Potential analysis of hydrogen storage systems in aircraft design

The substitution of fossil fuels with renewable energy sources such as hydrogen is a decisive factor in making aviation environmentally compatible. A key parameter for the use of hydrogen is the storage system. In the design of a flight-capable storage system, not only the mass but especially the volume of the hydrogen has to be considered. Therefore, in this paper different techniques are compared and evaluated from the point of view of their application in aircraft design. The analyses are performed on two reference aircraft, the Airbus A320 and the Embraer E 190, in the short- and medium-haul range. Simplified, it is assumed that the respective max. Take-off mass (MTOM) remains constant. The change of the necessary periphery has no influence on the MTOM. A tank concept could be designed, which can find applications in today's conventional aircraft design.     

Life cycle evaluation of hydrogen and other potential fuels for aircrafts

In the present study, hydrogen and some other alternative fuels (such as ammonia, methanol, ethanol, liquefied natural gas) are considered for aviation applications under a comprehensive life cycle assessment study and are evaluated comparatively with the conventional kerosene based jet fuel for various impact categories. Therefore, this study is performed with a well-to-wake approach to evaluate the overall life cycle of an aircraft running on these conventional and alternative fuels. Both conventional and renewable fuel routes are considered for the production of ammonia and hydrogen fuels. Although there are modifications required to fulfill the aviation fuel specifications for such alternative fuels, the long term viability and environmental sustainability make them attractive solutions for the future of aviation industry. This study uses a life cycle assessment of an average aircraft utilizing various alternative aviation fuels to determine the relative environmental impact of each life cycle phase. The life cycle phases included in the analyses are as follows: (i) production, operation and maintenance of the aircraft, (ii) construction, maintenance and disposal of the airport, (iii) production, transportation and utilization of the aviation fuel in the aircraft. The results show that hydrogen and liquefied natural gas represent more environmentally benign alternatives although fuel costs are higher compared to ammonia, jet fuel and methanol. The total GHG emissions from hydropower based ammonia and hydrogen are calculated to be about 0.24 kg CO₂ eq. per traveled tonne-km and 0.03 kg CO₂ eq. per traveled tonne-km, respectively. Renewable based ammonia and hydrogen fueled aircrafts can further decrease the overall environmental impact in many categories allowing a brighter future for aviation industry.     

Long range transport aircraft using hydrogen fuel

Hydrogen is since long seen as an outstanding candidate for an environmentally acceptable, future aviation fuel. Given that most comprehensive studies on its use in aviation were performed over two decades ago, the current article evaluates its potential as a fuel for long range transport aircraft at current and future technology levels. The investigations show that hydrogen has the potential to reduce the energy utilisation of long range transport aircraft by approximately 11%. The use of hydrogen namely allows a much smaller wing area and span since the wing size is not restricted by its fuel storage capacity. At a given price per unit energy content, the smaller wings lead to a reduction of around 30% in take-off gross weight and 3% in direct operating costs for a given fuel price per energy content. The hydrogen-fuelled aircraft are furthermore slightly more sensitive to a possible reduction in operating empty weight in the future and 20% less sensitive to further improvements in engine thrust specific fuel consumption.     

Parametric study on tank integration for hydrogen civil aviation propulsion

Hydrogen powered gas turbine propulsion will play a central role in the decarbonisation of civil aviation. A key challenge is the integration of large liquid hydrogen tanks into the aircraft, given the low density of liquid hydrogen. Hydrogen offers a quarter of the energy content, per unit volume and one third of the fuel weight, when compared to a conventional fuel. Optimising tank weight is seen as key to aircraft usefulness. A detailed evaluation of tanks for civil aviation is presented here, covering a very wide range of sizes and design solutions.For passenger air transport, if the choice is made not to vent, dormancy time (the time the tank can be allowed to operate without vapour or important fuel extraction) becomes a key design parameter. This paper highlights the interdependence of Maximum Allowable Operating Pressure and the amount of insulation with heating and venting, considering the influence of dormancy time.The resulting tank gravimetric efficiency is presented for cylindrical tanks with hemispheric ends (a very likely choice for tank design). Notwithstanding conservative analysis, tank gravimetric efficiencies of 65–70% can be achieved. This permits combined fuel and tank weights that are less than half of those of current aircraft. The issue that then becomes critical is the resulting large tank and aircraft volume.     

