Simulation of coupling a solar thermal system with ground source heat pump for a single house in cold weather with TRNSYS

Ground source heat pump system extracts energy from the ground via a borehole heat exchanger. The borehole heat exchanger is created by drilling a very long borehole (can be 100-300 m long) with a diameter of roughly 6-8 inches. A tube (doubled over so that it forms a “U”) is placed into the ground and surrounded by a grout that provides good heat transfer between the tube and the adjacent soil. A heat transfer fluid (water or water/glycol mixture) flows through the tube line and exchanges heat with the ground. In the winter time, energy is extracted from the ground and used for household space heating. While in the summer (when the air conditioner is used), energy is extracted from a house and deposited in the ground. The challenge we face is that in heating-dominated climates, such as Canada, more energy is extracted from the ground in the winter than is replenished in the summer. The outcome of this is that the soil will cool down over time, resulting in lower values of the heat pump coefficient of performance, and the overall system performance will decline. We will investigate the idea for solving this problem in this study relying on solar domestic hot water systems, which use solar thermal collectors to heat water for domestic use. These systems are relatively simple: collectors, piping, pump, hot water tank, and controllers. The collector area is sized to provide high solar fractions in the summer; however, the solar fraction in the winter is low. In Toronto, annual solar fractions (defined as energy from solar thermal system/total energy needed by the load) are around 50-60%. This work aims to use the solar thermal collector to recharge the ground in the summer. This would allow for larger collector areas, and thus better performance in the winter, and the excess energy in the summertime would be deposited in the ground. The current study has been conducted for a single house, so talking about the solar thermal collector area and being large should be around 5 m2. This would help with balancing the seasonal heat transfer to the ground, which could alleviate the long-term drop in performance that can occur with ground source heat pumps. The study shows that coupling the solar thermal system with the ground source heat pump would meet our target. Using this system, the temperature of the ground, whose initial temperature is 14 °C, would increase by 24% (from 10.88 °C to 13.49 °C) for a borehole heat exchanger length of 150 m in Toronto, ON, Canada.