System boundary for embodied energy in buildings: A conceptual model for definition

Buildings consume nearly 40% of global energy annually in their production, operation, maintenance, replacement and demolition stages. Energy consumed in their life cycle stages other than the operation is called life cycle embodied energy. Total life cycle energy constitutes the building's embodied and operational energy over its service life. Operational energy constitutes a relatively larger fraction of life cycle energy in a conventional building. However, with the emergence of larger number of low energy buildings the significance of embodied energy is expected to grow. Current embodied energy calculations exhibit problems of variation, inaccuracy and incompleteness. System boundary definition is a key parameter that differs across studies and causes these problems, as studies define their system boundary subjectively. Research studies have proposed various system boundary models that should be applied to the buildings for life cycle analysis; however, the extent of their boundary definition differs. This paper gathers and synthesizes relevant literature opinions to develop a comprehensive system boundary model that can be adopted while performing the life cycle energy analysis of a building. The purpose of developing this model is twofold. Firstly, it would provide a clear picture of the system boundary. Second, it would provide a model to quantify the embodied energy of a building. Three possible approaches to cover the proposed system boundary are also recommended.