Heat pump

from Wikipedia, the free encyclopedia
Diagram of the heat flow (large arrows) and the refrigerant (small arrows) of a compression heat pump (see compression refrigeration machine ):
1) condenser, 2) throttle, 3) evaporator, 4) compressor
Dark red: gaseous, high pressure, very warm
pink: liquid, high pressure, warm
blue: liquid, low pressure, very cold
light blue: gaseous, low pressure, cold

A heat pump is a machine that uses technical work to absorb thermal energy from a reservoir with a lower temperature (usually this is the environment ) and - together with the drive energy - as useful heat to a system to be heated with a higher temperature ( room heating ) transmits. The process used is basically the reverse of a heat and power process , in which thermal energy is absorbed at a high temperature and partially converted into useful mechanical work and the remaining energy is dissipated as waste heat at a lower temperature , mostly to the environment. The principle of the heat pump is also used for cooling (as with the refrigerator ), while the term “heat pump” is only used for the heating unit. In the cooling process, the useful energy is the heat absorbed from the room to be cooled, which, together with the drive energy, is dissipated as waste heat to the environment.

technical realization

Figure 1: Circuit diagram of a heat pump with a cold steam process
Figure 2: Ts diagram of the comparison process
Temperatures. T U = ambient temperature,
T V = evaporator temperature,
T K = condenser temperature,
T N / H = useful / heating temperature

Heat pumps are usually operated with media that evaporate at low pressure with the addition of heat and condense again after being compressed to a higher pressure with the release of heat. The pressure is selected so that the temperatures of the phase transition are at a sufficient distance from the temperatures of the heat source and heat sink for heat transfer . Depending on the material used, this pressure is in different areas. Figure 1 shows the circuit diagram with the four components required for the process: evaporator, compressor (compressor), condenser and throttle, Figure 2 shows the process in the Ts diagram . Theoretically, it would be possible to use the working capacity of the condensate when it is expanded to the lower pressure by a prime mover, for example a turbine. But the liquid would partially evaporate and cause such great technical difficulties with only a small energy gain, so that for the sake of simplicity a throttle is used here (relaxation with constant total enthalpy ).


In the case of heat pumps, physical effects of the transition of a liquid into the gaseous phase and vice versa are used. Propane, for example, has the property of being either gaseous or liquid, depending on the pressure and temperature, on the one hand, and, on the other hand, as a gas, it becomes hot when compressed and cools down when relaxed: Propane is gaseous at normal air pressure and cool outside temperature (e.g. 5 ° C) ; if you compress it, it becomes warmer, but remains gaseous. If you then cool it down to room temperature, it becomes liquid (the pressure drops again a bit). If you relax the liquid propane, it evaporates (it turns back into gas) and becomes very cold in the process.

This effect is used in the heat pump: The propane gas is compressed in the compressor by a motor and is heated up in the process. The hot, compressed gas can then transfer its heat to the water in the heating system in the heat exchanger. The compressed gas cools down and condenses into liquid propane (the heat exchanger in a heat pump is therefore called a condenser). During the subsequent passage through the so-called throttle (to put it simply: an extreme constriction in the pipe), the liquid propane is relaxed, evaporates and becomes very cold (significantly colder than 5 ° C). If the cold gas is then allowed to flow through a second heat exchanger (usually outside the house), which is kept at 5 ° C from the outside - for example through groundwater or the outside air - the very cold gas is heated to 5 ° C and the environment cools down by 1 or 2 ° C. In this way, the propane absorbs just as much heat from the groundwater or the outside air as it previously gave off to the heating water. It is then fed back into the compressor and the process starts all over again.

The energy required to drive the heat pump is reduced, i.e. the operation becomes more economical the lower the temperature difference between the ground temperature and the flow temperature of the heating system. Low-temperature heating systems meet this requirement best, which is why the heat in the living space is often given off by underfloor heating .

Depending on the design of the system, the heating energy consumption can be reduced by around 30 to 50%. By coupling with solar power, household power or natural gas to drive the heat pump, carbon dioxide emissions can be reduced significantly compared to heating oil.

Choosing the right heat source is particularly important because the efficiency of a heat pump depends on it. Since the heat pump often has a service life of more than 50 years, building it is an investment for generations.

“The annual performance factor is a measure of the efficiency of a heat pump. It describes the ratio of useful energy in the form of heat to the compressor energy used in the form of electricity. ”In good systems, this value is greater than 5.0 (direct evaporation systems). However, it must be taken into account that when calculating the annual coefficient of performance, neither secondary consumption nor storage losses are taken into account.

