Evaluating Closed-Loop Geothermal Projects: A Technical Guide from Eavor’s Geretsried Pivot

Overview

Closed-loop geothermal systems promise to unlock heat from hot dry rocks without the need for natural fractures or permeable aquifers. By circulating a working fluid through a sealed underground loop—often miles deep—these systems extract geothermal energy anywhere with sufficient subsurface temperatures. Eavor, a leading next-generation geothermal company, recently pivoted its flagship Geretsried project in Germany, a move that sent ripples through the clean energy community. This guide dissects that pivot and provides a framework for evaluating closed-loop geothermal projects yourself. You will learn the technical prerequisites, step-by-step assessment methods, and common pitfalls that separate successful deployments from costly experiments.

Prerequisites

Background Knowledge

• Geothermal basics: Understand heat flow, geothermal gradients, and the difference between conventional (hydrothermal) and enhanced/closed-loop systems.
• Heat transfer mechanisms: Familiarity with conduction, convection, and how rock thermal conductivity affects heat extraction.
• Drilling and wellbore design: Know the basics of drilling depth, casing, and completion—especially for deviated or multilateral wells.
• Thermodynamic cycles: Basic grasp of Rankine or Kalina cycles for converting heat to electricity.

Materials

• A hypothetical project dataset (temperature gradient, rock type, thermal conductivity values) for practice—we provide a representative case later.
• Access to simple simulation tools (e.g., COMSOL, FEFLOW, or even spreadsheet calculators for first-order estimates).
• Economic parameters: cost per meter drilled, interest rates, electricity prices.

Step-by-Step Instructions for Evaluating a Closed-Loop Geothermal Project

Step 1: Characterize the Subsurface Thermal Resource

Start by collecting temperature logs from nearby wells or boreholes. Compute the average geothermal gradient (°C/km). For Geretsried, the gradient is reported around 40–50°C/km, which is moderate. Then assess rock thermal conductivity (W/m·K) and volumetric heat capacity (MJ/m³·K). Use laboratory tests or literature values for the local lithology (e.g., granite ~3.0 W/m·K, sandstone ~2.5 W/m·K).

Code example (Python pseudo):
gradient = 45 # °C/km
depth = 4000 # m
bottomhole_temp = 20 + (gradient * depth / 1000) # 20°C ambient
print(f'Estimated reservoir temperature: {bottomhole_temp:.1f}°C')

This gives a first estimate of the heat available.

Step 2: Design the Closed Loop Geometry

Closed-loop systems typically use a U-tube or concentric pipe arrangement. Choose a well depth and lateral length (horizontal segments) to maximize heat exchange area. For Geretsried, Eavor initially proposed a deep vertical well connected to multiple laterals forming a closed loop. Key parameters:

• Loop depth: Usually 3–5 km to reach temperatures of 150–200°C.
• Well diameter: Larger diameters reduce pressure drop but increase cost.
• Lateral length: Several kilometers to provide sufficient surface area.

Calculate heat extraction rate using: Q = UA ⋅ ΔT_lm, where U is overall heat transfer coefficient (based on rock conductivity and pipe resistance), A is heat exchanger area (pipe surface), and ΔT_lm is log-mean temperature difference between rock and fluid.

Example calculation:
U = 200 # W/m²K (typical)
A = 2 * pi * r * L # e.g., r=0.1m, L=5000m
delta_T_lm = 50 # K
Q_extract = U * A * delta_T_lm / 1e6 # in MW
print(f'Estimated heat extraction: {Q_extract:.2f} MW')

Step 3: Model Fluid Flow and Pressure Drop

Choose a working fluid (e.g., water or supercritical CO₂). Compute Reynolds number to determine flow regime. Use Darcy-Weisbach equation to find pressure drop in the loop. Ensure pump power doesn't consume more than ~20% of the gross power output.

Equation: ΔP = f ⋅ (L/D) ⋅ (ρv²/2), where f is friction factor, L total pipe length, D inner diameter, ρ fluid density, v velocity.

Iterate flow rate to balance heat extraction and pump work. For Geretsried, Eavor likely faced high parasitic loads that forced the pivot.

Step 4: Convert Heat to Power (or Direct Use)

If electricity is the goal, select an Organic Rankine Cycle (ORC) or Kalina cycle. Efficiency depends on inlet temperature and ambient sink. Typical thermal-to-electric conversion efficiency for 150–200°C is 10–15%.

Simple efficiency estimate:
efficiency = 0.15 # approximate
net_power = Q_extract * efficiency - pump_parasitic_MW
print(f'Net power output: {net_power:.2f} MW')

Step 5: Economic Feasibility

Estimate capital costs: drilling (often >50% of CAPEX), well completion, surface plant. Use industry averages: $3–5 million per kilometer of well. For a 6 km deep loop with 10 km laterals, cost could exceed $60–80 million. Levelized Cost of Energy (LCOE) = (CAPEX * CRF + O&M) / annual MWh. Compare to local electricity price. Eavor's pivot might reflect that LCOE was too high for the projected heat extraction.

Step 6: Identify Red Flags and Pivot Options

If heat extraction is too low or cost too high, consider alternatives: increase well depth, add laterals, enhance rock permeability (though that blurs closed-loop distinction), or repurpose for direct heat (heat mining) instead of power. Eavor’s Geretsried pivot appears to involve scaling back the closed-loop ambition in favor of a more conventional hybrid approach.

Common Mistakes

Overestimating Thermal Conductivity

Many models assume homogeneous high-conductivity rock, but real formations have fractures, faults, and variable mineralogy that reduce effective heat transfer. Always run sensitivity analyses with ±30% conductivity.

Ignoring Scale and Friction

Long loops create enormous pressure drops. Neglecting parasitic loads can make a project appear viable when it is not. Apply a pump efficiency factor and recheck net power.

Optimistic Drilling Cost Assumptions

Drilling deep multilateral wells is expensive, and cost overruns are common. Use a contingency of 30–50% over initial estimates.

Neglecting Regulatory and Land Access Barriers

Geothermal projects require permits for drilling, water use, and disturbance. Geretsried faced local opposition and bureaucratic delays that contributed to the pivot. Include a timeline risk premium.

Summary

Eavor’s Geretsried pivot underscores the gap between closed-loop geothermal theory and economic reality. By following the systematic evaluation steps—resource characterization, loop design, fluid dynamics, power conversion, and cost analysis—developers can identify fatal flaws early. Common pitfalls like overestimating heat transfer, underestimating pumping costs, and ignoring regulatory hurdles can derail projects. The lesson from Eavor is clear: next-generation geothermal demands rigorous technical and financial due diligence, not just innovative drilling concepts.