Research on the Use of Solar Energy Made in the Soviet Union

V. A. Baum

doi:10.1073/pnas.47.8.1262 1961;47;1262-1270 PNAS

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RESEARCH ON THE USE OF SOLAR ENERGY MADE INTHE SOVIET UNION

BY V. A. BAUM

SOLAR ENERGY LABORATORY OF THE POWER INSTITUTE OF THE USSR ACADEMY OF SCIENCES

The features of the modern power industry which have developed over almost150 years will soon undergo very great changes, in view of the need to utilize newsources of energy and new methods for transforming energy. The power industryof the economically advanced countries is at present so immense that it is essentialto begin preparing for the changes well in advance.

Recently solar radiation has come to be regarded as one of the new promisingsources of energy.1-6 Yet only a short while ago a very modest part had beenaccorded to it in the future power balance. However, in view of the achievementsin physics, the production of new specialized materials, and the possibilities ofmodem industry, it can be expected that in the near future solar energy can be asupplementary source of energy. Then, as the methods of its utilization areperfected, it could come to play an ever-increasing role.The problem of utilizing solar radiation could be formulated as studying the

interaction of the radial energy of the sun with live and dead matter and determin-ing possible ways for the practical use of this interaction. However, when theproblem is formulated in such a general way, the volume of research becomes toolarge and indefinite. It is expedient to conduct the research first in those direc-tions which could be expected to produce the most important practical results inthe shortest space of time. From this point of view, we believe the primary direc-tions of research should be the following: (1) Studies of the Earth's climate andthe extent to which it could be controlled. These studies would include boththe elaboration of methods for controlling climatic conditions in large territoriesand also the changing of micro-climates. (2) Studying photochemical processes,particularly photosynthesis, both for the purpose of increasing crop harvests andfor determining the possibility of creating artificial chemical systems for obtainingfood products and energy. (3) Studying photoelectric and thermoelectric processesin order to create economically-acceptable devices for transforming the energyof solar radiation into electricity. (4) Studying the processes of heat and masstransfer in different solar installations. (5) Investigations connected with design-ing and manufacturing special materials and their products with definite properties(materials for photo- and thermo-elements, transparent and metal films, selectivecoverings, and so on). (6) The search for new methods of transforming andutilizing solar energy. (7) At the same time, engineering work should be con-tinued on designing prospective solar installations and calculating their techno-logical and economic characteristics.

These subjects, though different in their scientific and practical value as wellas in the methods and scale of research are closely interlinked.

In the Soviet Union there are many institutes whose work is connected to agreater or lesser extent with the problems of using solar energy. These are theinstitutes which study the theoretical and practical problems of photosynthesis,photochemistry, the physical and chemical properties of semiconductors, the

1262

VOL. 47, 1961 N. A. S. SYMPOSIUM: V. A. BAUM 1263

production and testing of semiconductors and other materials, the climate of theSoviet Union, and questions of heat and mass transfer. However, in addition toinstitutes, classified according to methods of research or the field of science inwhich they work, it is also considered expedient to have laboratories working onproblems as a whole or on their separate major aspects if practical results arelikely to be obtained.

In the Soviet Union there exist, in addition to the Helio Laboratory of thePower Institute of the Soviet Academy of Sciences, similar laboratories or groupsin some of the southern republics for which separate aspects of the problem havean economic significance.From the work done during the last 3-4 years by the Helio Laboratory of the

Power Institute of the Soviet Academy of Sciences, the following projects couldbe mentioned:The information collected on the number of hours of sunshine and radiation

(and in some cases also the temperature and wind velocity), as well as the data onthe efficiency of different solar installations, serve as the basis for the determinationof the prospects of utilizing solar energy in a given area.The Helio Laboratory has attempted to determine by statistical methods on

the basis of a large number of climatic studies the correlation between the numberof hours of sunshine To and the possible number of hours during which solar in-stallations could operate T. as well as the correlation between solar radiation 30and the amount of energy which could possibly be used 3 (ref. 7, pp. 18-26). Thisdependence was determined for eight places in the southern part of the SovietUnion for certain types of solar installations. For instance, for a solar heatingstation it was determined that 5.104 < 3o < 3. 105 kg cal/m2 month. The equationwould be 5 = 1.1 + 0.266 30 + 0.00118 So2 where S is the heat output per squaremeter of mirror surface a month and 3o the total radiation per square meter for amonth. The deviations from this equation for different geographical places arenot considerable, the correlation coefficient r being equal to 0.95.The transformation of solar radiation into mechanical or electrical energy is

one of the main questions. So far, it is realistic to speak of three ways of trans-formation: by means of the ordinary steam system, by solar thermoelectro-generators, and by solar photoelectric batteries.To obtain capacities from 1,000 to 3,000 kw in a single installation, it is at present

most economically advantageous to use the conventional steam system which wehave elaborated in detail. When solar radiation is transformed into the heat ofsteam, the efficiency is about 55-65 per cent, and when transformed into electricity,about 12-15 per cent. These installations could be regarded as prototypes of apower unit for reclaiming arid and semiarid ones and creating oases where theycould be utilized for producing electricity, for distilling or pumping water, and forother purposes.

