Extraction & Conversion

"Water is the common medium used to capture and transport the heat contained in the earth, or geothermal energy, to the surface for conversion to electricity."

Drilling

Sedimentary Rock

The easiest rock to drill into is sedimentary rock, rock that is formed by the erosion of older rocks into a form of compressed sand. It is within the porous confines of sedimentary rock that decaying vegetation was once trapped and heavily compressed.

Sedimentary rock provides two major advantages for geotheral energy production over volcanic rock and granite. Firstly, most of the World's sedimentary deposits for reserves of fossil fuels have been scoured, drilled, assessed and mapped.

Secondly, as sedimentary rock is mostly porous, it also traps water. Therefore in order to harness the energy from hot sedimentary rock there is no need to create a water circuit as it is already there.

Hot Rocks

Wells are drilled into rocks which are much hotter than normal and sufficiently hot to enable commercial generation of electricity.

Hydraulic fracturing is used to enhance the natural fracture permeability of the reservoir rocks.

Geothermal energy from hot dry rocks is recovered by drilling deep into the hot crystalline rocks (usually granitic) and forcing water down an injection well and through fractures forced open by the water pressure in the rocks at depth and back to the surface through fractures connecting to other wells drilled nearby. The water gathers heat and becomes superheated as it flows through the hot rocks.

Power Plants

To harness geothermal energy and transform this into electricity, geothermal power stations are used in a variety of designs.

The three main power plant designs which use geothermal as an energy source are; "dry steam", "flash steam", and "binary-cycle" power stations.

Dry Steam

When the geothermal resource produces pure steam, the steam directly drives a turbine and generates electricity. These resources are however relatively scarce.

Flash Steam

Geothermal resoures that contain water temperatures in excess of 200°C have the pressure of the fluid reduced until it begins to boil or flash. This produces both steam and water. The steam is used to drive the turbine; the water is injected back into the reservoir or used to drive a binary cycle power plant.

Binary Cycle

Where water temperatures are below 200°C, Binary Cycle power plants are used. Rather than flashing the geothermal fluid to produce steam, the hot geothermal water is passed through a heat exchanger which transfers the heat energy to a second liquid (usually an organic fluid) in the power plant which vaporises and drives an electricity generation turbine.

The organic fluid has a lower boiling point and higher vapour pressure than steam at the same temperature. Having passed through the turbine, the organic vapour is condensed and re-circulated in the plant. The geothermal water, which is now partially cooled, is pumped directly back down an injection well.

Binary cycle geothermal power plants are environmentally superior compared to others because the hot water (which may contain dissolved salts and gases) is never exposed to the atmosphere.

Binary-Cycle Power Plants

The type of power plant used depends on the temperature, pressure and chemistry of the water circulating through the rocks.

In general, Binary Cycle power plants (see left) are used for temperatures below about 190°C and Flashed Steam plants are used for higher temperatures.

The Binary Cycle plant is so named because the geothermal fluid circulates in a closed loop underground while a lower boiling point working fluid, which is heated by the heat from the geothermal fluid, circulates in a separate closed loop in the power plant at the surface.

By Courtesy of ORMAT

There are two main types of Binary Cycle power plants. Organic Rankine cycle plants use an organic fluid with a lower boiling point (eg isobutene, n-pentane) than water. Kalina cycle plants vaporise a mixture of water and ammonia which evaporates over a larger temperature range compared to the Organic Rankine cycle. Rankine cycle plants are in commercial operation in many localities around the world while the proponents of Kalina cycle plants claim them to be more efficient. Large scale commercially viable working examples of Kalina cycle plants are yet to be brought into production.