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Friday, November 21, 2008

District heating

District heating (less commonly called teleheating) is a system for distributing heat generated in a centralized location for residential and commercial heating requirements such as space heating and water heating. The heat is often obtained from a cogeneration plant burning fossil fuels but increasingly biomass, although heat-only boiler stations, geothermal heating and central solar heating are also used. District heating plants can provide higher efficiencies and better pollution control than localized boilers.

Heat generation

The core element of a district heating system is usually a cogeneration plant (also called combined heat and power, CHP) or a heat-only boiler station. Both have in common that they are typically based on combustion of primary energy carriers. The difference between the two systems is that, in a cogeneration plant, heat and electricity are generated simultaneously, whereas in heat-only boiler stations - as the name suggests - only heat is generated.

The combination of cogeneration and district heating is very energy efficient. A thermal power station which generates only electricity can convert less than approximately 50 % of the fuel input into electricity.[citation needed] The major part of the energy is wasted in form of heat and dissipated to the environment. A cogeneration plant recovers that heat and can reach total energy efficiency beyond 90 %.

Other heat sources for district heating systems can be geothermal heat, solar power, surplus heat from industrial processes, and nuclear power.
A cancelled Russian nuclear district heating plant in Fedyakovo, Nizhny Novgorod Oblast.

Nuclear energy has been suggested to be used for district heating. The principals for a conventional combination of cogeneration and district heating applies the same for nuclear as it does for any thermal power station. One use of nuclear heat generation was with the Ă…gesta Nuclear Power Plant in Sweden. In Switzerland, the Beznau Nuclear Power Plant provides heat to about 20,000 people.

Heat distribution

Insulated pipes to connect a new building to University of Warwick's campus-wide combined heat and power system.

After generation, the heat is distributed to the customer via a network of insulated pipes. District heating systems consists of feed and return lines. Usually the pipes are installed underground but there are also systems with overground pipes. Within the system heat storages may be installed to even out peak load demands.

The common medium used for heat distribution is water, but also steam is used. The advantage of steam is that in addition to heating purposes it can be used in industrial processes due to its higher temperature. The disadvantage of steam is a higher heat loss due to the high temperature. Also, the thermal efficiency of cogeneration plants is significantly lower if the cooling medium is high temperature steam, causing smaller electric power generation.

At customer level the heat network is connected to the central heating of the dwellings by heat exchangers (heat substations). The water (or the steam) used in the district heating system is not mixed with the water of the central heating system of the dwelling.

For the Norwegian district heating systems the yearly heat losses from distribution are about 10% of the total heat generated.

Pros and cons

District heating has various advantages compared to individual heating systems. Usually district heating is more energy efficient, due to simultaneous production of heat and electricity in combined heat and power generation plants. The larger combustion units also have a more advanced flue gas cleaning than single boiler systems. In the case of surplus heat from industries, district heating systems do not use additional fuel because they use heat (termed heat recovery) which would be disbursed to the environment.

District heating is a long-term commitment that fits poorly with a focus on short-term returns on investment. Benefits to the community include avoided costs of energy, through the use of surplus and wasted heat energy, and reduced investment in individual household or building heating equipment. District heating network, heat-only boiler stations, and cogeneration plants require high initial capital expenditure and financing. Only if considered as long-term investments these may translate into profitable operations for the owners of district heating systems, or combined heat and power plant operators. District heating is less attractive for areas with low population densities, as the investment per household is considerably higher.

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