These benefits, however, are far outweighed by its disadvantages. The CPC, particularly at high concentration ratios, is a long and therefore massive device. In addition its shape does not lend itself to effective radiation of heat and a secondary radiator would be needed. As a result, unless pointing accuracy or precision construction prove to be insurmountable problems for the Cassegrainian, the CPC is disqualified on grounds of weight. 2.2.4.3 Radiator Design Approximately 80 percent of the light absorbed by the solar cell must be disposed of as waste heat. Two methods of achieving this have been considered in detail. The first, a disk radiator, has the advantage of all aluminum construction and simplicity of design but limits the system to small cell sizes and/or relatively low concentration ratios. The second option, heat pipes, allow the use of larger cells and higher concentration ratios but they require significant amounts of non-lunar material and have a considerable technological risk associated with them. Both radiator systems are compatible with a simple annealing scheme. A sheet of reflective material is positioned behind cells to be annealed. This reflects heat back to the radiator, raising the temperature of the cell. This annealing system has not been designed in detail, but should require virtually no non-lunar material. 2.2.4.3.1 Disk Radiator The major advantage of the disk radiator is that it is already present in the form of the primary reflector. By making the primary reflector perform double duty as both reflector and radiator it is possible to cool the solar cell without any additional structures. As aluminum alone is not an efficient radiator (15,pp2-7), it is necessary to coat the back surface of the primary reflector with a high emissivity material. Aluminum oxide was chosen for this purpose because it has a reasonably high emissivity, consists totally of lunar materials and is easily applied by anodization. The thermal and mechanical properties of A1203 are, however, considerably different from those of aluminum. It was necessary, therefore, to require that the radiator be thick with respect to the A1203 coating. It is estimated that 25 microns of A1203 will be required to give the desired emissivity of .8 (14,ppll98-1201), however this emissivity value is heavily dependent on the method of anodization. A precise knowledge of this value would be essential for a more accurate determination of the mass efficiency of the concentrator. 2.2.4.3.2 Heat Pipes Two different configurations were considered for the application of heat pipes. The first involved placing several heat pipes as ribs in the primary reflector. This had all the advantages of using the primary reflector as the radiator, most notably little or no reradiation between unit cells.
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