An overview of the problems and solutions of soil thermal imbalance of ground-coupled heat pumps in cold regions

Ground-coupled heat pumps are widely applied in cold regions, but soil thermal imbalances in some projects greatly deteriorate the systems' practical performances and heavily inhibit the systems' reasonable applications. This paper presents a comprehensive overview of the main problems caused by soil thermal imbalance and the existing solutions targeting on them. The caused problems mainly present in the following aspects: soil temperature decrease, heating performance deterioration, heating reliability decline, and even system failure. Three kinds of existing solutions can be classified. Ground heat exchanger modified solutions including increasing borehole space, increasing borehole length, modifying borehole layout, and improving thermal properties, can retard the thermal imbalance to some extent, suitable to projects with slight imbalance. System-modified solutions integrating auxiliary energy sources like fossil fuel, solar energy, ambient air and waste heat with ground-coupled heat pumps or using ground-coupled absorption heat pumps, can eliminate serious thermal imbalance essentially by increasing soil thermal injection and decreasing the extraction. Thus, they are dominant in the applications and attract more attention along with the utilization of renewable energies. Apart from the modifications of system design, operation-modified solutions, like seasonal and intermittent operations, are also helpful to maintain better soil thermal balance. Based on the overview, the current application situations and economic performance of different solutions are discussed. Finally, some suggestions for the applications of ground source heat pumps in cold regions are proposed to make the overview helpful for the system research and applications. (C) 2016 Elsevier Ltd. All rights reserved.