7 Power Systems_________________ The concept of electric power collection and transmission from space has been around since Peter Glaser first introduced solar power satellites (SPS) in the late 1960s. This chapter will describe the technologies associated with solar power systems which include power collection, conversion, transmission, and reception. The collection and conversion technologies include photovoltaic and solar dynamic systems. The transmission and reception systems include microwave and laser technologies. Major technical advancements since the 1970s along with increased global energy consumption have scientists and businessmen revisiting the concept of collecting solar energy and transmitting the power to electric energy users. This chapter will discuss both the developed and new technologies of a solar power system. 7.1 Solar to Electric Conversion In this section technologies for power collection and power conversion have been reviewed. In particular photo-voltaic and solar dynamic systems have been examined in order to understand the differences in design and see how this affects the efficiency of the system, mass .cost and other factors which size the system. This has been divided into present technology and new or future technology so that trends can be identified and help us to understand where technology will be in 10-15 years time. Detailed designs of power collection and conversion systems are examined in the design example chapter where sizing of arrays, radiators etc has been calculated. 7.1.1 Photovoltaics The most important element in photovoltaic systems is the solar cell. Nowadays there are many kinds of solar cells, but crystal silicon (c-Si) is the most popular for space use. Gallium arsenide cells (GaAs) were developed and have been qualified for space use in the middle of 1980's. They have a higher efficiency than silicon solar cells. But the cost of GaAs is much higher than that of c-Si, so careful trade-offs between the two types of cell are needed in designing a solar array. Many kinds of solar cells are under development, for example amorphous silicon (a-Si), indium phosphide (InP), copper indium diselenide (CuInSe2:CIS), cadmium telluride (CdTe), etc. Moreover various research of multi-junction types have been performed in order to get more efficient solar cells, for example AlGaAs/Si, AlGaAs/CIS, GalnP/GaAs, etc. Every solar cell can be characterized by its efficiency, cost, temperature characteristics, mechanical characteristics, degradation caused by radiation, etc. When designing a solar power satellite, we must investigate various parameters of solar cells based on the requirements of solar array wings such as power generation, operation temperature, mission life, spacecraft orbit, cost, etc. Particularly when the solar array structure of Solar Power Satellite is large scale and has a long-life like the NASA/DOE model, factors of cells such as conversion efficiency, density, endurance for radiation, etc. produce a powerful effect on the total weight and total cost of whole system. In this section, the characteristics and the performances of the main types of solar cells and solar arrays are described first. Then the problems of large scale solar array construction and operation in space will be discussed. This consideration will be useful to understand the trade-off between photovoltaic systems and solar dynamic systems. c-Si & a-Si (Silicon) Almost all the solar cells used in space are silicon. This is the cheapest solar cell and they have been used for a long time in space. The research on single-crystal silicon is advanced in comparison with other types of cells and the actual efficiency is 24.2% (AM 1.5 : Air Mass 1.5), this is very near the theoretical limit (27% - 30%). Recently polycrystalline silicon produced by cast method has also been developed. This type has high reliability and its electrical performance is stable for a long time but now the actual efficiency is about 17% (AM1.5).[Bailey, 1992] For constructing many huge solar power satellites, silicon has a big advantage because there are large reserves of Si on both the Earth and the Moon. The general characteristics of c-Si are as follows.
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