expected usable energy density of the rotor itself is 40 Wh/kg. When bearings, motor-generator and containment masses are taken into consideration, kinetic energy storage systems are not likely to compete with batteries for most space applications. 43.2 Primary Systems The development of the fuel cell system for HERMES at Dornier represents by far the largest spacecraft energy storage activity currently underway in Europe. The main development effort is currently being carried out, following from the initial study phases of the HERMES definition programme. A number of candidate technologies are under consideration. These include mobile and immobile phase alkaline cells, and solid polymer electrolyte systems. The selection of the appropriate electrochemistry is intimately connected with the details of the required peripheral components. Some of these, e. g. gas-liquid separators, have yet to be developed for weightless operation. Elenco and Dornier are evaluating possible flight demonstrations of such gravity sensitive devices under the TDP programme. A lithium battery is likely to be the peak power/contingency primary power source for HERMES. Final selection of which battery technology will be used in this role will be greatly influenced by safety considerations. The primary cells for the CASSINI Titan probe battery were developed by Fiat and BTC-Venture Technology (now Dowty Batteries) under the TRP programme. The Lithium-sulfur dioxide couple was chosen because it is in an advanced state of development due to military use. In order to increase the shelf life from 3 years to the 8 required for CASSINI, emphasis was placed on seal technology. Storage tests began at ESTEC in 1990. The same cell (10 Ah capacity) may also be suitable for the ARTEP platform. 5. Perspectives ESA has made rapid progress in space related power system technology since its inception 25 years ago as ESRO (European Space Research Organization). This has principally been due to the enthusiasm and dedication of a relatively small number of staff working on an increasingly ambitious space programme, including strictly European elements as well as cooperative projects. Plans for the next decades highlight the need for imaginative technological and systems solutions as well as the need for continuing enthusiastic engineering support in order to meet these new challenges.
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