Theoretical Analysis And Experimental Research Of Carbon Dioxide Heat Pump Heating System

CO₂ has recently come into fashion as a natural refrigeration that is safe, environmental and sustainable. It has the potential to replace hydrofluorocarbon (HFC) and hydrochlorofluorcarbon (HCFC) refrigerants, causing more and more attention of researchers.A carbon dioxide heat pump heating system had been designed for space heating and hot water heating, considering the thermodynamic properties of trans-critical carbon dioxide heat pump cycle. The designed system can be used for combined space heating and hot water heating by two-step cooling and can heat conventional radiators, which is an important advantage when converting buildings under preservation order to renewable energy supply. A computational program for the analysis of CO₂ heat pump unit's thermodynamic performance was written by EES software. Using two methods of heat balance analysis and exergy analysis, the effect of various run-time parameters on the system performance was analyzed, and the system performances under different operating conditions were compared with four conventional medium-high-temperature vapor-compression heat pumps: R134a, R152a, R290, and R245fa. The exergy economic model of CO₂ heat pump heating system was established for the generalized evaluation based on both technical and economic viewpoints. After building experiment platform of CO₂ heat pump heating system, the performance tests were carried and the results were compared with the ones calculated using the computational program.By calculating, the COP and exergy efficiency of CO₂ heat pump are higher than the performance of the four conventional medium-high-temperature vapor-compression heat pumps in the operating conditions of heating 60℃hot water model and combined space heating and hot water heating model. Between the components of five medium-high temperature heat pumps, the exergy loss of compressors is always the highest value and the evaporator and regenerator's exergy loss can be negligible. The rate of exergy loss of CO₂ heat pump's gas cooler compared to other heat pumps'condenser is the smallest, but the rate of exergy loss of CO₂ heat pump's compressor is the biggest. When the pressure radio is greater than 2.3, the exergy efficiency in the model of combined space heating and hot water heating is higher than the separate hot water heating model with the same outlet water temperature and same running parameters of heat pump. When heating 60℃hot water alone in the typical condition, the unit exergy cost of CO₂ heat pump products decreases linearly with decrease of the initial investment and the price of electricity, decreases with increase of the evaporator temperature, and descends first and then raises as the cooler outlet temperature increases. Through experimental testing, we can come to the conclusion that in the model of combined space heating and hot water heating is 6.4% higher than the separate hot water heating model with the same outlet water temperature. A good agreement between the experiment and simulation with deviation less that 5% is demonstrated.