Block-type thermal power station

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Biogas CHP in Güssing , Austria
Mini-CHP in an encapsulated design

A combined heat and power ( CHP ) is a modular built plant for the production of electrical energy and heat , which is preferably operated at the site of heat consumption. Useful heat can also be fed into a local heating network. The system uses the principle of combined heat and power for this .

As a drive for the generator to internal combustion engines , d. H. Diesel , vegetable oil or gas engines , but also gas turbines , Stirling engines or fuel cells can be used.

The higher overall degree of utilization compared to the conventional combination of local heating and central power plant results from the fact that the waste heat from electricity generation is used. The efficiency of electricity generation is between 25 and 44% (based on the calorific value ) , depending on the size of the system . If the waste heat is used completely and locally, an overall efficiency with regard to the primary energy used of 80 to 90% (based on the calorific value) can be achieved. Condensing boilers achieve efficiencies of up to 100% (based on the calorific value), but cannot generate electricity. (See also on efficiency> 100% in general and different efficiency definitions for condensing boilers .)

Common CHP modules have electrical outputs between one kilowatt (kW) and a few tens of megawatts (MW). Below 50 kW one also speaks of mini combined heat and power (mini-CHP), below 10 kW of micro-CHP . Systems with less than 2.5 kW are also called nano-CHP units. Mini and micro CHP are used in residential and business districts, hospitals, swimming pools and apartment buildings, but also in businesses and in housing developments. The nano-CHP class in particular is also suitable for single-family houses. Combined heat and power is also used in thermal power stations , there typically with electrical outputs of a few hundred MW.

Operating modes

Ideally, the purchase of the grid is covered so that overproduction can be fed into the grid. The heat is used for heating purposes and hot water preparation.

Heat guided

If the power output of the CHP is based on the local heat demand, it is a heat-controlled CHP. By regulating the heating output, individual units are switched on or off in modular systems as required. In the case of a configuration with only one unit , either its output is regulated accordingly or a heat storage tank is loaded in intermittent operation. The memory is often designed so that the engine needs at least one hour to fully charge. Since the maintenance intervals are based on the operating hours of the system, interval operation is usually the more economical mode of operation than modulated operation. The electricity generated by such systems is used as far as possible; the surplus is fed into the public grid and offset accordingly.

Current operated

Emergency cooler (horizontal fans in the center of the picture) on the container roof of a biogas CHP

In the case of a power- controlled CHP unit, the output depends on the power requirement or the capability of the unit (max. Power generation according to EEG, see below). The heat that cannot be used during this period is given off as waste heat to the environment via an emergency cooler , although this reduces the efficiency. On the one hand, this operating mode is often found in island networks . In Germany, a number of CHP units are powered by electricity using renewable raw materials. The legally stipulated feed-in tariffs for the electricity produced ( Renewable Energy Act , EEG for short) enabled operators to maximize their profit by producing as much electricity as possible. The recovery of the resulting heat was often neglected. The more recent changes to the EEG have significantly reduced this possibility.

Stream oriented

In the case of electricity-oriented operation, the first control variable remains the heat demand. In terms of control technology, however, the electricity load is covered as far as possible. The CHP runs when there is a need for electricity, the heat is used directly or is temporarily buffered in a heat store for later use. The CHP does not have an emergency cooler. In times of low electricity load, the heat demand is primarily met from the heat storage, after which the CHP goes into operation. This operating mode maximizes the electrical self-use, without loss of efficiency and without condensation operation.

Network controlled

If the performance level is specified by a central point for several systems, it is referred to as a grid-controlled CHP. The head office optimizes the deployment planning of the decentralized CHP units across systems based on economic boundary conditions, such as joint gas and residual electricity purchase agreements. Network management is the core idea of ​​a virtual power plant . As with the power supply, a storage tank must be integrated for the temporal decoupling of heat generation and load.


Usually, the output of a heat-controlled CHP system is designed in such a way that it only covers part of the maximum heating energy requirement of the connected consumers even in full load operation, the required residual heat is supplied by a peak load boiler . This ensures that the expensive electricity-generating facilities are used better and achieve higher operating hours. At least 7,900 hours per year are aimed for, but often only around 3,000 to 5,000 hours are achieved.

