The kinetic energy produced by a turbofan engine is inseparable from the unavoidable generation of waste heat dissipated into the environment and the chemical exergy of exhaust gases. However, exergoeconomic cost analyses of these propulsion systems have focused only on the formation process of the functional product and not the cost of residue formation. In this study, symbolic thermoeconomics was applied to evaluate the impact of residue formation on the production costs of a turbofan engine and analyze the effect of component malfunctions on the fuel impact formula for diagnosing anomalies. The GE90-115B high bypass turbofan engine under takeoff conditions and a thrust requirement of 510 kN was considered as a case study. The total exergoeconomic cost of the engine was 26,754.28 USD/h: 61.04% corresponded to external resources; 0.14% and 33.07% corresponded to waste heat dissipated from the bypass and core engine, respectively; 3.28% corresponded to the chemical exergy of the exhaust gases; 2.47% corresponded to capital and operating costs. A malfunction analysis revealed that a 1% reduction in the isentropic efficiency of the compressor reduced the total kinetic exergy by −0.77 MW, increased fuel consumption by 0.49 MW, and generated irreversibility and residue of 0.80 and 0.45 MW, respectively.

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In an energy system, it is important to identify the origin of residue formation in order to implement actions to reduce their formation or to eliminate them as well as to evaluate their impact on the production costs of the system. In the exergetic cost theory, although there are several criteria to allocate the cost formation of residues to the productive components, no unique indication on the best choice has been defined yet. In this paper, the production exergy costs are determined by allocating the residue cost formation to the irreversibilities of the productive components from which they originate. This criterion, based on the Gouy–Stodola theorem, is an extension of the criterion of entropy changes, and unlike this, it avoids the existence of a negative production cost. This criterion is applied to a combined cycle of three pressure levels, and the production exergy costs are compared with the criteria of entropy changes, distributed exergy, and entropy. The results of the proposed criterion are in agreement with the compared criteria.

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This paper considers the criterion of minimum compression work to derive an expression for the interstage pressure of a multistage compressor with intercooling that includes the gas properties, pressure drops in the intercoolers, different suction gas temperatures, and isentropic efficiencies in each compression stage. The analytical expression for the interstage pressures is applied to estimate the number of compression stages and to evaluate its applicability in order to estimate interstage pressures in the operation of multistage compressors, which can be especially useful when their measurements are not available.