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Page 345
Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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6
Solar Energy

The comprehensive term “solar energy” embraces a wide variety of processes for converting the sun’s energy into high- and low-temperature heat, electric power, and liquid and gaseous fuels, on scales that can vary from small household systems to large centralized power plants. Their common feature is that they use the sun’s radiation as a source of energy—either directly (as in the increasingly familiar rooftop solar heating systems) or indirectly (as in wind power, ocean thermal energy conversion, wood burning, or those prospective technologies designed to turn plant matter, or “biomass,” into liquid and gaseous fuels).

Obviously, these technologies are a disparate lot. Some are essentially fully developed technically and under certain circumstances are competitive with the replacement costs of other forms of energy. Some are very far from economic and technical practicality, With appropriate subsidies many of the former could become valuable conservation measures over the next two or three decades; others can be considered as important energy sources only for the twenty-first century and beyond.

For convenience in this report, the many different solar options are grouped, on the basis of end-use, into the following three categories.

  • Direct use of collected solar heat for heating and cooling. This group of solar technologies is characterized by low to medium temperatures. Some of the designs and processes are fairly simple and lend themselves to such applications as domestic space heating, domestic hot water heating, and production of hot water or low-pressure steam for industrial and agricultural processes. “Active” solar systems transfer heat by working

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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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fluids circulated mechanically; “passive” systems make use of natural forces such as convection, conduction, and radiation. Efficient and economical solar cooling remains a difficult problem. This group of solar technologies is, in general, the most nearly economical today. Some of the methods can be considered fairly well developed and among the most probable candidates for widespread commercialization in the intermediate term.

  • Solar electric technologies. Solar radiation can be used in many different ways to generate electricity. Photovoltaic solar cells are one promising technology, though high cost and the lack of an economic electric storage technology are barriers to widespread commercial use today. Wind-powered generators are another means of converting solar energy to electricity. In favored locations they are technically practical already; however, their ability to displace utility generating capacity is limited by the intermittent nature of wind. In another solar electric method the rays of the sun are focused on large boilers with large arrays of tracking mirrors (heliostats); such an installation might cover thousands of acres and have a generating capacity of 10–100 megawatts (electric) (MWe). This approach is referred to as solar thermal central station energy conversion. The Department of Energy is building a 10-MWe pilot plant for this concept. More speculative is the ocean thermal energy conversion (OTEC) concept, which involves using the temperature differential between the tropical ocean’s surface and subsurface waters to run large heat engines, thereby generating electricity. This last alternative is only in the research stage. An attractive feature of OTEC is that it could be used for base-load generation without storage devices, since the ocean itself would act as the storage medium.

  • Solar production of fuels. Solar radiation can be used in a number of ways to produce solid, liquid, and gaseous fuels. Solar heat can be used,  though with low efficiency, to decompose water molecules and produce hydrogen for use as a fuel or as a chemical raw material in place of hydrocarbon fuels. Photochemical processes can in principle be much more efficient but are in the early stages of research. Solar energy can also be used in the form of biomass for direct burning or conversion to synthetic fuels. Given the coming decade’s anticipated fluid fuel supply problems (chapters 1 and 3), this group of technologies could become valuable in the intermediate term.

In dealing with these options, government energy planners face two critical issues: (1) the extent to which solar technologies can save oil and gas in the intermediate term (1985–2010) and (2) how best to assure that the appropriate solar technologies will be available to help meet the nation’s long-term energy needs.

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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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The answer to the first question depends largely on how much the public is willing to pay for the fuel conservation benefits of solar technologies over the next few decades. At present, the relatively high capital costs of even the most economical solar technologies form a barrier to wider use. The required initial investments are very high in comparison with those of conventional systems, which spread their main costs—for fuel—over many years. However, solar energy has attractive advantages. First, of course, are the potential economic and political benefits of conserving fossil fuels and thus in some measure reducing the need for oil imports. Also of great importance are the environmental benefits; solar energy technologies can be among the most benign of all energy sources in this regard. Furthermore, we are convinced of the necessity for diversity in the nation’s energy system; adding a solar component to the system would reduce the danger inherent in relying on one or two primary energy sources. Finally, most solar systems perform well in small-scale installations, so that the time lag between the decision to build and actual operation can be short in comparison with the lag involved in siting, building, and licensing very large coal-fired or nuclear power plants. This means that once the economic and technical availability of a particular solar technology is established, wide deployment can begin relatively quickly.

Today’s market for energy does not reflect these advantages very well Conventional energy sources, with their lower first costs, average (rather than replacement) fuel costs, and established industries, will be favored for some time to come unless the barrier to solar energy posed by high initial investments can be surmounted. Nonetheless, if national energy policy mandates it for social and environmental reasons, the more nearly economic solar applications can be rendered competitive with conventional alternatives by some form of subsidy. For example, California has adopted a 55 percent tax credit on solar heating systems intended to promote significant market acceptance at the current prices of competing fossil fuels.

Subsidies for energy technologies are, of course, not unknown. The various incentives that have been provided the oil and natural gas industry in the form of depletion allowances, intangible drilling benefits, and the like are examples. The nuclear industry has received support in the form of federal research and development support, limits on utility liability (under the Price-Anderson Act*), and federal uranium enrichment facilities. Other energy and nonenergy minerals are accorded similar subsidies. However, to ensure the market penetration of most solar technologies at present costs, subsidies for solar energy would have to be considerably greater than those that have been accorded other energy forms. The

*

See statement 6–1, by B.I.Spinrad, Appendix A.

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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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subsidies required for at least some of the solar technologies would, however, be expected to decrease greatly or become unnecessary in the future as a result of advances from research and development and commercial experience.

SOLAR ENERGY SCENARIOS

To illustrate the potential contributions that solar energy could make in the intermediate term, this study developed two special scenarios of future use of solar energy. The first scenario embodies estimates of solar energy use under the assumption that the prices of competing energy sources remain near present levels and that solar energy is given no special incentives. The second represents the potential solar contribution under a policy of vigorous government intervention in the market for energy. More detailed information on how these scenarios were developed is given in the report of the Supply and Delivery Panel’s Solar Resource Group.1

LOW-SOLAR-ENERGY SCENARIO

This scenario is based on the assumption that no policy other than the current federal tax credits is implemented to assist the entry of solar energy into the energy market, that the costs of other energy sources increase only slowly, and that the costs of solar energy technologies remain high (i.e., follow the estimates of the Solar Resource Group without breakthroughs in the costs of advanced solar technologies). This yields a very small market penetration of solar energy by 2010 (Table 6–1). Solar heating systems do not achieve significant market penetration until after 2000, at which time it is assumed that the prices of electricity and natural gas have risen greatly. No solar electric technology becomes economically competitive before 2010, and municipal waste is converted to fuels only in major urban regions.

HIGH-SOLAR-ENERGY SCENARIO

This scenario is based on the assumption that by 1985 a national policy decision mandates vigorous incentives to bring about the use of solar energy. This scenario is driven not by economic forces, but by government intervention in the energy market. It is assumed that this government policy mandates, independent of cost, adoption after 1990 of solar energy for heating all new buildings and for all industrial process heat where feasible; sets in force a mandatory schedule for deploying several solar electric technologies; and requires rapid adoption of technologies for

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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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TABLE 6–1 Solar Contributions in Low-Solar-Energy Scenario (primary energy displaced, quads)

Solar Application

1985

1990

2000

2010

Heating and cooling

0

0

0

0.3

Solar electricity

0

0

0

0

Fuels

0

0

0.1

0.3

TOTAL

0

0

0.1

0.6

converting municipal and agricultural wastes to fuels. This scenario leads to a solar energy contribution about equal to President Carter’s announced goal of 20 percent solar energy in the year 2000. The 20 percent figure includes present use of hydro and biomass, estimated to be about 5 quadrillion Btu (quads), to which about 13 quads of solar energy would be added to form a total of 18 quads (or nearly 20 percent of the Administration’s assumed total).*

Heating and Cooling by Solar Energy

It is assumed in the high-solar scenario that all new construction after 1990 uses solar space heating and water heating and that half the solar heat collected for this purpose is used also for solar total energy systems, which cogenerate2 heat and electricity. An accelerated schedule for use of industrial process heat is assumed,3 and it is again assumed that half this solar heat is used in total energy systems. With these assumptions, the schedule of energy contributions from direct use of solar energy for heating and cooling is as shown in Table 6–2 (not including the electricity produced in total energy systems).

Solar Electricity

A number of solar electric technologies are under active research and development, and it is impossible to predict which would be technically and economically most suitable under the policies assumed in this solar-intensive scenario. The high-solar scenario is based on mandated deployment of three technologies—solar thermal central station energy conversion, solar thermal total energy systems, and dispersed wind energy systems—without implying that these would necessarily be the specific

*

See statement 6–2, by H.Brooks, Appendix A.

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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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TABLE 6–2 Heating and Cooling in High-Solar-Energy Scenario (primary energy displaced, quads)

Application

1985

1990

2000

2010

Domestic water heating

0.2

0.4

1.2

1.3

Passive space heating

0.1

0.2

0.3

0,4

Active space heating

0.02

0.06

0.64

1.21

Nonresidential air conditioning

0.06

0.36

1.50

Industrial process heat

0.2

0.4

1.6

6.6

TOTAL

0.52

1.12

4.1

11.01

technologies used. Photovoltaic or ocean thermal conversion, for example, might serve instead.

