geothermal energy-Box 3

Published by

Australian Academy
of Science

Rocking on with hot rocks geothermal energy

Box 3 | Geothermal electricity generation systems

Three systems currently exist for converting geothermal energy to electricity: dry steam, flash, and binary cycle power plants. The latter holds most promise for Australia’s hot rock resources.

Dry steam power plants use steam from geothermal reservoirs directly. Hot steam is tapped by bore holes and used at the surface to turn a turbine to generate electricity (Box 2: Electricity generation). Dry steam systems have been used for more than a hundred years and are effective for hydrothermal resources where the water is at temperatures of 150°C or more. It is employed, for example, at The Geysers in northern California as well as in Italy, Japan, Indonesia and Mexico. Dry steam systems are unlikely to find much use in Australia, however, because of a lack of sufficiently hot geothermal fluid.

Illustration of a Dry Steam Power Plant - Geothermal steam comes up from the reservoir through a production well. The steam spins a turbine, which in turn spins a generator that creates electricity. Excess steam condenses to water, which is put back into the reservoir via an injection well.
(Image: US Department of Energy)

Flash steam power plants are the most common geothermal systems in use today. They are suitable for resources that are in liquid form at relatively high temperatures (180°C or more). The high-pressure water extracted from underground, enters a tank at the surface that is at a much lower pressure, causing the water to rapidly change to steam, or ‘flash’. The steam is used to drive a turbine and generate electricity. In some systems, left-over vapour can be fed into a second flash tank, enabling more energy to be captured, thus increasing the efficiency of the system.

Illustration of a Flash Steam Power Plant - Pressurized geothermal hot water comes up from the reservoir through a production well. The water enters a flash tank where it depressurizes and flashes to steam. The steam then spins the turbine, which in turn spins a geneator that creates electricity. Excess steam condenses to water, which is put back into the reservoir via an injection well.
(Image: US Department of Energy)

Binary cycle power plants, which are thermal power plants capable of utilising relatively low-temperature water, hold most promise in Australia. Heated water, for example from a hot rock system, flows under pressure into a binary cycle power plant. Heat is transferred from the water through a heat exchanger to another liquid with a lower boiling point than water. The vapour that forms from this secondary liquid is used to drive a turbine, which generates electricity. The cooled water is piped back underground into the geothermal system, where it is reheated and reused. The water and any dissolved substances in binary systems are in a closed loop so never come into contact with the other liquid or, more importantly, with the atmosphere. Recycling water also helps to maintain the heat and pressure of the geothermal reservoir.

(Image: US Department of Energy)