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Exergy as a resource efficiency indicator for industries

Abstract:

A research activity was carried out to identify the most important resource efficiency indicators used at the industry level for five main categories: Energy, material use, water use, greenhouse gas emissions and waste generation and management. Fifty eight indicators were found in the literature but only twenty of them were applicable to the process level. Additionally, a detailed analysis was carried out to justify the use of the exergy indicators for industrial processes and the important advantages it has over the use of other indicators. The most repeated limitation among the studied indicators is the inability to measure the quality degradation of the resources, which is one of the most important advantages of the exergy indicators, since exergy not only measures the quantity but also the quality of the resources. Moreover, exergy can also cover the five categories of resources and can be used in the micro as well as in the macroeconomic level. An example of the application of the exergy indicator was developed to support this argument based on an industrial process from a fertilizer manufacturer. This example was modeled using the exergy cost accounting methodology, based on the thermoeconomic theory. Three operation scenarios were proposed in order to quantify how the exergy indicators are able to measure the efficiency in resource use in an industrial process. A global indicator was used for this matter to identify the most efficient scenario (global unit exergy costs) as well as several local exergy indicators (associated to raw materials, to the use of fossil fuels in the process, to CO2 emissions and to electricity consumption). This analysis represents an effort to highlight the advantages of using exergy and the exergy accounting methodology to assess industries in relation with resource consumption, energy efficiency, waste valorization and re-use.