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Exergy Analysis as a Tool for Decision Making in Aircraft Systems Design

Luiz Felipe Pellegrini* Escola Politécnica, University of São Paulo, São Paulo, SP, 05508-900, Brazil

Ricardo Gandolfi† and Guilherme Araújo Lima da Silva‡ Embraer – Empresa Brasileira de Aeronáutica, São José dos Campos, SP, 12227-900, Brazil

Escola Politécnica, University of São Paulo, São Paulo, SP, 05508-900, Brazil

and

Silvio de Oliveira Jr.§ Escola Politécnica, University of São Paulo, São Paulo, SP, 05508-900, Brazil

Airplanes are designed to attend the customer needs with minimum fuel consumption by reducing the inefficiencies. The aeronautical industry has evolved to design extremely complex aircrafts, with highly integrated systems. The current methods to design and later optimize are based on trade-off studies. However, this type of analysis for new systems evaluation may present non-conclusive results and, due to the high level of integration between aircraft systems, the optimization of a single system may lead to sub-optimized solutions for the whole aircraft. Therefore, the search for an optimized aircraft becomes a search for commitment solutions for different systems. There is a need to develop a general methodology that allows a complete vehicle design as a system that contains sub-systems in the same basis. It is important to define aircraft design parameters (ex.: weight, fuel consumption, drag) as energy objective functions so that the problem may be classified as an optimization with objective defined by the customer. In this way, the design is committed to the maximum efficiency and minimum waste of useful energy (exergy destruction), if adequate constraints are considered. The concept of exergy analysis has already been successfully applied to evaluate, compare and optimize thermal systems and chemical processes in other industrial fields. This work shows a revision of several papers related to the exergy method on the design of aircrafts, and includes the different approaches and applications of design optimization. Also, a case study is included to illustrate the potential of the exergy tool.

Nomenclature

dragdE _& = destroyed exergy rate due to drag, kW

cabindE _& = destroyed exergy rate in the cabin, kW

ECUdE _& = destroyed exergy rate in the environmental control unit, kW

enginedE _& = destroyed exergy rate in the engine, kW

* Research Engineer, Ph.D Student, Mechanical Engineering Department, Av. Prof. Mello Moraes, 2231, luiz.pellegrini@poli.usp.br † Systems Engineer, Environmental Systems, Av. Brigadeiro Faria Lima, 2170 also Ph.D Student, Mechanical Engineering Department, Av. Prof. Mello Moraes, 2231. ‡ CFD, Simulation and Control Engineer, Environmental Systems, Av. Brigadeiro Faria Lima, 2170, also Ph.D Student, Mechanical Engineering Department, Av. Prof. Mello Moraes, 2231, AIAA member. § Prof. Dr.., Mechanical Engineering Department, Av. Prof. Mello Moraes, 2231, silvio.oliveira@poli.usp.br

45th AIAA Aerospace Sciences Meeting and Exhibit8 - 11 January 2007, Reno, Nevada

AIAA 2007-1396

Copyright © 2007 by L. F. Pellegrini, R. Gandolfi, G. A. L. Silva and S. Oliveira Jr. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

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airxE& = exergy flow rate of the air at the inlet of the engine, kW

bleedxE& = exergy flow rate of bled air from the engine, kW

exhaustxE& = exergy flow rate of the exhaust gases from the cabin, kW

flightxE& = flight exergy rate (kinetic plus potential), kW

fuelxE& = fuel exergy flow rate, kW

gasesxE& = exergy flow rate of the gases leaving the engine, kW

inletxE& = exergy flow rate of the high pressure air at the inlet of the ECU, kW

cabinQxE ,& = exergy rate used to heat or cool the aircraft cabin, kW

ambp = ambient external pressure, kPa

cabinp = cabin mean pressure, kPa

inletp = pressure of the high pressure air at the inlet of the ECU, kPa SEC = specific exergy consumption by the environmental control unit SFC = specific fuel consumption, lbm/(h*lbf)

