them would have to operate at temperatures below 350°C, with correspondingly reduced efficiency in the steam turbines. Improved selective coatings may allow planar collectors-which Meinel and his co-workers believe, in principle, to be the most effective in areas of the United States other than the cloudless Southwest-to be used. But most initial designs are based on the assumption that concentration of the sunlight will be necessary, and in these systems the fabrication, cost, and durability of the concentrators are the major concern.

The trade-offs between different types of collectors are not the only feature of the design of solar thermal plants still open to debate. Even with concentrating collectors, it may prove advantageous to operate the system at a reduced temperature, according to the Minnesota team. Their analysis shows increasing efficiency of the collectors, but decreasing efficiency of the thermodynamic cycle of the turbines as the operating temperatures are reduced, with the optimum temperature dependent on detailed design of the system and on the heat storage medium chosen. Heat pipes of the size envisioned have never been built, and other hardware details remain to be considered.

Both groups of investigators believe that the cost of solar-thermal plants will be not more than two or three times what fossil-fueled or nucleargenerating plants cost now, and that rising fuel costs will eventually tip the

balance in favor of solar-thermal plants whose fuel is "free." Before accurate estimates of costs can be made, they agree, more detailed engineering studies and some additional research are necessary. But Meinel, at least, believes that full-scale solar-thermal power plants could be built as early as 1985 with an adequate research effort. Other estimates are somewhat less optimistic, but a group of western utility companies is considering the development of a small solar-powered facility that could serve as a prototype for peak load applications.

Although solar energy has probably the fewest potential environmental problems associated with its use of any of the major sources of energy, some problems, none of which appear to be insuperable, do exist. Collecting surfaces absorb more sunlight than the earth does, and while this is not likely to alter the local thermal balance in household or other small-scale use, the larger expanse of collecting surface in a central power plant might. Thermal pollution will also be a problem if watercooled turbines are used-indeed, more so than with nuclear power plants because solar installations are expected to have even lower thermal efficiencies. If waste heat is returned to the atmosphere, it could help to restore the local thermal balance. The effects of small changes in the thermal balance would depend on the local meteorological conditions, but are expected to be small. The lack of particulate emissions

or radiation hazards might allow solarthermal power plants to be built close enough to towns or industrial sites so that their waste heat could be put to use. Finally, like other industrial facilities, large-scale plants would also carry some risk of accidents, with the attendant possibility of leaking heat transfer or storage media into the environment.

Yet another option for generating electricity with sunlight is direct conversion by means of photovoltaic cells. But the cells available now-which were developed for space applications— are relatively inefficient and very expensive to manufacture. As a longterm prospect, however, both cadmium sulfide and silicon cells are attracting considerable attention. This option, and the bioconversion of sunlight to fuels, will be discussed in future articles.

Space heating and cooling with solar energy are not available today. Solarthermal power plants have yet to be built on any but the smallest scale, and key elements of the necessary technology have not been adequately demonstrated. But both options appear to be close enough to practical tests of their economic feasibility to warrant increased efforts. The ancient dream of power from the sun may not, after all, turn out to be impossible.

-ALLEN L. HAMMOND

References

1. Proceedings of the United Nations Conference on New Sources of Energy, Rome, Italy (United Nations, New York, 1961).

2. R. Tybout and G. Lof. Natur. Resour. J. 10, 268 (1970).

New Scientist 5 April 1973

Although out of phase with the 11-year cycle of solar activity, interest in solar power here on Earth seems to wax and wane with a similar periodicity. But it could be that the Earthbound cycle is about to be broken. The resurgence of enthusiasm in the United States for harnessing the energy of the Sun has coincided and has, of course, in large part been brought about by-the age of environmental concern, and more particularly with the year of the "energy crisis".

While in the short-term the energy crisis is increasingly recognised as political and institutional (and, as such, in the words of one wellinformed observer, "could be solved by a stroke of the President's pen"), it has alerted the country to the alarming medium- and longterm problems of satisfying the country's voracious energy appetite. With an end to the known domestic oil and natural gas reserves now clearly in sight, and with nuclear power so far signally failing to live up to expectations, even the nation's politicians-President Nixon among them-have begun to cast around in some desperation for energy technologies that will see the United States through to the next millenium. Power from the Sun, while even on the most optimistic of forecasts not expec ted to provide more than a small fraction of these needs, is suddenly no longer regarded as a quaint, if intellectually appealing, technological backwater.

