Heat pump heating

from Wikipedia, the free encyclopedia
Air / water heat pump heating in a single family home
Brine-water heat pump in the basement of an energetically refurbished two-family house
First ground-connected heat pump heating in Germany by Klemens Oskar Waterkotte from 1972
The inside of a heat pump

A heat pump system extracts the environment (ambient air, groundwater / surface water or ground ) heat and lifts it by means of a heat pump to a useable higher temperature to heat so that buildings or other facilities. A distinction is made between electric and gas-powered heat pump heating systems.

Since electric heat pumps do not emit any CO 2 directly , but generate around 25 to 30% of the thermal energy using electrical energy, they can work with very low emissions compared to other types of heating when using a high proportion of carbon-neutral electricity. If, on the other hand, the electrical energy comes from fossil sources, then the ecological advantage over modern gas heating is very small.

When converting to a CO 2 -free heat supply ( decarbonisation ), the heat pump can play a major role if the electricity is generated from renewable sources. As part of the district heating supply, heat sources can be used that do not have a sufficient temperature for direct heat utilization. Industrial waste heat such as cooling water , mine water or near-surface geothermal energy can be used as a heat source. The higher the temperature of the heat source from which heat is extracted in the heat pump's evaporator, the higher the system's coefficient of performance .


The heat pump draws heat from a reservoir (air, groundwater, soil) and thus cools the heat source, but only along a temperature gradient . However, the efficiency of the heat pump - expressed in the coefficient of performance - decreases the lower the temperature of the source.

The heat pump is technically structured like a refrigerator with the difference that with the heat pump the warm side ( condenser of the heat pump) is used for heating. The lower the desired temperature difference between the heat reservoir (for example groundwater of 7 ° C) and the "flow temperature" (= "heating flow" = the temperature at which the water is fed into the heating circuit ), the more efficient it is becomes). As the temperature rise increases, the coefficient of performance of the heat pump decreases. Most heat pumps are designed for flow temperatures up to a maximum of 60 ° C.

Heat sources for heat pumps are water, moist soil or moist air. If the evaporation temperature falls below 0 ° C, ice forms on the heat exchanger surfaces . Ice is an insulating layer and significantly worsens the heat transfer. Thanks to newer technologies (gas cooling), heat pumps that extract heat from the outside air can currently be used effectively down to an outside temperature of −25 ° C. A heat pump that extracts heat from a water tank at a depth of 10 m (approx. 5 ° C earth temperature) can be operated independently of the outside temperature (below the freezing point of water, because ice is lighter than water and therefore floats on the surface).

For the heat yield, power must be applied ("input"). The ratio of yield (“output”) to input is called the performance figure. A performance figure greater than 4 is considered economical.

The energy can be supplied by means of electricity or fuels . The combustion of fuels can operate an absorption or adsorption refrigeration machine or be used in internal combustion engines which, like the electric motor, drives a compression refrigeration machine.

Technical details

Electric compression heat pumps are usually used in the lower output range to heat buildings, and gas engine heat pumps are also used for higher outputs. Absorption or adsorption heat pumps are also used. Heat pumps that use the Vuilleumier cycle are not yet ready for the market.

The functional principle can be compared with a refrigerator that cools inside and heats outside. Many of these systems can also be used for cooling in reverse operation. Since heat pumps sometimes have considerable starting currents, which can lead to network perturbations (voltage drops ), the connection must be approved by the energy supply company . The approval is usually granted with certain conditions ( starting current limitation , starts / hour limited).

The compressed refrigerant condenses in the condenser . This is a heat exchanger to which a heat transfer medium , usually water or a water-brine mixture (frost protection), is applied on the opposite side . The heat released when the refrigerant liquefies is absorbed by the heat transfer medium and transferred to the radiators or heating surfaces. The heat output that can be used at the condenser in relation to the electrical output of the compressor motor increases as the difference between the evaporation and condensation temperatures in the heat pump's refrigerant circuit decreases.

The ratio of heat output ( "Output") for electrical power ( "Input") is a coefficient of performance (a heat pump English. Coefficient of Performance , abbreviated COP ), respectively.

A low heat transfer medium temperature (flow temperature) can be implemented in particular with underfloor heating, as the heat transfer surface is very large. In addition, very good thermal insulation must be aimed for in the building to be heated in order to be able to run a low flow temperature of the heat transfer medium when there is little heat demand.

The heating surface and mean excess temperature (temperature differences ) of a radiator or underfloor heating are indirectly proportional to one another. This can be compared with the changed power output of storage tanks with increasing primary temperatures. This problem also causes the storage tank temperature to only be raised to a certain temperature by means of a heat pump. The maximum hot water temperature that can be generated depends on the maximum compressor high pressure.

When heating storage tanks with geothermal probes, it must be ensured that the geothermal probe is not loaded with more than 100 W (thermal) / m probe in order to avoid excessive icing of the probe. Since ice is a poor conductor of heat , the probe temperature drops too far and the coefficient of performance falls into the uneconomical range below 2.5.

Choice of refrigerant

Heat pumps available on the European market today for both household and industrial use are almost exclusively operated with HFCs (fluorinated hydrocarbons). Systems with refrigerants that are less problematic for the environment, such as B. CO 2 or propane have not yet found widespread use. Studies have shown that CO 2 can be used to generate high flow temperatures and achieve higher annual performance factors than with conventional systems. It is also non-flammable and less toxic. In Japan, CO 2 air-to-water heat pumps have been available on the market since 2001 ; For some time these have also been offered sporadically in Europe. When using CO 2 , components are required that can be operated at higher pressures. For this purpose, research projects are ongoing B. at the Technical University of Braunschweig (Chair of Thermodynamics) and the Technical University of Dresden . CO 2 air-to-water heat pumps can in part displace very expensive competing systems (geothermal probe, ice probe, brine heat pumps, etc.) in application niches.

Blocking times

When using a favorable heating tariff , the energy suppliers can switch off the heat pumps at times of peak load , for example in the morning and in the early evening, according to the technical connection conditions (TAB) up to three times a day for two hours each (also remotely controlled). However, many energy supply companies (EVU) can deviate from this option downwards, as they control the blocking times using the ripple control receiver based on the actual load profile . The blocking times are then relatively short, so that an increased technical effort ( e.g. buffer storage ) is usually not necessary for bridging the blocking time.

