In-Situ Experiment of Heat Extraction of An Open-Loop Deep Geothermal Single Well Coupled With Heat Pump System

Geothermal resources are abundant in medium-low temperature in China, the way of hot water extraction and reinjection after heat exchange has been the main direct use mode for many years. However, due to the technical bottleneck of reinjection into sandstone and mining rights, shallow and deep close-loop borehole heat exchangers (BHEs) have received more and more attention, and there are also many practical projects. Since the heat transfer between the formation and both the shallow and deep BHEs are heat conduction, the heat extraction rate is lower, and heat/cold accumulation will occur in areas with unbalanced cooling and heating loads, affecting the long-term operation of the system, especially for deep BHEs, the cold accumulation is more obvious. In contrast, the open-loop deep geothermal single well system (DGSW) has convective heat transfer and mass transfer with the formation, and the heat extraction rate is much higher than that of the closed-loop system with the same formation conditions. The idea of Deep Geothermal Single Well (DGSW) heat production has existed for many years, but there is no consensus on its applicability. So far, only the Geothermal Engineering Ltd (GEL) in UK has conducted a field trial of DGSW, and the heat output of a single well with a depth of 2000 m is 363 kW. In this paper, an in-situ test of an open-loop DGSW coupled with a heat pump system was carried out in Hebei Province, China, the total heat energy output from the well was dependent on the total flow rate, the delivery temperature and the return temperature. With the current configuration, it was shown that the well with a depth of 1398 m and bottom temperature of 67.2 oC would deliver a stable heat output of 450 kW. The heat output per unit length of this open-loop DGSW system is more than 3 times that of a close-loop system with similar formation conditions.