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Challenges and Benefits offered by Liquid Hydrogen Fuels in Commercial Aviation

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This paper aims to highlight the opportunities and challenges associated with the adoption of hydrogen fuels in aviation. An overview of the environmental and economic benefits and technological challenges is performed, including considerations in aircraft and airport design, operations and safety. A simplified model is subsequently introduced to quantify the benefits associated with the adoption of liquid hydrogen fuel in aviation. The model is used to evaluate the benefits of liquid hydrogen in aircraft of conventional configurations and encompasses the changes in volume, weights and environmental impacts. This paper concludes that hydrogen in cryogenic liquid form demonstrates great potential to become a highly sustainable commercial aviation fuel and to improve the safety of commercial air travel. However, with the implementation of this technology come many difficulties, which seemingly stretch beyond the current aviation capabilities. These include the identification of a sustainable production, storage and delivery systems that shall not dilute the nominal environmental benefits, and public and industry support to ensure financial feasibility.
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Corresponding Author - roberto.sabatini@rmit.edu.au.
Challenges and Benefits offered by Liquid Hydrogen Fuels in Commercial
Aviation
Stephen Rondinelli1, Roberto Sabatini1,, Alessandro Gardi1
1School of Aerospace, Mechanical and Manufacturing Engineering,
RMIT University, Melbourne, VIC 3000, Australia
Abstract
This paper aims to highlight the opportunities and challenges associated with the adoption of
hydrogen fuels in aviation. An overview of the environmental and economic benefits and technological
challenges is performed, including considerations in aircraft and airport design, operations and safety.
A simplified model is subsequently introduced to quantify the benefits associated with the adoption of
liquid hydrogen fuel in aviation. The model is used to evaluate the benefits of liquid hydrogen in
aircraft of conventional configurations and encompasses the changes in volume, weights and
environmental impacts. This paper concludes that hydrogen in cryogenic liquid form demonstrates
great potential to become a highly sustainable commercial aviation fuel and to improve the safety of
commercial air travel. However, with the implementation of this technology come many difficulties,
which seemingly stretch beyond the current aviation capabilities. These include the identification of a
sustainable production, storage and delivery systems that shall not dilute the nominal environmental
benefits, and public and industry support to ensure financial feasibility.
Keywords: hydrogen, sustainable aviation, cryoplane, environmental gains.
1. Introduction
The highly dynamic context of the air transport sector is driving the aviation industry to attain ever
rising economic, environmental and social standards. A major challenge is to establish and develop
the future of aviation beyond 2050. This will involve the adoption of innovative air vehicle designs and
systematic changes to the manufacture and operation of aircraft, including the type of fuel used,
engine performance, weight metrics, air traffic management (ATM) strategies and advances in safety.
The average annual growth rate of passenger and cargo traffic over the next two decades is
estimated at 4.1%, and this is a major driving factor promoting change in aviation. The rapid increase
of the overall market is mainly due to the estimated 3.2% annual increase in worldwide Gross
Domestic Product (GDP) over the next 20 years (Current Market Outlook 2014-2033). Furthermore
after the year 2042, it is expected that coal will be the only fossil fuel available (Singh & Singh, 2012),
highlighting the importance of timely change and progress towards sustainable development. One
highly anticipated and promising alternative relies on the use of hydrogen (H2) as the main fuel source
behind commercial aircraft engine propulsion due to its negligible environmental impacts. Combustion
processes utilising H2 only produce water (H2O) and reduced amounts of nitrogen oxides (NOX) as its
by-products (Contreras et al., 1997). However great challenges for aircraft and airport design and
operation as well as safety considerations are associated with the introduction of hydrogen fuels.
1.1. Historical overview
Hydrogen (H2) was featured during the 18th century in the voyage of notable gas balloons such as the
Charlière Hydrogen Balloon in 1783 (Brewer, 1991). In the 19th century Ferdinand von Zeppelin
utilised hydrogen for buoyancy of his rigid frame airships in conjunction with gasoline propulsion
systems. The 20th century saw a substantial exploitation of H2 propellants in space propulsion
systems (Kocer, 1994). Russia experimented H2 fuel for aviation in a customised TU-155 aircraft,
running one engine on H2 (Contreras et al., 1997). Russians subsequently united with Germans in
1991 in a joint program to develop a 200 passenger aircraft with a predicted range of 500 nautical
miles (Leonorovitz, 1990; Pohl & Malychev, 1997). Both the Airbus 310 and the TU-204 airframes
were evaluated as a reference platform. This cooperation led to a design placing the H2 tanks on top
of the aircraft fuselage and wings. Meanwhile, NASA was also developing its own cryoplane design,
which adopted twin spherical tanks. This configuration limited the surface-to-volume ratio and allowed
for 400 passengers travelling at Mach 0.85 for 5500 nautical miles (Contreras et al., 1997). The 21st
century saw 35 aviation industry partners come together under the guidance of Airbus Deutschland to
undertake a project known as 'Cryoplane’. The project was funded by the European Commission and
was aimed at initiating progress towards H2-fuelled aircraft. Over 25 months of study were undertaken
S. Rondinelli, R. Sabatini, A. Gardi
Challenges and Benefits Offered by Liquid Hydrogen Fuels
in Commercial Aviation
(Contreras et al., 1997), fostering political and industrial support for introducing H2 in aviation. The
study assessed the fundamental metrics associated with the introduction of H2. Safety standards of H2
were also evaluated and contrasted to that of jet fuels, highlighting need for special attention when
handling H2. However the overall safety standards, which could be achieved with H2, were well on par
with that of conventional jet fuels (Liquid Hydrogen Fuelled Aircraft - System Analysis, 2003).
