 1 and references therein), the system definition study for the 1 MW(el) solar thermal power station of the EEC was performed with the participation of British, French, German and Italian industry. Its results formed the basis of the project EURELIOS, the 1 MW(el) solar power station, which is now under construction by a Euro-pean industrial consortium. By the end of 1980/beginning of 1981, the plant shall be ready for an experimental programme, which foresees the operation of the plant and its

*Paper presented during a Course on Solar Thermal Power Generation held at the Joint Research Centre of the Commission of the European Communities, Ispra (Varese), Italy, in the framework of ISPRA Courses, September 3 - 7, 1979.

ability to deliver the energy produced into a public grid. It will be the first solar thermal power plant in the world with this operational capability [5, 6 ] .

2. ORGANISATION AND TIME SCHEDULE

The 1 MW(el) solar thermal power station EURELIOS is part of the so-called 'Programme B' within the Commission's Energy research and development programme section Solar Energy. As the project is done by industry and not by an institute of the EEC, only 50% of the programme is financed by the EEC. The other half is financed by the participating countries, the Federal Republic of Germany, France and Italy, through the ministries con-cerned.

The participating firms are ANSALDO and ENEL (Genoa, Italy and Milan/Rome, Italy, respectively),CETHEL (Paris, France) and Mes-serschmitt-Blkow-Blohm (MBB) (Munich, Germany). A consulting contract with the British firm GTS was concluded by the EEC for support supervision of the development work. The Italian firm ENEL acts as the host partner; the plant will be erected on ENEL's site at Adrano, Sicily, in the vicinity of Cantania.

The main areas of responsibility for the firms of the consortium are:

ANSALDO: receiver, steam cycle CETHEL: heliostat (CETHEL-type), heat

storage, electrical system ENEL: host partner: site with neces-

sary infrastructure, buildings, advice concerning Italian tech-nical regulations

MBB: heliostats (MBB-type) The firms have set up a management commit-tee which takes all necessary managerial and system-oriented decisions.

14

A detailed diagram of the organisation is given in Fig. 1, including the supervising Helioelectric Plant Consultative Committee, the project manager of the Commission and the British firm GTS as consultant to the project manager.

The overall programme was arranged in the following way.

Phase A, Feasibility study and system definition, concerning:

Definition of operating conditions. Layout of system and components. Preliminary subsystem and component

specifications.

Preliminary cost estimate and planning for phases and C. Phase A was completed in 1976.

Phase Overall system definition according to

results of phase A. Overall engineering design. Implementation of a management concept

of the plant. Planning and cost estimates. Detailed engineering and manufacturing

specifications. Phase took place between November 1977 and November 1978.

Consulting Firm GTS / UK

ITALI

ANSALDO / Italy Project Leader

Management System Engineering Receiver Steam cycle and Storage Interface Electrical System

{Part) Instailation Work

Direction XI1 Directorate General for Research, Science and Education

Germany BftfT

I t a l y

Hel ioe lect r ic Plant Consultative Committee

Project Leader

Management Committee Consortium of: ANSALDO / ENEL CETHEL / MBB

.J ITALY

CETHEL / France Project Leader

ENEL / Italy Project Leader

Management System Engineering Heliostats Typ 1 Tracking Master Safety Control Electrical System

(Part) Instailation and Commissioning Heat Storage

Management System Engineering

Observer Status for industrial partner

GERMANY BMFT via "Projektbegleitung"

MBB / Germany Project Leader

Consortium Members Host Partner

Management System Engineering Heliostats Typ II Tracking

Instailation and Commissioning

Commissioning Land Civil Engineering Infrastructure

Advice cone. Local and National Construction Rules

Communication Lines Organisation's Appointments for the HPCC

Fig. 1. Organisation for the implementation of EURELIOS.

15

Phase C - Construction and installation. - Acceptance testing.

Duration, November 1978 - November 1980. Phase D - Testing and experimental work. Phase E - Testing and potential modifications. At present we are in phase C of the project,

at a stage when civil works at the site at Adrano, Sicily, have started and when manu-facture of all parts is in progress.

3. TECHNICAL CONCEPT AND SYSTEM SUMMARY

As a result of phase A, a system for the 1 MW(el) solar power plant has been defined. This system is the basis for the design and development work of the present phase. A schematic diagram is shown in Fig. 2. It repre-sents the typical layout of a central receiver-multiheliostat system. Its major subsystems are:

heliostat field receiver and tower electrical power conversion system with - steam cycle including turbine, condensor, pumps, valves, etc.

- electrical system including alternator, transformers, etc. thermal storage

Two main aspects are important for this par-ticular project:

(1) Operational requirements: The plant is not only a facility with which to study the performance of certain components, e.g. helio-stats and a receiver, but it will produce elec-tricity and will deliver this electric power into a public grid. Such a demonstration power plant will allow the study of electric power generation and the grid interface of a solar power plant and is therefore a step beyond facilities where strictly development work is done and where the thermal energy generated by the sun is not used further. For the dem-onstration plant, a storage capability of 1/2 hour appears to be reasonable.

