Compression & Expansion

This is an idea I have been kicking around for a long time. I may even have written about it before. Why can't we make use of the waste heat from refrigerators to generate power, or to at least power themselves?

Your basic refrigerator and air conditioner work the same way. There is a compressor that compresses the "working fluid", typically freon. Compressing it makes it hot. The hot, high pressure fluid is allowed to flow through an external radiator where it gives up it's excess heat to the surrounding air. At the exit from the radiator there is the expansion valve that controls the amount of gas allowed through to the internal radiator. The pressure in the internal radiator is much lower than in the external one, so as the working fluid passes the expansion valve, it expands, and when it expands it gets much cooler. It gets so cold that it is colder than the inside of the refrigerator, so heat from the inside of the refrigerator is absorbed by the working fluid in the internal radiator. (I guess it's not proper to call it a radiator, since it is not radiating anything, but it functions just like a radiator: it maximizes the amount of surface area between the working fluid and the environment and so facilitates the transfer of heat energy from between the two.)  After the freon has worked it's way through the internal radiator, it enters the compressor, and the cycle starts over.

I always thought this was a little weird. Here you have this high pressure gas, which you have gone to a lot of trouble to compress, and you are just venting it. Why aren't we using it to run some kind of engine? An engine performs the same function as an expansion valve, plus you could get energy back from it.

I imagine there are probably two reasons it hasn't been done:

1. The present designs work. Companies build refrigerators and people buy them and feed them electricity. They do their job and people are willing to pay the cost of using them. Why spend a bunch of money on experimental research and engineering to build a product no one is asking for?
2. It may not be possible to produce a compressed gas motor that would be efficient enough to produce a tangible benefit.

Feeble excuses, both. On the other hand, I haven't spent any time or money actually trying to build one either, so I can't really be pointing any fingers.

There are a couple of items that bear on this.

Vane-Type Motor

Several (many?) years ago I saw an article in either Popular Science or Popular Mechanics about a new kind of air conditioner a guy had built. It had a vane type compressor and air motor combined into one. It operated directly on air, there was no freon. For 180 degrees it would compress the air. The compressed air was then fed into a radiator to discharge excess heat. The return air from the radiator came back into the combination motor/compressor at the very next point and drove the vanes of the motor for the next 180 degrees. The claim was that it used very little power, as it was using the pressure from the compressed gas to help drive the compressor, so it still used an electric motor, but it was smaller than it would be for a conventional air conditioner of similar size. I think the machine's failing was the precision needed to make the relatively large compressor/motor.

There have been some attempts to build power generating stations that use the difference in temperature of water on the surface of the ocean and water from a hundred or so feet down. I haven't heard anything about them lately. Here they are generating power from relatively small differences in temperature.

The last little bit is something I tripped over recently, and that is the thermodynamic cycle used by refrigeration engineers makes no use of the potential energy in the compressed gas. That seems like an awfully big hole in such an established theory.

P.S. Here's an interesting A/C idea.

Update February 2017 replaced missing picture.