by metal-organic chemical vapor deposition (MOCVD) on gallium arsenide substrate. These cells are currently available in 2 X 2 cm and 2X4 cm sizes, and are currently being installed on a number of satellites scheduled for launch in 1989-1992. Although currently about 10 times costlier than single-crystal silicon, savings due to their higher efficiency (typically 18% in production compared with silicon's 13%), much better resistance to cosmic and other radiation, and ability to operate at higher temperatures without degradation tend to balance out their high manufacturing cost. Moreover, current development efforts to replace the GaAs substrate with germanium (Ge) are expected to cut unit cell costs by about half, and will also allow the fabrication of much larger cells (e.g. 6x6 cm), simplifying panel assembly considerably. Other pertinent developments include research on ‘cascade' photovoltaic cells, which use very thin multiple layers of different photovoltaic materials to absorb photons with energies distributed over a large portion of the visible spectrum. Such cells could, in principle, reach near 100% efficiency; a practical goal, however, is 35%, about twice that of current gallium arsenide production-quality cells. Other prospects, important for military applications, include solar concentrator panels employing banks of cassegrainian reflectors or light funnels to focus sunlight at intensities up to 60 suns on small gallium arsenide photocells, currently being developed for the US SDI program under the project names SCOPA and SUPER. To reduce the size of space solar powerplants for drag reduction it is necessary to increase their overall (thermal) efficiency. Conventional ground-based dynamic power conversion subsystems using Rankine (vapor) or Brayton (gas) cycles can achieve overall thermal efficiencies in the 30-40% range. Several ground-based prototype systems have been built and operated using concentrated solar energy in place of conventional fuel-fired or nuclear furnaces. The use of concentrating solar collectors (up to 2000 Suns) to do the same in space has been developed extensively in recent years as a prospective supplementary power system to accommodate expected growth in space station power needs beyond the initial 75 kWe photovoltaic configuration. Chemical. Hydrogen-oxygen fuel cells, used on the US Apollo modules 20 years ago and the current US space shuttle, and batteries are the only chemical space power systems with operational experience. Current interest in generating very high power levels (e.g. 100-1000 MWe) for very short periods (e.g. 1 hour) has stimulated active development of hydrogen-fluorine and other chemical lasers and high-power-density open-cycle turbogenerators driven by rocket-type chemical gas generators. One interesting approach proposed over two decades ago, the Orbiting Energy Depot (OED), uses an onboard nuclear power source to electrolyze water into hydrogen and oxygen, which it then delivers to customers' spacecraft for use in fuel cells, simultaneously collecting the customers' waste water for recycling. This scheme could be combined with one currently being evaluated on the US Pathfinder technology program, the Cryogenic Fluid Depot. A somewhat similar concept has been proposed to meet the eclipse needs of large onboard solar power supplies. Here, instead of recharging batteries (which would be too heavy for high power levels), the solar energy available during the sunlit portions of the orbit is used to electrolyze water into hydrogen and oxygen, which is then consumed by fuel cells during the dark portion of the orbit. This regenerative fuel-cell scheme is slated for use aboard the space station. These concepts are of some interest to central station power systems in low- inclination Earth orbits where eclipsing occurs. They could also be used for dedicated power depots which transmit either electricity or chemical propellants via tether or are equipped with an orbit transfer vehicle (OTV) to deliver chemical fuels to customers
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