70 Schools for Zurich: Optimization of Embodied Energy and Greenhouse Gas Emissions in Construction Projects with Bio-Based Materials

Fabian Kastner was awarded the Culman Prize for his Master Thesis.

Abstract

The goal of this study is to investigate the optimization potential of embodied environmental loads in construction projects with bio-based materials. The study is conducted in cooperation with the City of Zurich, which provided data on planned school buildings. One of these school buildings is investigated in-depth in a case study. A project at the Institute of Structural Engineering (ETH Zurich) included a structural assessment of its wood structure (Schürpf & Parravicini, 2018), which is integrated in this thesis.

Indicators of the study are chosen in accordance with SIA 2040 (2011), which offers target values linked to the goals of the "2000-Watt society" in terms of embodied primary energy non-renewable and embodied greenhouse gas emissions. Environmental loads from a database entitled "Ökobilanzdaten im Baubereich" and Ecoinvent v3 (Moreno Ruiz et al., 2013) accessed via SimaPro® is used to assess selected materials. Additionally, material costs are added as a third indicator to analyze the economic feasibility of the optimized solutions. A multifunctional approach is used to compare different materials: Material combinations in timber-framed elements are assessed in terms of selected key functions within the context of a school building. As a result, data sets are generated representing all possible material combinations included in the wood structure in terms of each indicator.

A key question of this study is how the carbon storage potential related to (fast-growing) bio-based building materials affects the carbon balance on the building level. It is assumed that carbon neutrality does not necessarily lead to climate neutrality. Thus, an indicator entitled GWPtot, which includes the global warming potential of carbon dioxide emissions from bio-based materials is used. In combination with the PENR indicator an optimization on the material level is conducted. The optimization is based on a sensitivity analysis of the model parameters and the life cycle assessment of a selected product pool in school elements. In this study, the optimization process corresponds to the investigation of an adequate balance between the two indicators (PENR and GWPtot) in order to reach SIA 2040 (2011) targets. This is needed because the results showed a non-linear relation between the two indicators.

Finally, a comparison of four proposed clusters is made with target values shown in SIA 2040 (2011) and the currently considered material mix. Additionally, the results are compared to a case where greenhouse gas emissions from bio-based materials are assumed to be climate neutral. The results indicate a considerable potential of material combinations including (fast-growing) bio-based materials if carbon storage is considered: Specified material combinations meet previously surpassed target values and simultaneously indicate a potential economic benefit on the building level.

Impacts regarding a possible total project volume of 70 schools are estimated in a conclusion of the thesis: The results indicate the environmental and economic potential if the offered material mixes are considered for future wood structures in school buildings.