Last changed: 30/11/12
District heating is the term used to describe the transport of thermal energy from the generator to the consumer. This is mostly used to heat buildings and for the provision of hot water. A large-scale industrial application of district heating is in its use as process heat. The heat is transported using a thermally insulated system of pipes. District heating can be used to meet the heating requirements for whole towns or districts. If individual buildings, parts thereof or small residential estates are heated using their own heat generation systems, the term employed is “local heating”. Technically and legally, however, the correct designation in both cases is “district heating”. Large district heating networks are fed by co-generation plants, usually fired using fossil energy carriers. Smaller networks permit the deployment of renewable energies, such as, for example, biomass.
The use of district heating is basically only possible in places where a district heating network is already in place or can be built. In those places where district heating networks are already available, heating needs should, in particular from a climate protection point of view, preferably be met by the use of district heating. CO2 and primary energy savings are particularly noteworthy factors here. District heat is most commonly generated in co-generation plants, which, in comparison to separate generation, bring about savings in primary energy and lead to reductions in carbon dioxide emissions. In residential areas in particular, the maintenance of air cleanliness is an additional significant advantage, as heat supplied through district heating avoids the problem of heat generation in the residential areas themselves with its inevitable associated emissions. [UBA 2007b]
The use of district heating is supported by the amended CHP Act (KWK-Gesetz) which came into force on 1.1.2009. The expansion of existing, and construction of new, co-generation plants is fostered by a graded CHP bonus for the CHP electricity generated. The use of heat thus generated is additionally supported by an investment grant of a maximum of 20% for the expansion and construction of heating networks. Support for smaller co-generation plants with an output of up to 50 kWe comes within the framework of the climate protection initiative in the form of an investment subsidy. The heat generated can be distributed via a local heating network. This lends itself, for example, to biogas co-generation plants in rural areas.
The share of residences heated with district heat in 2003 was 13.7%. The proportion in the eastern Federal states is significantly higher than in the west of the country. In contrast to the share in the west of 9%, 32% of all households in the eastern states are supplied with district heat. Throughout the entire country the thermal output from district heating currently lies at approx, 57,000 MWth. The heat thus generated is distributed via 1,400 networks with a total length of approx. 19,000 kilometres. Roughly 550 of the 1,000 or so German energy supply companies supply households with district and local heat. [cf. AGFW 2008]
The costs of district heat mirror the prices of the energy carriers. Structural changes are also leading to an increase in prices. Responsible for this are also demographic developments in Germany. Fewer births, an ageing population and emigration-- with regional differences- are leading to an ever greater decline in population. Factors which oblige the energy companies to distribute their unchanging fixed costs over a dwindling demand for heating. At the same time, increasing numbers of buildings are undergoing energy-saving refurbishment, and solar thermal energy is increasingly being used to generate heat. The result: A reduced demand for heating. [UBA 2007a]
A further expansion in the use of district heat generated in co-generation plants could bring about additional emissions reductions, with particular reference to carbon dioxide emissions. Whilst it is certainly true that district heating networks have been expanded in recent years (network lengths and number of transfer stations), the amount of district heat actually supplied has been in decline. One reason for this lies in the improved thermal insulation of buildings and the installation of solar thermal systems, which has led to a reduction in demand. Expansion of the district heating networks should be expedited by the conversion of large power stations currently without district heat extraction and the connection of residential or public buildings to existing district heat supply lines that run close to them but to which only large-scale users are currently connected (greater network density). However, moves to implement the retrospective connection of such buildings often fail to surmount the obstacles presented by private reservations or investment costs.
In the planning of new heating networks for a locality or a region the heating requirements should be recorded along with the seasonally-determined load profiles and an optimised heating concept developed that takes existing conditions into account. The quest for a meaningful use of heat in the summer months presents a particular challenge. For institutions with large-scale refrigeration requirements, e.g. hospitals and shopping centres, the use of district cooling may be of interest. In such cases the client is supplied with hot water in the winter, whereby an absorption chiller is used locally to generate the refrigeration required.
