Seasonal heat storage addresses the natural seasonal imbalance between heat supply and heat demand. During the summer months, excess heat, such as that generated from cooling buildings, is stored. In winter, this stored heat is typically used for heating purposes, often with the assistance of a heat pump. In geothermal seasonal storage, the subsurface soil and rock, as well as the groundwater circulating within it, serve as storage media. The subsurface is accessed via boreholes or geothermal probes. If groundwater plays a significant role in the storage, the system is referred to as an aquifer storage.
When using geothermal probes, heat is injected and extracted via the closed fluid loop within the probes. The surrounding rock and groundwater act as the storage medium. A probe-based storage system can consist of a single or a few probes and can be used on a small scale, for example in a single-family home. For larger heat or cooling demands, extensive networks of geothermal probe fields with many probes can supply entire new residential areas.
Aquifer storage systems are charged and discharged directly through groundwater exchange. Typically, the doublet principle is applied, where the two wells alternate seasonally between production and injection. Depending on the hydrogeological conditions, heat and cold storage can utilize the same aquifer, but an adequate distance between production and injection points is required for optimal efficiency. If multiple groundwater horizons are available, different layers can be used for heat and cold storage.
The heat source for summer storage can include solar energy, unused waste heat from power plants or industrial processes, or simply the return flow from cooling circuits. Large-scale systems combined with solar thermal energy have been implemented, for example, in Neckarsulm with a probe field of over 500 probes, and in Crailsheim with a probe field of about 100 probes. The most prominent example of an aquifer storage system is at the Reichstag in Berlin, where the waste heat from the building’s combined heat and power plant is stored in a 280–315 m deep aquifer during the summer. Cooling is stored in a groundwater horizon approximately 60 m deep.
Heat supply from shafts or tunnels represents an effective integration of underground construction and geothermal energy utilization.
Heat can be extracted directly from water seeping into tunnels or from tunnel air, with temperatures depending on the thickness of the rock cover above the tunnel, according to the geothermal gradient. In transalpine tunnels beneath high mountain masses, temperatures of over 30 °C can easily be reached. Numerous examples exist in Switzerland, where one of the first tunnel geothermal systems was commissioned in 1979 at the Gotthard Road Tunnel to supply heat to a highway maintenance facility. Today, many tunnels in Switzerland use geothermal heat for district heating networks, swimming pools, and even more unusual applications, such as the tropical house and sturgeon farm for caviar production in Frutigen. A 1995 study by the Swiss Federal Office of Energy indicated that the 15 most productive tunnels completed at that time had a combined usable capacity of around 30,000 kW. With recent major projects like the Gotthard Base and Lötschberg tunnels, even higher energy yields are expected.
Another significant application, especially in large cities in Germany and across Europe, is heat extraction from the walls of road and subway tunnels or from wastewater pipes. In these cases, the tunnel or pipe walls serve as heat exchanger surfaces. Similar to the layout of shallow geothermal collectors, an absorber system runs through the walls of the tunnels or sewer pipes. For road and subway tunnels, most of the heat comes from the surrounding earth and rock, whereas in wastewater pipes, the greater heat potential is found in the wastewater itself. Early successful trials of heat extraction from tunnel walls have been reported in the Hadersdorf district of Vienna, where since 2004 the Hadersdorf secondary school and an adjacent kindergarten have been supplied with approximately 200 MWh of heat per year via a heat pump system. The heat source is a section of the adjacent Lainzer railway tunnel, lined with an energy fleece in the tunnel wall acting as a heat absorber.
In geothermal utilization of mining facilities, heat is extracted using infrastructure originally intended for other purposes. This includes not only mines but also decommissioned oil and gas wells. Oil and gas are often extracted along with water from boreholes ranging from several hundred to a few thousand meters deep, making these old wells suitable for use as thermal water production wells. Through deepening and stimulation measures, these wells can be further developed and even used for electricity generation. This concept has been implemented in the geothermal research project Groß-Schönebeck and in the GeneSys project. Results from GeneSys were used to develop concepts for geothermal heat supply for the Geo Center Hannover.
Decommissioned mines, especially the coal mines up to 1,500 m deep in Germany’s coal regions, also hold significant geothermal potential. During operation, mines already provide substantial amounts of warm air and mine water through ventilation and water management systems. Depending on the depth of the exploited seams, temperatures can exceed 60 °C.
After mine closure, the extensive underground tunnels and ventilation and extraction shafts provide largely developed heat exchange systems with very large volumes. Standard mine decommissioning plans often involve flooding the facilities, which generally allows for relatively simple adaptation for further use as hydrothermal reservoirs.
An example of geothermal utilization of a former coal mine is the Zeche Heinrich in Essen, where for nearly two decades, 22 °C mine water from shaft 3 has been used to supply heat to an adjacent senior residence.