(2) Receiver technology development by Francia: The receiver design is a cavity-type receiver, operating at approximately 500 C and using water/steam as a working medium. This design allows the use of a well-known simple power conversion circuit: use of water, possibility of feeding the turbine directly from the receiver, utilisation of conventional com-ponents.

Although it is acknowledged that systems with higher operating temperatures will oper-

INCIDENT SOLAR ENERGY . RECEIVER SUBSYSTEM

CENTRAL RECEIVER SOLAR THERMAL POWER SYSTEM

Fig. 2. Schematic diagram of EURELIOS.

16

ate more efficiently, the present system offers the advantage of a low development risk for the whole plant. The receiver in particular has been studied in several models of smaller size by Francia and ANSALDO and is not considered to represent development problems.

The selection of the receiver characteristics has immediate consequences on the heliostat field, i.e. its shape and dimensions and the pointing accuracy of the heliostats.

4. DESIGN CHARACTERISTICS 4.1. System parameter summary

The design of the 1 MW(el) solar power demonstration plant EURELIOS uses as key parameters the characteristics shown in Fig. 3.

A forecast of the overall energy balance on the basis of the nominal operating conditions is given in Fig. 4. It is obvious, however, that for conditions other than nominal (e.g. other values of insolation, other positions of the sun), different efficiencies will be experienced. One goal of the later experimental phases is to correlate experimental results with theoretical forecasts for the efficiency with respect to the power output.

4.2. The heliostat field Two types of heliostats will be used in the

project, one provided by MBB (Fig. 5), one by CETHEL (Fig. 6). The task of the heliostats is to reflect the rays of the sun into the aperture of the receiver, independent of the time. This is accomplished by a socalled tracking system (Fig. 7). In both designs a computer calculates the position of the sun and the angles for azimuth and elevation required. DC motors bring the heliostats into the predetermined position; deviations from the nominal positions will be detected by encoders and will be corrected as required.

Each single heliostat will have focusing capability, in fact each of the mirror elements will be bent: in one dimension in the case of CETHEL's, in two dimensions in the case of MBB's. A comprehensive development programme gave the assurance that the series of heliostats will meet the accuracy requirements of 4 mrad (1 ) at a nominal wind speed of 18 km/h.

Although MBB and CETHEL heliostats apply the same design principles and are subject to the same system requirements, they differ in many details, e.g. with respect to dimen

General characteristics Experimental plant of central receivermultiheliostat type Location 37.5 N, 15.25 E (Sicily, Italy) Design point: equinox noon and assumed insolation of 1000 W/m2 Power generated: 1 MW(el) into existing grid at design point

Design parameters (power figures at design point) Heliostat field: Heliostats: CETHEL type: MBB type:

Receiver/tower:

Steam cycle:

Thermal storage: Energy storage: Equipment:

Electrical system:

4800 kW(th) to receiver two types, two axiscontrolled, overall inaccuracy 4 mrad (1) ca. 52 m2, 8 focusing modules, 70 heliostats ca. 23 m2 , 16 square elements, 112 heliostats Cavitytype receiver, 4.5 m Q) aperture in 55 m height, 110 inclination. Receiver outlet steam conditions: 512 C, 64 atm, 4860 kg/h (5346 kg/h possible) Turbine connected to receiver (no intermediate heat exchanger). Nominal power: 1200 kW(mec) with steam of 510 C, 60 atm. Feedwater temperature at receiver inlet: 36 C. Cooling water temperature: 25 C max Reduced electrical output for ca. 30 min Vapour 300 kWh;Hitec: 60 kWh Pressurized (19 bar) water reservoir for 4300 kg. Vapour produced from 19 to 7 bar. Two storage tanks, containing 1600 kg Hitec (overall capacity). Heat exchangers for 19 bar, 480 and 410 C steam temperature Power generation: alternator for 1100 kW min for ca. 100 kW internal power and 1000 kW for external users. Transformers, emergency power supply. Interface to grid : equipment to connect transformers to public grid. Steamcycle control equipment. Command, operation and monitoring centralized in control centre

Fig. 3. Main design characteristics of EURELIOS.

17

ti^lt? H*

Cosin

Solar power f a l 1 mg on nomina 1 rror surfai_e

S96 KW

RefleL shad in b l o c k i

>

Solac power i n t e r c e p t e d by the l t ' r o r s

4800 \i

t i v i t y H e l i ] and racy "> Reco

nte >

Solac power r e f l e c t e d to the r e c e i v e r

4676 KW

i s t a t Accuver Recei cept E f f i c

y -

Solar Power e n t e r i n g the r e c e i v e r

444? KW

er Cycle ency E f f i c

>

The m a l power to steai'i

I n t e r n a l ency Power Consumption > >

1100 KW , ,,

El . power a t genera tor bus bar

El . power tn g r i d

P0ER PLA'IT E'.ERGV BALANCE REQUIREMENT ' , 1 Condi t ions )

',injl Conditions: equinox noon, 37,b north, 1000 ' insolation, wind speed ^ 18 km h

Fig. 4. Power plant energy requirements. (For different operating conditions, other efficiencies will be experienced.)

sions, mechanical/optical characteristics, and the control and tracking system.