2. Production of hydrogen
The energy usage and the pollutant emissions associated with the production must all be considered
when evaluating a potential source of H2, whilst the start-up, maintenance and operational costs must
be considered as they will inevitably impact the customers and utilisers of H2 fuel technology
(Khandelwal et al., 2013). Many economically and environmentally sustainable H2 production
strategies have been proposed. The most probable and realistic source of H2 involves fossil fuels,
such as gas and coal, and renewable sources, such as water, biomass, wind, solar or hydropower.
Various technologies and processes are proposed for H2 production. These include photolytic,
biological, electrolytic, thermo-chemical and chemical processes (Hydrogen production and storage -
R&D priorities and gaps, 2006).
3. Aircraft design
Hydrogen fuels propose challenges for designers in terms of mass and volume requirements, as well
as for fuel management and storage on-board aircraft. The high volume-to-energy characteristics of
Liquid Hydrogen (LH2) require hydrogen aircraft to carry a larger volume of fuel to that of conventional
fuel aircraft. The design of a successful hydrogen aircraft is mainly centred on identifying the optimal
tank configuration, in order to carry the required amounts of LH2. Amongst industry, several design
proposals have been identified. Tank configurations can be distinguished as either non-integral or
integral. Non-integral tank configurations are external to the fuselage of the aircraft. They are usually
mounted either on the airframe, above or under the wing. Non-integral tanks must be able to cope
with the aerodynamic and inertial loads, in addition to the fuel containment loads (Khandelwal et al.,
2013).
(a) (b) (c)
Figure 1 Proposed integral tank configurations for a regional aircraft (a) and for a long-range wide-
body aircraft (b), including the proposed catwalk (c) (Liquid Hydrogen Fuelled Aircraft - System
Analysis, 2003; Verstraete et al., 2010).
Integral tanks, as depicted in Fig. 1, are located inside the fuselage, hence their shape and
dimensions are interdependent with the fuselage design. Integral tanks are not required to withstand
aerodynamic loads, and on the other hand may enhance the structural integrity of the fuselage by
increasing the resistance to bending and shear forces. Integral tanks represent a more realistic and
feasible aircraft design for wide body or long haul aircraft (Khandelwal et al., 2013). The Cryoplane
project leaned towards an integral design for LH2-fuelled aircraft, mainly due to cryogenic
temperatures required for LH2 containment (Verstraete, 2013) and the need to provide the required
tank capacity for long-haul flights (Khandelwal et al., 2013). The length and width of the fuselage will
both increase to accommodate the integral LH2 tanks (Verstraete, 2013). The elimination of wing
tanks detracts the associated shear stress and bending moment alleviation. In order to compensate,
an approximate increase of 37% in the wing structure is required, leading to an overall weight
increase of 6% (Verstraete, 2013) to support and affix the integral LH2 tanks, but this will enhance
S. Rondinelli, R. Sabatini, A. Gardi
Challenges and Benefits Offered by Liquid Hydrogen Fuels
in Commercial Aviation
safety as the tanks are further protected by the supportive and rigid structure of the fuselage (Brewer,
1991). The increased drag and the boil-off issues also come into consideration as they impact range
and operating costs. For this reason a significant part of the Cryoplane project involved evaluating the
various possible tank configurations. Tanks over the fuselage and across the wing were also
considered. Though these configurations improved the overall volume exploitation and attainable tank
capacities, the LH2 containment loads significantly impacted the structural weights. Thus a spherical
or cylindrical design was preferred (Allideris & Janin, 2002). The spherical tank design minimises
surface-to-volume ratio and hence the passive heat transfer across the tank wall, minimizing the boil
off rate. For these reasons, spherical or quasi-spherical tanks have been frequently adopted in space
launchers and vehicle. However, it involves a larger frontal area for the same volume in comparison to
a cylindrical tank design (Mital et al., 2006). A cylindrical tank also provides greater volumetric
efficiencies through maximising space usage within the fuselage (Brewer, 1991). However, fuel
pressure loads are extremely inhomogeneous in a purely cylindrical tank. The ideal compromise is
therefore a cylindrical tank with its bases shaped into a semi-spherical design, as such design adopts
the best characteristics of both cylindrical and spherical shapes (Khandelwal et al., 2013). For a LH2
