An Exergy-Based Methodology for Decision-Based Design of Integrated Aircraft Thermal Systems

Abstract

This paper details the concept of using an exergy-based method as a thermal design methodology tool for integrated aircraft thermal systems. An exergy-based approach was applied to the design of an environmental control system (ECS) of an advanced aircraft. Concurrently, a traditional energybased approach was applied to the same system. Simplified analytical models of the ECS were developed for each method and compared to determine the validity of using the exergy approach to facilitate the design process in optimizing the overall system for a minimum gross takeoff weight (GTW). The study identified some roadblocks to assessing the value of using an exergy-based approach. Energy and exergy methods seek answers to somewhat different questions making direct comparisons awkward. Also, high entropy generating devices can dominate the design objective of the exergy approach. Nonetheless, exergy methods do provide information to aid design providing a ready estimate for efficiency on a component and system basis. The results from the two analyses did provide similar while not exact solutions. While the paper will illustrate the methodology and its implementation, further progress is necessary to validate the hypothesis that exergybased methods are advantageous for the design of integrated systems. INTRODUCTION In thermal systems, decisions are based in part on the thermodynamic behavior of the component or system. Traditional design procedures use an energy-based approach for decisionmaking. Essentially this is a thermodynamic first law analysis. Exergy-based methods deal with the simultaneous application of the thermodynamic principles of the first law and second law to component or system design. Exergy analysis yields an optimal solution based on an objective of entropy generation minimization. Energy-based methods are built on the fundamental concept that energy flows into and out of a system through heat transfer, work, and mass flow. That energy must be conserved is the basic premise of the first law. On the other hand, exergy represents the ability to do work or, put in a different way, the ability to bring about a desired change. Exergy is not conserved and, in fact, is partially or totally destroyed. The amount of exergy destroyed is proportional to the amount of entropy generated. It is the destroyed exergy that brings about the component or system inefficiency. Hence, a design process based on minimizing entropy generation reduces exergy destruction to improve efficiency. Exergy-based methods applied to the design of aircraft integrated systems have been discussed as being advantageous to traditional methods. The approach has been applied to components and to land-based power plant design [e.g. 1-3]. Tipton et al. [4] first applied exergy methods to the environmental control system of an aircraft and this paper follows up on that study. By looking at the irreversibilities associated with the entropy generation of each component, an attempt is made to meet design objectives that make best use of available energy. An attractive feature of the method comes from entropy as a property. Irreversibility within a system is related to the sum of the entropy generated by each component in the system. Constraints, such as size or weight, are readily imposed. However, the demonstration of a complete optimized design of an aircraft system using exergy methods has not been documented. The original motivation for this work was prompted by a study to evaluate the aircraft-level impact of using spray cooling technology in an avionics chassis, which was part of the environmental control system (ECS) of an advanced aircraft [4]. In particular, the aircraft level impacts of this new cooling technology were to be evaluated. While evaluation of this technology is not the focus of this paper, the avionics chassis remains within the model and its impact will be studied. More recently, participants from an Air Force sponsored workshop [5] identified several unresolved questions of importance in moving towards an innovative, fully integrated (c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. design methodology for ah vehicles. Of these, an unambiguous demonstration of exergy-based methodology advantages at the system and aircraft levels was a priority. This paper details the concept of using an exergy-based method as a thermal design methodology tool for integrated aircraft thermal systems.