A study of a transcritical multi-stage carbon dioxide heat pump
The author believes CO₂ to be a promising future refrigerant due to the threat to the HFCs of restriction or elimination posed by legislation planned in many countries. This thesis addresses the feasibility of using reciprocating compressors in a transcritical CO₂ heat pump working in cooling and/or heating modes through the use of computer modeling. A detailed simulation model of a reciprocating compressor is combined with semi-detailed gas cooler, evaporator and internal heat exchanger models to produce complete cycle model of systems having one and two stages of compression. Measured data from the open literature for CO₂ compressors and single-stage heat pumps was used to validate the models. Piston ring-cylinder leakage and valve dynamics are included in the compressor model. The influence of ring-cylinder clearance on heat pump performance is investigated as is the influence of heat pump running conditions on valve dynamics. Prior to this study, there were no models known to the author for CO₂ heat pumps which incorporate a detailed simulation of the compression process. Furthermore, there were no models for CO₂ heat pumps incorporating multi-stage compression. This study provides insight into the manner in which a CO₂ heat pump might perform in both cooling and heating modes for running conditions representing summer and winter. The models are believed to be of value to other research workers, plant designers and plant operators. As a result of this study, useful techniques have been provided for the design and manufacture of environmentally safe and energy efficient heat pump systems; it is hoped that they will make a positive contribution to the reduction of effects harmful to the environment and its inhabitants in the future.