Calculations made in the Helio Laboratory have shown that a steam engine ofa capacity of about 1-2 kw can hardly have an efficiency of more than 1 per cent.This is due to the fact that when the size of steam machines is decreased the com-parative heat losses are greatly increased as a result of the heat transfer betweenthe steam and walls of the cylinder of the machine. It is also known that even asmall admixture of noncondensing gases (air for instance) added to the steam

1264 N. A. S. SYMPOSIUM: V. A. BAUM PROC. N. A. S.

greatly reduces the heat transfer coefficient from the steam to the wall. TheHelio Laboratory conducted experiments on the condensation of steam from amoving air-vapor mixture. The following dependence was obtained8 for Re1,300-40,000 with the amount of steam in the mixture from 3 to 95 per cent (volu-metric):

11= Gd ~ =13.0 Re-02 (P8 12(8 P'0

(Dt + Dm)pm (Pm! PM !'

where D: and Dm are the coefficients of turbulent and molecular diffusion taken inregard to the partial pressures, G is the density of the steam flow in the directionof the condensation surface, d is the diameter of the pipe along which the mixtureis moving, ps, pw, and Pm are the partial pressures of the steam, the steam near thecondensation surface, and the air-vapor mixture, and Re is the Reynolds number.The coefficient of turbulent diffusion is determined on the basis of previous work.9

For preliminary calculations, when 700 < Re < 1,300, the following equationcould be used.

2(2 --0.06) P w10pm Pm

Using this relation, maybe it would in some cases be possible to improvesomewhat the efficiency of small steam machines.

Experiments carried out in the United States on building small steam engineshave shown that their efficiency is low.'0 This task is evidently a very difficultone. But it is probably too early to give the matter up altogether.The experiments in building solar thermoelectric generators showed that they

have a much higher efficiency of transforming solar energy into electricity thansmall steam machines (ref. 4, pp. 7-23). The intensive search for semiconductorsgives grounds for hoping that in the near future it will be possible to make solarthermoelectric generators of a capacity of 1-3 kw with an efficiency of about 5-8per cent which would be economically acceptable in some areas for such purposesas the pumping of water.The investigations connected with creating solar thermoelectrogenerators were

conducted in the Helio Laboratory in the following directions: choosing andinvestigating materials for thermal couples (ref. 7, pp. 78-87 and 99-110), thesearch for methods for distributing evenly the heat flow passing through separateelements of the battery (ref. 7, pp. 88-98), a study of the optimal conditions forthe work of such devices, designing mechanisms for following the sun, and engi-neering methods of calculation."

Recently our Helio Laboratory has investigated at various temperatures thethermoelectric properties of the following binary compounds of tellurium: FeTe,CoTe, GeTe, PdTe, AgTe, Ag3Te2, Ag2Te, InTe, In2Te, SnTe, Sb2Te3, PbTe, BiTe3.On the basis of measurements of the Hall effect, the electric conductivity of

and the thermoelectric force a, a calculation was made for the density of currentcarriers, their mobility, and their effective mass.'2 These investigations are im-portant for practical purposes, since the evaluation of the effectivity of the ma-

terials in regard to a, u, and the heat conductivity X measured at room temperaturecould lead to large errors. The value of z = qa2/X for some materials depends

VOL. 47, 1961 N. A. S. SYMPOSIUM: V. A. BAUM 1265

very little on the temperature of the hot junction, but for others it decreases verysharply. Figure 1 shows for some tellurium compounds with different admixturesthe dependence of a', a, X, and z on the temperature of the hot junction.Here a' is the electric conductivity of the material 1/ohm cm; a is the thermo-

electric force V/C', and X is the heat conductivity W/cm C'.It is to be noted that in Figures 1-4, X is given in heat units kg cal/m hr C0,

2002400'600 8005~~~~~~~~~~~~~~_

2

0200 400 600 800 /000 o,0

FIG. 1 -Efficiency of a thermoelectric generator at different tempera-tures and solar radiation concentrations.

and to calculate z, the heat conductivity X must be taken in W/cm C'.At given temperatures of hot and cold junctions of electric batteries, an increase

in the coefficient of concentration K = Fbat/Fmir (where Fbat = surface of batteryheated by rays in m2 and Fmir = surface of mirror in M2) leads to an increase in theefficiency of the solar thermoelectrogenerator, since at an equal amount of energyreceived by the battery its heat losses through convection and radiation are lessenedbecause the surface of the batteries heated by the rays is decreased. At a givencoefficient of concentration, there exists a maximum temperature of the hot junc-tion, since the heat losses by radiation and convection from the battery grow fasterthan the quantity of heat transformed into electricity does. However, even atsuch small concentrations as K = 500, this maximum temperature is considerablyhigher than the melting point of the existing.materials.