To study the behaviour of the two types of heliostats, advantage is taken of the fact that one half (east/west) of the heliostat field has essentially the same operational characteristics in the morning as the other half in the afternoon. Therefore the field is subdivided into a west field (CETHEL) and an east field (MBB). As the CETHEL heliostats will provide 57% of the power, some of them are also placed east of the centre line, where the MBB heliostats are essentially positioned.

The power contribution of a single heliostat, located at a certain position and considered at a certain time, depends on a variety of parameters illustrated in Fig. 8. The final layout has been evaluated taking into account the abovementioned aspects. The result is shown in Fig. 9. A description of the MBB computer program applied and of details of the calculations is given in ref. 7 and in a separate paper of this issue [8 ] .

4.3. Receiver and tower The receiver technology and its influence

on the system layout have been briefly discussed in section 3. The receiver was designed by Francia together with a team of engineers at ANSALDO and provides steam at 510 C and

64 bar (at nominal design conditions), the water inlet temperature being 36 C.

Extensive testing of a receiver model (Fig. 10) confirmed the design assumptions for the oncethrough boiler. A schematic diagram of the final receiver is given in Fig. 11.

The water/steam is guided through the boiler by two parallel pipes. The preheating zone is in the centre of the receiver, directly exposed to the incoming radiation flux, the boiling zone forms the wall, and the superheating zone is at the back of the receiver, protected against the direct radiation. Mineral wood protects the receiver against losses through the wall, and Pyrex tubes inside the boiler have the task of equalising the radiation energy density inside the cavity receiver.

The receiver is mounted at a height of 55 m on top of a steel tower.

The receiver control is included as a part of the control of the total steam cycle (see ref. 9).

4.4. Thermal cycle A schematic diagram of the steam cycle is

given in Fig. 12. The regulation of a water/steam cycle has

been described recently by Francia [9] . Under a wide range of insolation conditions the te

18

Heliostat ;or,i.o-J:it - Mirror

Mirror Structure

Dr i ve uni t inel. motors,gears 1 bear i ng for az :muUi

2 bear inqs for elevation

Column Foundation

Fig. 5. MBB heliostat and technical data: mirror surface, 23 m 2 ; mirror elements 16 elements, flat, 3 mm float glass, 1.20 m Xl.20 m; total height, 5.48 m ; mirror + mirror structure height, 5.01 m ; width, 5.60 m ; weight, ca. 1500 kg; ground clearance, ca. 0.50 m; height of elevation axis above ground, 2.97 m.

perature of the steam provided by the receiver shall be kept constant.

The means to control the temperature are the mass-flow control by the feed pump and the steam-temperature control by injection of water. These water injections have, in particu-

lar, the task of fixing the geometric position of the boiling zone in the once-through type of boiler.

It should be noted that passage of a cloud is not a trivial operational mode: to prevent the boiler from excessive temperature increase

19

Fig. 6. CETHEL heliostat and technical data: mirror surface, 51.8 m2 ; mirror elements 48 elements, 6 mm float glass, 1.8 m X 0.6 m; total height, 7.87 m; width, 8.84 m weight, ca. 4900 kg; height of elevation axis above ground 4.2 m.

when the sun is coming back, the heliostats have to be successively deviated from the re-ceiver pointing orientation during the cloudy period.

A bypass system allows receiver testing without the turbine. Whereas the cycle control involves a rather complex regulating system, the steam cycle itself is based on conventional

components, e.g. the turbine is based on a design used in a similar form for auxiliary power generation in naval applications.

Although the turbine is capable of accept-ing short-term temperature variations of 50 C, it is intended that the turbine should not be subjected to too much stress due to short-term inlet temperature variations. It is, however,

20

Fig. 7. Heliostat tracking system (MBB type).

Reduced Beam cross section (Cosine Effect]

Blocking Hehostctt Imperfections Shading and optical Errors

Fig. 8. Radiation power collected in the central receiver.

Sun Radiation Flux Date Hour (Atmospheric Effects)

Receiver

Height

Aperture Inclination

(Max Temperature )

Heliostats Location Dimensions

Accuracy Reflectivity

Operational Conditions

( Wind I ( Turbidity of Atmosphere )

foreseen that the turbine should be operated in a continuous mode at any temperature be-tween 410 and 510 C. This is of particular importance when it is operating together with the storage system as the maximum tempera-ture of steam provided by the storage system is 430 C.

4.5. Thermal storage To operate the power plant for 30 minutes

without using steam from the receiver (e.g. during the evening or during cloudy periods), thermal storage capacity is provided (for nu-merical values, see Fig. 3). For this purpose, two types of equipment are used:

21

4 : * U

D D D D D D D D D D D O G D D

D D D D D D D D D DDDDDDDDD

D D D D G D D D D D D D D D D G D D D D D D D D D D D D D D D

O

DDDD DDDD DDDD DDDD D D D G DDDD DDDD DDDD DDDD DDDD

DDDDDD D D D D D D D D D G D D D D D D D D D D D D DDG DD D

5 * 5 * C t

Fig. 9. Field configuration.

h' . . I' " " J

Fig. 10. Receiver model from ANSALDO during tests at San Ilario, Genoa.

a storage tank with pressurised water, two storage tanks filled with molten salt

(mixture of 53% KN03 , 40% NaN02, 7% NaN03; tradename: Hitec).

