Exergy assessment comparison of conventional and hybrid-electric aircraft propulsion systems

The purpose of this research is to develop an exergy-based method to evaluate and compare different aircraft propulsion systems architectures to assist the design engineer at the early stages of product development. The method was successfully applied to a case study comprised of a baseline regional aircraft powered by gas turbines, which was compared to a hybrid-electric propulsion (HEP) version comprised of the gas turbines hybridized with batteries. The highest exergy efficiency of 33.5% was obtained for a configuration that presented a 5% degree of hybridization (DOH), defined as “power coming from batteries divided by total power”, and 800Wh/kg battery density. This corresponds to an increase of 0.7% when compared to the 32.8% efficiency of the baseline gas turbine. On the other hand, the aircraft total weight increased 2,160 kg, or 7.1%. Also, both the exergy consumption and exergy destruction increased with hybridization. For the flight mission, a remarkable increase of 2% to 7% was obtained for these parameters, as hybridization increased from 5% to 15%. On top of that, the HEP configuration saves 23 kg of jet fuel or 1% of fuel burn along the mission in comparison with the baseline. CO\(_{2}\) emissions reduction was around 70 kg per flight mission, as expected, since emissions increase proportionately with fuel consumption. Exergy-based emission costs and exergy destroyed in the kerosene refinery plant and in the electric power generation plant were also evaluated. Finally, some possible means to re-use the exergy lost in the aircraft propulsion system were presented and discussed.

DOH:

Degree of hybridization

ECS:

Environmental control system

E/E:

Electro-electronic (equipment)

HEP:

Hybrid-electric propulsion

IATA:

International Air Transportation Association

ICAO:

International Civil Aviation Organization

GDP:

Gross domestic product

MDAO:

Multi-disciplinary analysis and optimization

MEA:

More electric aircraft

MTOW:

Maximum take-off weight

NASA:

National Aeronautics and Space Administration

ORC:

Organic Rankine cycle

SUAVE:

Stanford University Aerospace Vehicle Environment

\({b}{_{ch}}\) :

Chemical exergy of a substance, [kJ/kg]

\(\dot{B}\) :

Exergy flow rate of a mass flow rate, [kW]

\(\dot{B}{_{Dest}}\) :

Destroyed exergy rate of a component/system,[kW]

\(\dot{B}{_{Bat\_Heat}}\) :

Exergy rate associated with heat dissipation in the batteries, [kW]

\(\dot{B}{_{Bleed}}\) :

Exergy flow rate of extracted bleed air from the thermal engine, [kW]

\(B{_{Bleed}}\) :

Total exergy of extracted bleed air from the thermal engine, [kJ]

\(\dot{B}{_{ch}}\) :

Chemical exergy flow rate of a mass flow rate, [kW]

\(\dot{B}{_{Dest\_Bat}}\) :

Destroyed exergy rate in the batteries, [kW]

\(\dot{B}{_{Dest\_Engine}}\) :

Destroyed exergy rate in the thermal engine, [kW]

\(\dot{B}{_{Dest\_Gearbox}}\) :

Destroyed exergy rate in the gear box, [kW]

\(\dot{B}{_{Dest\_Inverter}}\) :

Destroyed exergy rate in the inverter, [kW]

\(B{_{Dest,Integrated,Mission}}\) :

Integrated destroyed exergy of the mission, [kJ]

\(B{_{Dest,Mission}}\) :

Total destroyed exergy of the mission, [kJ]

\(\dot{B}{_{Dest\_Motor}}\) :

Destroyed exergy rate in the electric motor, [kW]

\(\dot{B}{_{Dest\_Prop}}\) :

Destroyed exergy rate in the propeller, [kW]

\(\dot{B}{_{Eng\_Air}}\) :

Exergy flow rate of the thermal engine inlet air, [kW]

\(B{_{Eng\_Air}}\) :

Total exergy of the thermal engine inlet air for the mission, [kJ]

\(\dot{B}{_{Eng\_Gases}}\) :

Exergy flow rate of the gases leaving the thermal engine, [kW]

\(B{_{Eng\_Gases}}\) :

Total exergy of the gases leaving the thermal engine for the mission, [kJ]

\(\dot{B}{_{Fuel}}\) :

Exergy flow rate of the fuel flow, [kW]

\(B{_{Fuel}}\) :

Total exergy of the fuel consumed in the mission, [kJ]

\(\dot{B}{_{Gearbox\_Heat}}\) :

Exergy rate associated with heat dissipation in the gearbox, [kW]

\(B{_{Heat}}\) :

Total exergy associated with heat dissipation of the propulsion system for the mission, [kJ]

\(\dot{B}{_{Inputs}}\) :

Input exergy rate of a component/system, [kW]

\(B{_{Inputs}}\) :

Total exergy inputs of the propulsion system for the mission, [kJ]

\(\dot{B}{_{Inverter\_Heat}}\) :

Exergy rate associated with heat dissipation in the inverter, [kW]

\(B{_{Kerosene}}\) :

Total kerosene exergy consumed in the mission, [kJ]

\(\dot{B}{_{kin\_mass}}\) :

Kinetic exergy flow rate of a mass flow rate, [kW]

\(\dot{B}{_{Motor\_Heat}}\) :

Exergy rate associated with heat dissipation in the motor, [kW]

\(\dot{B}{_{ph}}\) :

Physical exergy flow rate of a mass flow rate, [kW]

\(\dot{B}{_{pot}}\) :

Potential exergy flow rate of a mass flow rate, [kW]

