5 English Summary, Miljøstyrelsen

CO2 as refrigerant in heat pumps

5 English Summary

The present report describes design, construction and test of a prototype heat pump with CO₂ as refrigerant. The project is phase 2 of a project commenced in 2000 with the title "CO₂ som kølemiddel i varmepumper" ("CO₂ as refrigerant in heat pumps"). In phase 1, it was demonstrated that components for transcritical CO₂ processes are commercially available today. There is however not a wide range of components, but as it appears from the present report, it is after all possible to design, construct and test a prototype system which lives up to the expectations of a CO₂ heat pump system.

The constructed heat pump of the project is a sanitary water heat pump designed for heating of water for sanitary use. Heat pumps of this type typically contain HFC-134a, which is a so-called greenhouse gas. Due to pressure from authorities and certain customer groups, the industry has focused on finding alternatives to the HFCs. The most obvious alternative is without doubt CO₂, which has excellent properties as refrigerant in heat pumps and sanitary water heat pumps in particular, where the requirement for high temperatures is available.

The project has demonstrated that certain components are difficult to obtain for transcritical CO₂ systems, especially small compressors for sanitary water heat pumps have turned out to be practically impossible to obtain. For that reason, a semihermetic piston compressor has been used in the project, which has a considerably higher performance than required. The compressor (i.e. the motor) has thus been equipped with a frequency converter, which makes it possible to reduce the performance of the compressor. But in spite of this, the compressor performance is still somewhat higher than desirable and certain corrections have been made in the analysis of measurement data.

Other components, which have been difficult to obtain, are the valves. Two valves have been used in the project, i.e. one valve for regulating the pressure in the gas cooler (high-pressure valve) and one for throttling. Both of the used valves are not at the present time considered as being commercially competitive.

In addition, a new control has been developed in the project for sanitary heat pumps with CO₂ as refrigerant. The control contains i.a. optimum control of the gas cooler pressure (the high pressure) which is necessary if an optimum regulation of the refrigerant system has to be achieved and if the expected efficiency has to be achieved.

In the project, a number of tests have been carried out of the constructed prototype, i.e. tests concerning refrigerant charge, valves, etc. and naturally part of the EN255-3 test. EN255-3 is the test standard used when the performance and efficiency of the sanitary heat pumps have to be determined.

All the tests demonstrate that if conditions (like e.g. the too large compressor) are compensated for, CO₂ is as expected better than R134a – it should however be noticed that only part of the EN255-3 test has been carried out.