Long-term energy performance of thermal caisson geothermal systems

In Canada, buildings account for a considerable portion of energy use and contribute heavily to Canada's greenhouse gas (GHG) emissions. As such, a transition towards energy-efficient heating and cooling systems is important. Sustainable ground-source heat pumps (GSHPs) are one type of energy-efficient heating and cooling systems. But, they have high capital costs and often decline in efficiency over time. In this study, we further develop a phase change material (PCM)-based thermal caisson (TC) system, and extend a numerical algorithm previously developed in the same research group to assess the long-term energy performance of these systems. Dual-purpose and low-cost TCs not only provide structural support to a building, but also provide high efficiency geothermal heating and cooling, through ground heat exchanger U-tubes and embedded PCMs. Linear regression analyses are also performed to formulate how values of the coefficient of performance (COP) of TC systems change over time based on results from the literature and the current model. The results show that the long-term energy performance of TCs is significantly influenced by the use of a PCM, which can improve the long-term COP of GSHP systems, by 22% on average. For the small residential building investigated in this study, the capital cost and the net present value (NPV) of a TC system can be up to 50% and 32% lower, respectively, than the cor-responding values for a conventional GSHP system. In addition, the TC system has a return on investment (ROI) of 65% which is significantly higher compared to the conventional GSHP system. Furthermore, over the heat pump system lifetime (assumed 20 years in this work), the CO2 emissions of the TC-based system are approxi-mately 19% lower than conventional GSHP systems. Overall, the techniques used in this study can be extended and applied to larger buildings and potentially lead to greater energy savings for these buildings, if TCs are widely adopted in the building sector