Consequential impacts of a net-zero carbon design: life cycle assessment of an active building

Life Cycle Assessment (LCA) is becoming the predominant means for determining if a building design meets a carbon emission target. These target values are set to help building designers meet aspirational net-zero carbon targets. Within LCA, there are two modelling frameworks. Attributional LCA (ALCA) assigns a portion of global emissions to a specific product or process. Consequential LCA (CLCA) assesses the impacts from a market’s response to a change in demand for a product or process. A case study building, located in Swansea, UK, has been assessed to investigate the differences between ALCA and CLCA. The case study building employs: a modular off-site construction building fabric; on-site energy generation; and, on-site energy storage – all strategies that may be adopted at large scale to decarbonise the built environment. Based on global warming potential assessed over a 100-year time horizon (GWP₁₀₀), the total upfront embodied impacts from CLCA are 19% higher than that from ALCA. Three differences exist within the rank order of building elements. The Frame presented the highest contribution to the GWP₁₀₀ within the CLCA results, whereas External Walls contributed the most within the ALCA results. The differences arise mostly from how electricity production is modelled within attributional and consequential datasets and whether substitution or cut-off are used within the background processes. CLCA can capture the environmental impacts of decisions taken to create a net-zero built environment. However, CLCA should not be directly compared to ALCA without appreciating and recognising how the methods and scopes differ.

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