Extraction & Conversion

"Water is 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 sandy, fractured or cavernous sedimentary rock that water is trapped and becomes heated by hot rocks or magma.

Sedimentary rock provides two major advantages for geotheral energy production over volcanic rock and granite. Firstly, as most of the World's fossil fuels are formed in sedimentary rocks they have been drilled, assessed and mapped.

Secondly, where sedimentary rock is porous, it usually traps water. Therefore in order to harness the energy from hot sedimentary rock there is no need to introduce water and circulate it through the rock 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 pathways of the hot reservoir rocks.

Geothermal energy from hot dry rocks is recovered by drilling deep into the hot crystalline rocks (usually granites) and forcing water down an injection well and through fractures forced open by the water pressure in the rocks 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.

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

Dry Steam

When the geothermal resource produces pure steam, the steam directly drives a turbine and generates electricity. These resources are however relatively scarce. An example is the Geysers field in the USA.

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 hot water is injected back into the reservoir or used to drive a binary cycle power plant.

Binary Cycle

In Binary plants, 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 underground 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

In general, Binary Cycle power plants (see left) are used for water 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. Organic 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. There are only two commercial scale Kalina cycle plants in operation; a 3-4 MWe capacity plant at Unterhaching, Germany and a 2 MWe plant at Husavik, Iceland.