For the purpose of defining this scenario it is assumed that public policy requires the use of specified amounts of solar electric capacity for new generating capacity, regardless of cost. The estimates for central station solar thermal conversion are based on an assumption that in 1990 three full-scale production facilities for heliostats (sun-tracking concentrating mirrors) are placed in operation and that the production of heliostats is doubled every 5 years thereafter. This is a 15 percent annual growth rate. The estimates for solar thermal total energy systems are based on the estimates given above for solar heat delivered for space heating, air conditioning, and industrial process heat. It is assumed that 50 percent of the solar heat delivered for space heating and industrial processes (Table 6–2) is used for total energy conversion, and 70 percent of the solar heat delivered for nonresidential air conditioning is so used. The heat rate of the total energy systems is assumed to be 17,000 Btu per kilowatt-hour (kWh). Table 6–3 gives estimates of solar electric contributions under these conditions. The estimates in the table correspond to an installed capacity in the year 2010 of 250 GWe of central station solar thermal plants (with storage for a load factor4 of 0.4), 74 GWe of total energy generation (load factor of 0.3), and 50 GWe of wind turbines (load factor of 0.4). (Total U.S. generating capacity in 1976 was 494 GWe.)5

Use of Biomass for Fuels

We assume in the high-solar scenario (Table 6–4) that a federal decision is made to mandate recovery of the energy content of municipal wastes and agricultural residues. By the year 2000, 95 percent of municipal waste is processed for its energy, and 35 percent of the energy content of

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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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TABLE 6–3 Solar Electricity in High-Solar-Energy Scenario (primary energy displaced, quads)

Solar Electric Technology

1985

1990

2000

2010

Central station

1.7

8.7

Total energy

0.07

0.5

1.9

Wind

0.1

0.5

1.4

1.8

TOTAL

0.1

0.57

3.6

12.4

agricultural residues is recovered as methane, yielding a total of 5.4 quads annually in addition to the 2.7 quads now used annually by the forest products industry.6 We do not include here any contributions of energy farms. Such farms could possibly provide another 3.4 quads in the year 2010, albeit at substantial ecological cost.

Combining all these estimates yields a total of about 29 quads of primary energy displaced in 2010 by solar energy in this high-solar scenario (Table 6–5).

The high-solar scenario was not intended to represent a recommended national strategy for solar energy development, but rather to indicate the upper bound of what is technically feasible. The total cost of implementing such a scenario, at today’s costs for solar technologies, might be around 3 trillion dollars, perhaps 2–3 times the cost of obtaining equivalent energy from conventional nonrenewable sources. The increment, if provided as a subsidy by government, would be much larger than the total amount provided to date by government to stimulate energy production by conventional means (including nuclear), which was estimated recently at about one tenth of a trillion dollars.7

TABLE 6–4 Use of Biomass for Fuels in High-Solar-Energy Scenario (primary energy displaced, quads)

Biomass Source

1985

1990

2000

2010

Municipal wastes

0.5

0.8

1.9

1.9

Agricultural residues

0.5

0.9

3.5

3.5

Energy farms

TOTAL

1.0

1.7

5.4

5.4

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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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TABLE 6–5 Total Solar Contributions in High-Solar-Energy Scenario (primary energy displaced, quads)

Solar Application

1985

1990

2000

2010

Heating and cooling

0.5

1.1

4.1

11.0

Solar electricity

0.1

0.6

3.6

12.4

Fuels

1.0

1.6

5.4

5.4

TOTAL

1.6

3.3

13.1

28.8

THE ROLE OF RESEARCH AND DEVELOPMENT

Preparing solar energy to take its place in the long-term energy system depends even more on government action than does planning for nearerterm contributions. At issue is the total amount of research and development effort and its allocation to various solar technologies now in their infancies. It would be unfortunate if national decisions about longterm energy options were to settle on a single option, such as the breeder reactor, merely because it is relatively more developed today. The government’s role should be to bring the state of knowledge of all longterm technologies to the point at which some appropriate combination of energy sources can be chosen on the basis of realistic comparisons. Decisions that restrict the variety of our long-term energy options should be deferred as long as possible.

Government solar energy research and development policy therefore should provide for exploring many long-term solar options, without concentrating too heavily on large-scale demonstrations of one or a few, far in advance of their expected deployment. Indeed, premature demonstrations could well prove counterproductive, since their results might well be taken as evidence that the technologies could never become economic. The government’s present concentration of funds on demonstrations of solar thermal central station power plants may turn out to be an example of such premature demonstration programs.

DIRECT USE OF SOLAR HEAT

Heating buildings and domestic water and providing industrial and agricultural process heat and low-pressure steam are by far the simplest and most economical applications of solar energy. (Solar space cooling, while it can be done, is not now efficient, reliable, or economical.) For these reasons, this group of technologies is the most suitable for

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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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deployment in the intermediate term as part of the nation’s energy conservation program.

However, these applications are in general more costly than conventional alternatives, Btu for Btu, and even more costly in terms of the initial investments in complete heating or cooling systems. Solar applications are likely to develop a large market before the end of this century only if the prices of alternative fuels rise quite sharply (either from market forces or from taxation) or if solar installations are subsidized heavily.*

The cost of heat from solar systems cannot be computed with certainty, and it is hard to generalize about the economic standing of solar systems as compared with alternatives. Their economic success depends on a number of variable and uncertain considerations, including the following.

  • The initial cost of the system, including storage.

  • The amount of sunshine available at the site.

  • The amount of the collected solar heat that is actually used. (Solar hot water or process heat systems, for example, operate all year long and thus yield fairly constant returns on the initial investments. Solar space heating systems normally do not, and this lowers their value.)

  • Local prices of conventional energy sources.

  • The total cost of providing backup energy.

  • Interest rates, taxes, and subsidies.

  • The performance, reliability, and maintenance and repair costs of the particular system.

Obviously, these factors vary from place to place and time to time, as well as from system to system. Most, moreover, are difficult to project, especially over the expected system lifetime of about 20 years. Judgments about the economics of installing a solar system rather than a conventional source of heat, therefore, depend on assumptions about these uncertain quantities. It can, however, be stated with confidence that most forms of solar heating are not economic now except in special circumstances, and that either fairly rapid changes in the prices of alternatives or a government incentive program larger than the one now in place will be necessary if solar heating is to attain much importance before the end of this century.

A consumer is more likely to decide in favor of purchasing a solar heating system if he considers the life cycle cost of the system, including estimated fuel costs, and is not deterred by the high initial cost. The decision is essentially a trade-off of present capital investments against future costs of fuel. In fact, consumer decisions are not often made in this

*

See statement 6–3, by H.Brooks, Appendix A.

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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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way. Because people tend to value the present more than the future, most near-term decisions would favor nonsolar approaches because of their lower first costs, even if the relative life cycle cost per million Btu of solar energy and the alternatives were equal. Consumers may also be deterred by expectations of technological improvements (hence lower costs) in solar systems.

However, if public policy so dictates, because of the social and environmental benefits from solar heating, it is possible to bring decisions on new heating systems more into line with minimum life cycle cost and thus to render consumers more likely to select solar systems on a Btu-for-Btu comparison with conventional alternatives. This can be done by means of additional solar tax credits, low-interest or interest-free loans, thermal performance standards for buildings or additional taxes on nonrenewable fuels. Such measures would help solar heating gain a significant market share earlier than it would otherwise. (There is now in place a federal tax credit of up to $2200, as a variable percentage of the system’s cost, for installing solar, wind, or geothermal equipment in the home.)

WATER HEATING

Domestic water heating is easily and efficiently performed by present solar technology. The water temperature desired is about 140°F, which is well within the capacity of simple flat plate collectors. Systems being marketed today are superior in performance to the thermosyphon systems once widely used in Florida.

The cost of a solar water heating system for an average single-family residence has stood at about $1500 or more in recent years, increasing with inflation. This compares to about $200–$300 for a conventional system. The life cycle cost of delivered heat from such a solar water heater might be as low as $7.50 per million Btu, although it is more likely to be at least $10 per miilion Btu. The latter cost is comparable to that of heat from electricity at its present national average residential cost (but about half the replacement cost of electricity). It is about twice the average residential cost of heat from natural gas or fuel oil. The cost advantage of natural gas and fuel oil would be increased if the efficiency of fuel-fired water heaters, now about 50 percent, is improved in the future. Such improvements in efficiency might offset fuel price increases for a decade or more and maintain the present relative cost relationship of solar and fossil fuel water heaters. Solar water heating is a mature technology, and no great reductions in the cost of units are anticipated, though experience with solar heaters should lead to more efficient use of labor for installation, slightly reducing the first cost of installed units.

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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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SPACE HEATING AND COOLING

Solar space heating and cooling technologies fall into two groups. In so-called active systems, water, air, or some other working fluid is forced through heat exchangers heated by the sun; the heated fluid is then moved by pumps or fans to where heat is required or to a heat storage volume, where the heat can be retained until needed. Passive systems involve careful design of buildings to take advantage of solar radiation, natural convection, shading, and the like to heat or cool without the need for special heat-transfer systems. Active systems are more versatile and allow better temperature control. They are also much more expensive in general.

Active Solar Space Heating

Active solar energy systems provide space heating for several hundred buildings in the United States, and the number will soon be in the thousands. Such systems collect solar heat in water, antifreeze, or air and move it through pipes or ducts to heat the building space directly or to serve as a source of low-temperature heat for further boosting by heat pumps. Some of these systems have been wholly or partly supported by governments or other organizations as demonstrations, but others have been paid for entirely by the building owners. Experience with these systems has shown the need for further performance and reliability.8

Is the existence of these solar-heated buildings proof of the adequacy of present technology for solar space heating? To answer this question, it is necessary first to define the objective of such systems. Such active systems are intended to serve as thermostat-controlled heat sources to replace some of the energy used by conventional heating units. Most of the buildings in which they are installed are in most respects of normal design and construction. In such buildings active solar space heating systems are in direct competition with conventional systems.

To make economic comparisons of solar space heating systems with conventional systems, it is obviously necessary to know the costs of installed solar systems. There is considerable uncertainty about these costs even at present. Following are assessments from several sources.