The solar optimists have recently had two opportunities to display their wares: at a conference on energy held last month at the Massachusetts Institute of Technology; and within the pages of the report of a National Science Foundation NASA panel devoted to solar energy as a national energy resource. With the drawback of a hefty R & D pricetag -$3500 million spread over the next 15 years --power from the Sun could. in the panel's opinion. economically provide, by the year 2020, 35 per cent of the energy required to heat and cool the nation's buildings. 30 per cent of the gaseous fuel. 10 per cent of the liquid fuel. and 20 per cent of the United States' electricity needs

The Sun's rays have several notable attractions as an energy source. They are not going to run out (or at least, when they do, mankind will have to face more pressing problems). They come already distributed. They provide as environmentally clean a source of energy as one could wish for And there is several orders of magnitude more than enough energy from the Sun falling on Earth to cope with all anticipated needs For instance, assuming a 10 per cent efficiency in the conversion of sunbeams to electricity, the total electricity consumption of the United States in 1969 could have been supplied by the solar energy incident upon 014 per cent of the US land area The NSF NASA panel identifies three fields

in which solar power could (given enough money) make a significant impact over the next 15 years or so. These are: the beating and cooling of buildings; the production of organic materials and their use directly to provide energy, or alternatively their con version to solid, liquid, or gaseous fuel; and the generation of electricity. In each case the obviously crucial task is to make solar power economically competitive with conventional energy sources, a task which looks more difficult as one works through the list.

Solar houses

Sun power in the home is unquestionably the prospect most likely to come to something. Already, solar water heaters are commercially manufactured in Australia, Israel, Japan, the USSR and, on a small scale, in the US Although further behind, space heating in the home by solar power 15 already techno logically feasible, and about 20 experimental buildings have been built. Technologists also have limited experience in the operation of absorption refrigeration systems using heat from solar collectors. The solar energy panel sees no technological and few economic barriers even now to the construction of homes with a combined system providing between about 50 to 80 per cent of all heating and cooling needs. A typical installation would involve a flat, solar collector on the roof. heating water pumped through it. Heat extracted from the stored hot water would be used to warm air in the winter and cool it in the summer. An auxiliary water beating system would have to cope with prolonged cold spells.

Two examples of the use of solar power in houses were presented at the MIT energy symposium, and together illustrate the pas sibilities and practicalities of exploiting the Sun in the home. The Environmental Quality Laboratory, at California Institute of Techno logy, is forging a coalition between local energy utility companies, building firms, and solar equipment manufacturers to demonstrate the feasibility of solar water heating in new apartment buildings constructed in Southern California With a 50 square-foot flat solar collector per unit, an optimised sys tem could reduce gas consumption to 20 per cent of present levels. The major obstacle to the introduction of such systems is not tech nological or even economic but institutional: the building trade is perhaps the most conservative in the land, and power compames are naturally highly suspicious of any innovation that will reduce the consumption of their output. By bringing these forces into the partnership, the EQL group hopes to over come these prejudices and hasten the intre duction of solar water beating by some five or ten years.

While Southern California has sunshine in abundance, Delaware, in the north-east of the country, has to cope with weather of the English type. Yet it is at the University of Delaware that perhaps the most ambitious feasibility study of solar power for the home is underway. The university is constructing an experimental single-family home which will provide its hypothetical occupants not only with thermal energy but also with a small proportion of their electricity requirements. The home has a total of 40 collectors on its roof and south wall, each employing cadmium sulphide/copper sulphide solar cells to convert sunlight directly into electricity as well as absorbing the Sun's heat to provide hot