Buffer storage tanks can only be used to a limited extent for bridging blocking times, as the utility company does not give a pre-signal for the switch-off time of the heat pump. Therefore, the temperature sensor in the buffer tank could just give the "On" signal to start the HP when the blocking time occurs. If this occurs, there is no or only a slight usable temperature gradient in the buffer tank. The probability that a building will cool down due to a blocking period is relatively low, but possible to a limited extent (cooling down 1–2 K).

  • A building with only a small amount of storage mass cools down faster than one with large storage masses;
  • a badly insulated building cools down faster than a well insulated one;
  • a large building cools down more slowly than a small one (better ratio of building surface to enclosed space).

Heating element control

In the event that the output of the heat pump is insufficient at low ambient temperatures and at the same time high heat demand, most heat pump heating systems have a simple electric heater ( immersion heater ) in the hot water circuit or storage tank.

The technical connection conditions (TAB 2007) stipulate in chapter 10.2.4 that the compressor and heating element may only be switched on six times per hour. Manufacturers implement this regulation by taking a ten-minute break after switching off. This fact must be taken into account during planning and design.

The temperature lift of the heating rod is determined by the mass flow , the specific heat capacity of the fluid and the heating rod output .

With water as the fluid , the temperature lift is at a mass flow rate of 1000 kg per hour per kW of heating rod output.

With a small switching hysteresis and a large temperature swing, the heating element is only switched on for a few minutes and switched off for at least ten minutes. The supposedly high connected load of the heating element cannot develop. By rearranging the above formula according to the time:

If the switching hysteresis is 1 K, the heating rod output is 1 kW and the mass of the water is 1 kg, the heating rod is switched off after 4.176 s.

Key figures

Performance figure and grade of quality

The performance figure ε - also known as the Coefficient of Performance (COP) - is used to assess heat pumps . It is the ratio of the heat output emitted to the drive power used by the compressor (also known as the compressor). The achievable coefficient of performance is a function of the temperatures used in accordance with the second law of thermodynamics limited to the reciprocal value of the Carnot efficiency for a lossless force heat engine, the Carnot coefficient of performance:

The ratio of the actual to Carnot performance figure is the quality grade . This is how the performance figure is calculated

Electric compression heat pumps for building heating achieve quality levels of around 50% in continuous operation under specified standard operating conditions. This value is mainly used to assess the quality of the heat pump itself. It does not take into account the rest of the heating system.

For a heat pump with a geothermal probe (evaporation temperature , approx. 0 ° C) and underfloor heating ( approx. 35 ° C flow temperature) one can calculate, for example:

If a radiator heater with 55 ° C ( ) flow temperature (evaporation temperature –0 ° C) is connected to the same heat pump circuit, the result is a significantly lower coefficient of performance:

When using a geothermal probe as a heat source, the evaporation temperature is independent of the ambient temperature.

A heat pump that uses the ambient air as a heat source has a significantly lower evaporation temperature than a system with a geothermal probe. As the heat demand increases, the ambient temperature and thus the coefficient of performance also decrease. In addition, the heat transfer coefficient from air to the evaporator surfaces is low. Ribbed tubes with the largest possible area are therefore used in the evaporator. A fan or fan is required to push the air through the evaporator surfaces.

If the dew point is frequently not reached in the evaporator, the condensate (water) that forms must be removed. If the condensate falls below the freezing point in the evaporator, the yield factor drops to zero due to the insulating effect of the ice cover. De-icing systems are energetically nonsensical, the same amount of energy is supplied that was previously extracted from the frozen condensate.

In the following calculation of the COP, an outside temperature of around 7 ° C and a temperature difference of 12 ° C between the air inlet temperature and the evaporation temperature of the refrigerant are assumed. With (equal to about −5 ° C) for the cold side we get:

It becomes clear that the coefficient of performance of a heat pump is strongly influenced by the design of the heat exchanger, condenser and evaporator. The icing of the evaporator is not considered. The system in the example calculation is only useful for outside temperatures greater than +12 ° C.

With the geothermal probe, a heat source with a relatively high temperature is available regardless of the prevailing outside temperature, while the outside air is an unfavorable heat source. On the side of the heat sink, a small temperature difference between room temperature and heat transfer medium flow temperature should be aimed for with the largest possible area. In the examples shown, the coefficient of performance varies by a factor of 1.8 between the geothermal probe / underfloor heating heat pump and the outside air / radiator heat pump.

Under standard conditions, commercially available heat pumps achieve COP values ​​in the range of 3.2 to 4.5 with ambient air as a heat source and 4.2 to 5.2 when using geothermal energy, and the trend is rising.

Annual performance factor (JAZ)

The so-called annual performance factor (JAZ), also called Seasonal Performance Factor (SPF), is used to evaluate the energetic efficiency of a heat pump heating system. It indicates the ratio of the heat released over the year to the drive energy consumed and should not be confused with the coefficient of performance determined under standardized laboratory conditions . To ensure comparability, it is important to be clear about the system boundary . The annual performance factor can contain the additional energy expenditure for the auxiliary drives (brine pumps, groundwater pumps or air fans, etc.), which, if incorrectly designed, make up a considerable part.

The annual performance factor is calculated using the following formula:

Many factors influence the annual performance factor. For example, manufacturers supply hardware and software of varying quality. The same applies to the work of installers. Furthermore, the temperatures under which the heat pump has to work change over the course of the year. On the sink side, for example, building heating usually dominates in winter, with a comparatively low temperature, whereas in summer, domestic hot water preparation with comparatively high temperatures. The entire design of a heat pump heating system, e.g. B. the depth of the geothermal probe, the choice of storage or heat distribution system, has an influence on its efficiency. Temperature fluctuations can also be observed on the source side, but these are strongly dependent on the source. The air temperature fluctuates strongly in the daily and seasonal course, but the soil and groundwater temperature hardly. The location and the climate are also relevant.

The JAZ in Germany is in the order of 3 to 4.5, in the case of groundwater systems it is also over 5. Outliers in both directions are possible.

Ecological balance

Heat pumps play an important role in sustainable heat generation, which is a fundamental part of the energy transition . Most studies on the subject come to the conclusion that heat pumps must play a central role in a climate-friendly energy system. The reason for this is that both decentralized heat pumps and large heat pumps in district heating systems reduce overall costs. They can also help to better integrate renewable energies into the energy system and, together with them, to decarbonise the heating sector .

