Investigation on CO2 Ground-coupled Heat Pumping System with Ejector
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After the proposal of CO2 transcritical cycle in 1980s by Professor Gustav Lorentz, researchers have looked into theoretical and experimental research, as well as commercial system development to improve transcritical system performance to a level similar to that of conventional heat pump systems. Over the years researchers are investigating for newer system concepts with transcritical CO2 cycle that can be implemented across the globe for different climate conditions. One of the major challenges of CO2 transcritical cycle is that the system COP is greatly dependent on the gas cooler outlet condition. Thus, application of such systems in wormer climate may result in poor system performance where ambient temperature is relatively high and unstable. However, the temperature of the ground remains comparatively stable and can be utilized as a heat sink to bring down the gas cooler outlet temperature to avoid low cooling performance and large expansion losses. These observations lead the concept of a hybrid system where part of the gas cooler heat is rejected to ambient air and rest to the ground using a ground-coupled heat exchange. Furthermore, incorporating an ejector instead of conventional expansion valve may increase the system performance. It is necessary to evaluate these system alternatives and figure out the maximum borehole length required for such systems to be functional and economically viable.This thesis investigates the performance of CO2 ground-coupled ejector cycle to conventional CO2 transcritical cycle with expansion valve when ambient air temperature constraints the cooling of supercritical CO2. After a theoretical analysis, the system configurations were implemented in Modelica for further simulation.