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This paper discusses the application of exergy analysis to a district heating network (DHN) currently in operation in Brescia (Italy). The purpose is to compare the existing system design with that resulting from an exergonomic analysis. To this aim a general mathematical model is formulated to account for irreversibilities due to the temperature and pressure losses as well as for the assessment of exergy costs for each stream. The objective function is stated as the sum of capital, operational, and maintenance costs of the whole DHN. Such a function is minimized by using two different approaches: an iterative procedure and a genetic algorithm; the latter proves to be particularly suited for this type of problem. Both methods lead to comparable outcomes in terms of minimum cost and plant arrangement. Finally, the optimum configuration is compared with the existing one and shows that the actual network is far from an optimum design both from a thermodynamic and an economic standpoint. Such a procedure could be used profitably in the design of any future DHN.

In this paper, exergonomic theory is applied to an air-conditioning system for optimization purposes. The investigation is addressed to an all-air system with air recirculation. The thermodynamic cycle includes a mixing plenum, a cooling and heating coil, chiller, and heater. The thermodynamic model is stated according to recent formulations of exergy for moist air streams, while the economic model is based on cost balance equations and real cost data for mechanical equipment. The objective function to minimize includes the following decision variables: fresh to total air rate, coefficient of performance for the chiller, inlet temperature of water for the cooling and the heating coils, temperature difference of the same streams. For the exergonomic optimization, the authors followed the approach proposed by Tsatsaronis (1984). The optimum configuration is obtained through an iterative procedure aimed at the design improvement. The results show that there is considerable room for improvement with respect to a system based on typical design parameters.