If the electricity required to operate electrically driven heat pumps is obtained from emission-free sources such as hydropower plants or wind turbines , they can be used to generate efficient and climate-neutral heating. Of all the individual technologies currently available on the market, heat pump heating is considered to be the one that could possibly make the greatest contribution to global greenhouse gas reduction in the future. The IEA assumes that the use of heat pumps alone can reduce global greenhouse gas emissions by 8% if 30% of the buildings are heated with heat pumps instead of fossil-fueled heating systems. The conversion of global heat generation to heat pump heating systems, which are supplied with electricity from renewable energies, would at the same time mean a considerable increase in global energy generation from regenerative sources and increase the efficiency of the energy system.

The environmental compatibility of a compression heat pump is influenced by several factors:

  • Type of electricity generation (CO 2 balance, pollutant emissions),
  • Type of gas generation (extraction, import, biogas processing ),
  • Losses in the conduct of electricity,
  • Annual performance factor of the heat pump,
  • Global warming potential of the refrigerant.

Greenhouse gas balance

The way in which the electricity required for operation was produced is decisive for the eco-balance of the electric heat pumps. Whether carbon dioxide is saved depends in particular on the annual performance factor and the emission intensity of electricity generation. Different fuels in power plants and domestic heating systems and their emission factors have to be taken into account, so that even with the same primary energy requirement, the CO 2 emissions during power generation are higher (e.g. with a focus on coal-fired power generation) or lower (e.g. due to a high proportion of renewable energy or Nuclear energy) can fail. In countries with a high proportion of emission-free energy generation such as B. Austria , where hydropower is the dominant source of electricity, already saves carbon dioxide emissions with an annual coefficient of performance of 1.0, in Estonia, however, only with an annual coefficient of performance of 5.1. In Germany the value is 2.2. A gas boiler with an efficiency of 95% and emissions of 213 g / kWh was used as a comparison value.

A study by the Technical University of Munich came to the result that the heat pump load profile shows a temperature-dependent, but almost even distribution of the electricity demand over the day. Therefore, the electricity demand of heat pumps is currently tended to be covered by base load power plants, especially coal-fired power plants. In the future, renewable energies would provide a large part of the electricity. The study assumes a JAZ of 3.1 and 3.9 for air / water or brine / water heat pumps. According to this, heat pumps currently reduce the use of fossil primary energy sources compared to other heating systems by 30% to 52%, while the savings in carbon dioxide emissions are between 14% and 45%. For 2030, primary energy savings of 73% to 83% and greenhouse gas savings of between 56% and 78% compared to oil and gas heating are expected.

If gas is used as the primary energy source to drive the heat pump, a distinction must be made between fossil natural gas and biomethane upgraded to natural gas quality . In the case of biomethane, the heating operation only releases as much CO 2 as was absorbed from the atmosphere when the plants were growing before fermentation in the biogas plant. By the end of 2017, almost 200 biogas processing plants with an installed feed-in capacity of 1 billion m³ / a were in operation in Germany.

Heat pumps can contain climate-damaging refrigerants such as R134a ( 1,1,1,2-tetrafluoroethane ), R404A (replacement refrigerant for R502 and R22 ( chlorodifluoromethane )), R407C (replacement for refrigerant R22) or R-410A . One kilogram of this refrigerant develops the same global warming potential as 1.3 to 3.3 tons of CO 2 . Incorrect recycling can lead to the release of these substances and corresponding greenhouse gas emissions. However, there are also climate-friendly alternatives such as R744 , R290 , R600a or R1270 .

Primary energy balance

How much primary energy is saved results from the primary energy balance of electricity generation. With the primary energy factor of 1.8, which has been in force in Germany since 2016, heat pumps with a heat source from outside air now also save primary energy compared to gas condensing boilers . Average values ​​for Germany are given in the table below. The greatest savings result when the heat pumps are operated with renewable energies , which produce electricity directly without thermodynamic losses. In the case of fossil power plants, gas and steam power plants ( combined cycle power plants ) perform best. Heat pumps with a JAZ of 3.5, which are operated with electricity from a gas-fired power plant, deliver 3.5 kWh of thermal energy with a primary energy input of 1.7 kWh.

Specific primary energy requirement for the production of one kWh of electrical energy
power plant Primary energy use electrical energy obtained from it Useful heat at JAZ 3.5
coal-fired power station 2.4 kWh 1 kWh 3.5 kWh
Combined cycle power plant 1.7 kWh
Hydroelectric power plant , wind power plant , photovoltaic , solar thermal power plant 1 , 0kWh
Nuclear power plant 3 , 0kWh
Power plant mix in Germany 2.4 kWh

Regardless of this primary energy consideration, heat pumps can also contribute to a reduction in certain pollutant emissions (carbon dioxide, nitrogen oxides, fine dust, sulfur compounds, etc.), because when using solid and fossil fuels ( hard coal , lignite ) in the power plant, a highly effective flue gas cleaning (at least with the same fuel) usually specifically lower emissions than local combustion causes.

System types and heat sources

Heat pump heating systems can be roughly categorized according to their heat source:

  • Air (outside air or exhaust air heat pump, if necessary with preheating via geothermal heat exchanger)
  • Geothermal energy (heat recovery via geothermal probes or collectors, see below)
  • Water (heat recovery from groundwater , surface water or wastewater)
  • Sun ( solar system heats brine storage)

A distinction is made according to the medium of heat generation and dissipation:

In low-energy houses, exhaust air (e.g. compact ventilation devices in passive houses ), wastewater and solar heat are increasingly used to generate energy, and in trade and industry also the process heat that occurs anyway. Several sources can also be combined in a heat pump system, for example via a common source-side brine circuit.

In addition to systems that heat individual houses, heat pumps can also be integrated into district or local heating networks . Such systems usually have an output in the MW range and are considered to be an important technology for linking the electricity and heating sectors in future energy systems with a high proportion of renewable energies, especially wind energy . In such systems, heat pumps play the role of supplying heat during times of high regenerative power generation, so that no (fossil) -powered boilers or heating plants have to be operated, which can increase energy efficiency . Such large heat pumps can be operated with different heat sources; among others are low-temperature waste heat from industry, supermarkets , wastewater (eg. for example, from sewage treatment plants ), drinking water, industrial water and groundwater, rivers, lakes and the sea water as a heat source in question.

