3.2.3 Component Level Requirements The need for greater area efficiency has always been an important solar cell technology driver. The need to reduce atmospheric drag on high power arrays in LEO for COLUMBUS has made this even more important. The area minimization requirement is often in conflict with mass minimization requirements, which are still extremely important for GEO and interplanetary missions. These requirementshave a major impact at the solar cell component level where there is competition between silicon and gallium arsenide technology for future applications. 3.3 Storage Requirements The shift in emphasis from geostationary communications satellites to spacecraft such as COLUMBUS requiring high power and long life in LEO has made energy storage requirements much more demanding. Another new requirement is for high energy density primary energy sources for relatively short duration applications such as for HERMES and the TITAN PROBE. 3.3.1 Secondary Storage for GEO Spacecraft The switch over from nickel-cadmium to nickel-hydrogen storage batteries for GEO spacecraft is nearly complete in the US, but is just beginning in Europe. It is driven by the lighter weight of the nickel-hydrogen batteries. Future power requirements for GEO missions are expected to remain constant except for an increase in required power system lifetime from 10 to 15 years. This is made possible by the introduction of ion propulsion for stationkeeping. Battery lifetime requirementswill increase from 1000 to 6000 cycles. Although this is a substantial increase, 6000 cycles is still low compared to LEO requirements. A 6000 cycle lifetime is expected to be well within the capability of current technology. 3.3.2 Secondary Storage for LEO Spacecraft Power and lifetime requirements are growing rapidly in LEO applications. For example, the COLUMBUS resource module needs about 10 kilowatts of eclipse power, and a four year lifetime between battery replacements is required. The 24000 charge-discharge cycles implied by a four year life, together with the high charge and discharge rates are quite challenging for current battery technology. This raises the possibility of using regenerative fuel cells, or (in the longer term) solar dynamic systems with thermal storage for eclipse power. 3.3.3 Primary Storage Primary energy sources have previously been confined to launch vehicles and minor payload applications. They are now emerging in several new and diverse areas. Deep space mission elements such as the TITAN PROBE on the American CASSINI mission require a power source operating for a short time (a few hours) far enough from the sun that solar arrays would be impractical. The power source
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