So, assume a compressor of some sort that compresses outside air into a copper coil that runs inside a house, with the other end going through the house’s envelope again and releasing the air back outside, with enough resistance (likely through some sort of expander that recuperates as much energy as possible from the still-pressurized exhaust) that the air stays under high pressure as it runs through the coil. Now, the compressed air going through the pipe will be hot, so it’ll transfer heat to the air inside the house. When it expands back at the end, it’ll be colder than it was when it got in.
Barring a misunderstanding on my part, we now have a theoretical heat pump that needs no specific refrigerant and thus avoids all the issues we have with those. It’s also a simple design, so there’s no way I’m the first one to come up with it. Why do we build heat pumps that operate on specific fluids within closed circuits?
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For efficient heat transfer, the gas you are using for refrigerant needs to condense into a liquid under reasonable pressures and the expected operating temperatures. It’s that phase change that absorbs and releases heat at either end of the loop.
To use regular air instead of a specific refrigerant, you’d be primarily using nitrogen (almost 80%) which at room temperature needs to be compressed to over 40,000psi to condense into a liquid. AC systems usually run between 60 and 200psi depending on the gas. A system to handle 40k psi would cost way too much and be a hazard to have in a home. That’s a hell of a lot of stored energy.
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