Types of heat pumps according to heat sources

Air heat pump

An air heat pump uses the outside air warmed by the sun to heat and process the hot water. At particularly low outside temperatures, the efficiency drops sharply. By bivalent - parallel operation of heat pumps with certain combination systems, the efficiency can be increased by switching on an alternative heating system in these cases in order to deliver the required peak load. Of course, this increases the costs. The term air source heat pump is used for different systems. Therefore, the following is usually divided into more differentiated terms:

  • Air-water heat pumps extract heat from the ambient air via a heat exchanger and transfer it to the existing heating and / or hot water circuits (underfloor heating, radiators, etc.).
  • Air-to-air heat pumps extract heat from the air and make it available to an air heating system or a ventilation system.
  • With the direct heat pump, heat is extracted from the air, which is fed into the underfloor heating pipes laid in the heating screed by means of direct condensation without additional heat exchanger losses. Unlike other air heat pumps, a refrigerant flows directly through the copper pipes of the floor heating. The direct heat pump has no circulation pump and no secondary heat exchanger. A direct heat pump is only suitable for new builds. The disadvantage is that it is almost impossible to control individual heating circuits.

Compared to other heat pumps, air source heat pumps are usually cheaper to buy, but more expensive to operate. Air-water heat pumps can be installed outside and inside buildings, depending on the make. The efficiency of the air source heat pump decreases the lower the outside temperature becomes. Air heat pumps can be installed in refurbished old buildings and in new buildings with surface heating circuits and operated in both monovalent and bivalent operation (see above section on refrigerants). The noise pollution of the environment is also relevant, which often makes installation near buildings problematic. A typical sound pressure level at a distance of one meter of for example 51 to 62 dB (A) (data sheet Viessmann Vitocal 300-A) is perceived as very annoying. In order to assess the noise emissions from air heat pumps, in the absence of a separate, norm-specific administrative regulation, the TA-Lärm responsible for industrial and commercial noise is used, which provides for different immission guide values ​​depending on the residential area designation. In common residential areas, 55 dB (A) during the day and 40 dB (A) at night in front of the window of a living room in need of protection (living room, bedroom, study and kitchen with dining table).

The annual performance factor of the modern LW-HP can be improved by using inverter technology. Nevertheless, there are still large deviations between the annual performance factors calculated according to VDI 4650 and the values ​​that can be achieved in practice.

Orientation values:

  • Underfloor heating flow temperature 30 to 35 ° C
  • Radiators / radiators flow temperature 50 to 55 ° C

Geothermal heat pump

Geothermal heat pumps use the sensible heat from the earth's body as an energy source. The heat extracted is mainly compensated for by the heating of the earth's body by solar radiation and rainwater. Only a small proportion comes from the interior of the earth.

In Germany, an ambient temperature of 0 ° C for near-surface geothermal collectors and 8 ° C for groundwater and deep geothermal probes is usually assumed for the calculation.

If the design is inadequate, brine heat pumps can freeze the ground in winter (see permafrost ).

Geothermal probes
(e.g. CO 2 probes) are boreholes in the ground up to several 100 meters. Most of the drilling is done up to 50 meters. If the output of a geothermal probe is insufficient, several boreholes are made based on the calculated extraction capacity. Boring is a simple and reliable method of operating a heat pump, as the entire garden does not have to be dug up (as is the case with collectors) and the extraction capacity is highest. The high costs for the drilling are disadvantageous.
Geothermal collectors
are " heating coils " laid in the ground at a shallow depth (approx. 1 to 1.5 m, spacing approx. 1 m) . The heat is mainly brought in by solar energy and seeping rainwater, which is why collectors close to the surface should not be laid under sealed surfaces. Only when the groundwater level is very high does this also contribute to heat recovery.

The extraction capacity depends, among other things, on the thermal conductivity and water storage of the soil, as well as on solar radiation and soil moisture. Collectors close to the surface should be planned so that the sensitive ground heat is sufficient for supply. When the surrounding area is iced up, additional amounts of heat (latency heat) can be extracted, but when the brine temperature falls (the electricity requirement increases by approx. 2.5% per degree Celsius).

Spiral collectors

Spiral collectors and geothermal baskets as heat exchangers require less space than geothermal collectors installed over a large area and are cheaper than deep drilling. There is also no need for an access option for a geothermal deep drilling rig.

Geothermal energy recovery from tunnels

Tunnels are increasingly being used to generate geothermal energy. Either via automatically flowing water or via brine pipes in the tunnel walls. According to a study by the Swiss Federal Office for Energy from 1995, around 30 MW of heat could be obtained from 130 of the 600 tunnels and galleries in Switzerland .

Water heat pump

Groundwater heat pump (water-to-water heat pumps)

Here, groundwater is taken from a delivery well and fed back through a so-called intake well . Here the quality of the water is of crucial importance for the reliability of the system. Either the groundwater is fed directly to the heat pump through the evaporator heat exchanger, or a heat exchanger is first connected between the groundwater and the evaporator heat exchanger. Before installation, a water sample should be taken and compared with the requirements of the heat pump manufacturer. Due to the groundwater temperatures, which are often around 7 to 11 ° C throughout the year, groundwater heat pumps can achieve annual performance factors of over 5. The problem is the clogging or corrosion of the system parts through which the groundwater flows in the case of water containing iron and manganese. As a rule, a permit under water law (water authority) is required, since the operation means an intervention in the groundwater balance.

Surface water heat pump

The water from seas, rivers and lakes is also suitable as an energy source for operating heat pumps. The potential of such heat sources is considered to be very large: With temperature fluctuations of ± 0.2 ° C, a heat output of one GW can be obtained from Lake Constance alone. The first such systems were installed in Lake Constance in the 1960s, but they are not yet very widespread in Germany, while there are significantly more systems in Switzerland and the use of the Alpine lakes for heat generation is being promoted politically. Such systems are also common in Scandinavia and Japan. In Great Britain it is assumed that several million households could be heated by means of heat pumps that get their energy from rivers and lakes. According to plans by the Ministry of Energy, a total of 4.5 million households are to be heated with heat pumps. A first system that generates heat from the Thames for over 100 households and other affiliated businesses went into operation in March 2014. As of 2016, the largest heat pump system that uses seawater is located in Stockholm . It supplies a district heating network to which 2.1 million people are connected and has an output of around 420 MW.

Sometimes geothermal energy from swimming ponds , so-called "energy ponds ", is used as a heat source or from salt water-filled "solar ponds".