The water tank is part of the thermal cycle subsystem. The principle of the salt storage system is shown in Fig. 13. To pump the liquid salt from the cold tank through the desuperheater into the hot tank (charging mode) a system with pressurised nitrogen is used (the

same applies for the analogous discharging mode).

The salt used is solid at ambient temperatures (freezing point 145 C), whereas normal operationing temperatures are between 480 and 240 C and lead to contamination if air (humidity!) can get access. Therefore important technological aspects are: avoiding component damage during melting/freezing, heating of the system, and avoiding contamination. This kind of technique has, however, been known for many years and is not expected to cause development problems.

4.6. The electrical subsystem The task of this subsystem is to convert the turbine shaft power into

electrical energy, to transform the voltages adequately for

internal and external users and to provide a distribution system,

to connect the plant to the public grid, to control the plant by means of the dif

ferent subsystem controls. Most of the equipment and many design

aspects are conventional. Special aspects, however, are the control cycles coping with the changing radiation intensities offered by the sun. These and the consequences of operation of the plant together with the grid will be studied experimentally during the next phases of the project.

22

0 4500

Fig. 11. Receiver design (ANSALDO).

Fig. 12. Schematic diagram of steam cycle (ANSAL-DO).

4.7. Site and civil work The site for the plant is in Sicily, close to

Adrano and near Catania. The site offers all facilities required (access roads, cooling water, grid, etc.). There is a slight slope to the south which is favourable because the shading and blocking effects of the heliostats are reduced.

The site layout, including the arrangement of the buildings, is presented in Fig. 14. Shown are the main elements of the heliostat field, tower and receiver, the machine house which also contains the heat storage system, and the main components of the electrical system in-cluding the control room. Attached to the ma-chine house there will be offices, etc.; a sepa-rate building provides a warehouse for storage of materials and equipment.

5. DEVELOPMENT STATUS

The project outlined in the preceding sec-tions is in its hardware stage, the so-called phase C. This means in particular that the civil works have already started at the site. The contracts have been placed for the manufac-ture of all hardware and work is in progress.

23

275" C 1672 kq/h

1010 kg/h J

I

300 C 315 kg/h h

16 bar 201 C

|1MI5 kg/h I V * ( | | ) 16 bar " 418 C

1W5 kg/h 50* C 3^ 15 kg/h ! \

II

I Cold tank I I Hot tank III Desuperheater exchanger IV Superheater exchanger V Nitrogen storage

Steam Hitec Nitrogen

E.E.C. SOLAR POVER PLAUT

CETHEL

WP 11IO SALT S70RA6

HOV 0IA6RAM

CAL/QUA P3 Ot UK *. *

Fig. 13. Molten salt storage (CETHEL).

The majority of the installation work will be done during the first half of 1980. We are confident that the plant will be ready for experimental work by the end of 1980.

REFERENCES

Vergleichende Analyse von Sonnenkraftwerkskonzepten im Bereich von 100 W bis 10 MW, compiled by MBB, May 1976, BMFT Vorhaben ET 4234 A, under contract from PLE Jlich. See also: P. Zahn, Conceptual design of solar thermal power plants (100 W 10 MW), Proceedings of UKISES Conference on Solar Thermal Power Generation, July 1978, UKISES, p. 24. F. Trombe and A. Le Phat Vinh, Thousand kW solar furnace, built by the NCSR in Odeillo, France, Solar Energy, 15 (1973) 57 61. G. Francia, Pilot plants of solar steam generating stations, Solar Energy, 12 (1968) 51 64.

4 L. L. VantHull and A. F. Hildebrandt, Solar thermal power system based on optical transmission, Solar Energy, 18 (1976) 34 ff.

5 A. Strub, The 1 MW(el) solar power plant of the European Communities, Proceedings of the International Symposium on Solar Thermal Power Stations, DFVLR, Cologne, April 1978.

6 J. Hofmann and J. Gretz, The concept of the 1 MW(el) solar thermal power plant of the European Economic Communities, 2. Internationales Sonnenforum/XII Zusammenkunft der Comptes, Hamburg, April 1978.

7 J. Hofmann and Chr. Kindermann, Heliostat field for central receiver solar power plants in the 1 MW(el)range, Proceedings of the Internat. Symp. on Solar Thermal Power Stations, DFVLR, Cologne, April 1978.

8 V. Hartung, Chr. Kindermann and J. Hofmann, The heliostat field layout of the EEC experimental solar power plant, Elect. Power Systems Res., 3 (1980) 7 7 8 9 .

9 G. Francia, Regulation of the watersteamcycle in a solar receiver, Rev. Int. Heliostech., Comptes, Marseilles, (1) (1979) 18 ff.

For Fig. 14 please see overleaf.

24

WtWAMtHK ah

Fig. 14. Layout of the site (ENEL): 1, CETHEL heliostats; 2, MBB heliostats; 3, tower and receiver; 4, machine room; 5, control room/service building; 6, warehouse; 7, parking lot; 8, main entrance; 9, transformers; 10, cool-ing tower; 11, protective wall.