regional airliner, several designs are possible. One layout incorporates a single tank at the rear of the
fuselage, which offers the greatest benefits in terms of weight metrics. However, this design might
frequently lead to weight and balance issues, which may in turn require increases to the tail planes
weight and dimensions. In the second considered layout, LH2 tanks may be positioned in both the aft
and front of the fuselage. However this poses problems in terms of crew access to and from the
cockpit, which may be rectified through implementation of a passageway within this design. Lastly, the
LH2 tanks may be configured along the top of the fuselage above passengers in conjunction with a
tank in the aft of the fuselage, impacting upon luggage storage (Verstraete et al., 2010). Currently,
aviation is undergoing a shift towards larger long-range aircraft in order to relieve congestion and
improve efficiencies (Current Market Outlook 2014-2033). Long-range aircraft typically have a wide-
body design, utilising multiple aisles in the passenger cabin. Possible designs for future long-range
hydrogen aircraft may therefore further exploit the fuselage cross section increase, including a tri-
story aircraft with LH2 tanks located in the aft and front of the fuselage. For this design, the fuel tank at
the front of the aircraft with contains approximately 40% of the total fuel in order to satisfy weight and
balance requirements (Brewer, 1991; Verstraete et al., 2010). Furthermore, in comparison to a
modern equivalent in a conventional fuel aircraft such as the Airbus 380 and Boeing 747, the size
metrics vary considerably. For instance in the case of fuselage diameter, a tri-story LH2 aircraft is
likely to be up to 8.5 m wide in comparison to the smaller 7.14 m and 6.1 m airframe width as seen on
the A380 and B747-8 respectively (Verstraete et al., 2010).
3.1. Tank structure and materials
A major aspect for the attainment of safety standard by LH2 tank designs is the insulation, which
upholds the safety standards of kerosene based conventional aircraft. Major development in new
materials is aimed at alleviating the boil-off of hydrogen. This occurs when cryogenic conditions are
compromised due to the inward heat transfer (Khandelwal et al., 2013). An effective insulation in LH2
tanks will reduce the boil-off rate of LH2 increasing operational efficiencies and improving safety.
Three types of insulation have been highlighted (Khandelwal et al., 2013) which include:
- Multilayer insulation (MLI): this layout consists of up to 100 layers of insulating material such as
polyester of glass fiber alternated with metal layers for fuel containment and radiative shielding,
arranged perpendicularly to the heat transfer direction. The outside of the inner layer consists of a
reflective foil to minimize radiative transfer. The effectiveness of this type of insulation is
dependant on factors including the composition and pressure of the fuel gas phase. MLI does not
operate effectively when experiencing pressures of more than 0.001 mbar (Allideris & Janin,
2002). It is also quite susceptible to manufacturing faults during production and is a heavy form of
insulation (Khandelwal et al., 2013).
- Vacuum Insulation: this layout involves a pumping system to maintain the vacuum within the tank
walls. Such a system must ensure that air does not interact with the vacuum walls as its freezing
would cause a seizure of the tanks vital systems (Colozza, 2002). The vacuum insulation is also
vulnerable to the external ambient pressure, as the walls may not withstand pressure spikes and
fail. Therefore further strength must be introduced through stiffeners, which increase the mass of
the tank (Millis et al., 2009). The vacuum insulation offers a promising alternative to the multilayer
design, having the highest potential in terms of minimising lost mass during boil off. However it
involves heavier structures and costs (Wilkins, 2002).
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Challenges and Benefits Offered by Liquid Hydrogen Fuels
in Commercial Aviation
- Foam Insulation: this layout involves insulating foam introduced between the inner and the outer
tank walls. The outer wall can consist of a thin metal sheet, which protects the foam and assists
the structure in maintaining structural integrity. The insulating foam contains good characteristics in
terms of low thermal conductivity whilst maintaining a low density (Cumalioglu, 2005). The
feasibility of foam insulation is dependent on certain factors. This insulation provides acceptable
boil off rates, tank weight and size characteristics. Foam insulation also represents a much
cheaper option to that of multi-layer or vacuum insulation. In comparison to a vacuum system, the
rate of failure of a foam system is also much smaller (Khandelwal et al., 2013).
