Geothermal & Heat Pump Wells

Why Geothermal?

The ground maintains a nearly constant temperature (10-16°C / 50-60°F) at depths below 3-6 m, regardless of surface weather. Ground-source heat pump (GSHP) systems exploit this thermal stability to provide heating, cooling, and hot water at 300-500% efficiency — for every 1 kW of electricity consumed, 3-5 kW of thermal energy is delivered.

System Types

Closed-Loop Systems

A continuous loop of pipe (typically HDPE) circulates a heat-transfer fluid (water or water-antifreeze mix) through the ground. No direct contact between the fluid and the aquifer.

• Vertical closed loop: U-bend pipes installed in boreholes 50-200 m deep, 3-6 m spacing — minimal land area required
• Horizontal closed loop: Pipes buried 1.5-2.5 m deep in trenches 200-600 m long — requires ample land
• Slinky loop: Coiled pipe in shorter trenches — higher heat exchange per meter of trench
• Pond/lake loop: Submerged coils in a water body — excellent heat transfer, lowest cost

Open-Loop Systems

Groundwater is pumped directly from an aquifer, passed through a heat exchanger, and returned to the aquifer via an injection well or surface discharge.

• Requires: Adequate aquifer yield (≥5 L/s), suitable water quality (low iron, no hardness scaling)
• Advantages: Higher efficiency than closed-loop, no antifreeze needed, simpler installation
• Challenges: Water quality treatment, discharge permits, aquifer sustainability
• Typical setup: Two wells — extraction well upgradient, injection well downgradient

Dual-Purpose Wells

Single well providing both domestic water supply and geothermal heating/cooling:

• Standing column well: Water circulates in a tall column — provides both heat exchange and water supply
• Thermal well: Heat exchanger co-located with water supply pump
• Energy pile: Heat exchange loops embedded in structural foundations (bridge piers, building piles)

Thermal Conductivity Testing

Before sizing a geothermal system, thermal response tests (TRT) measure ground thermal properties:

• Thermal conductivity (k): 1.5-6.0 W/m·K depending on rock/soil type
• Thermal resistivity: 1/k — important for backfill design
• Ground temperature profile: Measured during TRT — establishes baseline
• Test procedure: Circulate heated fluid at constant rate, measure temperature response over 24-72 hours

Drilling for Geothermal

Geothermal boreholes use similar equipment to water wells but with specific requirements:

• Borehole diameter: 100-150 mm (4-6") for residential, 150-200 mm for commercial
• Drilling method: Mud rotary or air rotary depending on formation
• Backfill (grout): Thermally enhanced grout (bentonite + sand) to maximize heat transfer
• Casing: Not required for closed-loop — U-bend pipes hang freely in borehole
• Depth: 50-200 m typical — determined by thermal load and TRT results

System Sizing

Geothermal system sizing depends on building thermal load, ground conditions, and climate:

• Peak heating load: Determines maximum borehole length needed
• Cooling load: May reduce borehole length if cooling-dominant (ground absorbs heat)
• Rule of thumb: 40-60 m of borehole per kW of heating load (vertical)
• Sizing software: GLHEPRO, EED,GroundLoop — use TRT data for accurate design

Environmental Considerations

• Groundwater protection: Closed-loop systems have zero aquifer impact
• Open-loop regulations: Most jurisdictions require permits for extraction and injection
• Thermal plume: Injection water temperature must typically be within 10°C of ambient
• Cross-contamination risk: Proper well construction and setbacks prevent water quality issues
• Seismicity: GSHP systems operate at pressures too low to induce seismicity