Hydrogen powered aircraft : The future of air transport

Progress in Aerospace Sciences ] (]]]]) ]]]–]]]

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Hydrogen powered aircraft : The future of air transport Bhupendra Khandelwal n, Adam Karakurt, Paulas R. Sekaran, Vishal Sethi, Riti Singh Department of Power and Propulsion, Cranfield University, Cranfield, Bedfordshire MK43 0AL, UK

a r t i c l e i n f o

Keywords: Cryoplane Hydrogen aircraft Future aircraft Hydrogen combustor Micro-mix combustor Hydrogen tank

abstract This paper investigates properties and traits of hydrogen with regard to environmental concerns and viability in near future applications. Hydrogen is the most likely energy carrier for the future of aviation, a fuel that has the potential of zero emissions. With investigation into the history of hydrogen, this study establishes issues and concerns made apparent when regarding the fuel in aero applications. Various strategies are analyzed in order to evaluate hydrogen’s feasibility which includes production, storage, engine configurations and aircraft configurations. & 2012 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Historical review of hydrogen aircraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Hydrogen production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Aircraft and fuel tank configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.1. Liquid hydrogen storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4.1.1. Tank configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1.2. Tank shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1.3. Tank insulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.1.4. Tank volume. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.1.5. Insulation material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.1.6. Multilayer insulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.1.7. Vacuum insulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.1.8. Foam insulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 5. Hydrogen aircraft configurations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 6. Hydrogen aircraft engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 7. Hydrogen combustors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.1. Lean Direct injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 7.2. Micro-mix combustion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 8. Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 9. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1. Introduction Abbreviations: AAN, army after next; CH4, methane; CO, carbon-monoxide; CO2, carbon-dioxide; GE, general electric; GH2, gaseous hydrogen; GWP, global warming potential; HALE, high altitude long endurance; H2O, water; LDI, lean direct injection; LH2, liquid hydrogen; MLI, multi-layer insulation; NOx, oxides of nitrogen; O3, ozone; UAV, unmanned aerial vehicle; UHC, unburned hydrocarbons n Corresponding author. Tel.: þ44 7411108151. E-mail address: [email protected] (B. Khandelwal).

According to leading experts the aviation industry is expected to grow continuously, at a rapid pace in the coming few decades which is shown in Fig. 1 [1–3]. Commercial sectors are projected to increase in the order of 5% and more than that for cargo transportation, despite the downturn in the current world

0376-0421/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.paerosci.2012.12.002

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Fig. 1. Expected growth in aviation industry.

Table 1 GWP regarding altitude [5]. Altitude (km)

GWP (CO2)

GWP (H2O)

GWP (NOx)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.24 0.34 0.43 0.53 0.62 0.72

 7.1  7.1  7.1  4.3  1.5 6.5 14.5 37.5 60.5 64.7 68.9 57.7 46.5 25.6 4.6 0.6

Fig. 2. Key challenges for a propulsion system.

economy [1–4]. This increase is due to the tendency in developing countries now requiring additional travel and cargo. Hence aviation appears to be the fastest growing industry for the next two to three decades. Also, there is a requirement to limit the dependency on fossil fuels. Although opinions about the exact date of perilously low levels may vary but the supply of fossil fuels are expected to be exhausted sometime in this century. Innovations in propulsion systems have been the primary driver for the progress in air transportation. Due to advances in propulsion performance and efficiency, aircraft have the ability to travel at higher speeds over longer distances with the capability of carrying larger payloads [4]. Present air traffic contributes to about 3% to the anthropogenic greenhouse effect. This number may alter due to an increase of air traffic in the near future and additionally the strategy to decrease major CO2 producers of today will vary this figure as well. Fig. 2 shows the challenges involved with future propulsion systems. Pollutants and particles that are emitted into the environment have a negative effect on our global climate. Whilst pollution is created directly from the combustion of fuel it is also formed by power production and consumption. The power produced is sourced from energy in fuel that is extracted from reserves, refined and then transported. During the extraction, refinement and transportation pollutants are being discharged before the fuel has even been combusted.

Table 2 Radiative force (mW/m2) [6]. Year

CO2

CH4

H2O

O3

Contrails

Sulfate

1992

18

 14

1.5

23

20

 3.0

This is usually over looked when comparing typical carbon based fuels with hydrogen. Hydrogen is a suitable energy storage medium that is free of carbon and other impurities; it is also the most abundant element in the universe allowing it to be easily sourced. Hydrogen was enthusiastically studied during the last fuel crisis, with the current trend of increased fuel prices together with environmental considerations; hydrogen is again being examined as an answer for a long term energy solution. Although hydrogen cannot be the answer alone, it must be utilized with current day technology to truly go into operation. Emissions of CO2 and H2O produced during hydrogen combustion are contributors to global warming, changing the radiative balance of the planet [6]. The water vapor formed, impacts the environment contributing to the formation of contrails. Table 1 shows when comparing the impact of CO2 to H2O, the impact of H2O emissions are negligible below 10 km, above this altitude the effect is minute whilst additionally increasing with altitude.

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The green house effects of both pollutants are dependent on altitude. Although when considering the residence times of both molecules the depiction is quite different. CO2 has a residence time of over 100 years that is not subject to altitude [6]. Whereas water vapor is present for 3–4 days at sea level and up to 6–12 months in the stratosphere [6]. With a slight decrease in altitude the global warming effect of water vapor can be eliminated with cruise optimization [5]. Studies on the effect of contrails are quite limited whilst its effects on global warming cannot be disregarded, as seen in Table 2 where the effect of contrails present

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a larger radiative force than CO2 [6]. With a slight decrease in altitude the global warming effect of water vapor can be eliminated with cruise optimization. Although hydrogen and conventional fuels yield NOx emissions which are of major concern to researchers and designers. NOx emissions are a challenging pollutant to reduce. NOx is an acronym for oxides of nitrogen which are dependent on residence time and are produced at high temperatures. NOx formations are an endothermic mechanism that is produced substantially above 1800 K [7]. Hydrogen combustors have the capability to provide

Fig. 3. Evolution of hydrogen in aviation industry.

