Systems based off Earth (row 8), which collect solar energy and broadcast it to users in space or on Earth, appear to offer fundamental advantages over the alternatives. The sun is a thermonuclear reactor which uses advanced reaction schemes we do not expect to duplicate on Earth. To support a 60,000 GW world we will intercept the energy released by complete thermonuclear solar burning of 21.2 tons/year of its mass. Conveniently, the sun retains the waste products over geologic time scales. Power gathering installations off Earth with space-to-Earth conversion efficiency of 5% would have a collection area in space perpendicular to the sun of about 900,000 km2 (or 925 km on a side in one area). For 50% overall conversion, possible in principal after much development, the collection area would be about 300 km on a side. Assuming power is delivered to Earth by microwaves beams with an intensity of 2 GW/km2 (twice sunlight) receiver area would total 30,000 km2 (173 km on a side). Of course, the power would be received by numerous smaller arrays as close as reasonable to the final users. The beam need only pass through the atmosphere once prior to final use rather than twice as expected with terrestrial photovoltaic systems using microwave redistribution of power. For microwave power reception the receiver mass can be divided into relatively simple support structures and electronic components. Support structures made of concrete and steel would have a mass per GW of received power about that of a coal power station. They would be far simpler in construction than the major concrete and steel components of a nuclear reactor. Wooden support structures might be usable. This would be a far more efficient use of the wood than burning it. The antennas and other electronics which receive the power would have a mass the order of 500 tons per GW, far smaller than any other power mass shown in Table 6. Columns D and E show how this mass efficiency translates into total space and Earth system mass and production rates. The concrete and steel mass of the microwave system is about that of the mass of coal, OTEC and biomass plants, but is of a much simpler nature and subjected to far fewer operational challenges. It is reasonable to expect receiver mass to decrease as the technology of space solar power collection, transmission, and reception and facilities construction are systematically explored. In terms of manipulated mass (facilities — C+C + C and fuels) space solar power systems are potentially 10 to 100 times more efficient than other reasonable large scale power schemes. Likely, as the SPS technology reaches maturity the very long facilities life times will be achieved. These global SPS numbers may well be high. Electricity is generally a more efficient source of end-use power than carbon combustion. Widely available and economical electric power could reduce energy use in Demandite manipulation and transport. Improving electronics and communications will likely reduce service industry and residential power use. Possibly the order of 20,000 GW (electric) could underpin material prosperity for a 6 E9 person world. We note, hopefully not in passing, that if the world inventory of thermonuclear/ nuclear devices (bombs) were consumed in civil power production (assume 7% conversion efficiency) they would produce an average of only 40 GW over a 30 year period (71). This number gives some scale to the weapons/radioactive waste potential of a 60,000 GW nuclear power system. Of course, most of the bomb power is thermonuclear energy that is not deemed accessible for civilian use. The primary challenge of space based solar power is to build the transducer for converting the dominant radiation of our natural fusion reactor (sunlight) into electricity for use in situ or at remote locations. Development of these transducers — there are many options — can be conducted on Earth in relatively small laboratories and at little cost. Much progress has already been made. Development of controlled
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