AP Syllabus focus:
‘Geothermal energy uses heat from Earth’s interior to heat water; the resulting steam drives a generator to produce electricity.’
Geothermal power plants convert underground heat into electricity by moving hot water or steam from deep rock to the surface, spinning a turbine, and returning cooled fluids underground for reuse.
Core idea: turning underground heat into electricity
Geothermal electricity generation depends on three linked elements:
A heat source in Earth’s crust (hot rock heated by magma or natural geothermal gradients)
A working fluid (water/steam, or a secondary fluid in some designs)
A power conversion system (turbine + generator) that turns thermal energy into electrical energy
Geothermal energy: Usable energy derived from heat stored in Earth’s interior.
In most plants, wells tap naturally occurring hydrothermal systems where hot water and steam circulate through permeable rock. The plant extracts thermal energy, converts part of it to electricity, and then manages the remaining heat and fluid.
From hot rock to steam: heating water underground
Geothermal fluids originate mainly from groundwater that has percolated downward and been heated at depth. Key subsurface features include:
Reservoir-scale schematic illustrating a geothermal system with a hot-fluid production well and a cooled-fluid injection well within layered geologic units. The diagram emphasizes how permeability structure and well placement control subsurface flow paths, supporting the production–reinjection circulation described in geothermal power systems. Source
Permeable reservoir rock that stores and transmits hot fluids
Impermeable cap rock that helps trap heat and pressure
Production wells that bring hot fluids to the surface
Injection wells that return cooled water back underground
Geothermal reservoir: A subsurface body of hot rock and fluids that can supply heat and/or steam to a power plant through wells.
Pressure and temperature determine whether the fluid arrives as mostly steam, mostly liquid water, or a mixture. At the surface, equipment routes that fluid through one of several plant designs to produce rotating motion for electricity generation.
Major geothermal power plant designs
Dry steam plants
Dry steam plants use steam that is already present underground.
Steam rises up the production well
Steam is cleaned of debris/moisture as needed
Steam expands through a turbine, causing it to spin
The turbine drives a generator to produce electricity
Steam is condensed to water and commonly reinjected
Dry steam systems are relatively straightforward but require rare reservoirs that naturally produce dry steam.
Flash steam plants
Flash systems start with very hot, high-pressure liquid water. When pressure drops at the surface, part of the water rapidly boils (“flashes”) into steam.
Hot water is brought up under pressure
A separator reduces pressure so some liquid becomes steam
Steam drives the turbine-generator
Remaining liquid (and condensed steam) is reinjected
Flash steam plants are common because many reservoirs produce hot water rather than pure steam.
Binary cycle plants
Binary plants keep geothermal water in a closed pathway and transfer its heat to a second fluid with a lower boiling point.
Schematic of a binary-cycle geothermal power plant showing heat transfer from geothermal hot water to a separate, low-boiling-point secondary working fluid in a heat exchanger. The secondary vapor drives the turbine–generator, then condenses and recirculates in a closed loop while cooled geothermal water is reinjected to sustain the reservoir. Source
Binary cycle: A geothermal system in which geothermal water heats a secondary fluid (with a low boiling point) in a heat exchanger; the secondary fluid vapour spins the turbine.
Basic process:
Hot geothermal water passes through a heat exchanger
A secondary fluid vaporizes and spins the turbine-generator
The secondary fluid is cooled and reused in a closed loop
The cooled geothermal water is reinjected
Binary designs allow electricity production from moderate-temperature resources by using a working fluid that vaporizes more easily than water.
Key components and the energy conversion chain
Turbine and generator operation
The central conversion step is mechanical rotation:
Expanding steam (or secondary vapour) pushes turbine blades
The turbine shaft spins the generator rotor within magnetic fields
The generator converts mechanical energy to electrical energy sent to the grid
Cooling and reinjection (maintaining circulation)
After leaving the turbine, vapour is cooled and condensed (or the working fluid is cooled in binary systems). Reinjection supports long-term operation by:
Maintaining reservoir pressure
Sustaining fluid circulation through hot rock
Enabling repeated heat extraction as water cycles underground
Operational flow path (typical sequence)
Drill and case wells into a hot reservoir
Bring hot water/steam to the surface via production wells
Route fluid through dry steam, flash, or binary equipment
Spin a turbine connected to a generator to produce electricity
Cool/condense used vapour and manage separated liquids
Return cooled water underground through injection wells
FAQ
Depth depends on local geology and where high temperatures occur.
Controls include the geothermal gradient, proximity to volcanic/tectonic zones, and reservoir permeability that allows fluids to flow to the well.
Scaling happens when dissolved minerals precipitate as pressure and temperature change.
Common triggers are flashing (rapid boiling), degassing, and mixing of fluids with different chemistries, which can reduce equipment efficiency and restrict flow.
Selection prioritises a low boiling point and suitable vapour pressure at operating temperatures.
Engineers also consider chemical stability, compatibility with materials, safety characteristics, and how efficiently the fluid condenses in the plant’s cooling system.
EGS creates or improves permeability in hot, dry rock so injected water can circulate and return heated.
Operation relies more heavily on controlled injection/production patterns to maintain flow paths and sustain heat extraction over time.
They track temperature and chemical “tracer” signals in produced fluids over time.
A faster-than-expected temperature decline, or early tracer return, can indicate short-circuiting between injection and production wells.
Practice Questions
Describe how a geothermal power plant uses Earth’s internal heat to generate electricity. (1–3 marks)
Heat from Earth’s interior heats underground water / produces steam (1)
Steam (or vapour) drives a turbine (1)
Turbine turns a generator to produce electricity (1)
Compare flash steam and binary cycle geothermal power plants in terms of how they produce vapour to spin a turbine and how geothermal water is handled. (4–6 marks)
Flash: hot, pressurised water is brought to surface and pressure drop causes some to “flash” into steam (1)
Flash: steam is separated and used to drive the turbine/generator (1)
Flash: remaining liquid and/or condensate is reinjected (1)
Binary: geothermal water transfers heat in a heat exchanger to a secondary low-boiling-point fluid (1)
Binary: secondary fluid vapour spins the turbine/generator in a closed loop (1)
Binary: geothermal water typically remains separate from turbine loop and is reinjected after cooling (1)