Fig. 4. Hydrogen production paths [21].

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extremely low NOx emissions without concern of CO2 emissions, as there are none, permitting for a near zero emission combustor that will be a solution to our climate concerns [8]. Liquid hydrogen has several advantages compared to other fuels. LH2 can be produced at a given rate because its primary source is water. As a result of combustion, H2 only releases water vapour and small amounts of nitrogen oxides, dependant on combustor design. Hydrogen has large energy content per unit mass which is imperative in aviation as this allows for a greater payload or increased aircraft range [6,8–10]. Hydrogen production can be based upon renewable energies, off grid optimization of power plants and syngas plants.

2. Historical review of hydrogen aircraft Hydrogen was used for the first time in aeronautics for the inflation of balloons. Earlier balloons flew using hot air as a lifting medium. On December 1, 1783 just two weeks after the ground breaking Montgolfier flight, the French physicist Jacques Charles and Nicolas Robert flew the maiden gas balloon using hydrogen. This model (Charlie re Hydrogen Balloon) was 26 ft in diameter was launched and carried two passengers [11]. Early in the 20th century, a German Count Ferdinand von Zeppelin pioneered a type of rigid airship known as A Zeppelin which was the first airship to fly with hydrogen as fuel. It was based on his design outlined in 1874 and was detailed in 1893 [12,13]. Figs. 1–5 shows the evolution of hydrogen in aviation as a fuel. Von Ohain is the pioneer, in using hydrogen as an alternate fuel for aero-derivative gas turbine. In 1937, he ran successfully a gas turbine fueled by hydrogen. It was tested in a rig and named as Heinkel-Strahltriebwerk 1 (HeS-1) experimental engine. The engine was a turbojet which produces 250 lb of thrust [13]. A couple of decades later, in 1956 Pratt and Whitney were asked by US Air Force to find the feasibility of liquid hydrogen fuel for aero engines. The research was done by modifying the J57 engine for hydrogen fueled injection. Further research work performed altitude test for 3 turbojet engines (J-47, J-65-B-3 and J71-A-11). As laboratory testing was not adequate to establish the reliability of using LH2 in aircraft, flight tests were executed establishing inflight performance statistics [14]. A modified J-65 turbojet engine with a separate hydrogen supply system was installed in a US Air force B-57 twin-engine bomber for the flight tests. It was the first aircraft to fly using liquid hydrogen pressurized with helium in one of its engines [15].

Since that time, the US has started several other projects like the CL-400 airplane, the US Space Program and the Space Shuttle Program all utilized liquid hydrogen. In the 1970s hydrogen as a fuel renewed its interest due to consequences of the oil crisis. Since the beginning of 1970s, several studies were carried out by General Electric (GE) and NASA to explore the procedure of hydrogen as an alternate fuel for use in gas turbines. GE evaluated unconventional cycles, using hydrogen for aircraft propulsion system [16]. In 1988, the Soviets tested hydrogen with a modified TU-154 aircraft (renamed TU-155), with one engine operating on hydrogen. During 1991, the Soviet Union and Germany announced their agreement to work together on liquid hydrogen fueled commercial prototype, which is similar to the A310 at an estimated range of 500 miles [15]. There are two different projects of cryoplane designs for subsonic aircraft adopted by NASA-Langley Research Centre and the Russian–German Cooperative Venture on the basis of an existing Airbus A310. The Russian–German Cooperative Venture established a design with hydrogen tanks on the top of the fuselage and small amount of fuel on the wings, which intently reduces wing size. The NASA project was to have two spherical tanks for hydrogen, in order to reduce the surface to volume ratio. It was designed for 400 passengers, at a cruise speed of Mach 0.85 and a range of 5500 nautical miles [17]. In 1998, the Army After Next (AAN) annual report states, ‘‘An absolute imperative exists to develop alternative fuels for AAN-era forces.’’ Furthermore, it is stated that ‘‘The development of hydrogen-based vehicles are a national necessity for AAN platforms.’’ In 2000, the European Commission funded a consortium of 35 partners from the aviation sector, led by Airbus Deutschland called CRYOPLANE project, for the system analysis of air craft fueled by LH2. The investigation aim was to have a strong platform for initiating large scale activity for the development of alternate fuel and the introduction of LH2 fuel for aviation. Different configurations of aircraft were studied and a transition of aviation fuel was investigated during these 26 months of study lead by Airbus Deutschland [18]. The focal matter of the project was to design a LH2 fuel system. The study concluded that for an equivalent amount of energy density, liquid hydrogen requires 4 times the volume of conventional aviation fuel. Hence, the fuel tanks must be 4 times larger when compared to conventional aircraft fuel storage. Due to this excessive surface area of the tanks, consumption of energy would increase from 9% to 14%. Overall operating costs of hydrogen fueled aircraft would increase from 4% to 5% based on fuel alone [18,19]. AeroVironment built and tested the world’s first LH2 powered Unmanned Ariel Vehicle (UAV) successfully during 2005 and is one of the pioneers in High-Altitude Long-Endurance (HALE) aircraft with development of a 50 foot wingspan Global Observer HALE aircraft on their merits. The prototype built, demonstrated the robustness and the practicality involved in enabling the concept of a Global Observer Operation System [20].

3. Hydrogen production

Fig. 5. Comparison of LH2 and kerosene regarding weight and volume [29].

