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BigWiggly1 t1_j18lp9z wrote

There are a few reasons the refrigeration cycle is particularly useful to us, and they do in fact relate to energy density and cost as you guessed. Another factor is operating range.

A major reason is that latent heat of vaporizations are orders of magnitude higher than sensible heat capacities. By using phase changes, we can move much more energy using much less fluid.

For example, imagine we're using steam cool a hot process. The heat capacity of steam (water vapor) around 100-200 C is about 1.9 kJ/kgC.

So in order to remove 1 MW of energy (1000 kJ/s), we would need about 10.5 kg/s of steam if we could get a 50 C temperature drop in our steam. 10.5 kg/s is a lot of steam. In my line of work we measure steam in 1000's of lbs/hr. 10.5kg/s is 83 klb/hr or 83,000 lbs per hour. Nothing to scoff at.

Alternatively, if we could use a steam condensing heat exchanger, we could accomplish the same energy transfer with 0.44 kg/s. That's 24x less steam. That means smaller pipes, smaller pumps and compressors moving the fluid around, smaller valves, etc. Those costs balloon quickly.

Handily, it also outputs condensate water instead of steam, which is easier to handle and pump than steam. It can be pumped right back to the boiler feedwater system.

A second reason we use refrigerants is because they offer convenient phase change temperatures. Water vapor is useless for space cooling, because it condenses at 100C. We'd have to use liquid water for cooling instead, which wouldnt be able to use the carnot cycle.

For space cooling with the carnot cycle, we'd need to use a compound that's a gas at usable room temperatures and high pressures like nitrogen. The problem is that we'd need to use a LOT of it to get any meaningful energy transfer out of the system. We'd need large high pressure storage cylinders, high pressure piping, large heat exchangers. We're talking 100's of thousands of dollars of equipment per house.

Refrigerants are easily contained, the equipment to pump them around is cheap since we don't need much per house. It keeps costs manageable.

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samskiter OP t1_j18od2r wrote

Excellent answer, thanks so much. I'm certainly curious in what the gas-obly system would cost/look like if we made it and whether it could outperform phase-chabgw systems. (If we spent those 100s of thousands as you said!)

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sorenabergard t1_j18rn23 wrote

A very practical reason that is implied by this explanation but not quite explicit is related to the quantity of fluid. Because the heat of evaporation is so high, you can achieve your cooling effect with less fluid (as the previous poster said). This means way smaller pipes, which are much cheaper to install, and smaller heat exchange surface areas, reducing equipment size.

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kilotesla t1_j1aic57 wrote

To expand on this point:

> Water vapor is useless for space cooling, because it condenses at 100 C.

That's at atmospheric pressure. Part of the concept of how a refrigeration cycle works is to change the pressure so as to change the boiling point. So then the question becomes, why not lower the pressure an lower the boiling point, and use water as the working fluid for an air conditioner? A problem with that is the low pressure needed (circa 0.01 atmosphere) would mean the gas would be very low density, and you'd need to flow and compress a very large volume of it to move significant heat. (The other problem is that you couldn't go below 0 C without the water freezing.)

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samskiter OP t1_j1cq9k9 wrote

Nice! So your system could dynamically flex the boiling point to match the desired working range (within reason)?

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kilotesla t1_j1da7rf wrote

It could within some range. It would be best as a heat pump for heating from moderately warm temperatures up to high temperatures, perhaps 40° C up to 100° C, for example.

Note that that's also how a conventional heat pump works, with an HFC refrigerant. Let's say it's operating between 10° C and 40° C. The evaporator pressure will be set up so that the boiling point of the HFC is around 10° C, and the condenser pressure will be high enough to make the boiling point there 40° C.

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QtPlatypus t1_j1bb7my wrote

>Water vapor is useless for space cooling, because it condenses at 100C. We'd have to use liquid water for cooling instead, which wouldnt be able to use the carnot cycle.

Though cooling is just the other side of warming. Steam is a great way to heat a room.

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