Experts: Lyesse Laloui (EPFL)
Geothermal energy from various depths can be used for heating and cooling buildings, but also for generating electricity. While Switzerland has a very high density of shallow geothermal energy installations by international standardds, the use of deeper, higher-temperature underground heat sources is still in its infancy. However, geothermal energy could make an important contribution to achieving Switzerland’s net-zero target by 2050. In order to tap its full potential, a coordinated national initiative to better understand the geology and to make use of geothermal energy would be helpful. In addition, ways to recover energy from underground infrastructure such as tunnels or underground car parks would need to be found as a matter of urgency.
Picture: Matt Palmer, Unsplash
Geothermal energy is natural heat energy from the Earth’s interior. It dates back to when the planet formed, but is also continually being generated by the radioactive decay of minerals, and by solar energy absorbed on the surface. In total, 99 percent of the Earth’s volume has a temperature of over 1000 degrees Celsius and just 0.1 percent has a temperature below 100 degrees Celsius. Geothermal systems are technologies that use this energy. One common classification system is based on utilisation depth.If this is between 0 and 400 metres, the systems are classified as shallow; beyond 400 metres, they are considered deep. Shallow systems operate at low temperatures of up to 25 degrees Celsius, while deep systems exploit medium to high temperatures from 25 to 200 degrees Celsius.
Geothermal systems comprise three main components: a heat source, a heat sink, and a heat exchanger that transports the heat between source and sink. Usually, i.e. for heating, the heat source is the ground and the heat sink is a building, but the reverse is also possible. Closed systems use a water-based mixture circulating through sealed pipes as a heat exchanger, while open systems extract or draw in groundwater for heat exchange.
Ground temperatures beyond 10 to 15 metres below the surface are relatively stable: warmer than the atmosphere in winter and cooler in summer. Shallow systems make use of underground temperatures around 25 degrees Celsius, which are tapped using common vertical geothermal boreholes with depths of 50 to 300 metres. In these closed-loop systems, the underground energy reservoir is used for heating and hot water – or in summer for cooling, by reversing the process and storing heat from the building underground. Single-borehole installations are limited to relatively small individual buildings, such as single-family homes, but can be used in almost any geographical location; borefields are needed to supply larger structures. Switzerland has a higher density of shallow boreholes than any other country in the world. In 2021, such geothermal systems supplied 5.5 percent of its heat requirement for heating and hot water.
Deep geothermal systems are based on underground temperatures between 25 and 200 degrees Celsius, and are used for heating, hot water and electricity generation. These systems are suitable for larger building projects, but unlike shallow systems, they are dependent on specific geological conditions. Hydrothermal systems use naturally occurring hot water at depths of 1000 to 4000 metres and extract it for district heating, but also to heat large industrial or agricultural buildings. Such applications are limited though, because of the need for hot water deposits underground. In contrast, petrothermal systems work without underground water; they inject water from the surface through boreholes, to depths of between 4000 and 6000 metres. The water circulates through cracks in the rock, heats up and is pumped back to the surface for use. The thermal energy available at such depths can be used to generate electricity.
The use of geothermal energy has so far been limited mainly by competition from cheaper alternatives and a lack of acceptance, rather than by technical progress. However, as fossil fuel prices rise and understanding of the available geothermal resources increases, geothermal technologies are likely to play an increasingly important role in the future electricity mix, as well as in the heating and cooling of buildings.
After solar energy, geothermal energy is the second most common primary energy source in the world. This renewable energy source is classified as sustainable and causes minimal greenhouse gas emissions. Moreover, it is continually available, regardless of the weather conditions or season. The use of geothermal energy could reduce CO2 emissions and dependence on energy imports, which is significant in view of the current energy crisis and Switzerland’s target of net-zero greenhouse gas emissions by 2050. This is because 75 percent of a building’s energy consumption is attributable to thermal energy, of which more than 50 percent is generated by fossil fuels like oil or gas. The 4.6 terawatt hours of heat generated geothermally in Switzerland during 2021 saved the equivalent of 403,000 tonnes of heating oil and prevented 1,274,700 tonnes of CO2 emissions, which is more than the city of Zurich produces annually.
Tapping the potential of medium-depth and deep heat sources is still in its infancy in Switzerland. In general, there is a need to better understand Switzerland’s geological subsurface and the associated geothermal potential. This would simplify new projects and the selection of suitable new drilling sites, and also minimise risks, such as the danger of triggering earthquakes while drilling for deep geothermal energy, as has already happened. Numerous research teams around the world, including in Switzerland, are working to improve understanding of artificial earthquakes. The Swiss project GEOBEST is monitoring the deep geothermal projects that have been realised to date; the data obtained provides the basis for models with which to predict artificially generated quakes. One risk not to be neglected with regard to tapping a geothermal reservoir is that of getting an insufficient thermal water production rate and/or encountering a deposit temperature that is too low. In such cases, the high initial costs do not pay off. Geothermal boreholes also entail a risk of negatively affecting or polluting the soil and groundwater. In addition, they are in competition with underground gas storage facilities and aspirations to store CO2 underground.
The public is sceptical about the technology and, above all, fear of induced earthquakes is widespread. In particular, there is inadequate awareness of the option of using geothermal boreholes to cool buildings in summer.
Within the current energy strategy, geothermal energy for electricity generation plays only a marginal role. Due to the time-consuming exploration and the uncertain chances of success, it is assumed that only the already planned deep geothermal projects will be realised by 2035. A coordinated national approach could help to better understand the geology and to make use of geothermal energy at local and regional level. The possibility of recovering energy from the built environment should also be given greater consideration and incorporated into planning. This involves the use of geostructures, i.e. building elements that are in direct contact with the Earth and can conduct heat. These include, for example, so-called ‘energy piles’, floor slabs, or walls of basements and underground garages, which act as heat probes. The fears among the general public could be combated by public relations work and transparent communication.