Electric Power Systems Research, 3 (1980) 25 39 Elsevier Sequoia S.A., Lausanne Printed in the Netherlands

25

Layout, Application and Economic Efficiency of Solar Farm Systems*

J. E. FEUSTEL, O. MAYRHOFER and U. WIEDMANN Firma MAN (Entwicklung), Dachauer Strasse 667, Munich (F.R.G.)

1. INTRODUCTION

Small solar power systems of the solar farm type use decentralized absorbers and operate from temperatures of about 200 C up to 400 C. To generate these temperatures collector systems with concentration ratios of 30 up to 50 suns are used in most systems. Because of the principle of decentralised absorption solar tower systems use central absorbers with high light concentration optimum operating temperatures are about 300 C, and overall efficiencies of solar farm stations producing electricity are between 8 and 12%. Concerning optimum system sizes there are certainly considerable economic advantages for having solar farms instead of tower systems in the range 30 1000 kW, possibly even up to several MW.

Figure 1 shows the achievable temperatures and system efficiencies for several collector types and power conversion principles. The following considerations refer to the most typical solar farm, namely the system using parabolic trough collectors. After description of some major considerations for the layout of collector fields and prime mover, some examples of applications of process heat generators and electricity producing systems will be given. Finally, the economical potential of solar farm stations will be described.

2. LAYOUT OF SOLAR FARM SYSTEMS 2.1. Overall system description

Typical solar farm systems consist of the four major subsystems comprising collector

*Paper presented during a Course on Solar Thermal Power Generation held at the Joint Research Centre of the Commission of the European Communities, Ispra (Varese), Italy, in the framework of ISPRA Courses, September 3 7, 1979.

0 100 200 300 400 500 600 700 UPPER PROCESS TEMPERATURE ( C )

Fig. 1. Efficiencies of solar power stations.

field, storage, power conversion and cooling. Figure 2 shows a functional block diagram

of such a typical system. In the collector field the collectors or col

lector modules are grouped to loops which are normally to some extent autonomously operating subsystems. The collector loops feed their heat energy to the main pipes of the field. Heat is accumulated in storage tanks

I Collector Module 2Hot Storage 3Cold Storage

Fig. 2. Functional block diagram of a typical solar farm plant.

26

which might be a thermocline storage or a two-tank version with hot and cold tank. The power conversion system consists of the main components comprising steam generator, prime mover, condenser and electro-generator, and a number of other smaller mechanical and electrical components.

The cooling cycle uses wet or dry cooling units; owing to non-availability of water, in most cases dry cooling is applied.

Figure 3 shows an artist's view of a small experimental solar farm plant. This test centre near Madrid (Spain) is used for the compar-ison of several collector types and for the optimisation of overall farm systems for generation of electrical energy with 50 kW rated output. The left-hand side of the field contains collectors with one-axis tracking, the right-hand part shows six 32 m2 collector modules with two-axis tracking and one large module (150 m2) with azimuth-axis tracking. In the building and the container in the fore-ground the complete power conversion sys-tem and the data acquisition system are arranged. Beside the building a thermal stor-age tank can be seen, including the necessary field equipment like pumps and valves.

_ 015-

Fig. 3. MAN/Auxini solar test centre near Madrid.

Solar farm systems seem to be very suitable for power production at fairly small sizes, and even plants of some 100 m2 collector area can be advantageously applied. But power stations of far bigger sizes can also be built according to the farm principle.

Figure 4 shows possible overall efficiencies for various amounts of electricity generated. If the efficiencies of the power conversion systems are increased, the overall efficiency

0 05 Collector Concentration Factor C = 0 Condensation Condition lor PCS 50 C Thermal Energy Storage Included

001 01 1 10 100 Net Electrical Power Output [MW]

1000

Fig. 4. Efficiency of electrical energy generation with solar farm plants.

grows with greater performances. This tendency is valid up to several MW; at larger sizes efficiency drops owing to increasing losses in the heat collecting network. It is therefore an advantageous characteristic of solar farm power stations that they are as well adapted to small decentralised systems as they are to fairly large central stations.

To complete the picture of the overall sys-tem, Fig. 5 shows the type of energy conver-sion in the major subsystems. This typical example shows that the collector field produces about 52% of usable heat out of 100% insolated energy. The losses are in the form of optical and thermal losses in the col-lector and as thermal.losses in the field piping.

Direct insolation 100%

Collector field

56%

optical and thermal losses

U %

heat losses of field

Rower Conver-sion System

1%| cycle and mechanical losses

1 %

^ plant internal consumption

W 10% net electrical power

Fig. 5. Energy flow chart of a typical solar farm.

27

The power conversion system has a relatively high thermodynamic loss; in this case about 11% remains as electrical output. Some part of this power is used for internal consumption, e.g. pumps, so that finally about 10% overall plant electrical efficiency seems to be realistic. The achievable thermal efficiency for a process power generation system is about 50%; for the combined production of electricity and heat the efficiencies are, according to the above considerations, between 10 and 50%.

