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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