Thermal performance of medium-depth borehole heat exchanger-coupled direct-expansion photovoltaic/thermal (PV/T) heat pump domestic hot water system

To address the instability and insufficient thermal output associated with single renewable energy heating systems, this study proposes a novel medium-depth borehole heat exchanger (MDBHE) coupled with a direct-expansion photovoltaic/thermal (DX-PV/T) heat pump system for domestic hot water supply. The proposed system comprises a 2000 m deep coaxial tube MDBHE, a PV/T evaporator, a compressor, a condenser, and an expansion valve, and operates in two modes: standalone DX-PV/T heat pump mode and hybrid heating mode. A thermal model of the MDBHE was established based on the segmented finite line source method and quasi-steady-state heat transfer theory, while a simulation model of the DX-PV/T heat pump system was developed using energy conservation equations. Both sub-models were validated against experimental data from the literature: the MDBHE model demonstrated good agreement with published results, and the DX-PV/T heat pump system model yielded an average coefficient of performance (COP) deviation of 5.3% and a maximum deviation of 9.6%. Parametric analyses were subsequently conducted to evaluate the thermal performance of each system component. The key findings are as follows: ① For the MDBHE, as the circulating water flow rate increased from 16 m³/h to 48 m³/h, the maximum heat extraction capacity reached 226.2 kW at a COP of 16.1, while pump power consumption simultaneously rose to 14.1 kW. When the inlet water temperature exceeded 34.5 ℃, reverse heat transfer occurred and heat extraction capability was lost. ② For the DX-PV/T heat pump system, as solar radiation intensity (SRI) increased from 200 W/m² to 1 000 W/m², condensing power and electricity generation increased by 59.7% and 344.3%, respectively, with a peak COP of 5.7; however, overall system efficiency declined from 89.5% to 54.5%. ③ Under typical operating conditions with a PV/T collector area of 2 000 m² and an SRI of 600 W/m², the hybrid system achieved an outlet water temperature of 50.9 ℃, a solar energy contribution of 63%, and a total heating capacity of 966.2 kW — approximately five times that of a standalone MDBHE system — with a system COP of 5.8, substantially outperforming conventional PV/T ground-source heat pump systems (COP ≈ 3) and air-source heat pumps (COP ≈ 3.5). When the SRI exceeded 600 W/m², the electricity generated by the PV/T modules was sufficient to meet the system’s own energy consumption, achieving near-zero grid dependency. These results demonstrate that the proposed hybrid system offers significant advantages in energy efficiency, operational stability, and environmental compatibility, providing a viable solution for domestic hot water supply in medium-scale residential and small commercial buildings.