Figure 2 shows the results of calculations of the efficiency of a solar thermo-electric generator t1 depending on the temperature of the hot junction and thecoefficient of ray concentration K for a conditional case where z = 0.001 (constant).The Helio Laboratory is continuing to work on methods of construction. This

is one of the most complicated and important problems in creating thermal bat-teries, since many of the materials investigated proved fragile and unstable. -Atworking temperatures, they warp and crack. In view of this, as well as becauseof intercrystalline corrosion, their electric resistance increases in time. Measure-ments were also made of the coefficient of linear expansion of materials duringheating which wholly satisfies the practical requirements.

1266 N. A. S. SYMPOSIUM: V. A. BAUM PROC. N. A. S.

0( S a A"6X4°o~~~~~~~~~~~tA<< re4.,O2,0

1, 1,0 - - -/00 M0s/00 ~ ~ 00 S60 --s- e-

60 -2 CZ, joc ___

S~ ~ ~~~~T 3 AOi'

-50 0 I/400 500 300 4to

___or 81Te4t -Vi% Gla B.- 42

I 44f~ A~--7200=.;+_iAFIG.2. evtr10C~~~~~~~ - 0 0 j,~~~~~~~~~~~ ~~04

10203010 10 ,2v50.

160d-t

/00ZooStv 4LW oJI3004.o 50 60

ordinate a is thermoelectric force and ordinate X is thermal conductivity.

The Helio Laboratory also studied the creation of conditions in thermal genera-tors such that the hot junctions of all elements of the battery have the same tem-perature at a given temperature of the cold junctions. These requirements canbe approximated as follows: (a) by making thermoelectric batteries with a specialform of hot surface or with thermo-elements of different length (an apiproximatemethod of determining this form was examined by D. Tepliakov'3); (b) by theuse of massive plates which by means of heat conductivity smooths the heat flowon the hot side of the battery (thus it was that A. Okhotin built an experimentalsolar thermoelectrogenerator); (c) by making the surface of the reflectors in aspecial form. R. Aaparasi, for instance, designed a special mirror that creates

VOL. 47, 1961 N. A. S. SYMPOSIUM: V. A. BAUM 1267

in the focal plane a more equal flow than is created by a paraboloid mirror. Theequation for this mirror in polar coordinates is

1 - K - p tan 22 1

p = Co a o+Ksin0'1 + K + p tan 2

2

where sp is the angle of divergence, K = A/I + tan2 0, 0 is determined from theequation for the focal distance f = p cos 0, CO is determined on the basis that 0 = 0;P = Po.

Mirrors which give a smoothed flow of radiant energy on the focus are importantalso for designing solar photoelectrogenerators with an increased concentration ofsolar radiation.The silicon photocells with a high efficiency made at first in the United States'4 15

are of great interest in those cases when the cost of the electricity produced isirrelevant, as for instance for man-made space bodies. These photocells are nowproduced in many countries. However, for the ordinary consumer they are tooexpensive. It would seem that the problem of creating economically acceptablesolar photoelectric generators had reached an impasse, since their working tempera-ture should not exceed about 1000C. However, experiments carried out in theSoviet Union have shown that by increasing the light falling on the photocells itis possible to increase its output considerably if it is cooled artificially.'6. 17 Thus,in some cases we have succeeded in increasing the output of a photoelement 70times by increasing the light falling on it 160 times. Under certain conditions itis profitable to concentrate the rays, since the cost of a unit of surface of the photo-cells is several hundred times greater than that of mirrors. We believe that byusing concentrating mirrors, cooling the photoelements, and improving the tappingof the current, it would be possible to solve the problem of creating an economicallyacceptable device for obtaining electricity from solar energy.