Sewage heat pump

A sewage heat pump is installed in the sewer system and uses the heat from sewage . Larger sewer pipes are particularly suitable for use. With these, however, high performance can also be achieved. In the sewer system, temperatures are largely evenly between 12 and 20 degrees Celsius over the course of the year. The earth around the pipes also insulates, which means that peak loads can be buffered. Larger systems that heat administration centers, hospitals, schools, housing estates or indoor swimming pools with a relatively constant heat requirement are considered economical. In the future, there are plans to temporarily store waste heat from industrial processes or power plants in the sewer system and to retrieve it when required using a heat pump.

Cold local heating

Cold district heating or cold district heating is a technical variant of a heat supply network that works with low transfer temperatures close to the ground temperature and thus well below conventional district or local heating systems . Since these operating temperatures are not sufficient for the production of hot water and heating, the temperature at the customer is raised to the required level by means of heat pumps. In the same way, cold can also be produced and the waste heat fed back into the heating network. In this way, affiliates are not just customers, but can act as prosumers who, depending on the circumstances, can either consume or produce heat. Like conventional geothermal heat pumps, cold local heating networks have the advantage over air heat pumps that they work more efficiently due to the lower temperature delta between the heat source and the heating temperature. Compared to geothermal heat pumps, however, cold local heating networks have the additional advantage that even in urban areas, where space problems often prevent the use of geothermal heat pumps, heat can be stored seasonally via central heat storage , and the different load profiles of different buildings, if necessary, a balance between heat and Enable cooling demand.

They are particularly suitable for use where there are different types of buildings (residential buildings, businesses, supermarkets, etc.) and there is thus a demand for both heating and cooling, which enables energy to be balanced over short or long periods of time. Alternatively, seasonal heat storage systems enable the energy supply and demand to be balanced. By using different heat sources such as B. Waste heat from industry and trade and the combination of heat sources and heat sinks can also create synergies and the heat supply can be further developed in the direction of a circular economy . In addition, the low operating temperature of the cold heating network enables otherwise hardly usable low-temperature waste heat to be fed into the network in an uncomplicated manner. At the same time, the low operating temperature significantly reduces the heat losses in the heating network, which in particular limits energy losses in summer, when there is only little heat demand. The annual coefficient of performance of heat pumps is also relatively high, especially compared to air heat pumps. An investigation of 40 systems commissioned by 2018 showed that the heat pumps achieved an annual coefficient of performance of at least 4 in the majority of the systems examined; the highest values ​​were 6.

Heat pumps according to the type of drive

As described above, part of the heating output of heat pumps is usually achieved through compression . The refrigerant heats up dissipatively due to the higher pressure and is then used for heating. Depending on the application, different types of drive may be more suitable.

Electric motor

The most common variant found in single-family homes is the electric motor. A motor controlled by a frequency converter drives a scroll or screw compressor . The advantages of the pure electric heat pump lie in the low possible power range of the systems and in the exclusive use of electricity as an energy source. There is no need for a chimney or fuel supply. Disadvantages are the lower efficiency at low ambient or groundwater temperatures and the one-sided dependence on the power grid.

Gas engine

Functional principle of a gas engine heat pump

Gas engines can be used for larger objects such as apartment buildings, commercial operations or supermarkets . Gas-Otto engines adapted for operation with natural gas or other gases (propane, butane, etc.) are installed as compressor drives. The somewhat more complex systems only pay off in larger buildings or in local heating networks, but offer three additional, simultaneous ones with the engine waste heat (between 75 ° C and 90 ° C) and the heat in the exhaust gas (several 100 ° C possible) as well as the cooling at the evaporator Temperature circuits for a wide variety of applications. By using biomethane processed into bio natural gas as fuel, gas engine heat pumps can also be operated in a CO 2 -neutral manner.

Another functional principle uses a fuel gas to drive the hot part of a Stirling engine, which on the cold side serves directly as a compressor for a refrigerant circuit without any further lossy conversions. With this principle, the performance range for single-family houses can also be covered in an economical and environmentally friendly manner. Due to the higher flow temperature compared to compression heat pumps driven by an electric motor, this technology is particularly suitable for the renovation of old existing buildings.

Gas burner

If a gas burner is used for thermal compression of the refrigerant ( sorption heat pump ), the dependency on the power grid can be further reduced. Here only a small, electric pump is used for pre-compression in addition to the pure circulation function. The actual compression takes place as a result of the heating by gas burners, which achieve a high, purely thermal efficiency without mechanical conversion losses. The reduction in moving components also results in lower internal electricity consumption. Like gas engine heat pumps, pure gas heat pumps are a bit more expensive to invest and only pay off from certain system sizes or in local heating networks. They can also be operated in a CO 2 -neutral manner using biomethane that has been converted into bio natural gas . The same also applies if a gas burner is added to an electric heat pump for peak load heating.

Hybrid and mixed systems

Solar ice storage heat pump / latent heat pump / direct evaporation heat pump

The solar ice storage system consists of a large water tank which, when freezing to 0 ° C, uses a coolant (e.g. a water-glycol mixture) to make the so-called heat of crystallization available for heat utilization.

The icing process that occurs when further heat is extracted is intentional, because the phase change from water to ice brings further energy gains. The temperature remains constant at 0 ° C, but a further 93 Wh / kg of crystallization energy is released, which can be used by the heat pump. This is the same amount of energy that is released when water is cooled from 80 to 0 ° C.

The system largely corresponds to that of the water-to-water heat pump. However, the cooled water does not simply continue to flow as groundwater, but also serves directly as a cooling medium in summer, which can be used in reverse operation (air conditioning system) via a circulation pump in the house heating without renewed energy-intensive, cost-intensive heat exchange process and thus partially regenerates the storage tank.

The regeneration takes place constantly through all energy sources that are warmer than 0 degrees.

The enthalpy - that is, the "heat" content of the "ice store" - is 333.5 kJ / kg or 85 kWh / m³ of ice. This is a good 8 liters of heating oil per cubic meter. The system must be dimensioned accordingly around the cooling coils, around which an ice jacket forms over time, which hinders the further extraction of energy.

Common models with a solar ice storage of approx. 12 m³ and 5 solar air collectors (à 2 m²) on the roof offer in monovalent operation about 1800 full load hours per year for the heating operation with a maximum heating load of 7.5 kW .

In theory, a water-to-water heat pump is still the first choice for year-round heating, including DHW heating. However, in summer the energy expenditure for cooling is probably more lucrative with the ice storage system. The systems therefore compete with each other for overall energy efficiency.

Under the reference conditions of Stiftung Warentest, the solar heating achieves an annual system performance factor (SJAZ) of approx. 5 (including power consumption fans, pumps, etc. and including directly used solar heat).

