cells. Providing thick cover glass for radiation protection of course conflicts with (1). 3) The cells must be efficient, since the SPS area required is inversely proportional to the cell efficiency. Thin cells tend to be less efficient than thick cells. A good temperature coefficient is also to be desired, i.e., the efficiency should not fall off rapidly as cell temperature is increased. 4) The cells (and the completed array of cells, substrate, connections, etc.) must be low in cost. Today's array costs are far too high for the SPS. These requirements are seen to be interactive. The requirements are also somewhat dependent on the configuration of the SPS. High solar concentration ratios decrease the cell area, reducing the array costs and allowing thicker cover glass to be used without a mass increase (since reducing cell area also reduces cover glass area). However, higher concentration ratios tend to increase the cell temperature, hence a good temperature coefficient is required to keep efficiency high. 4.7.2 Cell Performance Prediction 4.7.2.1 Efficiency Cell performance to be available in the initial SPS operating period was obtained by projection of the historical trend in efficiency improvement. This historical trend was researched back to 1957. Performances used were generally those of cells which could be purchased, not merely performances quoted in Photovoltaic Specialists Conferences or in press releases. Projection of the trend from 1957 to 1976 into the future resulted in the performances shown in Figure 4-28. Fig. 4-28. Solar Cell Performance Predictions Fig. 4-29. Fundamental Limitations May Enforce Performance Plateau
RkJQdWJsaXNoZXIy MTU5NjU0Mg==