ambT = ambient external temperature, oC or K

cabinT = cabin mean temperature, oC or K

inletT = temperature of the high pressure air at the inlet of the ECU, oC or K

electricW& = electric compressor power, kW

ECUex,η = exergy efficiency of the environmental control unit

I. Introduction nergy systems have become increasingly more complex, leading to higher levels of interaction between each sub-system. Consequently, there is a need to develop tools that would allow the design/synthesis of the

system in complete integrated way, allowing all demands of each sub-system to be reached in the best possible way1. The aeronautical industry also has evolved to design extremely complex aircrafts, with highly integrated systems, requiring more information in order to evaluate the whole system2.

Modern aircrafts are designed to attend the client needs, minimizing fuel consumption and inefficiencies - this approach is named "traditional optimization". Most of these studies rely on rules-of-thumb, individual experience and non-integrated, non-interdisciplinary approach of basic calculations, i.e. simple trade-off analysis 1. Such analysis relies on cost-benefit studies among different options, but not in the same basis. Thus, it may lead to sub-optimal solutions.

Many authors have criticized such analysis2-9. These authors agree that there must be a common basis to compare different alternatives in aircraft design, especially when developing new systems.

Moorhouse2 states that it is possible to define aircraft design parameters (for instance, weight, fuel consumption, drag) as energy functions. Thus, the initial optimization problem may be characterized as one of minimizing additional weight and losses constrained by the energy requirements of the client (weight to be transported). In this way, the project becomes committed to maximum efficiency and minimum waste of useful energy, considering the adequate constrains – parameters of an energy optimization problem.

According to Tsatsaronis10, Classical Thermodynamics provides the concepts of energy, energy transfer by heat and work, energy balance, entropy, entropy balance, among others. The Second Law of Thermodynamics complements and enhances an energy balance by enabling calculation of the true Thermodynamic value of an energy carrier, and the real Thermodynamic inefficiencies and losses from processes and systems. Therefore the concept of exergy is extremely useful for this purpose.

Exergy is defined as the maximum work obtained by an energy carrier, when it is brought into equilibrium with the environment, through reversible processes with common components of the environment11. Exergy is not only a measure of the true Thermodynamic value of an energy carrier, but it also might be related to the economic value of the carrier10.

Exergy analysis and minimization of exergy destruction can be used by themselves mainly in areas where the total cost of the installation is dominated by the cost due to Thermodynamic irreversibility4. Paulus and Gaggioli3 state that the design of vehicles do not have the same goal of energy systems designed to provide mass

E

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flow, heat or work transfer. Instead, they are designed to attend performance goals, and these are related to Thermodynamic inefficiencies.

The use of exergy indicates directly the location of the main sources of irreversibilities; hence it provides new information to the designer regarding the operation of the system. Also, exergy allows a more rational way to attribute costs in energy systems; therefore it is possible to associate irreversibilities and costs. Unfortunately, exergy analysis does not indicate directly the system alteration required to minimize these irreversibilities 4.

Some authors1,3,4,8 suggest the use of decomposition methods, based on the work of El-Sayed and Evans12, in order to overcome some drawbacks related to the design/synthesis of aircrafts based on exergy considerations. Others13, 14, 15 follow the "Entropy Generation Minimization" approach16, a more phenomenological one. However, the demonstration of a completely optimized design of an aircraft system using exergy methods have not been documented so far6.

Thus, a review of previous works is of crucial importance in order to understand the advantages and disadvantages of the proposed methods, and to develop new ideas.

II. Exergy Analysis in Industrial Applications The concept of exergy has its origins in the formulation proposed by Gouy and Stodola, which shows that the

maximum potential a system may perform, is a function of its internal energy, entropy and the environment conditions (pressure, temperature and composition)11.