2.2. Collector and collector field The collector is the major subsystem of a

solar farm plant. Parabolic trough collectors are usually applied in today's solar farms. As well as the line focus mirror system, some farms use point focus systems like paraboloid mirrors, and some others use Fresnel lenses or Fresnel mirrors, mostly as line focus collector systems.

Figure 6 shows a typical linear focus collector of the trough type using glass mirrors. The picture shows a collector developed by Auxini, Spain, which uses a horizontal tracking axis. The parabolic shaped reflector concentrates the direct sunlight on the straight absorber tube. Through the absorber tube flows the cooling fluid. The selectively coated absorber tube is covered by a lighttransparent glass cladding in order to decrease thermal losses.

Fig, 6. Auxini parabolic trough collector.

Figure 7 shows schematically the energy conversion and energy transport in the collector and gives a general definition of the collector efficiency.

Energy Balance Collected _ Insolated _ Insolation _ Heat Heal Energy Losses Losses

QC = IS lL QL

Collector Efficiency 2C. = n = 1 Hn.T.p.r! HT,A.t|m,A,a') = --\ 's ' is is

. _ hsolation /Radiative +Convective \ ' " Losses \Heat Losses Heat Losses/

Optical Losses

Thermal Losses

Fig. 7. Collector energy transport and definition of efficiency.

Collector efficiency can be described as the ratio of heat collected in the heat transfer medium to insolated energy to the collector area.

The energy balance can be described as follows: Qc ~ Is L^ Qi where Qc = collected heat, Is = insolated energy, Ih = insolation losses or optical losses, Qh = heat losses.

The theoretical determination of collector efficiency is relatively complex, depends on a great number of factors and cannot be described explicitly. The major determining factors give the equation in Fig. 7.

The optical losses can be described mainly by

reflection losses in the reflector (reflectivity )

interception losses in the reflector (intercept factor )

transmission losses in the collector cover and absorber envelope (transmittances )

absorption losses at the absorber tube surface (absorptance a)

The second group of losses are thermal losses of the collector, mainly radiation and convection losses. These are caused by the heat fluxes which go from the absorber to the surroundings instead of to the heat transfer fluid. For both types of losses, absorber tube temperature and absorber surface area are the important factors of influence. In addition, the emittance for longwave radiation (e) and the heat transfer coefficient between absorber surface and thermofluid (a') are important factors.

28

Some of these factors which can be grouped into design parameters and operational parameters of the collector will be discussed in more detail.

Concentration ratio, mirror reflectivity, absorber coating selectivity, absorber insulation, and reflector/absorber interception are typical design parameters. Degree of insolation, wind speed, tracking mode and accuracy, dust and shading are typical operational parameters that influence collector efficiency.

The concentration ratio is one of the most important factors influencing collector efficiency. It can be described as

(dM dE) X 360 C = dATi2e where C = concentration ratio, dM = mirror aperture, dE = diameter of absorber envelope, dA = diameter of absorber tube and = rim angle (in degrees).

Figure 8 shows, for a nonselective absorber, the strong influence of concentration ratio on collector efficiency. For achievement of a 200 C absorber temperature at 50% efficiency, a concentration of 20 suns is sufficient; to heat a thermofluid or steam up to 300 C which is a normal requirement for a good solar farm overall efficiency a concentration factor of 50 is required with again 50% efficiency. The higher the absorber temperature required, the faster the value of the concentration ratio increases to achieve adequate efficiencies.

An important factor for collector performance especially at high concentration is the intercept factor, which comprises losses

, vu 60

> D L L ILI

COLL

ECTO

R

^ o

c

c

\ \

0 2C ABSORBER

yc= \

\ 3

TEMPER/

D=800 T=20

" 1

\ \ ( \ ^ '

20 \30

X) u VTURE TA(

W/M2 C

:0NCENTR 3ATI0 C

uo \so

X) 5C C) -

ATION

due to dispersion of the solar picture, positioning inaccuracies of the absorber and, predominantly, losses due to reflector inaccuracies.

Simplifying, one can say that the degree of interception gives the part of reflected solar radiation which reaches the absorber.

Figure 9 shows the measured intercept factor for a warm bended glass mirror. The diagram shows the increasing value of interception loss with growing concentration ratio. Because of the finite diameter of a solar disk of about 0.5 degrees, high concentration ratios lead to a rapidly decreasing intercept factor. Concentrators with intercept factors in the range of 98% at concentrations around 40 suns are of excellent quality and require very careful design. As pointed out before, for good collector efficiencies at higher temperatures of about 300 C, concentration ratios in the range of 50 (at a/e = 1) are required. The reason for this is mainly the thermal behaviour of the absorber. Thermal losses of the absorber to the surroundings increase proportionally with the heatemitting outer surface area of the absorber. For radiation and convection a similar behaviour is found. Thus, as high concentration means small absorber area, collector efficiency increases with concentration ratio.

1.0

oe

h 06 o

02

Design ft)nt: c = 41 y0.976

Experimental Dala. Measured by Laser Test Equipment (DFVLR)

30 50 70 Concentration Ratb c()

x

Fig. 8. Influence of concentration ratio on efficiency.