  • Energy Research and Development Administration/Mitre: A current installed system cost of $20/ft2 of collector is assumed.9

  • Department of Energy/CS: Current installed system costs range from $25 to $40/ft2 of collector.10

  • Solar Energy Research Institute: Surveyed costs of actual installed systems are $39–$43/ft2 of collector.11,12

  • Lovins: Intelligently designed and installed systems in today’s market are $10–$15/ft2 of collector.13

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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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For the following calculations, we use a value of $20/ft2 of collector. The collectors themselves account for about $8 of this cost. The chances of large cost reductions in commercial, contractor-installed systems have been widely discussed in the past few years, and it appears unlikely that the cost of collectors can be brought down much below $8/ft2, though future collectors at this price may be more efficient than those available now. The costs of components other than collectors (e.g., pipes and valves) have been rising faster than the rate of general inflation. On-site labor hours can be reduced by the use of prefabricated units, but it can probably be anticipated that the hourly cost of construction labor will rise at or above the rate of general inflation. Thus, the installed cost of solar space heating systems will probably remain close to what it is today, though homeowners willing to invest their own labor can lower these costs greatly. Similarly, very large systems, built to provide heat to large buildings or entire neighborhoods, could take advantage of some economies of scale.

Given an assumed system cost of $20/ft2, the cost of energy from such a system can be calculated. For example, each square foot of collector, of average efficiency and at an average location, will provide annually about 0.12 million Btu of useful heat (i.e., delivered when heat is actually required; this quantity can vary ±50 percent with climate and design factors). If the interest rate on a 20-yr mortgage is 11 percent, then that square foot costs $2.50 annually in mortgage payments, and the cost of delivered energy is $2.50/0.12, or about $20 per million Btu. This is about 4 times the current national average price of heat from natural gas with a well-designed (70 percent efficient) furnace and about twice the present price of electric resistance heat (or about equal to the replacement cost). Note that the range of system costs given earlier would result in costs of delivered energy from half to twice this value.

The true cost to the consumer, of course, will also vary because of factors other than system costs. For example, the interest on the mortgage would represent an income tax deduction. Further, the owner of a solar heating system now receives a federal income tax credit (and in many states, a state income tax credit or rebate). Property taxes might be higher because of the added value represented by the system, though some states have passed laws exempting solar systems from property taxes. In addition, the yearly output of useful heat per square foot will vary from place to place; systems installed in very sunny, relatively cool climates where heating is necessary for longer periods each year would cost less per million Btu than those in less appropriate climates.

These uncertainties are compounded by the need to weigh the costs of the solar system against the expectation that costs of more conventional alternatives will rise more rapidly than general inflation over the assumed 20-yr life of the system. The reliability of the system and the cost of

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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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maintenance and repairs (and the prospect of technological advances that improve performance) must also be estimated. Expectations about its resale value also play an important part.

Each potential buyer of a solar system will, for individual reasons, make certain assumptions about these cost considerations. Some may choose to compare the cost of solar heat with that of using electricity rather than natural gas, even though gas is now cheaper, because they expect gas to be unavailable or extremely expensive in 20 years. Some may assume that the price of alternatives will rise fast enough over the 20 years to make solar heating economic. Some may install solar systems for noneconomic or personal reasons, just as consumers often prefer higher-cost electricity over cheaper fuels because of its perceived advantages in convenience or cleanliness. We can state no universal conclusion as to the relative attractiveness to consumers of solar space heating and conventional sources. However, because of the expected long-term economic, social, and environmental benefits, it is appropriate for the government to grant tax advantages or other subsidies that alter market economics, thereby accelerating the introduction of this technology.

Solar Space Cooling

No technically and economically adequate solar space cooling technology is now available for small residential applications. A number of approaches that are being explored are described in the report of the Solar Resource Group.14 None of these approaches yet meets the requirements of simplicity, low cost, efficiency, and effectiveness of waste heat rejection that will likely be necessary for consumer acceptance.

A fundamental problem is that it is very difficult to operate an air conditioner from a flat-plate solar collector when the waste heat must be rejected by a dry cooling unit (the type used by most residential air conditioners). The efficiency of such a unit is very low, and at other than design conditions, the unit may fail to perform at all. To solve this problem at present, one must either (1) operate the collector at higher temperatures (which requires more sophisticated collectors and some form of energy storage other than hot water) or (2) use an evaporative, or “wet,” cooling tower (which consumes significant amounts of water and requires frequent maintenance). Neither solution is satisfactory, and a new approach to solar cooling for individual residential buildings may be required.

Solar cooling of larger buildings or clusters of residential buildings is less difficult. This is because economies of scale allow use of fairly complex units and because such facilities already have maintenance arrangements for their heating and ventilation systems. A number of systems have been demonstrated successfully, and a commercial 25-ton solar cooling unit is

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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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now on the market. However, such systems (including collectors) cost too much at present to compete with gas-fired absorption air conditioners in regions where natural gas is available (gas-fired, rather than electrical, units dominate this market).

The use of solar energy to supplement conventional fuels in nonresidential cooling will be made generally feasible if low-cost solar collectors that provide low-pressure steam become available, since large cooling units are now generally run on low-pressure steam (usually obtained from combustion of natural gas). Such collectors are now under development for industrial process heat applications. If this development effort is successful in meeting its cost goals (see below), it will greatly improve the feasibility of solar space cooling in large buildings.

Impact on Utilities

If electricity is used to provide backup energy for a solar heating and cooling system, there is potential for a significant and adverse impact on the utility. It would be inefficient and uneconomic to use electrical generating capacity only to provide occasional supplemental energy, whether to solar energy systems or to other sources of intermittent demand such as all-electric homes. The Electric Power Research Institute investigated this question from the perspective of minimizing the total cost of the system including the building and the utility and concluded that solar heating and cooling systems are not inherently less efficient in their use of electrical generating capacity than conventional all-electric homes.15

INDUSTRIAL PROCESS HEAT

Another potentially important direct use of solar energy is to provide industrial process heat for use in small- and medium-size applications, as in laundries, food processing operations, and crop drying. At present about half of industrial process heat in the United States is supplied by natural gas, which is increasing in price and becoming less readily available. The temperature requirements of industrial processes range from warm water to very high-temperature (more than 1000°F) gases, but the heat is used mostly in the form of low-pressure steam.

Lower-temperature (less than 200°F) processes can be served by flat-plate collectors, so here the performance of present solar technology for fuel saving is adequate. However, for most industrial hot water applications the cost of present solar systems is not competitive with that of fossil fuels at current prices. If natural gas and fuel oil become much more expensive or government regulations limit their availability to industries, and at the same time environmental restrictions preclude on-site combus-

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tion of coal, then solar hot water systems may be selected over the principal alternative, electricity, on the basis of lower life cycle cost.

No adequate technology is now available for producing low-pressure steam with solar collectors. Significant contributions by solar energy to industrial processes will require development of collectors for producing steam at about $5 per million Btu. Steam-producing collectors are under development and should soon be available, but it is not known if or when this cost goal will be met.

There is also no available solar technology for producing very high temperatures (greater than 600°F) for industrial processes. Such technologies will be difficult to develop, because of requirements for high concentration of sunlight and for high temperature. They may therefore be more costly than systems using the relatively low temperatures provided by stationary collectors.

ACTIVE AND PASSIVE SOLAR ENERGY SYSTEMS AND ENERGY-CONSERVING BUILDING DESIGN

In the past few years most of the efforts to conserve energy in buildings have focused on alterations that can be made in existing buildings to reduce their conduction or infiltration heat losses in winter. However, over the time period considered by this study, much of this stock of existing buildings will be replaced. Thus, a more important question is how new buildings can be designed to use less energy.

The combination of energy-conserving design and passive solar design can reduce energy use per square foot of building area to half its present value, and in favorable circumstances perhaps to a third or less. This would reduce the energy requirements for building space conditioning from a major part of the nation’s energy demand (22 percent today) to a minor one.*

If buildings are to be properly designed, all factors determining their external environment must be considered. The external environment is usually thought of simply as causing part of the energy demand within the building, but it may also be used to displace some of the demand. Of particular interest are the possibilities of using the sunshine falling on the surfaces of the building to provide some of the functions of space conditioning, by so-called passive solar design.

In a passive solar energy system the transfer of solar heat takes place by natural convection, radiation, or conduction without the use of special mechanical pumps or blowers. Active solar systems generally provide

*

See statement 6–4, by B.I.Spinrad, Appendix A.

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better control of the use of solar energy in a building than do passive systems, but at a substantial increase in complexity and first cost. In some cases, the use of passive systems may lead to greater excursions in temperature and humidity than the more expensive active systems.

Active and passive solar energy systems are not mutually exclusive; that is, a building can be designed with passive solar concepts to minimize its energy demands for space conditioning and can use an active solar system to satisfy part of the remaining demand. Further, some systems that use air for collection and transfer of solar heat cannot be clearly categorized as either active or passive but represent a hybrid.

Passive solar energy systems can do more than simply provide heat. Those aspects of a building that constitute a solar passive heating system will also alter the other energy requirements of the building. For example, a carefully conceived passive system may decrease the cooling load in summer by using thermal convection exhausts to help draw breezes, or air from a cool storage volume, through the building in summer. The passive system may also improve internal lighting of the building by “daylighting” and thus reduce the air conditioning load normally due to waste heat from lamps. In designing a passive solar system, it is necessary to consider simultaneously all the energy requirements of the building, as well as the ultimate objective of creating pleasing and functional space in the building. This is not a trivial task, and the processes involved in the operation of a passive system are subtle. While a passive system is physically less complex than an active one and has fewer or no moving parts, it requires a more sophisticated match of building design to the external environment and the needs of the inhabitants. Active systems have heavy first costs in components and installation; the first costs of passive systems are in careful thought and delicate balance of design.

A number of design concepts related to solar active and passive design do not involve direct use of the sun. For example, cooling can be done by radiating heat to the night sky or blowing cool night air through crushed rock to provide storage for daytime cooling. Whether or not these concepts should be labeled as solar energy is irrelevant, since they are part of the repertoire available to the designer of a solar-heated building, and they work together with the solar part of the design in determining the overall performance of the building.

