Hydrogen Fuel Cell Inverter Matlab

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Comprehensive Investigation on Hydrogen and Fuel Cell Technology in the Aviation

And Aerospace Sectors

Ahmad Baroutaji1 (*), Tabbi Wilberforce2, Mohamad Ramadan3, Abdul Ghani Olabi4, 5

Abstract

The world energy consumption is greatly influenced by the aviation industry with a total energy consumption ranging between 2.5% and 5%. Currently, liquid fossil fuel, which releases various types of Greenhouse Gas (GHG) emissions, is the main fuel in the aviation industry. As the aviation industry grows rapidly to meet the requirements of the increased world population, the demand for environmentally friendly power technology for various applications in the aviation sector has been increased sharply in recent years. Among the various clean power sources, energy obtained from hydrogen is considered the future for energy generation in the aviation industry due to its cleanness and abundance. This paper aims to give an overview of the potential aviation applications where hydrogen and fuel cell technology can be used.

Also, the major challenges that limit the wide adoption of hydrogen technology in aviation are highlighted and future research prospects are identified. Keywords: Hydrogen, Fuel cells, Aviation, Renewable Energy, Clean Power

Introduction

Nowadays, the air transport community contributes significantly to the socio-economic development of various countries around the world. The capacity of transport by air has increased significantly over the last two decades and it is expected to grow by around 5% annually until 2030 . Inevitably, this will result in increasing the number of working jets.

According to the Boeing current market outlook , it is projected that by 2036 there will be around 47000 working jets compared to 23000 in 2016. The weight that a plane can hold is huge and it varies depending on the total capacity of the plane but the average total weight in most commercial flights is approximately 640 tons . In order for a plane to carry such a big weight, huge engines, which consume considerable amounts of fuel, are needed. The high demand for aviation fuel to support this huge load comes with various challenges and

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consequences pertaining to safety, cost, specific energy and efficiency. Most fuels used in the aviation industry are currently fossil based petroleum commodities. The fuel that can be used in the aviation industry must contain a combination of different properties such as high specific heat capacity, high energy density, good atomization, fast evaporation, excellent burning characteristics, low viscosity, high lubricity, low freezing point, good chemical and thermal stability, and low impact to the environment , . Among the various types of fuel commodities, kerosene is the cheapest and the most used fuel in the aviation industry. Kerosene is made up of 35% alkenes, 60% cyclic alkanes, 15% aromatics . Despite it is a cheap fuel, the kerosene has a significant impact on the environment as it releases different GHG emissions and for this scientists are currently working earnestly hard to find a clean alternative to it. The global aviation industry produces around 12% of carbon dioxide (CO2) emissions generated by the transport sector . In 2015, it was reported that nearly 781 million tonnes of CO2 were generated from flights . GHG emissions of the kerosene are CO2 and sulphur dioxide (SO2) . SO2 is considered a very dangerous emission due to its role in the formation of acid rain . The environmental effects of the kerosene are the crucial factors for seeking a new clean replacement of the fossil commodities for aviation applications. Hydrogen and fuel cell technology have a great potential to be used as a power source for various applications in the aviation industry –. The scientific community is currently championing several researchers into hydrogen as a source of power because it is readily available, has good specific energy properties and also friendly to the environment , , . The usage of hydrogen in aviation sector can be categorised into two main routes; the first is the use of hydrogen as a replacement to kerosene for big aeroplanes, and the second is using fuel cells in place of jet engines for small aeroplanes . The use of fuel cell technology for aircraft still in its early days but it is receiving increased attention every day from leading aircraft manufactures. Airbus and Boeing are currently exploring the possibility of using fuel cell technology to power the

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Auxiliary Power Units (APUs) of their aircraft that used to be powered by diesel engines. Also, the fuel cells have the potential to be used in place of batteries to power other devices and systems in aircraft. For instance, they can be used for cabin pressurisation, environmental control system, lighting, and instrumentation in the cockpit, wing anti-ice protection, control surface and landing gear actuation.

It is projected that hydrogen and the fuel cell technology will continue to receive interest and application in the aviation. A fuel-cell-powered commercial aircraft might become a reality within a decade or two after solving some major problems in the field related to high cost of the technology, hydrogen production, storage, and refuelling infrastructures, aircraft design, and so on. The aim of the current paper is to present the current status of hydrogen and its technology for the aviation and aerospace industry and also to highlight the main challenges that need attention in order to see wider adoption of these technologies in the sector.

Hydrogen As A Propulsion Fuel

Using hydrogen in aeronautical applications dated back to the late 18th century where it was used for hydrogen balloon and then, at early twenty century, for rigid airship such as Zeppelin due to its superb bouncy feature . Since then, different projects were launched in many countries to evaluate and develop hydrogen-powered aircraft, which use hydrogen as a propellant, such as Suntan (USA-1956), Tupolev Tu-155 (Soviet Union- 1988), CRYOPLANE (Europe- 2000), HyShot (Australia- 2001), NASA X-43 (USA- 2004), Phantom Eye (USA- 2013). In addition to its usage for aeronautics, hydrogen is the primary fuel for rockets because it has high specific energy. The fuel tank of a rocket is normally filled with liquid hydrogen (LH2) and liquid oxygen (LOX) that react together and gives a lot of power to thrust the rocket up. For decades, cold LH2 was used by NASA as rocket fuel for many of its space shuttle vehicles including Centaur and Apollo . Hydrogen was adopted as a fuel by most of rocket engines developers around the world such as RL10 (Aerojet Rocketdyne- USA), LE-5

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(Mitsubishi- Japan), HM7B (Snecma- France), YF-73 (CALT- China), KVD-1 (Russia), CE- 20 (HAL- India) . The usage of hydrogen as a fuel in place of fossil commodities has some challenges but its advantages outweigh the disadvantages. The hydrogen has the highest energy per unit mass compared to any other fuel obtained chemically. Its energy density is 2.5 times bigger than that of kerosene . Using hydrogen as an aviation fuel would eliminate most of the GHG emissions including all carbon-based emissions, soot, and sulphur oxides . The main by- products of hydrogen in a combustion process are water vapour (H2O) and nitrogen oxides (NOX) . NOx emissions are associated with the formation of smog, acid rain and particulate matter. Furthermore, NOx emissions result in the production of ozone (O3) which is a climate warming gas and have negative effects on tropospheric O3 and stratospheric O3 , .

