Geothermal

Types of geothermal resources

Geothermal resources are divided into 4 types - hydrothermal, geopressured, hot dry rock and magma.

The formation of hydrothermal resources requires three basic components, a heat source, an aquifer containing accessible water and an impermeable cap rock to seal the aquifer. Hydrothermal resources are made available when hot water or steam is formed in fractured or porous rock at shallow to moderate depths (100m to 4.5km), for example due to intrusion of molten magma from the earth's interior. The energy of a hydrothermal resource is tapped by drilling into the aquifer and extracting the hot water or steam. Hydrothermal resources with high-grade heat (temperature from 180 oC to 350 oC) are usually utilized for electricity generation, while resources with low-grade heat are used for direct heating.

Geopressured resources refer to of hot brine saturated with methane found in large, deep aquifers under high pressure at a depth of about 3 to 6 km. The temperature of the water ranges from 90 to 200 oC. Thermal energy, hydraulic energy (from the pressure) and chemical energy (from burning of methane) can be extracted from geopressured resources.

Under certain geological conditions, the subsurface rocks reach temperatures of 200-300oC within five kilometres of the surface. The energy stored in these hot dry rocks can by harvested by injecting pressurized cold water to cause hydraulic fracture of the rocks and to extract heat from the hot rocks.

Magma is molten rock found at depths of 3 to 10 km and deeper. It has a temperature from 700-1200oC. Since magma is lighter than the surrounding rock, it moves up toward the earth's crust, carrying the heat from below. The heat of the magma can be extracted by circulating a fluid in the top part of the magma chamber. The heat will then be used to power a closed Rankine-cycle machine to generate electricity.