Introduction
Double flash type geothermal power plants form one of the main three types of geothermal power station that are in commercial operation today; the others two types being the binary type and dry steam type. Geothermal power plant types are classed as renewable forms of energy and belong to the ‘green’ energy sector.
As a general rule, approximately 25-30% more energy can be extracted from a geofluid when using a double flash system instead of a single flash system.
Process
Hot water (geofluid) is extracted from an underground water dominated reservoir. The geofluid rises to the surface through the production well. A production well head valve can be used to isolate the underground part of the system from the surface part.
The hot geofluid is in a liquid state when it reaches the surface. Pressure reduction chambers are used to reduce the geofluid pressure. The reduction in pressure causes some of the geofluid liquid to evaporate (‘flash off’). One flash chamber is used to create high pressure steam whilst others are used to create low pressure steam. Approximately 20% of the geofluid can be flashed to steam whilst the rest is returned to the underground reservoir.
The generated steam is then used to drive a steam turbine and a generator that is connected on a common shaft. Once the useful working energy from the steam has been extracted, it is referred to as ‘waste steam’ and must be condensed in order that it can be pumped. There are a series of options for handling waste steam:
- The geofluid is dumped to a water body (river or ocean).
- The geofluid is pumped back into the underground reservoir.
For various reasons, both environmental and economical, the geofluid is almost always returned to the underground reservoir.
Cooling towers are used to cool the waste steam and condense it to condensate prior to it being returned to the reservoir. Cooling towers are almost always of the axial forced draft type.
System Components
This 3D model shows all major components associated with an idealised double flash geothermal power station, these include:
- Water Dominated Reservoir
- Production Well
- Injection Well
- Steam Turbine
- AC Generator
- Cooling Tower
Additional Resources
https://www.sciencedirect.com/topics/engineering/double-flash-system
https://www.energy.gov/eere/geothermal/electricity-generation
https://en.wikipedia.org/wiki/Binary_cycle