However, the amounts of NOx emissions released from burning hydrogen are extremely low compared to those released when burning kerosene . Generally, the overall reduction of NOx when using hydrogen fuel is due to the fact that the water vapour generated from burning hydrogen absorb most of the energy released from the combustion process and thus reduce peak combustion temperatures preventing the formation of NOx . The water vapour contributes to the formation of contrails which also has global warming effects. The percentage of water vapour increases when hydrogen is used in place of petroleum-based fuels in aeroplanes . Despite that burning hydrogen produces a larger amount of water vapour, which is considered as a GHG, it still provides a significant reduction in the net GHG emissions as a jet fuel compared to kerosene , . Furthermore, adoption of hydrogen as a jet fuel can provide long term stable level of the fuel prices and also enhance the reliability of the fuel supply because it can be obtained from different types of sources and this reduces the reliance on fossil fuel concentrated in only few regions around the world , . Thus, hydrogen is deemed suitable for the aviation industry because it is capable of providing clean, reliable and

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affordable energy supply that has extremely low pollutant emissions and thus has a low impact on the environment . In order for hydrogen to be viable for the aviation industry, its volumetric density or energy density per unit volume must be increased , . Comparing with kerosene, four times bigger volume of LH2 is needed to deliver the same amount of energy . However, despite this drawback, it has been reported that the hydrogen-powered aircraft are more weight efficient and involve lower operating costs than those aircraft powered with kerosene .

Generally, using hydrogen as a jet fuel necessitates modifying the design of both aircraft and engines. LH2 used in hydrogen-powered aircraft must be kept at very low temperatures and for this it can only be stored in highly-insulated tanks and not in the wings which have restricted space to accommodate LH2 and cannot be insulated properly. With the huge volume requirements of the hydrogen fuel tanks, aircraft design needs to be modified with a greater volume and heavier fuselage, which is the best location to place LH2 fuel tanks, as shown in Figure 1 . For short-to-medium range aircraft, the hydrogen tanks could be placed above the passenger cabin while for the long-range aircrafts the hydrogen is stored in two big integral tanks; one of them is located directly behind the cockpit and the second is placed at the far aft of the passenger cabin . The configuration of hydrogen tanks has a significant impact on the energy efficiency of hydrogen-powered aircraft . The top-tank design, used in short-to- medium range aircraft, may increase the energy use by 6-19% due to the greater weight of this type of tanks . On the other hand, the integral design, suitable for long-range aircraft, can increase the energy efficiency by 12% . Thus, it can be concluded that hydrogen fuel is more suitable for long-range aircraft. Since the fuselage of hydrogen-powered aircraft is used to store hydrogen tanks, a heavier and bigger fuselage is needed to support the loads generated from these tanks. The mass of fuselage in hydrogen-powered aircraft is almost 6% bigger than the normal aircraft . Additionally, since the wings in the hydrogen-powered aircraft are no

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longer used to store fuel, the wings could be designed to be smaller in terms of area and span. However, the weight of wings should be increased when using hydrogen in order to enhance their structural integrity against bending and vibrations generated by aerodynamic forces .

The smaller wings and greater fuselage of hydrogen aircraft may affect the aerodynamic efficiency negatively . In addition to changing the aircraft design, the engine has to be changed when converting into hydrogen due to variation in the combustion gases and properties between the kerosene and hydrogen. Smaller engines can be used for hydrogen fuel . The changes in aircraft and engine designs when using hydrogen entail an increase in the production and maintenance costs which could reach up to 25% .

Hydrogen Fuel Cell For Aviation Applications

A hydrogen fuel cell is an electrochemical device that generates electricity via an electrochemical reaction between hydrogen and oxygen. Fuel cells are silent, produce little vibrations and produce no NOx emissions which make them very attractive for wide range of applications. Among the various types of fuel cells devices, Proton Exchange Membrane Fuel Cell (PEMFC) and Solid Oxide Fuel Cell (SOFC) were mostly considered for the aviation applications. SOFC operates at high temperature and uses a dense ceramic layer as an electrolyte while PEMFC operates at low temperature and employs proton conducting membrane as the electrolyte. A summary of the working principles of these types of fuel cells is presented in Figure 5. Hydrogen fuel cells could be used on-board of aircraft to power different items and systems that are currently powered by batteries such as emergency door system, floor escape path lighting, Emergency Locator Transmitter (ELT), Flight Data Recorder (FDR), and Cockpit Voice Recorder (CVR). The main aviation applications where fuel cells have been tested are reviewed in the next sections

3.1 Apu In Aircrafts

APU of aircraft is a small gas turbine which functions as a source of electricity and compressed air for the operation of other components in the aircraft, such as jet engines and environmental systems, when the main engines are not running. APU is used as a back up to internal power during cruising and as the main source of power to the aircraft during stationary as it supplies the required power for air conditioning and lighting in the aircraft. It also supports the starting up of the main aircraft engines. APU is turned on when the aircraft is taxing from the runway to the gate or the parking . Around 20% of aircraft emissions are from traditional APU powered with gas turbines . One of the targets set by the Advisory Council for Aviation Research in Europe (ACARE) is the emission-free taxing phase. Such a target can be achieved by implementing a clean power source to run APU during taxing. Fuel cell devices have been identified as a suitable source to power APU and thus reducing the fuel consumption during aircraft taxing phase . The by-products of fuel cells including water and heat can be channelled to other components in the aircraft . Besides their environmental advantages, fuel cell powered APU can help also in reducing the noise levels .

