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Geothermal Energy and Ground Source Heat

Geothermal Energy and Ground Source Heat

Geothermal energy can be used for space heating (and cooling), district heating, heating of swimming pools, etc., and to generate electricity if the resource is hot enough.

In Ireland, use of geothermal energy is dominated by ground source heat energy together with heat pump technologies for heating houses, hotels, nursing homes and other business premises, public buildings and swimming pools, and horticulture amongst other uses. Cooling systems using the subsurface are used mainly in industry and offices.

Geological Survey Ireland has been involved in characterising geothermal resources since the 1960s. An upswing in activity in the 1970's was in response to the oil price crisis. Since about 2010, GSI's renewed focus on geothermal resources has been driven by the need to increase the use of renewable energy and reduce CO2 emissions.

To find out more about GSI's recent work on geothermal energy, please look at our Geoenergy projects.

Geothermal energy is heat energy within the earth. The heat energy can originate deep within planet earth ('deep geothermal'), or be mainly solar heat captured and stored by the ground ('shallow geothermal' or 'ground source heat') and heat energy that has reached the earth's surface from deep within.

The way in which the heat energy is used depends on the temperature of the geothermal resource:

1.  Shallow geothermal – also known as ground source heat: the available thermal energy resource is usually augmented through the use of a heat pump to provide heating (and/or in some cases cooling).

Heat energy is extracted from (or put into) the earth and then boosted using a heat pump. Shallow geothermal energy systems are connected to the ground thermal resource through either closed loop pipe networks (horizontal or vertical) or open loop systems with ground water being re-injected back to the subsurface or discharged to streams, rivers or the sea.

The amount of heat energy that can be extracted is up to four times the amount of electrical energy used by the heat pump (this is called the Coefficient of Performance, COP).

Because the temperature of the subsurface, including the groundwater, is relatively constant all year round, ground source heat pumps are more efficient for heating than air source heat pumps during low temperature periods, which is when heating is most needed.  They are also more efficient during hot weather when heat pumps are used in cooling mode. (This is called the Seasonal Performance Factor, SPF).

Ground or groundwater temperatures utilised by heat pump technologies are typically in the range of 5-15oC in Ireland. This geothermal energy source is also known as 'low enthalpy'.


2.  Deep geothermal: the energy source is usually hot enough to provide a direct source of heat to the end user or a district heating network or, if hotter again, the energy source could generate power through steam driven turbines.  

The geothermal gradient vertically down through the crust is 25–30°C per kilometre in much of the world.  In some parts of the world, heat from deep within the earth can reach the near-surface relatively easily due to the presence of a thinner crust or the upward movement of magma (as, for example, in Iceland).

Heat accessed generally deeper below the surface is generated within the earth. Earth's internal heat is thermal energy generated from decay of naturally occurring radioactive minerals within the mantle. Heat can also be generated within the earth's crust due to a higher concentration of radioactive heat-producing elements (for example, in certain granites).

In areas with a higher heat flux, the geothermal gradient will be higher, and greater temperatures can be reached at shallower depths below ground surface. Heat energy movement may be further augmented by fluid circulation, such as hydrothermal fluids (which are sometimes associated with warm or hot springs).

District heating schemes can utilise modest temperatures of 40oC or so (or lower), which can then be boosted by heat pumps or conventional boilers. Temperatures in typically excess of  90 – 100oC are needed for direct heat energy use in district heating schemes. Electrical power generation can be achieved with temperatures from 120oC to >300oC, with temperatures <200oC requiring pumping. The electrical turbines are commonly driven by a vaporised organic fluid that has been heated by the geothermal water in a heat exchanger.