Large Heat Networks in District Heating Systems

Sammanfattning: District heating (DH) systems have experienced three generations since the 1880s. In chronological order, high temperature steam and pressurized hot water above and below 100 degrees have been used to carry heat over these three generations. The drive to increase energy efficiency and reduce investment costs of these systems have been the principal incentives for shifting from one generation to the other. The future development of DH systems towards the fourth generation will involve an attempt to recover heat from low-temperature sources (e.g. industrial excess heat (EH)), the use of renewable sources and the integration into smart energy systems. In Sweden, DH currently supplies about 60% of the heat demand. For future DH developments, these systems need to be competitive compared to individual solutions (i.e. heat pumps and boilers) in supplying heat. They could also be incorporated in future sustainable energy systems by integrating renewables and establishing synergies with other energy sectors. There are currently some successful synergies between industry and DH systems but as one step towards the fourth generation of DH, industry-DH synergies could be further developed in order to recover still unused industrial EH. Due to the diversity of Swedish DH systems in terms of local fuel use and heat demand, their choice of heat production technologies is affected. Thus, the environmental and economic impacts of DH systems-industry synergies that allow for industrial EH use in DH systems or the DH use in industrial processes have often been studied in a small geographical scale, limited to the boundaries of local DH systems. However, because it is often transported over relatively short distances, biomass as the main fuel used in DH systems has often turned into a regional market. With increasingly stringent targets for climate change mitigation, biomass use is likely becoming more attractive not only in the heat but also in the power and transport sectors. Since synergies between local DH systems and industry affect the regional market for biomass and, consequently, the power and transport sectors, a regional level combined with an inter-sectoral approach might provide a comprehensive way to identify the impacts of DH-industry synergies. The aims of this thesis are, first, to develop a methodology for assessing an option for future DH development, i.e. a large heat network that would allow for long-distance industrial EH transmission for use in DH systems; and, second, to apply this methodology to assess energy systems, environmental and economic impacts of a large heat network between the cluster of chemical industries in Stenungsund and the DH systems of Gothenburg and Kungälv in West Sweden Region (Västra Götaland (VG)). The assessment has been carried out with the help of optimizing energy systems model MARKAL_WS, in which the DH systems in the VG Region are represented individually. In addition, options for transport biofuel production as competitors to regional biomass are included. The thesis is based upon two papers. In the first paper, energy system and CO2 emission impacts of the large heat network have been analyzed at a regional level. The results show that the heat network contributes to a reduction of biomass and fossil fuel use, and to a related reduction of CO2 emissions, in the DH systems. This outcome opens opportunities for the earlier production of transport biofuels but implies decreased electricity generation from combined heat and power (CHP) plants in the Region. In the short-term, total CO2 emissions increase, given an expanded systems view that effects on the DH systems, transport and European electricity system are accounted for, while in the mid-term they decrease. In the second paper, the long-term system cost and marginal cost effects of the large heat network have been assessed. The results show that the heat network is profitable under most assumptions and that the profitability increases with biomass competition and the phase-out of fossil fuel use while it decreases with higher CO2 charge, interest rates and the availability of other EH sources in the vicinity of the DH systems. The marginal cost of DH supply in the Gothenburg and Kungälv DH systems decreases during most seasons except for the cold seasons.