There are several different processes of hydrogen production available broadly, divided into three categories named from renewable resources, nuclear energy and fossil fuel [21]. A summary of all the processes have been shown in Fig. 4. Various chemical methods have also been studied by researchers for production of hydrogen [22]. There are two main methods which are being used today for the production of hydrogen, from current technology to state of the art. For the majority (97%) of hydrogen production natural gas steam reforming is used [23]. This is

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mainly due to the economic benefit of production by this method, which is unlikely to change in near future. The other major production method is electrolysis of water. The major benefit of hydrogen production with electrolysis of water is use of almost all sources of primary energy. Production of hydrogen from sunlight offers large environmental benefits, especially if the cost of production could be decreased and efficiency of production could be improved [24]. Electrolysis of water is a process where water is decomposed into oxygen and hydrogen by forcing electric current flow through the water. Production of hydrogen by electrolysis could be renewable or non-renewable depending on the source of electricity production. Electricity could be produced from renewable sources as solar, wind, geo-thermal or from non-renewable sources as gas, oil and coal. Normally hydrogen production by fossil fuel is the most efficient method, considering hydrogen production and delivery. The overall efficiency is estimated around just over 50% for fossil fuel, 40% for biomass and 53% for gases [25]. Sarigiannis and Kronberger [26] studied different renewable energy based methods for the production of hydrogen based on Life Cycle Assessments of technology. Production of electricity to fabricate hydrogen was considered being produced from renewable sources. They concluded that low emissions could be achieved by using wind and hydropower energy sources even for long distance transports. Production of hydrogen by biomass can also result in low emissions on the condition that biomass is produced locally for avoiding transportation. For production of hydrogen through electrolysis, large amounts of desalted water and electricity would be required. Kronberger [27] studied production of hydrogen by electrolysis. He found that for producing 50,000 kg/day of hydrogen, 105 MW of electricity and 28 m3/h desalted water would be required considering an efficiency of 80%. At the same time a liquefaction plant would consume 25,400 kW of electricity for main electrical power, 155 kW for control electrical power; for an output of 50,000 kg/day of hydrogen [28]. Kronberger [27] also studied uses of biomass for hydrogen. He found that for producing 50,000 kg/ day of hydrogen 490,000 kg of dry biomass would be required resulting in 179  106 kg biomass per year. Sevenson [24] compared the amount of energy, in terms of biomass, that would be required to power all aviation refueling in Sweden with hydrogen, with the potential of biomass supply in Sweden. He concluded that the amounts required for aviation are not unreasonably large. However, it requires that the biomass use would be enlarged. Sevenson [24] studied the potential of reducing the environmental impact of civil subsonic aviation by using liquid hydrogen. In his study he considered changing a fleet of aircraft to hydrogen based aircraft in Sweden. He predicted that the amount of electrical energy needed to power all aircrafts refueling with liquid hydrogen in Sweden in 2050, produced by electrolysis of water, would be about 20 TWh. This would be about 12% of the net electricity supply in Sweden in year 2000. In 2050 the electricity production would be substantially larger than in year 2000 and this gives an indication of the magnitude of electricity required for hydrogen production. His study has also considered the growth in aviation till 2050. One possible way to obtain that would be for the aviation industry to support the development of renewable energy sources. Another way to produce hydrogen would be to use power plants during off peak load hours. Production of hydrogen by fossil fuel is a good option, but it again leaves the problem of emissions and depleting fossil fuel resources [30]. Looking at the short term goal and cost effectiveness, hydrogen produced from fossil fuel reformation is an effective method. Under this transition phase it might be reasonable to employ this method to reduce production costs, particularly if the CO2 is extracted and sequestered in reservoirs or

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utilized [24]. Electrolysis of water and gasification of biomass are promising technologies for future. Considering the progress in research and development of hydrogen production methods it can be said that in the long run there are likely to be sustainable hydrogen production methods. Hydrogen productions in photochemical and photo-biological systems using sunlight are examples that probably will offer large environmental benefits in the future if successfully developed [6]. It is clear that civil aviation with hydrogen will not necessarily indicate that emissions of greenhouse gases are eradicated, since greenhouses gases may be released during the hydrogen production. Production of hydrogen by nuclear energy is also a substantially important production method for low emissions.

4. Aircraft and fuel tank configurations Hydrogen contains 2.8 times more energy than kerosene which is shown in Fig. 5. But this advantage will be compensated to some extent because the volume of hydrogen will be 4 times more than kerosene as shown in Fig. 5. For airborne application, hydrogen must be maintained in a liquid state (LH2, 20 K approximately). The cylindrical or spherical LH2 tank needs to permit the differential pressure and insulation requirements. This results in non-conventional configurations of tanks [19]. 4.1. Liquid hydrogen storage Though the timing remains debatable for depletion of fossil fuels, it is possible that in the 21st century there will be an urgent situation for alternate fuels. Primarily, the key factors act as a hurdle in using LH2 for airborne application, the issue is the storage tank as it has four times the volume compared to kerosene which in turn increases the weight of the tank [32,33]. Even nowadays light weight, durable and insulated LH2 storage tanks face the crucial challenges in utilizing LH2 for airborne applications [34,35]. According to Zuttel [34], LH2 has been used in space missions as of its high volumetric density (70.8 kg/m3). Even though LH2 tanks are presently being used for space missions, the same tank cannot be used for aviation application. In a space vehicle, the LH2 tanks have very short lifetime with very high fuel consumption rate and approximately the boil-off is 1.6% of LH2 by weight per hour. Since the cycle of LH2 is short, a higher boil-off rate is accepted. 0.1% by weight per hour or less is an acceptable rate of boil-off for aircraft application [33]. Hydrogen has the ability to provide a clean, reliable and affordable energy supply that can reduce the impact on the environment [35]. At room temperature, hydrogen will be in a gaseous form. But at atmospheric pressure, H2 can be maintained at liquid state under 20.4 K which is the cryogenic temperature below the critical temperature 33 K, 1.29 MPa [36]. Table 3 gives the comparison between compressed hydrogen gas and liquid hydrogen properties. Table 3 Comparison of compressed hydrogen gas with LH2 properties. Properties

Compressed hydrogen gas

Liquid hydrogen

Operating pressure Boil-off Cooling capacity Volume Tank cost Insulation Hydrogen permeation Liquefaction process Volumetric capacity

