Auxiliary equations for the determination of specific exergy revenues

Thermoeconomics allows the specific costs (the costs per unit exergy) associated with exergy streams to be determined within, and at the boundaries of, energy systems. These costs are determined through the simultaneous solution of monetary balances and auxiliary equations, with the values of the primary fuel and capital costs known. The methodology for determining the auxiliary equations, when product costs are to be calculated from known fuel and capital costs, has been formulated by Lazzaretto and Tsatsaronis. These costs and other thermoeconomic variables have been shown to have utility in the optimization of energy systems. In most applications, this has been applied to systems with a specified output and variable fuel input. The goal in the optimization of these systems is to minimize product costs; for a given output with given product price(s), this is equivalent to maximizing profit. However, some practical systems, such as a combined-cycle power plant, have an essentially fixed fuel input. In these cases, capital cost is traded against power output, and profit is maximized by taking the difference of the products times their prices and the capital costs. With a known price of the product, a value fundamentally different than a specific cost is calculated. In this paper, these values are called “specific revenues”. Owing to their difference in nature from specific costs, specific revenues require different auxiliary equations. Here, the differences between revenues and costs are explained, principles are given for writing the proper governing equations and both revenues and costs are given for a simple example system. Additionally, some suggestions as to how these specific revenues may aid in system optimization are given.