How effectively can a passive solar system provide the space conditioning of a building? Experience is still limited to only a few building types and climates, but the results are very promising. One example of the use of passive design in a large nonresidential building is a California state government office building in Sacramento. Some residential passive solar buildings constructed in moderate climate areas, such as the Skytherm House in Atascadero, California, require no other source of energy for

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space conditioning. Others use only small stoves (often wood burning) as backup heat sources. In climates with harsh winters, more backup is needed. In climates with hot, humid summers, passive design will not always provide sufficient comfort, and air conditioning will be required. However, on a national scale, the widespread use of solar passive design concepts can drastically reduce the amount of energy required for space conditioning.1618

How expensive is the energy provided by passive solar systems? Although a passive system provides energy in a way that cannot be directly compared with the workings of a normal heating and cooling system, the energy cost of a passive system can in principle be estimated by comparing the total initial cost of the building and its total energy requirements for all purposes with those of an otherwise similar building without passive solar design. Convincing analyses of this type have yet to be done. The type, layout, and quality of the internal space in a passive building depend on the special features of its passive design. For example, if the passive design includes a greenhouse placed on the south side of the building to serve both as a solar collector and for aesthetic purposes, what fraction of the cost of the greenhouse should be included in cost comparisons?

What needs to be done to bring about the use of solar passive designs? A number of things—some technical, others institutional—are required. Much knowledge is required concerning how various elements of building structures and heating, ventilation, and air conditioning systems interact with the building’s environment. This will require data and methods of analysis, design, and performance prediction that are not yet available. Research on these topics is essential. In addition, some new passive system components will need to be developed, though most passive concepts require only proper use of existing components.

Building standards that properly regulate the use of passive concepts in design need to be adopted. Some proposed changes in existing standards, intended to achieve energy conservation, might actually forbid or restrict passive designs, for example, by imposing arbitrary limits on amounts of window space. All changes in building standards intended to aid energy conservation should be properly related to the ultimate performance of the building, so as not to exclude passive solar design innovations that prove effective in certain situations. Training and education of architects, mechanical engineers, and contractors will be necessary to bring about well-informed use of passive concepts.

Finally—and this problem is shared with active solar systems and other methods of energy conservation in buildings—the obstacle posed by the present emphasis on first cost of a building must be resolved. Financial or regulatory arrangements to bring purchase decisions into line with life

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cycle costs are necessary to speed implementation of energy conservation in this and other ways, with the social and environmental benefits discussed elsewhere in this report.

Extensive future application of passive solar techniques and energy-conserving building design may have adverse implications for the economics of active solar heating and cooling. Successful use of passive solar and conservation techniques reduces energy requirements, which substantially limits the further savings obtainable by active solar systems. Such small savings could make the high capital costs of active systems unattractive. This might be true even though a smaller collector surface area would be required, since the cost of the noncollector parts of active solar heating systems do not decrease in proportion to collection area as size is reduced. This intimate technical and economic relationship among active and passive solar systems and energy-conserving building design makes it important that government-sponsored research and development and demonstration programs in these areas be integrated.

RETROFITTING FOR SOLAR HEATING

An important determinant of the rate at which solar energy can supplement the nation’s energy supplies is the extent to which retrofitting of existing buildings can contribute to the use of solar heating and cooling.

That existing buildings tend to be so energy inefficient is a major barrier to retrofitting them with solar energy. It would be economic nonsense to install a solar system to provide heat that leaks rapidly out of the building, just as it is to use increasingly expensive fossil fuels for the same purpose. Thus the first step in retrofitting should be proper insulation and leak tightening. Furthermore, when a building is thermally inefficient, a much larger area of solar collectors is required to heat it. This much free area is often hard to obtain on the roofs of existing buildings, generally cluttered with vents, pipes, and other obstructions. Thus a retrofit solar system will often provide only a small fraction of the total heating load.

However, the greatest barrier to retrofitting buildings with active solar energy systems is the high cost of installation in existing structures. It is expensive to add collector fittings and supports on roofs, to run pipes or ducts through walls, and to fit heat storage tanks into buildings. Further, existing roofs are not always properly exposed to sunlight. For these reasons, active system retrofits are sometimes placed externally to buildings, with the collectors on separate, specially constructed structures and the storage tanks buried in yards rather than in basements. The few demonstrations of retrofit of active space heating systems have cost in the range of $40–$84 84 in total system cost per square foot of collector area, which corresponds to paying roughly $40 per million Btu of space heat.19

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Another barrier to solar heating retrofit is the imposing level of understanding of solar design principles, architecture, and construction practice necessary to custom-design a retrofit system for an existing structure. Buildings vary so greatly that there are literally millions of special cases. To make a retrofit at minimum cost, all aspects of the existing structure must be used to advantage. Thus, designing a good retrofit is more difficult than designing a new house with a common type of active solar system. A further barrier to solar retrofit is encountered in financing the construction. When a solar system is included in the initial construction of a building, the cost can be included in the mortgage. However, for a solar retrofit a special loan must be arranged, generally with a higher interest rate than that typical for a mortgage on a new building.

It is worthwhile as part of the nation’s energy conservation effort to seek techniques for solar heating retrofit of buildings at reasonable cost, and ways to lower the barriers inhibiting their use such as by encouraging and financing such retrofits. Inability to establish a market for solar retrofits will greatly retard the potential market penetration of solar heating. Attaining significant contributions from solar heating and cooling by 1990, for example, would require retrofitting a substantial fraction of existing residences, because only about 25 percent replacement is expected from new construction during that period.

It is much easier to retrofit for domestic water heating, since the cost is almost as low as installation in a new building. The solar heater can be tied into the existing water heating system and needs little additional plumbing and no heat storage. Most roofs have room for the small collectors. Since in many regions as much energy is used for water heating as for domestic space heating, such retrofits could have significant impacts on energy consumption. A good solar water heating system can provide 75 percent of the energy used for water heating in a household, or on the order of 20 million Btu/yr in favorable circumstances.20

Some promising approaches to solar retrofit for space heating have begun to appear. These are generally passive, rather than active, design approaches. At present the most successful is the add-on greenhouse. These are light, frame structures with double glazing, which are added to the south sides of existing buildings. To temper the swings of temperature between day and night, thermal mass is added to the greenhouse in the form of crushed rock, water-filled drums, or the like. Warm air is exchanged with the building by natural convection or by fans and ducts. The result is a passive solar heating system that may not be as effective as it would be if the entire building had been designed initially for this, but it does provide significant energy savings. This approach is being rapidly adopted in northern New Mexico, where the clear, cold winters allow this

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system to work very well. Such units should also be able to contribute some useful space heating in other regions.

The widespread use of solar water heater retrofits and passive add-ons for space heating could make significant energy contributions by 1990. However, solar and conservation measures must be designed to complement each other; such retrofits should always be done in conjunction with other measures for energy conservation in the building. With the use of economic retrofits for conservation and solar heating (20 percent or better payback on investment),21 the average energy requirement per house can be cut by half or more; were 50 percent of U.S. residences so retrofitted, a savings of 3 quads/yr of oil and gas would be realized.* There are significant challenges to governments in designing building codes, incentives, and education programs to aid the adoption of such approaches to solar retrofit.

SOLAR ELECTRICITY GENERATION

Four concepts for generating electric power from solar radiation are under active development. Today the one that is receiving the most attention in government research programs is solar thermal conversion, which involves concentrating sunshine to achieve high-temperature heat. Next are photovoltaic generation with solar cells; wind power; and ocean thermal energy conversion, which uses floating power stations that exploit the temperature difference between the ocean’s surface and subsurface waters to run heat engines.

Each of these concepts has important potential for the long term as an inexhaustible source of power. Some wind power applications are technically well developed and fairly economical and could play significant roles in the intermediate term as fuel savers. However, solar-generated electricity, today costing several times average electricity costs, is unlikely to penetrate the market on a substantial scale much before 2010 unless the prices of competing fuels continue to rise rapidly or obstacles are met in installing more conventional power plants at the rate desired. In general, these solar technologies are so much less economic than other sources of power that their entry into the market would need to be massively supported by government, a policy that could be justified only if the environmental advantages over other forms of electricity generation or the need to conserve fossil fuels becomes of overriding national importance.

*

Statement 6–5, by B.I.Spinrad: My statement 6–4, Appendix A, also applies here.

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electric technologies with utility grids have been examined by a number of studies.22 However, general conclusions have not yet been attained because of the complexity of the issues. The actual situation depends on the region and type of resource (e.g., wind resources vary greatly between the Great Plains and New York),23 on the load profile of the utility, and on the generating fuel mix. In favorable circumstances, solar electric generation will have significant value as a fuel saver, even without capacity credit.24

Solar electric research and development has been neglected until recently. Although the federal research and development program has been growing rapidly, actual research and development projects, apart from analyses and design studies, have been under way generally only a few years, and it is too early to expect significant results. The next 5 years should provide the beginning of a strong technology base for solar electric conversion and should allow a much more realistic estimate of the extent to which solar technologies may be competitive with other sources of electricity. Present technology, lacking the benefits of extensive previous efforts in research and development, nonetheless provides some basis for estimating future capabilities and costs.

SOLAR THERMAL CONVERSION

Solar thermal electric conversion dates back to the early years of the twentieth century; several technologically successful plants have been operated over the past 70 years. The solar thermal conversion approach is typified by the central receiver concept now being developed by the U.S. Department of Energy and the Electric Power Research Institute, in which a large field of mirrors, built to follow the sun, concentrates heat onto a boiler atop a tower. Such a combination of mirror field and tower would be designed to provide from 10 to 100 MWe of peak electrical output and would occupy up to 1 square mile. This system could probably be designed to provide intermediate-load power to a utility grid. Because of the intermittent nature of sunshine and the lack of a practical energy storage technique, however, it would be unable to displace much generating capacity.

Solar thermal conversion systems for smaller-scale on-site generation would be roughly similar, but smaller. Such systems would be sized to match the requirements of local loads and might range from 1 to 10 MWe in electrical capacity. Note that 1 MWe requires about 6 acres; this amount of land may be difficult to find for local loads.

