Investigation of CO2 hybrid groundcoupled heat pumping system for warm climate
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Environmental sustainability, renewable and clean energy utilization and energy conservation have become key issues facing the development of modern society. In the field of refrigeration, air conditioning and heat pumping systems, the combination of sustainable energy technology with environmentally friendly refrigerants, will be an important trend. Carbon dioxide (CO2), or R744, as a natural refrigerant, shows great potential to be a main refrigerant in the future, because of its environmental characteristics and superior thermodynamic properties. The methods described in this thesis aims to improve the energy efficiency of a CO2 transcritical heat pumping system for indoor air conditioning application under warm climate conditions. Therefore, concepts, like ground borehole heat exchanger, two-phase ejector and separated gas coolers for gas cooling heat recovery, are introduced to the investigated CO2 heat pumping system. The state-of-the art CO2 as refrigerant, heat pumping system and hybrid ground-coupled heat pumping system indicates the necessity of further development of CO2 refrigerant to the building air conditioning sector. Moreover, it also shows that the transcritical CO2 heat pumping system has potential to integrate the technology concepts, like temperature and humidity independent control for indoor air conditioning and the hybrid ground-coupled heat pumping system, for the better energy efficiency. Regarding the methodology, steady and quasi-steady state analyses are performed on a CO2 heat pumping system. The steady state analysis provides the fundamental information for the system design procedure for the transient and practical models of the system, and the optimal control strategy of the gas cooler pressure is proposed for the investigated CO2 transcritical refrigeration cycle. The quasi-steady state analysis provides the practical information on the system’s energy performance. In addition, the experimental validation for the transient models shows the difference between the simulation and the measurements of the practical system. In the research work, the following solutions are suggested to improve the energy efficiency of a CO2 heat pumping system for air conditioning application. These include the technology concepts, i.e. ①.CO2 hybrid heat pumping system by integration of ground soil and ambient air as heat sinks; ②.CO2 two-phase ejector system for the indoor air temperature and humidity independent control; ③.smart utilization of the CO2 heat pumping system by the means of gas cooling heat recovery. Firstly, the quasi-steady state simulation calculates the annual energy efficiency of R744 hybrid ground-coupled heat pumping system. This hybrid system uses both ground soil and ambient air as heat sinks in the cooling mode. The simulation work also compares the annual energy performance of heat pumping systems using R410A and CO2 as refrigerant. The simulation results show that the seasonal cooling and heating coefficient of performance (COP) of the CO2 hybrid ground-coupled heat pumping (GCHP) system are 3.55 and 3.32. Moreover, the cooling performance is 42% and 23% better than both the CO2 air source heat pumping (ASHP) and the conventional CO2 GCHP systems. Therefore, the CO2 system shows superiority by integrating of two temperature levels of heat sinks, i.e. ground soil and ambient air. Although the annual system performance of the R410A ASHP system is better than that of the CO2 hybrid GCHP system, the cooling performance of the R410A system seriously decreases when the ambient temperature is higher than 30 ºC. The research work also investigates the CO2 two-phase ejector transcritical system with quasi-steady state simulation, based on the gas cooler outlet split (GOS) ejector refrigeration cycle. This CO2 GOS ejector refrigeration cycle can generate two levels of evaporating temperature for the indoor air temperature and humidity independent control. The theoretical steady state analysis shows that the COP of the CO2 ejector refrigeration cycle is 12~60% better than that of the standard isenthalpic expansion cycle when the humidity load ratio covers from 10~50% under the same boundary conditions. Meanwhile, the quasi-steady state investigation explains the operating characteristics with the different control strategies, and the low evaporating temperature control strategy is much more challenging than the high evaporating temperature one. The quasi-steady state performance of the CO2 GOS ejector system under different load conditions shows that the average energy performance is 30% better than that of the R410A system under hottest summer weather conditions. Finally, compared to the traditional standard ejector refrigeration cycle, with the GOS cycle, it is much easier to control the system performance and easy to switch the operating mode for the different ejector refrigeration cycles. At last, the research work involves the smart utilization of the CO2 heat pumping system for service hot water heating by the means of heat recovery. The developed system is able to operate under space cooling or heating mode, while simultaneously servicing hot water heating. The energy performance of the heat pumping system is investigated under both full and partial capacity, and the quasi-steady state simulation result shows that the combined COP for indoor air conditioning and service hot water varies from 3.0 to 5.5 with 65 ºC of service hot water supply. Moreover, the annual ground thermal imbalance for the CO2 hybrid GCHP system is studied for three cities in a warm climate. It is proved that the proposed CO2 system is suitable for reference buildings under two climate conditions without the problem of ground thermal energy accumulation due to overheating of the underground borehole. To summarize, the hybrid utilization of the heat sinks based on the CO2 transcritical refrigeration cycle improves the heat pumping system’s energy efficiency under warm climate conditions. In addition, some smart technologies, like work recovery from an expansion process by an ejector and heat recovery from a gas cooling process for servicing hot water heating, further improves the energy efficiency of the CO2 heat pumping system. In particular, the CO2 hybrid GOS ejector GCHP system offers the future air conditioning sector many possibilities for using the environmentally friendly CO2 refrigerant.