According to Tsatsaronis10, the modern development of exergy analysis was initiated by F. Bosnjakovic in Europe and J. H. Keenan in the United States, in beginning of the 20th Century. In 1950s and 1960s contributions to the exergy concept were also made by Z. Rant, P. Grassmann, W. M. Brodyanski, E. A. Bruges, M. Tribus, E. F. Obert, R. A. Gaggioli, R. B. Evans, H. D. Baehr, W. Fratzscher, J. Szargut, R. Petela and K. F. Knoche, among others. During this period, the calculation of exergy of fuels and the definition of reference states for calculating chemical exergy were introduced.

Since then, exergy analysis has been successfully applied to a great number of areas: a) Cryogenic Plants17; b) Desalination Plants18; c) Wastewater Treatment Plants19,20; d) Cogeneration Plants21,22; e) Process Industry23,24; f) Chemical and Petrochemical Plants25,26,27,; g) Power Plants28,29; h) Third Sector30,31; j) Society32.

The idea of using exergy for costing purpose was initiated by J. H. Keenan in 1932. The modern development of thermoeconomics began in the late 1950s with M. Tribus and R. B. Evans at the University of California, and by E. F. Obert and R. A. Gaggioli at the University of Winsconsin10. Different methodologies have been proposed and applied to different energy systems since then33-38.

Recently, exergy has also been used as an ecological indicator in order to assess the environmental impact of energy systems.39-46

III. Exergy Analysis in Aerospace Industry The exergy approach has been used for some years in aerospace industry involving isolated systems and

integrated aircraft-systems analysis, due to the reason that several systems and processes on an aircraft contribute to the destruction of all the exergy furnished by the fuel. Analyzing the direction of fuel flow to illustrate the method, the first exergy loss (about 30%) is due to the combustion process13. Associating this with the loss due to the irreversible operation of the engine, the exergy destruction in the engine can reach the values up to 50% of the total exergy destroyed15. In addition to propulsion system losses, aerodynamic drag and all electric, pneumatic and hydraulic power user systems generate irreversibilities. Figure 1 illustrates the application of the exergy analysis method to the environmental control system of an aircraft. The environmental control unit (ECU) pack provides to aircraft cabin and maintains temperature and pressure at comfort levels for crew and passengers. However, the ECU demands air or electric work from engine, and imposes penalties to airframe due to weight of equipment and aerodynamic drag caused by ram air inlets and exhausts air streams. Reliable numerical tools shall predict the airframe flight mechanics and engine operational condition in order to determine the aircraft thrust and speed. Similarly, all other user systems may be included in the analysis of Fig. 1 using the same methodology, i.e., by adding each system exergy demand from engine and penalties to airframe.

Considering system level approach, some authors applied the exergy methodology in environmental control systems, due to the evident exergy destruction and opportunity to optimize operation and design. Some authors13-15 addressed two basic issues in the Thermodynamic optimization of environmental control systems (ECS) for aircraft: realistic limits, the minimal power requirement (reversible cycle approach) and design features that facilitate operation at minimal power consumption. It is shown that the temperature of the air

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Thrust

SpeedEx_flight

Ex_fuel

Ed_drag

Ed_engine

Ex_gases

Ex_air

Weight + Aerodynamicdrag penalties Ex_bleed or Weletric

ECU pack Ed_ECU_pack

EQ_pack

Ex_exhaust

Ed_cabinCabin

EngineAirframe

Aircraft

Figure 1: Demand and Penalties Imposed by Environmental Control System to Engine and Airframe

stream that the ECS delivers to the cabin can be optimized for operation at minimal power. In their work, models of cabin heat transfer with different complexities were implemented and associated to an irreversible ECS bootstrap model.

Paulus and Gaggioli3 used exergy for the decomposition and detailed design of the subsystems and devices of a light experimental aircraft (alternator and engine). The method proposed used exergy analysis and decomposition as well as thermoeconomics to the selection of an optimum design.