The present state of central receiver solar thermal generation is indicated by the design for a federally funded 10-MWe pilot plant at

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Barstow, California, and by design studies for solar thermal repowering of existing oil- and gas-fired plants in the Southwest. In repowering, a solar thermal field provides supplementary power, using existing turbines and generators. Construction of the Barstow pilot plant is expected to cost $123 million, or over $10,000 per kilowatt (electric) (kWe). (This is about $1 kWh, assuming a load factor of 0.2 and an annual fixed charge rate of 15 percent.) A commercial solar thermal electric plant built with present technology and operational in 1985 would probably cost about one fourth this much and have generating costs about 5 times the current average. Such plants would probably be designed with more energy storage than Barstow. Increasing the plant load factor, which would reduce the cost per kilowatt-hour, depends on the development of a practical technology for storing high-temperature heat or electricity. Early experiments conducted on storage systems for the 10-MWe pilot plant are encouraging, but much needs to be done before high load factors can be achieved. An alternative to storage is the use of oil and gas for backup generating capacity, as in repowering.

One promising extension of solar thermal electric conversion designs is the so-called solar total energy system, in which the waste heat from the generator is used near the power plant. No commercializable technology for this has been demonstrated, but an experimental system is being built at Shenandoah, Georgia. The electricity from these early demonstrations will probably have costs of the same order of magnitude as the first electricity from central station solar thermal plants, or several hundred mills per kilowatt-hour. However, until the first solar thermal total energy system demonstrations are completed, it will be difficult to estimate costs precisely.

Environmental Health, and Social Considerations

These impacts have been considered by the Risk and Impact Panel,25 and the following material makes use of that analysis. An environmental assessment of solar energy is also being performed by the Department of Energy.26,27 The most significant environmental impacts of large solar thermal conversion plants would be their land and material requirements. Studies suggest that such a plant, with enough storage to generate 100 MWe at a 40 percent load factor, if located optimally (e.g., in the southwestern desert) would take up at least 1 square mile. This is about as much land as would be taken during the entire expected life of an equivalent coal plant (including underground mining) and an order of magnitude greater than that for the equivalent nuclear plant using high-grade ore (including mining, milling, etc.). Depending on assumptions

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about the efficacy of reclamation, a coal-fired plant based on strip mining may require more or less land than the solar plant.

The materials requirements of this approach are also great. A 100-MWe solar electric plant would use, for the arrays of heliostats alone, 30,000– 40,000 tons of steel, 5000 tons of glass, and 200,000 tons of concrete, as compared with about 5000 tons of steel and 50,000 tons of concrete for the construction of equivalent capacity with nuclear power. Equivalent capacity in a coal plant would require considerably less steel and concrete.28 Many of the air pollutants produced in mining and manufacturing the steel, glass, and cement for such a solar thermal plant—notably sulfur and nitrogen oxides, carbon monoxide, and particulates—would be comparable in kind and amount with 1 year’s effluents of an equivalent coal-fired plant using current control technology, except for the particulates, which would be an order of magnitude greater for the solar plant. Thus, over the (30-yr) lifetimes of the two systems, the solar plant would be an order of magnitude more benign in most pollutants. The solar plant would be somewhat worse in effluents over the lifetime of the plant than a natural-gas-fired electric generating station of equivalent capacity (with the exception of nitrogen oxide emissions and the long-term carbon dioxide hazard) and much worse than the equivalent nuclear generating capacity.

Ecological and environmental effects of this technology in a desert location would be considerable. Burrowing animals and their habitats would be destroyed during construction, and sites would have to be chosen to avoid dense wildlife populations or endangered species. The desert surface would be altered by construction, road building, off-road vehicle traffic, building of transmission lines, and so on. This would affect erosion in the region. Wind erosion would increase because the protective desert crust, or pavement, would be broken, and water erosion and runoff would also increase, especially along roads. The hydrological cycle would be affected by this and by modification of evaporation rates due to the heliostat canopy. Evaporative losses resulting from the use of wet-tower cooling or storage reservoirs would also significantly affect this cycle. Availability of cooling water (chapters 4 and 9) may restrict the deployment of this or any other electric generating option in the Southwest and elsewhere.

Central receiver electric generating plants could also alter local and regional climates by modifying the radiation balance of the natural desert. Considerable amounts of dust are likely to be introduced into the atmosphere by construction activity. The longer-term climatic effects of this would probably not be as significant as those of modifications of the radiation balance, but the temporary potential for distant effects might be

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greater, since dust clouds can travel long distances. Some fogging could occur in the vicinity of the plants if wet-tower cooling were used.

The social impacts of building central receiver plants in large numbers in the Southwest could be substantial. There might be significant population shifts to that region. Eventually, the availability of more energy in this region might attract major industries. Some people of the Southwest may object to added stresses on the environment, such as new demands on water supplies; others may welcome new industries.

PHOTOVOLTAIC CONVERSION

Photovoltaic cells are semiconductor devices that generate small electric currents when exposed to light. Power can be produced by wiring the cells together in arrays, either exposed directly to the sun or equipped with concentrating mirrors or lenses that track the sun. Such arrays have been used for a number of years in the U.S. space program and for power generation on earth in remote places. They are much too expensive at present to compete with conventional means of producing electricity. Because of the complex semiconductor technology by which the cells are made, and their relatively low efficiencies, silicon and cadmium sulfide cells now sell for $10,000–$30,000/kWe of peak output (depending mainly on quantity purchased) when used without concentrating mirrors or lenses.29 This corresponds to costs of about 1000–3000 mills/kWh, more than 20 times the prevailing cost of residential electricity. The Department of Energy has identified and is funding development of processes that, if integrated, could produce silicon cell arrays for $2000/kWe of peak output. These costs might be further reduced by technical breakthroughs; the federal photovoltaic program has a goal of reducing prices of photovoltaic arrays (not including associated electronics and storage) to $500 per peak kilowatt by 1986. Whether this is possible is the subject of some controversy, though most experts agree that these array costs could be brought down to $1000–$2000. The major thrust of this cost reduction effort is presently directed toward incremental advances in the technology for automated and mass production of silicon photovoltaic cells. However, it is argued by some that achieving the cost goals for large markets will require a new type of photovoltaic device which will come only from research on advanced device concepts, perhaps thin films or amorphous (rather than crystalline) materials. If so, the creation of a major industry based on silicon cells may be wasteful.30

Photovoltaic systems may be used in any size, from single households to large central generation stations. Photovoltaic conversion for on-site

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generation of electricity may also be used in a total energy system. The land requirements for large photovoltaic systems would be perhaps twice those of solar thermal systems unless highly efficient photovoltaic devices are developed.

Environmental and Health Considerations

Large-scale photovoltaic generators would probably be located in arid regions. They might be installed with concentrating heliostat fields or in multiple mass-produced arrays. Greater land areas would be required than for solar thermal plants unless efficiencies can be increased, but local ecological disruption and habitat destruction would be similar. The capsules of the solar cells might degrade in time, and their decomposition under normal conditions or during a fire could produce hazardous reaction products, depending on the materials of which they are made. These should be studied carefully. Some proposed solar cells contain environmentally dangerous substances, and the mining, manufacturing, and distribution of these must be handled with care. These hazards are roughly similar to those of other products using semiconductor components, though much greater in scale. With attention to these hazards during the development of advanced photovoltaic technology, it will be possible to ensure adequate safety.

WIND ENERGY CONVERSION

Wind turbines are fairly simple mechanical devices, and the problems of connecting wind generators to an electric grid have been successfully resolved by experience in Europe and the United States. The greatest use of wind-generated electricity was in fuel-short Denmark during World War II, when wind turbines generated 18 million kWh for local networks over the course of more than 7 years (an average power of about 300 kWe).

Wind energy conversion may be accomplished by horizontal-axis propellers, by vertical-axis turbines, or by various other types of devices. The resulting mechanical energy is easily converted to electricity. Units can range in size from half a kilowatt to several megawatts. Wind energy is more limited in choice of sites than other solar technologies, because available wind energy density varies greatly with average wind speed. To accommodate variable wind energy to the regular cyclic needs of a utility system, large amounts of electrical storage capacity are needed unless the delivered energy can be economically used when the wind happens to be blowing or if the wind energy is used in a supplemental mode. It has been

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proposed that some energy-intensive industries, such as nitrogen fertilizer production, may be able to use wind energy in an intermittent mode. Wind turbines are also useful for pumping water or compressing air. The promise of wind energy for operation in power grids is thus significantly enhanced where pumped hydroelectric storage or other energy storage capacity is available.

Small wind generators are now sold for several thousand dollars per kilowatt of rated output. Dominion Aluminum, Ltd., of Canada offered in 1977 to sell for $175,000 copies of a 200-kWe prototype machine built in Canada. This cost corresponds to about 37 mills/kWh on a good site. The value of such a machine as a fuel saver in a utility grid will generally be less than this cost would imply, because of the need either to supply additional energy storage or to maintain backup generating capacity for times when peak electricity demand and peak winds do not coincide.

The most immediate prospect for wind technology would be the development of a diversified design and manufacturing effort directed generally at machines with generating capacities of about 1 MWe. The market potential is likely to be highly differentiated and, relative to total domestic energy demand, modest. The Solar Resource Group31 estimated, in its high-solar scenario, that about 50 GWe of capacity from wind energy could be installed in the United States.

Environmental Considerations

The main environmental impacts of wind power are its use of land, local ecological disruption, and the aesthetic considerations of noise and cluttered landscapes. Because the devices are rather small in comparison with conventional power plants, it takes a large number of them to produce the power equivalent of a typical 1000-MWe fossil or nuclear power plant. These must be spread over a large area so that the individual devices do not interfere with one another, and the land area required would be several times that of a solar thermal power plant of equivalent output. The land required for 1000 MWe of installed wind electric capacity has been estimated to be between 200 and 500 square miles.32 (This land could still be used for other purposes, such as agriculture, so it need not be considered lost.) Spinning turbine blades could injure birds, and the noise could disrupt wildlife. A potentially serious problem is that spinning metal turbine blades cause interference with television video reception, especially in the UHF bands. Access roads for construction and maintenance, and the electrical interconnection of many units, would pose a severe environmental problem, especially in relatively primitive areas.

