In a geothermal power plant, what energy turns the turbine?

• A. Water
• B. Magma
• C. Air
• D. Steam

Answer: (D)

Geothermal plants rely on heat from Earth to create steam that drives the turbine. The force turning the turbine comes from the energy of that steam—the vapor expands and pushes on the turbine blades, converting thermal energy into mechanical energy that the generator then converts to electricity. Liquid water in itself mainly transfers heat and isn’t capable of sustaining the high-pressure expansion needed to spin the turbine. Magma is the underground heat source, not the working fluid, and air isn’t used to turn turbines in these systems. In some designs hot geothermal fluid flashes to steam or heats a secondary fluid to vapor, but the turbine is powered by the energy of the steam.