3.2. Blended Wing Body
Given the predicted timeframes for the widespread adoption of LH2 fuels and innovative propulsion
systems in aviation, NASA and other governmental, academic and industrial R&D entities have also
extended the study to encompass more innovative and futuristic aircraft configurations. A particularly
attractive configuration is the Blended Wing Body (BWB), offering higher aerodynamic and payload
efficiencies, greater airframe volumes, higher propulsive configuration flexibility and reduced noise
footprints. BWB will notably enhance the technological feasibility of hydrogen propulsion systems,
thanks to improved volumetric efficiencies and operational capabilities in terms of passenger and
freight movements, offering considerable environmental benefits in comparison to conventional
aircraft (Guynn et al., 2004). NASA’s model for a clean commercial aircraft of the future is based upon
the ‘Quiet Green Transport’ concept. Therefore, their project for an environmentally friendly blended
wing bodied aircraft involves a carbon-free fuel system that eliminates hydrocarbon and
carbon/sulphur oxides (COX/SOX) emissions. This is achieved through the electrochemical release of
hydrogen instead of gas turbine combustion. The hydrogen used is contained in insulated integral
tanks located inside the airframe. Concerning NOX emissions, they are notably associated with high
temperatures and pressures experienced in combustion chambers of conventional engines. With
hydrogen fuel cells, they are entirely eliminated, in addition to a significant portion of noise emissions.
Electric motors are powered by fuel cells, which turn a ducted fan generating thrust. The fuel-cell
based electric propulsion typically involves a relatively higher number of smaller engines to generate
the desired amount of thrust, leading to higher frequency noise with smaller amplitudes (Guynn et al.,
2004). The BWB airframe features top mounted ducted fans, improving noise shielding as well as
aerodynamic efficiency. The airframe also shows advancements in terms of noise mitigation through
the management of gaps and edges amongst the airframes flaps. This is achieved through the
continuous mold-line technology, which is incorporated into the flap system (Guynn et al., 2004).
Research into the BWB design by NASA also incorporated considerations for operational
improvements. An important contribution for the reduction of noise footprint is by increasing the final
approach slope angle by 9° (from 3° to 12°). This increases the altitude at which arrival traffic overflies
the ground on approach. In order to reduce degradation to the natural environment through contrail
formation, NASA has further suggested a reduction in cruise altitude for its BWB aircraft.
Conventional cruise altitudes in the upper troposphere provide ideal conditions for contrail formation
from the H2O exhausts (Guynn et al., 2004).
3.3. Systems impacts
In order to accommodate LH2 the present propulsion technologies will need to be partially redesigned.
This will particularly affect sub-systems including the fuel lines and combustion chamber. LH2-fuelled
Auxiliary Power Unit (APU) will also be proposed. This would eliminate CO2 emissions on the ground
when external power sources cannot be gained. Air Traffic Management (ATM) and operational
procedures will also have to evolve to allow the attainment of fuel, time, environmental and monetary
benefits. This will imply a redesign of procedures, en-route and terminal airspace (IPCC - Aviation and
the Global Atmosphere). During the aircraft start up, ambient air contamination within the fuel lines
poses the risk of flash back, which may be prevented through flushing with an inert gas such as
nitrogen. The flushing the lines should also occur upon shut down for analogous reasons (Dahl &
Suttrop, 1998; Khandelwal et al., 2013). A pre-heating of LH2 prior to entering the combustion
chamber is desirable, and can be performed in a heat exchanger that could capture the heat from
warm parts of the engine (i.e. turbine, exhaust and combustion chamber), improving the thermal
efficiencies and longevity of the engine. An electrical heater may be used to heat the fuel when the
engines are still cold. Furthermore, a tailored metering system will also be required to provide LH2 to
the engine in line with the throttle set by the flight crew (Dahl & Suttrop, 1998; Khandelwal et al.,
2013). Combustion of hydrogen in aircraft engines raises complications beyond that of simple fuel to
air mixing (Juste, 2006). The use of LH2 in commercial aircraft requires redesigning the conventional
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Challenges and Benefits Offered by Liquid Hydrogen Fuels
in Commercial Aviation
combustors in order to attain optimal efficiency (Dahl & Suttrop, 1998). Use of hydrogen in the
conventional kerosene combustors would lead to excessive NOX emissions due to unnecessary
increases in temperature during the combustion process (Dahl & Suttrop). Studies to reduce such
effects on board LH2 aircraft have been undertaken, with emphasis on improving combustion
efficiency, noise and flame stability. Current efforts of industry have highlighted potential combustors
as being the Lean Direct Injection (LDI) and Micro-Mix concepts. These two concepts are similar in
methodology and both have been proven as viable. Both aim to reduce the presence of large flames
in order to minimise NOX emissions, whilst reducing flashback. This is achieved through altering and
increasing the mix intensity since NOX is dependent on residence time and temperature (Khandelwal
et al., 2013).
