must also operate after a long quiescent period (8 to 10 years), which imposes severe shelf life and self discharge requirements. HERMES requires high power (4 kilowatts) during a relatively short mission of one week. Solar arrays would be mechanically impractical, and would offer a substantial mass penalty. The highest gravimetric energy storage efficiency available at present from electrochemical sources is from hydrogen oxygen fuel cells. Fuel cell systems are mechanically complex. This complexity presents a significant technological challenge when combined with the stringent safety and reliability requirements associated with manned spacecraft. HERMES must also have an auxiliary battery with a capacity of about 15 kilowatt hours to meet additional peak and contingency power requirements. Primary batteries will no doubt be employed as short duration and contingency power sources on various future spacecraft, due to their long shelf life and high energy density compared to secondary batteries (300 versus 50 watt hours per kilogram). For example, the proposed ARTEP platform would require 5 to 6 kilowatt hours of energy during a mission lasting a few days. 4. The European Space Power Development Programme ESA consistently prefers fully regulated power busses. Detailed system level mass tradeoffs have always shown that active battery discharge regulators result in a more well defined overall power system. This allows proper matching of the spacecraft batteries to the actual energy requirements. The resulting flexibility in the number of battery cells, the number of batteries and the selection of cell capacity and technology is key in allowing proper system mass optimization. Regulated busses also make equipment interfaces more well defined early in the design process, resulting in lighter, more efficient and lower cost payload hardware. Most new power systems studies generate an "expert system" like model. This allows rapid optimization of the power system topology to meet specific mission requirements. Considerable effort is being devoted to improving battery charge/discharge management. This is a key factor in extending spacecraft lifetime, especially in LEO. Power distribution and fault protection have often been neglected areas of power system technology. They have received considerable attention in the ESA program - especially during COLUMBUS, with its high power and high voltage power distribution system. Sophisticated, current limited and thermally protected switches have been developed. 4.1 Power Conditioning and Electronics Technology ESA has historically been at the forefront of developments in both power conditioning and control concepts as is shown in the following figures.
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