Ground source thermal loop with two-phase carbon dioxide (CO2) as heat carrier fluid to manage thermal energy in buildings

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Under the Pan-Canadian Framework on Clean Growth and Climate Change, Canada has committed to a low-carbon future through reduced use of fossil fuels and increased adoption of renewable energy. Buildings can have the greatest impact among all other sectors towards reducing Canadian greenhouse gas (GHG) emissions and energy use. Developing high performance buildings by integrating efficient end-use technologies and renewable energy can contribute to significant reductions of energy consumption and GHG emissions in the building sector. This research program proposes a new concept to manage thermal energy in buildings. It facilitates the integration of heat pumps and renewable energy sources such as geothermal. It consists of a thermal loop with two-phase CO2 as the heat carrier fluid through which all heating and cooling components (e.g. heat pump units, thermal storage systems, and air ventilation systems) will exchange heat, regardless of their physical distance within the building. The constant temperature two-phase CO2 loop ensures efficient building energy performance by exploiting heat recovery and improving the Coefficient of Performance (COP) of local heating and cooling units. Due to superior thermophysical properties of CO2 compared to other alternatives such as water in water loop system, the loop potentially offers additional advantages such as reduced circulation energy consumption, increased thermal energy transfer intensity, and reduced pipe sizes of the loop. The research program addresses the modeling and performance evaluation of a ground source two-phase CO2 thermal loop. Simulation models will be developed and validated against measurements in laboratory. Modeling of the ground source CO2 loop requires the development of coupled ground thermal response factors and two-phase CO2 fluid flow and heat transfer in both the loop and the ground heat exchanger. Improvements in simulation techniques will allow for development of a less computationally intensive direct expansion ground heat exchanger (DX-GHE) model and accurate evaluation of the potential energetic, environmental and economical benefits of the concept. A tool will also be developed and tested to measure thermal energy use of each local heat pump unit connected to the loop for evaluating cooling/heating shares of building zones, offices or apartments. Development of this new concept along with the thermal energy meter will also facilitate the building real-time intelligent monitoring, diagnostics, and troubleshooting of building HVAC systems. The proposed research will also contribute significantly towards the decarbonisation of the Canadian building sector by increasing building energy performance.