With the help of buffer storage, mini-CHP units for residential buildings are operated monovalently, i.e. without a peak load boiler. Such a CHP is not designed - as described above as usual - according to the base load of heat energy demand, but like a conventional heating system according to the peak load. This type of interpretation is propagated especially for mini-CHP . With such a design, the CHP unit is switched on and off (so-called "cycling"), which shortens its service life.

An existing heating installation can usually continue to be used with minor changes when converting to a mini-CHP unit. On the one hand, there is the option of using a monovalent CHP unit with the inclusion of larger heat storage units. If, on the other hand, the heating is not sufficient in the winter months, additional heating can be carried out with the existing burner or a peak load boiler (bivalent use). If additional heating is only rarely required, the installation of a simple electrical additional heater (immersion heater principle) can also be cost-effective. Larger CHPs can also be optimized through the use of heat storage. These serve to buffer the peak load and thus avoid the requirement of additional heat from non-CHP generation (boilers). Furthermore, they allow temporary current-controlled operation, i. H. a storage tank charging at times with high electricity prices.

Ecological and environmental aspects

The basic economic and ecological concept of heat-controlled operation is to use the heat generated in full and, if possible, also use the electricity on site. Unused electricity is fed into the public grid for a fee . Since less conventional power plant capacity is required for electricity generation in this way, the increased use of CHP replaces the electricity from fossil condensing power plants of the medium load (mainly coal) and thus enables lower carbon dioxide emissions. This should also be supported in Germany by legal regulations such as the KWKG .

A CHP unit achieves a significantly higher degree of utilization (useful energy electricity plus useful energy heat divided by energy input) compared to conventional mixed operation of local heating and central power supply. A modern, large-scale coal- based power plant, for example, achieves an efficiency of around 45%. That means: around half of the energy generated is waste heat. However, using them as district heating would entail considerable transport losses (10–15%) and require a complex and expensive pipeline network , since in large central power plants the heat generator and the heat consumer are usually far apart. Another 2 to 5% of the energy is lost through the transmission of electricity. CHP units have an electrical efficiency of approx. 25 to 38% (depending on size and type); the overall efficiency of approx. 90% can only be achieved if electricity and heat are used directly on site. The advantage of being able to use the process heat on site requires suitable customers. Residential buildings need different amounts of heat depending on the season, which is why the process heat can only be partially used in summer.

The effort involved in maintaining the engines (such as oil changes and air filters, the spark plugs in the case of gas engines, etc.) with the associated trips to and from the service staff has a negative impact on the economic balance, as it is necessary for smaller and decentralized systems, especially the micro- CHP , specifically, increases significantly.

The use of palm oil as a fuel is particularly critical. The cultivation of palm oil plantations takes place in many emerging and developing countries at the expense of the rainforest, often even through illegal logging, despite alliances such as the RSPO (Round Table of Sustainable Palm Oil). The last rainforests are shrinking massively due to the ever increasing demand for palm oil for fuel, food, cosmetics, etc. This destroys unique ecosystems and the habitats of innumerable animal and plant species irretrievably. In addition, the deforestation of the forests releases huge amounts of CO 2 every year .


Functional diagram of a motor-driven CHP plant

Originally, CHP systems were based on combustion engines, the heat of which from the exhaust gas and the cooling water circuit is used to heat the heating water. In the meantime, other systems such as the Stirling engine and steam engine with a relatively low level of efficiency are used to generate the mechanical energy (necessary for the generator) in CHP systems. Depending on the type of internal combustion engine, the use of combined heat and power plants is not limited to providing room heat , but is also used to generate process heat using steam, hot air or thermal oil or for air conditioning using an absorption heat pump . This uses the waste heat from the CHP system for cooling.

The main fuels used are fossil or regenerative hydrocarbons such as heating oil , vegetable oil (mostly palm oil ), biodiesel (for a diesel engine ) or natural gas or biogas (for a gasoline engine ), pilot injection engine or a gas turbine ( biogas engine ) . Wood chips and wood pellets as renewable raw materials in Stirling engines , steam power plants and wood gasifiers are also possible, the latter in connection with a gasoline engine (efficiency <36%).