Greenhouse gas emissions assessment of hydrogen and kerosene-fueled aircraft propulsion

The paper highlights the importance of hydrogen as a promising alternative for future aircraft fuel, with respect to reduced environmental impact, increased sustainability, high energy content and favorable combustion kinetics, since the rapid growth and dependence of aircraft propulsion on fossil fuels are unsustainable. This paper compares the environmental impact of hydrogen and kerosene-fueled aircraft, in terms of greenhouse gas emissions and other emission comparisons. Sample flights from Toronto to Montreal, and Calgary to London are examined. Emissions from a conventional aircraft are estimated and compared with the LH₂ (liquid hydrogen) aircraft. The environmental benefits and drawbacks of these systems are presented from safety and storage perspectives. Radiative forcing factors that compare conventional aircraft and LH₂ aircraft are included. It is shown that the amount of NO_x, HC and CO emissions for the trips with conventional aircraft for Calgary is 171.4, 41.9 and 32.2 kg, while Montreal is 56.17, 2.43 and 21.9 kg, and London is 251.7, 5.1 and 39.2 kg, respectively. These results are compared against hydrogen propulsion to show the promising capabilities of hydrogen as an aircraft fuel.     

Review of hydrogen storage techniques for on board vehicle applications

Hydrogen gas is increasingly studied as a potential replacement for fossil fuels because fossil fuel supplies are depleting rapidly and the devastating environmental impacts of their use can no longer be ignored. H₂ is a promising replacement energy storage molecule because it has the highest energy density of all common fuels by weight. One area in which replacing fossil fuels will have a large impact is in automobiles, which currently operate almost exclusively on gasoline. Due to the size and weight constraints in vehicles, on board hydrogen must be stored in a small, lightweight system. This is particularly challenging for hydrogen because it has the lowest energy density of common fuels by volume. Therefore, a lot of research is invested in finding a compact, safe, reliable, inexpensive and energy efficient method of H₂ storage. Mechanical compression as well as storage in chemical hydrides and absorption to carbon substrates has been investigated. An overview of all systems including the current research and potential benefits and issue are provided in the present paper.     

Route to zero emission shipping: Hydrogen,ammonia or methanol?

To achieve climate change targets, new ship orders should be capable of delivering zero emission propulsion from 2025. The pathway towards this is unclear and requires significant investment. This study analyses the engineering considerations of the storage of alternative fuels on board large scale international vessels, with a particular focus on ammonia, hydrogen and methanol. Analysis of raw shipping data shows the maximum expected propulsion demand per voyage was 9270 MWh. The volume and mass requirements for alternative fuels to deliver this are projected and compared to three further methods for estimating fuel storage considering: storage infrastructure; desired design range; both. This shows that a reduction of fuel storage quantities to closer to actual expected usage results in more realistic storage requirements. Also, hydrogen has a perceived low volumetric energy density, however the calculated volume required (6500 m³ for liquid storage) is not sufficiently high to be considered inviable.     

The thermal efficiency and cost of producing hydrogen and other synthetic aircraft fuels from coal

A comparison is made of the cost and thermal efficiency of producing liquid hydrogen, liquid methane and synthetic aviation kerosene from coal. These results are combined with estimates of the cost and energy losses associated with transporting, storing, and transferring the fuels to aircraft. The results of hydrogen-fueled and kerosene-fueled aircraft performance studies are utilized to compare the economic viability and efficiency of coal resource utilization of synthetic aviation fuels.     

Hydrogen fuel tanks for subsonic transport aircraft

Hydrogen is since long seen as an outstanding candidate for an environmentally acceptable, future aviation fuel. Since the first studies, the design of light yet highly insulated tanks for cryogenic liquid hydrogen has been identified as one of the key enabling technologies. Despite this early recognition, the design of the tanks is nowadays still seen as crucial as aircraft tanks differ significantly from existing tanks in the automotive or aerospace sector. To enable system level feasibility studies of hydrogen fueled aircraft, a preliminary design model for aircraft liquid hydrogen tanks is developed for both foam and multilayer insulations. This model is then used to design tanks for a small regional airliner as well as a large long range transport aircraft. Foam based and multilayer insulations are compared and the sensitivity of the tank weight to the fuselage diameter and the mission fuel load is assessed. The influence of a ‘hold’ period before take-off is analyzed too. As the developed model is intended for use in the preliminary aircraft design phase, structural design or attachment issues are not addressed.     

Hydrogen as the fuel of the future in aircrafts – Challenges and opportunities

The use of hydrogen as a fuel in civil aviation depends largely on the mass of the tank system. A key parameter for the evaluation of mass development is the gravimetric storage density. With the goal of gaining a maximum equal operating empty weight (OEW), the resulting total mass of hydrogen (fuel) and structural mass must be at most equal to the current total tank mass. The minimum gravimetric storage density required for this is determined in this paper. The passenger aircraft ATR 72–212A, which was developed for short-range operation, serves as a reference. For the first calculation, the assumption is made that the efficiencies of the propulsion systems do not change after the storage, and the other necessary systems have an equivalent mass. For a flight range of 715NM, which corresponds to the design range, a gravimetric storage density of 19 wt% is already sufficient to cover the flight distance and to load the maximum payload. In order to avoid increased OEW, a storage density in the range of 33–35 wt% is necessary.     

