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The promise of renewable energy has remained a major area of controversy. Advocates claim that feasible, widespread use of renewables plus proper conservation policies can satisfy US (or World) energy requirements in the near future. The only barrier is said to be that the energy establishment and governments are not trying hard enough to develop the cheap, non-polluting, and enormous power sources that are out there, waiting to be tapped.
However, skeptics of the broad utility of renewables point to their still minuscule contribution to US energy sources (apart from hydropower, which has been present for decades, is not expanding and is variable from year to year). Renewables today contribute little more in absolute energy supplies in the US than on the first Earth Day (and less, as a fraction of total energy consumption). The primarily causes for variation are the effects of long-term droughts and silting on hydropower, which is the largest renewable energy source (and by far the largest renewable source of electricity) in America1. However, this is only one aspect of the matter. Some renewable sources, such as biomass, are beginning to make significant contributions to energy supplies in several developed nations, including the US; photovoltaic cells are economically viable in some circumstances even today, and wind power, while still a tiny fraction of the energy mix, is rapidly expanding in the United States and in several European countries2.
One question is whether the rapid expansion of these sources over the past decade puts us early on the rising slope of the "S" curve, implying more rapid growth in the future, or whether we are approaching a plateau, with the absolute value of renewable energy production not likely to increase greatly for many decades still. Table II.3.1 shows the recent contributions of renewable energy to the energy mix in the United States.
Table II.3.1: U.S. Energy Consumption by Energy Source, 1990-1994 in quads/y, 1015 BTU/y.
Source: Renewable Energy Annual 1995, Energy Information Administration, p. 9, Table H1
|Energy Source (quads/y)||1990||1991||1992||1993||1994|
|Total Renewable Energy||6.163||6.265||6.106||6.403||6.350|
As usual, the truth lies somewhere between the extreme views sketched above. The question is, just where? A look at current basic technical and economic facts provides a good starting point to answer this question for photovoltaics, wind, solar thermal electricity, and biomass-produced electricity. Because of the ease with which electricity may be transported and converted to other energy forms, because of the great advantages that non-or low-polluting renewable electricity sources would bestow, and because of electricity's potential to contribute significantly more than it does today to the major sector of transportation, this chapter is confined to electricity production by the four renewable technologies listed above.
Of course other sources are of some importance. Geothermal energy is a quasi-renewable source that has been exploited for many years in the US, although at a relatively low level. Geothermal sources are not inexhaustible if too much energy per unit time (power) is removed from too small an area, or if the dry steam in place is removed without adequate replacement (as is happening in the Geyserville, CA geothermal field).
Both solar thermal and biomass also have obvious energy uses in producing heat and for some industrial and residential processes (such as cooking). The latter is a major source of energy in less developed countries, and the former is used in countries in all stages of economic development. Biomass also has clear applications in the transportation sector through ethanol and methanol production.
There is little disagreement that if renewable sources of energy were to prove technically practical and economically feasible in the quantities needed, they would be greatly preferable to current methods of energy production.
Table II.3.2. Estimated Emissions from Electric Power Generation in tons/GWatt-hour. Source: Renewable Energy Annual 1995, Energy Information Administration, p. xiii, Table FE1. (SO2 = sulfer dioxide, NOx = nitrogen oxides, PM10 = particulate matter with diameter less than 10 microns, CO2 = carbon dioxide, VOCs = volatile oganic compounds)
Fuel (tons/GWatt-h) SO2 NOx PM10 CO2 VOCs
Eastern Coal 1.74 2.90 0.10 1,000 0.06 Western Coal 0.81 2.20 0.06 1,039 0.09 Gas 0.003 0.57 0.02 640 0.05 Biomass 0.06 1.25 0.11 0 0.61 Oil 0.51 0.63 0.02 840 0.03 Wind 0 0 0 0 0 Geothermal 0 0 0 0 0 Hydro 0 0 0 0 0 Solar 0 0 0 0 0 Nuclear 0 0 0 0 0
On average, some 200 watts/square meter of sunlight reach the US, although the distribution is, of course, quite uneven. This level provides a natural limit on the energy production per area of land for all solar collection systems. Photovoltaic (PV) systems have made major progress over the last 10-15 years, with efficiencies reaching over 10%. But these fundamental limitations pose large area and, therefore, material requirements on widespread, large scale use of this technology. Nevertheless, in principle, 0.3% of the land in the US could supply all of its electricity.
In 1987, 22 MW (peak) of PV modules were produced worldwide; in 1989, this number reached 40 MW; in 1993, production reached 60 MW and, in 1996, about 100 MW is anticipated4 . In 1988, about 12 MW (peak) capacity of PV modules were produced in the US -- 0.002% of the total supply, neglecting the actual efficiency of insolation. By 1993, this number had grown to about 20 MW, of which 40% was for domestic use. A total of 78 MW (peak) has been shipped for use in the US since 1982, resulting in an estimated production of 0.002 quads in 1994. Clearly there is moderate growth in demand and capacity, although since national demand for electricity is roughly four orders of magnitude greater than the PV contribution, these numbers by no means indicate an imminent market takeover.
