7.1.3 Comparison of Photovoltaics with Solar Dynamic Systems for Power Collection This section provides an overview of the two main competing technologies for collecting solar radiation and converting it to electrical energy. It's purpose is to indicate the type of trade-offs that must be performed when designing a solar satellite power source. Efficiency One of the main technical design drivers is the efficiency of the collection process since this will determine the overall mass of the system used, i.e., the higher the efficiency, the smaller the mass. Photovoltaic systems which are presently used for solar arrays on satellites, are typically 10-15% efficient using Silicon solar cells with GaAs cells being developed being 18-20% efficient. In the long term it is possible to increase the efficiencies up to 30% using tandem cell technology but this considerably increases the costs. Solar dynamic systems (SDS) tested on the ground have typical efficiencies of 25-30 % depending on the cycle, used with up to 40 -50 % predicted in the longer term. Since SDS are a lot more efficient than photovoltaics then for generating the same power, the area required to collect the solar radiation can be considerable reduced.. Space Qualification Photovoltaics based on silicon solar cells have a very good space heritage having been used for solar arrays for the last 20-30 years. GaAs cells have been flown on satellites but mainly on an experimental basis to test the characteristics of these cells in a space environment. Solar Dynamic Systems have not been used in space although there is an extensive ground database on the use of certain type of heat engines in particular the Brayton Heat Engine has been tested extensively by NASA. Stirling Heat Coolers as opposed to Stirling Heat Engines , i.e., they are used to cool equipment, have been flown on a number of space missions. A number of space qualification missions would be required before an operational mission could be flown using space heat engines. Costing of System Photovoltaic systems are very expensive based on present manufacturing techniques. Using more efficient GaAs and tandem cell reduces the number of solar cells required to produce a given power but increases the individual solar cell cost and so a trade-off must be performed as to the lowest. SDS are inherently less expensive since they use mirrors in the form of concentrators to collect the radiation. For example for the Space Station Freedom it has been estimated that the SDS recurring cost would be half that of an equivalent photovoltaic system although there will be extra cost for the initial design and development of a SDS system. This is shown in Figure 8.12. The data is based on current 1875 kW photovoltaic and 25 kW Solar Dynamic power modules in a balanced station configuration. Orbit Selection For power collection, the orbit has two main impacts on the selection of either photo-voltaic or Solar dynamic systems. These are the radiation environment and the orbit itself. The radiation environment varies with the orbit the satellite is in. For Low Earth Orbit, the radiation dose is small but at geostationary and inter-mediate (especially orbits like Molniya) orbits, the radiation dosage can be very high. For photovoltaic systems, the effect of radiation varies with the type of solar cell being used. At geostationary orbit the impact is quite high for Si solar cells leading to between 10-20% overall power degradation. However new cell technology like InP and GaAs is inherently less radiation susceptible and therefore has very little radiation degradation. For solar dynamic systems there are no solar cells and therefore there is no power degradation.
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