Introduction
Binary geothermal power stations are one of the three main types of geothermal power stations used commercially, the other two being the flash type and dry steam type.
Unlike the other two types, binary plants can produce electricity from geofluid at a much lower temperature, as low as 57°C. The number of accessible water dominated resources at <150°C is far greater than >150°C, this means that binary plants have a much greater potential for wider applications than the other two types of geothermal plants. The initial setup costs for binary plants are also lower, as it is not necessary to drill as deep to reach geofluid at a suitable temperature; drilling typically accounts for 30-50% of a geothermal plants total construction cost.
Binary plants benefit from lower turbine maintenance costs as the working fluid is not corrosive and does not cause scale. Geothermal plants employing the dry steam or flash type generation method will suffer from scaling, corrosion and problems associated with non-condensable gases (NCG), but these problems have a lesser impact on binary plants.
Another benefit for binary plants is the ability to be deployed in package/modular form. It is not unusual to see 0.5-2MW plants shipped, installed and generating, within a small time frame (days or weeks).
Operation
Geofluid is pumped from an underground water dominated reservoir to a heat exchanger at surface level. The heat exchanger brings the working fluid (usually hydrocarbon based) into close proximity to the geofluid where heat transfer can take place.
The geofluid heats the working fluid above its boiling point and changes its state to vapour. The geofluid is then returned back to the underground reservoir via the injection line. The vaporised working fluid is then discharged to an organic Rankine cycle (ORC) turbine where it causes the turbine to rotate.
The rotation of the turbine also rotates an AC generator which is connected to the turbine on a common shaft; electricity is thus produced by the generator. Once the working fluid is discharged from the turbine, it is condensed back into a liquid using a condenser. It is necessary to condense the working fluid in order that it can be pumped back to the heat exchanger and the process can be repeated.
Cooling towers are employed to cool the working fluid vapour and condense it back into a liquid. The heat from the cooling tower is expelled to air, or, expelled to a water source such as a river or lake. Generally, induced or forced draft axial fans are used and these are installed in modular form as cooling cells.
System Components
This 3D model shows all major components associated with an idealised binary geothermal power station, these include:
- Production Well
- Injection Well
- Heat Exchanger
- Organic Rankine Cycle (ORC) Turbine
- AC Generator
- Cooling Tower
Additional Resources
https://geothermal-energy-journal.springeropen.com/articles/10.1186/s40517-017-0074-z
https://www.energy.gov/eere/geothermal/electricity-generation