Economic efficiency: When heating smaller residential buildings, heat pumps are driven electrically. If they are compared with gas heating from an economic point of view, the annual performance factor provides an indicator for a comparison of operating costs between heat pumps and gas heating. If the electricity price for the heat pump (in € / kWh) is higher than the gas price (in € / kWh) multiplied by the annual coefficient of performance, it is to be expected that the electricity costs for the heat pump are higher than the costs for burning gas . This also applies to the comparison of a heat pump with an oil heater.

With older coal-fired power plants still in operation, only one part of electricity can be obtained from three parts of thermal energy. For electricity-driven heat pumps, it is therefore advantageous to improve the use of electricity from renewable sources.

With direct electrical heating, for example with heating rods, the thermal energy generated corresponds exactly to the electrical energy used. However, the electrical energy is of much higher quality than thermal energy at low temperatures, because by using a heat engine , only part of the thermal output can be converted back into electrical output.

Performance balance of the heat pump: The COP describes the quotient of usable heat ( red ) and the electrical compressor output used for this ( yellow )

Heat can be extracted from the exhaust air, the outside air, the ground, the sewage or the groundwater using a heat pump. A multiple of the electrical power used for the heat pump can be withdrawn from the heat source (air, ground) and pumped to a higher temperature level. In the power balance, the heat pump is supplied with electrical power for the compressor drive and the heat extracted from the environment. At the heat pump outlet, part of the power supplied is available as heat at a higher level. The losses of the process must also be taken into account in the overall current account.

The ratio of the heat output in the heating circuit to the electrical compressor output supplied is called the coefficient of performance . The coefficient of performance has an upper value which cannot be exceeded and which can be derived from the Carnot cycle . The coefficient of performance is determined on a test bench in accordance with the EN 14511 standard (previously EN 255) and is only valid under the respective test conditions. According to EN 14511, the performance figure is also called COP (coefficient of performance) . The COP is a quality criterion for heat pumps, but does not allow an energetic evaluation of the entire system.

In order to achieve the highest possible coefficient of performance and thus high energy efficiency , the temperature difference between the temperature of the heat source and the useful temperature should be as small as possible. The heat exchangers should be designed for the lowest possible temperature differences between the primary and secondary side.

The designation heat pump is based on the fact that heat from the environment is raised (pumped) to a higher usable temperature level. The heat pump has a compressor that is driven electrically or by an internal combustion engine. The compressor compresses a refrigerant to a higher pressure, whereby it heats up. The energy released during the subsequent cooling and liquefaction of the refrigerant is transferred in a heat exchanger to the heat transfer medium of the heating circuit, usually water or brine . The refrigerant is then expanded at an expansion valve and it cools down. The cold refrigerant is fed to the evaporator (geothermal probes, air evaporator ) and turns into a gaseous state by absorbing ambient heat ( anergy ).

A disadvantage of the heat pump is the significantly higher expenditure on equipment. Effective evaporators (geothermal probes, buried surface evaporators) are particularly cost-intensive. The investments compared to a conventional gas or heating oil burner are significantly higher. In return, the regular effort for maintenance and repair is significantly lower, for example there are no cleaning and chimney sweep costs.

The heat pump process, called the Plank process after Rudolf Plank , is also known as a combined heat and power machine. The borderline case of a reversible heat engine is the left-hand Carnot process .

Refrigerant (working gases)

From 1930 to the early 1990s, chlorofluorocarbons (CFCs) were the preferred refrigerants. They condense at room temperature under easily manageable pressure. They are non-toxic, non-flammable and do not react with the usual materials. However, when CFCs are released, they damage the ozone layer in the atmosphere and contribute to the ozone hole . The use of chlorofluorocarbons was therefore banned in Germany in 1995. The fluorocarbons (PFCs) used as substitutes do not damage the ozone layer, but they do contribute to the greenhouse effect and are listed as environmentally hazardous in the Kyoto Protocol . Pure hydrocarbons such as propane or propylene are considered natural refrigerants , whereby their flammability requires special safety measures. Inorganic, non-flammable alternatives such as ammonia , carbon dioxide or water have also been used for heat pumps. Due to specific disadvantages, these refrigerants have not been able to establish themselves on a larger technical scale. Ammonia (NH 3 ) and carbon dioxide (CO 2 ) are generally used in industrial cooling systems such as cold stores and breweries. CO 2 is being considered instead of fluorocarbons for air conditioning vehicles and is already being used by the first manufacturers (as of 2017).