- Lean Direct Injection (LDI):
Marek et al. (2005) conducted several experiments aimed at evaluating NOX emissions and
combustion performance. The LDI system used featured quick mixing and multiple injection
points. In order to combat flashback, velocities were high and induced mixing times were
reduced. Results from these experiments demonstrated the capabilities of hydrogen to attain the
same NOX levels of modern advanced kerosene LDI combustors (Marek et al., 2005).
- Micro-mix combustion:
If managed correctly, the micro mix or miniaturised diffusive combustion process of hydrogen can
produce less NOX emission than that of conventional kerosene combustion (Heywood & Mikus,
1973). In this layout the number of local mixing zones between the fuel and air is increased in
comparison to conventional kerosene burner designs, improving the mixing intensity whilst
reducing its scale. Therefore the micro-mix combustion process involves thousands of miniature
diffusion flames reducing the likelihood of flashback (Dahl & Suttrop, 1998). Dahl and Suttrop
(1998) examined the effects of micro mix combustion on a modified KHD T215 gas turbine
engine on an Airbus 320. Their study highlighted the ability of hydrogen to be metered safely
whilst maintaining engine control during conditions similarly to that of kerosene fueled aircraft
engine. Their configuration also demonstrated hydrogen’s ability under a micro mix system to
produce less nitrogen oxides than kerosene combustion, all whilst adhering to a diffusive burning
process that demonstrates a reduced risk of flashback and engine failure. Safe start up and
engine ignition procedures were also demonstrated, with reduced risk of excess and dangerous
pressure and heat transfer. Furthermore the technology evaluated also proposes potential in
terms of its adoption in APU (Dahl & Suttrop, 1998).
4. Hydrogen aircraft operations
LH2-fuelled aircraft poses exciting prospects for the aviation industry by not only eliminating CO2
emissions from operations but also by the potential improvements on the operational costs for airlines
(Contreras et al., 1997). However in order to accurately evaluate the potential and effects of LH2
aircraft, an in-depth analysis of all factors is required, going beyond the scope of current knowledge.
This includes but is not limited to the traditional aircraft performance metrics such as payload and
range capabilities. All direct and indirect operating costs associated with such aircraft have to be
considered, including logistics and maintenance implications. The past five years have demonstrated
a rising trend in aviation fuel prices (Current Market Outlook 2014-2033). Recent spikes in the jet fuel
prices have made it become the greatest direct operating cost for most aircraft operators. Trends
have witnessed the fluctuation of fuel prices for airlines entail over 30% of operating expenses (Fact
sheet: fuel, 2014). In order to deal with such trends, aircraft operators and particularly the airlines are
left with limited options. The most sustainable solution to combat fuel expenses is alleviation from
fossil based conventional fuels. Historic knowledge shows that unless action is mediated in terms of
subsidising, an alternative fuel will one day reach the same price point as the fuel it is directly
competing with (Price, 1991). The adoption of a fuel such as LH2 in aviation may hold the key to
reducing fuel related operating costs. For instance, a kerosene price of $5 USD per gallon will allow
LH2 to be $0.7 USD more expensive to produce the same direct operating costs as conventional jet
fuel allowing for a 50% increase in acquisition and maintenance costs in the early introduction stages
(Verstraete, 2013). Benefits of the adoption of LH2 based fuels are also associated to its excellent
Energy Specific Fuel Consumption (ESFC) in comparison to conventional aircraft fuels. The high
ESFC of LH2 may allow for lighter engines which may lead to a 3% indirect savings in energy
consumption. Similar results have been highlighted by Verstraete (2013) and in several other papers,
providing support for the energy efficiencies of LH2 engines. The adoption of LH2 fuel may lead to up
S. Rondinelli, R. Sabatini, A. Gardi
Challenges and Benefits Offered by Liquid Hydrogen Fuels
in Commercial Aviation
to a 30% reduction in gross weight brought on by the lower mass of LH2 in comparison to kerosene.
Though the operating empty weight (OEW) of both aircraft would be similar, a long-range LH2 fuelled
aircraft would likely be about 7m longer. Coupled with a double deck fuselage and smaller wing size,
a comparable LH2 aircraft will see a reduction of approximately 15% in its cruise average lift to drag
ratio. However this increase in drag will be counteracted by an 11% improvement in terms of energy
usage (Verstraete, 2013), slightly reducing direct operating costs. Savings in direct operating costs
are also expected to be diluted in the early stages due to the predicted increases in aircraft purchase
price, maintenance and servicing in comparison to conventional aircraft (Verstraete, 2013). Airfreight
has a rising importance for the profitability of airline routes. In order to benefit from it, there must be a
dedicated cargo capacity in addition to passenger luggage requirements. In a comparison between
conventional kerosene fuelled aircraft and a LH2 aircraft of conventional configuration, it becomes
apparent that conventional aircraft have more volumetric capacity for payload. This potentially
economic disadvantage may be outweighed by a LH2 aircrafts extended range capabilities. The
weight and energy advantages of LH2 allow these aircraft to fly at greater distances to that of
conventional kerosene aircraft, as represented in Fig. 2 (Verstraete, 2013).