According to the DoE, the global production of PV modules is now 48% single-crystal silicon, 30% polycrystalline silicon, and 20% amorphous silicon. The latter category has been a long-term hope for cheaper modules, but amorphous silicon PV units been plagued by shorter lifetimes. On a hopeful note, a major gas company, Enron Corporation of Houston, is seriously examining building a 100 MW PV plant in the planned Solar Enterprise Zone in Nevada, on the Nuclear Test Site. They hope to achieve coats of 5.5 cents/kWh (bus bar cost) with non-concentrating thin-film technology5 . Present conventional electric costs range in the US from 3 to 15 cents per kWh.
The problems facing larger scale market penetration are principally cost and the need for backup storage systems. The latter add to cost and complicate matters for individuals who might otherwise contemplate purchase of PV technology for residential use6 . Government and industry sources anticipate further cost reductions in the future, to be achieved by improved manufacturing technologies and, possibly, from the development of more efficient and cheaper photoelectric cells.
The technology is relatively simple, relying on solar collectors/reflectors to heat a fluid from which energy is extracted. The fluid may be water, in which case steam may be used to drive a turbine, or a molten salt (having the advantage of being able to store the energy), which is then used to drive a heat engine. Luz built nine plants, with support and technical assistance and advice from the US Department of Energy, totaling 354 megawatts in generating capacity in the Mojave Desert between 1985 and 1991. At that time, the firm went bankrupt while constructing its tenth plant. It had succeeded, however, in reducing construction and operating costs considerably, to $3011 per installed kilowatt (quite competitive with many conventional electric generating plants) and 8 cents per kWh (a 50% reduction over 6 years). Luz's bankruptcy was attributed by Department of Energy sources to several factors, including a prospective loss of tax credits, higher property tax liabilities relative to gas-fired electric generating plants, overextending its own resources, and competition from unexpectedly low gas prices. The plants continue to produce electricity, operated by a utility consortium7
A small high-technology addition to U.S. solar thermal capacity was inaugurated in spring 1996 in the Mojave Desert. Solar Two, sponsored by the Department of Energy and private utilities, uses molten salt to boil water to drive a 10 MW steam turbine8 The molten salt has a time constant of several hours, permitting some storage capacity. In addition, on a much larger scale, there are proposals for constructing up to 841 megawatts of solar thermal electric capacity in the Solar Enterprise Zone in Nevada.
The range of useful wind velocities may be extended by improved turbine design, thus adding to effective capacity. Turbines (at least, those of lower cost) experience problems at low temperatures, especially in the presence of snow or ice. This hampers their development in some attractive, windy areas, such as the Dakotas (North Dakota alone could theoretically produce over a third of current electric demand in the US). Turbine lifetimes are estimated at 10 to 20 years. The DoE estimates that the US could produce some 20% of its electricity demand from wind power at those locations with average annual wind speeds of 16 mph or more. The area required would be 18,000 square miles (0.3% of the area of the lower 48 states), most of which, however, could still be used for farming or ranching. Less than 5% of this area would need to be devoted to roads and other infrastructure.
Electricity capacity based on biomass reached 11,000 megawatts in 19949 compared with 200 megawatts in the early 1980s. Waste burning facilities have been increased in capacity, but several were stalled or shut off because of pollution concerns (as the Hempstead plant in Long Island, New York, where PCV's among the waste material became an issue).
In principle, energy crops could be grown to provide fuel for biomass electric plants. For example, grass or willow or poplar trees could be grown for this purpose on some of the millions of acres set aside for cropland recovery under the Conservation Reserve Program. The technology to produce electricity is not an issue, but the economics will be determinant. Another issue will be the minimization of polluting emissions from such plants.
The first plant of a new wave of biomass power was a 50 megawatt plant built in Burlington, VT in 1994. Several hundred megawatts of capacity were built in California under a ten-year plan started in 1985, under which guaranteed prices for the generated electricity were contracted for by utility purchasers. However, after ten years, the plants were not advantageous economically and 500 megawatts of capacity were retired after the guarantee ran out. Currently, such plants produce electricity at between 4-7 cents per kilowatt hour, which is not competitive with coal-fired plants at the margin. A federal tax credit, instituted in 1992, of 1.5 cents/kWh for "closed-loop" biomass electricity, in which biomass crops are grown, burned for electricity, and renewed by the operator, might assist in the future, although there are no current plants operating in this mode. Improvements in technology could, in principle, increase the economic viability of this option.
In general, the Energy Information Administration reports that the future of renewable energy is uncertain10. Most potentially viable renewable energy technologies are not yet economically competitive and would require favorable tax treatment as well as federal and state support to develop. It can be argued that such tax incentives would be wise policy both to reduce pollution and to develop energy sources, which will be needed in the future. It is argued by many supporters that the renewable approach would be highly competitive if external costs (e.g., pollution, health and environmental effects, costs related to national security) were properly considered in setting energy prices. However, the Federal Energy Regulatory Commission ruled in early 1995 that states may not regulate energy utility prices in this fashion, so this major possibility of support for renewable energy production has currently been lost. One option for promoting renewable energy in the marketplace would be for governments at all levels to purchase preferentially energy from renewable sources. It remains to be seen whether technological improvements, future constraints in conventional energy supplies, or public demand for governmental intervention will permit renewable energy to achieve the major market penetrations that had once been envisioned by many enthusiastic supporters. Clearly, there are many areas in which further research and development would bring results. Recent trends in federal R&D funding for renewable energy research are discussed in section IV.1 of this report.