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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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.40–41 With this efficiency, which is more than 10 times the efficiency probably to be
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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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 The aim of the government’s solar energy program should be to place
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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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. 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. 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. 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. 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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. 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). 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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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Energy in Transition, 1985-2010: Final Report of the Committee on Nuclear and Alternative Energy Systems 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.
Representative terms from entire chapter: