**1.2. District heating in Sweden**

various kinds of heat and even cooling. District heating helps us utilising large amounts of

Thus, district heating is not only a technology for energy distribution but it increases the amount of available energy resources. District heating can utilise energy sources that are dif‐ ficult to use for individual buildings, such as unrefined biomass fuels, heat from waste in‐ cineration, heat from electricity generation in combined heat and power (CHP) plants and industrial surplus heat, for example heat from pulp and paper mills or production of auto‐ motive biofuel. Little of this energy could be utilised without district heating. Therefore, dis‐

District heating is used for heat supply to various kinds of buildings in villages and cities, primarily multi-family buildings and service premises, where the heat is used for prepara‐ tion of domestic hot (tap) water and for space heating, normally, through a central water‐

District heating systems connect energy sources and energy users. District heating can pro‐ vide affordable energy to consumers by using low-cost energy sources, such as surplus heat and waste. Many of these heat sources can be of local origin and promote local business and industry. What is the most suitable solution depends on the local conditions. By using vari‐ ous energy sources, district heating becomes a central component for waste management

Heat sources that cannot be used for separate houses can in a district-heating system be complemented by technologies that also are applicable at smaller scale, for example, fossil fuels, solar energy and electric heat pumps upgrading low-temperature heat. A small dis‐ trict-heating system can have one or two heating units, whereas a large system can host many different heat sources where, for example, a CHP plant fed with low-cost waste covers the base load throughout the year, a wood-fired heat-only boiler supplies most of the spaceheating demand in winter and a boiler using expensive oil covers the peak load during the

Base-load plants typically have a low heat production cost but require large investments. The low operation cost makes them suitable for being used during many hours a year. Bene‐ fitting from a lower heat cost than from other units pays back the heavy investment. Com‐ mon base-load supply comes from CHP plants, waste incineration and industrial surplus heat. Oil-fired boilers, on the other hand, have low capacity costs but high operation costs,

Figure 1 shows how heat production can take place in a Swedish district heating system during a year. In summer, heat demand and production are low because there is primarily need for heating of domestic hot tap water only but in winter heat production is much larg‐ er due to high space heating demand. The base load is covered by industrial surplus heat throughout the year because it has the lowest cost. The higher load in winter is mainly cov‐ ered by wood used in CHP plants and boilers but fossil CHP production and heat pumps

trict-heating expansion may be beneficial for economy and environment.

systems, forestry, power production and efficient energy use in industry.

which make them suitable for covering short periods of peak heat demand.

heat that now are wasted in Europe.

2 Sustainable Energy - Recent Studies

borne heating system for the whole building.

**1.1. Heat supply**

coldest days.

District heating is used extensively in Sweden. Sweden has nine million inhabitants. Fifty annual TWh of district heating cover one-half of the heat market. There is a district heating system in every municipality with more than 10 000 inhabitants and in total there are more than 400 systems. One-half of Swedish district heating is supplied to multi-family houses, the rest mainly to premises, such as schools and offices, and small but growing fractions to industry and single-family houses [3]. House owners chose whether to connect to a districtheating grid or not.

Figure 2 shows the energy sources used for district-heating supply in Sweden since 1970 [4]. The total supply varies between cold and warm years. The last year (2009) was a very cold year. The fuel use for district-heating production in Sweden has switched from almost only using oil in the 1970s to a present mixture with many heat sources. Now, two-thirds of Swedish district heating is produced from wood and waste fuel. Sweden utilises much in‐ dustrial surplus heat compared to most countries and heat pumps take heat from sewage water and lakes. Minor quantities of biogas and gas from ironworks are used but fossil fuels now produce less than 15% of the district heating (Fig. 2). The fossil carbon-dioxide emis‐ sions from district heating have been reduced significantly during the past decades because fossil fuels have produced a decreasing fraction of the heat. This transition has been facilitat‐ ed by an early introduction of a carbon-dioxide tax and other policy measures [5].

**2. Methods**

described [6].

costs [6].

sideration of heat-demand fluctuations.

bined heat and power production.

Favourable comprehensive solutions can be elucidated through system analysis and optimi‐ sation models. These methods can show the best way to use resources to satisfy aims. Com‐ mon aims are low costs and low environmental impact, which often can be conflicting. The essential features of an issue under study for a system can be represented in a model. Mod‐ els often help system understanding and reveal relations among components, such as be‐ tween district-heating production, solar energy extraction and wall insulation. The best solution according to a criterion and under certain conditions can be shown by an optimisa‐ tion model. For energy issues, an energy system optimisation model can be used to find the best design and operation of a system. In such models, many technical components can be

District Heating and Cooling Enable Efficient Energy Resource Utilisation

http://dx.doi.org/10.5772/51837

5

Examples of energy system optimisation models are MARKAL (e.g., [7]) and TIMES (e.g., [8]). These models were primarily developed for national energy-system analyses, whereas the model MODEST was originally made for optimising district-heating supply under con‐

MODEST is an energy system optimisation model, which uses the optimisation method line‐ ar programming to find the minimum cost for satisfying energy demand and presents the system design and operation that achieves the lowest cost. A large number of options for energy supply and conservation can be considered with this model framework. The user can make a comprehensive representation of the energy system under study with chosen level of detail. Many different energy systems can be analysed as long as the important properties of the system can be described by linear relations. An almost arbitrary set of parameter val‐ ues may be attributed to each component and energy flow in a system. A flexible time divi‐ sion makes it possible to reflect diurnal, weekly, monthly, seasonal and long-term variations of, for example, costs, capacities and demand. The modelling result presents the optimal in‐ vestments and the optimal operation of existing and new units as well as emissions and

MODEST has been most used for optimisation of electricity and district-heating production. MODEST has been applied to more than 50 district-heating systems, some regional energy systems and a few national power systems. Studied issues include introduction of waste in‐ cineration and combined heat and power production and connections between industrial and municipal energy systems [6], for example, how large CHP plant should be built, is waste or biomass the best fuel and should industrial surplus heat be utilised? The model has been used a lot to study impact of energy prices and policy instruments on investments and operations in energy conversion, for example, how emissions allowances influence com‐

**Figure 2.** Fuels etc. used for district-heating production in Sweden (TWh/year)

Oil and coal use decreased during the 1980s (Fig. 2) due to increasing taxes. There has been a carbon-dioxide tax in Sweden since 1991, which now is 100 euro/ton. An energy tax was in‐ troduced even earlier. There is natural gas only in south-west Sweden. The use of electric boilers for district-heating production increased when nuclear power expanded during the 1980s but decreased when the electricity was taxed in the 1990s. Use of biomass (e.g., wood‐ chips) was first promoted by the taxes on fossil fuels and later also by green electricity certif‐ icates and higher electricity prices that make biomass-fired CHP plants more profitable. Waste incineration increases (Fig. 2) because it is prohibited to dispose combustible fuel on a dump and district heating companies collect revenues for taking care of the waste. There have also been investment subsidies to some selected projects using local energy sources. The district-heating increase during the last decades reflects a political commitment to in‐ vest in infrastructure and reduce dependency on imported fossil fuels.
