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martinborgen t1_j18eb3l wrote

Apart from what others have said, the phase change can be seen as a nifty way to realize the carnot cycle. The phase changes are isotermic, and expansion valve is a simple way to get an almost isentrpoic expansion. Just the compressor to work on.

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samskiter OP t1_j18efbn wrote

Interesting. The carnot cycle would extract energy during the expansion. Is there a way to do that with the evaporator? Why don't we do it?

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saywherefore t1_j18gmh3 wrote

You absolutely could implement a turbine rather than an orifice to drop the pressure of the fluid in a phase change cycle, and in doing so you would recover some energy that is otherwise lost. However the added complexity and maintenance overhead is not worthwhile in many real world situations such as domestic refrigerators.

Thermodynamic efficiency is only one of several competing criteria. Another commenter already mentioned that the size of heat exchangers is massively lower in a system with liquid than a purely gas heat cycle. This is far more important than you might think, given the packaging constraints of say a domestic refrigerator.

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TackoFell t1_j18gahn wrote

Expanders are essentially compressors run in reverse (in fact can be made modified from such) and recover some energy. They aren’t used in many applications because the work recovered doesn’t make up for the cost and potential maintenance needs etc

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cheses t1_j18i5bp wrote

Most of the time when you expand via a valve in a refrigerant cycle you expand into the two phase region from a subcooled state. Expanding a liquid does not give us a lot of energy back. With co2 as a refrigerant there are ideas to use an expander to raise the efficiency of the cycle. But that's a special case as co2 has a flat saturation curve where we have a high vapor content while expanding. Also, expanding while having liquid content in the fluid can damage most expanders.

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TackoFell t1_j19l3md wrote

Also the comparatively very large pressure difference in a CO2 system helps! Say 1600 psi down to 600 psi, rather than say 600 down to 200. (Still may have flash gas…)

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RelativisticTowel t1_j1b44mn wrote

In many domestic refrigerators we do recycle a bit of heat. The external areas around the door gaskets are colder, and prone to a buildup of condensation if you don't heat them up somehow to match the rest of the exterior. We can and on occasion do put electric heaters there, but the best solution when possible is to simply route the starting section of the condenser (where the gas temperature is pretty high) through those areas. Bad move from a heat pump efficiency perspective since some of that heat leaks back into the cold side, but good for the efficiency of the appliance as a whole.

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