In CHP systems based on combustion engines or gas turbines, waste heat is generated in the cooling circuit and in the exhaust gas. It is transferred to the heating circuit of the building's central heating system via heat exchangers . A degree of utilization of up to 95% can be achieved, depending on the respective utilization of the motor and the motor efficiency. The pure electrical efficiency at full engine load is, depending on the fuel, size and construction (e.g. with or without turbo charger) of the engine and generator, between 20% (with mini-CHP ) and 43% (with diesel engines with outputs above 1 MW).

Small CHP ( micro-CHP ) up to around 10 kW electrical and around 20 kW thermal output are suitable for heating single and multi-family houses and small businesses. Medium-sized CHPs with an electrical output of several hundred kW are often used by municipal utilities to heat residential areas or indoor swimming pools or larger commercial operations. Large CHP units with gas engines with an output of several MW are suitable for supplying electricity and heat to larger residential and commercial areas as well as factories.


A large annual number of operating hours in the high load range of the engine (typically around 4,000 full load hours per year) is essential for the profitability of a CHP system. The aim is to balance the investments in the plant economically with the financial compensation for the amount of electricity and heat generated (or a reduction in the associated costs).

In the case of heat-controlled CHP units, the annual duration curve of the heat demand (a curve plotting how many hours per year which heat output is required for the building) is used to determine the heat output, which requires around 3000 hours per year. The peak heat output of the CHP is set to this output; as a rule, this is 25-30% of the peak heat demand. In order to be able to provide the output in stages, CHP units are usually equipped with several motors in a modular manner.

In the living area, the heating energy requirement fluctuates very strongly with the seasons; in midsummer it is only used for heating domestic hot water. At the moment there are only a few CHP models that can change the heat generated and thus also the coupled electricity ("modulating operation"). Fluctuations in demand - limited by heat storage - can, however, be compensated. A parallel operation with a conventional burner for the thermal peak demand and as a reserve in case of malfunctions or maintenance is common. If there are no measures to compensate for such fluctuations in the heat demand, the CHP system will be switched on and off ("cycling") more frequently, which leads to a reduction in efficiency and service life.

In a comprehensive profitability analysis, all investment and operating costs - i.e. depreciation , fuel and maintenance - are included and compared with the revenues for electricity and heat or the amounts saved. The VDI guideline 2067 "Economic efficiency of technical building systems" forms the basis of this profitability analysis.

In summer, for example, an absorption refrigeration machine, which has to be invested in addition, can be used to take off heat when there is no heating energy requirement , which generates cold for air conditioning. One then speaks of combined heat, power and cooling . In the future, the use of CHP units as a virtual power plant is also planned in which a large number of decentralized CHP units are centrally controlled. Due to the increasing share of wind and solar energy , which do not supply electricity as required and also cannot provide a base load, CHP units with a downstream heat buffer could work even more profitably: They provide the necessary electrical power locally, and heat storage units can be filled on the heating side .

A basic problem in marketing CHPs, especially in single and multi-family houses, is the higher purchase price compared to conventional natural gas or oil heating systems. The heating system comparison calculator of the consumer center in North Rhine-Westphalia compares the total costs of different heating systems (including CHP) for existing residential buildings from single-family houses to ten-family houses. The use of CHPs in factories and companies may be advisable. The Hanseatic City of Hamburg offers a good opportunity with the CHP check by specialist advisors. Here it is checked whether it makes sense to use it in the company, as there are still high energy and savings opportunities in these areas.

In comparison, gas engines have a higher electrical efficiency with lower specific investment costs, micro gas turbines can also use gas with a lower calorific value, tend to have a longer service life with lower operating and maintenance costs and lower operating noise.

Public funding

CHP, operated with cold-pressed rapeseed oil

Since January 1, 2009, CHPs have been promoted in Germany by the law for the maintenance, modernization and expansion of combined heat and power , or the Combined Heat and Power Act for short. Network operators are obliged to connect a CHP plant to their electricity network and to pay for the electricity fed into the public network. The remuneration is made up of the average price for base load electricity (baseload price) on the Leipzig electricity exchange EEX , a CHP surcharge in accordance with the CHP Act and a network charge for decentralized feed-in. In addition, from January 1, 2009, the CHP systems will also receive electricity for the B. is used in the supply property (hotel, residential building) itself, a CHP surcharge. CHP systems that were or can be categorized according to the (old) CHP Act of April 1, 2002, have also received a CHP surcharge since January 1, 2009 for electricity used outside the public grid. On July 19, 2012, the new CHP law came into force. For example, mini-CHP systems that were connected before July 19, 2012 receive a remuneration of 5.11 cents / kWh, systems that were installed afterwards receive a subsidy of 5.41 cents / kWh according to the new remuneration model .

