to direct current could be about 90 percent efficient, an SSPS antenna array would release into the biosphere only one-tenth as much energy as it would deliver for use. In contrast, a fossil-fuel or nuclear plant discards as waste heat about 1.5 times the energy which it delivers to the power lines. The land-use requirements for an SSPS ground antenna array would be only about 1/ 10 to 1/ 100 as great as for direct photovoltaic energy conversion of sunlight, because direct conversion is limited by low efficiency, the day-night cycle, seasonal variation of the day length, atmospheric absorption, and cloud cover. Economic Factors Delay in the realization of the SSPS concept appears to be due mainly to lift costs and power plant mass. The installed cost of an SSPS would depend primarily on four factors: the capital cost per kilowatt of the power plant for converting solar energy to electricity ($/ kw); the specific mass of the power plant in kilograms per kilowatt of output (kg/kw); the cost per kilogram of lifting the power plant from the earth to geosynchronous orbit ($/ kgsy); and the overall efficiency of converting electricity into microwave energy, transmitting it to the earth, and reconverting it into direct current or to a power line frequency (E). The installed capital cost of the installation, per kilowatt of power output from the antenna busbar on the earth, would be approximately I t [$/ kw + (kg/kw) ($/kgsy)] plus interest charges, development costs, and smaller additive terms for the ground antenna and the land it would occupy. Earth-Launched Power Satellites Solution of the microwave transmission problem (9, IO) for the SSPS appears to be progressing well: tests have already demonstrated a transmission efficiency f (direct current to direct current by a microwave link) of 55 percent. The goal is an efficiency of about 63 percent. For an earthlaunched SSPS, the economic problem lies in the remaining factors: capital cost, power plant mass per unit power, and lift cost. Two alternatives for the conversion of solar energy to electric energy in an SSPS ·have been considered: photovoltaic cells (solar panel arrays) and turbogenerators powered by mirror-concentrated sunlight. Glaser (3) has estimated that for an earth-launched SSPS the specific mass for solar panel arrays will have to be reduced to about 0.88 kg/kw. For comparison, the 944 value for photovoltaic solar cell arrays in operational satellites of the last decade has ranged from 78 to 107 kg/kw; one experimental satellite designed as a short-life test vehicle achieved 29 kg/ kw (/ /). For the Solar Electric Propulsion System space probe scheduled to fly in 1984, the specific mass is intended to be 13 kg/kw(/2). Present costs of solar panel arrays for space applications are based on manual assembly techniques and are therefore much too high for application to an SSPS; they are typically $175,000 per kilowatt (3). A more reasonable starting point is the 1971 figure of $5,000 per kilowatt for singlecrystal wafers 5 cm in diameter. The necessary target figure (3) for a competitive SSPS launched from the earth is about $220 per kilowatt, about half the present cost of a large, coal-fired central power station. As an alternative to a photovoltaic array for SSPS power, Woodcock and Gregory (/3) have considered the use of closedcycle helium turbines (/4) driving conventional electric generators. In that alternative the specific mass must be reduced from presently attainable values (IO kg/ kw) to about 5 kg/kw. To achieve that reduction, Woodcock and Gregory have assumed a development program in which the turbine inlet temperature could be increased to a value considerably higher than that used in current practice. For an earth-launched SSPS the cost of lifting components to geosynchronous orbit from the earth would be critically important. For a photovoltaic SSPS of 0.88 kg/kw, the necessary lift cost figure would be $220/kgsy (3). For a turbogenerator SSPS of 5 kg/kw and efficiency f of 70 percent, the performance demands on the lift vehicle would be even more severe: $75/ kgsy (/3). Launch Vehicles An advanced, chemically propelled "space tug" could bring from low earth orbit to geosynchronous orbit, as payload, about one-third of the total payload delivered to low earth orbit from the earth's surface. When the cost of space-tug operations is included, the cost of transport ($/ kgsy) from the earth to geosynchronous orbit can then be taken as roughly four times the cost of transport to low earth orbit. For simplicity, lift cost figures in the following discussion will refer to the overall transport from the surface of the earth to geosynchronous orbit ($/kgsy) and will be taken as four times the cost to low orbit. An additional uncertainty of about ± 30 percent is a consequence of this simplification . The target figure for the space shuttle, planned for operation in the early l980's, is $1400/kgsy, not including development costs of several billion dollars (/ 5). A heavy-lift launch vehicle (70-ton payload) using the same kind of engines that are already being developed for the shuttle, and therefore obtainable without large additional expense for development, is estimated to be capable of achieving $600/ kgsy to $1000/kgsy (/6). To summarize, for an economically viable earth-launched photovoltaic SSPS the specific mass (kg/kw) must be reduced by about a factor of 30 to 60 below the corresponding figure for satellites of the l970's; the lift cost to geosynchronous orbit must be reduced by about a factor of 4 below figures estimated to be attainable in the l980's without large additional development costs; the capital cost ($/kw) must be reduced by a factor of about 30. For an earth-launched turbogenerator SSPS the capital cost must be held equal to that of a present-day coal-fired plant; the specific mass must be reduced to about half the value currently attainable; f must be raised to 0.70; and the lift cost must be reduced by about a factor of I0 below the figure now considered to be attainable in the 1980's without substantial postshuttle development. Table I summarizes the values of these factors which have been assumed in several studies and, where the information is available, the resulting estimate of power cost. Extrapolations to vehicles more advanced than shuttle-derived rockets are necessarily subject to large uncertainties; new developments in engines, heat shields, reusable fuel tanks, and other components would all be needed before their construction. For a very large vehicle, capable of lifting 180 tons to low orbit, estimates of attainable recurring cost range from $80/kgsy to $900/kgsy, and estimates of development cost range from $5 billion to more than $25 billion (/ 7). Power Plant Economics In power generation, the busbar cost is crucial to the achievement of market penetration. Power plants are characterized as base load (operating nearly all the time), intermediate load (operating part of each day), and peak load (operating only during coincidences of maximum industrial and residential demands). Peak-load plants are normally simple and inexpensive to build and, when called into use, generate electricity at a cost up to 60 mills (that is, $0.06 per kilowatt-hour). Intermediateload plants are capitalized more heavily and generate electricity at 20 to 25 mills. SCIENCE, VOL. 190
RkJQdWJsaXNoZXIy MTU5NjU0Mg==