Hydrogen-powered aviation and its reliance on green hydrogen infrastructure – Review and research gaps

Aircraft powered by green hydrogen (H2) are a lever for the aviation sector to reduce the climate impact. Previous research already focused on evaluations of H2 aircraft technology, but analyses on infrastructure related cost factors are rarely undertaken.Therefore, this paper aims to provide a holistic overview of previous efforts and introduces an approach to assess the importance of a H2 infrastructure for aviation. A short- and a medium-range aircraft are modelled and modified for H2 propulsion. Based on these, a detailed cost analysis is used to compare both aircraft and infrastructure related direct operating costs (DOC).Overall, it is shown that the economy of H2 aviation highly depends on the availability of low-cost, green liquid hydrogen (LH2) supply infrastructure. While total DOC might even slightly decrease in a best LH2 cost case, total DOC could also increase between 10 and 70% (short-range) and 15–102% (medium-range) due to LH2 costs alone.     

Efficiencies of hydrogen storage systems onboard fuel cell vehicles

Energy efficiency, vehicle weight, driving range, and fuel economy are compared among fuel cell vehicles (FCV) with different types of fuel storage and battery-powered electric vehicles. Three options for onboard fuel storage are examined and compared in order to evaluate the most energy efficient option of storing fuel in fuel cell vehicles: compressed hydrogen gas storage, metal hydride storage, and onboard reformer of methanol. Solar energy is considered the primary source for fair comparison of efficiencies for true zero emission vehicles. Component efficiencies are from the literature. The battery powered electric vehicle has the highest efficiency of conversion from solar energy for a driving range of 300 miles. Among the fuel cell vehicles, the most efficient is the vehicle with onboard compressed hydrogen storage. The compressed gas FCV is also the leader in four other categories: vehicle weight for a given range, driving range for a given weight, efficiency starting with fossil fuels, and miles per gallon equivalent (about equal to a hybrid electric) on urban and highway driving cycles.     

Modification of NaAlH4 properties using catalysts for solid-state hydrogen storage: A review

Energy is an essential requirement in our daily lives. Currently, most of our energy demands are fulfilled by fossil fuels. After 20 years, non-renewable fossil fuels are estimated to plummet rapidly. The world will face energy shortage and will seek for a new environmental method of energy generation for transportation, economy and application. Hydrogen is a fascinating energy carrier that is considered as ‘hydrogen economy’ for the future. The key challenge in developing the hydrogen economy is the context of hydrogen storage. Storing hydrogen via the solid-state method has received special attention and consideration because of its safety and larger storage capacity. A light complex hydride, NaAlH₄, is considered as an attractive material for solid-state hydrogen storage owing to its high hydrogen capacity, bulk in availability and low cost. Sluggish sorption kinetics and poor reversibility have driven research into various catalysts to enhance its hydrogen storage properties. This review article examines the development of different catalysts and their effects on the hydrogen storage properties of NaAlH₄. The addition of catalyst offers synergistic catalytic effect on the dehydrogenation performance of NaAlH₄. Doping NaAlH₄ with catalyst promote promising results such as lower decomposition temperature, improved kinetics and reduced activation energy. Superior performance on the dehydrogenation performance of NaAlH₄ doping with the catalyst may be due to the nanosized catalyst particle and in situ formed active species that may serve as nucleation sites at the surface of the NaAlH₄ matrix and benefiting the kinetics properties of NaAlH₄.     

Explosion characteristics of n-decane/hydrogen/air mixtures

Hydrogen can be used in conjunction with aviation kerosene in aircraft engines. To this end, this study uses n-decane/hydrogen mixtures to investigate the explosion characteristics of aviation kerosene/hydrogen in a constant volume combustion chamber with different hydrogen addition ratios (0, 0.2, 0.4), wide effective equivalence ratios (0.7–1.7), an initial temperature of 470 K, and initial pressures of 1 and 2 bar. The results show that the explosion pressure and explosion time decrease linearly with increasing hydrogen addition ratio. The effect of initial pressure is also discussed. A comparison of the adiabatic explosion pressures indicates that the hydrogen addition effect varies at different initial pressures and effective equivalence ratios owing to heat loss. In addition, the maximum pressure rise rate and deflagration index increase with increasing hydrogen concentration, which is more obvious for rich mixtures and high hydrogen concentrations.     

