Water-to-water heat pump (geothermal energy using groundwater)

A water-to-water heat pump (groundwater geothermal) harnesses the stable temperature of an aquifer: groundwater is pumped from a production well, its heat (or cooling) is extracted via a heat exchanger, and then the water is reinjected into a reinjection well. On the building side, the heat pump provides heating, cooling (often very efficient), and sometimes domestic hot water. A major advantage is a virtually constant heat source year-round, guaranteeing high COPs and consistent power output, provided that hydrogeological and regulatory constraints are met.

Water-to-water heat pump: definition and operation

A typical installation includes: two boreholes (pumping / reinjection), a groundwater line (submersible pump, filters, flow/pressure/temp measurements), a plate heat exchanger that separates the groundwater from the building circuit (to limit scaling/fouling of the heat pump), then a water/water heat pump supplying a buffer tank, water law, 2/3-way valves and low-temperature emitters (floor heating, fan coil units, suitable radiators).

The system design is based on the aquifer's extraction capacity (permitted flow rate × allowable temperature difference), the water chemistry (iron, manganese, carbonates), the hydraulic distance between wells (to avoid short circuits), and the annual heat balance (to prevent the aquifer from being permanently cooled or heated). In summer, "free" cooling (free/geo-cooling) can be achieved by bypassing the heat pump: groundwater passes through the heat exchanger and cools the secondary network with very low energy consumption. Building management system (BMS) monitoring is recommended: SCOP/SEER, groundwater temperature difference, flow rates, filter clogging, and domestic hot water quality (anti-legionella treatment if local production).

Advantages, limitations and points to consider regarding water-to-water heat pumps

Interests

• High COP/SEER thanks to a stable source (not very sensitive to cold/hot waves).
• Passive cooling possible, very low energy consumption in summer.
• Silence and architectural integration (no visible external units).
• Significant decarbonization if low-carbon electricity is used.

Boundaries

• Authorizations and studies related to water law (declarations, environmental monitoring).
• Risks of clogging/scaling/biofouling (iron, Mn, carbonates).
• Requirement for reliable reinjection (permeability, load, thermal control).
• CAPEX and higher planning (drilling, analyses, pumping tests).

Points to consider

• Hydrogeology: pumping tests, well distance, controlled thermal plume.
• Treatments: filtration, flushing/blowing, anti-scale if needed, systematic intermediate heat exchanger.
• Hydraulics: minimum flow rate of the heat pump, balancing, target ΔT, anti-freeze protection on the secondary side.
• Emitters: aim for low temperatures to maximize efficiency.
• Supervision: energy/fluid metering, ΔP filter alarms, temperature trends.
• Operation: well maintenance plan (brushing, rehabilitation), water quality monitoring.

Anecdote — “The Rhine's water table cools the open-plan office”

In Strasbourg, an office building lacked rooftop space and wanted to avoid visible chillers. A groundwater solution was chosen: a pumping well in the rear courtyard, a reinjection well at the property line, an intermediate heat exchanger, and then a water-to-water heat pump. In summer, the free cooling from the groundwater is sufficient most of the time; the heat pump only kicks in during peak periods. The result: silence on the facade, a comfortably high seasonal COP, and satisfied neighbors… without a single outdoor unit. The moral of the story: when the site allows, the best system is underground.