system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we use phase change refrigerants?

So from my memory of thermodynamics, an ideal heat pump is the carnot cycle. This cycle uses an ideal gas on both the hot and cold sides of the pump. However in the real world we use the refridgeration cycle with an evaporator and a compressor.

I understand that the Carnot cycle is 'ideal' and therefore we can't get to Carnot efficiencies in real life.

But what real life factor means we can't try and use a gas both sides (with a turbine to replace the evaporator? Is it energy density? Cost? Complexity? Do space/military grade heat pumps with high performance requirements do something different?

Thanks!

Edit: just a quick edit to say thanks so much for all the responses so far, it's exactly the sort of detailed science and real world experience I wanted to understand and get a feeling for. I will try and respond to everyone shortly!

Edit2: bonus question and I think some commenters have already hinted at this: flip the question, what would it take / what would it look like to have an all-gas cycle and if money were no object could it outperform a phase change cycle? I'm assuming extremely high pressure nitrogen as the working fluid to achieve a good energy density... Enormous heat exchangers. Could it get closer to Carnot COPs?
gpt
Phase changes occur at fixed temperatures. When you introduce a saturated liquid in a heat exchanger and extract a saturated vapour from it, your temperature difference between the exchanger and your heat source has remained constant (assuming a sufficiently large source). Same goes for a saturated vapour entering a heat exchanger and leaving as a saturated liquid. This is more efficient than heating a vapour, as its temperature will increase, causing a smaller temperature difference between itself and its heat source/sink and with that a reduced heat flux. The latent heat of vaporisation for e.g. water is quite high, which means you can absorb or reject a lot of heat at a constant temperature.

This is readily apparent from the heat capacity of water vapour and the latent heat of vaporisation of water. The c_p of water vapour is roughly 1.8 or 1.9 kJ/(kg K) at 0 degrees Celsius. This means that adding 1.8 kJ to one kilogram of water vapour will raise its temperature one Kelvin. The latent heat of vaporisation for water at 0 degrees Celsius is about 2500 kJ/kg. Meaning one kilogram of saturated liquid water will absorb 2500 kJ before its temperature will start to rise.

So to answer your question more directly, yes, you can use a heat engine with only a gas as a working fluid, but phase transitions are an excellent way of absorbing or rejecting large amounts of heat quickly. An example of a real-world gas-only heat engine is the Stirling engine, which runs on the Stirling cycle.