Figure 2 - Payload vs. range curve of LH2 and kerosene-fuelled aircraft (Verstraete, 2013).
5. Airport design and operations
It is evident that airports will need to evolve to host regular hydrogen powered aircraft operations,
ensuring the required maintenance and support for hydrogen aircraft. The integration of LH2 fuel
systems will require airports to adopt new technologies and systems. This may involve an integrated
logistics and supply chain, which can meet the LH2 demands of aircraft operators, an onsite hydrogen
production facility, or the adoption of infrastructure which secures and safely houses the airports
reserve of LH2. For LH2 fuel systems to be introduced, there needs to be a reasonable demand by the
aircraft operators. Multiple airports will need to be equipped to supply LH2 in order for the fuel to be
commercially viable. During the implementation stage of hydrogen fuel systems at airports, airports
shall seamlessly accommodate hydrogen fuel infrastructure in conjunction with conventional kerosene
fuel delivery systems. For these reasons it is expected that the larger airports will be the first to adopt
such infrastructure, as the first LH2 aircraft are likely long-range transport category type aircraft (Janic,
2010). Modern international route structures are mainly based on the hub and spoke model, where
feeder flights are flown into a central location or hub, where passengers can benefit from a significant
number of flight connections. This network structure is particularly well suited for long range LH2
aircraft viability, allowing hubs to be major supply and maintenance centres for LH2 aircraft. Such
practices may alleviate demand for all airports to contain LH2 refuelling and maintenance capabilities,
instead concentrating the efforts on a steady and reliable supply where the fuel is mostly needed.
Future network planning will need to consider the regions that are supported by strong hydrogen
production capacities. The regions producing the most LH2 currently include North America, Japan
and Europe. Within these regions, the largest airports are likely to be the first integrators of LH2
technologies. Based on departure movements and location in correspondence to liquefiers, certain
cities stand out as plausible LH2 adopters. This includes Chicago, Los Angeles and Ontario in the
United States, Tokyo and Osaka in Japan, and Amsterdam in Europe (Stiller & Schmidt, 2010). The
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Challenges and Benefits Offered by Liquid Hydrogen Fuels
in Commercial Aviation
optimal fuel delivery systems for LH2 aircraft of the future would likely involve onsite production.
Careful consideration should be taken when locating the storage tanks (Janic, 2010; Schmidtchen et
al., 1997). The piping will need adequate insulation in order to the liquid hydrogen at -253° and may
consist of three pipes, satisfying the requirements to transfer the LH2, collect the boiled off H2 and a
allow for redundancy (Janic, 2010; Korycinski, 1978). Airports themselves also contribute
approximately 30 million tons or 5% of the total air pollution of the aviation industry (Cherry, 2008).
Contributing factors include aircraft, passengers, freight and airside/landside vehicle movements
(Janic, 2010). The widespread adoption of hydrogen fuels also for ground vehicles will provide great
environmental benefits to airport, not only restricted to the complete elimination of carbon emissions.
