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Last changed: 30/11/12
Combined heat and power, or cogeneration plants (CHP plants) generate both electricity and heat. When these plants are operated locally, or decentralised, they are known as district heating plants.
Some 74% of the electric power in Germany originates from large power plants, which convert on average only 41% of the energy supplied from coal, nuclear fuel, natural gas or petroleum into usable electrical power [UBA 2007]. The remaining energy input is lost as waste heat. In theory, this waste heat could provide the heating for all the buildings in Germany. 14.5% of electrical power stems from renewable energy plants, and only about 12% from plants based on cogeneration. [BMU 2009]
Cogeneration units produce both electricity and heat and have up to 90% thermal efficiency. The heat produced in the process of power generation is not released unused to the environment (as is visible from the cooling towers, for example, of coal power plants that produce electricity only). Therefore, the Federal government has decided to double the CHP share of electricity supply to 25% of total by 2020. [BMU 2008]
Widely varying technologies are applied in cogeneration units, ranging from combustion engine to gas turbine. The fuels used include natural gas, heating oil, biogas or vegetable oil, as well as wood pellets or wood chips.
CHP as a decentralised means of energy supply is very flexible. The size of an available CHP plant ranges from one that supplies a detached house to larger systems with an output ranging from 50 kilowatts to 2 megawatts of electric power that can supply blocks of flats or commercial property with heat and power. The heat produced in CHP is transported either directly or via a local heat distribution network to the final consumer. Surplus power is fed into the local power grid. CHP operators receive remuneration for this at a per kilowatt hour rate that is prescribed by the Combined Heat and Power Act (KWK-Gesetz) and the Renewable Energies Sources Act (Erneuerbare-Energien-Gesetz). The Acts regulate feed-in and compensation for power produced by CHP plants, payment of investment funds for heat networks, and apportionment to power consumers. CHP remuneration is derived from the price per kWh paid by the network operator, the monetary advantage that this decentralised feed-in offers (cuts out grid use cost), and the CHP supplement. The CHP supplement is paid by the grid operator to CHP plant operator for total kilowatt hours generated, less that needed to cover the plant’s own energy needs. The additional connection to the local network ensures secure power supply even at times of higher demand. [KWK Modellstadt Berlin]
As with a traditional boiler the size of a cogeneration unit is determined by the heating requirements of the target building. What distinguishes them from larger-scale technology is that heat arrives there as a waste product and can be decoupled. In heat-led cogeneration units power is produced almost incidentally. What is new is an idea by Lichtblick to link electricity-led cogeneration units with an electricity output of 20 kilowatts and thermal output of 34 kilowatts so that they act in concert as a large power plant and can supply power quickly at peak demand times. One communications unit can link up to 100,000 home power plants to one decentralised large-scale power plant via mobile communications or a DSL connection. [LichtBlick]
In cogeneration units operated with combustion engines or gas turbines, waste heat is produced in the exhaust gas, which heat exchangers transport to the heating circuit.
Operation of a heat-led CHP plant makes particular economic sense when there is a demand for heat year-round. This demand may exist among commercial enterprises that require heat for production processes. Cogeneration units are therefore well-suited for use in industry as well as hospitals, swimming pools and trade. Besides, CHP plants are also suited for backup power supply as well as refrigeration, and therefore to provide air conditioning in buildings. In combined cooling, heating, and power generation (CCHP) the generated heat is used to operate an absorption chiller. The international term for CCHP is ‘trigeneration‘ and thus reflects the idea of "cogeneration" in CHP.
An absorption chiller provides refrigeration by using heat in combination with a thermal compressor. The refrigerant is absorbed by a second substance in a solvent circuit at low temperature, and desorbed at higher temperatures. The most common solution is lithium bromide, which absorbs water, or water, which absorbs ammonia. The temperature-dependent physical solvency properties of both substances are used in this process.
