If you care about the heat coming out on the hot side rather than the heat going in on the cold side (i.e. the application is heat pump rather than refrigerator), then the theoretical limit is always greater than 1, since the work done gets added onto the heat absorbed:
COPheating=COPcooling+1
Cooling performance can absolutely be less than 1, and often is for very cold temperatures.
post:
A lot of MRI machines use >70 kW.
COPcooling=TCTH−TC
20 K300 K−20 K=0.071
If you care about the heat coming out on the hot side rather than the heat going in on the cold side (i.e. the application is heat pump rather than refrigerator), then the theoretical limit is always greater than 1, since the work done gets added onto the heat absorbed:
COPheating=COPcooling+1
Cooling performance can absolutely be less than 1, and often is for very cold temperatures.
Right. But I was using net efficiency values from papers on cryocoolers, not Carnot efficiency values.
That’s another issue with liquid hydrogen fuel:
combustion to gas: 119.93 kJ/g
vaporization: 446 J/g
ortho-para conversion: 670 J/g
Burning liquid hydrogen in a car engine would probably make less energy than it takes to liquify the hydrogen.