Thermoeconomic diagnosis and entropy generation paradox

In the entropy generation paradox, the entropy generation number, as a function of heat exchanger effectiveness, counter-intuitively approaches zero in two limits symmetrically from a single maximum. In thermoeconomic diagnosis, namely in the characteristic curve method, the exergy destruction is proposed as the dependent variable, along with a set of independent variables, to locate the actual cause of malfunction. This relies on the assumption that in case of an operation anomaly its exergy destruction rate strictly increases. We examine the behaviour of the diagnosis method with regards to the entropy generation paradox, as a decreased heat exchanger effectiveness (as in the case of an operation anomaly in the component) can counter-intuitively result in decreased exergy destruction rate of the component. Therefore, along with an improper selection of independent variables, the heat exchanger can be deduced to be working more effectively from the resulting indicator, when it is actually degraded. From an extensive analysis of the diagnosis method an alternative dependent variable was proposed in the form of exergy destruction rate normalised with the exergy fuel rate, which strictly increases in case of an operation anomaly in a component. The normalised exergy destruction rate as the dependent variable therefore resolves the relation of the characteristic curve method with the entropy generation paradox.