Rocking on with hot rocks geothermal energy

Glossary

alloy. A substance made of two or more metals, or a metal and one or more non-metals, that has mostly metallic properties. Alloys are often created to improve the properties of metals such as strength, resistance to corrosion and hardness. For example, steel is an alloy of iron with up to two per cent carbon and often small amounts of other elements. The properties of steel such as strength, malleability and machinability can be changed by adjusting the amounts of its component elements.

artesian. Describes a source of groundwater or aquifer that is under pressure. If an artesian aquifer is tapped by a well, water rises above the surface of the aquifer without the need for pumping. The Great Artesian Basin in Australia is one of the largest artesian basins in the world lying under 22 per cent of the country.  

carbon emission reduction scheme. A method such as a carbon tax or carbon trading scheme that reduces emissions of carbon (and often other greenhouse gases). A carbon tax is a tax imposed on the production or use of fossil fuels based on the carbon content of those fossil fuels. Trading schemes set a limit to the amount of greenhouse gases that can be released. Permits are then allocated to organisations for their carbon emissions; those reducing emissions below their quota can trade the excess to other organisations. For more information see Carbon currency – the credits and debits of carbon emissions trading (Nova: Science in the news).

electromagnetic induction. The production of electrical current in a conductor due to a changing magnetic field (moving magnet) or to the movement of a conductor through a magnetic field. For more information see electromagnetic induction (Florida State University, USA).

geothermal gradient. The rate of increase in temperature per unit depth in the Earth. For more information see Geothermal gradient (Absolute astronomy, USA).

granite. A hard, coarse grained, rock that is often used in building materials. Granite is an intrusive, igneous rock ie. it forms from magma underground. Like most other rocks, granite contains low levels of radioactive elements.

heat pump. By circulating water underground, a geothermal heat pump uses the relatively constant heat of the earth to alter the temperature of the circulated water. This water can then be used to heat or cool buildings. Heat pumps can extract heat from the earth for heating buildings in winter and deliver heat from buildings to the earth in summer.

hydrothermal. Describes geothermal systems with naturally occurring groundwater that has been heated by either heat-producing rocks or nearby volcanic activity.

impermeable. A substance that cannot be penetrated. A rock or material that stops the movement of water or other liquids through it.

megajoule. The unit of energy is the joule (J). It is defined as the work done, or energy expended, by a force of one newton moving one metre in the direction of the force. When we consider power generation, we use larger units. A megajoule is 1,000,000 joules.

radiogenic. Produced by radioactive decay, eg, the heat and isotopes produced as a result of the decay of uranium in the Earth’s crust are radiogenic.

Ring of Fire.  A region around the Pacific Ocean where volcanoes and earthquakes frequently occur, corresponding to the edge of the Pacific tectonic plate. For a map see Ring of Fire (US Geological Survey).

sedimentary. A class of rocks that are formed from sediments eg, sandstone or from precipitation of chemicals from solution eg, limestone.

tectonic plate. The Earth’s crust is broken up into a series of large areas or tectonic plates. These plates move slowly on top of the currents in the semi-liquid mantle below them. Volcanoes, earthquakes, geysers and hot springs frequently occur where plates are either colliding or pulling apart from each other. For more information see Plate tectonics (Nova: Science in the news).

thermal conductivity. A measure of the ability of a substance to conduct heat. A potential geothermal resource is evaluated in terms of heat flow, estimated from the geothermal gradient and thermal conductivity measurements. An overlying layer of rock of low thermal conductivity (eg, sedimentary rock) helps to maintain the heat of a geothermal resource.

turbine. A device in which a stream of water or gas turns a bladed wheel, converting the kinetic energy of the fluid flow into mechanical energy available from the turbine shaft. The earliest turbines were water wheels. Now, steam turbines are driven by jets of high-temperature steam; gas turbines are driven by burning fuel vapour; and wind turbines use the power of moving air.


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Posted July 2009, edited August 2012.