Dynamic Exergy Analysis for the Thermal Storage Optimization of the Building Envelope

Valentina Bonetti,Georgios Kokogiannakis

doi:10.3390/en10010095

Valentina Bonetti, Georgios Kokogiannakis

Open Access

https://doi.org/10.3390/en10010095

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Journal: Energies	Publication Date: Jan 13, 2017
Citations: 13	License type: CC BY 4.0

Affiliation: University of Strathclyde, University of Wollongong

Abstract

As a measure of energy “quality”, exergy is meaningful for comparing the potential for thermal storage. Systems containing the same amount of energy could have considerably different capabilities in matching a demand profile, and exergy measures this difference. Exergy stored in the envelope of buildings is central in sustainability because the environment could be an unlimited source of energy if its interaction with the envelope is optimised for maintaining the indoor conditions within comfort ranges. Since the occurring phenomena are highly fluctuating, a dynamic exergy analysis is required; however, dynamic exergy modelling is complex and has not hitherto been implemented in building simulation tools. Simplified energy and exergy assessments are presented for a case study in which thermal storage determines the performance of seven different wall types for utilising nocturnal ventilation as a passive cooling strategy. Hourly temperatures within the walls are obtained with the ESP-r software in free-floating operation and are used to assess the envelope exergy storage capacity. The results for the most suitable wall types were different between the exergy analysis and the more traditional energy performance indicators. The exergy method is an effective technique for selecting the construction type that results in the most favourable free-floating conditions through the analysed passive strategy.

Highlights

Exergy is a state function that combines the first and second law of thermodynamics through a reference environment
The guidance provided by the energy analysis typically performed to select the envelope during the design phase, discussed in Section 2.1, is compared to the support offered by a tailored exergy assessment for the case study described in Section 2.3 representing a common example in the Mediterranean climate
The outputs provided by the building energy and exergy analysis were compared for the case study and are presented in Sections 3.1 and 3.2, respectively

Summary

Introduction

Exergy is a state function that combines the first and second law of thermodynamics through a reference environment. The strong dependency of exergy from the defined reference creates a “co-property” of the system and the environment [1] and enables a quantification of energy quality, measured as “the maximum theoretical useful work obtainable as the system interacts to equilibrium, heat transfer occurring with the environment only” [2]. A simple way to obtain exergy fluxes is to apply “quality factors” q f as conversion factors of the energy fluxes. In the case of a conductive heat transfer Q occurring at a constant T, if the reference environment has temperature T0 , the associated exergy transfer B is directly derived from the Carnot efficiency as: T0.

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Conclusion