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∗
Methodology for Assessing Retrofitted Hydrogen Combustion
and Fuel Cell Aircraft Environmental Impacts
Khaled Alsamri, Jessica De la Cruz, and Melody Emmanouilidi †
University of California Irvine, 4200 Engineering Gateway, Irvine, CA, 92697
Jacqueline Huynh ‡and Jack Brouwer §
University of California Irvine, 4200 Engineering Gateway, Irvine, CA, 92697
Hydrogen (H
2
) combustion and Solid Oxide Fuel Cells (SOFC) can potentially reduce
aviation-induced greenhouse emissions compared to kerosene propulsion. This paper outlines a
methodology for evaluating performance and emissions trade-offs when retrofitting conventional
kerosene-powered aircraft with lower-emission H
2
-combustion and SOFC hybrid alternatives.
The proposed framework presents a constant-range approach for designing liquid hydrogen fuel
tanks, considering insulation, sizing, center of gravity, and power constraints. Liquid hydrogen
tanks enable comparison between H
2
-combustion and SOFC hybrid aircraft flying identical
ranges. A lifecycle assessment evaluates greenhouse emissions and contrail formation effects for
carbon footprint mitigation, while a cost analysis examines retrofit implementation consequences.
A Cessna Citation 560XLS+ case study shows a 5% mass decrease for H
2
-combustion and
0.4% for the SOFC hybrid, at the trade-off of removing three passengers. The overall lifecycle
of green hydrogen results in a 50.78% and 73.12% reduction in CO
2
emissions compared to
Jet-A fuel for H
2
-combustion and SOFC hybrid, respectively. Fuel costs decrease by 20.10%
per flight using gray H
2
-powered SOFC hybrid. The results suggest retrofitting aircraft with
alternative fuels could lower carbon emissions and fuel costs per flight, with the trade-off of
reducing passenger capacity for the same range.
∗
Presented paper number 1954.c1 at 2023 AIAA Science and Technology Forum and Exposition (AIAA SciTech Forum) on 26-Jan-2023 at
Gaylord National Harbor, National Harbor, MD
†
Graduate Student Researcher, Department of Mechanical and Aerospace Engineering, University of California Irvine/4200 Engineering Gateway
Irvine CA 92697, and AIAA Member.
‡
Assistant Professor, Department of Mechanical and Aerospace Engineering, University of California Irvine/4200 Engineering Gateway Irvine
CA 92697, and AIAA Member.
§
Director of Advanced Power and Energy Program, Department of Mechanical and Aerospace Engineering, University of California Irvine/4200
Engineering Gateway Irvine CA 92697, and AIAA Member.
1
I. Nomenclature
𝐶𝑝, 𝐴𝑖𝑟 = Specific Heat Capacity of Air
𝑑= Height of the Spherical Head
𝑑1= Width of Spherical Head
𝑑𝑜= Radius of Inner Tank
𝐸 𝐼 (𝑋)= Emission Index of Species X
𝑒𝑤= Weld Efficiency
𝐹𝑂 𝑆 = Factor of Safety
𝑔= Acceleration due to Gravity on Earth
𝐺= Mixing Line Slope
ℎ= Cruising Altitude
𝐻= Hydrogen
ℎ𝑓= Heat Energy Available per Unit Weight of Fuel
𝐾= Geometrical Constant
𝐾𝑖𝑛𝑠 = Thermal Conductivity of Insulation
𝐿= Length of the Cylindrical Part of Tank
𝜆𝑡 𝑎𝑛𝑘 = Total Length of Tank
𝐿𝑐𝑦 𝑙 = Length of Cylinder
𝐿𝐻𝑉𝑓 𝑢 𝑒𝑙 = Lower Heat Value of Fuel
𝐿𝐻2= Liquid Hydrogen Fuel
𝐿/𝐷= Lift-to-Drag Ratio
𝑀𝑏𝑜𝑖𝑙 𝑜 𝑓 𝑓 = Mass Boiloff
𝑚𝑓 𝑖𝑙 𝑙𝑒𝑑 𝑐𝑎 𝑝 𝑠𝑢𝑙𝑒 = Mass of Filled Capsule
𝑀𝐻= Mass of Hydrogen
𝑚𝑡= Mass of Tank
¤𝑚𝑎𝑖𝑟 = Mass Flow Rate of Air
¤𝑚= Mass Flow Rate
2
¤𝑚𝑓 𝑢𝑒𝑙 = Mass Flow Rate of Fuel
¤𝑚𝐻2= Mass Flow Rate of Hydrogen
¤𝑚𝐻2𝑂= Mass Flow Rate of Water
¤𝑚𝑠𝑡 𝑒𝑎𝑚 = Mass Flow Rate of Steam
𝑀 𝐴𝐶 = Mean Aerodynamic Chord
𝑁𝑈𝐷= Nusselt Number
𝑃= Pressure
𝑃𝑎= Ambient Pressure at Altitude
𝑃𝑑𝑒𝑠 = Pressure for Hydrogen Storage
𝑃𝑟 = Prandtl Number
𝑞= Heat Loss
𝑄= Heat Transfer Rate
𝑟= Radius
𝑟1= Radius of Inner Vessel
𝑟2= Radius of Outer Shell
𝑟𝑖𝑛𝑠 = Radius of Insulation
𝑅= Range
𝑅𝑒𝑑= Reynolds Number
𝑇= Temperature
𝑇𝑜= Outside Temperature
𝑇𝑖= Inside Temperature
𝑇1= Outside Temperature
𝑇2= Inside Temperature
𝑡𝑤= Wall Thickness
𝑡𝑤ℎ = Wall Hemisphere Thickness
𝑇𝑊 𝑊 = Tank-to-Wheel
𝑉𝑖= Excess Volume
3
𝑉𝑡= Tank Volume
𝑉𝑜𝑢𝑡 = Volume Out
𝑉𝑠𝑦 𝑠𝑡𝑒 𝑚 = Volume of tank system
𝑊𝑇 𝑇 = Well-to-Tank
𝑊𝑇 𝑊 = Well-to-Wheel
𝑊𝑡𝑜 = Maximum Takeoff Weight
𝑊𝑓 𝑢𝑒𝑙 = Fuel Weight
𝜖𝐻2𝑂= Molar Mass of Water over Mass of Dry Air
𝜂𝑜𝑣𝑒 𝑟 𝑎𝑙𝑙 = Overall Engine Efficiency
𝜆𝑡= Tank Sizing Constraints
𝜆𝑐𝑎𝑏𝑖𝑛 = Tank Sizing Cabin Constraints
𝜎𝑎= Tensile Strength of Material for Cryogenic Tank
𝜌= Density
𝜏𝑎𝑙𝑙𝑜𝑤 = Allowable Shear Stress
()ℎ= Property at Altitude
()𝑠 𝑡 = Property at Standard Temperature
()𝐻2= Property for Hydrogen
()∗= Per Segment
II. Introduction
As
airline traffic is predicted to grow by 4% between 2022-2040 [
1
], the environmental pressure and pollutants
near airports have become an emerging concern. According to the Aviation Sustainability report, alternative
fuel sources such as hydrogen, are predicted to reduce CO
2
emissions from 2% to 12% by 2050 [
1
]. Hydrogen
(H
2
)-combustion and fuel cell powered electric propulsion have been studied as leading alternatives for pollutant
reduction [
1
]. The broad availability and high volumetric energy density of hydrogen make this fuel a potentially viable
solution for carbon mitigation since H
2
-combustion produces mainly NO
x
and H
2
O greenhouse gas (GHG) emissions.
Such hydrogen combustion engine consists of a hydrogen powered turbofan, turboprop, or propeller that converts
chemical energy into mechanical energy by combustion. Another additional alternative consists of an electric power
train of a hydrogen powered SOFC hybrid that provides the necessary energy to run an electric propulsor such as a
4
turbofan. The benefits of the use of fuel cells in aviation vehicles include a fast filling time and increased efficiency
when paired with hydrogen fuel [
2
]. However, H
2
-combustion and SOFC hybrids require large tanks to carry hydrogen
in cryo-compressed liquid hydrogen (LH
2
) onboard the aircraft, as well as, more complex power trains. Such LH
2
requires large storage tanks and power systems that can lead to potential range and balance compromises. Therefore, a
framework methodology is needed to assess the trade-offs of implementing such alternative fuel and power sources on
modern aircraft.