Fig. 9. Intercept factor for a typical collector.

Besides this, at higher temperatures the selectivity of absorber coatings plays a deciding role for efficiency. The selectivity of the absorber surface is described by the absorptance for solar radiation and the emittance e for longwave radiation.

29

Figure 10 shows the influence of a selective coating on the absorber tube at different temperatures for 40 suns. If a collector has to be operated at elevated temperatures such as 200 400 C, selective coating is of extreme importance.

Fig.

500

ABSORBER TEMPERATURE T A ( C )

10. Influence of coating selectivity on efficiency.

Going back to the above example of an absorber temperature of 300 C, collector efficiency increases with selectivity a/e = 1 to 5 from ? = 35 to 52%; in other words, efficiency increases by almost 50% with the use of selective surfaces. The increase of efficiency is especially strong at low selective values and at high absorber temperatures.

An overall view of the design parameters' influence on collector efficiency is given in Fig. 11. Starting from the design point (D.P.) of a given collector the diagram shows the gradients of efficiency change in relation to the change of a selected parameter. The individual points in this diagram indicate the practical potential of efficiency increase by changing design parameters. A sensitivity analysis leads to the result that the best realistic increases of efficiency can be achieved by improving mirror reflectivity and emissivity of the absorber tube.

Besides these design parameters, operational parameters play an important role. For the installation of a field the chosen collector tracking system is of great importance.

Figure 12 shows the rear side of a collector platform with a twoaxis tracking system. The photo gives an impression of the structural equipment of such a module. A rigidly mounted pedestal is used as a base for a

. Absorber Absorber

Envelope -f Envelopp

, ' ' X Reflector Absorber

Mirror

2

Fig.

0.6 07 0.8 0.9 1 1.1 relative Lhange of parameter /

11. Potential of collector efficiency increase.

Fig. 12. Rear view of 32 m2 collector module HELIOMAN.

able collector structure on which mirrors and absorbers are mounted.

Figure 13 shows the front view of the same module. The projected glass mirror area is about 32 m2; the insolated energy is focused with a concentration ratio of 42 suns to 4 absorber tubes.

The performance of this collector module is described by its efficiency characteristic which is shown in Fig. 14. The graph shows that even at elevated temperatures good effi

30

Fig. 13. Front view of 32 m2 collector module HE-LIOMAN.

400 500 600 700 800 900 1000 beam irradiant solar flux ensily Eb [w/m]

Fig. 14. Heat collecting efficiency of the collector module HELIOMAN.

ciencies for heating up the thermal oil in the absorber can be achieved. The abscissa of the diagram gives the solar flux density normal to the collector area. This value obviously has to be as high as possible to get good collector efficiencies. This aim can be realised in the best way by using heliostatic tracking systems.

Figure 15 shows the influence of the track-ing mode on energy output. The diagram compares three different one-axis tracking systems with heliostatic or two-axis tracking collectors. Using azimuthal tracking the col-lector is turned around a vertical axis; collec-tors with a horizontal tracking axis can either be arranged in an east-west or north-south direction. The graph shows the insolated energy and the useful heat produced by equal collector areas. The heat output is optimal for two-axis tracking collectors. This maximum energy output has to be paid with higher col-lector costs owing to the use of a more expen-sive tracking system.

N-S HORIZONTAL AXIS

EQUINOX 37 LATITUDE

6 8 10 DAY TIME ( H ) -

daily average energy at equinox

TRACKING

RELATIVE NORMAL INSO-LATED ENERGY

RELATIVE HEAT OUTPUT

INCLINED COLLECTOR 37AXIS

HELIOSTATIC

1007.

100 7.

AZIMUTHAL

94 7.

93 7.

HORIZONTAL COLLECTOR AXIS

N-S

87 7.

84 7.

E-W

74 7.

687.

Fig. 15. Influence of tracking mode on energy output for typical collector fields.

Besides tracking, the geographical latitude is of major influence on collector output. The climatic conditions of different geographical latitudes are certainly of great importance but the geometric data of the path of the sun also have a strong influence, even for collectors with two-axis tracking. At a low position of the sun, collector shading in a field cannot be avoided.

Figure 16 shows the heat losses due to col-lector shading for two-axis tracking collectors. The collectors are arranged according to the sketch in an orthogonal way. The centre dis-tance in the north-south direction is 10.5 m ; the abscissa shows the relative distance in the east-west direction. The data of Fig. 16 give typical arrangements for most of the two-axis

11 dEw Dstance Ratio NS

Fig. 16. Influence of module spacing and latitude on collector module shading losses.

31

tracking collector systems. At equinox conditions, therefore, 2 5% losses due to collector shading have to be taken into account.

For a special collector arrangement, Fig. 17 shows the influence of geographical latitude on shading losses. In the zone between 20 latitude north and south, heat losses are in the order of 2.5% and are not strongly dependent on latitude. At higher degrees of geographical latitude the losses due to shading play an increasing role, especially during winter months. During summer months a site of 40 latitude is not at a great disadvantage compared with a site on the equator.

assumptions 065 exp 20 bar 1^300 C l 2 '

Fig. 17. Influence of latitude and date on collector module shading losses.

