What is Geothermal?

Geothermal energy is a form of energy stored as heat below the earth’s surface.


This heat has its origin in different ways. One type comes from heat generated during the formation of the planet and is still reaching the surface from the core. Another type is due to solar radiation and the climate in general, which results in the earth absorbing and accumulating this heat. Finally, another type of this geothermal energy originates from radioactive decay constantly occurring below the surface of the earth.

The geothermal gradient is the ratio that defines the variation of temperature with depth. In general, the temperature increases by 3°C for every 100 meters in depth. This value varies according to physical characteristics and the thickness of the lithospheric plate.

The earth receives energy from the sun, which heats the first few meters of the crust. However, below 10 meters, the temperature is very stable, without seasonality nor climatic dependencies.

Types of Geothermal


Stable temperature throughout the year

Blue: In winter, as the deeper one goes, the temperature increases until reaching a value close to 10°C.

Red: In the summer the reverse happens. The deeper one goes, the temperature declines, until reaching a value close to 10°C.

Green and  yellow: In spring and autumn variations tend to be smaller, reaching a stable temperature of around 10°C as one goes deeper.

This factor is extremely important because it means that from a certain depth, the temperature of the soil and matter beneath it is constant, regardless of time of year and season, and regardless of time of day, be it day or night. Therefore, just a few meters from us we can find a source of constant energy throughout the year and accessible in all places.

In Spain, the average temperature of the subsoil can reach 15-17°C

In the subsoil there is abundant energy that may not be useful for heating or air conditioning, but is useful for a geothermal heat pump, which transforms low temperature energy into high temperature energy useful for air conditioning and heating.

This can be utilized through the use of vertical perforations in closed circuits, open circuits (making use of groundwater), or horizontal systems, as seen below.


The heat pump

The heat pump’s operation is similar to that of a refrigerator.

The evaporator: A refrigerated fluid at a very low temperature absorbs the temperature of the earth or the groundwater by heating and evaporating it. This would be the inside of the refrigerator, as the cold plate placed at the bottom.

The compressor: This is responsible for compressing the gas, in turn raising its temperature

The condenser: Once the gas is heated for its purpose, this heat is exchanged to the emitter circuit (for heating or cooling). In this process, our gas loses the heat resulting in it cooling.

The expansion valve: Once the gas has once again cooled, it is expanded so that it cools further, enabling it to reabsorb heat. In the fridge, this would represent the engine area.

geotermia-6This occurs when the subsoil temperature is between 25°C and 100°C. This occurs when there is a thermal anomaly and the temperature of the terrain is above a normal temperature. Their use tends to be direct; for example in spas.

When subsoil temperature is between 100°C and 150°C, the ground water tends to be in a gaseous state (at normal pressure). Therefore, it can be used in the production of electricity through binary cycle. These types of plants utilize a second working fluid, such as isopentane, freon, isobutene, etc., which evaporate and are used to move the turbine, in turn generating electricity.

geotermia-8High enthalpy deposits in which the existence of a reservoir and a heat source results in the liquid being under the conditions of a pressure and high temperature (generally above 150°C), The thermodynamic characteristics of the fluid allow it to be used in the production of electricity.

Why geothermal?



  • It is an inexhaustible source of energy
  • It is respectful to the environment
  • It is a clean source of energy without direct emissions of CO2 nor other types of waste or greenhouse gases
  • It can be extended and combined with other renewable sources of energy


  • No need to store any fuel, reducing the space needed and eliminating bad odors
  • No external condenser units are required, reducing noise and vibrations emitted
  • It is compatible with existing and radiator and fan coils
  • It does not need a specially conditioned area, nor fireplaces, protection or ventilation
  • It doesn´t cause any negative aesthetic impact
  • It is compact and easy to set up
  • The system is fully tested in central and northern Europe


  • Short repayment period
  • The life of a heat pump is about 20 years, twice as long as a conventional boiler; and that of an exchanger with a radiating floor is around 50 years
  • Maintains constant power throughout the year
  • Revaluation of property in which it is installed
  • It is a system subsidized by administrations
  • All in one: heating, cooling, domestic hot water (ACS) and acclimatization of swimming pools

Mining and Geothermal

Mining activity

During the initial phase of mining, activity was carried out through mountain operations (access to the coal layers was carried out from the surface by means of entrances into the mountain rock at different levels). Subsequently, the mining was developed by means of vertical wells allowing access to new areas through a single point; this point being the well. These wells sometimes joined with each other to form a complex network of galleries beneath the surface of the basin.

In the case of the Barredo well and its different connections, the total length of its galleries amount to 400km!


Once mining activity ceased, all areas that had previously been drained in order to develop the industry became flooded naturally, forming a network of galleries and collapsed areas (corresponding to holes formed in the extraction of coal) in a large underground reservoir.

Physical conditions such as depth (recalling that temperature increases with depth), sealing and insulation provided a certain amount of heat to the water. This high temperature, as well as the volume of water, makes this mine water suitable for geothermal use.

Minería de montaña

Esquema de galerías correspondientes a los pozos Barredo y Figaredo

Caudales medios evacuados entre los diferentes pozos conectados.

Geothermal energy:  Barredo Shaft


In 1844 the Sociedad Fábrica de Mieres was founded, which would go on to mine the coal from the basin of the river Turón and build the Pozo Barredo in 1937. Activity in the well finally ceased in 1994, while drainage work continued. In 2008, the drainage of groundwater was organized, involving controlled flooding and the installation of submersible pumps.

In the case of the Pozo Barredo, due to the geothermal gradient, water stored inside reached temperatures close to 25°C. This temperature is constant throughout the year, without seasonal variation.

It is this energy, stored in the form of heat, which is used to control temperature and obtain cooling or heating, and to obtain hot water.

Available water flow rate

The estimation of total thermal potential represents a value of 36.8 Hm3/year

Regulating capacity

The different works of digitalization and analysis of the mining work have revealed that there is a total gap volume greater than 87.12 Mm3. This huge reservoir inside the mines will allow suitable management of the warehouse, which can calculate variable flow rates through each season.

Water quality

These are bicarbonate-sodium waters, which no fall below pH 7 and do not cause corrosion problems in pipes. Nevertheless, the elevated hardness of these mine waters is a drawback, and therfore the use of heat exchangers is neccesary.


The Future


According with the preliminary studies carried out, the set of mining works located in the central zone of Asturias has an average usable flow rate of about 36 Hm3/year, which could be used directly for different uses or indirectly, as a source of thermal energy.

The stable temperature and relatively low enthalpy of mine water contained in the abandoned and flooded coal mines have the potential to form a geothermal resource to be used in conjunction with heat pumps to provide heating and cooling for buildings.

The HUNOSA Group, due to the water flow from the mining drainage systems and the average water temperatures, has a high potential to supply geothermal energy by heat pump technology and their distribution through an district heating networks.

In comparison to a gas heater, the mine water source heat pump systems are much more efficient and produce much less carbon emission and consumes much less energy from the same heating demand.

The energy contained in the mine water of the Hunosa group is the equivalent  energy consumption by heating in:

21,900 households

87,600 inhabitants


geothermal energy and / or heat pumps

  • Geoexchange.org
    The Geothermal Exchange Organization (GEO) is The Voice of the Geothermal Heat Pump Industry in the United States.
    Spanish Geothermal Technology Platform
    The Institute for Diversification and Saving of Energy (IDAE) is an agency of the Ministry of Industry, Energy and Tourism through the Ministry of Energy.