Figliola and co-workers6 developed a model for the ECS of an advanced military aircraft, which includes seven integrated subsystems: 1) cold poly-alpha-olefin (PAO) loop; 2) vapor cycle system; 3) air cycle system; 4) hot PAO loop; 5) oil loop; 6) hydraulic system; 7) fuel loop. The closed-circuit liquid loop subsystem uses PAO as the liquid coolant. The model includes a methodology to analyze each subsystem, component by component, for each separate mission aspect, or as integrated over the mission. The main objectives are to reduce the entropy generation for the system while satisfying the mission performance requirements of the ECS and take off weight (fuel penalty), this one as a result of the energy analysis. In order to do that, the main variables such as evaporator effectiveness, cold PAO mass flow rate, etc. were expressed in terms of ground take off weight. A multi-objective optimization decision-based approach was applied in a tradeoff study between these two objectives (minimization of take off weight and entropy), and provided similar results.

At the same direction, Roth7 introduces the exergy methodology and shows the exergy usage during an F-5E subsonic area intercept mission. In this case, 90% of the total exergy is destroyed in the propulsion system and the vast majority of these propulsive losses consist of exhaust heat, irreversible combustion, and residual kinetic energy of the jet efflux left in the wake of the vehicle. The remaining 10% of the exergy is converted into thrust work and used to overcome vehicle drag. Moreover, the author establishes the bridge between aero-thermo performance and vehicle weight as it is possible to quantify losses incurred during the mission (such as drag work, engine inefficiencies, etc.) in terms of the fuel weight required to offset those losses, over an entire mission. In order to illustrate that, it is shown a comparison of conventional gross weight breakdown for the F-5E versus the chargeable gross weight breakdown (measured based on losses in thrust work potential, instead of exergy). Also, he shows that it is possible to allocate of fuel cost chargeability to the aerothermodynamics loss mechanisms. At the end, the author introduces the modeling structure at aircraft level to identify and quantify the losses in a vehicle design, allocating of the losses during the entire mission.

Moorhouse2 improves the idea of allocating losses during the mission as he introduces the methodology of quantifying the mission requirements in terms of energy. The method must consider the work necessary to take off, climb, cruise, descent, weight variations, speed variations, etc. An exergy brake down can be done to quantify the exergy destruction in each mission phase.

Prof. von Spakovsky advised various Master and Ph.D. works in the field of synthesis/design optimization of aircrafts1,9,47-50.

Muñoz1 developed different decomposition strategies and applied them to the integrated optimization of two sub-systems, which are part of an advanced military aircraft. The idea was to carry out the conceptual design of

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a low-bypass turbofan engine with afterburning and the full synthesis/design optimization of an air-cycle Environmental Control System (ECS).

Rancruel47 applied the methodology developed by Muñoz and von Spakovsky to the synthesis/design and operational optimization of the advanced tactical fighter aircraft. The total system was decomposed into five sub-systems as follows: propulsion system, environmental control system (ECS), fuel loop system, vapor compressor and PAO loops system , and airframe system of which the latter is a non-energy based sub-system.

Markell48 compared an exergy-based methodology to a more traditional based measure by applying both to the synthesis/design and operational optimization of a hypersonic vehicle configuration comprised of an airframe sub-system and a propulsion sub-system consisting of inlet, combustor, and nozzle components. Results of these optimizations show that the exergy method performs well when compared to the standard performance measure and, in a number of cases, leads to more optimal syntheses/designs in terms of the fuel mass flow rate required for a given task.

Butt49 applied energy and exergy-based methods to the integrated synthesis/design of an Air-to-Air Fighter (AAF) aircraft with and without wing-morphing capability. The morphing-wing and fixed-wing aircraft models were optimized using four different objective functions: the minimization of fuel consumed, the minimization of total exergy destroyed and lost, the minimization of the exergy destroyed and lost by the propulsion subsystem (PS) and the maximization of the thrust efficiency. Out of these four objective functions, the minimization of fuel consumption and total amount of exergy destroyed and lost produced the best performance results from the optimization runs. This is due to the fact that both of these objective functions in essence are seeking to minimize the use of the highest quality energy present on board the aircraft, i.e., the fuel exergy.

