Brayton cycle. As background temperature is raised, radiator area must be increased to reject the same amount of waste heat or the cycle temperature ratio must be reduced to elevate the radiator temperature. System mass is significantly affected. This can be illustrated by considering the component scaling data for the original BIPS and its cycle performance variation as temperature ratio is reduced (Fig. 5, effect of raising compressor inlet temperature). From these data, mass breakdowns of (mini-BRU) system design points sized to provide the same output power over the range of background temperatures (design points optimized for minimum mass) can be plotted (Fig. 9). The data illustrate how system mass, mainly due to increased radiator area and heat source which must be added as cycle efficiency falls, would rise as background temperature is elevated from 216 K to 290°K. The mass penalty comes from the increased radiator area which is required to reject heat at reduced delta T. Figure 10 shows area required (emissivity assumed is 0.8) to radiate one thermal kilowatt at two rejection temperatures, 330°K and 55O°K, as background temperature varies from 210°K to 290°K. The radiator area required for a 33O°K rejection temperature, which corresponds to the mean effective temperature of the MRSR Brayton radiator, changes by a factor of 2.6, while the area required for a 550°K radiator changes less than 10%. On the other hand a thermoelectric system, which rejects heat at a temperature of 55O°K, shows a much smaller change over the same range of sink temperatures. Figure 11 shows the effect of cold-junction temperature on RTG weight and power, system efficiency and specific power for a thermoelectric system [11] similar to the Mod-RTG (a figure of merit of 0.846 is assumed for the calculation). For a given fin geometry and radiator area, elevating sink temperature from 20°K (interplanetary space) to 290°K (Mars or lunar surface) raises the cold junction temperature by only 8°K; resulting in an overall performance reduction of only 1%.
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