Exergo-economic optimization of heat-integrated water networks

This paper introduces an exergo-economic optimization approach for synthesizing heat-integrated water networks (HIWNs). Most previous research focused on economic optimization, aiming for optimal network design with minimum total annualized cost (TAC). Exergetic optimization of HIWNs has rarely been studied in the literature. Thus, a novel approach is introduced by developing a nonlinear programming (NLP) model to minimize exergy destruction within the system. In order to manage network complexity (stream splits and extensive piping), exergy destruction caused by friction is added to the objective function. The NLP model produces good local solutions comparable to those obtained with the mixed integer nonlinear programming (MINLP) model with an economic objective function (TAC), with a relative discrepancy in TAC of about 0.4 % and solution time for the NLP model being only about one third of the time needed by the MINLP model. In addition, the exergy-based model does not depend on costs for freshwater, utilities, and equipment. The proposed methodology provides a set of local solutions from which the best one can be selected based on users’ criteria, such as minimum TAC, minimum exergy destruction or a solution in between.