Numerical study of geothermal district heating from a ground heat exchanger coupled with a heat pump system

Abstract

Two important problems are often encountered in the application of middle-deep geothermal heat pump (MD-GHP) systems: (1) large deviations between the operating and design conditions and (2) the lack of a theoretical basis for parameter control during actual operations. Accurate heat transfer and energy conversion models of MD-GHP must be established to solve these problems.The novelty of this work lies in its establishment of a complete unsteady heat transfer model of a ground heat exchanger (GHE) and the coupling of this model with the energy conversion model of a heat pump. The influences of key design and control parameters on the coefficient of performance (COP) of MD-GHP are then studied. The use of vacuum insulated tubes as central pipes can increase the COP of the system by approximately 10%. When the well depth increases from 2000 m to 3000 m, the maximum heating load increases nearly linearly from 500 kW to 960 kW. At the bottom 20% of the well, insulation of the central pipe does not obviously increase the COP of the system. An optimal circulation rate occurs in MD-GHP, and this rate increases with the well depth. When the well depth increases from 2000 m to 3000 m, the optimal circulation rate increases from 6.7 kg/s to 9.9 kg/s. When the average heating load is kept constant, the intermittent heating mode slightly affects the average COP of the system. The thermal conductivity of the cement sheath exerts minimal effects on the COP of the system.The overall merit of this work is that a mathematical relationship between the heating load and formation temperature distribution is established to predict the energy efficiency of the whole system. This study provides important theoretical guidance for the design and control of MD-GHP.