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Jamie1897 t1_j18h4np wrote

We do in fact have refrigeration systems that use gases as the working fluid. They utilized the Joule-Thomson effect (most gases cool upon expansion) and were called "Bell-Coleman machines" or "air cycle refrigeration machines". And they do exactly what you suggest, use a turboexpander as an energy recovery device rather than an expansion valve, the throttling losses in a gas system being too deleterious on performance to ignore. Even with this, the refrigeration COP was rarely greater than 1. This method is still used in air liquefaction devices in air separation plants, and for aircraft air conditioning where compressed air is readily available by bleeding from jet engine compressors. The low efficiency and the large circulating volume of gas needed per unit of refrigeration put them at a distinct disadvantage compared to mechanical vapor compression refrigeration. Here is a guy who built an air-cycle air conditioner powered by a vacuum cleaner and with a turbocharger as the turboexpander.

https://www.youtube.com/watch?v=f1FQjfyOifI

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samskiter OP t1_j18nw7y wrote

Awesome, bonus points for the Vidya. Will certainly watch later!

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mypoorlifechoices t1_j19un41 wrote

An important side note, his "lift" (the difference between his hot and cold side) was like 17 degrees F... A higher pressure air compressor will do more, but by carefully selecting your refrigerant to have a boiling point that changes rapidly with pressure, a heat pump can easily have a lift of 70 degrees, which is needed to heat a house in the winter, while still having manageable pressure ranges.

Tangentially, to convert this open loop system he has set up from cooling to hearing you'd have to move the radiator from outside to inside. To convert a heat pump, you just need one valve at the compressor that reverses the input and output, pretty much.

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General_Urist t1_j1acjxa wrote

Thanks for the run-down, and thanks for the video link: A good explanation and a fun piece of amateur engineering besides!

Why's this cycle still used for air liquefaction? No suitable material for a vapor-compression cycle that reaches such low temperatures?

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