Geothermal energy refers to energy stored in the form of heat beneath the surface of the Earth. This energy can be used for heating buildings and businesses, and even for electricity generation in some places.
Most of this heat comes from inside the Earth itself. The centre of the Earth is around 5,000 ˚C and heat is constantly flowing from the core out to the surface of the Earth. Much of the heat deep inside the Earth is created by the natural radioactive decay of elements.
The Earth is composed of three main layers: the crust, the mantle, and the core. As we move deeper into the structure of the Earth the temperatures increase. Some of the heat that the Earth contains is leftover from when the planet formed about 4.5 billion years ago. Heat energy is created inside the Earth due to the decay of radioactive elements in different rock types. The Earth's crust is brittle and acts like a blanket trapping heat inside the planet. Where the crust cracks at tectonic plate boundaries, there is higher heat flow as heat escapes to space. Volcanoes can form where tectonic plates meet and move apart from each other. Volcanoes can also form in areas known as hotspots. Hotspots are caused by very hot magma from a mantle plume. This hot magma melts through the rock of the Earth's crust resulting in higher temperatures closer to the surface (e.g., Hawaii, USA).
In volcanic areas such as Iceland geothermal energy is a main source of energy. In Iceland 87% of buildings obtain their heat requirements from geothermal sources. The hot rocks and hot springs from the active volcanic area heat water to produce steam which in turn drives large turbines to generate electricity.
Geothermal energy resources can also be found in areas where there is no volcanic activity, far from tectonic plate boundaries, such as France, Denmark, Belgium, Netherlands and even Ireland. In these lower energy settings deep drilling is often required to access high temperatures for large-scale heating projects.
Shallow soils, rocks and groundwater also store heat from the warmth of the Sun. The Earth's surface acts as a very large collector of solar energy, where the energy radiated from the sun is stored in the shallow layers of rock and soil near the Earth's surface. This stored energy can provide a stable source of low-temperature heat for individual home heating systems.
Geothermal energy resources are often divided into two categories, shallow and deep.
Shallow Geothermal Energy
This method harnesses both the solar energy supplied to the Earth's surface by the sun as well as the heat from deep inside the Earth. The method for extracting this geothermal energy uses a heat pump to harness the temperature difference between the surface and the ground below to provide heating and in some cases cooling. These systems can be 'closed loop' - where a fluid is circulated in a sealed pipe beneath the ground to collect the heat - or 'open loop' - where natural groundwater is pumped to the surface and reinjected back under the ground. In some cases the groundwater is discharged to streams, rivers or the sea.
Deep Geothermal Energy and hot springs
Deep geothermal methods involve drilling for thousands of metres to harness high temperatures deeper underground. The temperatures reached at great depths can be used to generate electricity. In most cases, naturally heated groundwater or even steam is pumped to the surface and the heat extracted. Sometimes, natural hydrothermal systems occur where the hot groundwater flows to the surface of it's own accord. These are called hot springs and have been used for bathing, cooking and heating for thousands of years.
In some areas the rock below is hot but dry (the water or steam does not rise to the surface). In these cases a deep well (3-5 kilometres) is drilled into the rock and cold water is pumped down. This water passes through fissures in the rock where it is heated to high temperatures. The hot water and steam then rise to the surface through another well and are used to drive large turbines to generate electricity.
Geothermal Energy in Ireland
Ireland has excellent shallow geothermal energy reserves all over the country. Our shallow groundwaters provide a stable resource of thermal energy that can be used to provide heating at very high efficiencies. Ground Source Heat Pumps (GSHPs) are becoming more and more popular and with sufficient insulation these can be a very efficient method of heating our homes and businesses.
Ireland also has recognised potential for deep low-to-medium temperature geothermal energy resources suitable for large-scale or district heating and cooling in municipal, residential and
industrial areas. Geothermal electricity production is possible given what we know of Ireland's geothermal resources, however these projects may not be economically feasible and more research is needed.
To learn more about installing a ground source heat pump in your home see our home owners manual or see our Geothermal programme