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Heat pumps with CO2 (carbon dioxide) as the working fluid instead of conventional refrigerants have been used extensively in Japan for the last 10 years and in Europe for the last 5 or so. They can output 190 degree water for use with DWH or could be used with radiators for hydronic heating. I would think that they would also work wonderfully as a geothermal heat pump.
I have tried to find a source for them in the US but haven't been able to. Does anyone out there have any suggestions?