Performance figure and grade of quality

The coefficient of performance ε, also referred to in the literature as the heating number of a heat pump, is the quotient of the heat that is released into the heating circuit and the energy used:

With typical COPs of 4 to 5, four to five times the output is available as usable heat output, the gain comes from the extracted ambient heat.

The coefficient of performance depends heavily on the lower and upper temperature level. According to the second law of thermodynamics , the maximum theoretically achievable coefficient of performance of a heat pump is limited by the reciprocal of the Carnot efficiency

The absolute values ​​are to be used for the temperatures .

The quality grade of a heat pump is the actual performance figure related to the ideal performance figure at the temperature levels used. It is calculated as:

In practice, heat pump grades in the range 0.45 to 0.55 are achieved.

Sample values

The lower temperature level of a heat pump is 10 ° C (= 283.15 K), and the useful heat is transferred at 50 ° C (= 323.15 K). In an ideal reversible heat pump process, the reverse of the Carnot process, the coefficient of performance would be 8.1. A coefficient of performance of 4.5 can actually be achieved at this temperature level. With one energy unit exergy , which is brought in as technical work or electrical power, 3.5 units of anergy can be pumped from the environment to the high temperature level, so that 4.5 energy units can be used as heat at a heating flow temperature of 50 ° C. ( 1 unit of exergy + 3.5 units of anergy = 4.5 units of thermal energy ).

In the overall consideration, however, the exergetic power plant efficiency and the network transmission losses must be taken into account, which achieve an overall efficiency of approx. 35%. The 1 kWh exergy required requires a primary energy input of 100/35 × 1 kWh = 2.86 kWh. If the primary energy is not used in the power plant, but is used directly on site for heating, then with a combustion efficiency of 95% you get 2.86 kWh × 95% = 2.71 kWh of thermal energy.

With reference to the example given above, in the ideal case ( coefficient of performance = 4.5) 1.6 times the fuel enthalpy used can be converted as thermal energy with a heating heat pump and 0.95 times with a conventional heating system. Under very favorable conditions, the detour power plant → electricity → heat pump can achieve a 1.65 times higher amount of heat compared to direct combustion.

On the test bench, a COP of up to 6.8 is achieved at a groundwater temperature of 10 ° C and a useful heat temperature of 35 ° C. In practice, however, the power value actually achievable over the year, the annual performance factor (JAZ) including losses and auxiliary drives, of only 4.2 is achieved. For air / water heat pumps, the values ​​are significantly lower, which reduces the reduction in primary energy demand. Under unfavorable conditions - for example with electricity from fossil fuels - more primary energy can be consumed than with conventional heating. Electric heating of this kind is neither in terms of climate protection nor economically efficient.

A heat pump with a JAZ> 3 is considered energy efficient. However, according to a study, the electricity mix from 2008 already saves carbon dioxide emissions from a JAZ of 2, with the further expansion of renewable energies and the replacement of older power plants with more modern and efficient ones, the savings potential, including existing heat pumps, increases further.

Data Sheets

In the data sheets for the various heat pump products, the performance parameters are related to the medium and source and target temperatures; for example:

  • W10 / W50: COP = 4.5,
  • A10 / W35: heating power 8.8 kW; COP = 4.3,
  • A2 / W50: heating power 6.8 kW; COP = 2.7,
  • B0 / W35: heating power 10.35 kW; COP = 4.8,
  • B0 / W50: heating power 9 kW; COP = 3.6,
  • B10 / W35: heating power 13.8 kW; COP = 6.1

After several measured COP values ​​on the WPT socket. Specifications such as W10 / W50 indicate the inlet and outlet temperatures of the two media. W stands for water, A for air and B for brine , the number behind it for the temperature in ° C. B0 / W35 is, for example, an operating point of the heat pump with a brine inlet temperature of 0 ° C and a water outlet temperature of 35 ° C.


after the procedure
after the heat source
after the use of heat
according to the working method

There are various physical effects that can be used in a heat pump. The most important are:

in building technology

Heat pumps are often used to heat water for heating buildings ( heat pump heating ) and to provide hot water . Heat pumps can be used alone, in combination with other types of heating, as well as in district and local heating systems . The latter includes z. B. the cold local heating . The following combinations are common (abbreviations in brackets):


