English Summary, Miljøstyrelsen

CO₂ som kølemiddel i varmepumper

5. English Summary

In this report, the possibilities of using CO₂ as refrigerant in heat pumps for domestic hot water are described. The purpose of the project has among other things been to lay down a status for the development of components for CO₂ and to describe the special operating conditions present when this refrigerant is used.

In Denmark, approx. 15-20,000 domestic hot water heat pumps have been installed. The development during the last few years in the Danish energy sector has caused the number of domestic hot water heat pumps sold to stagnate to approx. 600 per year. The majority of the Danish manufactured domestic hot water heat pumps are exported to countries within the EU. The total production of Danish manufactured domestic hot water heat pumps is today approx. 3,000. It is expected that the number of Danish manufactured domestic hot water heat pumps reaches approx. 5,000 per year in 2002.

The project described in this report is divided into two main phases of which the first main phase is described in the report. This main phase is divided into four subphases in which the most important elements such as examination of components and control strategy are carried out.

The project is initiated with a description of the requirements that apply to systems containing CO₂ and a description of the general requirements applying to domestic hot water heat pumps. One of the most important factors to be aware of when using CO₂ as refrigerant is the system pressure. E.g. in the evaporator, the pressure reaches up to 70 bar.

For investigating the advantage of CO₂ as refrigerant in preference to R134a, a static simulation program has been developed in the project. This program makes it possible to carry out a comparison immediately of the energy efficiency of heat pumps using CO₂ and R134a respectively. These comparisons clearly show that CO₂ is an ideal alternative to the HFCs in heat pumps and in domestic hot water heat pumps in particular where the large temperature glide in the gas cooler can be utilised. E.g. at an outdoor temperature of 2°C and an inlet temperature on the water from the heat pump of 65°C for CO₂, an improvement has been calculated in the refrigeration system’s efficiency (COP) of approx. 48% compared with R134a.

With regard to heat pumps used for space heating only, the advantage is somewhat minor, but in this case CO₂ is also considered to be an interesting alternative to the HFC refrigerants. The calculations show that at an outdoor temperature of 2°C and an inlet temperature on the water from the heat pump of 35°C for CO₂, a decrease in the refrigeration system’s efficiency (COP) of approx. 12% is reached compared with R134a. If the corresponding is calculated at an outdoor temperature of 2°C and an inlet temperature on the water from the heat pump of 55°C, an improvement for CO₂ in the refrigeration system’s efficiency (COP) of approx. 27% is reached compared with R134a. The latter operating condition is of interest with regard to space heating as many operating hours in the heating season occur at these conditions.

In the examination of suitable components for CO_2,a conference on natural refrigerants (4^th IIR-Gustav Lorentzen Conference on Natural Working Fluids in Purdue University, Indiana, USA – July 2000) recently held has primarily been focused on. At this conference, a number of projects were presented which currently are running internationally. The main emphasis of this examination has been on the status for compressors to CO₂. The examination shows that within mobile air conditioning and small air conditioning and heat pump systems in particular, a number of compressors are found which are usable. With regard to the larger systems, a number of usable compressors are found as well. Common to all of the compressors is however that these are still on the prototype stage and only a few compressors – all for subcritical operation - are commercialised.

As for compressors, it applies to valves (high-pressure and expansion valves) for CO₂that these are still on the prototype stage. Heat exchangers (evaporator and gas cooler (condenser) for CO₂ are possible to procure on the market or alternatively to produce yourself. The supply is however relatively limited, but as gas cooler for domestic hot water heat pumps, coaxial exchangers can be used and as evaporator, traditional components can be used (fins mounted on tubes).

The rest of the components for domestic hot water heat pumps containing CO₂ are also referred to in the report. A number of hot water vessels has among other things been investigated and it has been described how these should be designed in order to utilise the properties of CO₂ optimally.

As the properties of CO₂makes it possible to control the system in a somewhat different way than traditional refrigeration systems, the part on control strategy describes how it is possible to control the pressure and the temperature in the gas cooler independently. This possibility can be used if a wide-ranging capacity control of the system is desired (provided that there is a transcritical fluid in the gas cooler). In this way, it is possible to avoid the traditionally used methods for capacity control by means of control of the variable speed of the compressor (frequency control/demand control).

For describing the possible system constructions of a domestic hot water heat pump containing CO₂, a number of concepts have been considered. These concepts are all possible and the selection of the correct one is left to the reader as this depends on several conditions, among other things production methods, supplier connections and whether more applications are desired in the system.

On the basis of the analysis carried out, the following can be concluded:

CO₂ is an interesting alternative to the HFC refrigerants in domestic hot water heat pumps in particular. However, at the moment, there is not a large selection of components commercially available on the market for this refrigerant so that the manufacturer can implement a traditional product development. It is expected that within a very short period of time (2-5 years), a heavy increase in the selection of compressors and valves for CO₂will be seen. Until then, much of the development work within this area will be concentrated at universities and institutes, but the refrigeration and heat pump industry has already been recommended to initiate an actual product development of systems for CO₂. With the information provided in this report, it will be possible to contact suppliers of components for CO₂ with reference to an actual co-operation. And it is assessed that such contacts will speed up the development of components for CO_2.The reason for this is that the manufacturers will not normally develop and market components before there is a demand from the customers. And this is exactly where the industry plays an important part.