 Gas refrigeration is a type of refrigeration cycle where the working fluid remains in the gas or vapor phase the whole time. Typically this is air because it's typically used in situations where you want to cool air, but where you want to minimize the number of devices and the complexity and the weight as much as possible. Gas refrigeration works by first compressing the gas, allowing it to heat up as a result of the compression process, pushing it through a heat exchanger where it rejects some of the heat that it now has available to it to reject, and then expanding it again in a turbine to get it back to its initial pressure where it now has a lower amount of thermal energy than it started and a lower temperature as a result. Gas refrigeration is essentially the Brayton cycle backwards. In fact, it's often referred to as the Brayton refrigeration cycle. The analogy that I use in my head though flawed is a sponge containing water. If you imagine a sponge, which represents the air, full of water, which represents thermal energy, squeezing the sponge allows you to drip some of the water out, and then when you allow the sponge to expand again, it ends with less water than it started. As a result of the compression process, more of the energy is able to be rejected in the form of heat transfer. As a result of not jumping across the saturation dome, we don't have latent energy helping us in our refrigeration process, which means that the coefficient of performance of a gas refrigeration cycle is often relatively low. The fact that it is so inefficient means that gas refrigeration is pretty much only used in situations where inefficiency is more favorable than weight. The aircraft industry, for example, uses gas refrigeration. If you think about the fact that if you are trying to air condition air, then this low temperature air outlet can be used as the actual air you're conditioning in the first place. Furthermore, since what this takes to operate is just shaft work, you can attach this to a power takeoff on an engine and run the entire system with relatively little mechanical complexity. You have a compressor, a heat exchanger, and then a turbine. We can improve the efficiency of a gas refrigeration cycle a little bit by adding in a regenerator. The presence of regeneration improves the efficiency of the cycle a little bit. The downside is this isn't really able to be used in the same direct air conditioning style as the open gas refrigeration cycle. Instead, you are pulling heat out of a refrigerated space through a heat exchanger. So the takeaways of the gas refrigeration cycle are relatively low weight, relatively low mechanical complexity, and relatively low coefficient of performance. And again, the only place you typically find gas refrigeration is in situations where having less weight is more favorable than paying a little bit more work to operate the cycle.