Brewer50 applied exergy to synthesis/design of generic hypersonic vehicles, in which tradeoffs between dissimilar technologies are observed, is

proposed and measured against traditional methods of assessing highly integrated systems. A quasi-one dimensional hypersonic vehicle simulation model was designed and optimized over a formal mission with three objective functions: maximized thrust efficiency, minimized fuel consumption, and minimized exergy destruction plus fuel exergy loss. For the three objective functions, the minimum fuel mass and minimum exergy destruction and exergy fuel loss proved to be able to design and operate a vehicle which meets the mission constraints using nearly identical amounts of fuel. The optimized thrust efficiency objective vehicle consumed significantly larger amounts of fuel and destroyed and lost more exergy while promoting a broad, thrust maximizing body.

Periannan9 studied different objective functions based on energy and exergy considerations on the optimization of three subsystems: propulsion system (PS), environmental control system (ECS) and airframe subsystem-aerodynamics (AFS). The author concluded that an exergy-based approach is not only able to

Figure 2: Typical Three -Wheels Environmental Control Unit

(ECU)

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Table 2: Conventional and Electric Architectures Data For Exergy Analysis

Conventional Electric Engine Bleed Air Exergy [kW] 40.5 - Electric Power to Compressor [kW] - 34 Engine Shaft Power Extraction [kW] 14.9 45.4 Exinlet [kW] 40.5 30.2 Tamb=T0 [°C] -30.4 -30.4 Pamb=P0 [kPa] 21 21 Pinlet [kPa] 219.9 169.6 Tinlet [°C] 305.1 213.0 TECU [°C] 8.8 8.7 Pcabin [kPa] 75.3 75.3 Tcabin [°C] 21.1 21.1 minlet [kg/s] 0.140 0.138

Table 1: Cruise Flight Operational Conditions

Parameter Value Altitude [ft] 37 000 Outside Temperature [°C] -30.4 Mach 0.78 Heat Load per ECU [BTU/h] 22 000 Cabin Temperature [°C] 21.1 Number of ECU packs 2

pinpoint where the greatest inefficiencies in the system occur but produces a superior optimum vehicle as well by accounting for irreversibility losses in subsystems only indirectly tied to fuel usage.

IV. Case Study The air management system (AMS) for commercial aircraft is mainly composed of: a) pneumatic system; b)

air conditioning; c) ice protection system; d) engine starting system; e) pressurization system. Due to size and complexity of AMS, the present case study focused in the air cycle environmental control unit (ECU), which provides conditioned and pressurized air to cabin. Typically, in commercial aircraft, the engine is the pneumatic power source for the ECU operation. The architecture of conventional air cycle ECU is presented in aerospace technical literature.51-53

In the last 25 years, several aircraft manufacturers, equipment suppliers, government organizations and universities have made efforts do optimize the ECU performance in order to reduce fuel burnt and minimize weight penalties. One of the most researched solutions is the more or all electric aircraft, in which the engine drives an electric generator that supplies electric power to all or most systems, including ECU. Due to minimization of losses and optimum control of equipment (allowed by electronics), the electric power system can deliver what is demanded by the user systems in that particular operational condition. However, the main gain in the electric architecture, with no engine air extraction, is the extension of engine life and economy because tapping off the air may increase the turbine inlet temperature and disturb the flow field in the engine core. In addition, a more electric engine can be designed for an optimum condition with no bleed air, i.e., compressor pressure and mass flow can be what is required for the best combustion, turbine performance, shaft power and maximum thrust, not requiring the usual excess of air and pressure for ECS operation and pneumatic system losses 54,55.

