A techno-economic system approach for the energy renovation of residential districts built before 1945

Abstract: A key factor in the quest for sustainable development worldwide is reducing the energy use and greenhouse gas emissions from residential buildings. The residential and service sector represent 39% of the final energy use in Sweden. The housing stock in Sweden is fairly old with approximately 25% of the residential buildings built before 1945, many of which possess heritage values. Considering the generally poorer thermal performance in older buildings compared to newer ones, it is important to investigate the techno-economic energy efficiency potential in this part of the built environment.  The aim of this thesis is to develop a bottom-up approach and to analyze energy renovation of residential districts built before 1945 from a system perspective with regard to targets of life cycle costs (LCC), energy use and preservation of building heritage values. The developed approach includes a combination of economic and environmental impacts from a building owner and energy utility point of view. The approach includes analysis on four different levels, i.e., building level, cluster level, district level and city level.The results show that the developed approach is successful in integrating targets of LCC and energy use, as well as preservation of heritage values, during techno-economic energy renovation. By a further development of the change-point model, data related to building thermal power characteristics, such as Qtotal and balance temperature, can be calculated and used for analysis of a residential district. Moreover, the cluster with the initial poorest thermal performance, i.e., the single-family houses in stone, account for the highest decrease in specific energy use (70–78%) and LCC (34–37%) during energy renovation at LCC optimum. The corresponding figures for the buildings with the best thermal performance initially, i.e., the cluster with multi-family buildings in wood, are 23–24% and 14%, respectively. Furthermore, it is concluded that the cost-effective energy efficiency potential is highly correlated with initial building properties and preservation requirements, which significantly affects the stone buildings. This is because insulation on the inside of the external walls is cost-effective in these buildings, but not in wooden buildings, which consequently decreases the energy savings potential from 46–69% in a balanced energy renovation scenario to 8–30% in a restricted energy renovation scenario.  The findings also show that the environmental performance of the building district is closely linked with the selected energy system boundary. This can be exemplified by CO2 emissions of 0.7–1.1 kg CO2 eq./(m2·year) at LCC optimum for multi-family buildings when considering biomass an unlimited resource, compared to 28.9–40.0 kg CO2 eq./(m2·year) when considering biomass a limited resource with condensing coal power plants as the marginal user. Furthermore, on a city level it is concluded that the environmental performance of the district heating (DH) system is improved as a result of techno-economic energy renovation of a district, and that the net income is decreased (8%) despite a lower system cost (12–13%) due to less DH sold to end users. The global CO2 emissions are decreased by 3,545–3,737 tonnes/year and the primary energy use is decreased by 5.0–5.2 GWh/year.  Apart from the developed bottom-up approach for analysis of the energy renovation of residential districts built before 1945, this thesis has provided valuable results to the research community, building sector and authorities in terms of (1) the further development of the change-point model, which enables time-effective analysis of the thermal performance of residential districts; (2) the environmental benefits with techno-economic energy renovation of residential districts from a DH producer perspective and (3) the need to develop packages of EEMs that are profitable for both DH producers and end users of DH.