§ 4.1.5 Heat Rejection
The vacuum environment does bring one drawback, however. Industry on Earth often rejects heat to the environment by smokestacks and cooling pipes in lakes. In space, plain infrared radiation must fulfill this task. Any processes requiring rapid heat rejection would require using large radiators (since infrared radiation rejects heat at a rate proportional to the fourth power of temperature, T4).
Large radiators would be expensive to blast up from Earth, but they can be simple enough so that they are mass produced from asteroidal and/or lunar material for most applications.
Of course, not every application requires rapid heat rejection. Some applications will require insulation for slow cooling. However, some processes will be limited by the rate of heat rejection.
It's worth mentioning that extremely cold temperatures are also readily available in the shadows of space. However, it takes time to achieve very cold temperatures. However, if you want to store something in the cold for a long time, it's cheapest to do it in space. Once it's cold, it doesn't take any refrigeration work to keep it cold. Just keep it in a shadow produced by a reflector.
Different designs of radiators in space
Many studies focus on making low mass, highly efficient radiators for launching up from Earth. However, in a PERMANENT scenario, many of these designs are inappropriate. It's important to understand that the mass of the radiator is not as big of a problem for radiators made from asteroidal or lunar material as it is for a radiator made on Earth and launched up. For manufacturing the radiator in space, simplicity of design for manufacturing it in space is an important factor.
There are two kinds of radiators, "passive" (no moving parts) and "active" (with moving parts). Within these two types are a wide array of variations.
The simplest radiator is just a big metal hot plate with fins. No moving parts. It could be oriented perpendicular to another large object which casts a shadow. It would protrude away from the factory so that its radiation is not reflected by the factory and so it doesn't receive other radiation from the factory. Radiators are best made of metals which are good heat conductors and have a high rate of emissivity.
The most common active radiators pipe a fluid to materially move the heat from one place to another, usually a fluid that evaporates at the hot end and condenses at the other (called a "heat pipe radiator", utilizing heat of vaporization and fusion). Another concept is the "liquid droplet radiator" whereby a hot liquid metal is sprayed from the hot end towards a collector (no evaporation or condensation, just spray), the idea being that droplets have a higher surface area to radiate, per unit mass.
Regarding radiators with working fluids, micrometeors could puncture them, or a large leak could cause a disaster, so the fluid must be protected some way. One interesting design has the working "fluid" in active radiators consist of tiny metal balls rather than a liquid.
A fairly new design is a "moving belt radiator" whereby a drum is connected to the heat source and a long metal conveyor belt moves across the drum.
Passive radiators are generally much simpler and easily mass produced from asteroidal or lunar material. In any case, the radiators in space will be much bigger than radiators on Earth to perform the same amount of cooling.
Multiple uses of heat gradient in factory complex
Different industrial processes require different operating temperatures. It may be feasible to design an overall factory complex whereby one process utilizes the waste heat from another process, rejects its own waste heat to a third process, and so on. However, if too many processes are added, this results in a very complex factory design. The factory must be adaptable to the shutdown of a particular operation down the chain, as shutting down one operation could affect the operating temperature of the next process unless remedial measures are taken.
"Cogeneration" is a process whereby electricity is generated using thermal engines (e.g., Stirling, Brayton or Rankine engines) and their waste heat is used for thermal heat in factories. Likewise, the other way around, waste heat from high temperature materials processing could be used for electricity generation (thermal engines or solid state thermoelectric). The value of this in space is debatable in view of solar cells as an alternative electricity generation scenario. Indeed, a good place to put a radiator is in the shadow of the solar cell array.