A) Case Study Objective The objective of this case study is to

demonstrate the advantages of exergy analysis as a tool to compare different ECS architectures, and to define adequate strategies for system optimization. Two architectures were chosen for the present study: 1) conventional, engine bleed air driven; 2) electric compressor driven. The three-wheel air cycle machine (Fig. 2) is identical in the two cases (same ducting, valves, heat exchangers, turbine, compressor and fan) but the first case uses engine bleed air as pneumatic source and the second one uses air from electric compressor.

For simplification purposes, the present case study considers only one operational point during cruise to apply the exergy analysis, when the usual procedure is to analyze conditions in all flight phases and obtain time-averaged results by considering each condition duration and flight total time. The operational parameters used in present analysis are shown in Table 1.

Another simplification is that same weight and drag were used for both architectures. Despite this assumption is rough and inadequate for design purposes, it serves to show the advantages of exergy analysis, which is the purpose of the present case study.

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Specific Entropy (kJ/kg-K)

Tem

pera

ture

(K)

Conventional Electric

2

5

4

3

67

8

9

Bleed from the engine

Compressor Inlet

Cabin Air

Figure 3: Air cycle Temperature-Specific Entropy Diagram

On the other hand, in actual design situation, the weight and drag for each alternative must be considered. Figure 1 presents an example of exergy analysis that considers the exergy demand and penalties imposed by ECU to aircraft. Basically, it is necessary to evaluate the flight mechanics coupled with engine numerical simulation tool to find the thrust, aircraft speed and associated fuel consumption.

B) Exergy Analysis Applied to ECS The relevant data used for the exergy analysis of the environmental control unit (ECU) pack driven by bleed

air and electric power is shown in Table 2. The refrigeration cycles for conventional and electric driven ECU are presented in Figs. 3 and 4 in the form of Temperature-Specific Entropy and Specific Exergy-Specific Enthalpy diagram. The lowest point in Fig. 4 is the ambient condition, the last point is the cabin discharge and the highest points refer to the ECU inlet condition for both architectures. Figure 4 shows that conventional alternative requires much more exergy Exinlet (dashed line) than electric one (solid line). This result is also confirmed in Table 3.

The exergy analysis, as described by Kotas56, is applied to ECU pack and Bleed Air in order to obtain the exergy destroyed (Exd) in each equipment. The results Exd normalized by (Exinlet or Welectric) are shown in Table 3. In appendix, Figs. 5 and 6 present the Grassmann diagrams for the two alternatives studied herein. This kind of analysis is important to inform which equipment produces the greatest Exd , i.e., it gives the indication which part of the system shall be optimized. In case of conventional ECU, there are losses in precooler, bleed pressure regulator valve, primary heat exchanger and mixer. On the other hand, the electric alternative has exergy destroyed mainly in electrical compressor, primary heat exchanger and mixer.

The ECU exergy efficiency can be defined as the ratio of exergy used to cool the cabin to the exergy rate available at the high pressure air source (bleed air, in the conventional, and power consumed by the compressor, in the electric alternative):

( )electricinletcabinQECUex WorxExE &&&,, =η (1)

In addition, it is also interesting to know how much exergy from the aircraft fuel is consumed in the ECU, since it will give a measure of the ECU impact in overall aircraft performance. If this number is small, even a significant increase in ECU exergetic efficiency (Eq. 1) will lead to a not so important increase in aircraft fuel burnt. This index, named specific exergy consumption, is given by:

( ) fuelelectricinlet xEWorxESEC &&&= (2)

Equation (2) requires the knowledge of fuel mass flowrate consumption fuelm& that is obtained from the engine performance numerical tool. In addition to the data from Tables 1 and 2, the engine simulation will need the specification of the mass flowrate extracted from engine fan, if there is a precooler in the bleed system. This is the case of conventional ECU in the present case study.

