Preliminary experimental results have indicated that the GaAlAs cells may be self-annealing to radiation damage at operating temperatures of approximately 125°C and above. Since the array is operating at T = 106°C, an analysis was performed to evaluate the increase in solar array temperature as a function of the emissivity of the rear surface of the Kapton substrate. The results of this study are shown in Table 2.2-5. The results of the analysis indicate that the solar array operating temperature can be increased to 125°C by changing the Kapton substrate rear surface emissivity of 0.8 to 0.5. The change in emissivity can be accomplished by the vapor disposition of 60 angstroms of aluminum on the back surface of the Kapton. An alternative approach which could be utilized involves the intermittent shutdown of individual bays. This would result in the bay not generating any electrical energy (conversion n = 0) and the total incoming solar energy absorbed by the array would be converted to heat. An analysis of this mode of operation shows that for a rear surface emissivity of 0.8 the array temperature would be 125°C. After the cells were thermally annealed, the solar array could then be connected to the distribution bus. Another major concern in the SPS operation is the thermal gradients arising in the structure. If these are excessive, the structural integrity and transmission capacity of the SPS may be jeoparized. A simple thermal model of the SPS structural cross-section utilizing a 14-node model was developed for determining the steady-state temperatures. It was assumed that the emissivity of the rear surfaces was equal to 0.4 and the view factor computations assumed that the section porosity was 90 percent (for example, 90-percent void volume). A rear surface solar array emissivity of 0.5 was used in the calculation. The results of the calculation are shown in Figure 2.2-17 and the bottom structural members of the SPS are seen to be -222°C. It should be noted that the computed structural temperatures may be actually greater than shown in the figure because the emissivity value of 0.4 is probably higher than would be evidenced by stock aluminum. The open porosity of the structure relative to the array also may be somewhat larger than the assumed value of 90 percent. Table 2.2-5. Array Temperature as Function of Substrate Emissivity
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