Table 4-33 Heat Pipe/Fin Radiator Mass (Tapered Manifolds) of the cell, from and through the cover slip. Since cell efficiency is a strong function of cell temperature, other methods of cell cooling have been investigated. These are of great significance for solar concentration ratios of more than one, and are mandatory for gallium arsenide cells at very high concentration ratios (10 to 1000). Three radiator concepts have been investigated (see Figure 4-88). Fig. 4-88. Radiators for Solar Cells l)Fins. Here individual cell strips, up to approximately 3 cm (1.2 inches) in width would be bonded to aluminum fins which extend beyond the cell edges in the plane of the cells. These fins could possibly be used as current conductors. With this method the cooling area can be approximately four times the cell area. 2) Sheet Heat Pipes. It appears to be possible to fabricate plastic film heat pipes in sheets with a thickness of as little as 300 pm (12 mils). The working fluid will be a hydrocarbon, or possibly water. The cells are bonded to the evaporator zone of the heat pipe. The condenser (heat rejection) region of the heat pipe extends beyond the cell area. The cooling area can be up to ten times the cell area. 3) Pumped Manifold Radiators with Heat Pipe Panels. Individual cell strips are bonded to aluminum fins which conduct heat into a coolant line. The lines from a number of cell strips are brought together into manifolds which carry the hydrocarbon or water fluid to the radiator panel area. Pumping power is parasitic on the cell output. This method can provide heat rejection areas hundreds of times greater than the cell area. Fig. 4-87. Radiator Mass Comparison: Heat Pipe and Tube/Fin (Both With Tapered Manifolds) From Figure 4-87 it can be seen that below an inlet temperature of 700K (800°F) the heat pipe radiator has a lower mass than the tube/fin radiator; above this temperature the reverse is true. Thus for heat pipe radiators to compare favorably in mass with tube/fin radiators they must employ tapered manifolds. This means that they will also have some of the inherent drawbacks of the tapered manifold design. Differential expansion of input and output headers will cause panels to flex. Isolation valves will be required at each panel to prevent loss of NaK in the event of a panel manifold puncture. However, the heat pipe radiator panel will be much less vulnerable to catastrophic meteoroid damage than the tube/fin radiator panel since a much smaller part of the panel is carrying NaK. Puncture of a heat pipe would only mean loss of that heat pipe (one of many in a panel); puncture of a tube in the tube/fin panel would mean loss of an entire panel. 4.10.7 Radiators For Solar Cells Conventional solar cell cooling consists of provision of a black backside coating on the array, plus the heat rejection which occurs from the front
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