Currently, fuel delivery systems for kerosene aircraft are usually large and in some instances can be
quite complex, and typically involve a tank area or fuel farm within reasonable distance from the
apron. These tanks usually provide a fuel supply for 1-3 days and their fuel is supplied to the airport
via trucks or a system of underground pipes. Typically larger airport utilise underground piping to ease
congestion, however smaller or regional airports may utilise fuel tankers for simplicity (Janic, 2010;
Korycinski, 1978). It is important to note a fuel distribution based on fossil fuelled trucks emits
emissions throughout the whole process. Proposition as to the installation of distribution lines should
include entrenched yet open plans, which allows for the vent of potentially dangerous hydrogen gases
(Schmidtchen et al., 1997). Further improvements or reductions in airport related aircraft emissions
may come about through the adoption of LH2 powered APU, which could also contribute to aircraft
weight reduction through eliminating the need for generators within the engine assembly (Stiller &
Schmidt, 2010)
6. Safety
Aircraft fuelled by hydrogen have a reputation for being a dangerous endeavor. This was largely
brought on by the Hindenburg disaster. The flammable cloth of the containment bag back then is
vastly different from the highly insulated and structurally sound ergonomic tanks proposed for modern
LH2 applications (Brewer, 1983). Most recent in-depth studies highlight hydrogen as a safer
alternative to conventional kerosene fuels (Khandelwal et al., 2013). In the event of an aircraft crash,
liquid hydrogen is more likely to result in a safer outcome than that of a kerosene fuelled aircraft
crash, due to the rigidity of LH2 tanks, less likely to rupture, to the buoyancy of the gas, dissipating
quickly, and to the smaller heat and intensity of a hydrogen-fuelled fire (Brewer, 1983). Unlike
kerosene, hydrogen cannot contaminate the natural environment such as water or soil. Hydrogen in
its liquid form is much safer than its gaseous state due to the lower pressures in storage tanks, which
reduce the likelihood of fatigue induced structural failures (Schmidtchen et al., 1997). However
hydrogen’s ability as a gas to seep through containment lines or tanks unlike air or other gases
causes challenges in identifying leaks. Hydrogen can even engrain its self in solid materials such as
polymers through permeation, demanding careful consideration when selecting hydrogen containment
materials (Schmidtchen et al., 1997; Schmidtchen et al., 1994). Like almost all fuels, hydrogen
represents a flammability hazard. In its gaseous state, hydrogen has more potential to mix with air or
kerosene fumes and form a dangerous detonating mixture. However the heat from a hydrogen flame
represents about a tenth of that of a hydrocarbon fuelled flame. This not only reduces the extent of
possible damage caused during a major accident, but it also allows authorities to get closer to the
heat source. In the event of a leak, hydrogen in reasonable quantity may asphyxiate the air, starving
organisms of oxygen. Though hydrogen is still not corrosive or poisonous, its cryogenic temperatures
would injure a person upon touch (Schmidtchen et al., 1997). A liquefier incorporated into an airport
requires careful design considerations. Components such as pumps, connections and accessories for
LH2 require accurate engineering due to the cryogenic conditions they experience (Brewer, 1976;
Jones et al., 1983). Personnel in contact with such systems require specialist training, as contact with
any cryogenically cooled metals will likely result in injury. Airports must implement technologies,
procedures and policies for a safe and economical handling LH2. Consideration should encompass
the impact of disasters or emergencies. Current design requirement prescribe that accidents remain
at the lowest possible level and internal to the confines of the affected structure. Considerations
should nonetheless extend beyond that of internal emergencies. Such events may cause fire or flying
debris to reach areas of LH2 production and/or containment. Mitigation of this should be in the form of
suitable location of ground LH2 resources, by enforcing certain distances of safety or protection.
Likewise on aircraft, tank shape is an important aspect in terms of upholding the interests of safety.
Though there is not much difference in terms of operation of either a cylindrical or spherical tank,
there is a greater associated risk in manufacturing faults of spherical tanks due to its complexity.
Cylindrical tanks also offer more efficient use of capital resources through vertical installation.
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Challenges and Benefits Offered by Liquid Hydrogen Fuels
in Commercial Aviation
Likewise with any LH2 structure, the fuel tanks require protection from external elements. This may
come about in the form of partial submersion underground, with the top elements of the tank being
exposed to the qualified operators, however still protected via fortification. Further recognition should
also be devoted to security measures, implementing strict scrutiny and restriction in the access to LH2
reserves (Schmidtchen et al., 1997).
7. Environmental gains modeling
In line with all the factors described previously, we introduce a quantitative analysis of the potential
environmental and economic benefits associated with the adoption of liquid hydrogen (LH2) fuels for
aviation. As a practical reference, we assume conventional aircraft currently adopted in both regional,
medium-range and long-range commercial airline flights, evaluating their hypothetical retrofit for
conversion to LH2 fuel. For this analysis, we start from the Breguet range equation in its traditional
form:
 

(1)
where:
is the range;
is the overall propulsive efficiency;
is the nominal lift-to-drag ratio;
is the lower combustion heat;
is the initial mass;
is the final mass;
is the gravity;
We also define  
. The comparison is based on the following assumptions:
Aircraft are listed by their International Civil Aviation Organization (ICAO) code, which are:
o E190: Embraer E-190;
o A320: Airbus A320-200;
o B738: Boeing 737-800;
o A333: Airbus A330-300;
o B788: Boeing 787-800;
o B77W: Boeing 777-300ER;
o A388: Airbus A380-800;
The assumed characteristics for each aircraft are meant to represent an average of the
advertised or published ones;
Aircraft are configured for maximum range, therefore loaded with maximum fuel and a partial
payload;
 Is deduced from the advertised aircraft performance by means of the rearranged Breguet
range equation;
 is calculated as 85% of , to represent the increased drag associated with the
additional LH2 tank volumes, in line with the findings documented in section 3;
The Operational Empty Weight (OEW) of the hydrogen aircraft is increased by 6% to represent
the additional structural mass required for the LH2 tank, in line with the findings documented in
section 3;
The chemical composition of Jet-A1 is approximated as 99.7% in mass of , with a sulfur
content of 0.15% in mass, corresponding to half of the maximum regulatory threshold (0.3% in
mass);
The chemical composition of Jet-A1 emissions is calculated by assuming that 1% of the carbon
content is processed into CO and 0.5% originates unburned HydroCarbons (HC);
In order to economically represent their noxious effect, emissions charges are hypothetically set
to: 20 $/t for CO2; 200 $/t for CO and SOX; 2000 $/t for HC; 10 $/t for H2O. The carbon dioxide
charge is very closely related to the average value from a number of nations presently adopting
S. Rondinelli, R. Sabatini, A. Gardi
Challenges and Benefits Offered by Liquid Hydrogen Fuels
in Commercial Aviation
carbon taxation schemes. The remaining figures are meant to represent an educated guess
correlated to the noxious potential of the various substances to the environment and the living
beings.
