Ground Loop Types for Heat Pumps

Ground Loop Types for Ground-Source Heat Pumps

Ground loop configuration represents the most critical design decision for ground-source heat pump (GSHP) systems, directly affecting installation cost, thermal performance, and long-term reliability. The ground heat exchanger serves as the thermal reservoir, rejecting heat during cooling and extracting heat during heating. Selection depends on site geology, available land area, soil thermal properties, and groundwater conditions.

Fundamental Heat Transfer Mechanisms

Heat transfer between the ground loop and surrounding earth occurs through three mechanisms operating simultaneously:

Conduction dominates in direct soil contact, governed by Fourier’s law:

$$q = -k_{\text{soil}} A \frac{dT}{dx}$$

where $q$ is heat transfer rate (W), $k_{\text{soil}}$ is soil thermal conductivity (W/m·K), $A$ is surface area (m²), and $dT/dx$ is temperature gradient (K/m).

Thermal diffusivity determines transient response:

$$\alpha = \frac{k_{\text{soil}}}{\rho c_p}$$

where $\alpha$ is thermal diffusivity (m²/s), $\rho$ is soil density (kg/m³), and $c_p$ is specific heat (J/kg·K). Higher diffusivity allows faster heat dissipation but reduces thermal storage capacity.

Groundwater movement enhances heat transfer through advection when present, effectively increasing apparent thermal conductivity by 20-50% in saturated zones with significant flow.

Closed-Loop Systems

Vertical Borehole Configuration

Vertical loops utilize boreholes drilled 100-600 ft (30-180 m) deep, typically 4-6 inches (100-150 mm) diameter. High-density polyethylene (HDPE) U-tubes are inserted and the annular space filled with thermally-enhanced grout.

Advantages:

• Minimal land area requirement (150-200 ft² per ton capacity)
• Access to stable deep-earth temperatures
• Higher thermal conductivity in consolidated formations
• No seasonal performance degradation
• Suitable for retrofit applications

Thermal Performance:

Heat transfer per borehole:

$$q_{\text{bore}} = \frac{T_{\text{ground}} - T_{\text{fluid}}}{R_{\text{bore}} + R_{\text{grout}} + R_{\text{pipe}}}$$

where resistances are calculated per IGSHPA methodology. Typical extraction rates: 50-80 W/m in heating, 60-90 W/m in cooling, depending on geology.

Required Bore Depth:

$$L_{\text{total}} = \frac{q_{\text{net}} R_{\text{ground}}}{T_{\text{ground}} - T_{\text{loop avg}}} + \frac{q_{\text{pulse}}}{2 F_{\text{sc}}} \left( R_{\text{bore}} F_{\text{p}} + R_{\text{grout}} \right)$$

where $q_{\text{net}}$ is annual net heat rejection (W), $q_{\text{pulse}}$ is peak load (W), $R_{\text{ground}}$ is effective ground thermal resistance (m·K/W), $F_{\text{sc}}$ is short-circuit heat loss factor, and $F_{\text{p}}$ is pulse load factor.

Horizontal Loop Configuration

Horizontal loops employ trenches 4-10 ft (1.2-3.0 m) deep with HDPE pipes in single-pipe, two-pipe, or four-pipe (spiral) configurations.

Advantages:

• Lower installation cost in suitable terrain
• Easier installation without specialized drilling
• Simple expansion or repair access
• Leverages solar gain in shallow soils

Disadvantages:

• Extensive land area (1500-3000 ft² per ton)
• Performance influenced by seasonal surface temperature variation
• Vulnerable to landscape changes
• Frost penetration concerns in cold climates

Trench Depth Selection:

Below frost line but within active solar influence zone. Heat transfer affected by:

$$T_{\text{soil}}(z,t) = T_{\text{mean}} + A_s e^{-z\sqrt{\frac{\pi}{\alpha P}}} \cos\left(\frac{2\pi t}{P} - z\sqrt{\frac{\pi}{\alpha P}}\right)$$

where $T_{\text{soil}}(z,t)$ is soil temperature at depth $z$ and time $t$, $T_{\text{mean}}$ is annual mean surface temperature, $A_s$ is surface temperature amplitude, $P$ is period (365 days), and $\alpha$ is thermal diffusivity.

Slinky/Spiral Configuration

Overlapping coils increase heat transfer surface area per trench length by 2-4×, reducing land requirements but increasing pipe and pumping costs.

Spacing Requirements:

• Trench spacing: 10-20 ft (3-6 m) minimum
• Loop pitch: 12-24 inches (0.3-0.6 m)
• Prevents thermal interaction between adjacent loops

Pond/Lake Systems

Submerged closed loops in water bodies utilize water’s superior thermal properties:

• Thermal conductivity: 0.6 W/m·K (vs. 0.5-2.5 W/m·K for soil)
• Consistent temperature stratification
• Natural convection enhances heat transfer

Requirements:

• Minimum depth: 8-10 ft (2.4-3.0 m) to prevent freezing
• Minimum volume: 0.5 acre-ft per ton capacity
• Bottom placement for temperature stability
• Anchoring against buoyancy and ice forces

Open-Loop Systems

Open-loop systems pump groundwater directly through a heat exchanger, then discharge to a second well or surface water body.

