1987. A large number of different solar cells may be available for space use in the '90's. Because of this, ESA is conducting a comparative study of advanced space solar cells to help define the agency's program. 43 Energy Storage Technology 43.1 Secondary Systems The vast majority of ESA spacecraft have used nickel cadmium batteries. These are expected to continue playing a major role in small to medium size LEO and GEO spacecraft. ESA and CNES are conducting an intensive evaluation program to determine nickel cadmium performance in LEO. Further ongoing development at SAFT is intended to reduce cell mass by 10 to 20 percent. The memory effect, which causes a discharge voltage reduction after many cycles, is one problem area. Proper discharge voltage can be restored through deep discharge reconditioning - standard practice on GEO spacecraft, but very difficult to arrange properly on LEO spacecraft. The most problematic effect of memory is the reduction in available contingency reserve power that can result. Optimum battery lifetime is believed to depend strongly on avoiding excessive overcharging. For LEO spacecraft such as ERS-1, highly variable payload power demands complicate battery management. Considerable intervention by ground controllers is required. These controllers cannot always obtain advice from battery experts in a timely manner. Electronique Serge-Dassault is developing an expert system to automatically interpret battery telemetry data and provide appropriate advice. The temperature derivative method of battery charge control, invented at ESTEC, is under development in the TRP program at ElektronicCentralen and ESTEC since it may allow simple, autonomous battery management. Cadmium electrodes are generally considered responsible for cell aging and memory effects in nickel cadmium batteries. The nickel hydrogen cell replaces cadmium with the long lived, highly reliable alkaline fuel cell hydrogen electrode. This also leads to a 30 percent improvement in gravimetric energy density. Cell costs are several times higher than for nickel cadmium, and the expected lifetime improvements have not yet been fully demonstrated and verified. ESA and CNES (at SAFT) are conducting joint development programs to make cells for GEO applications. British Aerospace is developing a complete battery within the ASTP programme. This battery is compatible with the EUROSTAR platform, and will be the design basis for an experimental battery to be flown aboard SAT-2 in 1993. The use of nickel hydrogen batteries is new even in the US, where the first application is on the Hubble Space Telescope. Nickel hydrogen batteries are baselined for the COLUMBUS resource module. A comparative life test of three European nickel hydrogen cells is scheduled to begin in late 1989 as part of the ESA PSTP programme. These cells are manufactured by DAUG (Daimler-Benz), SAFT and Marconi Space Systems. All were developed under national programmes. Both
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