Application of the Exergy Method to the Environmental Impact Analysis: A Case Study

Abstract

The cumulative exergy method is used to analyze and optimize a heat pump distillation process with respect to the total environmental impact including resource use and emissions. The results from the proposed method are compared with the ones from thermoeconomics. It is found that such an extended exergy analysis offers a new insight and permits the identification of solutions which are more stable in time and independently of economic conditions. Introduction The increasing concern for the environmental impacts of human activities has stimulated the development of new methods for the analysis of industrial processes and the implementation of natural resource measures. Nowadays there are several tools for design or analysis of environmental objectives of industrial processes. Herein, a well established and comprehensive approach is the life cycle assessment (LCA). LCA has undeniable merits but, being essentially a first-law type of analysis, its validity is tainted by its inability to account for different types of energy carriers [1]. Various approaches have been proposed and developed in the last years with the intention to obtain environmental impact indicators based on both the First and Second Laws of Thermodynamics. The exergy method appears to be the most suitable to achieve this purpose, as the exergy of a system is by definition the potential of this system related to the environment [2]. The relationships between the exergy and the environment concern both the consumption of natural resources and the impact of the emissions on the environment. Several attempts have been made to combine exergy and LCA to quantify the environmental impact of industrial processes, such as cumulative exergy consumption (CExC) analysis, exergetic life cycle analysis (ELCA) and so on. These works advocated the use of exergy as the metric within the LCA framework [3]. The CExC analysis has been used for many years as a useful approach for evaluation of the overall consumption of all kinds of natural resources at every step of a production process. However, the numerous attempts to obtain a direct correlation between exergy and the indicators used for estimation of the impact of the emissions on the environment are still not quite satisfying [2]. In this paper, a unified process assessment is carried out to integrate various resources and emissions associated with the life cycle of industrial processes, based on the thermodynamic concept of exergy as a common objective measure. The cumulative exergy concept is applied to analyze and optimize industrial processes for emission abatement with respect to their total environmental impact. The heat pump distillation process is studied as an interesting example.