2.3. Power conversion system (PCS) Solar farm power stations can, in the near

future, produce heat economically in a temperature range 200 400 C. If this heat is to be used in a thermodynamic cycle for producing mechanical and than electrical energy at the abovementioned temperatures, only vapour processes can be usefully applied. The selection of the right vaporizing and condensatihg medium is of great importance for solar farm stations.

Among a great variety of media, Fig. 18 compares three typical ones which seem to be well suited to this application: steam, toluene and benzene. The picture shows the three media in relation to three selective criteria, namely the cycle efficiency, the usable enthalpy difference and the volume flowrate.

With respect to the Rankine cycle efficiency, toluene shows the best results. Regarding

60C

I

k.

Steam Toluene Benzene Fig. 18. Comparison of working fluids for power conversion cycles.

the usable enthalpy difference, the organic media produce values far below those of steam. In general, small enthalpy differences lead to simple turbines with a small number of stages. If water is expanded in a onestage turbine this leads, at a high enthalpy difference of the theoretical Rankine process, to a relatively low turbine efficiency. The volumetric flowrate at the engine entrance, especially at small performances, is of great importance. Small volume flowrates lead to small engine dimensions or partial admission, which normally is combined with difficult design problems and low expansion efficiencies.

Besides the above thermodynamic criteria, a number of other important factors like corrosiveness, safety requirements, biological decomposition, availability and price of the medium have to be regarded when selecting the best medium. A number of different organic media are applied today but for many good reasons the application of water steam is favoured, especially in solar farm power conversion systems.

Another important factor is the selection of the upper process temperature of the PCS. This temperature has to be optimised as there are two opposing tendencies which affect the efficiency of the overall system.

Figure 19 shows the efficiency of the collector field which drops with increasing temperature and the efficiency of the PCS which increases with temperature. For the overall

32

COLLECTOR FIELD

CONCENTRATING COLLECTOR C = 40 MULTISTAGE STEAM TURBINE *i = 8 0 0 W / m ? T = 20C

100 200 300 400 COLLECTOR FIELD EXIT TEMPERATURE [ C ]

Fig. 19. System efficiency optimisation.

system the product of both factors is important; in this typical example an optimum system efficiency is shown to be at about 280 C collector field exit temperature.

Figure 20 shows a number of different prime movers, namely three different expansion motors and four different turbines. For each prime mover the major application range is marked. Taking this into account, almost all prime movers except the closedcycle gas turbine could be applied in solar farms. But the selection possibility is also limited by the temperature range in which the solar farms operate.

power 100MW 1000MW pistontype steam engine is stifling engine screwtype steam engine impulse steam turbine reaction steam turbine open cycle gasturbine closed cycle gas turbine

kW "SBl Jsd

W m_

_M!| Ml JM

SL 3 lbw_

~m i s IMW

"" MW

JZ 50 MW ^ solar energy power plants today and in near future Fig. 20. Major application range of different prime

Figure 21 shows the temperature ranges in which vapour and gasexpansion engines normally operate. As the maximum temperatures in solar farms do not exceed 400 C, gas

Upper Process Temperature 500 1000 C

Steam Motors

Steam Turbines

Gas Motors (Stirling)

Gas Turbines

100 350

300 550

1 1

500 1000

1600 | 1200

1 1 solar tarm

plants todays luture

solar tower plants

Fig. 21. Major temperature ranges of different prime movers.

expansion motors and gas turbines can be excluded. Therefore vapourexpansion motors and vapourexpansion turbines are of primary interest. For performances of several hundred kilowatts up to the VIW range, steam turbines can be advantageously applied.

Figure 22 shows as an example a steam turbine of 3 MW rated power. This 18stage turbine achieves an excellent process efficiency. For smaller performances simpler turbines can be applied, but there is only a very reduced number of turbines with good efficiencies available on the market.

Fig. 22. GHH condensation steam turbine, rated power 3 MW.

In the power range 50 300 kW, two or threestage screw expanders operating with steam seem to be an attractive solution. Figure 23 shows a twostage screw expander of about 100 kW power output.

The main advantages for a water steam operated screw expander can be seen in good machine efficiency, excellent part load behav

33

Fig. 23. Two-stage screw expander.

iour, low rotor speed and low fabrication and maintenance cost.

The development of a two-stage screw ex-pander in the 100 kW range shows very pro-mising test results. A first solar operated one-stage prime mover of the screw type will be installed in the near future at the solar test centre near Madrid. It will operate between temperatures of 250 and 117 C with a later second stage down to 64 C and produce 35 kW shaft power with a machine efficiency of 68%.

3. APPLICATION OF SOLAR FARM SYSTEMS

3.1. Process heat generation The production of hot pressurized water or

steam for mainly process heat application seems to be a relatively simple, but neverthe-less promising, application of solar concentra-tors. Simple configurations consisting of the major subsystems collector field including pumps and control, heat storage and heat ex-changer can be chosen to fulfil this task.

Figure 24 shows a functional block diagram of such an arrangement. The heat to be gener-ated is normally in the t