State subsidies also include tax breaks. For the fuels natural gas, heating oil and liquid gas, the energy tax , formerly " mineral oil tax ", was fully reimbursed until April 1, 2012 when used in CHP systems with an annual efficiency of at least 70% . Due to a change in the law, the full relief will only be provided retrospectively from April 1, 2012 if the system has not yet been fully depreciated under tax law in addition to the minimum degree of utilization of 70% and is highly efficient within the meaning of Directive 2004/8 / EC of the European Parliament. In addition, no electricity tax (2.05 cents / kWh) has to be paid for electricity from CHP plants with an electrical output of up to 2 MW, which is used by the plant operator in the “spatial context” .

Since 2012, the Federal Environment Ministry has been promoting the use of highly efficient mini-CHP systems in the power range of up to 20 kilowatts of electrical power as part of the National Climate Protection Initiative. On January 1, 2015, the amended guideline for the promotion of CHP systems with an electrical output of up to 20 kW came into force. As a result, the basic subsidy in the small output range was increased, bonus subsidies were introduced for particularly energy-efficient mini-CHP systems and technical requirements were simplified.

In practice, the KWK law from 2002 did not generate the expected dynamism of the addition of small CHP plants (up to two MW according to § 3 (3) KWKG), to which the CHP plants generally belong, although there is great potential here lie fallow that could be exploited economically.

CHP electricity generated with biomass as a raw material, which is fed into the power grid under the Renewable Energy Sources Act (EEG), was remunerated with an additional bonus of three cents / kWh in accordance with the rules of the EEG (CHP bonus according to Appendix 3 EEG 2009). With the amendment of the EEG, this bonus no longer applies at the beginning of 2012. In addition to grants and loans from KfW and BAFA, many federal states, municipal energy providers and municipal utilities also provide direct funding for retrofitting to a CHP unit.


  • Wolfgang Suttor: Combined heat and power plants - A guide for the user. 8th revised edition. Fraunhofer IRB Verlag, Stuttgart 2014, ISBN 978-3-8167-9303-8 (guidelines for users in the housing industry, settlement planning, municipalities, energy providers, not house builders)

Web links

Commons : Combined heat and power unit  - collection of images, videos and audio files
Wiktionary: cogeneration unit  - explanations of meanings, word origin, synonyms, translations

Individual evidence

  1. Information on combined heat and power systems for single and multi-family houses from co2online , accessed on January 12, 2015.
  2. FfE: CHP displacement mix, 2008 (PDF; 386 kB)
  3. Overview report on the new VDI 2067 (CHP information center)
  4. Page no longer available , search in web archives: Economic comparison of heating systems , consumer center NRW / City of Düsseldorf.@1@ 2Template: Toter Link / www.verbü
  5. BHKWCheck / Hanseatic City of Hamburg ( Memento from September 3, 2013 in the Internet Archive ) (PDF; 631 kB)
  6. ^ Roman Lugmayr: Technical and economic comparison of a gas engine with a gas turbine ( Memento from May 12, 2013 in the Internet Archive ) (PDF; 1.7 MB), accessed on February 7, 2012.
  7. Only partial energy tax relief for CHP units. November 13, 2012, accessed December 4, 2012 .
  8. CHP plants will receive the energy tax refund again in the future. November 13, 2012, accessed December 4, 2012 .
  9. Changed application procedure for the energy tax relief. February 1, 2013, accessed December 30, 2014 .
  10. ^ Novella mini CHP impulse program. December 30, 2014, archived from the original on December 25, 2014 ; accessed on December 30, 2014 .
  11. Mini CHP impulse program: From 2015 significantly more funding for nano and micro CHP units. December 30, 2014, accessed December 30, 2014 .
  12. A cost curve for greenhouse gas reduction, McKinsey Quarterly, 2007 (PDF; 291 kB)
  13. overview of all current funding for CHP from co2online , accessed on July 25, 2014.