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Table 3: Ratio of Exergy Destroyed (Exd) to Exergy at Inlet (Exinlet) for each equipment of air cycle machine in present case

study conditions Exd/(Exinlet or Wcompressor) [%] Equipment Conventional Electric Electric Compressor - 12.0 Bleed (valves, precooler) 29.0 1.5 Primary Heat Exchanger 17.8 23.3 Compressor 2.1 2.6 Secondary Heat Exchanger 6.4 7.9 Condenser Cold Pass 0.9 0.8 Turbine 4.2 5.1 Mixer 12.5 16.5 Condenser Hot Pass 1.3 1.5 Total Exergy Destroyed 74.2 71.3

The normalization of fuel burnt, specific exergy consumption and specific fuel consumption – SFC – (fuel burnt per thrust) are performed as:

( ) alconventionfuelalconventionfuelelectricfuelnormfuel FFFF ,,,, −=Δ (3)

where F is the generic function to be analyzed, i.e., it can be fuelm& , SEC or SFC. Tables 4 and 5 indicate that the electric ECU architecture is more efficient and require less exergy for

operation, as this system destroys less exergy than the conventional one. Also, there is a decrease in the exergy consumed by the ECS to the condition in the cabin, comparing the exergy inlet of the bleed driven system and the compressor power in the electric system. As a result, the impact of the ECS on the aircraft fuel consumption is decreased. Furthermore, the consumption of fuel decreased as well.

It is interesting to notice that, although the reduction in the SFC index is not very significant as the reduction in the SEC index. However, the SEC index does not take into consideration the weight change imposed to the aircraft by the new electric compressor as

well as the removal of pneumatic system (ducts, precooler, valves, sensors, joints and associated structural elements). As defined previously, the design of an aircraft must minimize additional weight and losses constrained by the energy requirements of the client. Thus, the use of combined exergetic indexes and conventional one gives more information about the system than using each of them separately. In industrial applications, the same result was obtained as exergy cannot evaluate some aspects, such as pollutant toxicity57,58.

V. Conclusions A review of the possibilities to

apply exergy analysis to the decision making process in aircraft design has been shown. Exergy analysis may bring different aspects of aircraft design into a common basis. However, some of these must be further studied in order to understand how they could be addressed in the analysis. Still, the indexes based on exergy do show the location of the main sources of irreversibilities, allowing a comparison of the increase/decrease of them because of a change in the architecture of the system.

Specific Enthalpy (kJ/kg)

Spec

ific

Exer

gy (k

J/kg

)

Conventional Electric

2

5

4

36

7

8

9

Bleed from the engine

Compressor Inlet

Cabin Air

Figure 4: Air Cycle Specific Exergy-Specific Enthalpy Diagram

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Table 4: Overall Exergy Analysis Results Conventional Electric

compressorinlet WorxE && [kW] 40.46 34.00

ECUex,η [%] 1.42 1.69 SEC [%] 0.71 0.48

The case study presented allowed a validation of the conclusions above. Although, the reduction in the fuel consumption was not significant, because of the limited importance of ECS for aircraft overall fuel burnt, the results showed a better use of the exergy supplied to the ECS system in the electric architecture. Even with the simplifications assumed in this present case study (same weight and drag for architectures and one operational point instead of a mission profile), the fuel burnt rate and SFC index presented some reduction. In addition, it is fair to say that a significant reduction in exergy consumption did not lead to a great impact to engine performance parameters by itself.

In spite of adding a source of irreversibility (electric motor and air compressor), the electric architecture reduces the exergy destroyed in the pressure control system and precooler, which were located upstream the environmental control unit. Yet, there is an increase exergy destroyed ratio in the primary heat exchanger and mixer because they become the main exergy destructors in the electric ECS architecture. Therefore, both primary heat exchanger and mixer are the next candidates for an optimization in a further opportunity, i.e., in a new architecture design.

