A thermo-hydraulically efficient hybrid solar-geothermal heating solution for sustainable space heating in cold regions

This work introduces a hybrid system combining a coaxial borehole-type geothermal heat exchanger (G-HEX) with a rock bed solar-air heater (RBS-AIR), demonstrating the feasibility and enhanced thermo-hydraulic performance of coupling sensible heat storage with geothermal preheating. The study is conducted using average monthly winter climate data from November to March for El Paso, USA, to simulate realistic cold-season performance. A transient numerical model is developed to analyze system performance under varying conditions, complemented by a closed-form steady-state solution for estimating the system’s annual average behavior. Parametric analysis reveals that increasing air mass flow rate boosts net power output by 36.62% in the standalone RBS-AIR and by 45.46% in the hybrid system. The associated temperature penalty is 7.70% without G-HEX and only 1.96% with it, indicating improved thermal stability with integration. The system also benefits from a larger inner and smaller outer radius of the G-HEX, which favorably influence heat transfer. The results show only weak sensitivity to solver parameter variations in the transient model and to soil type in both transient and steady-state simulations. The cost of net power output is calculated as $8.97 per kWh. Overall, the hybrid system offers a reliable and sustainable heating solution by efficiently combining solar and geothermal energy sources for cold climate applications. Future work could explore advanced dynamic control strategies, latent heat storage integration, and multi-source renewable hybridization to further enhance system efficiency and flexibility.