Open-Loop Ground Source Heat Pump Systems

Open-loop ground source heat pump systems extract groundwater directly from aquifers, pass it through a heat exchanger, and discharge it back to the ground. These systems leverage the stable thermal properties of groundwater, which maintains temperatures between 45°F and 75°F (7°C to 24°C) depending on geographic location and depth.

Fundamental Operating Principles

Open-loop systems operate by extracting thermal energy from flowing groundwater rather than conducting heat through soil. The heat transfer rate depends on water flow rate and temperature differential:

$$Q = \dot{m} \cdot c_p \cdot \Delta T = \rho \cdot \dot{V} \cdot c_p \cdot (T_{in} - T_{out})$$

Where:

• $Q$ = heat transfer rate (Btu/hr or kW)
• $\dot{m}$ = mass flow rate (lbm/hr or kg/s)
• $c_p$ = specific heat of water = 1.0 Btu/(lbm·°F) or 4.18 kJ/(kg·K)
• $\dot{V}$ = volumetric flow rate (gpm or L/s)
• $\rho$ = water density ≈ 62.4 lbm/ft³ or 1000 kg/m³
• $\Delta T$ = temperature change across heat exchanger

The required water flow rate for a given capacity:

$$\dot{V}{gpm} = \frac{Q{Btu/hr}}{500 \cdot \Delta T_{°F}}$$

Typical design values use $\Delta T$ = 10-15°F (5.5-8.3°C) for heating mode and 10-12°F (5.5-6.7°C) for cooling mode.

System Configurations

Supply and Discharge Well Systems

The most common configuration uses two separate wells: a supply well extracts groundwater while a discharge well returns it to the aquifer.

graph LR
    A[Supply Well] -->|Groundwater 50°F| B[Pump]
    B --> C[Screen Filter]
    C --> D[Plate Heat Exchanger]
    D -->|Isolated Loop| E[Heat Pump]
    D -->|Discharge Water 40°F| F[Discharge Well]
    F --> G[Aquifer Reinjection]

    style A fill:#e1f5ff
    style E fill:#ffe1e1
    style G fill:#e1f5ff

Design Criteria:

• Minimum well separation: 50-100 ft (15-30 m) to prevent thermal short-circuiting
• Supply well depth: determined by aquifer depth and static water level
• Discharge well location: downgradient from supply well when possible
• Well spacing increases with higher system capacity and continuous operation

Standing Column Well (SCW)

Standing column wells use a single deep well (typically 200-1500 ft / 60-450 m) where water circulates in a column. A controlled bleed maintains thermal recovery.

Operating Characteristics:

The standing column operates under three conditions:

1. No-bleed operation: Water recirculates entirely within the well column
2. Partial bleed: Small percentage (1-5%) discharged to maintain efficiency
3. Full bleed: Maximum discharge during peak loads

Bleed rate calculation:

$$\text{Bleed %} = \frac{\dot{V}{bleed}}{\dot{V}{total}} \times 100$$

$$\dot{V}{bleed,gpm} = \frac{Q{deficit,Btu/hr}}{500 \cdot \Delta T_{groundwater,°F}}$$

Where $Q_{deficit}$ represents the thermal load exceeding the well’s natural recharge capacity.

graph TD
    A[Standing Column Well] -->|Water Drawn| B[Submersible Pump]
    B --> C[Heat Exchanger]
    C -->|Primary Return| D[Return to Well Top]
    C -->|Controlled Bleed| E[Discharge]
    D --> F[Thermal Stratification]
    F --> A

    G[Aquifer Recharge] -.->|Natural Flow| A

    style A fill:#e1f5ff
    style C fill:#ffe1e1
    style E fill:#fff4e1

Advantages:

• Single well reduces drilling costs
• No well separation requirements
• Suitable for sites with limited land area
• Effective in fractured bedrock formations

Limitations:

• Requires adequate aquifer recharge
• May need bleed during sustained peak loads
• Thermal capacity decreases over extended operation
• Bleed discharge requires approved disposal method

Water Quality Requirements

Water quality directly affects heat exchanger performance and system longevity. Critical parameters must meet IGSHPA standards.

Chemical Analysis Requirements

Fouling and Scaling Indices

Langelier Saturation Index (LSI):

$$LSI = pH - pH_s$$

Where $pH_s$ is the saturation pH calculated from water chemistry.

