is due to the fact, that with a realistic and lightweight storage design the storage cannot be discharged completely during eclipse. What maximum periodical variation is acceptable for the heat engine and for the user is the main question to be answered in order to optimize the power system. As can be seen in Figure 2 the eclipse time for a given altitude varies depending on the orbit inclination. For low inclinations (0°-30°) the eclipse time stays close to the maximum value, whereas for higher inclinations the eclipse changes from orbit to orbit between zero and the maximum eclipse. For an orbit with a high eccentricity the variation is even more extreme, depending whether the satellite passes the Earth's shadow at its apogee or perigee. Consequently, the system optimization, i.e. defining the allowed power output reduction diming eclipse, is generally more important for high inclinations and high eccentricities. As a conclusion and recommendation for circular orbits it can be stated that for inclinations within 0° and 30° and for altitudes below 1500 km the worst case design (storage capacity and collector area are designed according to the maximum eclipse) is an appropriate approach. For highly inclined (more than 30°) and/or eccentric and/or high altitude orbits (above 1500 km) the system optimization requires a compromise between power system mass and continuous power availability. The worst case design is far from an optimum, because the maximum eclipse is a rare situation. Another point is of importance for the solar dynamic system. One reason for favoring a thermal energy storage for the dynamic system is that this allows the PCU to operate continuously avoiding stop and go operation which would reduce the system's life drastically. Consequently, a solar dynamic system should be fed also during eclipse by that amount of heat which allows a continuous operation, at least in idle mode. Closed Brayton cycle idle mode operation requires about 20 % of nominal operation including all losses, like receiver aperture losses, radiation losses etc. This idle mode operation minimizes the temperature variation of the system thus ensuring an appropriate operational life and it allows a storage reduction of more than 80 % compared to the nominal design. This overall system optimization is a challenge for a team of power system engineers, power consumers and Space Station or satellite designers. Summary and Conclusion The advantage of solar dynamic systems results from a three times higher sun-to- user efficiency and a better storage performance. For the Space Station orbit this means a four times smaller collector area for the dynamic option. The masses are expected to be comparable for the photovoltaic and dynamic option.
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