14,000 kW absorption heat pump for utilizing industrial waste heat in an Austrian district heating plant.
The compression heat pump
uses the physical effect of the heat of vaporization. A refrigerant circulates in it in a circuit, which, driven by a compressor, alternately assumes liquid and gaseous states.
The absorption heat pump
uses the physical effect of the heat of reaction when two liquids or gases are mixed. It has a solvent circuit and a refrigerant circuit. The solvent is repeatedly dissolved or expelled in the refrigerant.
The adsorption heat pump
works with a solid solvent, the "adsorbent", on which the refrigerant is adsorbed or desorbed. Heat is added to the process during desorption and removed during adsorption. Since the adsorbent cannot be circulated in a cycle, the process can only run discontinuously by changing cyclically between adsorption and desorption.

Electrically driven compression heat pump

The inside of an evaporator in an air-to-water heat pump

The electrically driven compression heat pump is the main application of heat pumps. The refrigerant is guided in a closed circuit. It is sucked in by a compressor, compressed and fed to the condenser. The condenser is a heat exchanger in which the condensation heat is transferred to a fluid - for example to a hot water circuit or to the room air. The liquefied refrigerant is then fed to an expansion device (capillary tube, thermal or electronic expansion valve). The refrigerant is cooled down by the adiabatic expansion. The suction pressure is set by the expansion device in combination with the delivery rate of the compressor in the heat pump so that the saturated steam temperature of the refrigerant is below the ambient temperature. In the evaporator, heat is thus transferred from the environment to the refrigerant and leads to the evaporation of the refrigerant. The ambient air or a brine circuit that absorbs the heat from the ground can be used as a heat source. The evaporated refrigerant is then drawn in by the compressor. From the example described above, it can be seen that the use of the electrically operated heat pump at the assumed temperature level does not allow a significantly higher thermal efficiency compared to conventional direct heating. The ratio improves in favor of the electrically driven heat pump if waste heat at a high temperature level can be used as a lower heat source or geothermal energy can be used at a high temperature level using a suitable geothermal collector .

Heat pump with oil or gas engine drive

A significantly higher thermal efficiency can be achieved if the primary energy can be used as gas or oil in a motor to generate technical work to directly drive the heat pump compressor. With an exergetic efficiency of the engine of 35% and a utilization of the engine waste heat to 90%, an overall thermal efficiency of 1.8 can be achieved. However, the considerable additional effort compared to direct heating must be taken into account, which is justified by significantly higher investments and maintenance costs . However, there are already gas heat pumps on the market (from 20 kW heating / cooling capacity upwards), which manage with service intervals of 10,000 hours (normal maintenance work for the engine) and every 30,000 operating hours for the oil change and thus have longer maintenance intervals than boiler systems. It should also be noted that certain manufacturers of motor-driven gas heat pumps manufacture these in series production, which in Europe have a service life of more than 80,000 operating hours. This is the case due to the sophisticated engine management, the low speeds and the optimized device processes.

Detailed description of heat pumps for heating buildings


Two-stage piston compressor installed in Saline Bex in 1877 / Wirth 1955 /
Heat pump unit built by Brown Boveri with eight-stage radial compressor and a heating output of 2,000 kW. Of these, two units were installed for a district heating network in Zurich in 1942.
1968: First central heat pump unit in Germany by Klemens Oskar Waterkotte

The history of the heat pump began with the development of the vapor compression machine. It is referred to as a cooling machine or a heat pump depending on the use of the heat supplied or removed. For a long time, the aim was to create ice artificially for cooling purposes. Jacob Perkins , who came from the USA, was the first to succeed in building a corresponding machine in 1834. It already contained the four main components of a modern heat pump: a compressor, a condenser, an evaporator and an expansion valve.

Lord Kelvin predicted the heat pump as early as 1852 when he recognized that a "reverse heat engine" could be used for heating purposes. He realized that such a heating device would require less primary energy than with conventional heating thanks to the extraction of heat from the environment (air, water, earth) . But it would be around 85 years before the first heat pump for space heating went into operation. During this period, the functional models of the pioneers were replaced by more reliable and better designed machines on the basis of a rapidly advancing scientific penetration, especially by Carl von Linde and the progress of industrial production. The refrigeration machines and systems were manufactured into industrial products and on an industrial scale. By 1900, most of the fundamental innovations in refrigeration technology for ice cream production and, later, the direct cooling of food and beverages were already available. The heat pump technology could also build on this later.