The methodology presented in this paper consists of a comparison between a conventional kerosene powered
aircraft, a retrofit H
2
-combustion powered aircraft, and a retrofit Solid Oxide Fuel Cell (SOFC) powered aircraft. Such
framework consists of a retrofit by designing liquid H
2
hydrogen tanks and a SOFC power train that meets the power and
feasibility constraints for an already existing kerosene powered aircraft. These hydrogen powered technologies are not
precisely drop-in technologies, mainly due to the requirement for large tanks and changes to the power train. Thus, this
framework utilizes a lifecycle emissions assessment, as well as, mission implementation costs to compare the trade-offs
between implementing hydrogen, fuel cell-hydrogen hybrid, and conventional power sources for this methodology. A
sample study on a Cessna Citation 560XLS+ business jet is demonstrated to assess current opportunities for emissions
and contrail mitigation, as well as, performance and feasibility compromises. Business jets share 0.04% of all annual
carbon emissions [
3
] and they have the potential of being the first commercialized zero-emission aircraft, since a greatest
34% net energy consumption reduction is observed for hydrogen powered business jets [
4
]. Such higher efficiency and
potential to lower emissions motivate the study of the methodology covered in the following sections.
III. Methodology to Assess Emissions and Performance Trade-Offs for a Retrofitted Solid
Oxide Fuel Cell and Hydrogen Powered Aircraft
The methodology to model the alternative fuel emissions for a proposed aircraft vehicle is presented in Fig. 1.
The inputs to the modeling framework include the aircraft characteristics, such as empty and takeoff weights, overall
efficiency and lift-to-drag ratio. In addition, the alternative fuel type is defined by the heat energy available per unit
weight of fuel and mission characteristics such as range and cruising altitude. These parameters define the aircraft
cruising performance in the flight profile module. Within the flight profile module, the weight of the fuel necessary
to complete the mission is determined and inputted into the H
2
tank configuration module and the emissions module.
The tank configuration module models the shape, insulation, and volume of an H
2
cylindrical tank that meets the
power requirements defined by the weight of the fuel. The tank volume and mass are then outputted into the center of
gravity Module. This module determines the center of gravity (CG) change within the flight envelope of the aircraft by
simultaneously placing the tanks in the interior layout. A tank sizing constraint is fed back into the tank configuration
module if such CG requirements are not feasible for the same number of passengers. The tank configuration module
updates the tank design and the weight of the fuel is remodeled to account for the weight of passenger removal. If
5
such changes occur, either a refueling stop is required or a second flight of the same mission will keep the number of
passengers constant for the same range. Such consequence is accounted for in the lifecycle emissions and cost modeling
covered in detail in the following section.
Furthermore, the weight of the fuel, the mission atmospheric conditions, and the power plant for each alternative
fuel type are inputted into the emissions module. Within this module, the emissions per segment are analyzed by
their emission indexes, greenhouse gas emissions, and contrails. Such segment emissions are then inputted into the
environmental impact module. This module implements the mentioned lifecycle assessment and cost analysis to output
the trade-offs between alternative fuel power plants per mission. The details of this framework are further discussed in
the following sections.
Aircraft
Characteristics
Fuel Type
Mission
Atmospheric
Conditions
Flight
Profile
Module
Tank Configuration
Module:
Emissions Module:
Emission Indices
Greenhouse
Emissions
Contrails
Environmental Impacts
Module:
Lifecycle Assessment
Cost Analysis Comparison
Tradeoffs
between
alternative fuel
emissions
Center of Gravity
Module:
Center of Gravity Analysis
Interior Layout
Geometrical Model
Mechanical Model
Thermal Model
Fig. 1 Modeling framework of the methodology to assess emissions and performance trade-Offs for a retrofitted
SOFC hybrid and H2Powered Aircraft
A. Flight Profile Module
The methodology presented in the previous section consists of a baseline range mission profile to compare the
alternative fuel sources with a baseline kerosene gas turbine combustion flight procedure. A constant range approach
analysis is implemented in order to design an alternative fuel tank and power train that satisfies insulation, center of
gravity and power constraints. The Breguet range equation determines the weight of the fuel required to fly the given
mission for the baseline and alternative fuel sources.
Hydrogen combustion would require some changes to the design of the engines due to the different properties of
hydrogen such as higher adiabatic temperature and faster flame speeds. Such changes include a smaller combustion
chamber, the addition of a pump, supply pipes, control valves, and turbine systems, as seen in Fig. 2. In addition, a
heat exchanger must also be added to heat the cryogenic hydrogen liquid fuel before combustion [
5
], as seen in Fig.
6
3. Cryogenic hydrogen tanks become very heavy depending on the design parameters, stored pressure, temperature,
and acceptable boil-off rates. Fortunately for aircraft applications, less insulation is required for short periods of flight
at a relatively high boil-off rate. Design choices of a number of tanks and storage locations affect the final mass and
volume of the hydrogen storage system. The high gravimetric energy density of hydrogen of a 120 MJ/kg is favorable
since mass reduction is critical during flight. Hydrogen needs to be stored at its critical temperature and pressure of
33.15 Kelvin and 188.55 psi. However, the main challenge in aviation lies in the mass and volume that such cryogenic
tanks occupy. Hydrogen density varies between a low of 0.08375 kg/
𝑚3
in gaseous form and a high of 81 kg/
𝑚3
in
cryo-compressed liquid form [
6
]. Such densities are low when compared to the densities of kerosene variation from a
low of 775 kg/
𝑚3
to a high of 840 kg/
𝑚3
. The aforementioned hydrogen combustion system replaces the conventional
turbofan for the H2-combustion powered aircraft studied in this paper.
𝑯𝟐
Pump
To
Afterburner
3rd Turbine
2nd Turbine
1st Turbine
Combustion Chamber
Heat Exchanger
Fig. 2 Design changes to the engine due to the different properties of hydrogen
𝐻2
𝑯𝟐
Combustion Chamber
Heat Exchanger
Fig. 3 Addition of a heat exchanger for the cryogenic liquid fuel combustion
Another alternative SOFC hybrid power plant configuration is evaluated for a constant range mission. Such SOFC
hybrid includes a battery and liquid H
2
tanks to provide electrical power with zero emissions. Proton exchange membrane
fuel cells (PEMFC) and SOFC advantages include independent power and energy scaling at efficiencies up to 60%.
Unfortunately, fuel cells lose efficiency with altitude due to lower atmospheric pressure. Hence for aircraft applications,
a hybrid SOFC gas-turbine system can convert fuel cell waste heat to electric power and pressurize a fuel cell [
7
]. The
7
overall power system efficiency has been shown to provide slightly higher efficiencies in the range of 10% to 20%
approximately for a conventional aircraft. For the SOFC hybrid, the power trains of this system consist of a gas turbine,
heat exchangers, a compressor, a generator, a battery, and a LH
2
tank. The power train designed in this methodology for
the SOFC hybrid for medium range and long range can be seen in Fig. 4. Power assumptions of the fuel cell, battery,
and motor-specific densities are assumed according to state of art (SOA) technology that can be commercially available
[
8
]. The SOFC has gravimetric and volumetric power densities of 2.5 kW/kg and 7.5 kW/kg respectively, as determined
by NASA Glen Research Center [
9
]. The SOFC hybrid designed by NASA has five and seven times higher gravimetric
and volumetric power densities than the state-of-the-art commercially available designs. The fuel cell and motor specific
densities are found in more advanced research to be 4.0 kW/kg and 10.0 kW/kg [
10
]. In addition, the hybrid power train
assumptions are shown in Table 1 where superconducting motors and lithium-ion batteries are used. The fuel cell is
assumed to power the throttle cruise at 75% power of the total energy required for this mission. The remaining 25% of
power is assumed to emerge from the battery during non-cruise flight segments.
Table 1 Power train for SOFC hybrid
Parameters Values
SOFC (kW/kg)(SOFC SOA) 2.5
SOFC (kW/L) 7.5
SOFC Exit Temperature (◦𝐶) 944
Motor Density (kW/kg) 7.064
Battery V Density (kWh/L) 0.67
Battery G Density (kW/kg) 0.35
GT-SOFC Cycle Efficiency (%) 70
GT V Density (kg/𝑚3)8000
GT G Density (kWe/kg) 4.4
Motor Density (kW/kg) 7.06
Cryo-cooler Density (kg/kW) 3
Gas Turbine Power (kW) 538
8
Compressor
Turbine
Generator
DC/ACBattery
Electric
Motor
Propeller
Anode
SOFC
Cathode
Fuel
Pump Fuel Heater
Combined HX
Combustor
: electrical energy
: mass flow
: mechanical energy
Fig. 4 Power train SOFC hybrid for Medium-Range and Long-Range aircraft designed for fuel cell hybrid
The SOFC hybrid power train system consists of multiple components such as an electric motor, the SOFC, a
generator a pump, a cryogenic tank, and other components seen in Fig. 4. The cryogenic tank stores liquid hydrogen
fuel which vaporizes once vented from the tank. The hydrogen is then heated in a heat exchanger (HX) that acts as a
fuel heater. The HX recycles heat that exits the turbine, and a fuel pump pressurizes the H
2
that is inserted the anode.
