Cheap surplus heat is available from various sources. Internationally, the strongest driving force for district heating is the possibility of using heat rejected from thermal power stations (CHP- plants). Other heat sources that can be used in district heating systems are waste heat recovered from industrial processes and waste heat from refuse incineration. District heating can also be used to utilize cheap fuels that are difficult to handle and burn (as coal, wood waste and peat).
The energy sources used for heat generation in the Swedish district heating systems have varied by the years due to variations in the prevailing conditions. This is especially the case with the utilization of combined heat and power and its relation to the Swedish power generation.
Between l965 and l980, thermal power was needed in order to fulfil the power balance. During these years, both industrial CHP and municipal CHP was used to a large extent in order to reduce the demand of condensing power. Industrial CHP is mainly connected to steam demands in the Swedish pulp and paper industry, while municipal CHP refer to CHP in connection with district heating. All municipal CHP- plants and most of the industrial plants were oil-fired during these years.
In 1981, when several new large nuclear power stations came into operation and when the oil price had risen, the total share of CHP in the power balance sunk to 4,5 % from being almost 10 % in 1980, see figure 4. Depending on supply of hydropower and demand of electricity in the country, the share of CHP in the power balance has varied between 3 and 4,5% during the 80's. Many municipal CHP-plants converted to coal-firing during the first half of the decade in order to reduce the cost of heat and power generation.
As bulk power generator, the Swedish State Power Board (SSPB) has had a competitive attitude towards municipal CUP. During the 60's, several large municipal energy utilities received long power pur chase contracts with substantially lowered electricity prices in order to discourage CHP and district heating (Luouml;nnroth et al 1980) Sometimes the municipal energy utility had to promise not to build a CHP- plant or even not to start a district heating system in order to get the favourable purchase contract. Municipal energy utilities with CHP-plants were also of the opinion that the price of reserve capacity from the SSPB for municipal CHP-plants was too high. The purpose of these SSPB activities was to obstruct power generation in competition and assure a safe and expanding market for their expansion plans concerning future power generated by nuclear energy.
During the 70's, the share of heat generated in conjunction with power generation in CHP-plants was 40-50 % in the total heat balance of the district heating systems, figure 5. This share has fallen to 15-20 %, during the 80's due to the lower utilization of CHP. This implies that the Swedish district heating systems were not mainly based on the internationally strongest driving force for district heating during the last decade.
The annual national overall electricity yield is defined as the difference between power generation and electricity consumption in the district heating systems divided by the total heat generation. Electricity consumption consists of power for distribution pumps , electric boilers , large heat pumps etc. During the 70's, this national electricity yield varied between 0,12 and 0,20, as can be seen in figure 6. This implies that the individual electricity yields of the existing CHP-plants were moderate. During the 80's, the national electricity yield has gone negative, since CHP generation has fallen and some new heat generation sources have been based on electricity consumption (electric boilers and heat pumps). This interesting conclusion reveals that the Swedish district heating systems have become power-consuming during the 80's from being power-generating during the 70's (Eriksson 1988).
Heavy fuel oil was the main fuel during the first three decades of district heating in Sweden. The presence of CHP and a price difference between light and heavy fuel oil gave competitive district heating prices. Nowadays, fuel oil is only used for peak loads and reserve capacity.
Coal as fuel in CHP-plants appears in Västerås, Norrkäiping, Stockholm, Helsingborg, Örebro, Linköping, Uppsala, and Borås. Major users of coal for heat generation only are Södertalje/Södertörn, Malmö, and Göteborg.
Wood chips consist of wood waste from the forest industry and are delivered by forest owners' societies, forest companies, and saw mills. Major users of wood chips are Borås, Linköping, Växjö Eskilstuna, and Östersund.
Peat is extracted from peat bogs. Major users of peat are Uppsala, Karlskoga, Växjö and Östersund. In November 1990, the intermediate peat storage in Uppsala was accidentally burned in a huge fire.
Major refuse incineration plants appear in Göteborg, Stockholm, Uppsala, Malmö, Linköping , and Umeå (Bergström & Gustafsson 1987). Some small refuse incineration plants exist, but new environmental demands reduce the economy of small units. Presently, a refuse incineration plant must burn at least 100 000 tons of refuse per year in order to cover its own costs.
Waste heat recovered from industrial processes is utilized in Göteborg, Helsingborg, Malmö, Landskrona, Köping, Piteå, and Vänersborg. In Göteborg, the heat source is the Shell refinery (Eriksson 1980), while a ferro alloy furnace is the heat source in Vänersborg.
Large heat pumps with heat capacity outputs in the range between 5 and 40 MW have been popular during the 80 ' s. During the first half of the decade, the electricity price in Sweden was very low compared with the oil-price, giving a driving force for installations of large heat pumps. Major installations of large heat pumps have been made in Stockholm, Göteborg, Solna/Sundbyberg, Lund, Örebro, Borlänge, Eskilstuna, and Malmö. The heat pumps built use heat sources like sea water and purified sewage water. In Lund, low- temperature geothermal water is used. During 1988, the average overall coefficient of performance for all large heat pumps installed in Swedish district heating systems was 3.1. This means that 1 MWh of electric energy fed into the heat pumps gave 3.1 MWh of heat generated.
Electric boilers were built during the first half of the 80's in order to utilize surplus electricity for heat generation. This electricity is mostly available in Sweden during the spring, summer, and autumn seasons due to surplus capacity of nuclear power and hydropower without water storage possibilities.
Natural gas arrived in the south of Sweden in 1985. Nowadays, natural gas is used by Malmö and Göteborg for heat and power generation in existing CHP-plants. Natural gas for heat generation only is used by Lund. But in this town, a simple cycle gas turbine with heat recovery is being built. A small combined cycle CHP- plant with natural gas as fuel is being built in Ängelholm. Further expansion of the natural gas system in Sweden is being discussed. Due to a favourable energy tax, LPG for heat generation has been interesting since 1987/88. But nothing lasts forever, the energy tax of LPG was increased by 307% on January 1, 1991. A CHP- plant in Luleå has used blast furnace gas since 1982. Minor utilization of digester gas from sewage water treatment and refuse gas from refuse disposals also occur for generation of heat.
Nuclear energy for district heating in Sweden began to be discussed in the mid-50's after the release of confidential technical information about nuclear energy at the Geneva Conference in 1955. These discussions later became the Ågesta nuclear heat and power station, south of Stockholm. This CHP-plant of 12 MW (electrical) and 68 MW (thermal) was in operation between l963 and l974. The heat was delivered to Farsta, a suburb of Stockholm. Extractions from the steam turbines of the nuclear stations Forsmark, Ringhals, and Barsebäck have been studied during the 70's for heal transmission to Stockholm, Göteborg, Malmö and Lund, respectively. But none of these studies have resulted in any heat generation for district heating.