Performance Analysis of CO2 Heat Pumps in Different Applications

Abstract: This study focuses on researching the performance of CO2 heat pumps in different real-time applications and in some studies, it compares the performance to synthetic and other natural refrigerants based on heat pump data provided from buildings. The research on the performance of the CO2 heat pump is performed based on Sweden's climatic conditions. The study consists of four different case studies each focusing on the CO2 heat pump used for four different buildings. The first study evaluates the performance of air source CO2 heat pump installed in a residential building and performs cost benefit in comparison to district heating energy consumption. The second study investigates the performance of the air source CO2 heat pump for the district heating application and compares the same with other refrigerant heat pumps. The refrigerants compared with include Ammonia (R-717), Propane (R-290), R-134a (1,1,1,2-tetrafluoroethane). The third study examines the performance of air source CO2 heat pumps in a commercial building with the field measured data obtained directly from the heat pump sensors through the online portal “itop”. The fourth study analyses the performance of a CO2 heat pump with that of a propane (R-290) heat pump for a commercial swimming pool application.  The study is performed using a simulation model created using Microsoft Excel Sheets and Cool Prop add-in, a thermophysical property database. The simulation model makes use of formulae of heat pumps to analyse the performance of the heat pump systems. The climatic data for Stockholm is taken from ASHRAE IWEC 2 database. The results of the study show advantages of CO2 heat pumps when used for combined purposes like space heating, space cooling and domestic hot water over the heat pumps using other refrigerants for their operation, as these refrigerants when operated at high condensation temperature led to low Coefficient of Performance (COP). The first study on residential building CO2 heat pumps showed a cost savings of about 116,000 kr per year even in high-pressure operations concerning the annual cost of district heating, which is about 30% of the total cost district heating with auxiliary equipment. The study also examined the energy saving over the usage of an ejector used in the heat pump which reached an average energy saving of 8%. The second study shows the dominance of the performance of CO2 over other refrigerants for district heating purposes. The third study indicates the performance of the CO2 heat pump in the application using real-time measure data. The fourth study illustrates an increase in overall COP of about 10% from the CO2 heat pump in comparison to that of propane refrigerant for swimming pool application. These results show that when the domestic hot water demand is higher, the CO2 heat pump performs better than other refrigerants specifically because the COP of other refrigerants is lower at high condensation temperatures.