Why does a binary cycle plant use a second working fluid?

Binary cycle power plants utilize a second working fluid—typically ammonia or hydrocarbons—to enable electricity generation from lower-temperature geothermal resources that would otherwise be insufficient to drive a turbine directly.1

In this process, geothermally heated water is passed through a heat exchanger, where its thermal energy is transferred to the secondary working fluid. Because this secondary fluid has a lower boiling point than water, it vaporizes at these lower temperatures, allowing the resulting vapor to drive the turbine and electrical generator. This closed-loop system is particularly effective in technologies like the organic Rankine cycle, which can operate with water temperatures as low as 85–90 °C (185–194 °F).1