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.
BigWiggly1 t1_j18lp9z wrote
Reply to Why do we use phase change refrigerants? by samskiter
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.