Seasonal underground storage plus heat pump

In the case of underground storage, it is possible to use this as a long-term energy storage. This consists of an insulated underground storage tank with a defined system of plastic pipes running through it. Surpluses from other heat sources such as solar thermal energy are buffered. This results in an increase in the source temperature for the heat pump by an average of 10 ° C compared to ground collectors. Heat sources with relatively low temperatures that cannot be used directly for heating can also be fed to the underground storage tank. In addition to (brine) or a water-glycol mixture, pure water can also be used as a heat transfer medium.

Operation without antifreeze enables use in drinking water protection areas. The basis for this is the controlled temperature level in the underground storage, which is approximately between +5 ° C and +23 ° C over the seasonal change.

The system largely corresponds to the brine-water heat pump with special control technology and can heat and cool like comparable systems. The primary heat sources are surpluses from solar systems or process heat.

An underground storage tank of approx. 100–120 m³, a coordinated heat pump and approx. 12–14 m² solar thermal flat-plate collectors cover a heating load of approx. 10 kW in heating mode.

The underground storage system is used in new buildings i. d. Usually installed under the floor slab in order to use synergies with any work that has to be done anyway, such as foundation, frost protection , foundations, insulation of the floor slab, etc. The use in existing buildings as well as in inner-city areas appears problematic, since the necessary areas may not be available there. The area in which the underground storage tank is installed should, if possible, not be flowed through by groundwater, otherwise increased demands are placed on the sealing.

Long-term energy storage is only required to be notified to the lower water authorities, as the installation usually takes place only 1.20–1.50 m below the floor slab and the ground is not used as a heat source. Due to the low installation depth, no groundwater-bearing layers are usually penetrated.

BAFA promotes ice storage systems and insulated geothermal energy storage systems such as the eTank as part of the innovation promotion “Heat pumps with improved system efficiency”. A minimum storage volume must be adhered to and the achievement of the system annual performance factor (SJAZ) of 4.1 must be proven by simulation. The manufacturer of the seasonal underground storage tank eTank was nominated for the innovation award of the states of Berlin and Brandenburg in 2015.

Air / water brine / water heat pump (hybrid heat pump)

The air / water brine / water heat pump is a hybrid heat pump that uses only renewable energy sources in its design. It combines air heat and geothermal heat in one compact device. This distinguishes this hybrid system from other systems that also use at least two heat sources. These mostly form a mix of conventional heating technology (gas condensing technology) and renewable energy sources.

The air / water brine / water heat pump (hybrid heat pump) is equipped with two evaporators (an outdoor air evaporator and a brine evaporator), both of which are connected to the heat pump circuit. This makes it possible to use the most economical heat source at the current time in comparison with the external conditions (e.g. air temperature). The hybrid system automatically selects the most effective operating mode (air heat or geothermal heat). Depending on the mode of operation, the energy sources air and geothermal energy can be used in parallel or alternatively.

Operating modes

As a rule, a distinction is made between three operating modes:

  • Monovalent operation = heat pump only, suitable for all low-temperature heating systems up to a maximum flow temperature of 55 ° C.
  • Bivalent operation = heat pump and an additional heat source (e.g. solar collectors, gas boiler, electric immersion heater and the like)
    • Bivalent-alternative = the heat pump supplies all of the heating energy up to a specified outside temperature. If the value falls below this, the heat pump switches off and a second heat generator takes over the heating.
    • Bivalent-parallel = as with the bivalent-alternative operating mode, the heat pump delivers the entire heating output up to a certain value, but the heat pump only switches off after a second limit value. In between, a second heat generator is switched on. In contrast to bivalent-alternative operation, the share of the heat pump in the annual output is significantly greater.
  • The mono-energetic operation = heat pump and electrical additional heating (mostly in inexpensive models). The heat pump provides the necessary heat output for most of the year. At very low temperatures (below −7 ° C) the heat output is insufficient and a heating element is switched on.

Reverse operation

Many heat pumps can also operate in reverse to cool the house. A distinction is made between passive cooling with groundwater or depth probes and active cooling through process reversal. Classic radiators are not suitable for room cooling, however, as the relatively small area of ​​the radiator only allows limited heat transfer.

Structure of the cycles

The system types can be distinguished by the number of fluid circuits . The decoupling of the circuits by indirect supply of the evaporation heat from the environment and the dissipation of the condensation energy via a hot water heating network are advantageous in terms of control technology (but with energy losses), the amount of refrigerant and the probability of leaks are low.

3-circuit system

Heat pump heating systems used this form of system for a long time. Brine is used, in the form of a deep hole or a surface collector. Here, brine circulates as a transmission medium in a closed circuit and absorbs the heat from the ground in order to transfer it to the refrigerant circuit in the heat pump. In the third circuit, space heating, water circulates, which is heated by the heat pump via a heat exchanger. With this type of system, a CO 2 probe can also be used as a collector in a deep borehole. The advantage (in terms of efficiency) compared to the brine in a deep borehole is that energy is not required to circulate the medium in the collector.

2-circuit system

They are also called direct systems because they do not have a separate brine circuit. There is no heat transfer from the collector circuit (brine) to the working circuit of the heat pump. The refrigerant absorbs the heat directly (direct evaporation). This brings an energetic advantage of at least 5 degrees. The elimination of the brine circulation pump reduces power consumption. When using ground spikes as a heat source, direct evaporation is not possible; a brine circuit must be used.

1-circuit system

The refrigerant circulates in the pipes of the room heating, in the heat pump and in the collector in the garden in a shared closed circuit. There is thus no heat transfer to water as a heating medium in the house. This system has energetic advantages since the circulation pump and the temperature drop at the heat exchanger to the heating circuit are not required. The refrigerant is usually fed to the underfloor heating collectors as hot gas and condenses in the condenser system. Problems with this arrangement are:

  • significantly higher refrigerant fill quantities,
  • the complex piping results in a higher probability of leaks,
  • problematic oil return from the floor collector,
  • load-dependent refrigerant distribution in the overall system,
  • difficult regulation and mutual influence of the floor collector surfaces.

Only a few (around two or three) manufacturers dared to implement this type of system in 2007 because it was difficult to control in terms of system technology (pressure and temperature of the refrigerant and running time of the heat pump).

Heating water distribution / interim storage

Water tank for an air-to-water heat pump heater in a single-family home

If the heat supplied by the heat pump is temporarily not consumed / used sufficiently, the hot water can be temporarily stored; this takes place in a large thermally insulated tank, a buffer storage . This tank holds i. d. Usually several hundred liters of water. The water flow now circulates between the tank and the radiators or underfloor heating for heating. The heat pump heats the water in the tank.