\(\dot{B}{_{Useful\_Outputs}}\) :

Exergy rate of useful outputs of a component/system, [kW]

\(B{_{Useful\_Outputs}}\) :

Total exergy of useful outputs of the system for the mission, [kJ]

\(\dot{B}{_{Prop\_Air}}\) :

Exergy flow rate of the air leaving the propeller, [kW]

\(B{_{Prop\_Air}}\) :

Total exergy of the air leaving the propeller for the mission, [kJ]

\(\dot{B}{_{Thrust}}\) :

Exergy rate associated with propeller thrust, [kW]

\(B{_{Thrust}}\) :

Total exergy associated with propeller thrust for the mission, [kJ]

\(C_{CO_{2},Electricity,generation}\) :

\(CO_{2}\) emitted to obtain one unit of exergy of electricity, [g/kJ]

\(C_{CO_{2},Kerosene,generation}\) :

\(CO_{2}\) emitted to obtain one unit of exergy of Kerosene, [g/kJ]

\(c_{T,Kerosene}\) :

Kerosene unit exergy cost \(c_{T,Ele}\) Electricity unit exergy cost

g :

Gravitational acceleration, \([m/s^2]\)

h :

Enthalpy, [kJ/kg]

\(\dot{m}\) :

Mass flow rate, [kg/s]

\(M_{CO_{2},Electricity,generation}\) :

Total \(CO_{2}\) emitted associated with the electricity generation, [g]

\(M_{CO_{2},Flight}\) :

Total \(CO_{2}\) emitted by the thermal engine during flight, [g]

\(M_{CO_{2},Kerosene,generation}\) :

Total \(CO_{2}\) emitted associated with the Kerosene generation, [g]

\(M_{Fuel}\) :

Total kerosene consumption during flight, [g]

s :

Entropy, [kJ/(kg.K)]

T :

Thrust, [N]

\(T_{0}\) :

Temperature of the reference environment, [K]

\(T_{stag}\) :

Atmospheric stagnation temperature, [K]

\(T_{static}\) :

Atmospheric static temperature, [K]

V :

Aircraft speed, [m/s]

\(\dot{W}{_{Bat\_Power}}\) :

Electric power generated by batteries, [kW]

\(\dot{W}{_{Electric}}\) :

Mechanical power extracted from the thermal engine to the aircraft electric system, [kW]

\(W{_{Electric}}\) :

Total mechanical power extracted from the thermal engine to the aircraft electric system in the mission, [kJ]

\(\dot{W}{_{Electric\_Motor}}\) :

Electric power from the electric inverter to the electric motor, [kW]

\(W{_{Electricity}}\) :

Total electricity from the electricity mix consumed in the mission, [kJ]

\(\dot{W}{_{Mec\_Engine}}\) :

Mechanical power extracted from the thermal engine to the gear box, [kW]

\(\dot{W}{_{Mec\_Gearbox}}\) :

Mechanical power from the gear box to the propeller, [kW]

\(\dot{W}{_{Mec\_Hydraulic}}\) :

Mechanical power extracted from the thermal engine to the aircraft hydraulic system, [kW]

\(W{_{Mec\_Hydraulic}}\) :

Total mechanical power extracted from the thermal engine to the hydraulic system in the mission, [kJ]

\(\dot{W}{_{Mec\_Motor}}\) :

Mechanical power from the electric motor to the gear box, [kW]

z :

Altitude above sea level, [m]

\(\eta\) :

Exergy efficiency of a component/system

\(\eta _{Mission}\) :

Exergy efficiency of the mission

\(\eta _{Integrated,Mission}\) :

Integrated exergy efficiency of the mission

\(\Delta t_{Phase}\) :

Time duration of the flight phase, [s]

The authors would like to thank Embraer S.A. for the support to make this work possible. The third author acknowledges CNPq (Brazilian National Council for Scientific and Technological Development) for the grant 313885/2020-6. The fourth author acknowledges CNPq for the grant 306484/2020-0.

Authors and Affiliations

1. R&T, Embraer S.A., Av. Brigadeiro Faria Lima, 2170, Sao Jose dos Campos, SP, 12227-901, Brazil

   Walter Affonso Jr & Ricardo Gandolfi

2. Aerospace Engineering Division, Instituto Tecnológico de Aeronáutica - ITA, Praça Marechal Eduardo Gomes, 50, Sao Jose dos Campos, SP, 12228-615, Brazil

   Roberto Gil A. da Silva

3. Mechanical Engineering Department, Escola Politécnica da Universidade de São Paulo, Av. Prof. Mello Moraes, 2231, Sao Paulo, SP, 05508-030, Brazil

   Silvio de Oliveira Jr.

Contributions

Walter Affonso Jr. contributed to conceptualization, data curation, formal analysis, investigation, methodology, project administration, resources, validation, visualization, writing—original draft, and writing—review and editing. Ricardo Gandolfi contributed to data curation, formal analysis, investigation, methodology, resources, software, validation, visualization and writing—review and editing. Roberto Gil A. da Silva contributed to conceptualization, funding acquisition, methodology, project administration, supervision, and writing—review and editing. Silvio de Oliveira Jr. contributed to conceptualization, funding acquisition, methodology, supervision, and writing—review and editing.

Corresponding author

Correspondence to Walter Affonso Jr.

competing financial interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare that the case studied in this work is not related to any actual or future Embraer product. It is a conceptual approach of a typical regional aircraft.

Technical Editor: Guilherme Ribeiro.

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