2. STATE OF THE ART TECHNOLOGY

Geothermal resources

There are four types of geothermal resources: hydrothermal, geopressured, hot dry rock and magma. Of the four types, only hydrothermal resources are currently commercially exploited.

Hydrothermal resources

Hydrothermal, or hot water, resources arise, when hot water and/or steam is formed in fractured or porous rock at shallow to moderate depths (100m to 4.5km) as a result of either the intrusion in the earth’s crust of molten magma from the earth’s interior, or the deep circulation of water through a fault or fracture. High temperature hydrothermal resources, with temperatures from 180 °C to over 350 °C, are usually heated by hot molten rock. While low temperature resources, with temperatures from 100°C to 180°C, can be produced by either process.

Deep underground, fluids absorb heat from their environment and move upwards to ground level either naturally, for example as geysers or springs, or through drilled wells. When water in the porous rocks and cracks reaches the surface, it becomes either dry or wet steam, or hot water. High-grade resources are usually used for electricity generation, while low grade resources are used in direct heating applications.

Hydrothermal resources require three basic components (see below) a heat source (e.g. crystallised magma), an aquifer containing accessible water, and an impermeable cap rock to seal the aquifer. The geothermal energy is usually tapped by drilling into the aquifer, and extracting the hot water or steam.