As you say, integrating heat pumps into existing district heating networks designed for combustion based heating is suboptimal, but redoing the network for lower temperature operation is incredibly expensive as well.Ĭurrently, in the case referenced above, during the winter the heat pumps are AFAIU the "first leg" when the cold water returns to the plants the output of the heat pumps is then routed via the fossil fueled CHP plants for topping up (depending on how cold it is, it's heated up to IIRC ~120C) before it's sent out in the network again. In general, the evolution in district heating networks is towards lower temperatures, allowing better efficiencies with things like heat pumps. Here's a brochure from the heat pump manufacturer about the plant. This is an optimization problem: as you increase the size of the heat exchanger, it gets more expense, but improves performance. 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. 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).Įdit: 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. 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. Based on the refrigerant and COP of 3 (18/6), its probably around 50C. Just to dig into your example a bit: It kind of depends on what the return temperature is of the 90C heating fluid is. Interesting, sounds like a water to water heat pump (takes heat from sewage and pumps it into district heating). You'd much rather heat air, and skip the secondary fluid loop. The problem is really that hydronic heating is a bad fit for heat pumps in general. CO2 is actually very good for domestic hot water production, where the hot water is not returned. These problems really aren't specific to CO2 hydronic systems, and also appear with HFC refrigerants when heating water. 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). 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. 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. The main problem is that most hydronic (water based) heating solutions return fairly 'hot' water after heating the space. I've done some theoretical work in this area.
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