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OCEAN THERMAL ENERGY CONVERSION

Ocean thermal energy conversion (OTEC) uses the approximately 40°F temperature difference between the surface and deep waters of tropical oceans to drive heat engines. A single plant might produce 100 MWe or more of electrical output. The energy could be transported to shore as electricity or as fuel generated by electrolysis of water, or it might be used on the ocean site for energy-intensive industrial processes. This is the only earth-based solar electric technology that is naturally suited for base-load generation of electricity; all others require special storage devices or other backup systems.

A small ocean thermal conversion experiment was operated briefly in 1929. However, no plant of the closed-cycle type now under consideration by the Department of Energy was built and operated until 1977, when the Mini-OTEC plant was demonstrated in Hawaii. This plant generated 10 kWe from a gross output of 50 kWe. (The balance was consumed by water pumping and other plant functions.) If present experiments on heat exchangers are successful, a multimegawatt pilot plant may be built in the 1980s.

There is now no basis for cost estimates except conceptual designs that assume various structures and operations that have not been used in the marine environment. Such estimates correspond to a busbar cost of 70 mills/kWh for electricity delivered to shore. There is great uncertainty and controversy about the cost estimates for ocean thermal conversion. Resolution must await systems tests under realistic conditions. There is also technical uncertainty about the potentially serious problem of fouling of the heat-transfer surfaces in the plant by organisms.33 This problem is being addressed by current Department of Energy research.

Some attention is being given to deriving power from ocean waves, tides, and currents. Research and development on wave energy converters is under way in England, and some limited use has been made of energy from tides, but these resources are too small to contribute significantly to U.S. energy needs.

Environmental Considerations

Two OTEC designs, differing radically in their environmental implications, are being considered. In one, cold water is pumped from a depth of more than 1000 ft and released at the surface. This could bring to the surface nutrients that could support a plankton bloom. It could also lower the surface temperatures, possibly altering local climate and commercial fish populations in the region. Evaporation rates would be reduced in the ocean

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near the plant; if many plants were built, large-scale precipitation patterns could be altered. Biocides might be used to prevent biofouling, but their release could be harmful to marine life. Working fluids could also be harmful if released accidentally. It has been suggested that the upwelling of cool water from the ocean depths might release substantial amounts of carbon dioxide into the atmosphere (at a rate about one third that of an equivalent fossil fuel plant). Because of the complexity of the ocean carbonate chemistry involved, this is only speculation.

The second design would pump the cold water back down to about the depth from which it was taken. This would avoid some of the problems mentioned above but would be more expensive.

SOLAR ENERGY SYSTEMS IN SPACE

Proposals have been advanced for development of space-based solar energy systems designed to generate terrestrial electricity with microwaves beamed from space. A considerable amount of ingenuity and analysis has been devoted to design concepts and performance criteria for such systems, which might be constructed from earth-based materials transported aloft by space shuttles, or perhaps from lunar or asteroid material. In the distant future such a system might become the most suitable energy source for large electricity grids if energy sources like breeder reactors, nuclear fusion, and earthbound solar electric technologies should prove to be unattractive. The attraction derives from a longer solar day, freedom from weather, and a solar flux about twice that of the southwestern desert on a clear day. In our judgment this technology, at huge estimated cost, cannot possibly become a substantial energy source until major earth-based competitors are shown to be unattractive or insufficient. Funds spent on satellite power for some decades, except for limited research and development, would be funds spent prematurely.

Space solar systems would require much initial space shuttle traffic, which would introduce significant amounts of energy and effluents into the atmosphere at all levels, though the effluents might be largely water vapor. The environmental consequences of this are not known. Microwave beams appear to be the most practical way to transmit the energy to earth, and the effects of this on the upper atmosphere might be significant. The effects of microwaves on living things are cause for concern also, although present plans call for fairly diffuse beams.

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SOLAR FUEL PRODUCTION

As the availability of natural oil and gas declines, the need for liquid and gaseous substitutes will become critical. Large quantities can be provided by coal conversion and perhaps oil shale conversion, as discussed in chapters 3 and 4, but at rather substantial environmental costs. There are a number of solar technologies, most of them using energy trapped in plant matter (biomass), that could supply useful amounts of these fuels in the intermediate term. Other solar technologies can produce fuels over the long term.

At present the federal solar research and development program places too little emphasis on these technologies; only 6 percent of the program’s funds go to support biomass technologies.34 Some research on generating fluid fuels from organic municipal and agricultural wastes is supported, but this is given a rather low priority. The potential, however, is large; this study has estimated that by the year 2010 an annual total of 5.4 quads of energy could be drawn from municipal and agricultural wastes alone.

Production of plants especially for their energy content, on so-called “energy farms,” could conceivably contribute another 3.4 quads, but the potential is limited by competition with conventional agriculture for land, nutrients, and water and by the severe ecological impacts involved. Growth of algae or other water plants in ponds or in the open ocean may be a more practical alternative.

For the long term, the most attractive potential solar alternative for fluid fuel production is direct photochemical decomposition of water to produce hydrogen. The hydrogen could be used either directly as a fuel or to synthesize hydrocarbon fuels from various sources of carbon, including carbon dioxide from the atmosphere. The approaches to photochemical conversion can be divided into three categories: biological, biochemical, and synthetic. In the biological approach, microorganisms are used to separate the oxygen and hydrogen in water. In the biochemical approach, enzyme systems are extracted from such organisms and used in reactions to decompose the water molecules. In the synthetic approach, chemical systems would be devised without using components from living organisms. Each of these approaches is under development, but none can be considered commercializable before the end of this century.

In addition, there are other techniques that are not uniquely suited to solar energy, but could be driven also by other energy sources, such as nuclear fission or fusion. These include thermochemical processes and electrolysis. Neither approach is very promising at this time, for reasons given below.

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ENERGY FROM BIOMASS

The solar energy stored in biomass, whether obtained from municipal and agricultural wastes or grown to be used as an energy crop, can be made available either directly through combustion (at 85–90 percent efficiency) or indirectly. (When the feedstocks are waste materials, efficiency refers to net yield of energy. However, when biomass is grown specifically for energy, the energy inputs for production must be deducted to give the net energy yield.) Indirect methods that have been proposed and studied are pyrolysis to produce mixed liquid and gaseous fuels, anaerobic fermentation to produce gaseous fuels, and hydrolysis of carbohydrates to produce glucose followed by fermentation of the glucose to produce alcohol. The maximum possible energy recovery efficiencies of these conversion processes are about 50–60 percent for anaerobic fermentation, about 35 percent for pyrolysis, and about 55 percent for hydrolysis and fermentation. Many of these processes are technically well developed.

Municipal Wastes

The use of municipal wastes in such processes holds great promise for the near-term future. Such wastes are already being collected and carried to central locations, often near municipal power plants that could use them or their conversion products as fuel. In addition, disposing of these wastes is becoming increasingly difficult and expensive in many cities, and the energy conversion projects would have value as means of disposal in addition to the value of the fuel produced. Furthermore, such wastes are increasingly being sorted to recover various nonfuel materials, and this operation would at least partially offset the expense of energy conversion.

The economics of recovering energy from municipal wastes thus depend strongly on the value of the nonenergy materials recovered and on the credit given to the operator for disposal of the wastes, which would otherwise be a cost to the municipality. A recent economic analysis of methane production from municipal solid wastes cited a total cost of $5.93 per million Btu for the facility and operating expenses. The credits to the operator correspond to $3.84, so that the net cost of the methane produced is only $2.09, a quite competitive cost for gas. While this pattern varies from place to place and from process to process, it is a reasonable estimate of the cost of converting municipal wastes for their energy content. A variety of processes are now being developed and tested, and it is yet unclear which will be the best in practice.35

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Agricultural Wastes

Agricultural wastes include the inedible portions of food crops, animal manures, and unused portions of trees harvested for paper or lumber. The total amount of energy contained in such wastes is much more uncertain than the energy content of available municipal wastes. Neither the gross availability of such materials (the total amount produced by U.S. agriculture and forestry) nor their real availability (the amount that could be economically collected, transported, and fed into a conversion process) can be precisely determined. The Solar Resource Group estimates that the gross annual availability of these wastes is about 8 quads and that their real availability is about half that value.

The energy available as fuel from agricultural wastes depends on the conversion process. The decision on which process to use in a given situation will be based on the character of the wastes, the ease with which they can be brought to large central conversion plants or converted in smaller plants on site, and the local prices of various fuels and residues from the processes. The decision will sometimes, but not always, be made in favor of the process with the highest net energy yield. The Solar Resource Group estimates that in about a decade collection and conversion of agricultural wastes (with a recovery factor of 25 percent, to take account of both real availability and conversion losses) could yield about 2.2 quads of methane or some equivalent fuel. This value sets the order of magnitude for the following discussion.

The U.S. food system consumes significant amounts of energy, only a fraction of which is used in the actual production of food. Most is used in transporting and processing food after production. The amount of energy used on farms for producing food has been estimated to be about 2.1 quads/yr.36 Of this amount, about half is actually consumed as fuel on the farm, and about half is used in producing fertilizer, pesticides, and other farm chemicals. The amount of energy used in U.S. agriculture has increased rapidly over the past few decades. This increase has been associated with a decrease in the labor inputs to agriculture but reflects also a trend to centralized activities. Cattle feedlots and “chicken factories” are but two examples of this centralization.

This trend has had two major consequences. One is increased consumption of energy; the other is a growing problem of waste disposal from large central facilities. For example, the control of water pollution from chicken factories has been a major goal of the Environmental Protection Agency for a number of years. Disposal of these wastes is costly, and the wastes contain both energy and nonenergy (nutrient) values. Thus, the economic situation is similar to that of municipal waste conversion, except that

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colocation of the waste with a major energy conversion facility is less likely.

