kilotesla t1_j1aic57 wrote on December 22, 2022 at 9:34 PM

Reply to comment by BigWiggly1 in Why do we use phase change refrigerants? by samskiter

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.)

samskiter OP t1_j1cq9k9 wrote on December 23, 2022 at 9:24 AM

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

kilotesla t1_j1da7rf wrote on December 23, 2022 at 1:26 PM

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.