Modelling environmental impacts of buildings – energy, material, and dynamics

Abstract

Buildings are responsible for a considerable share of global environmental impacts. Therefore, the building stock is an important stakeholder for reaching environmental targets. In Switzerland the vision of the 2000-Watt Society has been adopted by several cantons. It states that by 2050 the annual per capita consumption of primary energy must not exceed 17.5 MWh and greenhouse gas emissions remain below 1.0 ton of CO2-equivalents. This corresponds to a significant reduction, compared to today’s values. Based on the current literature on the assessment of buildings and building stocks, three main research gaps are identified. The first concerns the relevance of the different aspects causing environmental impacts in the construction sector. In particular material-related impacts have been rarely included into sensitivity analysis of whole buildings and the role of thermal mass is unclear. Secondly, new methods for building stock models are required. The most commonly used type are archetype bottom-up models, which have limitations concerning granularity, stock dynamics, quantification of uncertainty, and building representation. The third question deals with the implications of future material and energy flows of building stocks. Is it realistic to reach the targets of a 2000-Watt society? The central objective of this thesis is to close these research gaps and enhance the understanding and assessment of the building-environment system by exploring the complex relations between energy, material, environment, and building stock dynamics. To reach this objective, this thesis (i) investigates parameter sensitivity and identifies the most relevant drivers of impact, (ii) assesses the importance of construction material with regard to environmental life-cycle impact and accounting for dynamic thermal behaviour of buildings, (iii) quantifies different indicators of environmental impact, (iv) proposes a modelling approach for spatio-temporal assessment of building stocks, (v) defines and assesses prospective scenarios, and (vi) compares the results with environmental targets. Each chapter of this thesis presents a building model with specific features. The first model connects two specialised assessment tools with one another to capture the complexity of the building-environment system. On the one hand, a state-of-the-art dynamic thermal simulation tool, capable of accounting for thermal inertia, determines building energy demand. On the other hand, the resulting life cycle material and energy inventories are assessed with a life cycle assessment (LCA) tool. Furthermore, Monte Carlo sensitivity analysis is used to determine the influence of twenty-eight stochastic input parameters, describing aspects of building design, material, operation, and exogenic factors. The second model investigates the future energy demand of the Swiss building stock. It quantifies the related environmental impacts and investigates scenarios of future development. The results are then compared with the targets of the 2000-Watt society. The third model calculates space heat demand and the related environmental impacts of each building in a Swiss municipality. It investigates the characteristics of highand low-emission households by means of cluster analysis. The fourth model quantifies material stocks and flows for the entire Swiss residential building stock from today until 2055 and quantifies the related environmental impacts. The national model finds that the projected decline in population growth will lead to important changes in the building stock. On the one hand, this development helps to halt the increase in building energy demand and stabilize the related greenhouse gas emissions. On the other hand, the overlying dynamic of the building stock leads to the situation in which input material flows will decrease, but material waste flows increase. That means the building stock transitions from a growth state into a maintenance state. Unlike the energy-related emission, this does, however, not lead to a reduction of environmental impacts. Although the impacts due to concrete for new constructions reduce significantly, new impacts occur. The reason is the building stock renewal and the associated increase in impacts due to insulation and mineral material. As the quantity of input and output material flow are practically identical in the future, an important opportunity for circular material flows arises. However, new recycling technologies are required. Overall the importance of construction materials for the environmental performance of buildings will increase in the next decades. The targets of the 2000-Watt society can only be met by means of more ambitious strategies than the ones that are implemented today. Especially, an increased rate of refurbishment and demolition is required. This, however, leads also to a considerable increase of material-related impacts, unless energy-intensive materials (e.g. concrete) are substituted with low-impact materials, such as wood. The methods for building stock modelling are improved in different ways. Firstly, a new approach for tracking the evolution of the building’s thermal envelope is introduced. That allows to determine building space heat demand based on physical properties and thermal building simulation. Secondly, the change of electricity and thermal energy sources in buildings is considered by means of diffusion models. Using an approach that is based on life cycle assessment, it is possible to quantify also “hidden” environmental impacts, due to infrastructure, transport, etc., and greenhouse gases other than carbon dioxide. Thirdly, this thesis demonstrates the feasibility of modelling the material stock and flows of the entire Swiss residential building stock. This was possible by merging three-dimensional building data and by using a geospatial relational database. Stochastic modelling was successfully applied to perform sensitivity analysis, close data gaps, and define future scenarios. The combined building assessment method, using thermal simulation and life cycle assessment, allows to account for the interaction of material and energy related parameters. The building stock models, presented in this thesis, show a number of innovations that will facilitate future work. For instance, they illustrate how GIS bottom-up models can be implemented for large building stocks and prospective scenario assessment. The modelling approaches, presented in this thesis, are relevant for future scientific work. In a next step the spatial-dynamic material flow analysis and the energy demand model should be coupled in order to investigate the trade-off between material and energy demand for the entire building stock. This is important for identifying possible rebounds of building stock renewal strategies. This thesis contains a number of results that are relevant for decision-makers in the building sector. These include specific building design recommendations as well as the identification of risks that hinder the achievement of environmental targets. Energy supply, space heat demand, and construction material are identified as the prime drivers of environmental impacts. The reduced thermal inertia of wooden buildings leads to increased space heat demand, but the negative influence on environmental impact is overcompensated by the lower embodied impacts of the construction material. Another driver of environmental impacts in building stocks is the per capita floor area demand, because it directly scales with energy and material demand. The results can be translated into a number of recommendations. The first priority for planners should be to use clean energy sources for a building. Energy demand is second most influential and mostly determined by ventilation rate. Therefore, buildings should have good air-tightness and a sound ventilation strategy (e.g. using CO2-sensors and heat recovery in the case of mechanical ventilation). Construction material ranks higher than many of the energy-related parameters. Wooden construction is practically always beneficial and should be prioritized. Service life is also very relevant and durability of materials should be ensured. Particularly buildings with a short lifetime should be built from low-impact materials, such as wood, since the initial impact of a concrete construction is relatively high. Buildings with high anticipated cooling loads (e.g. sun-exposed office buildings) may profit from a higher thermal mass. Concerning policy implications, the LCA-based assessment of construction material should be included into future construction regulations, similar as it is the case for space heat demand today. The models, presented in this thesis, can be used to develop national and also regionalised resource and refurbishment strategies. Finally, a significant risk for a ‘lock-in’ situation exists. Due to the very slow turnover of the building stock, it is imperative to implement forward-looking policies. In other words, the buildings for the 2000-Watt society need to be built today.

My PhD thesis 😸.

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