High Moderate Less Medium Medium Medium High Not required 0.030 kg/L

Low High High High High High Medium Required 0.070 kg/L

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Usually LH2 storage tanks are preferred to be a thin wall pressure vessel with an operating pressure inside the tank varying from 0.1 MPa to 0.35 MPa which reduces the mass of the tank [31,37]. Hydrogen posses a tremendous amount of energy, but it is also an excellent heat sink for component cooling. Hydrogen cooling capacity is about 4.9 times the cooling capacity of Jet-A and about 2.8 times the cooling capacity of Methane. LH2 tanks are directly dependent on the airframe and the propulsion system configuration. The airframe imposes constraints on the shape and diameter of the tank. The propulsion system contributes with requirements of LH2 for a particular mission and impact the tank length. Hence a detailed tank design has to be considered and trade off studies should be carried out. A secondary problem with LH2 storage tanks is mass of boil-off. To compensate the mass of boil-off the tank volume, weight and tank cost is affected. Boil-off must be minimal or eliminated for safety and cost effectiveness. Volumetric capacity of LH2 tanks decreases because of insulation which is required to avoid boiloff. 4.1.1. Tank configuration The key driver in tank configuration will be to maximize the hydrogen available in a lightweight and low boil-off system. Storage of hydrogen can be of pressurized gas, as a hydride or in as liquid hydrogen (LH2). But liquid hydrogen has been considered because of its properties given in Table 3. Other forms of storage forms appear to be impossible for airborne application because of their excessive weight or volume [11,33]. Fig. 6 shows the tasks to be considered during selection of tank configuration. 4.1.2. Tank shape LH2 has a very low density, with respect to other liquids which results in its larger volumes. Hence the weight of LH2 required to

be carried on-board for a particular mission will be less than any other cooling fluid. The tank shape depends upon several issues mainly, fuselage and the type of tank matters a lot. Non-integral tanks act as a fuel container and have to be mounted in a conventional fuselage/skin/frame structure. Hence, this type of configuration has to resist the loads associated with fuel containment. In this method, the tank is kept outside the fuselage. The main constraints in designing non-integral tanks are the aerodynamics effect and the integration problem. Fig. 7 shows an aircraft with non-integral tank configuration. Integral tanks forms as an integral part of the airframe structure. Hence, it should resist stresses such as fuselage axial, bending and shear forces resulting from aircraft loading. The main constraint in designing an integral tank is that the fuselage drives the geometry (diameter) of LH2 storage tank. Fig. 8 shows an aircraft with integral tank configuration. Integral tank configuration seems to be only feasible configuration for wide body aircraft. In Cryoplane project, the tanks were kept over the fuselage and across the wings. This gave the chance to increase the LH2 carrying capacity but this lead to a thick and heavy tank wall and many stiffeners since the tank shape was not adopted well according to the pressurization requirement. Therefore, possible shapes are spherical and cylindrical tank shapes with diameter equal to the fuselage diameter [38]. Spherical LH2 tanks are being used in space application because it requires less surface area for the given volume. The boil-off rate is less because there is lesser passive heat flux into the spherical tank shape. Given that advantages provided by spherical tanks it has a problem in manufacturing and it has a higher frontal area compared to cylindrical shaped tanks [33]. On the other side it is easier to manufacture a cylindrical tank shapes but the drawback it has higher surface area to volume ratio which results in higher passive heat load into the tank. Cylindrical tanks are easier to be integrated inside the fuselage and they give higher volumetric efficiency. Hence, the space inside the fuselage can be used in an optimum way [11]. The problem with cylindrical shape tank is that the pressure inside the tank is not equally distributed. To overcome the problem the tank has been designed to have end cap which is semi-sphere on both the ends. Fig. 9 shows the cylindrical tank with an end cap. This type of tank has the advantage of a sphere as well as the cylindrical shape. Hence, the cylindrical tank with end cap has been adopted for further work.

Fig. 6. Vital factors for selection of tank configuration.

Fig. 8. Integral tank [18].

Fig. 7. Non-integral tank [18].

Fig. 9. Cylindrical tank with end cap as semi-sphere.

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4.1.3. Tank insulation There are two types of insulations that can be applied for a tank. They are internal insulation and external insulation. In case of internal insulation, the insulation is exposed always to LH2 which is at cryogenic temperature but due to heat transfer effect the LH2 changes its state to GH2. The GH2 causes diffusion into the tank wall which will inturn increase the thermal conductivity of the insulation, thereby crippling its effectiveness. Another problem involved with this insulation method is the system must be impermeable to GH2 [11]. When the tank is insulated on the outside then it is known as external insulation. In this case, primarily there will be an expansion and contraction of tank, as LH2 is filled and utilized. Secondly, there is a problem of attachment because of structural support system and the dimension of the tank increases as well. External insulation is easily subjected to mechanical damage and

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it has to withstand the impact load [11]. In the Cryoplane project, Air Liquide also adopted the external insulation method [38]. But these drawbacks can be solved when compared to the problems with internal insulation system. Hence, the external insulation system is being used for further work because of advantages in external insulation. 4.1.4. Tank volume The amount of LH2 required to carry onboard depends upon the mission of the aircraft. Once the mass of LH2 required is estimated, tank volume can be determined straightforward. Hence, depending upon the requirement of mass of LH2 to be carried the tank has to be designed accordingly. During the design phase, a trade-off can be done between the length of tank and available space on the fuselage to fit the tank to carry the same mass of LH2. 4.1.5. Insulation material The key challenge involved in LH2 storage is mass of boil-off which leads to loss of hydrogen. Boil-off is the phenomena that occur when liquid boils and changes its state into its gaseous form because of heat transfer and escapes by permeation. LH2 boil-off depends upon thermal insulation and tank geometry [39,40]. Tanks need to be equipped with effective insulation in order to minimize boil-off [41]. The design parameters for LH2 storage tanks are determined by LH2 temperatures, operating pressure and insulation thickness. There are three types of insulation methods available which are shown in Fig. 10.

Fig. 10. Different types of insulation method.