• LSI > 0: Water is supersaturated, scaling likely
• LSI = 0: Water is balanced
• LSI < 0: Water is undersaturated, corrosive

Ryznar Stability Index (RSI):

$$RSI = 2 \cdot pH_s - pH$$

• RSI < 6.0: Severe scaling
• RSI 6.0-7.0: Moderate scaling
• RSI 7.0-7.5: Balanced
• RSI > 7.5: Corrosive

Mitigation Strategies

For marginal water quality, implement isolation strategies:

Plate and Frame Heat Exchanger Isolation:

The groundwater passes through one side of a plate heat exchanger while a closed glycol or water loop connects to the heat pump. This protects the heat pump from direct exposure to poor-quality water.

$$UA_{total} = \frac{1}{\frac{1}{h_1 \cdot A_1} + \frac{t_{plate}}{k_{plate} \cdot A} + \frac{1}{h_2 \cdot A_2}}$$

The additional thermal resistance reduces overall system efficiency by 5-15% but eliminates heat pump fouling risk.

Discharge and Disposal Methods

Reinjection Wells

Return water to the same aquifer through a properly constructed discharge well. IGSHPA requires:

• Minimum 50 ft (15 m) separation from supply well
• Well screen in same aquifer zone
• Proper well development to maintain injection capacity
• Quarterly monitoring of injection pressure and flow rate

Injection capacity testing:

$$K = \frac{Q}{2\pi \cdot H \cdot s}$$

Where:

• $K$ = hydraulic conductivity (ft/day)
• $Q$ = injection flow rate (ft³/day)
• $H$ = saturated thickness (ft)
• $s$ = drawdown/buildup (ft)

Surface Discharge

Where permitted by local regulations, surface discharge to:

• Storm sewers (thermal limits typically 68-86°F / 20-30°C)
• Surface water bodies (requires NPDES permit in USA)
• Drainage ditches or retention ponds

Temperature limits prevent thermal pollution:

$$T_{discharge} = T_{supply} \pm \Delta T_{system}$$

Most regulations limit $T_{discharge}$ to ambient water temperature ±5°F (2.8°C).

Regulatory Considerations

United States:

• Underground Injection Control (UIC) regulations (EPA)
• State well construction codes
• Local groundwater protection ordinances
• NPDES permits for surface discharge

Design Review Requirements:

• Hydrogeological assessment
• Well yield testing (minimum 8-hour pump test)
• Water quality analysis (complete mineral analysis)
• Thermal impact modeling for large systems
• Permit applications and approvals before construction

Well Yield and Sizing

Minimum well yield must exceed peak system flow rate with safety factor:

$$\dot{V}{well,required} = \dot{V}{system,peak} \times 1.25$$

For a 20-ton (70 kW) cooling system:

$$\dot{V}_{system} = \frac{20 \text{ tons} \times 12,000 \text{ Btu/hr/ton}}{500 \times 10°F} = 48 \text{ gpm}$$

$$\dot{V}_{well,required} = 48 \times 1.25 = 60 \text{ gpm minimum}$$

Pump sizing must overcome static lift, friction losses, and system pressure drop:

$$TDH = h_{static} + h_{friction} + h_{system} + h_{velocity}$$

Where total dynamic head (TDH) is measured in feet of water column.

Performance Considerations

Advantages of Open-Loop Systems:

• Higher efficiency than closed-loop (stable source temperature)
• Lower installation cost for adequate aquifers
• No ground loop size constraints
• Excellent performance in heating and cooling modes

Limitations:

• Requires suitable aquifer with adequate yield
• Water quality impacts heat exchanger maintenance
• Regulatory approval requirements
• Potential for well fouling over time
• Disposal method must be permitted and sustainable

Typical Performance:

• Heating COP: 3.5-5.0 (EER 12-17)
• Cooling EER: 15-25 (COP 4.4-7.3)
• Source water temperature stability: ±2°F (1°C) annually

Open-loop systems deliver exceptional efficiency when aquifer conditions and water quality support sustainable operation. Proper design requires thorough hydrogeological investigation, water quality testing, and compliance with applicable regulations.

References:

• IGSHPA Design and Installation Standards (2021)
• ASHRAE Handbook - HVAC Applications, Chapter 34
• EPA Underground Injection Control Regulations (40 CFR Parts 144-148)
• NGWA Guidelines for Geothermal Heat Pump Well Construction