Miljøprojekt, 1074 – Kombineret brugsvands- og rumvarmepumpe med CO2 som kølemiddel

Kombineret brugsvands- og rumvarmepumpe med CO₂ som kølemiddel

6 English Summary

This project report describes the implementation of the Danish project called

"Combined heat pump for sanitary hot water and space heating with CO₂as refrigerant". In the course of the project, a combined heat pump has been developed for heating sanitary hot water and producing domestic space heating.

The project shows that CO₂has excellent properties in systems where a high temperature is desired on the gas cooler side and that it is possible to combine the production of sanitary hot water with the production of domestic space heating.

During the project, a number of system solutions have been analysed and at the end of the project a prototype was built. It was tested in the laboratory according to a current Dutch standard for heat pumps for sanitary hot water. The prototype was constructed without the space heat part which solely has been analysed according to calculations. The reason is that there currently are no applicable European standards for the testing of combined systems and as the total efficiency of the system mainly depends on the temperature out of the gas cooler it was decided not to spend resources on the construction of the combined system in the prototype version of the unit. Instead, a number of proposals have been submitted to how the system with a space heat section could be constructed.

The main components used in the prototype (compressor, exchangers, valve, control and tank) are all partly commercially available and therefore focus has been on the system construction. During the project, a number of CFD calculations have been carried out on the gas cooler in the hot water tank and the results show how important it is that the gas cooler is designed and placed correctly.

The laboratory tests carried out on the unit show that the COP of the heat pump plant in connection with sanitary hot water tapping (according to Dutch standard) is 1.4 – 1.5 which is not immediately satisfactory. But when it is considered that the unit is a prototype heat pump where product ripening/optimisation subsequently should take place the result is considered relatively reasonable.

Proposals for improvements:

Heat loss from the compressor should be reduced – the heat loss is relatively large and will principally be lost in the process.
Improvement of gas cooler so a lower discharge temperature is obtained from the gas cooler. Precisely the discharge temperature is Alpha and Omega in the effort to optimise the process.
Optimised determination of refrigeration charge that suits the current discharge temperature from the gas cooler. The realised determination of charge has been carried out at discharge temperature of the gas cooler = 36 °C
The pressure pipe should run at the top of the tank and not at the bottom so the hot pressure gas emits heat (at high temperature) exactly in that part of the tank where a high temperature is needed.

By using the residual heat from the gas cooler for heating purposes the actual discharge temperature from the gas cooler can be lowered substantially (down to app. 30 °C) which would improve the total process by up to 70%.

Summed up the results of the project are:

In the project, the construction of a prototype heat pump with CO₂as refrigerant has been a success. If the commercial conditions are available it will – with certain technical adjustments - be possible to start an actual batch production.