Fuel cell systems could be used in place of the traditional APU or combined with them to form a hybrid APU system . Both PEMFC and SOFC can be used for AUP systems . However, SOFC is more favourable for aircraft applications particularly for powering APU . SOFC is more tolerant of fuel impurities and can operate using hydrogen generated from kerosene reforming . Additionally, the high operating temperature of SOFC allows the internal reforming process of jet fuel and increases the efficiency . However, the weight of the SOFC powered APU is greater than PEMFC or the traditional APUs because it requires bigger balance-of-plant (BoP) items such as reformer, compressor, pumps, heat exchangers and so on . PEMFC can only be used for powering the APU if the source of hydrogen is installed on the aircraft.

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Main aircraft manufacturers, including Airbus and Boeing , are currently running projects aiming for using electrical power generated by hydrogen fuel-cell devices to power all non-propulsion systems in the aircraft. Such an approach might help in reducing the loads on the main engines and thus the planes become more fuel-efficient . Boeing stated in its report that using solid oxide fuel cells in APU will reduce the fuel consumption during cruising and stationary by 40% and 75%, respectively . The CRYOPLANE European project also estimated that SOFC APU will reduce aircraft ground NOX by 80% .

In summary, using fuel-cell-powered APU can be considered as a great solution for reducing fuel consumption, pollution and GHG emissions.

3.2 Ground Support Equipment (Gse)

With the increased interest in improving the air quality around airports, fuel cells devices have received increased attention due to their potential in powering different Ground Support Equipment (GSE) and other ground vehicles working in the airports, such as passenger shuttles and refuelling trucks, in the near future .

GSE or aircraft handling devices are used to service passengers, cargo, facilities and aircraft while they are in the airport between flights. GSE contain different service systems such as air start unit of the engine, air conditioning unit, cargo loaders, pushback track, stairs for passengers, baggage tractors, trolleys, etc. Each of these devices has different power requirements. Air start unit is considered as the most energy consuming device among all the aircraft handling devices as it consumes approximately 450 kW . The next energy consuming device is the pushback truck which is also approximately 200 kW. A number of other aircraft handling devices, such as forklifts, baggage tractors, water trucks, deck loaders and air conditioning units, have a power range between 10 kW to 50 kW .

of Energy (DOE) has announced that around 250 million US dollars will be used to deploy fuel

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cells for baggage vehicles at airports . The medium size fuel cell powered forklifts have been already tested in major airports such as Toronto Pearson, Hamburg, and Munich airports . These forklifts incorporate many advantages such as low noise, no emissions and less maintenance. They have almost the same dimensions as traditional batteries powered ones but they do not require battery swapping which is very challenging for most conventional forklifts. Additionally, refuelling time was seen as a major advantage for fuel cell powered forklift over battery ones as it can be completed within minutes while recharging the battery needs hours . The fuel cells can also be used for more than eight hours without requiring any refuelling. Thus, it can be concluded that replacing traditional batteries with fuel cells in forklifts is viable economically with respect to the initial capital cost as well as the operating costs. The Danish airport is currently using luggage trolleys that functions using fuel cells. In 2015, FedEx has rolled out the first fuel cell powered GSE cargo tractors that can pull around 15 tonnes of cargo .

In addition to GSE applications, the fuel cells are used to power the airport vehicles such as passenger buses. In Tokyo, some of the buses working on the route between Tokyo central and Tokyo airport are a hydrogen fuel cell/battery hybrid system . International Airport in Hawaii set a plan to use a hybrid fuel cell/ battery bus to shuttling passengers between airport terminals and the car rental facility .

3.3 Unmanned Aerial Vehicles (UAVs) and passengers aircraft Currently, most of Unmanned Aerial Vehicles (UAVs) are powered by batteries because the conventional power sources, such as reciprocating engines and gas turbines, exhibit very low efficiency for small-scale vehicles. However, the batteries have a low energy density, i.e. low energy per unit mass, which might lead to a significant increase in the weight of UAV if longer flight time, or greater endurance, is required. Fuel cell has been regarded as a possible new power source alternative to existing batteries. Belmonte et al compared the performance

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of batteries and fuel cells with respect to powering UAVs. The authors reported that the cost is the most attractive factor of battery-powered UAV as it is almost half the price of the fuel cell. However, increasing the weight of the battery required to achieve a long flying time has been reported as a limiting factor of battery-powered UAV. The main advantage of the fuel cells for powering UAV includes allowing extended mission time and distance due to superior energy density, providing quiet and reliable operation, reducing lithium battery charging and transport obstacles . PEMFCs are preferred for UAVs applications as they can operate at low temperature and this means smaller BoP requirements. In 2005, AeroVironment has developed the first fuel cell UAV . The plane, that has a wingspan of 15 m, was powered using a PEMFC propulsion system and managed to complete more than one hour of flying using only liquefied hydrogen. Following that in 2006, Bradley et al designed and tested a fuel cell powered UAV using compressed hydrogen. With a wingspan of 6.58 m and a total mass of 16.4 kg, the authors demonstrated that the plane can be powered using a 500 W PEMFC.

