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?
In: 1
You want a refrigerant that will evaporate/condense within the working temperature range. That latent heat will increase the amount of energy that can be pumped without needing to change the temperature of the heat reservoir.
The heating and cooling happens when the working fluid changes phase from gas to liquid. The cool thing about refrigerants is that the temperatures required for their phase change is easily controlled with a pressure change.
Air isn’t going to change to a liquid unless under an extreme amount of pressure.
For the simple fact that these fluids can absorb and distribute far more heat than air.
Ultimately the efficiency of the unit depends on how much heat energy it can transfer from one location to the other. By using a fluid that has a high specific heat capacity, the transfer rate is increased significantly.
The fluid used in heat pumps has over 100X the specific heat of air, even compressed air. Which means it has over 100X the ability to transfer heat.
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.
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.
Latest Answers