Power from Moderate Heat
Most of Earth's accessible geothermal heat exists at moderate temperatures — 80 to 180°C — too cool for conventional steam turbines but perfect for binary-cycle power plants. These plants use the organic Rankine cycle (ORC), substituting a low-boiling-point working fluid for water. Hot geothermal brine heats the working fluid through a heat exchanger; the vaporized fluid drives a turbine; and the condensed fluid is recycled. This simulator models the thermodynamic performance of a binary-cycle geothermal plant.
The Organic Rankine Cycle
In the ORC, an organic working fluid (isobutane, isopentane, or a refrigerant) undergoes four stages: pumping to high pressure, evaporation by geothermal heat, expansion through a turbine (generating electricity), and condensation. The choice of working fluid critically affects performance — its boiling point, critical temperature, and saturation curve shape must match the geothermal source temperature for optimal heat recovery.
Efficiency and Carnot Limits
Thermodynamics imposes strict limits on energy conversion. The Carnot efficiency — set by the ratio of cold and hot absolute temperatures — is modest for geothermal sources: typically 15-30%. Real-world irreversibilities (heat exchanger pinch points, turbine losses, parasitic pump loads) reduce actual thermal efficiency to 8-15%. Yet binary plants remain economic because geothermal 'fuel' is free — no combustion, no fuel costs, no emissions.
Environmental Advantages
Binary-cycle plants are arguably the cleanest electricity source available. The geothermal brine circulates in a sealed loop from well to heat exchanger and back — no gases, minerals, or fluids contact the atmosphere. The working fluid is also contained in a closed loop. Combined with small physical footprints and 95%+ capacity factors, binary plants offer reliable baseload renewable power with near-zero environmental impact.