If possible, consideration of the issue of heating requirements should not be restricted to public bodies: the heating needs of private individuals and companies should also be taken into account. The long lifetime and depreciation period of systems installed to supply district heating make it important to take into consideration demographic developments in the area in which the installation is to be carried out. In the eastern Federal states above all it must be assumed that population levels will continue to decline. The long lifetime and depreciation period of systems installed to supply district heating mean that such a development can have a significant influence on the planning of such systems.
The current situation is that all of the Federal states are considering the possibility of enacting legislation to make the connection to, and use of, district heating systems compulsory, whereby the state of Bavaria is alone in its intention to restrict this to new buildings and renovation areas. The basis of authority of the introduction of such a compulsion is to be found in the by-laws of old Federal states and, in the new states, in the law on the self-government of local authorities and rural districts in the GDR (local authority constitution of 17 May 1990). The essential premise is that such compulsion must be required by the general good or the health of the population or, as the case may be, the maintenance of air cleanliness. Compulsion on the part of the local authorities to connect to, and use, district heating systems can also be enacted on the basis of general climate protection, as has been clarified in Section 16 EEWärmeG (Act on the Promotion of Renewable Energies in the Heat Sector), and on that of a land development plan pursuant to Section 9 No. 23 of the Federal Building Code (Baugesetzbuch). Compulsion on the part of the local authorities to connect to, and use, public district heating systems for reasons of general climate protection is, according to a ruling of the German Federal Administrative Court (Bundesverwaltungsgericht) of 25 January 2006, Case no: BVerwG 8 C 13.05), lawful.
In practice, however, it has become increasingly rare for local authorities to take the step of introducing a compulsion to connect to, and use, district heating systems. For the introduction of such a compulsion leads not only to an obligation to connect and supply but also to a monopoly on the part of the supplier, who can then set prices at will. It is for this reason that most local authorities and supply companies prefer to use a good-value product, high levels of customer friendliness and a broad palette of services to consolidate and improve their position in the energy supply market.
For heating with district heat, warm water is conducted in thermally insulated pipes from the power station to the point of use. Such pipes are mostly buried in the ground, although free-standing pipes are sometimes also to be found above ground. The pipes that run from the heat source to the heat customers (heat sinks) are designated as feed pipes, and those that run from the heat sinks back to the heat source as return pipes. In order to protect the inner surfaces from corrosion and the build-up of mineral deposits, the water in the pipes is softened. The water is often also desalinated, as desalinated water, in comparison with softened water, gives rise to significantly lower rates of corrosion.
Normal operating temperatures for the feed pipes in a district heating network are 80-130 °C at an operating pressure of 1.6-2.5 MPa (16-25 bar). In smaller district heating networks with lower feed temperatures of 80-90 °C, the operating pressures of 0.4-1.0 MPa (4-10 bar) are correspondingly reduced.
In old buildings with existing central heating systems the connection to the district heating network is straightforward. A district heating substation, hardly bigger than a washing machine, is installed in the basement. The existing heating installation generally remains in use. Boiler, oil tank, coal cellar and chimney become redundant. In apartment blocks with individual heating systems a conversion has to be undertaken to a collective central heating system. [Berliner Energieagentur 2009]
District heat is generated in district heating stations, large combined heat and power plants with co-generation of heat and power, in waste incineration plants or in smaller co-generation plants. Fuels used are coal, natural gas, biogas, oil, wood and wood products, but also waste in different compositions and formats. As far as possible, the waste heat of industrial plants, for example, refineries and steelworks, is used as a heat source. In Iceland and parts of central Europe geothermal power stations are also used to generate district heat.