4.1.6. Multilayer insulation Multilayer insulation system uses a number of thermal radiation shields perpendicular to the direction heat flow. MLI usually consists of reflective foil over the outer side of inner tank wall to minimize the transport of radiation heat. Fig. 11 shows an example of multilayer insulation. Generally, the radiation shields are alternate layers of metal foil and a thin insulating material like glass fiber, polyester etc. to avoid metal to metal contact. With increase in number of heat shields, additional heat transfer takes place due to conduction. The optimal numbers of layers that can be used are about 60 and 100 layers. MLI insulations performance depends on the pressure and type of residual gas in the insulation. The thermal behavior of MLI degrades quickly for pressures higher than 0.001 mbar [32]. MLI insulations are very sensitive

Fig. 11. Multilayer insulation system [42].

Attachment to airframe

Level probe

2.0 in. vent line Centre support post Gusset

MLI insulation Vacuum jacket 102.4 in. diameter aluminium tank, 0.02 in. thick

0.5 in. liquid withdrawal

Fig. 12. Vacuum insulation system [43].

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to the layer density so that local compression must be avoided during manufacturing. The most important parameter for an insulation system to be used in aeronautical applications is low thermal conductivity, low emissivity and a low density. Overall the finished product of a LH2 storage tank with MLI layers will be heavier in weight. 4.1.7. Vacuum insulation The vacuum process seems to be a perfect solution for minimizing the mass of boil-off. But practically it is impossible to attain a vacuum and therefore venting equipments are necessary in the vacuum region [31]. The interaction between LH2 and the air has to be avoided by sealing materials to prevent air entering and freezing inside the tank system. If the air freezes in the flow lines, LH2 flow will be blocked [31]. Fig. 12 shows an aluminum vacuum jacketed insulation tank. The tank wall thickness must be sufficient enough to withstand the buckling, since the vacuum jacket is subjected to external pressure. Additional stiffeners are required between the (vacuum jacket shell) outer wall and the inner wall which increases the weight of the tank [43]. External equipment is required to suck the air and maintain the pressure at the vacuum chamber. The Vacuum insulation technique is adopted in space applications for the storage of LH2. Several research activities have taken place for vacuum insulation and it seems to be a promising solution for a LH2 storage tank. This type of concept is well established but heavier tank walls are required, which are expensive to implement and to maintain the temperature and pressure in the vacuum [35]. 4.1.8. Foam insulation Generally, the materials used for foam insulation have very low density and thermal conductivity. A schematic view of foam insulation is shown in Fig. 13. The rigid foam insulation is applied outside the inner tank wall and a thin metal wall required to be surrounded around the foam to maintain its structural stability to withstand and protect it from external forces [44]. The foam insulation concept is more resistant to catastrophic failure than

the vacuum-jacketed insulation [39]. The insulation thickness of the tank depends upon the insulation material properties, ank size, allowable boil-off and overall allowable tank weight. Foam insulation is low cost, easy to implement and light weight. Vacuum-jacketed and multilayer insulation has been investigated for quite some time and in the case of loss of vacuum it might cause catastrophic failure whereas in foam insulation the chances of catastrophic failure are less.

5. Hydrogen aircraft configurations Hydrogen powered aircraft must comply with some practical configurations so that the typical performance and handling requirements of airline operation could be met. This has to be done without undue need for infrastructure and ground equipment inconsistent with current aviation industry. Sefain [23] studied various configurations of hydrogen powered aircrafts to incorporate large LH2 fuel volume with minimum penalty and optimum performance benefits. In his study medium range aircrafts were considered. A team of researchers and experts worked in this study to work-out several configurations. Twin Tail-Boom and Tail-Tank Concepts as shown in Fig. 14 are selected as most appropriate configurations from a pool of various configurations studied. It is proposed that in Twin-Boom configuration external slender booms were utilized as hydrogen fuel tanks and also as a structural booms interconnecting the wing and tail surface together. In a Tail-Tank configuration, a tank was placed above the fuselage, physically separated it from the aircraft. Tank is interconnected by an above-fuselage pylon and the tail plane. Three different concepts have been shown in Fig. 16 which explains the different configuration of aircraft for LH2 storage. In Fig. 15(a) and (b) hydrogen tank storage has been shown at the top and the end of fuselage, whereas in Fig. 15(c) it is proposed that hydrogen is stored in front and end of the fuselage [18].

6. Hydrogen aircraft engine Inner wall

Insulation foam

Outer wall

LH2 Storage Tank

Fig. 13. Foam insulation tank [44].

Hydrogen has a tendency to flash back and produce high temperature flames which in turn lead to higher NOx emissions. Fig. 16 represents a comparison between Hydrogen and Kerosene flame stability limits. Whilst Hydrogen has a much higher temperature at its stoichiometric ratio, it can burn stable at significantly leaner ratios. Leaner equivalence ratios will attain low temperature flames avoiding the high temperatures associated with stoichiometric conditions. With leaner equivalence ratios the mixing intensity is required to increase, as to eliminate local hotspots and to enable the fuel to effectively mix producing a balanced flame profile. Burning the fuel requires changes in the combustor to avoid high temperatures and to provide effective mixing to take full

Fig. 14. Twin tail-boom and tail-tank concepts [23].

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Fig. 15. Hydrogen aircraft with different hydrogen tank configurations [18].

Fig. 16. Temperature characteristics hydrogen and kerosene [46].

advantage of hydrogen’s attributes at leaner conditions. Although if we take into account hydrogen’s attributes such as its high flame speeds, its low temperature from its cryogenic state and its major concern of flashback with flame propagation then we must consider systems to establish redundancy and reliability. Considering hydrogen’s journey right from the cryogenic tanks, from startup it can be given that the fuel lines are unquestionably void of H2 due to its likely hood to escape over time and there will most likely be ambient air present. With ambient air present in the fuel lines a large risk of flask back is presented immediately, during the start up cycle of the engine. To eliminate this risk, the fuel lines must be purged with an inert gas, cheaply nitrogen can be used to flush the lines [45], although the solidification of gases via liquid hydrogen must be considered which will cause fuel flow issues. This will also be required during shut down, flushing the fuel lines will eliminate this flashback risk entirely, Dahl and Suttrop [45] have tested this method proving its reliability. Although before the fuel has even entered the combustion chamber it must be preheated to ensure the

fuel has fully vaporized under its maximum flow rate from the cryogenic storage tank. To perform this effectively and safely a heat exchanger is required to provide extra energy. Placing fuel lines in hot sections of the engine are not recommended as fuel leaks will immediately ignite causing a large flammability risk. A heat exchanger will transport heat away from hot sections of the engine, be it the exhaust spokes, turbine section, combustion chamber or the high pressure compressor stage. Taking heat away from the hot engine sections will aid in reducing the amount of energy required for combustion, ensuring the fuel is completely in a gaseous state before injection. This will provide more advantages increasing thermal efficiency, increasing component life and resilience to temperature abuse, taking full advantage of the heat sink potential. During start up the engine will require an electrical heater to provide an increase in fuel temperature and once the engine has reached idle rotational speed the heat exchange can resume operations. A metering system will be required to alter the liquid and gas fuel flow rate under different power conditions of the engine.

