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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Refrigerants are used because they will change phases in the target range. That phase change transports a lot more heat per circulation than a gas would.
Take water for example refrigerant: if you’re just using steam to go from 100c to 200c in the cycle, each gram is going to transport 418.2 Joules (unit of energy). If you’re going from water at 100c to steam at 100C, it would transport 2257 Joules.
So to match the transporting ability of a phase changing refrigerant with a simple gas, you’d either have to pump ~5x the refrigerant, or pressurize it enough to elevate to ~5x the total cycle’s temperature. So you could, but it’s far less efficient.
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