Currently, there are considerable research efforts worldwide to design, develop, and construct fuel cell powered UAV. These efforts have resulted in a significant increase in endurance which exceeded 48 h . Reducing the weight of fuel cells, improving their reliability and durability, speeding their startup, and increasing their power density will make the fuel cell technology more attractive for UAV . A list of fuel-cell-powered UAV with fuel cell type, storage method and endurance is presented in Table 6.

The fuel cell technology has also been studied as a potential power source to manned aircraft after the 20th century . The first successful fuel cell powered light manned aircraft has been tested in 2008 by Boeing Research & Technology Europe (BR&TE) . A two-seat light aircraft with a 16.3 m wingspan, which has been powered by PEMFC, has successfully completed a 20 minutes flight at a speed of 100 km/h . In 2016, a research group from DLR German aerospace centre developed and tested a fuel cell powered four-seat passenger aircraft

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namely HY4 . A PEMFC, assisted by a lithium battery to meet the requirements of peak loads during take-off and climbing, was able to provide the required power to run HY4 that has a maximum weight of 1500 kg and average cursing speed of 145 km/h.

3.4 Space Applications

For a long time, NASA has used fuel cell technology as a power source and energy storage device in the space shuttles. Onboard fuel cell power plants in space shuttle generate all the electrical power required for the vehicle from launch through landing rollout . The fuel cell power system is integrated with several subsystems responsible for the distribution of reactants, cooling of fuel cell system, storing of generated water, and distribution of electrical power. The water generated during the fuel cells operation is usually used for drinking by the astronauts.

The first use of fuel cell by NASA dated back to 1965 in the Gemini V spacecraft. PEMFC was used as the main power source and it was integrated with silver-zinc batteries to meet the peak loads. The Gemini fuel cell system was subsequently used on six manned flights including Gemini 7, 8, 9, 10, 11 and 12 , . In 1963, Alkaline Fuel Cells (AFC) was selected for the Apollo mission. The AFC stack consisted of 31 individual fuel cells operated at a temperature of 206 °C and connected in series to generate a maximum output power of 2300 watts . The use of Apolo fuel cell was expanded to include all Apollo missions, the Apollo/Soyuz mission and Skylab. AFC and PEMFC are the main technologies used by NASA for energy storage applications. NASA has replaced batteries with fuel cells in almost all manned missions . Fuel cells are preferred over batteries for manned missions because such missions require primary energy storage with longer discharge times and higher power levels and these requirements can be better met using fuel cell technology .

Hydrogen Production And Storage

Despite the hydrogen can be considered as carbon-free fuel, the emissions generated during its production might be very high based on the production method . Thus, the full

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environmental benefits of using hydrogen as a fuel for applications in aviation and aerospace industries require generating the hydrogen using clean and renewable sources. Also, with its low volumetric density, storing hydrogen onboard of aircraft forms a big challenge for the hydrogen-powered aircraft. Due to their significant role in advancing the hydrogen for various aviation applications, the main hydrogen production and storage technologies are reviewed in this section.

4.1 Overview Of Hydrogen Production Technologies

Hydrogen is the most common element on the earth but it does not exist alone . The generation of hydrogen usually occurs by removing the other elements it combines with . The hydrogen can be obtained from several feedstocks including fossil fuel, such as coal and natural gas, as well as renewables sources, such as water and biomass . Obtaining hydrogen from fossil fuel is a matured approach. Around 90% of the global hydrogen demand is currently produced from fossil fuel including natural gas and coal . There are two main routes for obtaining hydrogen from natural gas including steam reforming and partial oxidation processes . In the steam reforming process, a natural gas, such as methane (𝐶𝐻4), and water vapour are transformed into carbon monoxide (CO) and hydrogen through an endothermic reaction,

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The CO produced as a by-product can further be transformed into 𝐶𝑂2 and hydrogen by water

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In the partial oxidation method, which is an exothermic process, there is a partial burning of methane with oxygen gas and this generates 𝐶𝑂, hydrogen, and heat as shown in Equation 3

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The water gas shift reaction, Equation 2, is then used to convert 𝐶𝑂 to 𝐶𝑂2 and hydrogen. There are several methods for converting coal to hydrogen through an endothermic gasification process such as fixed bed gasifier, fluidized bed reactor, entrained flow gasifier, and plasma gasifier . For most gasification reactions, the fuel and agents for the gasification are transformed into a mixture of 𝐶𝑂 and hydrogen as demonstrated in Equation 4

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The production of hydrogen from fossil fuel is always associated with emitting CO2 and CO which causes air pollution. The negative effects of these gases on living organisms formed the motivation towards developing a technology to extract hydrogen from a clean and renewable source such as water and biomass .

The separation process of water into oxygen and hydrogen is known as electrolysis and it is conducted by means of electric energy via a redox (oxidation-reduction) reaction . Equation 5 shows the chemical equation for water electrolysis

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The electrolysers could be either low temperatures such as alkaline and Proton Exchange Membrane (PEM) electrolysers; or high temperature such as solid oxide electrolyser. The low- temperature electrolysers often operate at temperatures below 100oC while the high- temperature ones function at higher temperatures up to 1000oC . In an alkaline electrolysis, the water in the cathode consumes electrons to form hydrogen. The hydroxide ions flow through the solution in the direction of the anode and during this process the electron is released. Equations 6-8 show the chemical equation for alkaline electrolysis .