In the period before 1875, heat pumps were first pursued for vapor compression (open heat pump process) in salt works with their obvious advantages for saving wood and coal. In 1857, the Austrian engineer Peter von Rittinger was the first to try to implement the idea of ​​vapor compression in a small pilot plant. Presumably inspired by Rittinger's experiments in Ebensee, Antoine-Paul Piccard from the University of Lausanne and the engineer JH Weibel from the Weibel-Briquet company in Geneva built the world's first really functioning vapor compression system with a two-stage compressor in Switzerland in 1876 . In 1877 this first heat pump in Switzerland was installed in the Bex saltworks . Around 1900, heat pumps remained visions for some engineers. The Swiss Heinrich Zoelly was the first to propose an electrically driven heat pump with geothermal energy as the heat source. For this he received the Swiss patent 59350 in 1919. But the state of the art was not yet ready for his ideas. It took around twenty years until the first technical implementation. In the USA from 1930 air conditioning systems for room cooling with additional options for room heating were built. However, the efficiency of space heating was modest.

During and after the First World War, Switzerland suffered from very difficult energy imports and subsequently expanded its hydropower plants. In the period before and especially during the Second World War , when neutral Switzerland was completely surrounded by fascist-ruled countries, the coal shortage once again became a major problem. Thanks to their leading position in energy technology, the Swiss companies Sulzer , Escher Wyss and Brown Boveri built and commissioned around 35 heat pumps between 1937 and 1945. The main sources of heat were lake water, river water, groundwater and waste heat. Particularly noteworthy are the six historic heat pumps from the city of Zurich with heat outputs from 100 kW to 6 MW. An international milestone is the heat pump built by Escher Wyss in 1937/38 to replace wood stoves in Zurich City Hall . To avoid noise and vibrations, a recently developed rotary piston compressor was used. This historic heat pump heated the town hall for 63 years until 2001. Only then was it replaced by a new, more efficient heat pump. The companies mentioned had built another 25 heat pumps by 1955. The steadily falling oil prices in the 1950s and 1960s then led to a dramatic slump in sales of heat pumps. In contrast, the vapor compression business remained successful. In other European countries, heat pumps were only used sporadically with simultaneous cooling and heating (e.g. dairies). In 1968, the first ground-coupled heat pump for a single-family house in Germany in combination with low-temperature underfloor heating was realized by Klemens Oskar Waterkotte .

The oil embargo of 1973 and the second oil crisis in 1979 led to an increase in the price of oil by up to 300%. This situation favored the heat pump technology enormously. There was a real heat pump boom. However, this was suddenly ended by too many incompetent suppliers in the small heat pump sector and the next drop in the price of oil towards the end of the 1980s. In the 1980s, numerous heat pumps powered by gas and diesel engines were also built. However, they were unsuccessful. After a few years of operation, they had to deal with frequent breakdowns and high maintenance costs. In contrast, the combination of combined heat and power plants with heat pumps, known as “total energy systems”, prevailed in the area of ​​higher heat output. For example, Sulzer-Escher-Wyss implemented a 19.2 MW total energy system with a utilization rate of 170% at the ETH-Lausanne based on the concept of Lucien Borel and Ludwig Silberring . As the largest heat pump system in the world with seawater as the heat source, Sulzer-Escher-Wyss supplied a 180 MW heat pump system with 6 heat pump units of 30 MW each for the Stockholm district heating network in 1984-1986 . The range of heat sources was expanded to include thermoactive building elements with integrated pipelines, sewage, tunnel sewage and low-temperature heating networks.

In 1985 the ozone hole over the Antarctic was discovered. Then in 1987 the Montreal Protocol was a global concerted action to rigorously phase out CFC refrigerants. This led to global emergency programs and a rebirth of ammonia as a refrigerant. The chlorine-free refrigerant R-134a was developed and used within just four years . In Europe, the use of flammable hydrocarbons such as propane and isobutane as refrigerants has also been promoted. Also, carbon dioxide enters increasingly used. After 1990, the hermetic scroll compressors began to replace the reciprocating compressors. The small heat pumps became less voluminous and had a lower refrigerant content. However, the market for small heat pumps still required a certain “self-cleaning effect” and concerted accompanying measures for quality assurance before a successful restart was possible towards the end of the 1980s.

