lightweight substrates for nickel oxide electrodes have been under way at NASA LeRC for the past few years and have been initiated at Hughes under NASA contract NAS 3-22238. Britton has recently described studies using substrates from several sources: Fibrex from National Standard, Metapore nickel felt from Soropec and machined sintered nickel fiber mat from Nippon Seisen [29]. A weight reduction of 40% in nickel electrode weight appears possible. Using a 1.1 C charge, 1.37 C discharge regime, 4534 cycles at 100 % depth of discharge have been recorded. At present it is uncertain if the lightweight substrate technology can be used for LEO applications, but results thus far suggest that specific energies approaching 80 watt-hours per kilogram (Wh/kg) for batteries in geosyncrhonous orbit may be achievable with successful implementation of these substrates and other weight reduction measures. Comparison to Other Technologies Other technologies in competition with nickel-hydrogen batteries for spacecraft energy storage systems include regenerative fuel cell systems, sodium-sulfur batteries and nickel-cadmium batteries. Solar dynamic systems and nuclear power systems have particular advantages/disadvantages over electrochemical/photovoltaic systems, but are not yet used extensively in the USA. They will not be addressed here. One of the most recent comparisons of regenerative fuel cell systems with nickel-hydrogen and sodium-sulfur batteries has been made by Taenaka et al. in USAF sponsored effort performed at Hughes Aircraft [30]. They concluded that the energy storage system specific energy (which includes the dedicated solar array, thermal menagement, power electronics, etc.) projected for a regenerative fuel cell system (~31 Wh/kg) in a mid- to high-altitude orbit satellite is intermediate between that presently attainable with flight-qualified nickel-hydrogen batteries (~24 Wh/kg) and those projected for the new sodium-sulfur technology (>53 Wh/kg) (system) or >100 Wh/kg at the battery only level). For large power systems (>2000 W) the advanced nickel-cadmium battery is not competitive with the nickel-hydrogen type, but in cases where power levels less than 1 kW are required, nickel-cadmium batteries are more weight and volume efficient and cost less than nickel-hydrogen batteries, and will continue to be used in small science satellites. However, if the cycle life projections made by Lim & Thaller [31] are valid, advanced nickel-hydrogen cells will have five times the cycle life. Conclusions In the past 18 years since their conception, nickel-hydrogen batteries have made large inroads into the space economy storage area, virtually replacing nickel-cadmium cells for all power applications greater that 1.5 kW. This trend is not restricted to the USA, but is catching hold in Europe [32] and Japan [33] as well. Advanced nickel-cadmium cells are expected to be used in small science satellites for some time. With improvements, regenerative fuel cells will probably be used in some mid-altitude and higher orbits since their specific energy becomes attractive in these cases. Because of their high specific energy and moderate cycle life, sodium-sulfur cells will replace nickel- hydrogen cells for some applications in the mid- to late-nineties. However, if lightweight nickel substrates prove feasible, nickel-hydrogen batteries can offer some formidable competition.
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