An experimental investigation of the influence of intermittent and continuous operating modes on the thermal behaviour of a full scale geothermal energy pile

Ground sourced heat pumps coupled with geothermal energy piles operate either continuously or intermittently depending on the heating or cooling needs of a built structure, giving rise potentially to variable thermal loads in the piles. This paper presents an experimental investigation on the influence of intermittent and continuous operations on energy extracted, ground and pile temperatures, and axial thermal strains and stresses of a full scale geothermal energy pile for daily cooling operation. Experiments were conducted for a 24 hours continuous operation (24 h mode), 16 hours operation with 8 hours rest (16 h mode), and 8 hours operation with 16 hours rest (8 h mode). It is generally found that the lower operating hours lead to higher energy extracted from the ground with lower thermal loads on the pile and the ground. The energy extracted was 40.9% and 14.8% higher in the 8 h and 16 h modes, respectively, compared to the 24 h mode. The ground temperatures for the 8 h and 16 h modes were 14.5% and 5.9% higher than the 24 h mode, respectively. The average thermal stresses in the pile for the 8h and 16 h modes were 42.3% and 12.2% lower than the 24 h mode, respectively. The results of the present study also showed that the soil did not affect the elasticity of the axial thermal strains between end of cooling and recovery of the intermittent modes for daily thermal cycles.