Annually newly installed heat pump heating systems in Germany (DE) and Austria (AT)
year DE AT
1995 1,200 5.124
1996 2,300 5,312
1997 3,600 4,957
1998 4,400 4,819
1999 4,800 4,612
2000 5,700 4,795
2001 8,200 5,590
2002 8,300 5,780
2003 9,890 6,935
2004 12,900 7,968
2005 18,900 9,795
2006 44,000 13,180
2007 44,600 15,148
2008 62,500 18,705
2009 54,800 18,138
2010 51,000 16,962
2011 57,000 16,398
2012 59,500 18,861
2013 60,000 19,175
2014 58,000 21,439
2015 57,000 23,014
2016 66,500 22,994

Heat pumps play an important role in EU energy policy in order to increase energy efficiency and reduce energy consumption and greenhouse gas emissions . Their market share is on the rise, not least through subsidy measures. Important sales markets are France, Sweden, Norway, Germany and Finland. In 2010 a total of 750,000 heat pumps were installed in the EU-20, the energy savings of which are estimated at 36.6 TWh.


The market share of heat pump heating in new buildings is very country-specific and averaged 10% nationwide in 2005, with the ground-based heat pump being the most successful with a share of around 40%. In 2010, the share of heat pump heating in new buildings in Germany was 23.4%. Due to newer technologies and more efficient construction methods and functional principles, heat pumps are growing rapidly as alternatives to conventional systems based on fossil fuels. The market share of air-to-water heat pumps in particular has risen sharply, as government subsidies, a fixed and subsidized heat pump electricity price and, last but not least, the lower investment costs make these systems attractive. In 2012, air-to-water heat pumps achieved a market share of 62.7% of all electrically driven compression heat pumps, excluding cooling units with an additional building heating function. These systems are increasingly preferred to conventional boilers, especially in new buildings and renovations. With 66,500 heat pumps, more environmentally friendly heating systems were sold in Germany in 2016 than ever before. Record sales year 2016: The high government subsidies for heat pumps and the stricter requirements imposed by the Energy Saving Ordinance are responsible for the increased sales figures , according to the Bundesverband Wärmepumpe (BWP) eV . The biggest winners are earth-coupled systems: Compared to the previous year, they increased by 21.8% (2015: 17,000 devices; 2016: 20,700 devices). Air heat pumps increased by 14.5% from 40,000 units in 2015 to 45,800 in 2016. As in the previous year, monoblock devices were particularly popular with an increase of 19.5% (2015: 21,000; 2016: 25,100). The growth in split devices was slightly lower at 8.9% (2015: 19,000; 2016: 20,700). The market shares between ground-based heat pumps and air heat pumps hardly change compared to the previous year: Air heat pumps again dominated sales in 2016 with 68.9% (previous year: 70.2%). Geothermal heat pumps increased by almost% (2016: 31.1%; 2015: 29.8%). Sales of hot water heat pumps remained unchanged from the previous year at 12,500 units. A total of around 750,000 heat pumps are installed in Germany. The BWP announced that the sales figures for heat pumps increased significantly in 2017. In total, the manufacturers sold 78,000 heating devices. Air source heat pumps experienced the greatest growth: their sales grew by 20% compared to 2016. But the demand for groundwater and geothermal heat pumps also rose by 11% to 23,000 units. With an increase of 8% and 13,500 devices, hot water heat pumps also benefited from the upswing in the industry. Thanks to them, total sales rose to 91,500 units.


A total of 190,200 heat pump systems were installed in Austria between 1975 and 2005. Most annual heat pumps were installed in 1986 and 1987 (with over 13,000 heat pumps per year).


In Switzerland, the market share for new buildings is around 75%. The spec. The cost of heating with a heat pump using geothermal energy is 3.9  Rp / kWh. (about 3.2 cents / kWh), while conventional oil heating with spec. Costs of 7.9 cents / kWh (around 6.6 cents / kWh) should be estimated. State funding is therefore superfluous.


Direct investment

The initial investment in heat pump systems is higher than in conventional boilers that burn gas or oil. However, there are no additional costs such as installing a chimney in a new building. There is also no need for a storage room for fuel for oil, pellets or wood.

Heat pump heating systems based on geothermal collectors or geothermal probes are very cost-intensive due to their installation (several boreholes up to at least 50 meters or large-scale excavation) and can only be used economically for new buildings. Ground collectors in particular require relatively large plots of land, which can hardly be achieved in metropolitan areas. For small property areas and for existing buildings, spiral collectors / geothermal baskets are an alternative, for example in the course of an energetic renovation of the old building.

Even with heat pumps that use groundwater as an energy source, the investment costs and the demands on the property area are high. As a rule, a delivery well and an injection well (at a distance of at least about 15 m approximately in the direction of the groundwater flow, depth to sufficiently below the groundwater level) as well as the underground connection line to the system must be built. The wells are drilled with a diameter of 15 to 30 cm or, if the groundwater is high, they are designed as a well shaft up to around 4 m. Instead of the sink well, sometimes only a cheaper drainage shaft is built, which, however, changes the property's water management and is therefore usually not permitted. Furthermore, a slightly higher pumping capacity of the feed pump is necessary, since the height energy of the water pumped up is lost. In some areas, however, the simultaneous use of groundwater for summer garden irrigation is permissible. The costs vary greatly depending on the structural conditions. In addition, there are additional costs for a soil survey and the approval process.

Systems that are based on air-water or air-air have lower investment costs, because the costs for acquisition and installation are significantly lower. However, with air-water or air-air systems, a significantly poorer coefficient of performance in winter can be expected, which means that the operating costs are higher than with earth systems. An air-to-water heat pump is therefore well suited for bivalent operation with an existing fossil heating system that covers peak loads and very low outside temperatures.