Oxidation reactions occur within the anode and compressed air from the compressor is then heated in the combined
HX. Such air then inlets into the cathode where the reduction reactions occur. Compressed air flow helps maintain and
increase the fuel cell performance at flight altitude. The turbine is utilized to power the compressor and generator while
the generator produces electricity that can be stored in the battery or used for propulsion in the electric motor.
The aforementioned H
2
-combustion and SOFC hybrid system are utilized to power the constant range from the
baseline kerosene flight procedure. The Breguet range equation heat energy available per unit weight accounts for such
changes within this module and results in the fuel weight outputted into the tank module. A sample implementation of
this methodology for both H2-combustion and SOFC hybrid system is performed on a business jet in Section IV.
B. Tank Configuration Module
Given the design fuel weight from the previous module, tanks are modeled for a retrofitted aircraft in the tank
configuration module. The design of such tanks follows the approach in Fig. 5. The tank module evaluates geometrical,
material and thermal models that serve as feasible variables within the design space [
11
]. Such tank modelling is
9
governed by equations 1 to 9.
satisfies &
Mechanical Model
Tank geometry
,
Tank Design:
Tank
Geometry
Yes
No
No
Yes
: Input, Output
: Process
Geometrical Model Thermal Model
: Model
Fig. 5 Tank configuration module flowchart
1. Geometrical Model
The geometrical model defines the tank geometry and volume of storage required for power constraints. The
tank geometry is defined as cylindrical with hemispherical ends, as hemispherical heads provide the best pressure
distribution and are widely used for pressurized vessels [12]. The excess Volume Viis defined to be 7.2% to maintain
constant pressure during boil-off with equations 1, 2 and 3. The volume of the capsule is delimited by a tank wall and a
hemispherical insulation wall. The thickness of the wall is modeled through equation 4 and 4, where P
des
is the pressure
at which the hydrogen is stored,
𝜎a
is the tensile strength of the material chosen, e
w
is weld efficiency. Equation 5
models the thickness of a cylindrical tank with hemispherical ends.
𝑉𝑡=
𝑀𝐻2(1+𝑉𝑖)
𝜌𝐿𝐻2
(1)
𝑉𝑡=
4·𝜋𝑟3
3+𝑟2𝜋𝐿 (2)
𝑚𝑓 𝑖𝑙 𝑙𝑒𝑑 𝑐𝑎 𝑝 𝑠𝑢𝑙𝑒 =𝜌 𝜋𝑟2
1(𝐿1−2
3𝑟1)(3)
10
𝑡𝑤=
𝑃𝑑𝑒𝑠 ·𝑑𝑜
2·𝜏𝑎𝑙𝑙𝑜𝑤 ·𝑒𝑤+ (0.8·𝑃𝑑 𝑒𝑠)(4)
𝑡𝑤ℎ =
𝑃𝑑𝑒𝑠 ·𝑑𝑜·𝐾
2·𝜏𝑎𝑙𝑙𝑜𝑤 ·𝑒𝑤+ (2·𝑃𝑑𝑒𝑠 · (𝐾−0.1)) (5)
𝐾=
1
6(2+𝑑
𝑑1
)(6)
2. Mechanical Model
The geometry outputted along with material choices defines the thickness of the tank walls in the mechanical model.
The choice of material for the tank walls is Aluminum (4.4 % Cu) 2014-T6 and evacuated aluminum foil separated with
fluffy glass mats for insulation, as suggested by [
13
]. The factor of safety (FOS) for the chosen material is set to 1.3
which is within a reasonable engineering margin. The weight of the cryogenic tanks is usually within 15% to 30% of
the LH
2
weight and can reach less than 15% with low enough hydrogen vaporization rates [
12
]. The inner vessel is
placed within a vacuum with the defined geometrical thickness and passed into the thermal module to set the insulation
thickness.
3. Thermal Model
The thermal model designs the wall insulation thickness by defining the material, acceptable boil-off rate, and
consequently acceptable rate of heat transfer modeled by equations 7 to 9. In addition, the insulation layer is defined by
an acceptable boil-off rate of 0.1% per hour as suggested by [
12
]. This design’s high boil-off rate minimizes insulation
and reduces cost and mass. This methodology sets for 20% of the stored hydrogen to be vented per hour with a 288.15
Kelvin outer surface temperature. Such temperature maximizes range and flight time. The inner vessel is placed within
a vacuum with the defined geometrical thickness and insulation to define the tank sizing constraint
𝜆tanks
, as seen in Fig.
6. This constraint is then outputted into the center of gravity (CG) module.
𝑁𝑈𝐷 =
0.60 +0.287 𝑅𝑒
1
6
𝑑
h1+ ( 0.559
𝑃𝑟 )9
16 i8
27
2
(7)
11
𝑄=¤𝑚×ℎ 𝑓 𝑔 (8)
𝑄=𝑄𝑐𝑦 𝑙𝑖𝑛𝑑 𝑒𝑟 +𝑄𝑠 𝑝ℎ𝑒𝑟 𝑒 =
2𝜋 𝐿𝑘 (𝑇𝑜−𝑇𝑖)
𝑙𝑛 (𝑟1·𝑟2)+4𝜋𝑟1·𝑟2𝐾(𝑇1−𝑇2)
𝑟2−𝑟1
(9)
𝐿𝐻!
Outer Shell
Inner Vessel
Fuel
Out
Refuel
MLI + Vacuum
𝑟
!
𝑟
"
𝑟
#$%
𝐿&'(
𝜆)*+,
𝑡-.((
Fig. 6 H2Cryogenic Tank geometry definition
C. Center of Gravity Module
1. Center of Gravity
A weight and balance analysis evaluates the feasibility of the tank design outputted from the tank configuration
module. The change in CG location from the operational limits of the retrofitted conventional kerosene powered aircraft
is modeled from an already exciting FAA-approved operational envelope found in [14].
The net change in CG is modeled to determine if the new retrofitted CG is within the minimum and maximum limits
of the aforementioned envelope. Assuming the CG lies at 25 percent Mean Aerodynamic Chord (MAC) in the existing
weight and balance diagram, the change in CG is determined with the shifted weight and potential moment arm, as
suggested by the FAA weight and balance handbook [
15
]. Such moment arm is simultaneously obtained in the interior
layout of the aircraft within this module. The weight per passenger is estimated to be 205 lbs for domestic flights, as
suggested by Shevell [
16
]. The weight of a fully stocked refreshment center is assumed to be 324 lbs, with two full carts,
while the weight of the lavatory is estimated to be 132 lbs. The change in weight from each alternative retrofitted fuel
configuration is obtained by summing all changes in moments from either removing a seat or adding a tank, among
others.
12
2. Interior Layout
Simultaneously within the center of gravity module, a potential change in the moment arm is obtained from an
interior layout map of the existing aircraft. A sample case interior layout for a business jet is used in Section IV, to
obtain the dimensions of the interior, the baggage compartment, and the overall aircraft specifications for a Cessna
Citation 560XLS+ [
17
]. Such dimensions are used to evaluate and constrain the size of the tanks by placing them in a
position that results in a feasible CG within the aforementioned envelope limits and FAA aisle width and seat pitch
regulations. After a feasible tank sizing constraint is reached in the tank configuration module, the final weight of the
fuel is inputted into the emissions module. Such weight of the fuel will account for passenger weight removal in case
passenger seats need to be removed to make room for tanks.
D. Emissions Module
The analysis for the emissions of kerosene combustion compared to the retrofitted H
2
-combustion and SOFC Hybrid
system powered aircraft is modeled by inputting such power plants for the propulsive systems, and the atmospheric
conditions into the emissions module. Within the emissions module, the aircraft’s greenhouse emissions are determined
per segment for a single flight and are dependent upon the engine type and engine thrust load. The emissions analyzed
for combustion of kerosene assume complete combustion emits CO
2
and H
2
O while incomplete combustion emits CO,
NO
x
, SO
x
and HC. The emissions analyzed for H
2
-combustion assume H
2
O is emitted for complete combustion while
incomplete combustion mainly results in NO
x
with no CO, HC or SO
x
emissions. However, hydrogen leaked or vented
into the atmosphere still can be an emission concern [
18
]. The SOFC hybrid is also responsible for H
2
O and NO
x
emissions respectively due to the use of H
2
fuel. The details of this analysis are presented in the following subsections.
