Ground/water heat pump (geothermal probes)

A ground-source heat pump (geothermal system with probes) extracts heat from the ground via a field of closed vertical probes (U-shaped tubes extending to a depth of approximately 80–200 m, filled with a water-glycol heat transfer fluid) and transfers it to a hydraulic network for heating (and possibly domestic hot water). In summer, the system can provide passive cooling (geocooling) simply by circulating water through the probes, with very low electricity consumption. Compared to the outside air, the ground offers a stable temperature year-round, resulting in high COPs and consistent comfort… provided the probes are correctly sized and the system is properly managed.

Ground/water heat pump: definition and operation

A typical system combines: vertical geothermal probes (boreholes, casings, wellheads, collectors), primary circuit (water-glycol, pumps, expansion vessels, antifreeze protection), water/water heat pump in technical room (plate heat exchanger), and on the building side, buffer tank, water law, 2/3 way valves and low temperature emitters (underfloor heating, fan coil units, well-sized radiators).

The system design is based on the building's annual load, geology (thermal conductivity), probe spacing (to avoid interaction), and ideally, a thermal response test (TRT) to calibrate the available power. The field is regenerated in summer (passive or active cooling) to maintain ground temperature and long-term performance. During operation, the building management system (BMS) controls flow rates/setpoints, temperature difference (ΔT), and schedules; SCOP (heating) and EER/SEER (cooling) provide information on seasonal efficiency. Antifreeze valves, proper water quality, and precise loop balancing ensure reliability and longevity.

Advantages, limitations and points to consider regarding ground/water heat pumps

Interests

• High yields and performance stability (soil at almost constant temperature).
• Passive cooling possible (very low consumption).
• Quiet comfort, without outdoor units on the facade/roof.
• Significant decarbonization (especially with decarbonized electricity).

Boundaries

• Significant CAPEX for drilling + technical room.
• Longer project time (studies, permits, drilling coordination).
• Extent for the drilling field and site constraints (drilling access).
• Performance is dependent on geology and dimensioning.

Points to consider

• Upstream studies: TRT, conductivity, hydrogeology, probe spacing.
• Hydraulics: minimum heat pump flow rate, loop balancing, target ΔT, antifreeze.
• Summer regeneration: plan for geo-cooling for the sustainability of the field.
• Emitter compatibility: aim for low temperature on the heating side.
• Building management system monitoring: actual SCOP/SEER, alarms, trends, water law optimization.
• Administrative framework: drilling declaration/authorization, prevention plan, spoil management.

Anecdote — “Under the cobblestones of Lyon, coolness”

In Lyon, an office building lacked sufficient rooftop space for a conventional cooling system. The building owner opted for 18 ground-source heat pumps, each 120 meters long, in the courtyard, coupled with a water-to-water heat pump and a chilled floor. In summer, the geothermal cooling system covers most comfort needs without the need for a chiller unit; in winter, the heat pump maintains a high seasonal COP thanks to the stable ground temperature. An unexpected bonus: a facade without outdoor units, resulting in a simpler permit and more peaceful neighbors. The moral of the story: sometimes, the best system is underground.