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A metering system was also described by Dahl and Suttrop [45] that provided a reactive hydrogen feed to the engine under fluctuating power conditions.

7. Hydrogen combustors Although the combustion of hydrogen is more complicated than just an adjustment of the air to fuel ratio and is predominantly dependent on combustor geometry [47]. The use of hydrogen addition to conventional fuels produce improved results although pure hydrogen use in conventional combustors have results that are inferior to conventional fuels [45]. This is due to the combustors geometry being inadequate to effectively mix the fuel and air. When hydrogen is combusted in a conventional chamber large diffusive flames are formed where stoichiometric ratios appear around the flame causing very high temperatures and in turn high NOx emissions [48]. To elude inadequate combustion all flame attributes of combustion must be considered such as flame stability, combustion efficiency, acoustics and other vital diagnostics. For this reason studies have been performed to design and research novel combustor concepts to effectively combust hydrogen realizing its full potential. There are two concepts of hydrogen combustors that pose the most likely designs to be further adapted into combustor configurations. The two concepts are the Lean Direct Injection (LDI) investigated by NASA with Marek et al. [49] and the Micro-mix concepts investigated by Dahl and Suttrop [45]. Both designs have been proven as concepts with actual combustion tests performed. The two concepts are quite similar in their methodology. Both concepts have established that flashback is of primary concern with the desire to increase fuel mixing. The mixing intensity of hydrogen and air is greatly increased in both designs to avoid large diffusion flames forming that result in higher NOx emissions. By increasing the mixing intensity, the flame length will be reduced having completed combustion earlier with reduced residence time. As NOx is both dependant on temperature and residence time, increasing the mixing intensity will enable very low NOx emissions. It was established that the injection methods of H2 are crucial in acquiring effective combustion, for this reason further investigations have been performed to illustrate the attributes of alternative fuel injection. 7.1. Lean Direct injection The Lean Direct injection combustors were examined by Marek et al. [49]. The designs delegated, immediately incorporated flashback avoidance by utilizing a H2 inlet that is less than the fuels quenching diameter. Marek et al. [49] discussed how the hydrogen LDI combustors perform very well with results that are outstanding compared to advance Jet-A LDI combustors. The investigations concluded that a non-premixed fuel flow was preferred to avoid randomly confined flame contours and flashback risks. The injectors employ a small pre-mixing channel that is present after the injector. Various fuel injectors were examined during the LDI experimentation. Fig. 17 illustrates the injectors to be studied. Fig. 17(a) shows the NASA N1 injector. Each air inlet has two Hydrogen inlets of 0.5 mm diameter spaced 1801 apart. This design was established by Marek et al. [49] using the jet cross flow program. The remaining designs are established by reputable manufactures, either for rocket propulsion or low emissions Jet-A LDI. C1 shown in Fig. 17(b) was designed for rocket propulsion. The design has a centered cross flow of hydrogen jets that mix with the eight surrounding angled air inlets. C2 Fig. 17(c) is a similar design to the N1 injector although it has incorporated triangular

Fig. 17. LDI Injectors [49].

conduits to enable an additional H2 inlet. This should aid in increasing the mixing intensity of hydrogen into the main stream flow. C3 Fig. 17(d) is more of a conventional design having a single centered fuel injector. C4 also shown in Fig. 17(d) is modified to have no swirl velocity. This is to aid in reducing the pressure losses and the H2 inlet is replaced to have four radial injectors [48]. The concepts are very well thought concerning all aspects, from methods of combustion, to methods of producing the combustor. The mainstream airflow and fuel are kept entirely independent, eliminating all risks of flashback. Combustion tests were performed at the NASA GRC RCL-23 facility. This facility is capable of providing conditions that are present in gas turbines as to obtain reliable results. The combustion tests will examine the flame profiles, temperatures and propagation into the premixing channels. With sampling temperature probes the NOx emissions are determined for the different fuel injection concepts over various equivalence ratios [50]. The importance of rapid mixing was noted as hydrogen reacts seven times faster than Jet-A with higher temperatures at stoichiometric conditions. These high temperature conditions can occur whilst hydrogen and air are still unmixed at the injector faces which will cause extreme stresses leading to material failure. The data presented in Fig. 18 shows all the injectors studied compared to LDI Jet-A and well stirred reactors. It is immediately notable that the NASA N1 injector with a 3.5 in. liner has the worst NOx emissions, this is partially due to larger recirculation zones which in turn caused longer residence times allowing greater NOx production, it also has higher pressure losses than the 2.5 in. liner, up to 10% more. In its 2.5 in. liner configuration, more of a balanced result is present [49]. Injector C3 has relatively high NOx emissions but provides a safe, simple and durable injector design with results similar to Jet-A LDI. Whilst C4 has a design very similar to the configuration of C3, its NOx emissions are very low for temperatures below 2500 1F, it also has less pressure losses by removal of its swirler, moreover it is durable with an insignia of considerable mixing radially. C1 and C2 designs showed the best reductions of NOx more than half of use with Jet-A. Although the C1 injector failed during testing and could not be completely tested over the desired pressure range. C2 has a unique design that encourages very fast mixing. Though,

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Fig. 18. NOx emissions from all injectors studied [49].

