Life cycle greenhouse gas emission analysis of single-family Norwegian Zero Emission pilot Buildings and concepts
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Can a life cycle single-family Zero Emission Building (ZEB-OM) be achieved in Norway? What have we learned from designing and building single-family Zero Emission pilot Buildings with respect to greenhouse gas (GHG) emission reductions? By examining emission loads and paybacks from Zero Emission pilot Buildings, this thesis studies the challenges and possibilities for achieving a life cycle zero emission balance (ZEB-OM) for single-family buildings in Norway. To combat climate change, GHG emissions from the building sector need to be reduced. Currently around one fifth to one fourth of global GHG emissions are related to energy and material use in buildings. With focus on achieving the Zero Emission Building-Operation and Materials (ZEB-OM) ambition level, the aim is to minimize greenhouse gas emissions from energy and material use and offset emission loads using on-site renewable electricity generation. The approach used in the present thesis analyses GHG emissions of single family Zero Emission pilot Buildings, ZEB concepts and their installed PV systems. The main objective is to increase our understanding of how Norwegian single-family ZEB pilot buildings are performing with respect to GHG emissions and identify potential improvements. The thesis is carried out within the Research Center on Zero Emission Buildings in Norway (www.zeb.no). The thesis consists of five individual research papers. The papers contribute towards the common objective of the thesis with different aspects and analyses: In paper I, the GHG emission loads of two net-ZEB concepts (an office building and a single-family building) were analyzed. It was revealed that the most prominent embodied emissions drivers for the single-family building concept were the photovoltaic system (30%) and the ground foundation (20%). The term "embodied emissions" refers to the GHG emissions that occur in raw material extraction and transportation and manufacturing of the materials used in the building. A ZEB operational balance (ZEB-O) was achieved for the single-family concept house by applying a simplified all electric symmetric weighting approach, using the symmetric weighting factor ZEBel equal to 132 grams CO2eq/kWh. However, a ZEB-OM balance ("O" operation and "M" materials) was only achieved with a higher symmetric emission factor, for instance, 361 or 531 grams CO2eq/kWh. When a symmetric weighting factor of 38 grams CO2eq/kWh was used, representing the Norwegian electricity grid, it was found that the embodied emissions were the dominant emission driver (85-90%) and a ZEB balance was not achieved for either concept. In paper II, the design phase GHG emission calculations for a Zero Emission pilot building were documented. The pilot building, with a 200 m2 heated oor area, had a 150 m2 roof mounted photovoltaic system. With the efforts made on embodied emission reductions and emission compensation, a ZEB-OM balance (with the simplified symmetric ZEBel approach) was nearly achieved for the building. The embodied emission reductions were achieved with the use of PV modules with partial use of recycled/upgraded materials, low carbon concrete, timber structures and glass wool insulation (ground foundation), and reused bricks. In paper III, a life cycle emission analysis was performed for the GHG emission loads and GHG payback times (GPBT) of three different installed phoiv tovoltaic systems in the single-family Zero Emission pilot buildings. Emission loads per square meter of the systems were found to be between 150-350 kg CO2eq, while emission per kWh were 30-120 grams CO2eq. The variation in emissions was mainly due to different emissions resulting from different modules, and best and worst case emission scenarios. The study showed that emissions from the mounting systems varied from 10-25 kg per m2 of photovoltaic module area, mostly due to different amounts of aluminum used. The GPBT for PV systems is dependent on the local grid emissions. With the static ZEBel of 132 grams CO2eq/kWh, the emission payback times were 8-18 years for the different systems. From the analyses it is clear that the availability of reliable life cycle information for photovoltaic modules on the market should be improved. In Paper IV, a comparative life cycle emission study was carried out for eight single-family buildings in Norway. The following buildings were compared: three ZEB buildings aiming for ZEB-OM, one ZEB aiming for the ZEB-O ambition, one active house, two passive houses, and one case building serving as a reference built to Norwegian building code standard (reference case). A harmonized methodological framework was applied e.g. for the inventories, climate and simulation tools used. To broaden the ZEB emission balance perspective, the study focused on a monthly emission balance for the use stage. Within the baseline scenario (where use stage emissions were based on a monthly balance perspective), the best ZEB case resulted in emissions of 5 kg CO2/m2 per year while the reference case resulted in emissions of 12 kg CO2eq/m2. The other low energy and ZEB cases had emissions around 7-8 kg CO2eq/m2. Total GHG emissions from the best case, which was the smallest ZEB, were around 23 metric tons of CO2eq, compared to the reference case of around 62 tons CO2eq (30 year reference study period). The study confirmed the importance of the embodied emissions for Norwegian ZEBs, where the embodied emissions share was around 60-75% of the total emission load. The share of embodied emissions of total emissions was found to be sensitive to the use stage emission scenario. Also, the study highlighted the importance of area efficiency: the largest ZEB (200 m2) had almost double the embodied emissions as in the smallest ZEB (102 m2). Paper V compares embodied and operational emissions (loads and benefits) from selected solutions applied in the four previous ZEBs (pilot and concept buildings) with the aim of improving the initial ZEB-OM model. The results showed that it was possible to increase the electricity production, with relatively low additional embodied and operational loads, and at the same time reduce embodied emissions in the ground foundation. No significant improvements in life cycle emissions were found when the insulation thicknesses and the heat supply systems were changed compared to the original designs of the ZEBs. Even with the improvements, the previously defined ZEB-OM ambition was not reached for the new model. However, for example, by redefining the boundaries for how to interpret the materials included in the "M", e.g. by only including the product stage embodied emissions, a ZEB-OM balance would be achieved. The overall conclusion is that it is possible to reduce GHG emissions from the current practices for single-family buildings in Norway by focusing on achieving a ZEB-OM balance. However, it is diffcult to achieve this balance in Norway given the low emissions from electricity. In Norway, the embodied emissions share of life cycle emissions is high, and should get increased priority in research and policy making. In the European context, a ZEB-OM is achievable for a single-family house, and how it is defined should be further developed.