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to different scenarios that assume different combinations of factors such as the population growth, the gross world product (GWP), energy demand in the future, technology cost and dynamics, and environmental taxes. Table 1.1 allocates the world production among various energy sources. The argument that nonrenew-able fossil fuel resources on Earth will run out in a few generations cannot be dismissed as a mere doomsday prediction. Alternate energy technologies that can potentially replace fossil fuels are emerging, but only very slowly. Hydropower, which currently accounts for less than 10% of commercial energy worldwide (and accounting for only 9-10% of the electricity generated in the United States in 1995), is a highly efficient (~85%) clean technology. But, it is unlikely to ever supply more than about 10 percent of the global demand.17 The best locations worldwide for siting the hydroelectric plants have already been exploited. Factors such as the high up-front cost of new plants as well as the impact of the plants on fish and wildlife, sediment buildup, and the impact on watershed characteristics discourage further expansion of this technology. Existing hydroelectric power projects, however, produce the most cost-effective power in the United States today.

The enormous amount of solar radiation that reaches Earth's surface is more than adequate for our needs only if it could be effectively captured and stored. Depending on the latitude, 100-300 Wm2 of solar radiation reach Earth's surface. This is a quite substantial quantity in that, at least in theory, a desert stretch of a few hundred square miles equipped with the inefficient solar cells of today would collect enough energy for the present global needs! In practice, however, cost remains a problem. Photovoltaic systems, the second fastest growing renewable energy source in the world, relies on collection plates and are therefore land intensive—requiring around 5-10 acres of land per megawatt of installed capacity due to the low (~15%) efficiency of the collectors. The up-front costs of installing these collector plates are also quite high. Dramatic reductions in cost have been achieved over the past decade, but solar-generated electricity still costs several times more per kilowatt compared to the conventional product. The thermal solar plants18 (such as the 10-MW Solar Two demonstration plant that came on-line in 1996) also have high up-front costs.

Wind power, while a promising source in the United States (as well as in countries such as Germany, Netherlands, and Denmark), has been slow in developing. It is a nonpolluting, free source19 of power, but the viability of the technology in the absence of government subsidies is not clear at this time. Low efficiency, high variability of production (as the availability of wind itself varies) and the danger posed to wild birds are the disadvantages of the technology. Even Denmark, the clear leader in this technology, produces less than 10% of its domestic power from this source. Globally wind power is the fastest growing renewable energy supply, based on 1990-1998 data [11].

The biomass-derived fuels including not only wood waste, but also peat wood, wood sludge, liquors, railroad ties, pitch, municipal solid waste, straw, landfill gases, fish oils, and other waste products, are important sources of energy in many parts of the world. It is also an increasingly scarce source as more and more forests worldwide are used for agriculture. It is difficult to envision the

17 Although its share globally is small, individual countries rely heavily on hydroelectric power. Hydroelectric power accounts for more than half the energy produced in 63 countries!

18 Thermal solar energy plants rely on sunlight concentrated using parabolic surfaces to operate a heat engine that converts the power into electricity. Photovoltaic technology directly converts the solar energy into electricity using solar panels (no moving parts).

19 Note, however, that wind plants also have high up-front costs though their maintenance costs are relatively low. Wind, though a free resource, is an erratic unreliable source of power. Consequently, the plants tend to operate at low capacity levels, around 23% in 1994. Fossil fuel and nuclear power plants typically operate at 75-90% of capacity [62].

prospects of large-scale utilization of biomass as a source of energy in the United States. Other renewable sources of energy such as tidal power, geothermal energy (not strictly a renewable resource), ocean temperature gradient energy, and wave power have been even slower in developing.

Nuclear fission and/or fusion are likely to one day economically supply nearly all the global energy needs.20 Presently the nuclear fission plants in the United States generate electricity at a cost lower than most other sources and is one of the cleanest technologies used for the purpose. It does not produce carbon dioxide in the process and when properly operated is virtually pollution free. The energy in a single gram of uranium-235 is equivalent to that in 2-3 tons of coal! In 1995, about 23% of the nation's electricity, amounting to 673 billion kilowatt hours, was supplied by nuclear fission power plants. In some European countries this percentage is much higher (e.g., 78% in France), but globally it currently accounts for only about 6% of primary energy. In the 1990s the global nuclear capacity grew by a mere 5% [12]. Safety concerns and the fact that reactors produce radioactive waste material that needs to be disposed of are the main drawbacks of this key source. With improved technology, cheaper and safer nuclear power plants will be a reality provided the issue of spent fuel disposal is resolved. Despite the well-known disasters at Three Mile Island and in Chernobyl, the average number of fatalities associated with producing energy is much lower for nuclear plants compared to that for coal, oil, or even the renewable sources. This is particularly true of plants operated in the United States. The problem with nuclear fission is that of disposing nuclear waste. A safe repository for highly radioactive waste storage is needed if growth of nuclear energy is to be a reality in the United States. (Perhaps the one presently under planning for Yucca Mountain, Nevada, will finally address this shortcoming, paving the way to more nuclear energy in the future for the United States). Given the known reserves of uranium, the fissionable material in nuclear power plants, we are likely to enjoy a steady supply long after oil, gas, and coal supplies have all been exhausted. This would allow a reliable energy source well into the next century before the fusion processes have been developed.

Research on nuclear fusion, regrettably and incredibly ignored in recent federal funding in the United States, may hold the eventual key to clean abundant energy in the distant future. The very real prospects of relatively clean abundant energy from this future technology should spur on its development. A promising and practical fusion reaction uses deuterium and tritium (at about a 100 x 106 °C) contained perhaps in a "magnetic field container." The reaction yields 17 MeV of energy (per mole of each reactant) that is carried by neutrons produced during the reaction. Given the high energy associated with the reaction and flammable materials (particularly the lithium used to make tritium) used in the process, fusion reactors are likely to be at least as hazardous as the fission reactors.

20 Nuclear fission requires mining and purification to generate the reactor-grade fissionable uranium-235. It also yields spent fuel that needs to be disposed of safely. About 50,000 Mt of nuclear waste lie in pools as spent fuel at the 109 operating and 20 closed power plants around the country.

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