Generate electricity from geothermal systems in residential areas

Near-surface geothermal energy

Near-surface geothermal energy uses the subsurface to a depth of approx. 400 m and temperatures of up to 25 ° C for heating and cooling buildings, technical systems or infrastructure facilities. For this purpose, the heat or cooling energy is obtained from the upper layers of the earth and rock or from the groundwater. In addition to classic forms of application for providing space heating and hot water, near-surface geothermal energy is also used to heat greenhouses and to de-icing points or parking lots.

In Germany, near-surface geothermal energy is used in over 440,000 single or multi-family houses, public institutions, hospitals, schools or commercial operations. Around 20,500 near-surface geothermal systems are added every year (as of 2020).

In Central and Northern Europe have Geothermal probes prevailed as the most common system types. Geothermal probes are drilled as vertical boreholes, into which the pipes are embedded and fixed in place with a kind of cement. In Germany, double U-pipes made of polyethylene are mostly used for this. These are filled with a heat transfer fluid, usually water with a special antifreeze agent, which absorbs the heat from the ground and transports it to the surface to the heat pump. In this country, geothermal probes are usually installed at a depth of 50-160 meters. One or two holes are sufficient to heat a single-family home. Complete residential areas can also be supplied in this way. At around 12 centimeters, its diameter is comparable to a CD and the space required is therefore very small.

For larger systems, for which many geothermal probe holes have to be drilled, a so-called thermal response test is carried out before such a probe field is created. It provides data about the subsurface such as the thermal conductivity of the soil. In this way, a geothermal planner can calculate how many boreholes and with what depth are required. As a result, drilling meters and thus costs can be saved and it can be ensured that the performance of the individual probe bores is not impaired.

A technical variant of the geothermal probes are so-called CO2-Ground heat pipes. They consist of a pressure-resistant, flexible stainless steel or copper pipe that is filled with liquid and gaseous carbon dioxide. Like conventional probes, they are inserted vertically into the earth. The pipe contains liquid carbon dioxide, which evaporates from the ground as it absorbs heat. Because of its low density, it rises again inside the pipe without having to be pumped. In the head of the pipe the CO2 the heat is withdrawn, the gas becomes liquid again and flows in the probe back to the bottom of the probe. Then the cycle starts all over again. CO2-Probes are used, among other things, in the railway network, where they keep tracks and switches free of ice. Since no additional energy is required for this, CO2- Probes have a decisive cost advantage compared to conventional point heating systems based on natural gas.

Groundwater heat pumps and wells

Due to the constant groundwater temperatures of 8-11 ° C in Germany all year round, groundwater can represent an energetically efficient heat source, depending on the hydrogeological conditions on site. In settlement areas, the groundwater temperatures are even slightly higher.

Two wells around 20 meters deep are required. The groundwater is pumped to the surface through a well. There, the heat from the groundwater is transferred before the water is returned to the ground via an injection well. Well systems require a certain degree of care and often filter devices to prevent foreign substances in the water from clogging the suction wells.

Groundwater heat pumps can therefore usually only be set up economically if they have a minimum size (approx. 35 kW heat requirement). Then, however, they are very cheap due to the comparatively high heat output per well bore. For larger buildings, groundwater heat pumps are therefore an interesting alternative. If there is enough groundwater available, groundwater well systems in conjunction with heat pumps can also be used to supply entire residential areas.

Geothermal collectors are laid horizontally at a depth of 80 - 160 centimeters in serpentine lines. As in geothermal probes, a mixture of water and antifreeze flows here. At these depths, the seasonal temperature fluctuations have an impact on the subsurface temperatures. The usable temperatures are thus lower in winter than with geothermal probes, but are sufficient for efficient heat pump operation. The collectors should be laid in a subsurface that can hold moisture. Buildings should be avoided as the heat supply from the rainwater is also used by the collectors to supply heat. A variant are spiral collectors, so-called geothermal baskets, which are inserted into the ground at appropriate intervals and usually require less excavation work.

Concrete components in contact with the ground and energy piles

Concrete components or foundation piles can not only be used as load-bearing or architectural elements, but also for heating and cooling purposes. For this purpose, heat exchanger pipes are inserted into the concrete during the construction of the building. The term “energy pile” has established itself for this technology. The economic advantage arises primarily from the fact that components that have already been planned are used and therefore the additional effort is relatively low. Energy piles are used in particular in large office buildings.

How the geothermal heat pump works

The heat pump contains a working fluid that evaporates at very low temperatures. This gas is compressed by an electrically driven compressor. This increases the pressure and the temperature. A heat exchanger absorbs the heat and transfers it to the heating system. The gas cools down and liquefies. The pressure is reduced via an expansion valve. A refrigerator works the same way, except that the heat is transferred from the inside to the outside.

The efficiency of ground-coupled heat pumps results from their annual performance factor. This indicates the amount of heat that is generated from one unit of drive energy, e.g. B. electricity is generated. Today, annual performance factors of up to 4 or 5 are achieved with earth-coupled systems, i.e. H. with 1 kilowatt hour of electricity, 4 or 5 kilowatt hours of heat can be provided.

Heat pumps work most efficiently with a flow temperature of up to 45 ° C. It therefore makes sense to combine it with surface heating (e.g. underfloor heating or wall heating) or with fan convectors, which are radiators in which fans distribute the heat in the room.

Inexpensive air conditioning directly from the earth

The subsurface can be used not only for heating, but also for cooling. In contrast to winter, the subsurface is cooler than the ambient air in summer. This effect can also be felt in caves. Only a few additional measures are necessary to take advantage of this pleasant coolness.
Air-conditioning can be conducted directly from the ground into the building via geothermal probes, energy piles, etc. Only the heat transfer fluid circulating in the system is used, or pumps are used to circulate it in the building. The energy consumption is limited to the power consumption of these pumps. Conventional units for generating air conditioning are no longer necessary. With 1 kWh of electrical energy, up to 100 kWh of thermal energy can be provided. This variant is called passive cooling. It is a very inexpensive alternative to conventional air conditioning units.