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PEM electrolyser contains a Solid Polymer Electrolyte (SPE) to conduct protons between the cathode and anode of the electrolyser. The high ionic conductivity of SPE makes the efficiency of PEM electrolyser higher than the alkaline one . The main chemical reactions that occur

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In a solid oxide electrolyser that operates at high temperatures typically between 500 and 850 °C, water is reduced, in the cathodic region, to generate hydrogen and oxygen ions. The hydrogen gas is collected from the cathode, while the oxygen ions are transferred through a dense electrolyte to the anode side to produce oxygen. The main reactions taking place in solid- oxide electrolyser are presented in Equations 12-14

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In addition to electrolysis technology, hydrogen can be produced from water via thermochemical water splitting processes that use heat from solar power or wasted heat of nuclear power reactions to drive a series of chemical reactions for splitting water into hydrogen and oxygen .

The second main renewable source of hydrogen is biomass resources such as agriculture residues, animal wastes, and organic municipal solid waste . Biomass gasification process, which involves using heat, steam, and oxygen together without combustion, is normally used to extract hydrogen from these organic materials.

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A comparison of the main technologies used for the hydrogen production, excluding those technologies that are still at early stages of research such as microbial biomass conversion and photobiological, is presented in Table 1 and Table 2. Different research and development activities that can lead to further advancement of these technologies are also presented in Table 3.

4.2 Overview Of Hydrogen Storage Technologies

Despite the potential benefits of using hydrogen as a fuel for the aviation industry, its low density, approximately 0.089 kg/m3 at standard temperature and pressure, and associated storage problems continue to remain an attractive field for research and developments. With the constraints imposed by aircraft weight and volume, it is challenging to store, produce and use hydrogen efficiently on aircraft. Storage of hydrogen is often categorised into two main sections; physical and chemical storage as shown in Figure 2. Hydrogen can be stored as a pressurised gas, cryogenic liquid, or in the solid state in combination with other materials such as metal hydrides and carbon materials.

Storing hydrogen in its gaseous state is very common and forms the simplest and most efficient solution in terms of fast filling-releasing rate. To increase its density, the gaseous hydrogen is normally compressed and store at pressures between 35 MPa and 70 MPa .

Despite that increasing the storage pressure seems to be an advantage for low-density gaseous hydrogen, the temperature rise during the fuel filling becomes a major challenge at very high pressures . Metallic tanks, such as steel, are normally used for gaseous hydrogen storage.

However, the pure hydrogen at high pressure may have serious deleterious effects on the mechanical properties of metals such as promoting localised plastic processes and accelerating crack propagation rate . Today, tanks made of composite material, as shown in Figure 3, are increasingly employed for storing hydrogen. Such tanks are light in weight and can withstand high pressures and thus they are very attractive for transportation applications as in

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aircraft. The tank consists of a high-density polyethene liner wrapped with carbon fibres composites shell. However, composite tanks are generally expensive with price tags between 500–600 USD/Kg H2 . Also, hydrogen compatibility and durability of composite vessels with polymeric liner still require further research and validation.

Hydrogen can be stored as a liquid in cryogenic tanks. Hydrogen needs first to be liquefied at −253 °C and this process consumes up to 40% of the chemical energy stored in the hydrogen. A cryogenic vessel, shown in Figure 4, uses a very complex vacuum insulation technique consisting of approximately 40 layers of metal foil to maintain the temperature of −253 °C for the hydrogen in the tank and limiting the boil-off loses . LH2 is difficult to store for long period as up to 3% of hydrogen is lost on daily basis due to evaporation caused by the heat gain from surroundings. The main factors that affect boil-off loses, i.e. loses due to evaporation of LH2, are the geometry of the vessel, thermal insulation, the ambient conditions, quantity of hydrogen and the length of storage time. The materials that can be used to construct a cryogenic tank wall should have high strength, high fracture toughness, low density and low permeation to both liquid and gaseous hydrogen. Fracture toughness property is of particular importance as many materials become brittle at the cryogenic temperatures . The density of cryogenic hydrogen is 70.8 kg/m3 which is almost twice the density of pressurised hydrogen gas. The high density of the LH2 is the key feature that allowing using this storage technique for various aerospace applications whether at subsonic or hypersonic speeds . The Boeing unmanned aerial system, Phantom Eye, employed lightweight cryogenic hydrogen tank to power hydrogen internal combustion engines .

The final storage method of hydrogen is to store it chemically in the solid state as in metal hydrides such as Mg2NiH4, FeTiH1.95, LaNi5H6.7, or in chemical hydride such as alkali metals borohydrides (LiBH4 or NaBH4). In this technique, the hydrogen is first absorbed reversibly by solid compounds, such as Li, Na, Mg, Ti, under certain temperature and pressure conditions

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and then it can be released when needed to by increasing its temperature or decreasing the operating pressure. The hydrogen content in metal hydrides is around 7.6 wt% while it reaches 18.4 wt% in borohydrides , . The main demerit of chemical hydride is the need to convert the by-products of the chemical reaction off-board in a suitable location . The chemical storage technology, using sodium borohydride NaBH4, was tested by Korea Advanced Institute of Science and Technology to obtain 5 hour flight time for a blended wing body micro air vehicle (MAV) combined with two 25W fuel cells. It was proved that the propulsion system made of a fuel cell with NaBH4 tank can provide an energy density of 1000 Wh/kg .

A comparison of hydrogen storage methods is shown in Table 4 and Table 5. Generally, for the aviation and aerospace applications, cryogenic tanks that can store LH2 continue to remain the most attractive storage method as it provides high hydrogen content for low weight and reasonable volume capacities. Hydride and gaseous storage methods are impractical because of the excessive weight or volume of the tanks .

