Geotechnical heat transfer performance and system operation with differences in external heat exchange

Shallow geothermal energy is a clean and safe resource that has recently garnered significant attention because of how efficiently it can be utilized. However, its use faces challenges, such as the current poor understanding of thermophysical characteristics in geotechnics and difficulty coordinating the user and ground heat exchanger (GHE) sides. To clarify these issues, nine groups of thermal response tests (TRTs) were conducted using orthogonal test design to analyze the factors that influence geotechnical thermal conductivity (7s). A thermal conductivity instrument (TC3000E) was used to measure 7s under various lithological conditions, levels of moisture content, and test voltages. Furthermore, a small ground source heat pump system was built to adjust flow rate on the user side, and the operating characteristics of both the user and GHE sides were subsequently analyzed. The results indicated that in constant heat flow TRTs, the most influential factors in descending order are the heating power, test time, and fluid velocity. Specifically, the heating power exerted a significant impact. Higher levels of heat exchange led to higher values of 7s for different lithologies and moisture content. For instance, for a heat exchange of 97.4 kJ, 7s was 1.377 W/(m & sdot;K), whereas it increased to 1.960 W/(m & sdot;K) for a heat exchange of 346.2 kJ. Moreover, heat exchange on the user side changes synchronously with that on the GHE side, affecting the geotechnical thermal response characteristics. To maintain balance between the two, the heat pump unit must increase its power consumption, reducing the coefficient of performance (COP). For example, when the user-side flow rate decreased from 0.9 to 0.3 m3/h, the COP decreased significantly, from approximately 5 to 3. This study provides valuable insights for the development of shallow geothermal energy systems.