# Residue Cost Formation of a High Bypass Turbofan Engine
## 1 Introduction  
The aviation sector contributes significantly to economic growth and an increasingly globalized society [1], but at a cost of 2-3% of total fossil fuel consumption and 2% of total greenhouse gas emissions worldwide [2]. In this sector, major priorities are not only to operate with the maximum cost effectiveness, but also to reduce fuel consumption and emissions of greenhouse gases and pollutants. Several studies have been conducted in which energy, exergy, and exergyeconomic analyses were applied to aviation propulsion systems as important tools for understanding and optimizing their operation to identify operational, economic, and environmental improvement opportunities [1, 3, 4, 5, 6, 7, 8]. In these studies, the most frequently used method was specific exergy costing, which is based on algebraic cost balance equations obtained from conventional economic analyses and complemented with auxiliary equations [9]. However, these studies have been conducted with a focus only on the formation of the functional product of aircraft engines (i.e., kinetic exergy) without considering theunavoidable formation of residues (i.e., waste heat dissipated from the engine and the chemical exergy of combustion gases). For these energy systems, residues are considered an exergy loss because they cannot be reused and they contribute to environmental pollution. Thermoeconomics is a powerful analytical tool for cost accounting that is based on exergy cost theory (ECT), which combines the second law of thermodynamics and economics [10]. It provides a rationale for assessing the production cost of energy systems in terms of the consumption of natural resources and their impact on the environment, money, and system irreversibilities [11]. Thermoeconomics can be used to help design, diagnose, and optimize complex energy systems. More precisely, it can help to determine how energy and resources degrade, identify which systems work better, improve the design to reduce consumption, and prevent residues from damaging the environment [12]. Exergy cost theory offers a procedure for determining the production costs of productive components of a system. This theory has been extended to dissipative components, but does not present a general procedure for identifying residue formation costs and their effect on the production costs [10]. The best residue distribution criteria among possible alternatives are still an open question [13; 14; 15; 16]. [PERSON] et al. proposed an exergoeconomic methodology known as symbolic thermoeconomics to determine the process cost of functional products and residues and establish a mathematical basis for the production cost assessment. It develops a productive scheme (i.e., productive structure) of the exergy flow distribution throughout the system and its interaction with the environment, which are obtained from its physical structure [10; 17]. This methodology formulates two alternative representations for the productive structure of a system: fuel-product-residue (FPR) and product-fuel-residue (PFR). These respectively use the external resources and plant product of the system as known information. Both approaches lead to the same results in terms of costs. The FPR representation is mainly adopted for cost accounting, whereas the PFR representation is highly useful for thermoeconomic diagnosis [18]. Thermoeconomic diagnosis is focused on identifying and interpreting anomalously functioning components and evaluating the effect of each component on additional fuel consumption. Symbolic thermoeconomics also includes the identification of malfunctions and dysfunctions to account for the impact of anomalies on fuel consumption [19; 20]. The main motivation of this study is to address the lack of exergoeconomic analysis on the contribution of residue formation to the production cost of turbofan engines. This paper is organized into six different sections. In Section 2, a brief description of a turbofan is presented.