According to figures made available by the German Heat & Power Association (AGFW), 7.5 million fewer tonnes of the greenhouse gas CO2 were emitted in 2002 than would have been the case with individual combustion. Inner-city air quality is also improved by the use of district heat, replacing as it does many incidences of single combustion and thus preventing emissions in residential areas. With centralised generation the harmful substances which arise in the combustion process can be more easily reduced by appropriate flue cleaning than is the case with decentralised heating plants. In any case, regardless of the quality of thermal insulation, heat losses occur over the length of the distribution pipelines which then reduce the overall efficiency of the heating station-district heating system. [AGFW 2008]
Table 1: Greenhouse gas emissions from CHP heat and CHP power in different supply systems.
Emission factors for CHP Systems
CHP system CO2 equivalents in g/KWhoutput (including upstream chains)
CHP heat CHP electricity
Eta = efficiency
Gas micro-cogeneration unit
Coal cogeneration unit-extraction condensing turbine
Oil cogeneration unit-extraction condensing turbine
Gas cogeneration unit-extraction condensing turbine
Wastes-cogeneration unit-extraction condensing turbine
Gas gas turbine-cogeneration unit
Gas gas and steam turbine-cogeneration unit
Biogas manure-feed-in-micro-cogeneration unit
Biogas-regrowable resources-micro-cogeneration unit
Wood-woodchips-short rotational plantations-cogeneration unit
For comparison: reference systems (gas boiler for heat, national mix for electricity)
Source: [UBA 2008]: The determination of specific greenhouse gas emissions factors for district heating.
In the heating station primary energy carriers such as coal, natural gas or biomass are burnt in order to generate heat for hot water supply and space heating. A heating station does not supply electricity. Pure heating stations are becoming ever more unusual. The idea is for them to be supplanted by co-generation plants which produce heat and power simultaneously (combined heat and power). 83% of district heat is currently produced in CHP plants (cf. AGFW sector report 2008). This simultaneous production is particularly energy efficient, with primary energy savings of up to 40% - and corresponding reductions in emissions of the greenhouse gas carbon dioxide. Biomass co-generation plants and solar local heating systems have a particularly favourable CO2 footprint.
Fossil energy carriers (coal and gas) are still by far the most widely used fuels in district heat generation.
Burning coal generates the largest amounts of CO2 per usable energy content of all fossil energy carriers: Approx. 430 g CO2/kWh in the case of lignite, approx. 375 g CO2/kWh in the case of anthracite. The combustion of coal also releases sulphur dioxide. This compound is responsible for acid rain. In modern coal-fired power stations, flue gas desulphurisation plants prevent the direct release of sulphur dioxide into the environment. Lignite is excavated in open-cast pits which require very large areas of land.
Burning natural gas leads to lower carbon dioxide emissions than burning coal, as the reaction of oxygen with hydrogen from methane also releases thermal energy. The end product of this secondary reaction is water. The favourable carbon-hydrogen ratio of 1:4 leads the combustion of natural gas to generate less CO2, approx. 219 g CO2/kWh. [Öko-Institut 2007]
Natural gas is used particularly efficiently in combined gas and steam power plants. The gas and steam turbine technology combines the principles of a gas turbine power station and a steam power station. A gas turbine serves as a source of heat for a downstream waste heat boiler which itself then functions as a steam generator for the steam turbine. Combined power stations, with efficiency ratings of up to approx. 60%, are among the most efficient conventional power stations. Thanks to short start times and fast load gradients they are ideal medium-load power plants.
In rural areas, increasing numbers of small local heating networks are being built, which are used, for example, to distribute the heat from biogas co-generation units in the locality. The support for smaller co-generation plants with an output of up to 50 kWe comes within the framework of the climate protection initiative in the form of an investment subsidy.
The use of biomass to generate district heat offers the following options:
As liquid fuels from biomass, biogenic fuels (biodiesel, bioethanol) and plant oils (rapeseed or sunflower oil) come into consideration. The use of liquid bio-energy carriers offers the greatest flexibility in respect of plant location and size and can build on tried and tested existing technologies. From the technical point of view this therefore offers great potential for networked heat provision.
