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A simplified heat transfer method to analyze the thermal performance of large-scale boreholes ground heat exchangers (GHE) has been proposed considering the geometric symmetry of borehole layout. Taking the ground heat exchanger with matrix layout of vertical boreholes as the example, the ground temperature distribution is analyzed under the conditions of no groundwater advection by the proposed simplified method. Based on the simulation results, a representative boreholes matrix can be proposed to replace the original large-scale ground heat exchanger for heat transfer analysis. Meanwhile, the results show that the representative boreholes matrix can be determined by the thermal influence radius of a single borehole by assuming all the boreholes have the same thermal loads and also in the same geological conditions, geometry parameters and operating conditions. It is indicated that the thermal influence radius of a single borehole can be obtained according to the allowed error range of the ground temperature responses, which can correspondingly determine the number of the representative boreholes matrix.

Although ground-coupled heat pump (GCHP) systems are becoming attractive air-conditioning systems in some regions, the significant drawback for their wider application is the high initial cost. Besides, more energy is rejected into ground by the GCHP system installed in cooling-dominated buildings than the energy extracted from ground on an annual basis and this imbalance can result in the degradation of system performance. One of the available options that can resolve these problems is to apply the hybrid ground-coupled heat pump (HGCHP) systems, with supplemental heat rejecters for rejecting extra thermal energy when they are installed in cooling-dominated buildings. This paper presents a practical hourly simulation model of the HGCHP system by modeling the heat transfer of its main components. The computer program developed on this hourly simulation model can be used to calculate the operating data of the HGCHP system according to the building load. The design methods and running control strategies of the HGCHP system for a sample building are investigated. The simulation results show that proper HGCHP system can effectively reduce both the initial cost and the operating cost of an air-conditioning system compared with the traditional GCHP system used in cooling-dominated buildings.