the Hubble Space Telescope and the COLUMBUS resource module are designed for on orbit service. Since nickel hydrogen cells are pressure vessels, considerable attention is being paid to astronaut safety during cell qualification. Nickel hydrogen battery designs with series connected cells inside a common pressure vessel should lead to significant mass and cost savings for large future LEO energy storage systems. Marconi has completed the first step in such developments. A battery containing two series stacks was fabricated in the PSTP programme. This battery does not offer advantages compared to conventional (individual pressure vessel) designs at the power levels currently projected for COLUMBUS. This design could be scaled up to make an actively cooled module should the need arise. It is unclear which secondary battery system might replace nickel hydrogen in the future. Most high energy density systems are ruled out by the high cycle life requirements of most space applications at present and projected levels of maturity. Sodium sulfur (and related cells such as sodium and nickel chloride) appear to be the most promising for low to medium power applications. These cells are under intensive development in Europe for terrestrial applications (at ASEA-Brown-Boveri, Beta R&D, CGE, and Chloride Silent Power). Such cells have yet to demonstrate adequate lifetime, reliability, freeze-thaw capability, or vibration resistance for space use. They operate at high temperatures (200 to 400°C) which will require new approaches to thermal control. Such high temperature operation may prove to be an advantage since a much smaller passive radiator would be required. These new cell chemistries might lead to a 25 to 100% increase in gravimetric energy density compared with nickel hydrogen (depending on the practically achievable depth of discharge). ESA is sponsoring an evaluation of sodium - nickel chloride technology at Harwell as a part of the TRP programme, and is about to begin testing prototype cells. Regenerative fuel cell systems appear to offer the most promising gravimetric energy density for high power applications where their relative complexity would be acceptable. They might also be integrated with the life support (oxygen and water supply) and propulsion (hydrogen-oxygen motors) systems. Regenerative fuel cell systems have a relatively low round trip energy efficiency, and operate at moderate temperatures of 70 to 100°C. In order to compare a regenerative fuel cell system with more conventional battery systems, systems trade offs including the solar array, thermal control and propulsion systems must be performed. Dornier has completed a thorough study or regenerative fuel cell systems under the TRP programme to evaluate their promise and to identify the required technology development effort to create flight qualified hardware. A number of European subcontractors contributed their special expertise to this effort. It was assumed that the fuel cell technology for HERMES could be reused in a regenerative fuel cell system. It is expected that a follow on programme will produce a prototype electrolyzer which will be delivered for evaluation in 1991. Kinetic energy storage (in the form of rotors) might offer a viable alternative to batteries for applications requiring extremely high charge and discharge rates. Volvo has developed a light weight, high strength rotor which is ready for testing. The
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