Most of the wastes from the existing U.S. agricultural system occur not in central facilities of the type just described but in widely dispersed locations. Their disposal is not seen as a costly process. In fact, the wastes produced at dispersed sites often are used to provide nutrients and other factors important for soil quality (e.g., crop residues are plowed under, and the nutrients are returned from manure on sparsely grazed ranges). While it is possible (though costly) to collect wastes from such dispersed locations and convert them to fuel, it is not clear that there will always be a net energy gain from doing so. If these wastes are not used to maintain soil quality, they must be replaced by other inputs, such as fertilizers. The result may be a net energy loss.

It is interesting, though perhaps not in itself important, that the energy estimated to be available from all agricultural wastes (about 2.2 quads) is about equal to the present energy inputs for agriculture. This fact does gain some importance when it is considered in light of the trade-offs possible in agriculture between energy recovery from wastes and reuse of wastes at the site of production to reduce energy inputs. The degree to which the use of wastes in the field can replace the use of fertilizers or other farm chemicals is not really known, even though it is widely debated.37

It is possible that in future years, through some combination of reuse of wastes and conversion of wastes to fuels, the agricultural sector will be made essentially self-sufficient in energy. However this balance works out in detail, the agricultural sector will not be a major producer of fuels for other U.S. energy needs. The resource is simply too small to have significant impact on a national scale. Conversion of waste materials, however, may reduce or eliminate agricultural energy demands and thus eventually make a net contribution of a few quads to the U.S. energy system.

Growth of Biomass for Energy

Existing farming methods could be used in “energy farms” to produce significant amounts of biomass. Since in an energy farm the goal is to produce the maximum amount of biomass, crops such as eucalyptus trees, rubber plants, or sunflowers might be used because of their rapid growth and high energy content. The energy efficiency of land agriculture is quite low; typically less than 0.5 percent of the solar energy is stored as biomass. However, the photosynthetic process is capable of higher efficiencies, up to 12 percent being noted in some experiments. The efficiency of energy farms could be greatly improved if crops can be developed to perform

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photosynthesis efficiently in full sunlight and not lose excessive amounts of energy by respiration. Furthermore, in the United States competition with food production for land, water, and nutrients is a major consideration. Energy farming would, like other agricultural operations, require substantial amounts of water. The availability of vital minerals such as phosphorus may also be a limitation. Crops now being raised command higher prices as food or fiber than as fuel, and this will probably not change in the near future. Those studying the world food situation do not foresee a period of general crop surpluses such as would be required to provide low-cost biomass from food crops for conversion to fuel.

Competition with conventional agriculture might be avoided by producing biomass in the open ocean. The surface waters there have low natural productivity, but if nutrients could be provided (for example, by pumping nutrient-rich deep ocean water to the surface), a biomass crop like giant kelp could be grown. An experimental test of the concept is under way, but technical and economic feasibility have yet to be proved.

A variety of cost estimates for energy farming have been published. They range from about $0.50 to $2.00 per million Btu of “raw biomass” energy. (If the same land, water, nutrients, etc., could be used with the same efficiency for producing food crops, sale of the crops would provide a return of several times this estimate.) If this biomass is to be converted to either liquid or gaseous fuels, the conversion cost must be included. Estimates for these vary widely, depending on the technologies, plant capacities, and conversion efficiencies assumed. The range of conversion cost estimates is about $0.70 to $3.50 per million Btu. This means a gaseous fuel (methane) from biomass production could cost as much as $5.50 per million Btu, or as little as $1.20 per million Btu (less than the present wellhead price of natural gas).38 It should be emphasized, of course, that these estimates are not based on proven and tested applications of existing technology.

The exemption of “gasohol” motor fuel (90 percent gasoline, 10 percent ethyl alcohol) from federal taxes, and in some cases state taxes, has given a great incentive to the fermentation of grain to fuel alcohol. However, the total amount of fuel available from grain is small, and such crops have high value in international trade. It is unclear whether this use of grain is a net producer of liquid fuels in the United States.39

Deployment of a biomass production technology in marginal U.S. lands would cause drastic environmental alterations. The natural ecosystems of the regions would be replaced by plant monocultures. The impacts on insect and animal life would also be drastic. If the plant type used grows extremely rapidly (as it must for high efficiency) there should be no trouble maintaining it in competition with native plants. However, plant monocultures are very susceptible to infestations and disease. The methods used to

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control infestations and disease are major sources of the pollutants released by food-producing agriculture, and these pollutants should be expected to be a significant feature of biomass production. One possible problem is escape of the biomass plant species from the confines of the energy farm. A plant that grows extremely rapidly under marginal conditions, with no extra water or nutrients, has little to prevent it from becoming widely dispersed and displacing native plant species.

Another question would be raised in the event that a very fast growing plant that prospers in marginal conditions can be developed. There would then be great incentive to adapt this plant to produce food for human consumption, or at least feed for livestock. We assume that efforts in this direction would be made, and that if successful they would put biomass production and food crops again in direct competition for even marginal land.

Another approach to biomass growth is to use the nutrients in liquid wastes from homes, industry, and agriculture to support the growth of algae in ponds. Such systems use bacteria to oxidize waste materials and produce nutrients to feed algae, which in turn collect solar energy to produce biomass. The advantage of this method is that it can be done with efficiencies of better than 5 percent and that it does not compete with agriculture for nutrients. In fact, it consumes waste nutrients that are responsible for eutrophying bodies of water. However, algae produced in such ponds might have a higher value as animal feed than as either a source of energy or a chemical feedstock.

PHOTOCHEMICAL, THERMOCHEMICAL, AND ELECTROLYTIC CONVERSION

In the long term, solar energy can be used with a number of chemical processes that supply fuels without the need for producing or gathering biomass. All must be considered speculative for one reason or another; some await a great deal of technical progress, while others are barred from near-term use by their high costs. The methods being investigated hold the promise of supplying fuels in virtually unlimited amounts, however, and much greater attention should be paid them in federal research and development programs. Commercialization is not likely in this century.

Photochemical Conversion

One attractive long-term alternative to the growth and conversion of biomass is direct solar fuel production by photochemical conversion. The processes now being most widely investigated involve photolysis, the decomposition of water to produce hydrogen by means of radiant energy.

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This can be done by a number of biological, biochemical, and synthetic methods.

In a biological approach, living organisms are used to decompose water into hydrogen and oxygen; thus the term biophotolysis. However, the liberation of molecular oxygen generally inhibits the activity of hydrogenases, the biological enzymes that produce molecular hydrogen. Thus, an important task for research in this area is to find species or mutations with hydrogenases that are effective in the presence of oxygen. One option is to produce the hydrogen and oxygen separately, as has been done with cultures of a blue-green alga. However, in all known cases of biophotolysis the rate of hydrogen production is extremely small, and great progress will be required before practical conversion schemes can be designed.

In a biochemical approach, enzyme systems would be obtained from biological organisms and then combined in an appropriate reaction cell to perform all the steps involved in collecting energy and driving the water-decomposing reactions. Production of hydrogen, at least at low rates for short periods of time, has been demonstrated, but more basic research on the biochemical mechanisms of photosynthesis, and inventive ideas for incorporating molecular components into systems, will be required for this technique ever to become practical.

In a synthetic approach, a complete chemical or electrochemical system for photolysis would be designed and synthesized without using any components taken from plants or algae. This has a great potential advantage in that problems of instability of biological components would be avoided. There are some promising ideas about the form such chemical systems might take, and some electrochemical systems have been demonstrated in the laboratory, but this must be considered a long-term research problem.

The achievement of a practical technology for photochemical conversion to produce fuels will depend on significant advances in our fundamental understanding of primary photochemical processes and the subsequent processes involved in the transfer of the energy of electronic excitation to stable chemical products. Experiments performed thus far on biochemical systems have suffered from very low efficiencies (often less than 0.1 percent) and from instability of the reactant systems. Some electrochemical systems have shown higher efficiencies, at least for short periods of operation. The difficulties to be overcome in the development of a practical technology of photochemical conversion are formidable. However, the potential rewards are great.

Theoretical considerations indicate that efficient photochemical processes should be possible. The attainable conversion efficiency might be on the order of 20–30 percent, based on incident solar energy.4041 With this efficiency, which is more than 10 times the efficiency probably to be

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attained through biological conversion in energy farms, the quantity of fuels required by a technological society could be obtained from a relatively small amount of land. For example, production of fuels with energy content equal to present U.S. consumption of petroleum and natural gas (55 quads) would require only about 20,000 square miles (about 0.6 percent of the coterminous U.S. land area) at a conversion efficiency of 25 percent. Furthermore, photochemical conversion would not require the great amounts of water and nutrients required by energy farming. However, attainment of a photochemical technology will demand a long-term commitment to both basic and applied research in this area; even then, ultimate success is uncertain.

Thermochemical Conversion

Processes for thermochemical decomposition of water to produce hydrogen are being actively investigated because of their potential for use with high-temperature nuclear reactors. Hundreds of possible processes that use various reactants in closed cycles have been investigated by computer simulation. Concentrating solar collectors could provide temperatures high enough (above 1350°F) to drive these cycles. However, these thermochemical processes are complex and might be impractical. Large amounts of reactants; high temperatures and perhaps high pressures; and extensive mixing, reaction, and separation steps are required. The design of a practical process will be a challenging task.

Electrolytic Conversion

An alternative to thermochemical production of hydrogen is to use any solar electric technology to generate electricity initially and then use the electricity to produce hydrogen by electrolysis of water. Electrolysis is a proven technology that has been used commercially in both small and large applications. Efficiencies are good, with only about 115 kWh of electricity required to produce 1000 ft3 of hydrogen, which corresponds to an energy efficiency of 83 percent. Once the hydrogen is produced, it can be converted to other fuel forms, such as methanol. However the economics are poor: If the cost of solar-produced electricity is 70 mills/kWh, this contributes $25 per million Btu to the cost of the hydrogen.

CONCLUSIONS

  1. The aim of the government’s solar energy program should be to place

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the nation in the best possible position to make realistic choices among solar and other possible long-term options when choices become necessary. This requires continuing support of research and development of many solar technologies. Comparisons of the present costs of various solar technologies and other long-term technologies should not be regarded as critical at the present stage of development. Of more importance is the potential for significant technical advances.

