Science & Technology: Shallow Geothermal Energy is Source of Clean, Renewable and Virtually Inexhaustible Energy

Shallow geothermal energy: Geological energy for the ecological transition and its inclusion in European and national energy policies (European Geologist Journal)

By Rubén Esteban Pérez

Of all possible forms of energy substitution of oil and gas, geothermal energy is the one most closely related to geology. Shallow geothermal energy is a source of clean, renewable and virtually inexhaustible energy that is directly related to geographical areas, where heat fluxes and temperature gradients can vary due to several factors. This document summarises the current situation of the policies on the use of shallow (low temperature) geothermal energy in Europe to generate hot water and air conditioning. The current status of legislation in some European countries and the promotion of shallow geothermal energy in the EU28 are also discussed.

The geothermal surface energy can be divided according to the depth and constant temperature of the subsoil in several types that are directly related to the indicated heat output (Krarti et al., 1995):

• Up to 0.5 m depth: The surface of the soil exchanges heat with the atmosphere and is undergoes daily temperature variations.
• Up to around 10 m depth: Seasonal temperature variations are noticeable in the ground. Some authors consider that in the south of Europe from 5 m depth, the temperature is around 15°C with small variations (e.g. Burroughs, 2003). From 10 m deep and depending on the characteristics of the subsoil the temperature of the ground can remain constant throughout the year.
• At a depth of 15 m: The soil is considered to be at constant temperature all year round with a value that, depending on the external climatic conditions, may be slightly higher than the average annual surface temperature. From this depth, the temperature of the subsoil does not depend on seasonal variations in temperature, only on geological and geothermal conditions.
• Below 20 m depth: The temperature increases at a rate of 3 °C per 100 m (average geothermal gradient), being 25–30 °C at 500 m depth in most of the planet.

With all the exposed so far, it can be said that from 15 m depth the temperature of the subsoil will be constant throughout the year, being higher than the atmospheric temperature in winter and much colder in summer (obviously this will depend on the climatic zone where the geothermal installation is located).

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Shallow geothermal energy: Geological energy for the ecological transition and its inclusion in European and national energy policies.  By Rubén Esteban Pérez.  European Geologist Journal 47 – Geology and the energy transition.