Table 1 summarized the assumed aircraft characteristics, the estimated Jet-A1 gaseous emissions
and the corresponding hypothetical charges. Table 2 presents the results of the analysis based on the
assumptions in terms of changes of weight, volume, and economic savings.
Table 1. Assumed aircraft characteristics and calculated emissions for Jet-A1 fuel.
Aircraft Model
E190
A320
B738
B77W
A388
Range [nmi]
2400
2950
3060
7930
8500
Total length [m]
36.2
37.5
39.5
73.9
72.7
Hydraulic diameter of fuselage
[m]
3.15
4.04
3.76
6.2
7.75
Approximate 
3.6
4.4
4.3
6.2
6.0
OEW [t]
28.1
42.6
41.4
167.8
276.8
Payload [t]
11
16.2
17
37
40
MTOW [t]
51.8
78
79
351.5
575
Generated CO2 [t]
39.4
59.5
63.9
454.8
800.4
Generated CO [t]
0.1
0.2
0.2
1.5
2.6
Generated SOX [t]
0.0
0.1
0.1
0.4
0.8
Generated HC [t]
0.1
0.1
0.1
0.7
1.3
Generated H2O [t]
16.3
24.6
26.4
187.8
330.5
Total charge [USD]
$1,110
$1,678
$1,800
$12,822
$22,567
Table 2. Results of the analysis.
Aircraft Model
E190
A320
B738
B77W
A388
Assumed 
3.1
3.7
3.6
5.3
5.1
TOW of corresponding
hypothetical
LH2-powered aircraft [t]
45.9
69.1
69.2
270.0
429.0
Total LH2 mass [t]
6.8
10.3
10.8
65.2
112.2
Total LH2 volume [m3]
96.3
145.7
151.9
918.8
1580.8
Equivalent fuselage length [m]
12.4
11.4
13.7
30.4
33.5
Fraction of the total length
34.1%
30.3%
34.6%
41.2%
46.1%
Weight savings
11%
11%
12%
23%
25%
Generated H2O [t]
61.0
92.3
96.2
581.9
1001.1
Total environmental
charge [USD]
$610
$923
$962
$5,819
$10,011
Total savings per flight [USD]
$500
$755
$838
$7,003
$12,556
8. Conclusions
This paper overviewed the main benefits and challenges associated with the introduction of hydrogen
fuels in aviation. The paper introduced a simplified model for the estimation of the environmental
gains in realistic operational conditions. The results highlight the remarkable economic and
environmental benefits associated with hydrogen fuels, even considering the lower aerodynamic
efficiency and higher structural mass. The worsened volumetric efficiency and the challenges
associated with the production and supply are nonetheless substantial and will require significant
technological and political support. Future research will extend and integrate the models in the novel
avionics and air traffic management systems being developed (Gardi et al., 2013; Gardi, Sabatini,
Ramasamy, et al., 2014; Ramasamy et al., 2014; Ramasamy et al., 2013), to estimate the
S. Rondinelli, R. Sabatini, A. Gardi
Challenges and Benefits Offered by Liquid Hydrogen Fuels
in Commercial Aviation
environmental gains associated with enhanced flight trajectories and operations. Future research
activities will also consider the actual pollutant concentrations around airports obtained with the
researched systems (Gardi, Sabatini & Ramasamy, 2014; Gardi, Sabatini & Wild, 2014; Sabatini &
Richardson, 2008, 2010, 2013; Sabatini et al., 2012). Particular consideration will be given to
identifying the combined benefits and the additional challenges associated with the adoption of
hydrogen fuels in advanced aircraft configurations (Marino & Sabatini, 2014) and in more electric
aircraft configurations (Seresinhe et al., 2013).
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