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Role of hydrogen technology in a sustainable aviation economy Aviation continues to remain one of the hardest sectors to decarbonise . Airports around the world face several challenges related to the quality of air, noise pollution, and energy efficiency . Additionally, airports operations need to handle thousands of travellers and aircraft on a daily basis and this makes the balancing between such intensive operations and their impact on the environment more complex than ever. The contribution of the aviation industry to global CO2 emissions has received increased interest in the last two decades .

The main target of the aviation industry worldwide is to achieve 50% reduction in the net CO2 emissions by 2050 compared to 2005 levels . Thus, in order to achieve such ambitious target, more sustainable energy resources and technologies, such as jet biofuels , hybrid propulsion systems , and hydrogen energy , are required to be deployed in the sector.

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Hydrogen and fuel cell technologies have a great potential to tackle the challenges of ensuring a cleaner future for the aviation industry. Hydrogen as a jet fuel provides many advantages over kerosene such as higher specific energy, less operating and engine maintenance costs, zero CO2 emissions and less NOx emissions . Additionally, using fuel cell technology to power various airport vehicles and equipment will mitigate the impact of pollution and enhance the quality air around the airports.

Currently, hydrogen is only produced economically using natural gas reformation process which cannot completely solve the issues of air pollution and sustainability , . Satisfying the economic and environmental requirements of adoption hydrogen for aviation sector requires that the majority of hydrogen should be produced from a clean energy source without the need to construct a long network of pipelines to transmit it from the production sites to the consumption centres. This can be achieved by producing hydrogen in locations close to the airports using renewable energy sources such as bioenergy, wind, geothermal, solar, and ocean , . Andrews and Shabani have identified three principal types of clean hydrogen production centres that can be used to produce hydrogen from different renewable energy sources including coastal hydrogen centres (CHCs), off-shore hydrogen centres (OHCs), inland hydrogen centres (IHCs). CHCs and OHCs use wave, wind, and/or tidal stream to produce electricity which then can be used in electrolysers to produce hydrogen from seawater. IHCs can employ other renewable energy sources such as solar radiation, solar thermal systems, wind power, and biomass resources to generate hydrogen. Thus, the type of renewable energy sources that can be used in a specific airport for hydrogen production purposes depends mainly on the location of the airport. Airports located close to a coastal region might use the wave and wind energy while those inland airports could rely more on the solar and bioenergy to produce hydrogen. Many airports across the world have already constructed hydrogen fuelling stations to supply hydrogen for fuel cell vehicles such as those

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in Berlin airport , Oslo airport , Orly Airport Paris , and Narita International Airport . The Berlin airport hydrogen facility produces hydrogen on-site via electrolysis using solar and wind energy .

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Challenges and future developments of hydrogen technology in the aviation

Industry

The aviation community continues to face complex challenges in using hydrogen and fuel cell technologies for the various applications in the sector. On the large scale, there is a lack of infrastructure required to provide the hydrogen fuel, as an energy carrier, for the aviation industry. The transition to hydrogen-powered aircraft will not only alter the aircraft design but also the airport operations. Special refuelling and storage structures will be required for handling and storing LH2 in the airports. The temperature of LH2 should be kept below 25 K and for this tanks with excellent insulation have to be built in the airports. Also, the airports will need special trucks with insulated fuel tanks for refuelling the aircraft with LH2.

Additionally, refuelling process of hydrogen-powered aircraft is more complex and takes longer time than the traditional one due to high risk of hydrogen leaking during the process . Thus, very strict safety regulations are required to control any hydrogen leaking issue.

Such problems can only be addressed by the respective international authorities who need to develop proper hydrogen fuel infrastructures and safety regulations after reaching a consensus regarding the transition to hydrogen utilisation for civilian transport applications .

Among the other challenges is the ability to produce high-quality hydrogen in a cost- effective manner and deliver it safely to the end users. One of the major problems in the production of hydrogen is its quality for any specific demand. For example, PEMFCs use platinum catalyst for the electrochemical reaction. This catalyst can easily be poisoned by any impurity in the hydrogen and this reduces its efficiency . Therefore, hydrogen production technologies must be able to produce pure hydrogen or a purification process will be needed

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and this will increase the overall cost of the hydrogen. The effects of hydrogen impurity on fuel cell performance could be reduced by using cost-effective non-platinum catalysts such as molybdenum nitride (Mo2N/C), tungsten nitride (W2N/C), carbon/polyaniline/cobalt (C-Pani- Co), iron acetate (FeAc), etc. These novel catalysts have received intensive research efforts in the past ten years. Detailed background about such catalysts is available elsewhere .

Another challenge is the cost of hydrogen production as the initial and operational costs of the majority of hydrogen production technologies are very high compared to other types of fuels . Production of hydrogen using renewable energy sources, particularly for large-scale LH2 suitable for the aviation industry, is still more expensive and less attractive than the conventional conversion methods which have a greater environmental impact . Redesign the production process, reducing its steps, and improving materials and equipment should help in reducing the high cost of hydrogen production. In terms of hydrogen transportation, the pipelines network responsible for hydrogen delivery should be made from special materials that can withstand the permeation effects of the hydrogen and resist the embrittlement caused by it.

Developing a light-weight, cost-effective, and safe hydrogen storage technology is another important requirement for the widespread commercialisation of hydrogen fuel cells technology in aviation applications. A light-weight hydrogen storage solution is needed for manned and unmanned vehicles. Among the various hydrogen storing technologies, only the cryogenic tank that store LH2 appears to be practical in the near future for the aviation industry. A tank that is suitable for aviation and aerospace applications differs significantly from those developed for ground applications. The tank’s insulation should be designed to withstand excessive thermal loads for longer time particularly in supersonic flight scenario and thus developing durable and light materials is very crucial for the success of the storing process.