Another investment to be considered when using the cheaper heating current is the installation of a second electricity meter, which in existing buildings can result in an expansion of the electricity box.

operating cost

Heating oil

One liter of heating oil currently costs around 50 cents (as of October 19, 2016) and contains around 9 to 10 kWh of thermally usable energy. This results in a price of around 5 to 6 cents / kWh for oil. Oil condensing boilers have an average efficiency of around 90% during operation. This results in a price of 5.6 to 6.6 cents / kWh of heat for generating useful heat. The energy requirement of the compression pump belonging to the oil burner and the fan that mixes the atomized oil with air are not included .

natural gas

The fuel price for natural gas in September 2014 was 5 ct per kWh with an annual requirement of 20,000 kWh. However, according to an Öko-Institut study , gas condensing heating systems with efficiencies related to the calorific value of over 100% still require 1.114 kWh of primary energy per kWh of useful energy. This also includes the electricity that is also required for the exhaust fan. They therefore cause costs of around 5.6 cents / kWh useful heat.

Low tariff electricity

With a current gross electricity price of 22.51 cents / kWh ( heat pump electricity tariff , as of 04/2013, including all taxes and charges) and an annual performance factor JAZ of the heat pump heating of 4.0 in the best case, the generation of useful heat using air-water costs -Heat pump at best 5.6 euro cents / kWh (gross). There are also variable tariffs that vary depending on the current electricity supply. Since the end of 2010 every electricity provider in Germany has had to use such a tariff. By owner-occupied photovoltaic electricity ( electricity generation costs ct about 12 / kWh (6/2014)) Operating costs can be further reduced.

There are no costs for the chimney sweep if there is no additional tiled stove or similar.

The heat pump tariff offered by the electricity supplier is considerably cheaper than the household tariff used. From an economic point of view, the higher investment costs of the heat pump compared to an oil or gas burner, the electricity price offered for the heat pump and its running times, and the heat pump's coefficient of performance must be taken into account as in any economic calculation.


In particular, the extraction of geothermal energy from geothermal wells is associated with risks. In Staufen im Breisgau, after drilling for geothermal heat pumps in 2006 and 2007, the ground rose significantly in the historic town center. Buildings got cracks . The estimated amount of damage is 50 million euros. Some of the boreholes had created a connection between the groundwater layer and the gypsum keuper layer. When the water penetrated into the anhydrite embedded in the gypsum keuper layer , a chemical reaction took place and gypsum was formed. This is associated with a significant increase in volume. The ground rose in the center of town. Similar cases occurred in Böblingen, Kamen, Rudersberg and Schorndorf. The ground did not always rise, in some cases the boreholes also caused the subsoil to sink. Geothermal wells in Basel were canceled because of unexpectedly strong earthquakes triggered by the wells.

Economic importance

From an economic point of view, gas and oil heating systems make them dependent on other countries, since over 90% of German natural gas and mineral oil consumption has to be imported. In addition, these resources are finite and affected by sometimes drastic price fluctuations. Depending on the performance figure of the heat pump as well as the efficiency and fuel of the marginal power plant, the consumption of heating oil or natural gas shifts from the house combustion to hard coal or lignite in fossil-fired thermal power plants . This reduces the dependence on the import of expensive energy resources such as oil and natural gas. With the increasing share of renewable energies (share of the electricity mix in 2017: 36%) and the construction of more efficient conventional power plants , the dependence on energy imports also falls further in the heating sector.

In addition, a study on behalf of the Federal Ministry of Economics came to the conclusion that heat pump heating systems can contribute to better grid integration of renewable energies, especially wind energy , as well as to decentralized load management in the electricity market. With the grid-friendly operation of heat pumps, the grid integration of fluctuating feeders could be made more economically favorable.

In connection with photovoltaic systems

An alternative to storing solar power in solar batteries is to store the energy in thermal storage . For this purpose, solar power z. B. recycled with a heat pump to heat domestic water, which is temporarily buffered in a heat storage tank (similar to an insulating jug ). The thermal energy stored in this way is then not converted back into electrical energy, but fed into the heating system or used to heat water . With the drop in PV module prices, such a system is often more cost-effective than a solar thermal system and offers the flexibility to use both electrical and thermal energy.

See also

Web links


Individual evidence

  1. See Valentin Crastan , electrical energy supply 2 , Berlin - Heidelberg 2012, p. 58.
  2. Volker Quaschning , Regenerative Energy Systems. Technology - calculation - simulation . 8th updated edition. Munich 2013, p. 339.
  3. Volker Quaschning: Renaissance of the Heat Pump , published in Sonne Wind & Wärme 09/2006 pp. 28–31
  4. a b Heat supply system with CO 2 heat pump (PDF; 4.1 MB). Website of the Institute for Thermodynamics, Technical University of Braunschweig . Retrieved March 8, 2012.
  5. CO2 heat pumps for passive houses . Website of the Technical University of Braunschweig. Retrieved March 13, 2016.
  6. David Fischer, Hatef Madani: On heat pumps in smart grids: A review . In: Renewable and Sustainable Energy Reviews . tape 70 , 2017, p. 342–357 , doi : 10.1016 / j.rser.2016.11.182 .
  7. Valentin Crastan , Electrical Energy Supply 2 , Berlin - Heidelberg 2012, p. 303.
  8. Andreas Bloess et al .: Power-to-heat for renewable energy integration: A review of technologies, modeling approaches, and flexibility potentials . In: Applied Energy . tape 212 , 2018, p. 1611-1626 , doi : 10.1016 / j.apenergy.2017.12.073 .
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  13. Archived copy ( Memento of the original dated December 22, 2017 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.biogaspartner.de
  14. Viktor Wesselak , Thomas Schabbach , Thomas Link, Joachim Fischer: Handbuch Regenerative Energietechnik , Berlin / Heidelberg 2017, p. 73.
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  21. A local energy source that needs to be grasped ( memento of the original from October 12, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , at geothermie.ch @1@ 2Template: Webachiv / IABot / geothermie.ch
  22. Warm water Furka tunnel . A project by the municipality of Obergoms (Oberwald): Heat from the Furka tunnel
  23. Geothermal energy from the tunnel "heats" the school in Penzing
  24. GeoTU6, University of Stuttgart ( Memento of the original from October 12, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  @1@ 2Template: Webachiv / IABot / www.uni-stuttgart.de
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  27. The battery in Lake Constance . In: Stuttgarter Nachrichten , August 5, 2014. Accessed August 23, 2014.
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  31. Solarteiche from Achmed Khammas: The book of synergy , online
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  46. bwp: Bundesverband Wärmepumpe eV - sales figures 2012
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  51. ^ Report on geothermal damage in Rudersberg. (No longer available online.) Archived from the original on September 16, 2016 ; accessed on September 15, 2016 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.geothermie-nachrichten.de
  52. Damage to a school and more than ten houses in Schorndorf after geothermal drilling. Retrieved September 15, 2016 .
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