1. Emission Indices
The International Civil Aviation Organization (ICAO) Engine Emissions Databank (EED) is utilized to obtain
the Emission Indices (EI) for the non-cruise portions of flight for kerosene powered aircraft. The cruise incomplete
combustion EI of HC, and CO are taken at an average of 0.4 g/kg and 0.6 g/kg respectively, as presented by Schumann
[
7
]. This analysis neglects SO
x
emissions since the ICAO databank does not include data for the SO
x
EIs . This paper
focuses on the main emissions that are in common with the three technologies that are investigated. Typically NOxEI
production ranges between 12 to 16 g/kg as supported by Schumann [
7
]. The EI of NO
x
is dependent upon the engine
design flame temperature. However, such design details are not taken into account in this analysis since a mid-value
between a maximum and minimum of 14 g/kg is taken for the cruise portion of the flight. For the purpose of comparison,
the EI for pure hydrogen combustion is utilized since pure kerosene combustion is also assumed. More recent research
has been able to reduce the EI
NOx
to 4.3 g/kg, as well as, determine a 90% reduction of NO
x
emissions when compared
to kerosene combustion [
19
],[
20
]. Such technologies include water injection to reduce temperatures and thus reduce
13
thermal NO
x
. Nevertheless, these technologies are harder to implement on flight compared to ground turbines since
the stability of combustion is prioritized in the air. Moreover, the EI
NOx
is assumed to be also at 14 g/kg in the SOFC
hybrid for the consistency of all technologies analyzed. The NO
x
emissions in the SOFC hybrid are not coming from
the fuel cell but as a result of a hydrogen combustor/micro gas turbine system that operates at high temperatures. For the
kerosene combustion emissions, the composition of the fuel can also affect H
2
O and CO
2
emissions since a higher H/C
ratio produces more water and lower CO
2
. The EI of CO
2
is modeled by accounting for the percentage of carbon in the
fuel, the molar mass of CO
2
, and the molar mass of carbon resulting in 3.15 kg/kg respectively. The EI of H
2
O is also
obtained which is 1.25 kg/kg following the same process.
2. Emissions
The CO
2
and H
2
O emissions of kerosene are compared to the retrofitted H
2
-combustion and SOFC Hybrid system
powered aircraft. Such an emissions model assumes a constant percent thrust per segment and a constant aircraft
Thrust-specific fuel consumption (TSFC). Each segment emission is modeled by dividing the flight profile into the
segments seen in Table 2. The thrust per engine is taken at 100% for takeoff, 85% for climb, 30% for approach, and 7%
for descent and idle, as suggested by the ICAO standard landing and takeoff cycle regulations [
21
]. The time to climb
and descent is assumed to be 30 minutes, as referenced from Textron Aviation [
22
]. Although Taxi/Idle time varies
by airport, an average value of 23 minutes is assumed for this analysis. For the cruise portion of the flight, equation
10 models the mass fuel burned to obtain the complete emissions of CO
2
, H
2
O, CO, HC, and NO
x
. A sample of
implementing this methodology for modeling emissions is demonstrated in detail on Section IV.A.
𝐸𝑥=𝑚·𝐸 𝐼 (𝑋)(10)
Table 2 Assumed flight profile segments [23][24]
Segment Duration (min) Thrust (%)
Takeoff 0.7 100
Climb 30 85
Descent 30 7
Approach 4 30
Taxi/Idle 23 7
14
3. Contrails
The likelihood of contrail formation using kerosene, H
2
-combustion fuel, and a SOFC hybrid-powered aircraft
is modeled using mass and energy balances to determine the mixing line slope G. An aircraft exhaust plume mixes
isobarically with exhaust air and can lead to the possibility of contrail formation [
21
]. Contrails may form by the mixing
of hot and humid air with cold ambient air below a critical temperature threshold, as defined by the "Schmidt-Appleman"
criterion [25], which is modeled by equation 11.
𝐺=
𝑃𝑎 ·𝐸 𝐼 (𝐻2𝑂) · 𝐶 𝑝𝑎𝑖 𝑟
𝜖𝐻2𝑂·𝐿𝐻𝑉𝑓 𝑢𝑒 𝑙 · (1−𝜂𝑜𝑣𝑒𝑟 𝑎𝑙𝑙 )(11)
Such contrails are evaluated since they can increase the overall warming effect due to trapped heat in the atmosphere
and affect cooling from reflected sunlight [
26
]. The overall efficiency of the aircraft is assumed constant for all three
configurations. The H
2
-combustion and SOFC hybrid are expected to have a shallower slope than kerosene due to a
higher LHV
fuel
value of 120 MJ/kg. Such value is higher when compared to the conventional lower 43 MJ/kg kerosene
LHV
fuel
, as seen in Section IV.A. However, an increase in the mixing slope G arises from the higher EI of H
2
O when
using liquid hydrogen fuel. The persistence of contrails is not explored due to the location dependence of atmospheric
conditions at every point of the duration of a single flight.
E. Environmental Impacts Module
1. Lifecycle Assessment
A complete lifecycle analysis (LCA) of CO
2
evaluates the environmental effects of a conventional kerosene powered
aircraft, a retrofit H
2
-combustion aircraft, and a retrofit SOFC hybrid powered aircraft. The lifecycle emissions are
modeled for the various stages of fuel extraction, transport, processing, and storage sectors known as Well-to-Tank
(WTT), and a combustion sector known as Tank-to-Wheel (TTW), as seen in Fig. 7. Such LCA evaluates the
consequences of eliminating the dependency of aviation upon dwindling crude oil resources, as well as, the overall
contribution of aviation to the anthropogenic greenhouse effect [27].
The carbon intensity of kerosene fuel can vary depending on the region, the refinery and the crude oil well. Various
studies have estimated that the carbon intensity of jet fuel ranges from 85 to 95 grams of CO
2
per MJ [
28
]. The
combustion of fuel contributes to a portion of 73 grams of CO
2
e/MJ, while the rest is generated by transportation,
processing, and the refinement process [
28
]. The Well-to-Wheel (WTW) CO
2
emissions for kerosene fuel are modeled
at 84.5 gCO
2
e/MJ with an 87% in combustion emissions, as supported by Wang [
29
]. Finally, the complete lifecycle of
kerosene WTW is found by adding WWT to TTW CO
2
emissions of kerosene and LH
2
fuel sources from the extraction
of crude oil or fuel to its combustion during flight.
15
The WTW for both H
2
-combustion and the SOFC hybrid is estimated using green and gray hydrogen. Green
hydrogen refers to the hydrogen produced via renewable energy, while gray hydrogen refers to the hydrogen produced
using steam methane reformation without any gene house gas (GHG) emissions capture. More than 95% of hydrogen
produced today is produced using fossil fuels like natural gas and coal [
30
]. Meanwhile, green hydrogen requires a
renewable energy-powered grid which is not yet available in many parts of the world. However, most countries have
plans to reach 100% renewable grids within the next 30-50 years [
30
]. The LCA estimation utilizes the Greenhouse
Gases, Regulated Emissions, and Energy Use in Technologies (GREET) model to estimate the transportation lifecycle
emissions via a mathematical framework that accounts for various pollutants such as CO
2
[
31
]. In addition, green
hydrogen solar electrolysis is assumed to emit 41.29 grams of CO
2
e/MJ for the full lifecycle, as referenced by Al-Breiki
[
31
]. Similarly, the gray hydrogen solar electrolysis full lifecycle is assumed to emit 75.6 g CO
2
e/MJ, as sourced by [
32
].
The mentioned LCA model does not include the production or life expectancy of lithium-ion batteries and the
SOFC. The model is thus focused on the fuel WTW lifecycle. Although, the environmental effects of producing those
components are mainly from mining, not enough current data and research are available on the lifecycle analysis of the
SOFC hybrid system. TTW CO
2
emissions for all alternative fuel sources are modeled from the weight of the fuel
inputted from the flight profile module as discussed in Section III.D.