Deployment In Out-Of-Position Situations

D. Bendjaballah1, A. Bouchoucha1, M. L. Sahli1,2* and J-C. Gelin2

Abstract

Side-impact collisions represent the second greatest cause of fatality in motor vehicle accidents. Side-impact airbags have been installed in recent model year vehicle due to its effectiveness in reducing passengers’ injuries and fatality rates. In meeting these requirements, simulations of folding and deploying airbags are very useful and are widely used. The paper presents a simulation method for the deploying airbags using three materials in different working conditions. Finite element analysis is primarily used to evaluate this concept. In these simulations, the gas flow is described by the conservation laws of mass, momentum, and energy. The numerical results indicate that the FE method in this paper is capable of capturing airbag deploying process accurately.

Keywords: Airbag simulations, Out-of-position, Crash, Modeling, Out-of-position

Background

The passive safety of cars has become a very high prior- ity issue for the automotive industry. Today, there are not only one or two airbags in a car; certain models have ten times more than that. With the increasing usage of airbags, the number of accidents where the airbag itself can cause an injury to the occupant also increases

(Augenstein Et Al. 2003; Gabauer And Gabler 2010;

Audrey et al. 2011). As is well known, safety belts are also now devices designed to provide protection to the users of vehicles during crash events, minimizing the loads necessary to adapt their movement to the move- ment of the car (Freesmeier and Butler 1999; Schmitt et al. 1997). In general, the seat belt is designed to restrain the occupant in the vehicle and prevent the

Occupant From Having Harsh Contacts With Interior

surfaces of the vehicles. The airbag acts to cushion any impact with vehicle structure and has positive internal pressure, which can exert distributed restraining forces over the head and face. As a safety component of auto- mobile, an airbag decreases occupants’ injury likelihood effectively in case of an accident (Ruff et al. 2007). These safety elements can reduce the death rates on the roads, and its protection effects have been widely approved (Crandall et al. 2001; Teru and Ishikawa 2003). With computational tools such as finite element methods designed for dynamic contact problems, crashworthiness simulations can now be used with reliable accuracy to evaluate occupant protection in various collision condi- tions with safety metric/parameters such as acceleration, head injury criteria, intrusion distance, intrusion vel- ocity, and neck forces (neck injury risk or whiplash).

Thus, new types of airbag products are being developed to handle different collision scenarios.

Become Standard Equipment On Most New Passenger

vehicles (Braver and Kyrychenko 2004; Teng et al. 2007; Yoganandan et al. 2007). The airbag cushion is com- posed of a woven fabric which is rapidly inflated during a car crash. The airbag dissipates the passenger’s kinetic energy thereby reducing injury through biaxial stretching of the fabric bag and escaping gas through vents. There- fore, the performance of the airbag is greatly influenced by the mechanical properties of the fabric. Generally, air bags are designed to deploy in a crash that is equivalent to a vehicle crashing into a solid wall at 8 to 14 mph.

Air bags most often deploy when a vehicle collides with another vehicle or with a solid object like a tree. There are various types of airbags: frontal, side-impact, and curtain airbags. In general, the passenger side airbags are usually larger than the driver airbags (see Fig. 1).

Besançon, France

© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Bendjaballah et al. International Journal of Mechanical

Doi 10.1186/S40712-016-0070-2

Extensive studies have shown that the airbag deploy- ment in load cases consists of two occupant loading phases: a punch-out effect where the airbag bursts out of its container with the airbag and airbag module cover accelerating towards the occupant and a second loading phase during which the airbag is taking on its deployed shape and volume (membrane-loading effect). Bankdak et al. (2002) developed an experimental airbag test system to study airbag-occupant interactions during close proximity deployment. The results provided insight for simulating the effect of inflation energy and mass flow on target response. Bedard et al. (2002) found that while left-side (driver-side) impacts accounted for only 13.5% of all crashes, the fatality rate among these

Crashes Was 68.3% In Comparison To Front Impact

(48.3%), right-side impact (31.3%), and rear impact (38.4%). These studies underscore the importance of oc- cupant safety during side-impact collisions. In the last years, the current market requested to reduce the time and cost airbag development. In order to achieve this result, virtual simulations play an important role since they allow to minimize the number of experimental tests (Pei et al. 2013; Cao et al. 2014). Several simulation models of airbag were established (Wang et al. 2007). It is feasible to optimize the parameters of airbag deploy- ment using simulation technology. Experimental and numerical studies have quantified injury risks to close- proximity occupants from deploying side airbags. These studies have focused on the prevention of the most ad- verse effects of airbag deployment (Duma et al. 2003).

Other studies have proposed airbag characteristics to minimize particular biomechanical responses (Haland and Pipkorn 1996). In a more recent study, Marklund and Nilsson (2003) compared deformation patterns with experimental data as well as the computational costs associated with three different airbag deployment simu- lation methods; they concluded that the SPH method is relatively inexpensive and produces incremental deform- ation patterns that compare most closely to the experi- mental results. The process of inflation of an airbag is one of the determining factors in saving lives. The duration from the initial impact of the crash to the full inflation of an airbag is about 40 ms, and during this time, the airbag goes from being in a folded state to a fully inflated state, with a high internal pressure. After achieving this state, the airbag begins to deflate, thus providing a nice cushion for the body impacting it.

Ideally, the person in the crash should come into contact with the airbag at this time. In the present study, a large volume passenger side airbag model is developed to handle different collision scenarios. The main aim is evaluate the performance of deploying of passenger side airbag using finite element methods (FEM).