Several small experimental designs of photoelectrogenerators were made by theHelio Laboratory in 1960.The photoelectrogenerator consisted of a photobattery with a surface of 100

cm2 which worked from concentrated solar energy reflected from an aluminummirror with an effective surface of 1.2 M2. The reflection coefficient of the mirrorwas equal to about 0.7. The photobattery was placed in a chamber with a glasslid. Water passing through the chamber cooled the face surface of the photo-battery. The useful capacity of this device, at a radiation of 3o = 700 kg cal/m2 hr, was about 10 watt. Without the mirror, the photobattery would havegiven a useful capacity of 0.5 watt, that is, one twentieth as much.

In some of the problems connected with the use of solar radiation, the so-calledselective surfaces are of very great significance, and so a great deal of attentionwas devoted this research.'8 The Helio Laboratory examined the theoreticalaspects of this problem and conducted experiments on creating selective surfacesfor solar heating installations. It proved feasible to introduce a concept of anideal selective surface which in the range of waves from X = 0 to X = X0 is, forinstance, absolutely black A = 1, and in the range X = Xo to X = o is absolutely

1268 N. A. S. SYMPOSIUM: V. A. BAUM PROC. N. A. S.

white. As has been done in a number of similar works, 19 we shall call Xo the thresh-old length.For an ideal surface, X0 must be such that the ratio between the quantity of

absorbed radiation and the quantity of lost energy should be optimal. If, forinstance, solar radiation of an absolutely black body at a temperature T, =6,0000K, then we find that Xo can be determined in microns from the followingformula

C2 tT,-T2\X TjT2S' k TT, I

In M + 41n

where M =y-o(T24 - T04), C2 is the second constant of Planck's law equal toS0

0.01438 m deg, T, = 6,0000K, T2 is the temperature of the selective surface inOK, To is the temperature of the ambient in OK, a = 4.9 X 10-4 the radiationcoefficient of an absolutely black body, and 3o is the solar radiation near the earth'ssurface in kg cal/M2 hr.Table 1 gives the results for calculations of Xo depending on T, and 35.

TABLE 1(kg cal\ T,-temperature of surface in°K

Jo \ m2hr) 600 500 400 350700 1.91 2.34 3.04 3.61810 1.93 2.37 3.08 3.661000 1.95 2.40 3.14 3.73

It can be seen that the value of 30 has little influence on X0.The X0 was also determined graphically on the basis of real solar radiation and

it can be seen that the results obtained coincide fairly well with those given inTable 1.

In order to compare the selective absorbing surface with the blackened surface,a number of calculations were done. The efficiency of the heater with a blackenedsurface decreases much more quickly than that of a heater with a selective surface.The results of the calculations also showed that for improving the efficiency of aheater with a selective surface, it is very important to try to decrease the heatlosses by free convection in the air.Experiments with black selective surfaces were carried out by covering different

freshly-polished surfaces of metallic plates with electroplating methods and byspraying with nitrate paints. On the basis of these experiments, it can be con-cluded that the optimal thickness of the covering layer lies between 0.2 and 0.5microns if produced by electroplating methods, and the optimal weight of a sprayedblack nitrate paint is between 0.2 and 0.4 mg/cm2.The results of some experiments are given in Tables 2-5, in which E is the fraction

of emission and at8 is the fraction of absorptivity compared to those of an idealblack body.On the basis of existing experiments and calculations, the Helio Laboratory

determined the technical characteristics of the different methods of distilling saltwater with the help of solar energy.'1 22 The capacity of solar distillers of the

VOL. 47, 1961 N. A. S. SYMPOSIUM: V. A. BAUM 1269

TABLE 2 TABLE 3STAINLESS STEEL COVERED BY A LAYER ALUMINUM COVERED BY A LAYER OF

OF COPPER OXIDES COPPER OXIDESThickness Thicknessof layer in of layer inmicrons microns C a0.15 0.18 0.71 0.39 0.2 0.620.23 0.2 0.78 0.54 0.27 0.680.3 0.22 0.8 0.9 0.38 0.820.35 0.24 0.86 1.09 0.41 0.820.4 0.26 0.87 1.3 0.43 0.84

TABLE 4 TABLE 5STAINLESS STEEL COVERED BY A LAYER OF ALUMINUM SPRAYED WITH BLACK

COPPER OXIDES NITROPAINTThickness Weight ofof layer in sprayed paintmicrons . in mg/cm2 (is0.168 0.19 0.81 0.25 0.34 0.830.237 0.25 0.84 0.29 0.39 0.880.34 0.30 0.86 0.34 0.43 0.900.38 0.31 0.88 0.57 0.60 0.930.475 0.35 0.90 0.87 0.70 0.941.04 0.40 0.93

hot-box type was determined by a computing machine, and the results givenin graphic form, showing its dependence on solar radiation, the ambient tempera-ture, and the quality of the glass. The capacity of a distiller was determined fromsuch a big solar station as was described in references 4 and 23. I shall mentiononly a few of the characteristics of this station. It consists of a tubular screen-boiler with a surface- of 15 X 9 M2. The rays of the sun are reflected from 1,300flat mirrors having a total surface area of 20,000 M2. The amount of steam ob-tained in the boiler could be calculated by taking the efficiency of the boiler to be60-65 per cent and the efficiency in the production of electricity as 15 per cent.

