Optimal GSHP–ASHP hybridisation for large-scale tertiary buildings with imbalanced loads

Peer-reviewed paper 8-017-26

Borehole heat exchangers for ground-source heat pumps (GSHPs) can be installed almost anywhere, theoretically enabling widespread deployment of this efficient energy source. However, development remains limited, primarily due to drilling costs. The specific cost of geothermal energy—dominated by drilling—depends heavily on system design: borehole length is driven not only by capacity but by total extracted energy, borehole spacing, and ground temperature depletion under imbalanced loads. This causes diminishing economic returns, leading to an optimal geothermal coverage ratio for a given field size, beyond which hybridisation with an air-source heat pump (ASHP) becomes more efficient.

This paper presents a parametric study of hybrid GSHP–ASHP systems using a TRNSYS model interfaced with a custom Python framework. Approximately 1,000 simulations over 25-year horizons are performed for office buildings of 500 to 2,000 kW under two French climates with varying cooling-to-heating ratios.

Two hybridisation strategies are compared. In an alternating monovalent model where the GSHP covers peak load, borefield surface can be reduced by 30–50% with reductions of 5–17% in the levelised cost of energy (LCOE). A base-load model is then explored to further improve the economics by raising the energy extracted per metre of borehole: with balanced loads, the borehole energy yield reaches up to 289 kWh/(m · yr)—roughly three times the reference yield for the same building—substantially improving the cost-competitiveness of geothermal, but is limited by ground cooling in heating-only climates.

Results show that the optimal hybridisation strategy depends on thermal load balance, climate, and fluid temperature constraints.