Shallow geothermal energy uses the subsurface up to a depth of around 400 meters and temperatures of up to 25 °C for heating and cooling buildings, technical facilities, or infrastructure installations. The heat or cooling energy is extracted from the upper layers of soil and rock or from groundwater. In addition to conventional applications for providing space heating and domestic hot water, shallow geothermal energy is also used for heating greenhouses and for de-icing switches or parking areas.
In Germany, shallow geothermal energy is used in more than 480,000 single- and multi-family houses, public facilities, hospitals, schools, and commercial enterprises. In 2024, around 15,000 new shallow geothermal systems were added.
In Central and Northern Europe, geothermal probes have become the most common type of installation. These probes are installed as vertical boreholes into which pipes are inserted and then sealed in place with a type of cement. In Germany, double U-shaped polyethylene pipes are typically used for this purpose. They are filled with a heat transfer fluid, usually water mixed with a special antifreeze, that absorbs heat from the ground and transports it to the surface for the heat pump. In this country, geothermal probes are usually installed at depths of 50 to 160 meters. One or two boreholes are sufficient to heat a single-family house, and entire residential areas can be supplied in this way. With a diameter of around 12 centimeters, roughly the size of a CD, the probes require very little surface area.
For larger systems that require many geothermal probe boreholes, a so-called Thermal Response Test is carried out before constructing such a probe field. It provides data about the subsurface, such as the thermal conductivity of the ground. This allows a geothermal planner to calculate how many boreholes are needed and how deep they must be. As a result, drilling meters and thus costs can be saved, while also ensuring that the individual boreholes do not interfere with each other’s performance.
A technical variant of geothermal probes is the so-called CO₂ geothermal pipe. It consists of a pressure-resistant, flexible stainless steel or copper pipe filled with both liquid and gaseous carbon dioxide. Like conventional probes, these are installed vertically into the ground. Inside the pipe, the liquid carbon dioxide absorbs heat from the ground and evaporates. Due to its low density, it rises within the pipe without the need for pumping. At the top of the pipe, the CO₂ releases its heat, condenses back into liquid form, and flows down again to the bottom of the probe, restarting the cycle. CO₂ probes are used, among other things, in railway networks, where they keep tracks and switches free of ice. Since no additional energy input is required, CO₂ probes offer a significant cost advantage over conventional natural gas-based switch heating systems.
Due to the year-round constant groundwater temperatures in Germany of 8–11 °C, groundwater can serve as an energetically efficient heat source, depending on the local hydrogeological conditions. In urban areas, groundwater temperatures are often slightly higher.
Two wells of around 20 meters in depth are required. Through an extraction well, groundwater is pumped to the surface, where its heat is transferred before the water is returned to the ground via an injection well. Well systems require regular maintenance and often use filtration devices to prevent foreign particles in the water from clogging the injection wells.
Groundwater heat pump systems are generally only economically viable above a minimum size (approximately 35 kW of heat demand). However, thanks to their relatively high heat output per borehole, they are very cost-effective at that scale. For larger buildings, groundwater heat pumps are therefore an attractive alternative. If sufficient groundwater is available, groundwater well systems combined with heat pumps can even be used to supply entire residential areas. Similarly, surface water bodies can be utilized in comparable ways.
Geothermal collectors are laid horizontally in serpentine patterns at a depth of 80–160 centimeters. As with geothermal probes, a mixture of water and antifreeze flows through them. At these depths, seasonal temperature fluctuations affect the ground temperature. The usable temperatures are therefore lower in winter than with geothermal probes, but still sufficient for efficient heat pump operation. The collectors should be installed in soil that can retain moisture. Overbuilding should be avoided, as the heat contribution from rainwater collected by the collectors is also used for heating. One variant is spiral collectors, so-called geothermal baskets, which are installed in the ground at appropriate intervals and usually require less excavation work.
Concrete structural elements or foundation piles can be used not only as load-bearing or architectural components but also for heating and cooling purposes. For this, heat exchanger pipes are embedded in the concrete during the construction of the building. The term “energy pile” has become established for this technology. The economic advantage mainly arises from the fact that already planned structural elements are utilized, making the additional effort relatively small. Energy piles are particularly used in large office buildings.
The heat pump contains a working fluid that evaporates even at very low temperatures. This gas is compressed by an electrically driven compressor, which increases its pressure and raises its temperature. A heat exchanger then absorbs the heat and transfers it to the heating system. During this process, the gas cools down and condenses. The pressure is reduced via an expansion valve. A refrigerator works in the same way, except that it transfers heat from the inside to the outside.
The efficiency of ground-coupled heat pumps is expressed by their annual performance factor. This indicates the amount of heat produced over the course of a year from a unit of input energy, such as electricity. Today, ground-coupled systems achieve annual performance factors of up to 4 or 5, meaning that 1 kilowatt-hour of electricity can provide 4 to 5 kilowatt-hours of heat.
Heat pumps operate most efficiently with a supply temperature of up to 45 °C. Therefore, it is sensible to combine them with surface heating systems, such as underfloor or wall heating, or with fan convectors, which are radiators that use fans to distribute heat throughout a room.
The ground can be used not only for heating but also for cooling. Unlike in winter, the ground is cooler than the ambient air in summer, a phenomenon that can also be felt in caves. Only minimal additional measures are needed to take advantage of this pleasant coolness. Cooling can be delivered directly from the ground into a building via geothermal probes, energy piles, and similar systems. Only the heat transfer fluid circulating within the system, or circulated by pumps within the building, is used. The energy demand is limited to the electricity consumption of these pumps. Conventional units for producing cooling are not needed. With 1 kWh of electrical energy, up to 100 kWh of thermal energy can be provided. This method is referred to as passive cooling and represents a very cost-effective alternative to conventional air conditioning systems.