Recovery
and
Extraction
Raw Material
Transport
Jet Fuel
Processing
Aircraft Tank
Transport and Storage
Aircraft Operation
Combustion
𝐶𝑂2
𝐻2𝑂, 𝑁𝑂𝑥
Well-to-Tank (WTT) Tank-to-Wheel (TTW)
Well-to-Wheel (WTW)
Production 𝐻2
(e.g. Gray, Green)
Transport in
Gas Form
Pressurize 𝐻2gas
into liquid
Aircraft Tank
Transport and Storage
Aircraft Operation
Combustion
𝐶𝑂2
𝐻2𝑂, 𝐶𝑂2, 𝑁𝑂𝑥,𝐶𝑂,HC
Fig. 7 Lifecycle Assessment (LCA) boundary of Jet-A Fuel (Top) and LH2fuel (Bottom)
2. Cost Analysis
The change in fuel cost of implementing alternative fuel sources for one constant range flight profile is determined
to further analyze the trade-offs of implementing a retrofit. The fuel burned per segment from the emissions module is
utilized to model the fuel price per flight for this mission, in addition to the change in capital cost of the alternative fuel
16
source.
The cost for kerosene is determined from the full-service average kerosene Jet-A fuel price per gallon for the U.S.
western pacific region for the current year. The price at the pump is assumed to already contain the production and
transportation costs of kerosene. The cost of utilizing LH
2
for the proposed flight is modeled per segment in order to
compare the change in fuel cost from a conventional kerosene powered flight.
The H
2
-combustion change in fuel costs are estimated for both green and gray hydrogen. The cost of production for
green hydrogen (electrolysis) was set to 5.5 USD/kg while the production for gray carbon capture hydrogen was taken at
1.55 USD/kg, as suggested by [
33
]. The cost liquefaction of both was set to 2.75 USD/kg as suggested by [
34
], while
the cost for transportation was set to 5 USD/kg, as referenced by [
35
]. In addition, the capital cost of implementing the
cryogenic tanks designed by this methodology is determined. Such costs are estimated from cryogenic tank market
prices and are taken at 34 dollars per lb of maximum LH2fuel weight, as suggested by Yang [36].
The SOFC hybrid cost is modeled per segment for the purpose of comparison with LH
2
prices are determined as
stated above. In addition, the stack cost at a high production volume of SOFC is determined to be 238 USD per kilowatt
of energy, as suggested by Scataglini [
37
]. A 500 kW microturbine is assumed to be in a mid-range market price of 900
dollars per kW following the California Distributed Energy Resources Guide on Microturbines as resourced by Capehart
[
38
]. The lithium-ion battery cost is estimated to be 135 USD per kilowatt hour for the current year as determined by
Statista [39].
IV. Methodology Demonstration for Alternative Fuel Retrofit on a Business Jet
The developed methodology in the previous section evaluates the potential to lower emissions for a single flight
by utilizing a retrofit analysis. When compared to an existing aircraft, business jets show a greater 34% net energy
consumption reduction in emission values when utilizing H
2
fuel, a suggested by Bhupendra [
4
]. Therefore, a business
jet is chosen for this study since they have the greatest energy consumption reduction and a greater potential to lower all
emissions including water vapor emissions. As global demand for private jet activity has risen by 7% in 2021[
40
], the
implementation of the aforementioned methodology on the Cessna Citation 560 XLS+ business jet presents a potential
opportunity for carbon mitigation. A summary of key mission and performance specifications for the mentioned aircraft
are found in Table 3:
17
Table 3 Cessna Citation 560 XLS+ performance specifications [22]
Parameter Value
Cruise Range 2,100 nmi
Maximum Number of Passengers 9
Maximum Speed Limit 0.75 Mach
Maximum Operating Altitude 45,000 ft
TSFC 0.44 lbf/lbhr
The methodology presented in Section III is utilized to model the performance and emissions of the standard kerosene
powered Cessna Citation 560XLS+ in order to compare the trade-offs resulting from a retrofitted H
2
-combustion fuel
and SOFC hybrid powered aircraft. In the flight profile module, these two alternative fuel power sources are examined
for the same mission profile as the kerosene baseline procedure. The weight of the fuel required for this mission is
determined for all three power plants as a function of heat energy available per unit weight of fuel, range, and other
Breguet range equation parameters as seen in Section III.A. Such weights are utilized to design the tanks as stated in
Section III.B and evaluated for feasibility in the center of gravity module, as shown in Section III.C. A few passengers
might be dropped if tank sizing volume constraints are required to power the same mission or a refueling stop might
be added. A new fuel weight that accounts for such changes is then outputted into the emissions module. The flight
emissions are then used to assess the lifecycle assessment and costs of implementing each retrofit. An overall analysis of
the trade-offs in performance and emissions by a retrofit methodology is outputted.
A. Analysis of Results
The conventional kerosene, the H
2
-combustion, and the SOFC hybrid powered retrofit aircraft are all able to power
the cruise mission specifications from Table 3. The fuel weights obtained from the flight profile module in Section III.A
are seen in Table 4:
Table 4 Fuel weights for cruise
Cruise Weights Jet-A (lbs) H2-combustion (lbs) SOFC (lbs)
𝑊𝑠𝑡 𝑎𝑟 𝑡 20,331 19,140 20,250
𝑊𝑒𝑛𝑑 17,955.48 18,254.41 19,652.06
𝑊𝑓 𝑢𝑒𝑙 2,375.52 885.59 597.94
18
The power requirements and constraints of the H
2
-combustion fuel and SOFC hybrid powered aircraft follow the
energy assumptions described in Section III.A, and are seen in Table 5. The power rating of the electric propulsion
system is defined based on the maximum takeoff velocity of the aircraft and the thrust of the conventional aircraft. The
Battery size is defined as providing maximum thrust for 15 minutes. Such parameters and the fuel weight are used as
design constraints in the tank configuration module.
Table 5 Power and SOFC energy requirements
Parameters Values
Thrust Per Engine (lbf) 4,119
Maximum T/O Velocity (knots) 124
Engine Max Power (kW) 2,344.96
Energy Required by H2-combustion (MJ) 32,546.51
Energy (kJ)(kWh) 9,040.70
Fuel Cell Power (75%)(kW) 1,758.72
Battery Power (25%)(kW) 586.24
Battery Size (kWh) 146.56
Cryocooler Maximum Power (kg/kW) 23.45
The hydrogen cryogenic tanks are designed with insulation and altitude pressure as added design constraints. The
resulting tank materials, properties, and characteristics are seen in Table 6. The design of insulation maximizes flight
temperature as specified in the Thermal module in III.B.3.
The tanks specified above are then evaluated for feasibility in the center of gravity module. The three interior layout
arrangements that satisfied the maximum and minimum CG envelope limits are seen in Fig. 8. The FAA minimum
12 in aisle width regulation (for airplanes less than 10 passengers [
41
],[
42
]) is exceeded for passenger comfort and
evacuation regulations in all three configurations. The conventional arrangement of the Cessna 560 XLS+ is seen in Fig.
8a with a forward refreshment center and an aft lavatory. The LH
2
tank design and layout results in six small tanks each
of 61.8 inches distributed in the forward section of the cabin, and four aft tanks, two of small size, one of mid-size and
one of large-size all with 106.7 inches in length o as seen in Fig. 8b and Fig. 9. Such tank designs are subject to sizing
and feasibility constraints and are seen in Table 6. However, passenger tables between seats 3 and 5 and 4 and 6 must be
removed in order to fit the 6 small forward tanks. Nonetheless, the FAA minimum required first-class seat pitch of 38
inches is exceed all seats after such removal [
43
]. The H
2
-combustion and SOFC layout required the removal of seats
19
1,2 and 9, as well as, the removal of the aft closet in order to fit the aft mid-size LH
2
tank 10 inches into section a-a of
the cabin aft section, as seen in Figs. 8b and 8c. However, the SOFC required the shift of the forward lavatory and
refreshment center since the aft section of the cabin is used to house the SOFC power train seen in pink in Fig. 8c.
Table 6 Cryogenic LH2tanks
Parameters Front Tanks Aft Tanks
𝑆𝑖𝑧𝑒 𝑆𝑚𝑎𝑙𝑙 (6)𝐿𝑎𝑟𝑔𝑒(1)𝑀𝑒𝑑𝑖𝑢𝑚(1)𝑆𝑚𝑎𝑙 𝑙(2)
𝑟𝑡𝑜 𝑡 (in) 10.08 23.7 9.09 7.90
𝐿𝑡𝑜 𝑡 (in) 61.81 106.69 106.69 106.69
𝑉𝑡𝑜𝑡 (gal) 76.08 103.53 103.77 79.14
𝑡𝑤(in) 0.035 0.0830 0.0319 0.0277
Insulation Thickness (in) 0.0031 0.00314 0.00315 0.00315
𝑊𝑡/𝑊𝑓(%) 23.8 23.5 24.1 24.1
20
(a) Jet-A
a
a
61.8’’ 106.7’’
(b) H2-combustion
a
a
106.7’’
Lavatory
Kitchen
𝐿𝐻2Tanks
SOFC Power Train
(c) SOFC hybrid
Fig. 8 Interior layouts for retrofit analysis
𝐿𝐻2Tanks
(a) (b)
Fig. 9 Cross section of fuselage: (a) forward six small tanks (b) aft four tanks: one large size, one medium, and
two small tanks
21
The final design for both the retrofitted H
2
-combustion and the SOFC hybrid both resulted in six passengers. For the
H
2
-combustion these changes result in a 5% decrease in overall aircraft weight when compared to the conventional
aircraft. For the SOFC such changes result in a 0.4% decrease in mass when compared to the conventional aircraft. This
change of mass is observed due to the more energy-dense hydrogen, the choice of SOA materials, and the loss of three
passengers, their seats, and luggage. The highest weights in the H
2
-combustion aircraft are the empty weight and the
weight of the passenger and bags, while the main weights in the SOFC are the empty weight and the fuel cell mass, as
seen in Fig. 10.