Materials

The tensile specimens were made in different airbags (P: Peugeot, R: Renault, and VW: Volkswagen) with a length of 200 mm long and a width of 40 mm. Table 1 shows the mechanical properties of the airbag.

Tensile Tests

To determine the mechanical properties of the material of airbag used in the test pieces, tensile tests were performed on Lloyd EZ20 universal testing machine in Constantine. These tests were conducted using rect- angular samples. The axial force and axial displacement acquired during a test are converted into stress and the strain in order to be used for the fabric material model.

The continuous recording of the stress-strain data was performed during both the load and unload phases. A minimum of five samples were made in order to check the repeatability of the measurements. All the data was collected by using a PC-based data acquisition system and analyzed by commercial software. The picture frame test device that is made for this study is shown in Fig. 2.

Fig. 1 a Frontal and side airbags. b Oblique view of facet occupant model in sitting posture following airbag deployment (Lim et al. 2014)

0.150

Bendjaballah et al. International Journal of Mechanical and Materials Engineering (2017) 12:12

Page 2 Of 9

Figure 3 shows the stress-strain relationship of the airbag sample under axial tensile loads. The results are showing a linear increase in extension with the increas- ing stresses. This is an expected output and it confirms with the theoretical behavior of a sample subjected to tensile stress. The rupture strain values for different airbags (R/P/VW) were 0.322, 0.441, and 0.472, respect- ively. The measured elastic parameters (i.e., Young’s modulus E and initial yield strength) and Poisson’s ratio are summarized in Table 2. The tensile tests of the woven fabrics can show differences on mechanical prop- erties because woven fabrics can resist in-plane shear loads once the yarn lock-up angle has been reached. The differences of material property on material direction can affect the shape of fully deployed bag (see Fig. 3b).

Theoretical Background

Numerical simulations of airbags use very complex and techniques such as an orthotropic model to identify the mechanical behaviors during the airbag inflation and the fluid mechanics (gas flow) to describe the inflator gas flow (pressure gradient) and improve the representation of the pressures within the airbag. To model the airbag as an orthotropic model, three material constants have to be provided. Assuming a plane stress condition, the

Ð1Þ

where σ is the normal stress and τ is the shear stress, the subscript refers to the principal material directions, i.e., the fill and warp directions. Also, ε and γ are the strain components. The material elastic constants Qij are

Ð2Þ

where E1 and E2 are the Young’s modulus in the fill and wrap directions and G12 is the shear modulus of the fabric material. νij is the Poisson ratio of the material.

The gas exerts a pressure load on the airbag causing it to expand. This expansion puts the airbag under tensile stress lowering the expansion rate. In this study, heat conduction and heat transfer is not taken into account.

Fig. 2 A photograph of Lloyd EZ20 universal testing Fig. 3 Stress versus strain using Lloyd EZ20 machine for a three different airbags at 0° and 90° and b VW airbag test specimens at

Different Angles

Table 2 Physical and mechanical properties of the airbag

Page 3 Of 9

In the deployment of an airbag, an inflator supplies high velocity gas into an airbag causing it to expand rapidly. The gas inside the airbag is assumed to be ideal, to be of constant entropy, and to satisfy the equation of state:

Ð3Þ

Here p, ρ, and e are respectively the pressure, density, and specific internal energy, and γ is the ratio of the heat capacities of the gas. The gas flow is described by the conservation laws for mass, momentum, and energy that

Ð4Þ

here, V is a volume, A is the boundary of this volume,

N Is The Normal Vector Along The Surface A, And U

denotes the velocity vector in the volume. Applying Bernoulli’s equation in the case of an ideal gas with

Ð5Þ

Here, the subscript ex denotes quantities at the throat of the tube. Furthermore u, p, and ρ denote the quan- tities inside that part of the tube that is supplying mass.

Materials And Boundary Conditions

The airbag system mainly consists of three parts: the airbag itself, the inflator unit, and the crash sensor or diagnostic unit. Thus, to study the behavior of the airbag using FE simulations, we need to have an FE model of the airbag in the folded position. A FE model of the airbag was used to simulate the test condition as shown in Fig. 5. LS-DYNA® material model FABRIC (MAT_34) is used to simulate the airbag material. It is a variation of the layered orthotropic material model. Additionally, in the LS-DYNA® material model, fabric leakage can be accounted for. However, for this CAB material, the leak- age is almost negligible and therefore no leakage is specified. The mechanical properties can be determined from the physical test. Typical material properties for airbag fabrics are taken as given in Chawla et al. (2004a) (Table 3). These properties are used to simulate inflation process of airbag (see Table 1). The car dashboard is modeled as the rectangular thin plate using a MAT_RI-

Gid Material, And The Degrees Of Freedom Are Con-

strained in all the directions. The similar properties of thermoplastic polymer are assigned for contact purposes. The porosity of the fabric is assumed zero. The nitro- gen gas is taken for inflating the airbag. Properties of nitrogen gas and initial bag conditions are shown in Table 4. The example on which we perform the study is a typical passenger side airbag. The geometric de- tails have been measured from a commercially avail- able airbag. The initial state of the airbag is a closed rectangular whose sides are to be finished to 482 × 635 mm2 and is shown in Fig. 4.

Table 3 Material properties of airbag and rigid plate used in FE

–

Table 4 Initial values used for FE simulation of the swelling of

3.33 × 10−4

Fig. 4 The initial airbag geometry in the form of a rectangular Bendjaballah et al. International Journal of Mechanical and Materials Engineering (2017) 12:12

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