Station Air Conditioning
Station air conditioning refers to a variety of systems that keeps space stations cool. In a vacuum, cooling is a much more difficult problem than heating and many different solutions have arisen to combat it. Modern systems are energy efficient, quick-acting, and highly automated, making life on board a a space station as comfortable (and at times more comfortable) than life planetside.
Space stations build up heat in a variety of ways. The most common method is through direct sunlight. Depending on a station's distance from the local star, the star's luminosity, and the station's orbit around its celestial, direct sunlight can be a major source of heat or contribute next to none. Stations far from the star, or in the shadow of their celestial, rarely need to worry about solar heating, while it is a major concern for those close-in and day-side.
Stations also generate their own heat, through their local power plants. Stations with high energy requirements can generate extreme temperatures in their reactor rooms, often in the realm of several thousand degrees Kelvin. This heat must be dissipated somehow, else it can cause even the coldest station to quickly overheat.
Finally, humans and space ships are a small but not-insignificant source of heat. Stations that get high amounts of traffic can see their internal temperatures increase by several degrees simply from body heat and space ship exhaust if proper precautions aren't taken beforehand.
The vast majority of stations cool themselves through simple thermal radiation. Heat pumps inside the stations carry gas or liquid coolants throughout the station, which is then carried to large radiator fins outside of the station. The coolant is allowed to radiate its heat out into space, then is recycled back into the station to start the process over again.
This system can be quite complex. For stations that rotate or orbit a celestial body, the radiator fins rotate so they are always in the station's shadow. Outside of the direct light of the sun, in the vacuum of space, temperatures quickly drop to match the cosmic background temperature, allowing cooling to occur even when an entire side of the station is bathed in sunlight.
However, for some stations close to extremely hot stars, simple thermal radiation via heat sinks is not sufficient to keep the station cool. In these cases, there are a variety of solutions. Some stations simply have small thrusters on them to keep them in the shadow of their celestial. Others simply vent their coolant directly into space, carrying away accumulated heat. In this case, a station requires a massive supply of coolant to replace that vented into space.
Both methods have their drawbacks. Venting coolant is expensive and also requires cleanup to keep large amounts of waste coolant from collecting in globules outside of the station. Moving a station can disrupt local traffic and requires constant monitoring.
Finally, some stations are equipped with highly reflective surfaces that simply reflect solar radiation off into space. These reflective panels must be constantly kept clean, else their ability to disperse heat is sorely diminished.