Field tests on thermal response characteristics of micro-steel-pipe pile under multiple temperature cycles

Abstract

Energy piles have dual function of supporting a superstructure and acting as a shallow geothermal heat exchanger. Field tests on the thermal response characteristics of micro-steel-pipe piles under five operating temperature conditions (summer intermittent: 12 h for heating and 12 h for stopping as a cycle, winter intermittent: 12 h for cooling and 12 h for stopping as a cycle, summer continuous, recovery and winter continuous) were conducted. The heat transfer efficiency, the coefficient of performance (COP), the temperature and thermal induced stress of micro-steel-pipe piles were measured and analyzed. The results indicated that values of the heat transfer efficiency and COP were decreased with the process of cycles. The average COP and the average heat exchange rate per unit length under summer intermittent condition were relative higher than those in the corresponding period under summer continuous condition (the same trend in winter). The heat transfer efficiency in winter conditions was relative lower than that in summer conditions. In this field test condition, the additional compressive stress in micro-steel-pipe piles was increased by 200 kPa/°C, and the average heat exchange rate per metre was −49 W/m under Condition 5, 95 W/m under Condition 3.