Binary cycle power plant
A geothermal power plant in which hot brine heats a separate low-boiling-point working fluid that drives the turbine, enabling generation from resources of roughly 100 to 180 °C.
In a binary plant the geothermal fluid never touches the turbine. It passes through a heat exchanger, where it vaporises a secondary working fluid, typically an organic compound such as isopentane, isobutane, n-pentane or a refrigerant (Organic Rankine Cycle, ORC), or an ammonia-water mixture (Kalina cycle). The vapour drives a turbine, is condensed in air-cooled or water-cooled condensers, and pumped back. The cooled brine is reinjected into the reservoir.
Advantages:
- Usable at moderate temperatures (about 100–180°C), far more common worldwide than the 200°C-plus resources needed for flash steam
- Closed loop with near-zero direct emissions of CO2 and H2S, since non-condensable gases stay dissolved in the reinjected brine
- Modular units of 1–30 MW
The trade-off is low efficiency, bounded by the small temperature difference:
Parasitic loads from brine pumps and air-cooler fans can absorb 15–30% of gross output, more in hot weather when air-cooled condensers lose performance.
Binary is now the most common technology by number of units and the dominant type for new plants in the US (Nevada), Türkiye, Germany (Bavarian Molasse Basin) and in EGS projects. Geothermal plants typically reach capacity factors of 70–90%. ORC technology is also used for industrial waste heat recovery, biomass and OTEC.
Related terms
All terms →- Geothermal gradient
The rate at which temperature increases with depth in the Earth's crust, averaging about 25 to 30 °C per kilometre away from tectonic plate boundaries.
- Enhanced geothermal system (EGS)
A geothermal reservoir created by stimulating hot, low-permeability rock, usually by hydraulic fracturing or shearing, so that water can be circulated between wells to extract heat.
- Capacity factor
The ratio of the energy a power plant actually produces over a period to the energy it would produce running at full nameplate capacity for the whole period.
- OTEC (Ocean Thermal Energy Conversion)
Power generation from the temperature difference between warm tropical surface seawater and cold deep water, typically about 20 to 25 °C, using a heat engine cycle.
- CHP (Combined Heat and Power)
Simultaneous generation of electricity and useful heat from one fuel input, reaching total efficiencies of 75–90% versus about 35–60% for power-only plants.