  1. In the intermediate-term future, the direct use of solar heat can contribute significantly to the nation’s energy system. Solar heating technologies should be viewed, along with many conservation measures, as means of reducing domestic use of exhaustible resources. The role of the government program should be to support the development and assist the implementation of the most cost-effective solar techniques, used wisely in combination with energy conservation. In particular, the government should stimulate the integration of solar heating into energy-conserving architectural design in both residential and commercial construction through support and incentives for passive solar design. Since all solar energy technologies are capital intensive, uses that are distributed throughout the year, such as domestic water heating and low-temperature industrial process heating, are likely to be economically competitive earlier than uses for which there are large seasonal variations in demand.

  2. Under present market conditions, solar heating systems are usually not competitive with other available technologies, and therefore market forces alone will bring about little use of solar energy by 2010—probably less than 6 quads even if average energy prices quadruple.42 Nevertheless, important social benefits would accrue from the early implementation of these systems: They would contribute to the nation’s conservation program, they are environmentally fairly benign, and they would increase the diversity of the domestic energy supply system and its resilience against interruption. National policy should stimulate the early use of solar energy by intervening in the energy market with subsidies and other incentives.

  3. Many solar energy applications require long-term development, and these technologies should properly be compared with breeder reactors or fusion. It would be unfortunate if alternatives to the breeder were rejected because too little is known about them today to count on them. It would also be wrong to assume that the choice will or should fall on a single long-term option. Diversity in the nation’s long-term sources can provide valuable resilience in the face of interruptions in the supply of a single fuel or technology. Decisions that restrict the variety of our long-term options should be deferred as long as possible.

  4. The cost picture for a number of solar technologies is likely to change radically in the future, with successes and failures in development.

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Competing technologies will display parallel trends. The costs of many factors of production are likely to change, affecting various technologies differently. In most cases, the economics of solar energy depend critically on advances in ancillary technologies, such as energy storage. It is important that the benefits of these ancillary developments be assessed for other energy technologies on the same basis as for solar, however. For example, cheap energy storage systems would benefit the economics of all systems containing capital-intensive generating technologies.

Large-scale government demonstrations of long-term solar technologies, such as the planned demonstration of a solar thermal central station power plant, could be counterproductive if undertaken prematurely. Such projects may suggest (possibly incorrectly) that the technologies could never become economically competitive, whereas waiting for additional technical developments could result in a considerably more favorable outlook.

  1. An imbalance exists in the federal solar energy program in favor of technologies to produce electricity at the expense of those to produce fuels. The program overemphasizes technologies for the production of electricity, yet electricity accounts for only about 11 percent of energy end-use. Much more attention should be given to the development of long-term solar technologies for fuels production, although there is at present no prime candidate besides biomass production (which is limited by ecological considerations).

  2. The diversity of solar technologies is so great that it is difficult to make decisions among alternatives in a centralized way. To a great extent, the actual choice of which solar technologies to deploy should be made in as decentralized a manner as possible. In other words, the decisions should be left to private industry and individual consumers. The government’s role should be development of a broad scientific and technological base in support of solar energy (much as it did for nuclear energy prior to 1960 and for aeronautics after World War I) and provision of economic incentives that favor solar alternatives.

NOTES

  

1. National Research Council, Domestic Potential of Solar and Other Renewable Energy Sources, Committee on Nuclear and Alternative Energy Systems, Supply and Delivery Panel, Solar Resource Group (Washington, D.C.: National Academy of Sciences, 1979).

  

2. See chapter 2 for a description of cogeneration and its advantages.

  

3. Supply and Delivery Panel, Solar Resource Group, op. cit.

  

4. The load factor of a power plant can be thought of as the proportion of the time that the plant is actually producing power at its rated capacity. It is thus the ratio of its actual output

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to its potential output. There are 8760 hours in a year, so that a plant operating at a load factor of 0.4 runs about 3500 hours/yr.

  

5. National Electric Reliability Council, Fossil and Nuclear Fuel for Electric Utility Generation: Requirements and Constraints (Princeton, N.J.: National Electric Reliability Council, August 1977).

  

6. U.S. Department of Energy, Report of the President’s Domestic Policy Review of Solar Energy (Washington, D.C.: U.S. Department of Energy, 1979).

  

7. An Analysis of Federal Incentives Used to Stimulate Energy Production (Richland, Wash.: Pacific Northwest Laboratory (PNL-2410), March 1978).

  

8. Ibid.

  

9. Energy Research and Development Corporation/Mitre Corp., An Economic Analysis of Solar Water and Space Heating (McLean, Va.: Mitre Corp. (M76–79), November 1976).

  

10. U.S. Department of Energy, An Analysis of the Current Economic Feasibility of Solar Water and Space Heating, Assistant Secretary for Conservation and Solar Applications (Washington, D.C.: U.S. Department of Energy, November 1977).

  

11. Solar Energy Research Institute, Economic Feasibility and Market Readiness of Eight Solar Technologies (Golden, Colo.: Solar Energy Research Institute (SERI-34), June 1978).

  

12. D.S.Ward, Solar Heating and Cooling Systems Operational Results Conference, Summary (Golden, Colo.: Solar Energy Research Institute (SERI/TP-49–209), 1979).

  

13. Amory Lovins, “Soft Energy Technologies,” in Annual Review of Energy, ed. Jack M. Hollander, vol. 3 (Palo Alto, Calif.: Annual Reviews, Inc., 1978), pp. 477–517.

  

14. Supply and Delivery Panel, Solar Resource Group, op. cit.

  

15. Arthur D.Little Co., Individual Load Center, Solar Heating and Cooling Project (Palo Alto, Calif.: Electric Power Research Institute (ER-594), December 1977).

  

16. A.H.Rosenfeld, Building Energy Compilation and Analysis (Berkeley, Calif.: Lawrence Berkeley Laboratory, 1979).

  

17. B.Anderson and M.Riordan, The Solar Home Book (Harrisville, N.H.: Chesire Books, 1976).

  

18. E.Mazria, The Passive Solar Energy Book (Emmaus, Pa.: Rodale Press, 1979).

  

19. Ward, op. cit.

  

20. Ibid.

  

21. Rosenfeld, op. cit.

  

22. E.Kahn, “The Compatibility of Wind and Solar Technology with Conventional Energy Systems,” in Annual Reviews of Energy, ed. Jack M.Hollander, vol. 4 (Palo Alto, Calif.: Annual Reviews, Inc., 1979), pp. 313–352.

  

23. W.D.Marsh, Requirements Assessment of Wind Power Plants in Electric Utility Systems, vol. II (Palo Alto, Calif.: Electric Power Research Institute (ER-978 V.2), 1979).

  

24. J.W.Doane et al., A Government Role in Solar Thermal Repowering (Golden, Colo.: Solar Energy Research Institute (SERI/TP-51–340), 1979).

  

25. National Research Council, Risks and Impacts of Alternative Energy Systems, Committee on Nuclear and Alternative Energy Systems, Risk and Impact Panel (Washington, D.C.: National Academy of Sciences, in preparation).

  

26. U.S. Department of Energy, Technology Assessment of Solar Energy (Washington, D.C.: U.S. Department of Energy, 1979).

  

27. M.Yokell et al., Environmental Benefits and Costs of Solar Energy (Golden, Colo.: Solar Energy Research Institute (SERI/TR-52–074), 1979).

  

28. M.Davidson, D.Grether, and K.Wilcox, Ecological Considerations of the Solar Alternative (Berkeley, Calif.: Lawrence Berkeley Laboratory (LBL-5927), February 1977).

  

29. D.Costello et al., Photovoltaic Venture Analysis (Golden, Colo.: Solar Energy Research Institute (SERI/TR-52–040), 1978).

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30. H.Ehrenreich, Solar Photovoltaic Energy Conversion (New York: American Physical Society, 1979).

  

31. Supply and Delivery Panel, Solar Resource Group, op. cit.

  

32. U.S. Energy Research and Development Administration, Wind Energy Mission Analysis (Washington, D.C.: U.S. Energy Research and Development Administration (COO/2578– 1/2), February 1977).

  

33. National Research Council, Selected Issues of the Ocean Thermal Energy Conversion Program, Assembly of Engineering, Marine Board, Panel on Ocean Thermal Energy Conversion (Washington, D.C.: National Academy of Sciences, 1977).

  

34. U.S. Department of Energy, Solar Energy Research and Development Program Balance (Washington, D.C.: U.S. Department of Energy (DOE/IR-0004), February 1978).

  

35. D.L.Klass, Energy from Biomass and Wastes (Chicago, 111.: Institute for Gas Technology, 1978).

  

36. J.S.Steinhart and C.E.Steinhart, “Energy Use in the U.S. Food Supply System,” Science 184 (1974):307–316.

  

37. S.Flaim, Fertility and Soil Loss Constraints on Crop Residue Removal for Energy Production (Golden, Colo.: Solar Energy Research Institute (SERI/RR-52–324), 1979).

  

38. Supply and Delivery Panel, Solar Resource Group, op. cit.

  

39. D.I.Hertzmark, A Preliminary Report on the Agricultural Sector Impacts of Obtaining Ethanol from Grain (Golden, Colo.: Solar Energy Research Institute (SERI/RR-51–292), 1979).

  

40. J.R.Bolton and D.O.Hall, “Photochemical Conversion and Storage of Solar Energy,” in Annual Review of Energy, ed. Jack M.Hollander, vol. 4 (Palo Alto, Calif.: Annual Reviews, Inc., 1979), pp. 353–402.

  

41. G.Porter and M.D.Archer, “In Vitro Photosynthesis,” Interdisciplinary Science Reviews 1, no. 2 (1976): 119–143.

  

42. Supply and Delivery Panel, Solar Resource Group, op. cit.

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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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Suggested Citation:"6 Solar Energy." National Research Council. 1980. Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems. Washington, DC: The National Academies Press. doi: 10.17226/11771.
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