• Heat Pump
• CO2 Heat Pump
• Project Presentation

• Thermodynamic cycle
• Heat Pump Design
• Sanyo Eco Cute
• Swedish Report (KTH)

• EES Model
• COP Regression
• Defrost Cycle Model
• Future Dynamic Model ?

• Finnish Case Study
• Our Own Monitoring
• 3 Scottish Case-Studies
• Data Analysis
• Discussion
• Download

• Economical Aspects
• Environmental Aspects

• Potential of CO2 HP
• Recommendations
• Further Work

Conclusions

In this section of the web-site we have drawn together our conclusions and realised the deliverables that we set out to achieve at the start of the project. This section will be split into four sections. On this page we will highlight the main conclusions of the project. The other pages accessible through the menu on the left hand side will cover the potential of CO2 heat pump in the UK, our recommendations and the possible further work.

Summary of the work carried out in the project

• A literary review was undertaken, looking at the previous work done on R-744 and CO₂ heat pumps. The standards and best practice guides for testing and installing heat pumps were also studied to gain an insight into how CO₂ heat pumps fit into the standards set for traditional refrigerants.
• Monitoring was carried out in three separate households around Scotland, all of them with operating Sanyo Eco-cute heat pumps.
• Modelling of the CO₂ cycle was done using the thermodynamic software EES.
• Data was collected from a Swedish study on the Sanyo heat pump and from a current Finnish study using the same heat pump with live monitoring.
• Regression modelling was carried out with the data collected to provide evidence and trends in the performance values of the heat pump under different conditions.
• The defrost cycles have also been studied in details with the monitoring data collected.

Main conclusions for our project

From the results of our project, we are able to draw the following conclusions for this project:

• The performances of a CO2 heat pump in operation are influenced by the outside temperature and the energy used for the defrost cycles, as every conventional heat pumps. However, the main specificity of the CO2 thermodynamic cycles is that the performances will be very sensitive to the return water temperature to the heat pump which should be as low as possible.
• The coefficient of performance of a CO2 heat pump in operation is mainly determined by the outside temperature and the return water temperature to the heat pump. This was verified for the Sanyo CO2 heat pump whose COP has been expressed in function of these two parameters.
• Considering a particular heat pump, the frequency and the energy required for the defrost cycles are mainly influenced by the outside temperature and relative humidity. It is however a quite complex problem of heat transfer, psychometry and ice formation. Modelling accurately the defrost cycles is not straightforward.
• The performances of the CO2 Sanyo heat pump given in the technical manual from the manufacturer are confirmed. However some improvement will be required so that the potential of the CO2 technology are actually fully used. In particular, a better design could enable to decrease more efficiently the return temperature to the heat pump.
• In the case of the Sanyo Eco Cute system, the water return temperature to the heat pump is mainly determined by the water returning temperature of the heating system and the occurrences of the sterilisation cycles. The domestic hot water demand doesn't seem to influence the water return temperature to the heat pump since the cold water inlet from the city is located in the middle of the tank.
• The use of underfloor heating helps the CO₂ heat pump run efficiently since it operates at lower temperature in general than the other heating system. Even if the Sanyo system enables to produce water at quite high temperature, the use of the system with radiators will induce a decrease of the performances since the return temperatures from the radiators tend to be quite high. Otherwise, the temperature drop in the radiators must be maximized as much as possible.
• The system runs a sterilisation cycle after every 4^th compressor start where it takes the water flowing from the heat pump to the tank up to around 64^oC. This is to try and inhibit the growth of Legionella bacteria in the hot water system. It would be our conclusion though that this sterilisation cycle happening after every 4^th compressor start is excessive and decrease significantly the performances.
• The position of the sensors and the design of the tank are not optimised to limit the starts and stops of the heat pump and to meet an important domestic hot water demand without using the electric back-up heater. The tank is mainly used as a buffer storage for the domestic hot water but not for space heating.
• With an average COP estimated at 2 for the installations in Athelstaneford and Oban during the weeks of monitoring, the expectation for the seasonal performance factor (average annual COP) are not extraordinary high. If there is a potential concerning the use of the CO2 heat pump, the design of the tank will have to be improved to optimise the performances.
• The use of the other tank for the installation in Ballencrief doesn't bring any significant improvement in comparison with the Sanyo tank unit.
• A better understanding of the properties of the heat pump resulting in improved sizing and installation will allow the heat pump to operate as efficiently as possible for each individual dwelling.
• Studies have been carried out, looking at the design of the heat pump and hot water tank relationships. In particular a paper by Jorn Stene talks about the use of a "tripartite" system to maximise the efficiency of the heating system, more information on this paper can be found here .