Substances are separated by heating the solution. The refrigerant vaporises initially due to low evaporation temperature. The refrigerant is then decompressed to the desired evaporation pressure, cooled, and liquefied. The refrigerant then evaporates through absorption of the ambient heat which comprises its useful effect. After separation from the refrigerant the solvent is released, cooled and can then reabsorb refrigerant vapour. The materials are then reheated and separated, starting the cycle anew.
The disadvantages of an absorption chiller compared to an electric compressor refrigerator are the higher investment costs and its larger overall installed size. On the other hand its advantages are that it serves as a heat sink in summer and thereby boosts the annual operating hours and efficiency of a CCHP plant. The heat generated in thermal photovoltaic systems or geothermal energy systems is also used in absorption chillers. Absorption chiller units require virtually no servicing on account of the absence of mechanical moving parts.
Larger-scale cogeneration units are not operated at partial load (no modulating operation), which is why a number of cogeneration units are used and switched off individually if demand is low. Smaller cogeneration units on the other hand are operated in power modulation, which allows for adjustment to demand. In general, cogeneration units are designed to operate at about 40% of maximum heating load (base load), to ensure efficient operation. An additional heat generator used for peak periods in the winter is necessary in this case. To achieve a high seasonal efficiency the cogeneration unit should, if possible, be in operation for many hours, that is between 4,000 and 8,000 hours per year. A buffer to absorb surplus heat serves to offset any fluctuations in demand. This design of cogeneration unit supplies between 50 and 75 percent of the total heating and warm water needs of one building.
Upon procurement of a cogeneration unit it should be kept in mind that its vibration does make considerable noise. Therefore, a call to tender for a cogeneration unit must address the issue of noise emissions and stipulate that it be set up in a separate structure or in the cellar if at all possible.
Tests are currently in progress in natural gas-operated Stirling-engine cogeneration units with 1 kW electrical and 12 kW thermal output, which can generate up to 30,000 kWh of heat per year, a volume that corresponds to 3,600 hours of operation. In addition, 1,000-3,000 kWh of electricity is also produced, which covers about 25-50% of one household’s power needs. These systems are heat-led and operated without an additional boiler for peak periods (monovalent). The main purpose of the installation is to supply the heat for a house while supplying some of its electricity. For 2011, a number of producers have announced the set-up of decentralised systems with Stirling engines with a thermal output in the range of 3.5 to 6 kilowatts. [BMU 2009]
Range of application and size of CHP
| Supply of: | Electric power in kW | Thermal output in kW | Final energy |
| Residential flat/Semi-/detached house | ca. 1 | 4 – 10 | Heat/Power |
| Block of flats | 5 – 30 | up to 100 | Heat/Power |
| Terraced house block | 5 – 30 | up to 100 | Heat/Power |
| Nursing home | 10 – 30 | up to 200 | Heat/Power |
| Hotel, small trade | ca. 30 – 50 | up to 300 | Heat/Power/Refrigeration |
| School | up to 50 | up to 300 | Heat/Power |
Source: [BMU 2009]
A building licence for CHP plants is only required over and above a certain size, which varies from one federal state to the next. For more information, contact the local building authority. A licence pursuant to air pollution control law is required in a combustion engine-operated cogeneration unit with a rated thermal input of 1 MW and above. For these cases the notification of approval sets emission limit values, which are usually oriented towards the Technical Regulations on Air Quality (TA Luft).
The following tax provisions apply for operation of a cogeneration unit:
For the sake of climate protection and productivity of resources, the cogeneration of heat and power is superior to their separate production because the emitted carbon dioxide and pollutant volumes per unit of energy are generally far lower. The core of most cogeneration processes is the production of heat energy. After completion of a process at high temperature and high pressure (e.g. operation of a turbine or a motor used to generate electricity) heat energy can be used for other energy purposes in low-temperature processes, e.g. space heating. This multiple use accounts for the overall greater efficiency of cogeneration processes as concerns use of the energy contained in the source.