water

Growing fuel

Fossil fuels represent a concentrated stored form of solar energy Because these fuels are now being rapidly depleted, the NSF/NASA panel looked into the possibility of mimicking the processes by which fossil fuels were originally generated as a way of producing renewable, clean, synthetic fuels The panel points out that if through advanced management techniques, including the exploitation of modern developments in plant genetics, crops of trees, grass or algae could be raised that convert solar power into the stored heat energy of plant materials with an efficiency of 3 per cent, then less than 3 per cent of the land area of the US would produce enough stored solar energy to meet all the nation's expected energy needs in 1985. The plant material could be burned directly to power electricity generators, or converted into convenient fuels. Among the possibilities are anaerobic fermentation of plant matter to produce methane, pyrolysis to produce solid. liquid and gaseous fuels; and chemical reduction to provide oil. Perhaps before the deliberate growing of plants for conversion into fuel (arguably not the best use of land in a world two-thirds hungry) some of the same conversion techniques could be insti tuted on a large scale to convert organic

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wastes to useful fuels. Although unlikely to be profitable, the income earned by selling the fuel from municipal pyrolysis plants, disposing of urban solid wastes in this manner, would probably allow the plants to be operated at no cost to the community

Solar power is most commonly thought of in connection with electricity production, either directly via solar cells or indirectly through the generation of steam for genera. tors. While solar cells offer the simplest, cleanest, and most efficient means of converting solar to electric power, the cost of the silicon solar cells used to power spacecraft must fall by a factor of 100 or so before the method becomes feasible on Earth. But the potential benefits are enormous: solar cells of around 10-15 per cent efficiency could col. lect from the roof of a house in the north-east of the US more than three times the electricity needs of the home. The energy arrives without transmission lines, and, with a technological breakthrough in electricity storage, could not only run the home but also power the family's electric cars

Technologically more accessible is indirect generation of electricity from solar power. A number of research and development projects are already underway, and one of the most promising was discussed at the MIT symposium. Under the NSF contract, the Aerospace Corporation is designing-with present day technology-a solar to thermal energy conversion (STEC) system which could be producing about 1000 MW(e) by 1990. Set in the sunny southwest, the system would consist of modular reflectors, focusing the Sun's rays on a pipe carrying either liquid sodium or high-temperature eutectic. To provide a reliable base-load capacity, the plant would have to include some means of storing the hot working fluid so that steam generators could be run through the day and night. One attraction of the system, however, is that electricity demand in the southwestern US. where everything is air-conditioned, also follows the Sun

The STEC solar farm epitomises the problem of exploiting the Sun, whether it be via the homespun technology of solar water heating or grandiose schemes like giant heat engines exploiting the temperature difference between the ocean surface and its depths, or the literally far-out concept of solar satellites in geostationary orbit. While the fuel for all these proposals is free, their capital costs are high. Yet in the end it is the peculiar economics of solar power that may prove to be its greatest asset. While oil and natural gas have been abundant and cheap, solar power has been unable to edge itself into the market place. If anything is certain, however, it is that oil and gas prices are going to climb steeply over the next decade. Even coal, of which the US still has enormous reserves and to which people are looking with increasing interest as a means of salvation. has very high environmental costs which in future will be reflected in its price. The Sun's rays-clean, abundant, limitless, and freewill become increasingly attractive simply by being there

tive Present costs for a Learnt are about $250 per kilosatt tkw of installed capacity. (The $250 per kw costs are those estimated by the nuclear power industry. but probably are too low. At the same time. cost estimates made by proponents of solar power systems probably are also too low. As a result, the comparative basis for the relative costs of the two approaches may be adequate.) A silicon cell system would cost in the range of $100,000 per kw. Approaches for reducing this cost are discussed in the references above. It is suggested that it might be possible to bring the cost down to a competitive level.

In addition to reducing the cost of fabricating the silicon cells, a savings could be achieved by using lenses to concentrate the sunlight and thus reduce the number of cells required. Moreover, the 10 percent efficiency is a factor of two to three below the theoretical efficiency of such cells. Costs could be reduced by improved conversion efficiency. Finally, it may be pos sible to fabricate (at an economic cost) sandwiched cells which are able to utilize a greater fraction of the solar energy spectrum. One such cell could achieve an efficiency of 60 percent.

