System monitoring and evaluation and thermo-mechanical characteristics of energy piles during winter operation

To address the winter heating demand of a 25 m²-area building room in Yichang, Hubei Province, a ground source heat pump system coupled with energy piles (referred to as "energy pile system") is constructed, which includeds energy piles, a water collector and distributor, heat pump units and a circulating water pump. Under the actual continuous or intermittent operation of the energy pile system, the thermo-mechanical characteristics of energy piles and system performance are studied. The tests on the conventional borehole buried pipe-based ground source heat pump system and the air source heat pump system are conducted for comparison and analysis as well. The study focuses on the similarities and differences of thermo-mechanical characteristics of energy piles under actual operation and the traditional TRT or TPT test conditions, and the heating performances and COP of the energy pile system are monitored. Under the test conditions in this study, corresponding to the continuous tests, the maximum temperature reduction value of 3.78℃ and the maximum thermally induced stress of 0.70 MPa (about 57.5% of the upper limit of the fully confining stress) occur at the pile top. The maximum observed strain differences between the operating piles and the non-operating piles corresponding to the cap structures at the upper and lower layer are 19.98με and 17.78με, respectively. The measured COP of the energy pile system is about 3.03, which is about 12.2%~21.2% higher than that of the related air source heat pump system (the normative reference COP is 2.50~2.70). The energy pile system has a faster start-up speed than the conventional air source heat pump, which reaches the preset temperature about 2.5 h earlier.