Primary energy efficiency and CO2 mitigation in residential buildings

Sammanfattning: In order to control climate change it is important to limit the atmosphericconcentration of carbon dioxide (CO2). Increased energy efficiency, as well as ashift from fossil fuels to renewable resources can reduce net CO2 emission. Theenergy required for constructing and operating buildings is significant in manycountries, and it is thus important to design energy efficient buildings and energysupply systems.Improvements in existing buildings are needed in order to achieve short-termemission reductions. The Swedish building stock expanded greatly during the1960s and 1970s. The energy efficiency of these houses was often quite low, andmany of them were built with resistance heating. In this thesis increased energyefficiency in such buildings is studied, as well as conversions from resistanceheating to other heating systems, and various technologies and fuels for theproduction of electricity and heat. The effects of these measures are analysed withrespect to primary energy use, CO2 emission and societal cost. The studies wereperformed using process-based systems analysis in a life-cycle perspective. Thesystem boundaries include energy chains from the natural resources to the usefulelectricity and heat in the houses. The results show that the choice of heatingsystem in the house has a greater effect on the primary energy use than measureson both the house envelope and the energy supply chains. District heating basedon cogeneration of heat and electricity and bedrock heat pumps were found to beenergy-efficient systems. The net emission of CO2 is dependent on the fuel and theCO2 emissions from these systems are comparable to those from a wood pelletboiler, if biomass-based supply chains are used. Conversion from resistanceheating to any of the other heating systems studied is also profitable from a societaleconomic perspective.The decision to implement energy-efficiency measures or install a new heatingsystem in a detached house is taken by the house owner. In order for successfulimplementation the alternatives must either be sufficiently attractive or incentivesor policy instruments that affects this large, inhomogeneous group must beimplemented. In this thesis, the house owners’ economic situation when changingthe heating system and implementing energy-efficiency measures on the buildingenvelope is analysed. The economic analysis includes current Swedish policyinstruments, such as an investment subsidy for heating system conversion, anincome tax deduction for replacing windows, levying a consumer electricity taxand increasing real estate tax. House owners’ perceptions of different heatingsystems are analysed through the results of comprehensive questionnaires. Societaleconomy, private economy and individuals’ perceptions are compared. Theconversion subsidy provides some incentive to house owners to act according tothe national energy policy, as does the electricity tax, which has a significantinfluence on consumer costs. The use of economic instruments seems efficient inpromoting systems in line with environmental goals since environmental factorsare ranked much lower by the home owners. However, the effect on the annualcost of most of the policy instruments studied is smaller than the price variationsbetween different energy suppliers. Energy suppliers thus have considerableopportunity to influence house owners.To achieve long-term changes in the building sector new houses should beconstructed with as low primary energy use and emission as possible, seen overtheir entire life cycle. The primary energy use is analysed for both the productionand operational phase of several types of residential buildings. When the demandfor operational primary energy decreases, due to a high energy standard orenergy-efficient supply, the relative importance of the energy required forproduction will increase. The amount of primary energy required for theproduction of a new low-energy building is significant compared with the primaryenergy required for space heating. One way of reducing both primary energy useand CO2 emission in the production phase is to use constructions with woodframes instead of concrete.The energy supply system is nevertheless still important also for low energybuildings. A new house built to passive standard, heated with fossil-fuel-basedresistance heating gives rise to higher primary energy use and CO2 emission than aconventional detached house from the 1970s that is heated with an energy-efficientbiomass-based heating system. The results thus indicate that wood-framed houseswith a high energy standard, together with efficient energy supply systems, couldbe an option for sustainable residential construction.