Another possibility for reducing the cost of this system is the use of another type of solar cell. Considerable effort is now being expended on the cadmium sulfide cell. The advantage of this material is that it functions as a thin polycrystalline film and hence does not require growth of large single crystals As a result, the fabrication costs are expected to be at least 100-fold less than silicon cells. It is anticipated that these cells may become practical for individual homes."

Since these systems will produce electricity only when the sun is shining. they would either have to be augmented by other systems or would have to in clude an energy storage system. One storage system would involve the elec trolysis of water and storage of hydrogen

In summary, while solar cells represent an existing technology that has been used extensively in the space program, the high cost of cell fabrication is prohibitive for commercial power production. The cost of the cells would have to be reduced at least 100-fold. There are reasons to believe that this could be accomplished.

Solar Heat Systems

These systems absorb solar energy as heat. The heat can be stored in high heat capacity materials in insulated con tainers and then subsequently used as heat or converted to electrical energy

June 1973

Systems of this type fall into two Classes small systems for individual dwellings (solar home systems) and large commerical systems.

Solar Homes. These systems simply absorb the solar energy as heat and store it in insulated bins as heated water or rocks. Such systems have been in use for some time for home water heating and even for space heating in homes." They can also be used for air conditioning through the application of absorptive refrigeration. Space heating and air conditioning represent some 15 percent of our present energy consumption, electrical and otherwise. This is a substantial amount of energy; it is larger than our total electrical power gen. erated today.

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In some areas, these systems are already competitive with conventional systems. As the cost of energy continues to increase and as solar technology is improved, these systems can be expected to come into wider use. It has also been suggested that a solar heat system could be coupled with a solar cell array and thereby supply a home with all of its power re

quirements.

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13

Commercial Systems. A large com. mercial solar heat system has been proposed by the Drs. Meinel of the University of Arizona. The system incorporates an advancement in solar absorption technology called "selective" surfaces These surfaces have high absorptive properties but low emit tances. Hence they would retard infrared re-emission as the temperature rises and thus produce a "super greenhouse" effect Theoretically., such surfaces could be made to approach temperatures of 1.000 degrees Fahrenheit, but the present state-of-the-art falls below this and some means of concentrating sunlight by a factor of two to four is needed It is estimated that 90 percent of the incident solar radiation could thus be used. The energy would be stored in liquid sodium at a temperature of 1,000 degrees Fahrenheit. The heat would then be used to generate electricity through a steam turbine cycle The Meinels estimate an overall efficiency of 30 percent for this system. An early estimate of the cost of this system was some $300 per kw of installed power as compared to $250 per kw for nuclear power. Their estimate was based on using selective absorbers without concentrating lenses or mirrors Nevertheless, they continue to propose that this scheme could produce competitive power when using lenses or

mirrors

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1་

The Meinels also present a plan for a millon megawatt average power (3.4 million megawatt peak power) national solar power system. This is equivalent to

18

Another large-scale solar heat system has been proposed by Drs. Ford and Kane of the University of Massachusetts. The authors emphasize that this is only a proposal and thus uncertain. They suggest that sunlight could be concentrated by using inexpensive Fresnel lenses made of plastic. This concentrated energy could then be used to heat water to some 1,500 degrees centigrade (2,732 degrees Fahrenheit), at which temperature a small fraction of water will dissociate into hydrogen and oxygen. The hydrogen would be absorbed into some chemical compound from which it could later be released. This system then would produce hydrogen as a fuel. They suggest that hydrogen could be marketed at an equiv alent price with natural gas on a heat per pound basis. In this case they assume a 10 percent efficiency in the overall process.

To summarize, terrestrial solar heat systems offer the prospect of supplying, economically, a significant portion of this nation's and the world's energy supply. The 30 percent efficiency suggested by the Meinels would certainly greatly improve the prospects. At the same time, some combination of the Meinel and Ford-Kane proposals, vis-aVis concentration by inexpensive Fresnel lenses, may lead to efficiencies greater than 10 percent and to a viable system. The ultimate economy of these systems would seem to depend upon developing the technology for producing economically competititve absorptive surfaces.