it’s cooling and durability was compromised, leading to imminent failure resulting in uniform mixing and increased mixing intensity. All of the LDI tests that were performed by Marek et al. [49] were exceptional and very stable. The LDI method resulted in reduced NOx levels. With no flashback or auto ignition phenomena occurring. Taking into consideration complexity, durability and low NOx emissions, C4 performed exceptional, at low equivalence ratios below 0.3 very low NOx emissions are achievable with reduced complexity and also desirable considering manufacturing costs. This design is not suitable at temperatures above 2500 1F leading to an exponential increase in NOx emissions. If there are cooling improvements with reduced injection complexity in configurations C2 and C3 then their low NOx potential can provide a very effective LDI injector at higher temperatures that will be present for high power settings. 7.2. Micro-mix combustion Studies of micro-mix combustors were formerly performed by Dahl and Suttrop [45] establishing hydrogen combustion, demonstrating safe combustion of the fuel with an effort to minimize NOx production. The objective of the investigations was to convert an A320 APU GTCP 36-300 to function on Hydrogen safely [45]. The combustor design used for H2 operations was utilized on the fundamental of miniaturized diffusive combustion. Miniaturized diffusion increases the amount of local mixing regions. This enhances the overall mixing concentration of the combustor. Miniaturized diffusive combustion was established to counteract the results of H2 combustion with large diffusive flames, which result in very high temperature zones. As thermal NOx is dependent on residence time, the amount of oxygen and nitrogen present and temperatures that increases emissions exponentially above 1800 K. It is very important to avoid these conditions in combustor design. The micro-mix combustor avoids this by the use of heterogeneous mixing, which generates numerous small diffusive flames. This is done by utilizing miniaturized diffusion by having a large number of fuel/air inlets, whilst taking the standard of 3–4% DPD into consideration limiting the amount of fuel and air inlets. The improved mixing has taken advantage of turbulent formations and eddy breakdown reducing the local

Fig. 19. Micro-mix cross-section [48].

residence times of the flame and avoiding stoichiometric conditions [51]. A schematic of a micro-mix combustor is shown in Fig. 19. Numerous tests were performed to establish the NOx emissions for micro-mix combustion over various equivalence ratios. The investigation also examines conventional combustors with kerosene and hydrogen combustion to establish a benchmark. The combustor used in the A320 APU is shown in Fig. 20. Examining the results it is clear that interesting data has been produced by Dahl and Suttrop [45]. The largest NOx emissions were acquired through hydrogen combustion. Although with data from Fig. 21 high temperatures will occur when hydrogen is burnt at stoichiometric conditions. To conclude this phenomenon stoichiometric conditions are present in conventional diffusive combustion of hydrogen. Whereas the micro-mix results, which primarily avoid stoichiometric conditions by improving the local mixing intensity

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has the lowest emissions of NOx. The success of this test was established by Dahl and Suttrop [45] utilizing rapid mixing of hydrogen, stressing heavily on safety issues of flashback establishing measures for safe ignition of H2 fuel. Further investigations were performed by Dahl and Suttrop [45] examining the performance of various configurations of micromix combustion. Dahl and Suttrop [48] were able to show further reductions of NOx are possible. This is accomplished with modifications to micro-mix combustion. This incentive lead to conceptual micro-mix injector designs to be examined with computational fluid dynamics at Cranfield University. Other micro-mix injectors have also been studied with various inlet geometries [8,53]. Fig. 22 displays the three different models of varrious micro-mix injectors studied by Murthy et al. [52]. Model 1 is a conventional micro-mix combustor which has much lower NOx emissions than hydrogen combustion in conventional cans. Although it is clear from Fig. 22 that high temperature hot spots are present in the flame profile. These high temperature zones are primariliy responsible for the thermal NOx productions. Further improvements of injectors have moved the H2 injector locations along the main flow conduit as seen in model 2. Lower NOx emissions are present with this design, although critically the

Fig. 22. CFD analysis of micro-mix injectors [52].

high temperature location is at the injector face. This will present a large potential risk. The high temperatures at the injector of model 2 will ineveitably lead to failure resulting in a large flammability risk. Utilizing a frustum incoroprated into the design is present in model 3. The small hot spot is now present further down stream in the flame profile. This will enable a relible and durable ocombustor for further experimentation. The NOx emissions from this design are substantially reduced 80% lower than model 1 [52].

8. Safety

Fig. 20. A320 APU GTCP 36-300 combustors [47].

Fig. 21. Comparison of NOx emissions with a modified [48].

Hydrogen has been shown to be a very advantageous fuel for the future of aviation, enabling reduced emissions whilst providing exceptional performance. The fuel is flammable like any other fuel, although there are certain folklore associated with hydrogen that entail the fuel to have a high risk quality, ‘‘images of the Hindenburg and the hydrogen bomb often cloud meaningful discussion of hydrogen’s safety as a fuel’’ [53]. In most cases a comparison of fuels will show hydrogen to be the safest and least devastating. Concerns associated with the fuels that are pressing is hydrogen persistence to escape from enclosures. This is unwanted for obvious reasons as this is a primary loss of energy. Whilst the slow dissipation of hydrogen does not pose a large flammability risk when correctly ventilated. This is due to the fact that hydrogen is the lightest element, allowing it to be quickly dissipated as it rises into the atmosphere. This is very similar when fuel leaks occur. Investigations performed at the University of Miami [53] have compared a hydrogen fuel leak with a kerosene fuel leak. Both fuels are ignited after a certain period of time, enabling the fuel to spread. Whilst kerosene is a liquid that will pour and fill out as much space as possible, hydrogen is a gaseous fuel that will be localized to its leak, rising into the atmosphere in a controlled manor. The investigations done by the University of Miami are seen in Fig. 23 with a hydrogen fuel leak seen in the left vehicle and a kerosene leak with the vehicle on the right. The two images above show that the hydrogen leak will continue to combust in a controlled manner until the tank is empty. Whereas the kerosene leak has engulfed the entire vehicle after one minute. If this test was performed on aircraft the kerosene fire would have an extremely high risk with likely loss of life. Whilst the hydrogen aircraft would still remain intact only requiring maintenance on the fuel tank and localized structure. This enables the aircraft to return to service faster and cheaper, when otherwise

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Fig. 23. Flammability comparison of hydrogen and kerosene [54].