An H
2
-combustion and SOFC hybrid powered aircraft can be designed where the same number of seats and cabin
area is maintained. However, this would require a refueling stop and result in higher energy requirements and higher
emissions for both the H2-combustion and the SOFC hybrid aircraft.
1% 2%
4%
11%
6%
59%
8%
82%
<1%
10%
8%
2%
2%
5%
Fig. 10 Resulting fractional weights from implementing a retrofit on a H
2
-combustion (Left) and a SOFC
hybrid (Right) powered Cessna Citation 560XLS+
As expected, H
2
-combustion and the SOFC hybrid produce zero CO
2
emissions, as seen in Fig. 11 Such figure also
shows that kerosene fuel CO
2
and H
2
O emissions are the highest during the cruise segments, with the second highest
during the climb. Such a result is expected since emissions from these segments are dependent on how much time is
spent while fuel is being burned. In comparison, higher emissions of CO and HC occur during idle and descent than
CO
2
and H
2
O emissions, due to incomplete combustion. H
2
-combustion and the SOFC hybrid both results in higher
water vapor emissions and could therefore have a likelihood of contrail formation. When compared to a conventional
22
aircraft, the G-factor increases due to high vapor emissions and the possibility of the low static temperature of the
exhaust. In addition, fuel cells can produce condensation phenomena at the earth’s surface if the weather is cold and
close to frost. However, these are short-living phenomena, which will disappear after a few seconds (outside of fog) and
thus the term "contrail" should not be used for such a transient phenomenon.
0% Takeoff and
Approach 0% Takeoff, Climb and
Approach
8%
32%
60% 55% 23%
21%
2%
78%
11%
12%
60%
36%
4% 8%
32%
60%
61%
61%
32%
35%
4% 7%
5%
2%
77%
21% 19%
76%
0% Takeoff and
Approach 0% Takeoff, Climb and
Approach
8%
32%
60% 55% 23%
21%
2%
78%
11%
12%
60%
36%
4% 8%
32%
60%
61%
61%
32%
35%
4% 7%
5%
2%
77%
21% 19%
76%
0% Takeoff and
Approach 0% Takeoff, Climb and
Approach
8%
32%
60% 55% 23%
21%
2%
78%
11%
12%
60%
36%
4% 8%
32%
60%
61%
61%
32%
35%
4% 7%
5%
2%
77%
21% 19%
76%
0% Takeoff and
Approach 0% Takeoff, Climb and
Approach
8%
32%
60% 55% 23%
21%
2%
78%
11%
12%
60%
36%
4% 8%
32%
60%
61%
61%
32%
35%
4% 7%
5%
2%
77%
21% 19%
76%
0% Takeoff and
Approach 0% Takeoff, Climb and
Approach
8%
32%
60% 55% 23%
21%
2%
78%
11%
12%
60%
36%
4% 8%
32%
60%
61%
61%
32%
35%
4% 7%
5%
2%
77%
21% 19%
76%
0% Takeoff and
Approach 0% Takeoff, Climb and
Approach
8%
32%
60% 55% 23%
21%
2%
78%
11%
12%
60%
36%
4% 8%
32%
60%
61%
61%
32%
35%
4% 7%
5%
2%
77%
21% 19%
76%
0%Takeoff and
Approach
0%Takeoff, Climb
and Approach
Fig. 11 CO
2
, CO, HC, NO
x
and H
2
O emissions per segment of conventional kerosene (Top), H
2
-combustion
(Middle), and retrofit SOFC-powered aircraft (Bottom)
As seen in Table 7, the water vapor (H
2
O) emissions per flight are 9,511 lbs, 13,118 lbs, 7,047 lbs for the conventional
kerosene powered aircraft, H
2
-combustion and SOFC hybrid powered aircraft, respectively. The contrailing of the water
vapor emissions depends on the environment, combustion temperature, altitude, and mixing line "G" shown in equation
11. The NO
x
emissions per flight are 21.68 lbs, 20.25 lbs, and 10.9 lbs for the conventional, H
2
-combustion and SOFC
hybrid aircraft respectively. Thus, hydrogen combustion has the highest water vapor emissions as it produces about 2.6
times more water vapor than conventional kerosene fuel per unit of energy. Moreover, NO
x
emissions are highest for
23
kerosene combustion due to more fuel being burned for a single flight.
Table 7 NOxand H2O total emissions per flight
Emissions Jet-A (lbs) H2(lbs) SOFC Hybrid (lbs)
NOx21.69 20.25 10.91
H2O 9,511.21 13,118.51 7,047.80
The full lifecycle of CO
2
results are categorized in two cases, as seen in Table 8. Case (i) stands for one flight
with nine passengers for the conventional kerosene powered aircraft, one flight with six passengers for the retrofit
H
2
-combustion-powered aircraft, and one flight with six passengers for the retrofit SOFC hybrid powered aircraft.
Whereas, case (ii) models taking an additional flight for the full lifecycle of the retrofit H2-combustion and the retrofit
SOFC hybrid aircraft. Such a model is obtained by keeping the same original amount of passengers (9) for the same
range and adding an additional flight for both alternative fuel configurations. The results seen in Table 8 also show the
full lifecycle as a function of the hydrogen sourcing production technique to compare emissions from both sourcing
gray and green. As seen in Table 8, 86.8% of CO
2
emissions for the kerosene powered aircraft happen during the
combustion process in the TTW path of the fuel seen in Fig. 7 in Section III.E.1. Such results are consistent with the
values supported by Wang [
29
], as stated in Section III.E.1. However, if tank sizing constraints did not require a second
flight for the H
2
and the SOFC powered aircraft, the results would have been closer to the values obtained for all case (i)
instances. In case (ii), the green retrofit SOFC hybrid powered aircraft has the highest WTW by 46.23% followed by the
gray retrofit SOFC hybrid with only 1.55%, when compared to the CO
2
emissions of the kerosene powered aircraft.
A not-so-satisfactory percent difference in CO
2
emissions between the conventional kerosene powered aircraft in the
WTT path comes from case (ii) for both gray 45.43% and green 0.08% fuel sources of the retrofit H
2
, in favor of the
lower emissions found for the kerosene powered aircraft. Such results arise from the carbon emissions during extraction,
sourcing, transportation, and storage, as seen in the WTT path in Fig. 7 in Section III.E.1. However, case (i) shows
a significant percent loss in WTW CO
2
emissions for three out of four configurations of the retrofit H
2
-combustion
aircraft (green), and the retrofit SOFC-powered aircraft (gray and green) when compared to the conventional kerosene
powered aircraft WTW CO
2
emissions. These percentages are: 49.96% for the green retrofit H
2
-combustion-powered
aircraft, 50.78% and 73.12% for the gray and green retrofit SOFC hybrid powered aircraft respectively. Lastly, the least
significant percent loss in carbon emissions comes from the gray retrofit H
2
-combustion-powered aircraft with an 8.38%
reduction compared to the conventional kerosene powered aircraft WTW CO2emissions.
