Dear Andrew,
Reading your request, I thought it might be a good idea to clarify some issues regarding the use of CO
2 heat pumps in hot water and space heating, as I’ve received quite a lot of similar requests:
HEATING SANITARY WATER
Sanitary water needs to be heated up from a low temperature (i.e. +10°C) up to storage temperature (60 to 80°C or more). A conventional heat pump, for example a heat pump using R134a as a refrigerant, releasing heat at constant temperature, requires a high condensing temperature, at the same level, at least, of final temperature of water.
The above is not true in absolute, as it is possible to design, for instance, a system with R134a as a refrigerant where also subcooling and superheating of refrigerant are used for heating water, obtaining a significant improvement in efficiency. Final temperature of water could be in that case slightly higher than condensing temperature. It means however that heat pump should be designed for heating water in one passage (once though) while most of the heat pumps today are not of this type. Heat pumps using CO
2 as a refrigerant are well suited for heating sanitary water in one passage from a relatively low temperature up to high temperature, as an example from +10°C up to 70°C or more, as the efficiency depends significantly from outlet temperature of refrigerant at the end of heat rejection process, in other words it depends strongly on inlet temperature of the water.
In combination with a stratification storage vessels – containing directly the water to be used – a CO
2 water heating heat pump has another significant advantage over state of the art heat pumps, as it is possible to have readily available a lot of water at high and constant temperature, as cold water entering the storage vessel doesn’t mix with warm water reducing its temperature.
Conventional heat pumps with HFC refrigerants, being not “once through” type, cannot be used with a stratification vessel, and they are normally used with a mixing storage vessel. The direct use of stored water has another advantage : for some time there is a huge “power” available. For instance if a relatively small heat pump with 20 kW output “produce” 3500 liters of warm water at 70°C – heated from 20°C - during a nighttime period of 10 hours and this water is used at 45°C in a hotel from 7,00 to 8,30 in the next morning it corresponds in average to a power of 130 kW, sufficient for about 150 showers of almost 50 liters. It is clear that in practice this is the most reasonable way to use a heat pump for heating water. A mixing type vessel would be enormous (at least 4 times as big) and impractical. Moreover the use of an “inertial” vessel would add inefficiency, due to additional temperature differences, and also there wouldn’t be, with an heat exchanger, the flexibility given by direct use of hot water.
A heat pump with 20 kW output as in the above example, operating with a COP of more than 4,0, would have an electrical power input of about 5 kW, so electricity input required is quite low. So definitely for this application, heating water for sanitary use or in general heating water dissipated in a process, it is better to use a C
O2 heat pump.
SPACE HEATING
It must be stressed that many people, listening about the features of this product, are attracted by high temperatures of water (i.e. 80°C) at the end of process and erroneously believe that it is possible to use a CO
2 heat pump to supply hot water to a space heating system operating, as an example, with radiators, with return temperature relatively high, i.e. 55-60°C.
This is not possible, as a CO
2 heat pump is not suited for high temperature space heating system, due to high return temperature required by these solutions. High return temperature means higher temperature of CO
2 at the end of heat rejection process (i.e with return water at 50°C CO
2 temperature at gas cooler out would be approximately 55°C) and consequently very poor efficiency and capacity.
Nor it is possible or convenient, for the same reason, to use a water heating heat pump as a back up system to lift water temperature of a vessel heated by solar panels. When solar radiation is no more sufficient to obtain a given temperature of the water. There are of course solutions for integration of a heat pump for water heating with solar thermal panels, but certainly a “series” connection of solar panels (meaning that “out” of solar panels is “in” of heat pump) with heat pump is definitely not the right solution.
It is obviously possible to produce an efficient heat pump for space heating using CO
2 as a refrigerant – and some companies are working on that – but it will always be designed for a low temperature heating system (i.e. radiant or floor heating), so with a water return temperature not higher than 30-35°C, as with normal heat pumps for space heating. In case, for low energy houses becoming now the standard, there will be in any case a significant advantage compared with HFC heat pumps, considering that energy required for heating of sanitary water will be predominant.