These results may indicate that the new generation of engine, which is adopted in more or all-electric aircrafts, must have additional net gain on fuel consumption by providing only shaft power and not bleeding air. Moreover, due to high exergy destruction rate inherent to current jet propulsion systems, the engine performance gains may not be sufficient to reach significant fuel consumption reduction values (20% level). Therefore, an integrated exergy analysis for all or more electric aircraft must consider the effects of the new generation of engines and its new user systems plus weight decrease due to new materials, structure optimization and breakthrough electric system architecture as well as less drag due to improved aerodynamics.

The combined use of aeronautical traditional indexes (such as fuel burnt, SFC and take off weight) and exergetic ones proved to be more interesting since it provides more information regarding the whole system. Such integrated analysis allows the comparison of different architectures, helps the engineer to find the equipment to be optimized in the plant, and provides understanding on how much the system is important for the aircraft and compared to other systems. Again, future works must study ways to address non-exergy related aspects into the exergy analysis framework.

Acknowledgments L. F. Pellegrini wishes to acknowledge Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

for the financial support received (Grant 03/12094-8). R. Gandolfi and G. A. L. da Silva would like to thank the Embraer Technological Development Leader, Eng. Luis M. C. Acosta and Environmental Systems Manager, Roberto Petrucci for the opportunity.

References 1 Muñoz, J. D., "Optimization Strategies for the Synthesis/Design of Highly Coupled, Highly Dynamic Energy Systems",

Ph.D Dissertation, Faculty of Virginia Polytechnic Institute and State University, Blacksburg, VA, 2000. 2 Moorhouse, D. J., "Proposed System-Level Multidisciplinary Analysis Technique Based on Exergy Methods", Journal

of Aircraft, Vol. 40, No. 1, Jan-Feb 2003, pp. 11-15. 3 Paulus, D. and Gaggioli, R., "Rational Objective Functions for Vehicles", AIAA Paper No. 2000-4852, 8th

AIAA/USAF/NASA/ISSMO Symposium on Multidisciplinary Analysis and Optimization, Long Beach, California, September 6-8, 2000, pp. 1-11.

4 Bejan, A. and Siems, D. L., "The Need for Exergy Analysis and Thermodynamic Optimization in Aircraft Development", Exergy, Vol. 1, No. 1, 2001, pp. 14-24.

Table 5: Fuel, SFC and SEC changes between conventional and electric

architectures

Electric to

Conventional Change Ratio

alconventionfuelfuel mm ,&&Δ [%] -0.12

alconventionSFCSFCΔ [%] -1.88

alconventionSECSECΔ [%] -32.39

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5 Muñoz, J.R. and von Spakovsky, M.R., "Decomposition in Energy System Synthesis/Design Optimization for Stationary and Aerospace Applications", Journal of Aircraft, Vol. 40, No. 1, Jan-Feb 2003, pp. 35-42.

6 Figliola, R. S., Tipton, R. and Li, H., "Exergy Approach to Decision-Based Design of Integrated Aircraft Thermal Systems", Journal of Aircraft, Vol. 40, No. 1, Jan-Feb 2003, pp. 49-55.

7 Roth, B., "The Role of Thermodynamic Work Potential in Aerospace Vehicle Design", ISABE-2003-1199, XVI International Symposium on Air Breathing Engines (ISABE), Cleveland, 31 Aug - 5 Sep, 2003.

8 Rancruel, D. F. and von Spakovsky, M. R., "Decomposition with Thermoeconomic Isolation Applied to the Optimal Synthesis/Design of an Advanced Fighter Aircraft System", International Journal of Thermodynamics, Vol. 6, No. 3 September, 2003.

9 Periannan, V., "Investigation of the Effects of Various Energy and Exergy-Based Objectives/Figures of Merit on the Optimal Design of High Performance Aircraft System", Master Dissertation, Faculty of Virginia Polytechnic Institute and State University, Blacksburg, VA, 2005.

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Appendix

Figure 6: Grassman Diagram for Electric Driven ECU Pack

Figure 5: Grassman Diagram for Conventional ECU Pack