For the conditions of Tashkent, where the annual amount of solar energy fallingoin a surface perpendicular to the rays amounts to 1,700,000 kg cal/M2 hr, weobtain the following results:

1. If all the steam produced by the boiler is channeled into an 8-9 stage dis-tiller, then the annual productivity of the installation will amount to about 1.6 X105 tons of distillate from water containing any amount of salt.

2. If the steam from the solar boiler is used for a condensation turbine, itsgenerating capacity will be 2,500 kw. If this electricity is used to purify waterby means of an electro-ionic purifier, the following quantity of water containingabout 350 mg of salt per liter will be obtained.

Initial salt content in water (mg/liter) 885 4,635 10,000 .35,000Annual productivity of installation in

tons 7 X 106 1.8 X 106 0.85 X 106 -0.28 X 106

The productivity of a hot box distiller with a surface of 20,000 m2 would be about1.7 X 104 tons of distillate a year.In recent years, considerable attention is being paid in many countries to the

utilization of solar energy. Substantial results have been obtained. Of course,there are still many difficulties which have to be overcome. But they are notdifficulties of principle.

1270 N. A. S. SYMPOSIUM: V. A. BAUM PROC. N. A. S.

Just imagine for a moment that mankind had based his power industry on solarradiation, not fuel, and then the proposal to use different kinds of fuel was putforward. Probably, there would have been very many objections.One could imagine that one of the most important arguments in defense of solar

energy would be formulated as follows:Solar radiation is a "noble" form of energy and it was under its influence that

life originated and continues to develop on earth; therefore its use, no matteron what scale, could represent no danger or inconvenience for either the flora orfauna of the world. The use of any other kind of fuel would inevitably be con-nected with the poisoning of the atmosphere, water, and land. Fuel should beused only where there are no other possibilities of obtaining energy, and in thesunny regions of the world the energy of the sun should be used.

1 Trofimov, K., Ispolzovanie solnechnoi energii (Using of Solar Energy), collection of papers, IV,Tashkent, USSR, 1935.

2 Proceedings of the Symposium on Wind and Solar Energy, New Delhi, 1954 (UNESCO, 1956).3 Daniels, F., and J. Duffie, Solar Energy Research (Madison: University of Wisconsin Press,

1955).4Ispolzovanie solnechnoi energii (Using of Solar Energy), collection of papers, I, Academy

of Sciences of the USSR, 1957.5 New Sources of Energy and Economic Development, United Nations, New York, 1957.6 Proceedings of the World Symposium on Applied Solar Energy, Phoenix, Arizona, 1955.

Transactions of the Conference on the Use of Solar Energy, Tucson, Arizona, 1955.7Ispolzovanie solnechnoi energii (Using of Solar Energy), collection of papers, II, Academy

of Sciences of the USSR, 1960.8 Baum, V., and P. Brdlik, Teploenergetica, No. 1, 1957, Moscow.9 Baum, V., Izvestia Academii Nauk, OTN, No. 2, 1952, Moscow.

10 Hsu, S. T., and B. S. Leo, J. Solar Energy, 2, No. 3-4, p. 7 (1958).1' Baum, V., and A. Okhotin, Ingenerno-phys. Journ., No. 11, 1959, Minsk.12 Telkes, M., Solar thermoelectrogenerators, see references 3 and 6.13 Tepliakov, D., Ingenerno-phys. journ., No. 4, 1958, Minsk.14 Chapin, Fuller, and Pearson, J. Appl. Phys., 25, 676 (1954).16 Chapin, Fuller, and Pearson, Bell Lab. Rec., 33, 241 (1955).16 Vavilov, V., Malovetskaja, and Galkin, Atomic Energy, No. 5, 1958, Moscow.17 Baum, V., R. Borovikova, and A. Okhotin, Ingenerno-phys. Journ., No. 8, 1960, Minsk.18 Tabor, H., see reference 6, vol. 2, part 1, sect. A.l9 Trivich, D., and P. Flinn, see reference 3.20 Ge Sing-shi, see reference 7, pp. 133-141.21 See reference 7, pp. 122-132.22 Baum, V., Solar Distillers (to be published).23 Baum, V., Aparasi, R. R., and Garf, B. A., J. Solar Energy (1957).