After overcoming the “burned-child effect” in small heat pumps, heat pump heating began to spread rapidly in 1990. This success is based on technical advances, greater reliability, quieter and more efficient compressors as well as better control - but no less also on better trained planners and installers, seals of approval for minimum requirements and, last but not least, on a massive price reduction. Thanks to power regulation using more cost-effective inverters and more complex process management, heat pumps are now able to meet the requirements of the renovation market with high energy efficiency.

See also


  • Hermann Recknagel, Ernst-Rudolf Schramek, Eberhard Sprenger: Pocket book for heating air conditioning technology. 76th edition. Oldenbourg, Munich 2014, ISBN 978-3-8356-3325-4 .
  • Maake-Eckert: Pohlmann Taschenbuch der Kältetechnik. CF Müller, Karlsruhe 2000, ISBN 978-3-7880-7310-7 .
  • Marek Miara et al .: Heat pumps - heating - cooling - using environmental energy. BINE-Fachbuch, Fraunhofer IRB Verlag, Stuttgart 2013, ISBN 978-3-8167-9046-4 (basics with a focus on systems engineering, monitoring experience, current technology).
  • Klaus Daniels: Building Technology, A Guide for Architects and Engineers. VDF , Zurich 2000, ISBN 3-7281-2727-2 .
  • Heat from renewable energies, save costs - increase living quality - protect the environment. Brochure from the German Energy Agency , Berlin 02/2007, pp. 33–36 ( online PDF 46 pages 2.6 MB ).
  • Thorsten Schröder, Bernhard Lüke: Heat sources for heat pumps. Dortmund book, Dortmund 2013, ISBN 978-3-9812130-7-2 .
  • Martin Kaltschmitt, Wolfgang Streicher: Renewable energies in Austria. Basics, system technology, environmental aspects, cost analyzes, potential, use. Vieweg + Teubner, Wiesbaden 2009, ISBN 978-3-8348-0839-4 .
  • Jürgen Bonin: Handbook heat pumps. Planning and project planning. Published by DIN , Beuth, Berlin / Vienna / Zurich 2012, ISBN 978-3-410-22130-2 .

Web links

Individual evidence

  1. a b c d Heat from renewable energies, save costs - increase living quality - protect the environment , brochure from Deutsche Energie-Agentur GmbH (dena) (www.dena.de) 02/2007, pp. 33–36.
  2. a b Landolt Börnstein, New Series VIII / 3C, keyword: Heat pumps, pp. 608–626.
  3. ↑ Annual performance factor of heat pumps .
  4. Energy saving in buildings: state of the art; Trends, at Google Books , page 161, accessed August 16, 2016.
  5. Energy economic evaluation of the heat pump in building heating
  6. WPZ book
  7. ^ A b c d e Zogg M .: History of the heat pump - Swiss contributions and international milestones, Federal Office of Energy, Bern 2008. ( admin.ch [accessed on August 4, 2020]).
  8. ^ Thomson W .: On the Economy of Heating and Cooling of Buildings by Means of Currents of Air. In: proceedings of the Philosophical Society. No. 3, 1852, pp. 269-272.
  9. Wolfinger U .: 125 years of Linde - a chronicle, Linde AG, Wiesbaden 2004. ( vhkk.org [PDF; accessed on August 4, 2020]).
  10. ^ Thevenot K. "A History of Refrigeration Throughout the World, International Institute of Refrigeration, Paris 1979."
  11. Wirth E .: From the history of the development of the heat pump, Schweizerische Bauzeitung 1955, vol. 73, no. 52, pp. 647-650. ( e-periodica.ch [accessed on August 4, 2020]).
  12. ^ Zogg M .: History of Heat Pumps - Swiss Contributions and International Milestones, Swiss Federal Office of Energy, Berne 2008. ( admin.ch [accessed on August 4, 2020]).
  13. Waterkotte, K. (1968): Earth-water heat pump system for a single family home . ETA elektrowärme int. 30 / A, pp. 39–43, Essen.
  14. Pelet X., D. Favrat, A. Voegeli: Experience with 3.9MWth Ammonia Heat Pumps - Status after Eleven Years of Operation. In: proceedings of the Workshop IEA Annex 22, Gatlinburg, TN, USA, Oct. 2-3, 1997.
  15. ^ Zehnder M., D. Favrat, E. Zahnd, J. Cizmar, D. Trüssel: Heat pump with intermediate injection in scroll compressors, final report, Federal Office of Energy, Bern 2000. ( admin.ch [accessed on August 4, 2020]).