Large thermal solar plants with seasonal heat reservoirs have the task of making available solar heat during the winter months. The heat trapped by the solar collectors is transported to central heating stations and distributed directly to the buildings where it is needed. As a rule the collectors are mounted on the roofs of the buildings or integrated into the roof structure itself. The collectors should be located as close as possible to the central heating station. Solar local heating plants with long-term reservoirs can compensate for the time discrepancy between the maximum availability of solar radiation in the summer and the greatest need for heat in the winter. The heat is stored either directly underground or in artificial containers. To date no optimal solution for the construction of long-term reservoirs has been established. It is preferable to integrate solar-supported local heating systems with short-term heat reservoirs into the heat supply of large apartment blocks or entire residential areas. The lower systems costs in relation to the roof area result in cost advantages for larger solar installations in comparison to smaller plants.
Geothermal energy from rock beds at depths of two kilometres or more can be used to supply settlement areas or commercial energy consumers with heat. In many part of central Europe the temperature increases by more than 30 °C per kilometre of depth. This heat is continually available to geothermal heating installations and power stations. According to estimates made by the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, deep geothermal energy could provide up to 330 TWh of heat energy per annum. This corresponds to roughly 20% of the nation's heat requirement. In the past, deep geothermal energy projects have led to seismic activity, which shows just how essential careful planning is.
Waste from residential areas is burnt in domestic waste incineration plants. Heat is generated that can be used for the production of electricity and useful heat. This cuts down on the use of fossil fuels. Waste incineration plants thus prevent the release of roughly 9.8 million tonnes of carbon dioxide per annum. Of these emissions, 4.69 million tonnes per annum are apportionable to electrical output and 5.14 million tonnes per annum to heat output. However, taking into account the carbon dioxide emissions that result from the treatment process, a net reduction in emissions is returned of roughly 3.6 million tonnes of carbon dioxide per year. [Bilitewski 2006]
Manufacturing processes in industrial and commercial operations can generate heat, hitherto untapped, that can be used for heat supply. If the distances between such heat sources and users of heat are not too great it can be cost-effective to transport the waste heat along with district heat in order to heat buildings. What must also be taken into account is that the time of heat production should correspond as closely as possible with the times of demand.
In Germany there is no ecolabel for district heat.
A heat supply contract forms the basis of the provision of district heat. This is based among other sources on the German Civil Code (Bürgerliches Gesetzbuch – BGB) and the Verordnung über allgemeine Bedingungen für die Versorgung mit Fernwärme (AVBFernwärmeV, the directive of the Federal Minister of the Economy of 20 June 1980 [BGBI. I, p.742] - amended by the Ordinance on the changes to the statutory regulations on energy saving (Verordnung zur Änderung der energiesparrechtlichen Vorschriften) of 19 January 1989 BGBI. I, p. 112).
The contract partner is whoever has the sole power of disposal over the house connection, as a rule the owner of the apartment or building. (§ 2 section. 2 AVBFernwämeV, according to the ruling of the district court of Frankfurt/Main RdE 1989, page 165f.)
Anyone interested in being supplied with district heat should firstly clarify the following points:
The largest district heat networks in Germany are in Berlin, Flensburg, Hamburg and Mannheim.
The district heating network in Berlin is the largest in Western Europe. 27% of the capital’s buildings are heated using district heat. Berlin’s district heating network is more than 1,500 kilometres long. Roughly 326,000 cubic metres of water circulate in the system. In the district of Mitte one single co-generation plant supplies more than 60,000 apartments and 500 large-scale customers with heat. At the heart of the network are several large gas and steam turbines. [BEA 2009]
Flensburg boasts the most favourable district heating prices at a connection density of more than 90%. Work started on expanding the supply network as early as 1969. The city’s own power station was converted in 1971 into a co-generation plant providing 170 megawatts of electrical and 800 megawatts of thermal output. Four reserve co-generation plants guarantee supply security. Large amounts of CO2, dust and other emissions have successfully been prevented over a period of more than 30 years. [Duske 2007]