Inside out : a shift in perspective for building exergy analysis

Bonetti, Valentina; (2018) Inside out : a shift in perspective for building exergy analysis. In: ECOS 2018. University of Minho, PRT. ISBN 978-972-99596-4-6

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In many countries, our lives are lived under a roof. The indoor space is constantly growing and its thermal conditions are kept relatively stable, within comfort requirements, by a variety of passive strategies and mechanical systems. Despite the many energy-efficiency programmes and conservation efforts, the inevitable consequence of the rise in complexity of the built environment is its increasing energy consumption, which poses urgent sustainability concerns. Within this scenario, many optimisation methods are attempted in building design. Among them, exergy analysis - with its roots in thermal power plants - provides a quantification of energy quality and a deeper insight into the causes of inefficiencies. However, buildings are substantially different from machines and some fundamental aspects of their exergy analysis are controversial, such as the impact of the reference-state selection on a dynamic study, or the practical benefits of achieving high exergy efficiencies. Exergy assessments are complex and do not currently hold a prominent role in building design. Nonetheless, second-law approaches preserve their attractiveness and many possible methods are still unexplored. After acknowledging the thermodynamic differences between buildings and power machines and their particular aims, a simplified theoretical framework (built on the current methods) is proposed and formally compared to a state-of-the-art dynamic exergy analysis. The indoor environment is recognised as the fulcrum of all building energy interactions and the reference to assess energy quality. A case study shows the quantitative differences between the proposed idea and the classical approach for the first fundamental steps of the building energy chain from the exergy demand to the emission system. Keeping the design focus on exploiting the local exergy budget, rather than minimising consumption, contributes towards flexible and low-cost sustainable buildings for integrated energy systems. Substantial further research, assisted by virtual representations and real-world testing, is required to explore meanings and practical implications of the proposed perspective.


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