The term geothermy is derived from two Greek words: geo, meaning ‘earth’ and thermos, ‘heat’. Geothermal energy is the energy contained as heat in the Earth’s interior. Geothermy refers to all applications that make use of this natural heat source. Two commonly known applications are heat pumps and the use of steam from hot earth layers to produce electricity.
Source of the Earth’s heat
The Earth’s heat is governed by three processes. A first part originates from the period the Earth was formed, roughly 4.6 billion years ago. Dust, that was circulating around the sun started to clench. Gravity forces between matter caused the circulating cloud to become increasingly denser, ultimately forming the planets we know today. During this formation process, particles and larger fragments hit and impacts occurred regularly. The enormous heat that was set free during such impacts caused part of the matter to melt. The outer, lighter part of the ‘cloud’ cooled, forming the Earth’s crust. The interior however remained hot, forming the current Earth’s core with a radius of 3470 km and a temperature of around 5000 °C to even 7000 °C. This primordial heat, generated during the Earth’s formation, dissipates slowly to the surface of the planet. The Earth’s heat flow is the amount of heat that is released into space from the interior through a unit area in a unit of time. It varies from place to place. At places where heat and magma (molten rock) well up, the earth surface tends to break up and plate boundaries diverge. These locations are regions of intense volcanism and extreme forms of geothermy like geysers, pools of boiling water and mud and fumaroles.
As soon as magma rises from depth towards the Earth’s surface it starts cooling and solidifies forming new rock. Its composition is marked by heavy elements like iron and magnesium and the rock will tend to sink again. The sliding of plates and rocks, under and past each other along faults at the Earth’s surface and at kilometers depth causes deformation and warming up. This friction heat is a second source of natural heat that dissipates to the surface. Where these processes take place, one can observe natural hot springs or even volcanic activity.
The outer layer of the Earth’s structure, the crust, is relatively cold. It can be compared to the peel of an apple, with a thickness of 7 km on average under ocean basins and 20 to 65 km under continents. Here the third process of heat generation is commonly dominant; radioactive decay. Rocks in the Earth’s crust are enriched in radioactive elements like uranium (U), thorium (Th) and potassium (40K). They are characterized by the fact that they are not stable and disintegrate, on a long time scale, into more stable components. This decay process sets free energy in the form of heat. The average heat flow from the continental crust is 57 mW/m², and 99 mW/m² through the oceanic crust (Barbier, 2002).
The processes of cooling down the Earth’s interior and radioactive decay are slow on a human time scale and are often considered to be ‘constant through time’. In this sense, geothermal energy can be considered renewable. When exploited in an industrial way however, hot fluid production rates might exceed the rate of warm fluid recharge, limiting the lifetime of a commercial plant if not managed well.