Design, simulation and control of air- and ground-source variable refrigerant flow heat pump systems

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Variable refrigerant flow (VRF) systems are heat pump systems where a single outdoor unit (i.e. the evaporator in heating mode) is connected to multiple indoor units (i.e. the condensers in heating mode) on a single refrigerant network. The indoor units are distributed across the multiple zones of a building and provide the necessary heating or cooling to their zone. VRF systems have been shown to reduce energy consumption for space cooling when compared to variable air volume systems, but they are rarely considered for heating dominated applications in cold climates. In the context of the Paris Agreement, VRF systems offer a great potential for the reduction of Canada's greenhouse gases emissions. The research program addresses the design, modelling and control of both air-source and ground-source VRF heat pump systems. A simulation model will be developed and validated against measurements in laboratory experiments with controlled environmental conditions (air temperature and humidity). This will allow for the development of new control methods to optimize the operation of VRF systems based on knowledge of the system configuration (e.g. size, number and position of indoor units). The simulation of VRF systems coupled to geothermal heat exchangers necessitates the consideration of short-term effects that arise due to changes of the operating conditions of the system (e.g. fluid flow rate) and long-term effects due in part by thermal interference between individual geothermal heat exchangers. Improvements in simulation techniques of geothermal heat exchangers will allow for the evaluation of the energy performance of ground-source heat pump systems and the potential energetic, environmental and economical benefits of using these systems. In the context of the Montreal Protocol and the Kigali Amendment, alternative refrigerants are needed to replace the current generation of refrigerants, including hydrofluorocarbons (HFCs). With the high refrigerant charge of VRF systems, the use of carbon dioxide (CO2) as the refrigerant could significantly reduce the life-cycle greenhouse gases emissions, especially from leakage and decommissioning. Building upon the simulation tools developed in the research program, a prototype transcritical CO2 VRF system will be designed, built and tested. This project will contribute to the advancement of the next generation of heat pump systems using low-impact refrigerants.