24
Table 8 CO2emissions for full lifecycle analysis of all configurations
Path Case Jet-A (lbs) Gray H2(lbs) Green H2(lbs) Gray SOFC (lbs) Green SOFC (lbs)
Well-to-Tank lbs CO2(i) 1,905.92 13,223.45 7,222.17 7,104.18 3,880.05
(ii) 26,446.91 14,444.35 14,208.36 7,760.10
Tank-to-Wheel lbs CO2(i) 12,526.59 0 0 0 0
(ii) 0 0 0 0
Well-to-Wheel lbs CO2(i) 14,432.51 13,223.45 7,222.17 7,104.18 3,880.05
(ii) 26,446.91 14,444.35 14,208.36 7,760.10
Table 9 Total fuel cost per segment
Segments Case Jet-A ($) Gray H2($) Green H2($) SOFC H2Gray ($) SOFC H2Green ($)
Takeoff (i) 44.11 64.06 89.54 20.81 29.1
Climb (i) 1,339.06 1,944.63 2,718.30 631.63 882.92
Cruise (i) 2,472.67 3,735.78 5,222.06 2,522.35 3,525.87
Descent (i) 110.28 159.96 223.60 52.02 72.71
Approach (i) 75.62 109.81 153.50 35.67 49.86
Taxi/Idle (i) 101.45 147.31 205.92 47.86 66.89
Total Fuel Cost (i) 4,143.18 6,161.55 8,612.92 3,310.33 4,627.34
Total Fuel Cost (ii) 12,323.10 17,225.84 6,620.66 9,254.68
Two cases were evaluated following the same approach as the lifecycle emissions for cases (i) and (ii). From the
economic point of view, a significant fuel cost change per flight results from replacing kerosene with alternative fuel
sources, with a greater fuel cost reduction for the SOFC gray H
2
in case(i), as seen in Table 9. The cost of gray
H
2
-combustion is 28.46% cheaper than green H
2
due to the increased cost of green H
2
production for cases (i) and (ii).
The SOFC gray H
2
shows a 20.10% reduction of fuel cost per flight while the H
2
gray shows a 32.76% increase when
compared to kerosene for case (i). Although both SOFC green and SOFC gray H
2
hybrid cost per flight is cheaper when
compared to the cost of H
2
-combustion for green and gray in case (i), a greater overall implementation cost is to be
considered. The change in capital cost for purchasing the SOFC hybrid includes a total of 919,497.27 USD for the
cryogenic tanks plus the SOFC power train, while the change in capital costs for the H
2
-combustion aircraft is 49,661.50
USD from the cryogenic tanks. Furthermore, the SOFC gray H2shows a 37.42% increase in fuel cost per flight while
25
the H
2
gray shows a 66.38% increase when compared to kerosene for case (ii). Such costs change significantly for case
(ii) since two flights are required to carry the same number of passengers and therefore the change in fuel cost per flight
increases.
V. Conclusion
The proposed methodology models the performance, emissions, lifecycle and costs of a retrofitted H
2
-combustion
and a retrofitted SOFC hybrid powered aircraft. Such methodology consists of a constant range and airframe analysis to
design liquid hydrogen fuel tanks that satisfy insulation, sizing, center of gravity, and power constraints. The interior
layout analysis results in a 5% and 0.4% decrease in takeoff weight for the H
2
-combustion and SOFC hybrid aircraft
respectively. However, the resulting mass change is achieved at the cost of removing a few passengers and their luggage
to account for cryogenic tank sizing and weight constraints for the same range. Therefore, neither H
2
-combustion nor
the SOFC hybrid aircraft are able to carry the same number of passengers for the same range as the kerosene powered
aircraft. Although kerosene powered aircraft can transport a greater amount of passengers per trip, carbon emissions
are higher since conventional kerosene combustion has the highest WTW CO
2
lbs emissions of 14,432 lbs per flight.
However, a great advantage for potential carbon mitigation arises from utilizing hydrogen alternative fuels since kerosene
combustion also produces other GHG emissions besides NO
x
, CO
2
and H
2
O that all systems share. The NO
x
emissions
are highest in conventional aircraft and lower in the hydrogen combustion and SOFC hybrid aircraft consecutively.
Gray and green hydrogen combustion result in 8.38% and 49.96% lower WTW CO
2
lbs emissions in comparison to
kerosene. Likewise, gray and green hydrogen powered SOFC hybrid have 50.78% and 73.12% reduction in WTW CO
2
lbs emissions respectively. Therefore, the SOFC hybrid aircraft powered by green hydrogen is the best option from a
CO
2
emissions perspective. However, other greenhouse emissions must be evaluated when comparing the SOFC hybrid
to the H
2
-combustion. H
2
O TTW emissions are highest for the H
2
-combustion aircraft and therefore have a likelihood
for contrail formation. Water vapor may not have a permanent climate effect like CO
2
emissions, but radiative forcing
caused by contrailing has an effect of the same order of magnitude as CO
2
emissions from kerosene combustion [
44
].
Hence, avoiding night-time flights and flying at lower altitudes are potential solutions to addressing those concerns.
Another concern that might arise is the change in fuel cost per flight for replacing kerosene power aircraft. Both the H
2
gray and the SOFC H
2
gray hybrid show a 28.46% cost reduction than H
2
green and SOFC H
2
green due to the increase
in the cost of green H
2
production. The cheapest change in the price of fuel per flight is obtained from the SOFC H
2
gray hybrid, however, a more expensive one-time capital cost of 919,497.27 USD comes from purchasing the SOFC
power train. The SOFC hybrid powered aircraft has a greater potential to lower carbon emissions although it was found
to have a slightly higher change in the cost of fuel per flight than kerosene. Such is a potential trade-off that aids carbon
mitigation in the near future for the cost of dropping a few passengers for the same range. However, other variables such
as aircraft size, and engine types namely turbofan, turbojet, and turboprop, will have to be considered for market-wide
26
implementation of alternative propulsive and aircraft power systems. The methodology presented in this paper can be
applied to any aircraft category and engine type but such variables will also need to be assessed for retrofitting other
aircraft.
27
References
[1] Wyman, O., “Aviation Sustainability: Our Future,” , 2022.
[2]
Gong, A., and Verstraete, D., “Fuel cell propulsion in small fixed-wing unmanned aerial vehicles: Current status and
research needs,” International Journal of Hydrogen Energy, Vol. 42, No. 33, 2017, pp. 21311–21333. https://doi.org/https:
//doi.org/10.1016/j.ijhydene.2017.06.148, URL https://www.sciencedirect.com/science/article/pii/S036031991732503X.
[3]
VistaJet, “Aviation’s Impact on Tomorrow,” VistaJet: Sustainability in Aviation, 2022. URL https://www.vistajet.com/
globalassets/documents/vistajet_sustainability_report_2022.pdf.
[4]
Nojoumi, H., Dincer, I., and Naterer, G., “Greenhouse gas emissions assessment of hydrogen and kerosene-fueled aircraft
propulsion,” International Journal of Hydrogen Energy, Vol. 34, No. 3, 2009, pp. 1363–1369. https://doi.org/https://doi.org/10.
1016/j.ijhydene.2008.11.017, URL https://www.sciencedirect.com/science/article/pii/S0360319908015048.
[5]
Liu, Y., Sun, X., Sethi, V., Nalianda, D., Li, Y., and Wang, L., “Review of modern low emissions combustion technologies for
aero gas turbine engines,” Progress in Aerospace Sciences, Vol. 94, 2017. https://doi.org/10.1016/j.paerosci.2017.08.001.
[6]
Ahluwalia, R., Hua, T., Peng, J.-K., Lasher, S., McKenney, K., Sinha, J., and Gardiner, M., “Technical assessment of
cryo-compressed hydrogen storage tank systems for automotive applications,” International Journal of Hydrogen Energy,
Vol. 35, 2011, pp. 4171–4184. https://doi.org/10.1016/j.ijhydene.2010.02.074.
[7]
douglas, i., “Aircraft emissions - nav node DLR Portal,” , 2002. URL https://www.dlr.de/pa/en/Portaldata/33/Resources/
dokumente/mitarbeiter/Schumann_Encyclopedia_Aircraft_Emissions_c415-_o.pdf.
[8]
Tornabene, R., Wang, X.-y., Steffen, J, C., and Freeh, J., “Development of Parametric Mass and Volume Models for an
Aerospace SOFC/Gas Turbine Hybrid System,” 2005. https://doi.org/10.1115/GT2005-68334.
[9] NASA, “High power density solid oxide fuel cell,” Online, 2016. Https://technology.nasa.gov/patent/LEW-TOPS-120.
[10]
Baroutaji, A., Wilberforce Awotwe, T., Ramadan, M., and Olabi, A. G., “Comprehensive investigation on hydrogen and fuel
cell technology in the aviation and aerospace sectors,” Renewable and Sustainable Energy Reviews, Vol. 106, 2019, pp. 31–40.
https://doi.org/10.1016/j.rser.2019.02.022.
[11]
Winnefeld, C., Kadyk, T., Bensmann, B., Krewer, U., and Hanke-Rauschenbach, R., “Modelling and Designing Cryogenic
Hydrogen Tanks for Future Aircraft Applications,” Energies, Vol. 11, 2018, p. 105. https://doi.org/10.3390/en11010105.
