Research on geothermal energy: using collectors under farmland

Two research projects are under way to investigate how innovative geothermal collectors can be used to harness heat from agricultural land. The scientists involved are also exploring how this heat can be used to supply the local area.

Geothermal energy used on farmland.© Doppelacker GmbH

As part of the research projects, collectors are being installed under farmland where they absorb temperatures of between five and 15 degrees. The heat generated can be used to provide surrounding households and commercial areas with either heating or cooling, while the field above the collectors can be used as normal.

Instead of using conventional district heating, municipalities would also be able to utilise this technology to harness enough geothermal energy to meet their needs and could even operate their own plants. Given the fact that temperatures in soil layers close to the surface are comparatively low, the collectors can be used for either cooling or heating as required. In the future, such systems could also go be used as storage mediums and absorb energy whenever supply exceeds demand.

No interference with land use: fields can be re-used as soon as collectors installed

Due to the fact that ground collectors are installed at shallow depths, they often interfere with other types of land use. This is the reason why this type of collector has typically been installed across small and unused areas. By cooperating with one another across these projects, research and industry are now setting out to change this precedent by developing innovative new ways of using collector technology and harnessing energy.

The aim of the research is to advance and significantly simplify the installation of large-scale geothermal systems at shallow depths. As part of the model project EnVisaGe, which is being coordinated by the University of Applied Sciences in Stuttgart, the project partners have implemented the agrothermal collector technology for the very first time. In the municipality of Wüstenrot (Baden-Württemberg), collectors were installed across an area of around 5,000 square metres. The facility, which is the size of a football field, supplies the entire local area.

While the first collectors were installed using a tractor, in future a special installation machine will be used. This machine is being developed as part of the KollWeb 4.0 research project at the University of Applied Sciences in Dresden under the coordination of Doppelacker GmbH, an engineering company specialising in low-temperature supply systems. With the help of the new technology, near-surface collectors of the desired size can be 'ploughed' into the ground at a depth of two meters with minimal intervention. As the collectors are installed below the depth reached by agricultural machines, the area can be used again for farming immediately after the collectors have been laid.

Collectors offer many advantages: cold heating networks

The heat generated by the collectors is then fed into a 'cold heating network' and then distributed. 'Cold' is used to refer to the temperatures of between five and 15 degrees Celsius found all year round where the collectors are installed – compared the higher temperatures in deeper layers of the earth. Once the energy has been transported through to the consumer, the temperature is raised using heat pumps so that it can be used for heating buildings or hot water, for example. Thanks to the low temperatures, the system can also be used for cooling – either directly or by means of refrigeration machines. By using heating and cooling simultaneously, additional synergy effects are created that can increase the efficiency of heat pumps and chillers.

Another advantage is that traditional district heating networks usually supply heating on the basis of supply. In contrast, the cold heating networks are able to deliver a needs-based supply of geothermal heat for heating and cooling at any time of the day.

As the required temperatures are only generated once the energy has reached the consumer, there is hardly any heat loss during transport – especially compared to district heating networks. Traditional district heating is usually produced as a by-product of fuel-based electricity production using combustibles. Agrothermal collector systems are also able to fully regenerate themselves. This is because the geothermal energy that is extracted from close to the surface largely comes from solar radiation over the course of the year.

The Federal Ministry for Economic Affairs and Energy is initially providing €2.6 million in funding for the 'EnEff: Wärme Kollweb 4.0' project up to the end of 2019. The project 'EnEff:Stadt – EnVisaGe' (municipal network-connected energy supply – Vision 2020 based on the example of the municipality of Wüstenrot) was provided with €3.4 million in funding up to the end of 2017.