System Components

graph LR
    A[Supply Well] --> B[Well Pump]
    B --> C[Plate Heat Exchanger]
    C --> D[Heat Pump]
    C --> E[Injection Well or Discharge]

    style A fill:#e1f5ff
    style B fill:#fff4e1
    style C fill:#ffe1e1
    style D fill:#e1ffe1
    style E fill:#f0e1ff

Design Criteria

Flow Rate Requirement:

$$\dot{m}{\text{water}} = \frac{q{\text{peak}}}{c_p \Delta T_{\text{water}}}$$

Typical $\Delta T_{\text{water}}$ = 10-15°F (5.5-8.3°C) through heat exchanger.

Well Capacity:

• 1.5-3.0 gpm per ton cooling capacity
• Continuous pumping capability for peak load duration
• Adequate drawdown recovery between cycles

Water Quality Considerations:

Advantages and Limitations

Advantages:

• Lower first cost (single well vs. bore field)
• Excellent thermal performance
• Minimal land area
• No thermal degradation concerns

Limitations:

• Requires suitable aquifer with sufficient yield
• Regulatory restrictions on water use/discharge
• Fouling risk requires maintenance
• Potential water rights issues
• Environmental concerns with discharge

Ground Loop Comparison

Site Assessment and Selection Criteria

Geological Investigation

Thermal Conductivity Testing:

In-situ thermal response testing (TRT) measures effective ground thermal conductivity:

$$k_{\text{eff}} = \frac{q}{4\pi m} \quad \text{where} \quad m = \frac{dT_{\text{fluid}}}{d(\ln t)}$$

Test duration: 48-72 hours minimum. Critical for vertical loop sizing accuracy.

Soil Classification:

Groundwater Considerations

Static Water Level: Affects saturation zone and thermal performance. Seasonal fluctuation monitoring essential.

Hydraulic Conductivity: Determines groundwater flow contribution to heat transfer. Values >10⁻⁵ m/s significantly enhance performance.

Site Constraints

Decision Flow:

flowchart TD
    A[Site Assessment] --> B{Adequate Aquifer?}
    B -->|Yes| C{Water Quality OK?}
    B -->|No| D{Land Available?}

    C -->|Yes| E{Regulations Allow?}
    C -->|No| D

    E -->|Yes| F[Open Loop System]
    E -->|No| D

    D -->|Yes, >1 acre| G[Horizontal Loop]
    D -->|No| H{Water Body?}

    H -->|Yes, >8 ft deep| I[Pond Loop]
    H -->|No| J[Vertical Loop]

    style F fill:#90EE90
    style G fill:#90EE90
    style I fill:#90EE90
    style J fill:#90EE90

Heat Exchanger Sizing Methodology

Closed-Loop Pipe Sizing

Pressure Drop Constraint:

$$\Delta P = f \frac{L}{D} \frac{\rho v^2}{2}$$

Target: <10 ft H₂O per ton (29.9 kPa/ton) total loop pressure drop to maintain pump efficiency.

Reynolds Number:

$$Re = \frac{\rho v D}{\mu}$$

Maintain turbulent flow (Re >4000) for optimal heat transfer. Typical velocities: 2-4 ft/s (0.6-1.2 m/s).

Pipe Diameter Selection:

Borehole Thermal Resistance

Total Thermal Resistance:

$$R_{\text{total}} = R_{\text{pipe}} + R_{\text{grout}} + R_{\text{bore}}$$

Borehole Resistance (shape factor method):

$$R_{\text{bore}} = \frac{1}{2\pi k_{\text{grout}}} \ln\left(\frac{r_{\text{bore}}}{\sqrt{2} r_{\text{pipe}}}\right)$$

Typical values: 0.10-0.20 m·K/W for thermally-enhanced grout, 0.30-0.50 m·K/W for bentonite.

Open-Loop Heat Exchanger

Plate Heat Exchanger Design:

$$UA = \frac{q}{\Delta T_{\text{lm}}}$$

where $U$ is overall heat transfer coefficient (typically 500-1000 W/m²·K for plate exchangers) and $A$ is heat transfer area.

Log-Mean Temperature Difference:

$$\Delta T_{\text{lm}} = \frac{(T_{\text{water,in}} - T_{\text{loop,out}}) - (T_{\text{water,out}} - T_{\text{loop,in}})}{\ln\left(\frac{T_{\text{water,in}} - T_{\text{loop,out}}}{T_{\text{water,out}} - T_{\text{loop,in}}}\right)}$$

Approach temperature: 2-5°F (1-3°C) for efficient heat transfer.

Design Standards and Guidelines

IGSHPA Design and Installation Standards provide comprehensive methodology for:

• Thermal conductivity testing procedures
• Loop length calculations
• Grout specifications
• Pipe fusion requirements

ASHRAE Handbook - HVAC Applications, Chapter 35 covers:

• Load calculation procedures
• Ground temperature estimation
• System design parameters
• Performance expectations

International Ground Source Heat Pump Association (IGSHPA) certification ensures installer competency and quality assurance.

Conclusion

Ground loop selection fundamentally balances site conditions, thermal performance requirements, and economic constraints. Vertical boreholes offer superior performance and minimal land impact at higher installation cost. Horizontal loops provide economical solutions where land availability permits. Open-loop systems deliver excellent efficiency but face regulatory and water quality challenges. Proper site characterization through thermal testing and geological investigation ensures accurate sizing and long-term performance reliability.

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