Thermal Management in Space

Abstract

Thermal Management in Space Abe Hertzberg The vehicles and habitats associated 'with space industrialization and the exploitation of nonterrestrial resources will inevitably require energy systems far exceeding the current requirements of scientific and exploratory missions. Because of the extended duration of these missions, it is not possible to consider systems involving expendables such as nonregeneratable fuel cells. Therefore, these missions become hostages to the capability of continuous-power energy systems. These systems will need to provide hundreds of kilowatts to tens of megawatts of electrical power to a product fabrication system, whether it uses terrestrial or nonterrestrial raw materials. Because the power system will be located in an essentially airless environment, rejecting waste heat becomes a limiting aspect of it. In the following paragraphs, I will review space-based or asteroidal and lunar based power generating systems, as well as the capability of existing technologies to dissipate this heat into the airless environment of space. It should be pointed out that in a vacuum environment, convection is no longer available and the only mechanism of rejecting heat is radiation. Radiation follows the Stefan-Boltzmann Law E = T 4 where E = the energy rejected , the Stefan-Boltzmann constant, = 5.67 W m-2 K-4 T = the temperature at which the heat is radiated That is, the total amount of heat radiated is proportional to the surface area of the radiator. And the lower the radiation temperature, the larger the radiator area (and thus the radiator mass, for a given design) must be. The radiator can only reject heat when the temperature is higher than that of the environment. In space, the optimum radiation efficiency is gained by aiming the radiator at free space. Radiating toward an illuminated surface is less effective, and the radiator must be shielded from direct sunlight. The rejection of heat at low temperatures, such as would be the case in environmental control and in the thermal management of a materials processing unit, is particularly difficult. Therefore, the design and operation of the heat rejection system is crucial for an efficient space-based energy system. Space-Based Power Generating Systems In a previous paper, space-based power generating systems have been described in detail. Solar photovoltaic systems have a generating capability of up to several hundred kilowatts. The power output range of solar thermal systems is expected to be one hundred to perhaps several hundred kilowatts. While in principle these power systems can be expanded into the megawatt region, the prohibitive demands for collection area and lift capacity would appear to rule out such expansion. Megawatt and multimegawatt nuclear power reactors adapted for the