The autoignition in air of some binary fuel mixtures containing hydrogen

A predictive model for the autoignition and combustion of fuel–air mixtures employing detailed full chemical schemes was used to examine the autoignition and combustion characteristics in air of hydrogen in the presence of a range of common fuels. These included the gaseous fuels: methane, carbon monoxide and the higher hydrocarbon fuel n-heptane. A wide range of relative concentrations of the fuel components in the binary mixtures with hydrogen for different values of initial mixture temperature and pressure were considered under constant volume adiabatic conditions. It is shown that the presence of hydrogen in turn with these fuels can bring about complex changes to the autoignition behaviour of the fuel mixtures that show hydrogen may behave as an accelerant or retardant depending on the fuel, initial temperature, pressure and equivalence ratio considered.     

Conceptual design and optimization of a general aviation aircraft with fuel cells and hydrogen

In this paper¹, an aircraft design from scratch and optimization is carried out to investigate the potentials of using liquid hydrogen and fuel cells in general aviation. The focus is set on finding an efficient aircraft configuration that considers all drive train-related components such as the hydrogen tank, fuel cells, and electric motors as well as passenger seats and cargo hold. The paper starts with the definition of the design requirements and specifications. Following, the baseline design of the aircraft in the Suave tool is presented. This includes making extensions to the Suave tool regarding the compatibility of fuel cells and hydrogen energy networks. Eventually, a multidisciplinary design optimization process of selected design variables together with the design constraints is carried out in Suave. Finally, the design is evaluated in terms of performance and emissions by drawing a comparison with conventional general aviation aircraft.     

How close is a hydrogen hypersonic commercial aircraft?

There is a growing interest in hydrogen hypersonic commercial aircraft, which is claimed could permit travel from Sydney to London in less than 1½ hours with reduced environmental and economic costs compared to current subsonic airliners fueled with conventional hydrocarbon jet fuels. As here discussed, this development faces many technical hurdles, from thermal management to materials for extreme environments, from maneuverability to communication, and from sonic boom noise to efficient propulsion over a range of Mach from 0 to 10 and above. The development of this disruptive modality of air travel is constricted by resource availability to tackle these numerous technological challenges. Ultimately, the success will depend on social, political, and economic aspects, from the relevance of mass vs. elite air travel, the developments in the global economy, the synergies with military hypersonic propulsion, and the commitment toward general hydrogen aviation employing hydrogen for subsonic airliners.     

Refinement of motivity factor in comparison of transportation fuels

Hydrogen is potentially the fuel of the future, with the transition to a hydrogen economy driven by the concerns about the climate change and the depletion of fossil fuel resources. Hydrogen is better than almost all other fuels when the fuels are evaluated on the basis of several metrics, including safety and versatility of use. Hydrogen is also presumed to have a superior motivity factor, a performance measure arising out of a combination of acceleration and drag forces. The motivity factor is quantified on the basis of the gravimetric and volumetric energy densities of a fuel. The present paper offers a refinement of the motivity factor which incorporates the effects of the mass of the storage matrix in these energy densities. It is shown that other fuels may have higher motivity factors than hydrogen when quantified on the basis of these refinements.     

Overview of systems considerations for on-board chemical hydrogen storage

Recent advances in chemical hydrogen storage systems are reviewed. Factors impacting design and implementation of chemical hydrogen storage systems for on-board vehicular use are highlighted. The physical and chemical characteristics of chemical hydrogen fuels and their spent fuel counterparts are considered to provide guidance to future technology developers. Heat management, fuel stability, reactor design, and fuel morphology are examples of issues that must be considered for the future of chemical hydrogen storage systems.     

Hydrogen supply chain and challenges in large-scale LH2 storage and transportation

Hydrogen is considered to be one of the fuels of future and liquid hydrogen (LH2) technology has great potential to become energy commodity beyond LNG. However, for commercial widespread use and feasibility of hydrogen technology, it is of utmost importance to develop cost-effective and safe technologies for storage and transportation of LH2 for use in stationary applications as well as offshore transportation. This paper reviews various aspects of global hydrogen supply chain starting from several ways of production to storage and delivery to utilization. While each these aspects contribute to the overall success and efficiency of the global supply chain, storage and delivery/transport are the key enablers for establishing global hydrogen technology, especially while current infrastructure and technology are being under development. In addition, while all storage options have their own advantages/disadvantages, the LH2 storage has unique advantages due to the familiarity with well-established LNG technology and existing hydrogen technology in space programs. However, because of extremely low temperature constraints, commercialization of LH2 technology for large-scale storage and transportation faces many challenges, which are discussed in this paper along with the current status and key gaps in the existing technology.