This reduced consumption of primary energy provides significant relief to the environment. The supply of heat and power from a cogeneration unit emits 34% less CO2 to the atmosphere than does their individual production with the use of fossil fuels. Considerable volumes of other pollutant emissions are avoided when CHP units are fired with natural gas instead of coal: hardly any sulphur dioxide is produced, and the volumes of nitrogen oxides and carbon monoxide are usually lower than in coal-fired power plants. Moreover, transport and distribution losses are kept to a minimum as production occurs locally on the user’s site. [BMU 2009]
CO2 Emissions of the usable energy of 34 electricity units, 56 heat units
District heating plant = BHKW
CO2 reduction = CO2 Minderung
Power plant = Kraftwerk
Boiler = Heizkessel
Energy supply = Energiebereitstellung
Source: [BMU 2009]
When combustion-engine CHP operate with biofuel, heating oil or vegetable oil, however, a much more differentiated impression results: it is precisely small biogas plants that are not bound to compliance with the Technical Regulations on Air Quality (TA Luft) that often emit more NOx and CO than modern coal power plants. CHP operated with heating or vegetable oil also generally produce higher NOx emissions than coal power plants; better CO emissions outcomes as compared to coal power plants can only be achieved with efficient flue gas cleaning. Furthermore, liquid fuel-fired plants with no flue gas cleaning normally have higher soot emissions than coal power plants.
The Blue Angel is the oldest official ecolabel that is used voluntarily to mark environmentally friendly products. The best products available on the market in a certain category are eligible for award of the Blue Angel. The label provides an incentive to develop products that are safer for the environment and health.
Small gas-fired cogeneration units, RAL-UZ 108
Cogeneration units marked with the Blue Angel save primary energy and reduce CO2 emissions. Other units besides cogeneration operated with gas or diesel engines might include Stirling engine-operated units.
Award criteria are developed by the Federal Environment Agency in concert with manufacturers, testing institutions, and other experts and members of consumer associations. An independent Environmental Label Jury reviews and determines award criteria. The award itself is granted by the RAL gGmbH on behalf of the Federal Environment Agency.
The emissions levels listed in the tender recommendation table for gas-fired cogeneration units are more stringent than those of the award criteria for the Blue Angel environmental label for small gas-fired cogeneration units (RAL-UZ 108). The current award criteria are under review in 2010 for adaptation to the state of the art, which will result in a tightening of emissions levels.
For the procurement of liquid fuel-fired cogeneration units the requirements of the (currently expired) climate protection impulse programme to promote small CHP plants by the German Federal Ministry for Environment can be applied. The programme foresees primary energy savings of at least 10% as compared to the separate generation of heat and power. Furthermore, the seasonal efficiency must be at least 80%. For plants with less than 1 MW rated thermal input, the requirements of the applicable Technical Regulations on Air Quality (TA-Luft) apply. [BMU 2008]
Note: The bidder declares that the individual requirements have been met and presents testing documentation of an independent testing institute complete with identification of testing equipment and method and all tolerances.
One of the Berliner Energieagentur’s (BEA) modern cogeneration units has supplied heat and power since October 2009 to three Berlin fire stations (Jagowstraße, Katzengraben and Grafensteinstraße) held by the BIM Berliner Immobilienmanagement GmbH. Thanks to this highly efficient technology an annual total of 425 tonnes of CO2 is saved as compared to traditional production of energy.
The three cogeneration units supplement the energy supply of the existing heating system. More than 90% of the primary energy input, which is natural gas in this case, leaves the small cogeneration units again as usable end-user energy in the form of electricity and heat. Per year they supply 1,650 megawatt hours of heat and 800 megawatt hours of electricity. The BEA has assumed the costs for the cogeneration units as contractor, making it responsible for the set-up and operation as well as service and maintenance of the installation. The three compact power sources each have an electrical output of 48 kW and thermal power output of 97 kW. The power generated by the cogeneration unit is used on site. Excess power is fed into the public grid and compensated. These funds are in turn used to refinance the installation.
Contact person:
Volker Gustedt
Berliner Energieagentur GmbH
Tel.: 0 30 / 29 33 30 - 19
Fax. 0 30 / 29 33 30 - 97
Email: gustedt@berliner-e-agentur.de
www.berliner-e-agentur.de
[BEA 2009]