Water will be injected under pressure down the first geothermal well
into the hot crystalline rocks to open up fractures at depth and create
a sub-surface reservoir.

This process, called fracture stimulation or "fraccing", will utilize commercially available technologies derived from the international petroleum industry to enhance the permeability in
tight reservoirs at depth. Permeability increases of 3 to 4 orders of magnitude have been inferred from flow testing at HDR test sites in France, the USA, UK, and Sweden. Permeability can be significantly enhanced by the self propping effected by small displacements of fractures caused by the fluid injection. Given encouragement from that fracture stimulation, another well will be drilled and fracture stimulated so that the well bores at the reservoir horizon are about 500 metres apart.

Download The Downhole HDR System Overview here
(Modified from the US Department of Energy)

Drilling

Techniques have been developed to drill to depths of 5 to 6 kilometres and for directionally controlled wells.

Power Plants

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

In general, Binary-Cycle power plants (see below) 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.