A binary cycle power plant is a type of geothermal power plant that allows cooler geothermal reservoirs to be used than is necessary for dry steam and flash steam plants. As of 2010, flash steam plants are the most common type of geothermal power generation plants in operation today, which use water at temperatures greater than 182 °C (455 K; 360 °F) that is pumped under high pressure to the generation equipment at the surface. With binary cycle geothermal power plants, pumps are used to pump hot water from a geothermal well, through a heat exchanger, and the cooled water is returned to the underground reservoir. A second "working" or "binary" fluid with a low boiling point, typically a butane or pentane hydrocarbon, is pumped at fairly high pressure (500 psi (3.4 MPa)) through the heat exchanger, where it is vaporized and then directed through a turbine. The vapor exiting the turbine is then condensed by cold air radiators or cold water and cycled back through the heat exchanger.
A binary vapor cycle is defined in thermodynamics as a power cycle that is a combination of two cycles, one in a high temperature region and the other in a lower temperature region.
Introduction to Binary Cycles
The use of mercury-water cycles in the United States can be dated back to the late 1920s. A small mercury-water plant which produced about 40 megawatts (MW) was in use in New Hampshire in the 1950s, with a higher thermal efficiency than most of the power plants in use during the 1950s. Unfortunately, binary vapor cycles have a high initial cost and so they are not as economically attractive.
Water is the optimal working fluid to use in vapor cycles because it is the closest to an ideal working fluid that is currently available. The binary cycle is a process designed to overcome the imperfections of water as a working fluid. The cycle uses two fluids in an attempt to approach an ideal working fluid.
- A high critical temperature and maximum pressure
- Low triple-point temperature
- A condenser pressure that is not too low (a substance with a saturation pressure at the ambient temperature is too low)
- A high enthalpy of vaporization (hfg)
- A saturation dome that resembles an inverted U
- High thermal conductivity (good heat transfer characteristics)
- Other properties: nontoxic, inert, inexpensive, and readily available
Rankine Vapor Cycle
The Rankine cycle is the ideal form of a vapor power cycle. The ideal conditions can be reached by superheating the steam in the boiler and condensing it completely in the condenser. The ideal Rankine cycle does not involve any internal irreversibilities and consists of four processes; isentropic compression in a pump, constant pressure heat addition in a boiler, isentropic expansion in a turbine, and constant pressure heat rejection in a condenser.
This process is designed to reduce the thermodynamic losses incurred in the brine heat exchangers of the basic cycle. The losses occur through the process of transferring heat across a large temperature difference between the high temperature brine and the lower temperature of the working fluid. Losses are reduced by maintaining a closer match between the brine cooling curve and the working fluid heating curve.
“Power is extracted from a stream of hot fluid, such as geothermal water, by passing the stream in heat exchange relationship with a working fluid to vaporize the latter, expanding the vapor through a turbine, and condensing the vapor in a conventional Rankine cycle. Additional power is obtained in a second Rankine cycle by employing a portion of the hot fluid after heat exchange with the working fluid to vaporize a second working fluid having a lower boiling point and higher vapor density than the first fluid.”
There are numerous binary cycle power stations in commercial production.
- Olkaria III, Kenya
- Mammoth Lakes, California, United States
- Steamboat Springs (Nevada), United States
- Te Huka Power Station, New Zealand 
Binary cycle power plants have a thermal efficiency of 10-13%.
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