Analysis Of Soil Heat Balance Under Long-term Operation Of Ground-Source Heat Pump In Hot Summer And Cold Winter Region

As the energy situation becomes increasingly serious, more countries are paying attention to the development and utilization of shallow geothermal energy. Ground-source heat pump (GSHP) is favored as an important way to use shallow geothermal energy and widely applied to building heating, air-conditioning, domestic hot water and so on. However, the heating period in winter is shorter than the cooling period in summer in hot summer and cold winter region and the heat ejected to soil is more than the heat absorbed from soil, which results in that soil temperature increases year after year. So far, it has become one of the main bottlenecks in development and application of GSHP in hot summer and cold winter region. Meanwhile, the research on heat balance and feasibility of GSHP under long-term operation in hot summer and cold winter has not been effectively carried out.In this paper, the soil temperature field and heat balance under long-term operation of GSHP in hot summer and cold winter region is studied by numerical method. Firstly, a building load model specific to hot summer and cold winter region, a GSHP unit model and a three-dimensional mathematical model of coupled heat transfer between U-tube ground heat exchanger and its surrounding soil are established. Based on the three models an analytical model of GSHP system for hot summer and cold winter region is established. Secondly, the operation characteristics under long-term operation of GSHP system are studied by the analytical model, including soil temperature field, reverse heat transfer length, heat transfer quantity per U-tube, heat pump unit performance and energy consumption. Then the effect of different operation modes on soil temperature, heat balance and GSHP system is discussed by comparing different transition seasons and different ratios of cooling load born by GSHP sysytem, respectively. At last, soil temperature field and heat balance under different arrangements styles are analyzed by discussing different tube spans and covering reverse transfer sections with thermal insulation material at different depths.The results show that the deteriorating situation of soil temperature field and heat balance can be improved to some extent by adjusting the time division of transition season and working season and the ratio of cooling load born by GSHP system; when GSHP system bears30%-35%cooling load, the soil is considered to reach enengy balance; the effect of different tube spans on soil temperature is little, but larger tube span is preferred to improve long term performance of heat pump unit; reverse transfer can be improved by covering thermal insulation material, but further study is needed.The analytical model in this paper can be easily applied to practical projects and be of some significance in forecasting the soil temperature field and heat balance and designing the U-tube heat exchanger.