irradiated components cannot be “switched off' at will. In addition, breeding of nuclear fuel would be necessary to meet the power needs of the world. Breeding rates achieved to date are too long to permit growth of an adequate fuel supply for an energy “rich” world by even 2050 (30). Inevitably, energy conversion technology are being driven by supply-demand forces, and possibly environmental concerns, to major reliance on renewable (solar) and possibly virtually limitless energy sources such as direct fusion. The demonstrated systems either have relatively low net energy yields or provide only small flows of power compared to world usage. The technical feasibility of most proposed solar energy conversion systems has been established or demonstrated. Comparative economic analyses can be performed against many current and projected fossil fuel systems. Massive initial and continuing investments would be required for terrestrial solar power systems (TPS) with significant output on the world level (30). For example, a terrestrial solar power system (TPS) with an average 10 GW output over the course of a year would have a total construction mass the order of Grand Coulee dam. The collectors would be spread over an area greater than 30 km on a side which is approximately the surface area of the lake behind Grand Coulee. This large area structure would have to be made of thin, cheap, low mass (1 kg/m2) solid state components which could dependably operate with little repair work for 30 years or longer. Even if the solar cells were free, the costs of wires to collect the power, mechanical structures and ancillary equipment would make the TPS more expensive than conventional power systems. In addition, terrestrial solar power systems (TPS) do not receive sunlight at night and receive much reduced sunlight during bad weather or when the sun is near the horizon. Thus, a TPS must have excess power and energy storage capacity for the worst expected conditions during the longest periods of bad conditions. Alternatively, TPSs spread over the world could switch power from clear to cloudy areas by converting the electricity to microwaves. Microwaves would be beamed to mirrors above the Earth which would reflect the beams back to Earth for collection and use at distant regions (29). Such microwave beams would have to pass through the biosphere twice and there would be significant stray energy near the transmitters and receivers, the latter because the required extremely large transmitting antennas can likely not be built on Earth due to ground motions, weather effects and other factors. Sunny regions would have considerable influence over regions undergoing prolonged severe conditions. They would also influence the cloudy tropics. Some systems (e.g., biofuels production) for directly obtaining solar power and others for indirectly obtaining solar power (wind energy, ocean thermal) are of the same general massiveness and areal extent as TPS or deal inefficiently with huge quantities of matter to obtain a limited flow of power. Solar energy results from nuclear fusion in the sun. Controlled nuclear fusion power, on the other hand, faces formidable obstacles even to reach technical feasibility status and “very preliminary” estimates of eventual economic practicality are “at this point" generally discouraging. Controlled nuclear fusion requires construction and operation of an entire power system (provision of fuel, furnace, conversion of power to useful form, transmission, disposal of waste products, construction and maintenance, etc.). Direct access to solar fusion power is dependent on conversion of the available power to a usable form. In TPS the engineering focus is on the energy transducers, energy storage, distribution, construction and maintenance. However, solar power transducers on Earth simply require too much land area and associated areal mass to be attractive (33). Dr. Peter Glaser first proposed in 1968 that we go to space to efficiently obtain
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