Fig. 24. Danger zone of spilled liquid gas [55].

with kerosene it would be economically unfeasible to restore such detrimental damage. Although a large hydrogen liquid leak will spread more than a gas leak, but it still will be the least hazardous when compared to other fuels as seen in Fig. 24. Hydrogen is a much safer fuel than conventional fuels when provided with the effective systems. As previously described fuel metering systems and fuel purging during startup and shutdown are required to enable effective and reliable combustion. Purging is a very important system that must be incorporated with gaseous fuels avoiding all risks of flashback. Like any other fuel the location of fuel lines must be delegated with care. Ensuring that the fuel pipes are far enough from areas of high temperatures and any other possible ignition source. The most effective way to fight a hydrogen fire is to shut off the flow of gas. If it is necessary to extinguish the flame a dry powder extinguisher is recommended [53]. However, if the fire is extinguished without stopping the flow of gas, an ignitable mixture may form, creating a more serious hazard should reignition occur from the hot surfaces or other ignition sources. The usual firefighting practice is to prevent the fire from spreading and let it burn until the hydrogen is consumed. Dry powder fire extinguishers should be available in the area. A fire blanket should be conveniently located also. The local fire department should be advised of the nature of products handled and made aware of the best known methods for combating hydrogen fires. There are several other studies [56–63] which have been done on use of hydrogen in aircraft.

9. Summary Hydrogen is the most likely energy carrier for the future energy economy. Hydrogen provides a clean energy system that will supply energy for mobile applications. There are no issues of

CO2 associated with hydrogen combustion and PEM fuel cells. This allows all other pollutants to be eliminated from backup generators and power production etc. Hydrogen combustion has emissions of only H2O and NOx which must be considered. NOx has the most potential to cause damage to the environment, whilst water contrails can be reduced easily by decreasing altitude. Although with hydrogen as a energy carrier the fuel must be stored in a liquid cryogenic state to attain a reasonable energy density. This provides one of the largest challenges with the fuel. Further investigations are required to establish the most feasible materials and insulation that provide the ability to contain high pressures for ground and aero applications. As weight is a very important issue with aero applications the design of the aircraft and cryogenic storage of hydrogen must been established with the least amount of boil off for the minimal weight. The NOx emissions are a major factor and must be considered. The only way to reduce the NOx emissions with hydrogen combustion is to improve the local mixing intensity. This is dependent on combustor design and the equivalence ratio of the fuel. As seen in Fig. 16 we can burn hydrogen at significantly leaner ratios. Although if we combust H2 at low equivalence ratios hot spots will still form in the flame profile if the mixing intensity is inadequate. The LDI and micro-mix concepts of combusting H2 provide the best methods avoiding flashback, having the fuel and air feed to the chamber along separate paths and utilizing small but rapid mixing channels proving an effective solution. There are risks associated with hydrogen like any other fuel. Although as hydrogen is a gas it will react differently to liquid fuels. If a leak has ignited in an aircraft the flames will be localized to the leak and the flames will rise. This makes hydrogen a very safe gas removing the large fire risks associated with conventional fuels that will flow to the ground and consume the vehicle spreading in all directions. This gives the incentive to locate the fuel tanks above the passengers as to further protect them from fire. Propelling our future aircraft by hydrogen is a viable option considering various constraints.

10. Conclusions It is a reputable consensus that for a long term future solution to environmental concerns and energy dependency, hydrogen is the only truly viable option. Whilst hydrogen combustion only regards NOx and water vapor. Water vapor that escapes high into the atmosphere will contribute to global warming; its effect in aero applications can be eliminated by reducing the altitude of aircraft producing no contrails. NOx emissions on the other hand are a very difficult pollutant to reduce. The primary properties of hydrogen enable fuels to be combusted stably at leaner equivalence ratios, reducing the residence times. Thus reduced emissions are acquirable with hydrogen combustion.

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Hydrogen is no stranger to aircraft it has successfully been used in flight test such as the Hes1 tests and Tupolev TU-155 tests. All flight tests performed were successful with no complications. Although it was established that whilst hydrogen has a very high specific energy its energy density is quite low. This required cryogenic storage during flight tests, maintaining the fuel in a liquid state. It was recognized that to enable hydrogen use effectively in aviation, viable cryogenic storage is required to provide low weight, whilst concerning boil off and storage densities. Further hydrogen investigations have shown viable cryogenic strategies enabling current aircraft to incorporate small tanks to take advantage of hydrogen addition, reducing emissions of current engines. Whilst more elaborate strategies show full modifications to conventional aircraft and also unconventional future aircraft concepts. Further studies with cryogenic storage are required to establish light structures that are safe, capable of withstanding high loads experienced during take-off and landing. Once the storage configurations and materials of insulation are established the combustive performances must be regarded, reducing NOx emissions as much as possible. The two most viable methods of hydrogen combustion for gas turbines are LDI and micro-mix methods. Both of these concepts establish improved local mixing intensities that enables lower equivalence ratios to combust absent of local hot spots that would otherwise form in the flame profile resulting in high NOx emissions. The two concepts have been tested and show effective methods of eliminating flashback risks associated with hydrogen’s high flame speeds. Whilst flashback can be avoided with suitable combustive modifications, fire risks must be considered. Although hydrogen when compared with other fuels is regarded to be especially safe. This is because hydrogen will quickly dissipate into the environment avoiding ignition. When fuel leaks are ignited the flames of a hydrogen fire will be localized to the leak, whereas kerosene fuel leaks will spread engulfing its storage vehicle.

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Please cite this article as: Khandelwal B. Hydrogen powered aircraft : The future of air transport. Progress in Aerospace Sciences (2013), http://dx.doi.org/10.1016/j.paerosci.2012.12.002i