The term "renewables" aggregates several sources of energy that have little in common except their reliance on a resource that is renewed by natural processes: hydropower, geothermal, wind, solar, and biomass. The total demand for renewables is understated somewhat in the DOE/EIA projections — there is not enough comprehensive data to allow consideration in the projections of 1) use of non-commercial fuels from plant and animal sources (important energy sources in developing countries) or 2) use of dispersed renewables (renewable energy consumed at the site of its production, e.g., heating water with solar panels). Currently, the predominant source of renewable energy is hydropower used for generation of electricity. It is a "clean" source of energy but is not without disadvantages. Large hydroelectric projects have high costs of construction and often have environmental and ecological effects. The other renewables are competitors, or potential competitors, for shares of various parts of the markets served by oil, natural gas, and coal Geothermal is a source of heat; wind is used for direct power and to generate electricity; solar is used for heat and electricity; and biomass is a source of heat or liquid fuels. All of these except biomass are clean sources of energy — direct bunting of biomass or its derivative liquid fuels produces carbon emissions. In broad terms, geography and climate determine where exploitable renewable resources will be found and what their availability will be. Seasonal changes or normal variations in weather can cause wide variations from average conditions. Except for biomass, renewable resources must be used where they are found, either directly (mechanical energy for a wind-powered pump) or to produce a form of energy that can be transmitted elsewhere (electricity from solar panels). Hydro and biomass can be stored for later use and unused geothermal remains available. Wind and solar energy must either be used when available, or converted to a storable form. Renewables can be applied to energy requirements of any size, from heating and cooking in a single household to generation of electricity for input to a grid. Dispersed renewables (those used at the point of production) are particularly advantageous in small or remote sites, where distribution of other types of fuel is impracticable or unduly expensive. Nuclear energy is projected to decline in percentage of total energy consumption from about 6% in 1992 to about 5% in 2010, with consultation increasing from 21.5 quadrillion Btu in 1992 to 24.4 quadrillion Btu in 2010 (nominal case). The increase is the smallest of the increases for any of the five sources of energy. The predominant use of nuclear energy is to produce steam for generation of electricity, almost always in light water reactor plants. The real environmental advantage of nuclear power plants over burning fossil fuels is more than offset by public perceptions of risk in operating nuclear plants, and the risk and cost issues associated with disposal of spent fuel and radioactive waste. In the United States, regulatory and pubhc concerns have led to increasingly high investment and operating costs in comparison with those for fossil-fired plants. Therefore, it is unlikely that any new U.S. nuclear power plants will be built. As existing nuclear power plants complete their service lives, their owners are expected to turn to other sources of energy for power generation. Current licenses of United States nuclear power plants will expire over a 33-year period beginning in 2000. Some current licenses may be extended by as much as 20 years. The United States Nuclear Regulatory Commission projects that the 100 net gigawatt maximum dependable capacity of U.S. commercial nuclear power plants will decline to zero in about 2030 (no life extension/license renewal) or 2050 (with life extension/license renewal). There are similar situations in many countries that now have nuclear power plants. The consequence is a leveling off of nuclear power generation capacity and reduction in percentage of total energy demands to be supplied by nuclear plants.
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