[12]
Verstraete, D., of Engineering, C. U. S., and Ramsden, K., Potential of Liquid Hydrogen for Long Range Aircraft Propulsion,
Theses 2009, Cranfield University, School of Engineering, Gas Turbine Engineering, 2009. URL https://books.google.ae/
books?id=7MQUyAEACAAJ.
[13]
Colozza, A. J., and Kohout, L., “Hydrogen Storage for Aircraft Applications Overview,” Contractor report (cr), Analex Corp.
Brook Park, OH United States, November 2002. NASA/CR-2002-211867.
28
[14]
“Model 560XL Operating Limitations,” https://www.smartcockpit.com/docs/Cessna_Citation_Excel-Limitations.pdf, 2015.
FAA Approved Section II.
[15]
U.S. Department of Transportation, “Federal Aviation Administration Aircraft Weight and Balance Handbook,” Online, 2016.
URL https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/faa-h-8083-1.pdf.
[16] Shevell, R. S., Fundamental Principles of Flight, Prentice Hall, 1989.
[17] Inc., T. A., “Citation XLS Gen2,” Textron Aviation Inc. Official Website, 2016.
[18]
Ocko, I. B., and Hamburg, S. P., “Climate consequences of hydrogen emissions,” Atmospheric Chemistry and Physics, Vol. 22,
No. 14, 2022, pp. 9349–9368.
[19]
Khan, M. A. H., Brierley, J., Tait, K. N., Bullock, S., Shallcross, D. E., and Lowenberg, M. H., “The Emissions of Water
Vapour and NOx from Modelled Hydrogen-Fuelled Aircraft and the Impact of NOx Reduction on Climate Compared
with Kerosene-Fuelled Aircraft,” Atmosphere, Vol. 13, No. 10, 2022. https://doi.org/10.3390/atmos13101660, URL https:
//www.mdpi.com/2073-4433/13/10/1660.
[20] Seeckt, K., and Scholz, D., “Jet versus Prop, Hydrogen versus Kerosene for a Regional Freighter Aircraft,” 2009.
[21]
Gierens, K., “Theory of Contrail Formation for Fuel Cells,” Aerospace, Vol. 8, No. 6, 2021. https://doi.org/10.3390/
aerospace8060164, URL https://www.mdpi.com/2226-4310/8/6/164.
[22] “Cessna - Textron Aviation, CITATION XLS GEN2 Brochure,” Online Brochure, 2022. Accessed: 06-01-2022.
[23]
Stettler, M. E. J., Koudis, G. S., Hu, S. J., Majumdar, A., and Ochieng, W. Y., “The impact of single engine taxiing on
aircraft fuel consumption and pollutant emissions,” The Aeronautical Journal, Vol. 122, No. 1258, 2018, p. 1967–1984.
https://doi.org/10.1017/aer.2018.117.
[24]
Chati, Y., and Balakrishnan, H., “Analysis of Aircraft Fuel Burn and Emissions in the Landing and Take Off Cycle using
Operational Data,” 2014.
[25]
Whelan, G., Cawkwell, F., Mannstein, H., and Minnis, P., “The use of Meteorological Data to Improve Contrail Detection in
Thermal Imagery over Ireland,” 2009.
[26]
Spangenberg, D., Minnis, P., Bedka, S., Palikonda, R., Duda, D., and Rose, F., “Contrail radiative forcing over the
Northern Hemisphere from 2006 Aqua MODIS data,” Geophysical Research Letters, Vol. 40, 2013, pp. 595–600.
https://doi.org/10.1002/grl.50168.
[27]
Koroneos, C., Dompros, A., Roumbas, G., and Moussiopoulos, N., “Advantages of the use of hydrogen fuel as compared to
kerosene,” Resources, Conservation and Recycling, Vol. 44, No. 2, 2005, pp. 99–113. https://doi.org/https://doi.org/10.1016/j.
resconrec.2004.09.004, URL https://www.sciencedirect.com/science/article/pii/S0921344904001508.
29
[28]
Nikita Pavlenko, S. S., “Assessing the sustainability implications of alternative aviation fuels,” 2021 INTERNATIONAL
COUNCIL ON CLEAN TRANSPORTATION, 2021.
[29]
Wang, M., Elgowainy, A., Lu, Z., Bafana, A., Benavides, P., Burnham, A., Cai, H., Dai, Q., Gracida, U., Hawkins, T., et al.,
“Greenhouse gases, regulated emissions, and energy use in technologies model®(2020. Net),” Computer software]. https://doi.
org/10.11578/GREET-Net-2020/dc, Vol. 20200913, 2020.
[30]
Kakoulaki, G., Kougias, I., Taylor, N., Dolci, F., Moya, J., and Jäger-Waldau, A., “Green hydrogen in Europe – A
regional assessment: Substituting existing production with electrolysis powered by renewables,” Energy Conversion and
Management, Vol. 228, 2021, p. 113649. https://doi.org/https://doi.org/10.1016/j.enconman.2020.113649, URL https:
//www.sciencedirect.com/science/article/pii/S0196890420311766.
[31]
Al-Breiki, M., and Bicer, Y., “Comparative life cycle assessment of sustainable energy carriers including production,
storage, overseas transport and utilization,” Journal of Cleaner Production, Vol. 279, 2021, p. 123481. https://doi.org/https:
//doi.org/10.1016/j.jclepro.2020.123481, URL https://www.sciencedirect.com/science/article/pii/S0959652620335265.
[32]
Howarth, R. W., and Jacobson, M. Z., “How green is blue hydrogen,” Energy Science & Engineering, Vol. 9, No. 10, ????, pp.
1676–1687. https://doi.org/https://doi.org/10.1002/ese3.956.
[33]
“Types of hydrogen production,” Johnson Matthey Journal, 2019. URL https://matthey.com/products-and-markets/energy/
hydrogen/types-of-hydrogen-production.
[34]
Connelly, E., Penev, M., Elgowainy, A., and Hunter, C., “Current Status of Hydrogen Liquefaction Costs,” DOE Hydrogen and
Fuel Cells Program Record, 2019, pp. 1–10.
[35]
Hoelzen, J., Silberhorn, D., Zill, T., Bensmann, B., and Hanke-Rauschenbach, R., “Hydrogen-powered aviation and its reliance
on green hydrogen infrastructure – Review and research gaps,” International Journal of Hydrogen Energy, Vol. 47, No. 5, 2022,
pp. 3108–3130. https://doi.org/https://doi.org/10.1016/j.ijhydene.2021.10.239, URL https://www.sciencedirect.com/science/
article/pii/S0360319921043184, hydrogen Energy and Fuel Cells.
[36]
Yang, B., Mane, M., and Crossley, W., “An Approach to Evaluate Fleet Level CO2 Impact of Introducing Liquid-Hydrogen
Aircraft to a World-Wide Network,” 2022. https://doi.org/10.2514/6.2022- 3313.
[37]
Scataglini, R., and et al, “A Total Cost of Ownership Model for Solid Oxide Fuel Cells in Combined Heat and Power and Power
Only Applications,” , 2015.
[38] Capehart, B. L., “Microturbines,” , Dec 2016. URL https://www.wbdg.org/resources/microturbines.
[39]
Statista, “Lithium Ion Battery Pack Costs,” , Jul 2022. URL https://www.statista.com/statistics/883118/global-lithium- ion-
battery-pack-costs/.
[40]
WingX, “2021 Marks Busiest Year on Record for Global Business Jet Activity,” , Jan. 6, 2022 [Online]. URL https://wingx-
advance.com/2021-busiest-year-on-record-for-global-business-jet-activity/.
30
[41]
“Code of Federal Regulations Title 14 § 25.815 Width of aisle,” The Federal Register journal, 1976. URL https://www.govinfo.
gov/content/pkg/CFR-2022-title14-vol1/pdf/CFR-2022-title14-vol1-sec25-815.pdf.
[42] Schaufele, R. D., Elements of Aircraft Preliminary Design, Aries Publications, 2000.
[43] Liebeck, R. H., “Airplane Design Lecture notes,” , March 1995.
[44]
Sanz-Morère, I., Eastham, S. D., Speth, R. L., and Barrett, S. R. H., “Reducing Uncertainty in Contrail Radiative Forcing
Resulting from Uncertainty in Ice Crystal Properties,” Environmental Science & Technology Letters, Vol. 7, No. 6, 2020, pp.
371–375. https://doi.org/10.1021/acs.estlett.0c00150, pMID: 32596410.
31
