[torpfactory]

I've done some theoretical work in this area. The main problem is that most hydronic (water based) heating solutions return fairly 'hot' water after heating the space. In order to get the most energy out of your CO2 refrigerant gas, you need your other heat exchange fluid to be as cool as you can manage. A designer could choose to use a different style of radiator which would allow the water to get closer to room temp, but that is not the type of system currently being manufactured. This would also not work for in-floor heating, as you would have parts of your floor very close to room temperature, not what people expect (warm floors).

These problems really aren't specific to CO2 hydronic systems, and also appear with HFC refrigerants when heating water. CO2 is actually very good for domestic hot water production, where the hot water is not returned.

The problem is really that hydronic heating is a bad fit for heat pumps in general. You'd much rather heat air, and skip the secondary fluid loop.

[jabl]

I recently toured a municipal heat pump plant. It took cleaned sewage at IIRC 12C, dropped it to 6C or abouts, and the heat was pumped into the district heating network at about 90C.

The plant had 6(?) pumps, each producing 18 MW heat while pulling 6 MW electricity (and also some district cooling). Not CO2 though, each pump had 9000 kg R134a as the working fluid.

[torpfactory]

Interesting, sounds like a water to water heat pump (takes heat from sewage and pumps it into district heating). Just to dig into your example a bit: It kind of depends on what the return temperature is of the 90C heating fluid is. Based on the refrigerant and COP of 3 (18/6), its probably around 50C. If they were able to find a use for the process fluid so that it returned at 20C, they would probably have a COP closer to 5. You can still build 'relatively' efficient heat pumps for hydronic heating, but they would be significantly more efficient if we did a better job dumping all of the heat from the water (which would require a larger, more effective heat exchanger).

Edit: I guess what I'm trying to say is that most of the radiator technology we use (like the in room part of the heating system) was designed with combustion-based heating in mind. Combustion based heating sort-of works just as well if the return water is 50C or 20C, but you would get a substantial boost in heat pump hydronic heating performance if you were to redesign heat exchangers to get as much temperature out of the fluid as possible. This is an optimization problem: as you increase the size of the heat exchanger, it gets more expense, but improves performance. There is an optimum in there somewhere...