Binary Cycle

Most of the geothermal reservoirs contain moderate-temperature water, where energy is extracted from the fluids in binary-cycle power plants. Binary cycle plants are used where the temperature of the reservoir is not hot enough to flash sufficient steam.

In the binary system, the water from the geothermal reservoir is passed through a heat exchanger to heat another, secondary or "working fluid" (e.g. isopentane, butane).

This binary liquid boils at a lower temperature than water. It is vaporized by heating and used in order to spin the turbine. The vapour is then re-condensed and reused repeatedly. The geothermal water and the "working fluid" are placed in separate circulating systems and never come into contact with each other. Furthermore, as it is a closed-loop system, virtually nothing is emitted to the atmosphere.

 

 

The advantage of the binary cycle plants is that they can operate with lower temperature waters. If kept under pressure by applying a closed-loop system, they can use chemically impure geothermal fluids without producing air emissions. Moderate-temperature water is by far the most common geothermal resource; therefore most future geothermal power plants are expected to be binary cycle plants.  An example is the Mammoth Pacific power plants at the Casa Diablo geothermal field where binary cycle power generation systems are deployed.