Jørn Stene
2000-12-21
17 sider
Price NOK 350,-
Order report

This report presents some preliminary calculations regarding the expected Coefficient of Performance (COP) for residential ground coupled CO2 heat pumps for combined low temperature floor heating (35-40ºC) and hot water heating (65ºC).

In new or renovated residences with high insulation standard, a low temperature floor heating system is regarded to be the optimum heat distribution system due to the high thermal comfort. On the other hand, the hot water demand in residences constitutes a relatively large share of the total heating demand. Consequently, future residential heat pump systems should be able to cover both heating demands at a high energy efficiency. The most energy efficient alternative available today is represented by propane water-to-water heat pumps using desuperheaters for hot water production. This design reduces the required condensing temperature compared to systems that preheat hot water with condenser heat, and the units typically achieve a COP of 4.0.

The calculations in this report have been made by using Coolpack and Refprop 6.0. The high-side pressure for the CO2 heat pump was set at 80 bar, and an isentropic compressor efficiency of 0.65 was selected. Moreover, the countercurrent (tube-in-tube) gas cooler was divided into three sections: 1) Preheating of hot water, 2) Space heating (floor heating system) and 3) Reheating of hot water, in order to achieve a large temperature glide and the lowest possible CO2 gas cooler exit temperature. At this low supercritical pressure and subsequent heat rejection to the floor and hot water heating systems, the CO2 process is rather similar to a conventional subcritical heat pump process with heat rejection by subcooling, condensation and superheating of the working fluid.

The preliminary calculations indicate that the CO2 heat pump process is well suited for combined low temperature space heating and heating of hot water, and a ground coupled CO2 heat pump may achieve the same energy efficiency as the state-of-the-art residential propane and HFC heat pumps – i.e. a COP between 3.3 and 4.0. Generally, the COP will increase with increasing hot water demand. In order to assess the real potential of this technology, more detailed work is required regarding evaluation of system design, system simulation, design and optimisation of the gas cooler as well as laboratory measurements/verification.