Solar Cell Production During the study it became evident that the major part of the ACE of a photovoltaic SPS results from the fabrication of the photovoltaic modules including solar cells and structure. Therefore, the solar cell production and the required energy is discussed briefly in this chapter. Figure 3 illustrates the ACE for the production of photovoltaic modules made from multi-crystalline silicon solar cells. The total energy' required sums up to 548 kWh/m2. For amorphous silicon (a-Si) photovoltaic modules this value amounts to 253 kWh/m2, as illustrated in figure 4, and is hence more than 50 % smaller than for the silicon modules. These data were obtained from a detailed investigation of ACE values by analyzing the individual production procedures for terrestrial solar power plants [7], The results are based on the assumption that a production line with a technology status available in 1995 is used to 100 % capacity (4 shifts operation). According to this investigation the higher ACE values of high efficient photovoltaic modules are usually not fully compensated by their higher energy production during operation. The more sophisticated the solar cells, the higher is the ACE for their production. The major trade-offs with regard to the energetic optimization of a SPS are determined by interrelations between: • the specific mass in kg/m2 of the solar collector/panels • the specific mass in kg/kW of the power converters • the efficiency of the power converters and • the accumulated consumed energy in kWh/m2 or kWh/kg for their production. Usually, a solely efficiency optimized system is not the best solution for space application. NASA/DoE Reference Systems In the late seventies and early eighties NASA and DoE (Department of Energy) led several studies concerning the technical feasibility of a SPS [2], [3], [8], Two reference systems were selected and considered in detail (figure 1). Both reference systems were designed to deliver 5 GW of electricity to a electrical net on Earth. The satellites were assumed to operate in a geosynchronous orbit (GEO), using photovoltaic solar electricity converters, one with mono-crystalline silicon (c-Si) solar cells and the other one with gallium-arsenide (GaAs) solar cells. To transfer the energy down to Earth a 2,45 GHz microwave was selected requiring a rectenna diameter in the order of 10 km.
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