Geothermal Direct Use Applications

Geothermal direct use harnesses low to moderate temperature geothermal resources (50-150°C) for heating applications without conversion to electricity. These systems provide cost-effective, sustainable thermal energy for space heating, industrial processes, agriculture, and aquaculture operations.

Temperature-Application Relationships

Direct use applications depend on available geothermal fluid temperature. The table below shows typical temperature requirements for various applications based on DOE Geothermal Technologies Office data.

District Heating Systems

District heating represents the largest direct use application globally. Geothermal district heating systems distribute thermal energy from production wells through insulated piping networks to multiple buildings.

System Configuration

graph TD
    A[Production Well<br/>60-100°C] --> B[Production Pump]
    B --> C[Primary Heat Exchanger]
    C --> D[Distribution Network<br/>Supply 70-90°C]
    D --> E[Building Substations]
    E --> F[Space Heating]
    E --> G[Domestic Hot Water]
    D --> H[Distribution Network<br/>Return 40-60°C]
    H --> I[Peaking Boilers<br/>Optional]
    I --> C
    C --> J[Injection Well]

    K[Cascaded Use] --> L[Greenhouse Heating<br/>35-55°C]
    K --> M[Aquaculture<br/>25-35°C]
    H -.-> K

    style A fill:#ff6b6b
    style D fill:#4ecdc4
    style H fill:#95e1d3
    style L fill:#ffd93d
    style M fill:#6bcf7f

Heat Exchanger Sizing

The primary heat exchanger transfers heat from geothermal fluid to the clean distribution loop. Design calculations follow standard heat exchanger theory.

Heat transfer rate:

$$Q = \dot{m}{geo} c{p,geo} (T_{geo,in} - T_{geo,out})$$

Where:

• $Q$ = heat transfer rate (W)
• $\dot{m}_{geo}$ = geothermal mass flow rate (kg/s)
• $c_{p,geo}$ = specific heat of geothermal fluid (J/kg·K)
• $T_{geo,in}$ = geothermal inlet temperature (°C)
• $T_{geo,out}$ = geothermal outlet temperature (°C)

Log mean temperature difference (LMTD) for counterflow:

$$\Delta T_{lm} = \frac{(T_{geo,in} - T_{dist,out}) - (T_{geo,out} - T_{dist,in})}{\ln\left(\frac{T_{geo,in} - T_{dist,out}}{T_{geo,out} - T_{dist,in}}\right)}$$

Required heat transfer area:

$$A = \frac{Q}{U \cdot \Delta T_{lm}}$$

Where:

• $A$ = heat transfer surface area (m²)
• $U$ = overall heat transfer coefficient (W/m²·K)
• Typical $U$ values: 1000-2000 W/m²·K for plate heat exchangers with clean fluids
• Typical $U$ values: 500-1000 W/m²·K for shell-and-tube with scaling potential

For geothermal fluids with high mineral content, fouling resistance must be included:

$$\frac{1}{U} = \frac{1}{h_{geo}} + R_{f,geo} + \frac{t_{wall}}{k_{wall}} + R_{f,dist} + \frac{1}{h_{dist}}$$

Where $R_f$ represents fouling resistance (0.0001-0.0005 m²·K/W typical for geothermal applications).

Greenhouse Heating

Geothermal greenhouse heating provides consistent, economical warmth for year-round crop production. Heating systems maintain optimal growing temperatures (15-25°C) using low-temperature geothermal resources.

Heat Distribution Methods

Floor heating systems distribute heat through embedded piping in concrete floors or gravel beds:

$$q_{floor} = \frac{T_{fluid} - T_{soil}}{R_{total}}$$

Where $R_{total}$ includes pipe-to-fluid, pipe wall, concrete, and soil resistances.

Pipe spacing calculation:

$$S = \sqrt{\frac{k_{concrete} \cdot t_{slab} \cdot (T_{fluid} - T_{soil})}{q_{required}}}$$

Fan-coil units provide rapid temperature response for high-value crops:

$$Q_{coil} = \dot{V} \cdot \rho \cdot c_p \cdot (T_{out} - T_{in})$$

Design considerations:

• Maintain minimum 35-40°C supply temperature for adequate heat output
• Size for worst-case outdoor design conditions (-20°C to -30°C depending on location)
• Include backup heating for temperatures below geothermal resource capability
• Design for 50-80 W/m² heat loss typical of modern greenhouse construction

Aquaculture Applications

Geothermal aquaculture maintains optimal water temperatures for fish, shrimp, and algae production. Temperature control directly affects growth rates, feed conversion, and survival.

Species Temperature Requirements

Pond Heating Design

Heat requirement for raceway or pond:

$$Q_{pond} = Q_{surface} + Q_{ground} + Q_{water}$$

Surface heat loss (radiation, convection, evaporation):

$$Q_{surface} = A_{surface} \left[h_c(T_{water} - T_{air}) + \epsilon \sigma (T_{water}^4 - T_{sky}^4) + h_{evap}\right]$$

Ground heat loss:

$$Q_{ground} = \frac{A_{bottom} \cdot k_{soil}}{d_{soil}} (T_{water} - T_{ground})$$

Fresh water heating requirement:

$$Q_{water} = \dot{V}{makeup} \cdot \rho \cdot c_p \cdot (T{pond} - T_{source})$$

Heat exchanger submerged in pond or raceway provides direct heating. For titanium plate heat exchangers with geothermal fluid:

$$A_{HX} = \frac{Q_{total}}{U \cdot \Delta T_{lm}}$$

Typical $U$ = 1500-2500 W/m²·K for clean geothermal water in titanium plate exchangers.

Cascaded Use Systems

Cascaded use maximizes energy extraction by utilizing geothermal fluid sequentially through progressively lower temperature applications. A typical cascade:

1. High temperature (80-100°C): Industrial process heat or district heating supply
2. Medium temperature (50-80°C): Greenhouse heating or secondary district heating
3. Low temperature (25-50°C): Aquaculture or balneology
4. Residual (20-25°C): Ground source heat pump source

Energy utilization factor:

$$\eta_{cascade} = \frac{Q_{extracted}}{Q_{available}} = \frac{\dot{m} c_p (T_{well} - T_{injection})}{\dot{m} c_p (T_{well} - T_{ambient})}$$

Well-designed cascaded systems achieve 60-75% utilization compared to 30-45% for single-use applications.

Economic Considerations

Direct use applications provide favorable economics due to:

• Low parasitic power consumption (5-10% of thermal output)
• High capacity factors (80-95% for heating applications)
• Minimal environmental emissions
• Long equipment life (30-50 years for wells, 20-30 years for heat exchangers)

Levelized cost of heat for geothermal district heating typically ranges from $15-35/MMBtu compared to $25-50/MMBtu for natural gas heating in suitable geological regions.

Design Best Practices

1. Specify corrosion-resistant materials: Titanium, stainless steel 316L, or polymer heat exchangers for geothermal fluid contact
2. Include filtration: 50-100 micron strainers prevent scaling and fouling in heat exchangers
3. Design for full reinjection: Closed-loop systems with complete reinjection minimize environmental impact
4. Provide redundancy: Parallel heat exchangers or backup heating for critical applications
5. Monitor chemistry: Continuous monitoring prevents scaling and corrosion issues
6. Optimize cascade: Maximize energy extraction through sequential temperature reduction

Geothermal direct use applications provide reliable, cost-effective heating for diverse applications where suitable resources exist. Proper heat exchanger sizing, material selection, and cascaded system design maximize economic and environmental benefits.