SPS Feasability Study SD76SA0239-2

The results of GaAs cell performance after testing in a radiation environment are shown in Figure 2.2-8. The cells tested were 1 x 1 cm or smaller in size. After determining the cell performance, the cells were annealed by heating them at various temperatures. It was found that at annealing temperatures about 125°C some of the cells recovered and appeared not to retain any "memory" of radiation degradation effects. The radiation test data then were used to plot Figure 2.2-9 comparing GaAs cells with 4- and 10-mil Si cells. Figure 2.2-9 shows the radiation environment for 5-year and 30-year missions at GEO using the TADA radiation model. For GaAlAs cells operating at temperatures below approximately 125°C, a performance degradation of approximately 31 percent is experienced for the 30-year mission. It appears from the test data, however, that if the GaAlAs cells were operated continuously at approximately 125°C or above, they would be selfannealing and would not experience any radiation degradation. Based on the test results, the performance curve shown by the shaded area is assumed for the GaAlAs cell for the post-1985 SPS. Utilizing the cell efficiency versus operating temperature and radiation effects, an analysis was performed to determine if the output of the array would increase by trying to capitalize on the effects of cell annealing on a continuous basis, as shown in Figure 2.2-9. The results of the analysis are presented in Figure 2.2-10, which shows the array output in watts per square meter as a function of operating temperature and including the effects of radiation degradation and cell annealing. The curve shows a significant performance improvement by operating the GaAlAs cell above the annealing temperatures. Array output at 30°C is 400 w/m2, while at the self-annealing operating temperature of approximately 125°C, the array output is 480 w/m^ for an overall increase of 20 percent in power output. In the study, an array operating temperature of 125°C was used for sizing the system. Band pass filters were not employed and a surface coating will be required on the back surface of the array to prevent the temperature from dropping below 125°C. A 60- angstrom coating of aluminum sprayed on the backside of the Kapton substrate will result in an emissivity of 0.5 and the array will operate at a temperature of 120°C. Solar Cell Weight The GaAs cell efficiency as a function of junction thickness is shown in Figure 2.2-11 compared with the Si cell. The GaAs cell requires approximately 5- to 6-pm to generate power and attain the high efficiencies. Because of the thin junction thickness, it is postulated that the GaAs cells can be developed for the small thickness on lightweight substrates and has the potential for low solar array blanket weights. The weights of 2-mil Si and GaAs cells and a 1-mil GaAs cell are shown in Table 2.2-2. The previous SPS studies of Spectrolab/Arthur D. Little indicated that a 2-mil Si Solar array weighs 29 mg/cm2 (0.059 Ib/ft^). The Rockwell study resulted in the weights of a 2-mil and equivalent 1-mil GaAs cell array of 50.5 mg/cm2 (0.102 lb/ft2) and 29.4 mg/cm2 (0.060 lb/ft2), respectively. A Spralon cover of 1/2-mil was used to protect the cells from low-energy proton damage. FEP for a cover material has a temperature capability of 80°C before it starts to crack or blister. Lockheed is developing