**5. Cooling**

Heat can be recovered for repeated utilisation at decreasing temperature levels in industry and finally for space heating. Although it is known that there is surplus heat in certain facili‐ ties, in some cases little is known concerning the volume and the quality of the heat. Identi‐ fying and measuring surplus heat resources would therefore be necessary before any evaluation can be made. For instance, a study based on energy auditing showed that as much as 500 GWh/year heat energy of different quality is wasted in a single pulp and paper mill [10]. However, knowing heat quantity and quality does not automatically mean that the surplus of heat can be used. Other factors, such as time of availability, heat demand, infra‐ structure, technology and costs, play decisive roles in determining if the surplus heat can be

District heating offers an outstanding opportunity to utilise surplus heat which otherwise would be wasted. Though the share differs from country to country, the Nordic district heating systems are good examples of using surplus heat [11]. From a consumer's perspec‐ tive, district heating systems with significant share of surplus heat in its fuel mix offer rela‐ tively low heating costs to their consumers. In some municipalities in Sweden, the rather low heat costs can be attributed to surplus heat supply from industries. Availability of sur‐ plus heat during summer when the heat demand is low opens an opportunity to produce

Utilising surplus heat in district-heating applications is not quite easy since such an endeav‐ our has different issues that need to be resolved. One of the main issues is how to bring about a co-operation platform between players where the use of industrial surplus heat is understood in the light of a broader system perspective. In this case, a municipality, or a re‐ gion with several municipalities, can be the system boundary when considering energy cooperation. In regions where district heating is well established and where there is relatively high concentration of industrial activities, there might be a need to develop a regional heat market to encourage efficient utilisation of energy resources. Though the core business of an industry is not selling surplus heat, such a market could be a driving force which in turn enables players to take measures that might promote the use of surplus heat. This would mean that industries with substantial amounts of surplus heat can play significant roles as heat suppliers in local or regional markets. This is also important when considering invest‐ ments in new generation facilities for electricity, steam and heat. In this case, regional ener‐ gy system optimisation would be valuable to maximise the efficiency of resource utilisation.

Thorough studies that focused on this subject are given in [12,13]. In both studies, several district-heating systems and industrial energy systems of a region are considered and the MODEST model is applied (Sect. 2). The second study [13] was more in detail and it in‐ cludes several scenarios where measures, such as investment in new facilities, process inte‐ gration and energy efficiency measures, are considered. In general, both studies indicate an overall system benefit of connecting the various energy systems in forms of reduced total cost, efficient use of plants and reduced carbon-dioxide emissions, but the latter depends strongly on the carbon accounting method applied. Interesting to note is that an enlarged system boundary, encompassing all the district-heating systems and industrial energy sys‐ tems, enables efficient utilisation of surplus heat that is available within the system. De‐

district-heating-driven cooling for buildings during summertime (Sect. 5).

utilised or not.

8 Sustainable Energy - Recent Studies

Cooling of rooms now increases due to higher comfort requirements in, for example, countries in Northern Europe, as well as from the middle class in transitions countries where the living standard is rapidly rising. The desire to cool the rooms is also enhanced by the global warm‐ ing and the growing number of electric appliances that supply waste heat in the rooms [14].

Normally, electricity-driven refrigeration machines are used to produce cooling. But districtheating sources can also produce cooling for indoor climatisation in absorption cooling ma‐ chines, either in central plants (e.g., waste incineration plants) supplying a district cooling network or in distributed units situated in the buildings that are to be cooled, which are fed from the district-heating network (Fig. 4). Such solutions mean that electricity is not re‐ quired for the cooling. It increases the low heat demand during summer and also the elec‐ tricity generation in CHP plants if the heat is produced there. Thus, cooling can become a basis for electricity generation instead of consuming electricity [2]. Absorption cooling is most suitable when a low-cost fuel, such as waste, can be used.

**Figure 4.** Cooling with heat through absorption cooling. One of the networks is required.
