**8.2. Using district heating at low demand**

cause district heating often comes from surplus resources, such as waste or waste heat,

Now, heat demand is decreasing due to higher outdoor temperatures caused by the en‐ hanced greenhouse effect, as well as policies that promote low-energy houses, which makes district heating a less suitable form of heat supply. All new buildings in the European Union are supposed to be *nearly-zero-energy* buildings in 2020 [19]. Low-energy houses often have thick wall and attic insulation, windows transmitting little heat, ventilation with heat recov‐ ery and solar heating. These more advanced installations cause higher investment costs but

Lower heat demand should reduce the use of natural resources, such as fossil fuels, and ena‐ ble biomass to be used for other purposes than space heating, such as production of automo‐ tive fuel. But the heat demand reductions are a challenge for district heating and therefore also for the possibilities to utilise energy sources that need district heating to be used, such as industrial surplus heat. Therefore it is important to analyse the interplay between energy supply and energy conservation and between district-heating companies and buildings.

Energy-efficiency measures, such as improved wall insulation and better windows, primari‐ ly reduce heat demand in winter and, hence, decrease seasonal demand variations. This may be favourable from a heat-production viewpoint because high-load plants are needed less but base-load plants (Sect 1.1) may be used more, which would reduce operation costs and environmental impact. But base-load plants would also be affected, which could decrease ef‐

Åberg and Henning [20] studied the impact of a potential heat-demand reduction due to ex‐ tensive energy-efficiency measures in existing buildings on district-heating and electricity production by using the energy system optimisation model MODEST (Sect. 2). In the Swedish city under study, the heat-demand reductions would primarily decrease heat-only produc‐ tion, whereas CHP production would be less reduced. The *electricity-to-heat output ratio* for the system would even increase, that is, generated electricity per unit of delivered district heating would increase. Local carbon-dioxide emissions would be lowered by the energy-efficiency measures because less fossil fuel would be used. Global carbon-dioxide emissions would also be reduced though less efficient coal-fired condensing power plants would need to replace the electricity that can no longer be produced in the CHP plants due to reduced heat sink in the buildings. However, only the existing electricity and district-heating production plants are considered in this study [20], whereas a process of gradual heat-demand reduction in present houses would run in parallel with a restructuring of the heat supply system probably includ‐ ing a transition to even larger use of renewable fuels. In such a future system, energy-efficien‐

In a similar study of another city [21], the combined effect of energy-efficiency improve‐ ments in existing multi-family buildings and the connection of new low-energy multi-family

ficient electricity generation in combined heat and power plants.

cy measures might not reduce carbon-dioxide emissions.

when there is most solar radiation.

the lower energy use reduces operation costs.

**8.1. Lower demand**

12 Sustainable Energy - Recent Studies

To make it favourable to use district heating in areas with low heat demand and, thus, to enable utilisation of the energy sources that can only be used through a district-heating sys‐ tem, as much district heating as possible should be used in such an area while still using the heat efficiently.

Besides the traditional purposes domestic hot water and space heating, district heating can be used for industrial processes and all heat supply to household appliances (e.g. dish wash‐ ers, washing machines, tumble dryers and towel dryers), which now often, at least partly, use electricity for heating. Solar rooftop energy extraction could yield electricity instead of heat, because the latter would reduce the demand for district heating supply.

Henning [22] outlined scenarios for more sustainable energy supply for a development in a Swedish town. In two cases, the buildings were supplied by district heating. In one of these scenarios, district heating was used only in the traditional way for domestic hot water and space heating in normal, but not inefficient, houses. In the other scenario, there were lowenergy buildings where district heating also was used for household appliances. Energy that in the first case only disappeared out of the buildings was in the other case utilised for heat supply to dish washers, washing machines, tumble dryers and towel dryers. The first sce‐ nario meant more climate-dependent space-heating demand partly covered by expensive high-load fuels (mainly forest wood chips in this town), whereas the latter scenario included more base load in the household appliances, which could be covered by fuels with lower costs (wood waste, [22]).

Many industrial processes have heat demand that partly or wholly can be covered by dis‐ trict heating but now is supplied through fuel or electricity. When required, district heating can be supplemented by boilers to obtain desired high temperatures. Heat demand in indus‐ trial manufacturing processes is often more or less independent on outdoor temperature and only has little seasonal variations (besides holidays) in the same manner as domestic hot wa‐ ter. Industrial processes can, therefore, constitute a base demand, which favourably could be covered by base-load plants, such as waste incineration or CHP plants [23].

With lower heat demand, the temperature in district-heating networks can be lower, which means that surplus heat of lower temperature can be utilised. Also, more electricity can be produced in CHP plants because the heat that is extracted after electricity generation can be of lower temperature. A larger fraction of fuel energy can yield high-quality electricity in‐ stead of low-quality heat.

district heating or other forms of heating where renewable sources are used should be seen as a necessary measure to achieve energy and environmental targets. From a Nordic per‐ spective, where the share of renewables in power production is high, a shift from electricitybased heating would offer tremendous opportunity to meet national and international policy targets through letting the not used electricity displace less environmentally benign power production. The transport sector, now being one of the main carbon-diode emitters, might alternatively benefit from energy-carrier switching to electricity. However, this de‐

District Heating and Cooling Enable Efficient Energy Resource Utilisation

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

15

District heating is a comprehensive concept for heat supply from fuel through heat produc‐ tion and distribution to consumers. District heating systems are valuable assets, which ena‐

The main advantages with district heating are low primary energy demand due to high en‐ ergy efficiency, high security of supply through utilisation of domestic renewable energy re‐ sources, if available, as well as small carbon-dioxide emissions thanks to low fossil fuel use

District heating enables utilisation of energy resources that are difficult to use in single buildings and that otherwise may be wasted, such as industrial surplus heat, municipal

Incineration of waste with heat recovery to district heating may be used at very low cost. Surplus heat from industries can, instead of being wasted to air or water, be utilised in dis‐ trict-heating systems. District heating also gives opportunity for cogeneration of power and heat with high efficiency. District heating, thus, enables profitable heat supply with less en‐

District cooling from, for example, absorption-cooling devices saves electricity and may in‐ crease power production in CHP plants. To efficiently utilise biomass for energy purposes, it could be supplied to poly-generation plants where it yields, heat, steam, electricity, cooling

The use of, for example, biomass fuel decreases the dependency on imported fossil fuels. Ef‐ ficient plants need less fuel, which decreases the vulnerability of energy supply. Global warming and better houses reduce heat demand. Using district heating for additional pur‐ poses enables increased utilisation of energy resources that otherwise may be wasted.

Companies and organisations in well-developed district-heating countries have much knowledge that can facilitate district-heating development elsewhere. Such actors could

waste and heat from generation of electricity in combined heat and power plants.

pends on the maturity and the efficiency of the technology for electric vehicles.

**10. Conclusions**

vironmental impact.

and automotive fuel.

**11. Outlook**

ble efficient resource utilisation.

and the high conversion efficiency.
