CO2 systems generally operate in the supercritical region by design, although the pressures involved are high enough to make DIY impractical. At the other extreme, water vapor operates at too low of a pressure (and chemical incompatibility issues) to easily build practical systems with, even though it is a very efficient refrigerant when properly used.
As a general guideline, higher critical temperatures and lower operating pressures (within reason) correlate with higher efficiency.
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To my surprise, shortly after Naomi Wu gave me a bit of fame for making good use of solar power, Allie Moore got really jealous of her...
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