Frequently, when the troubling reality of climate change comes up, people talk about aviation’s role in greenhouse gas emissions. Despite huge progress made in air transport efficiency (design, engines, aerodynamics, etc.), the sector is facing an increase in emissions due to rising demand for air transport. This underscores the powerful need for more radical innovation. One of the opportunities that many put on the table is using hydrogen as a potentially appropriate solution for aviation. Its use as an energy vector is an opportunity to significantly reduce carbon emissions in this important sector for the global economy.
Hydrogen-powered aircraft are emerging as an element to consider in the puzzle of sustainable aviation.
Air transport emissions have been on the rise, although increases have been relatively modest. According to recent IBA data, CO2 emissions in the commercial aviation industry averaged 144.2 grams per seat per mile (267.06 g per seat/kilometre) in February 2022, with an increase in carbon intensity of almost 0.3% per seat. This marginal increase is due in part to a reduction in operations for cutting-edge aircraft and a drop in flight stage length.
Moreover, the International Energy Agency (IEA) indicates that although fuel efficiency has clearly improved, it has not improved enough to offset the growth in energy demand over recent years. Between 2010 and 2019, fuel efficiency improved an average of 1.8% per year, but demand grew at a rate of more than 5% year over year. To stay on track for net-zero emissions, efficiency will need to improve at a rate of 2% per year until 2030.
The balance between energy efficiency and growing demand for flights remains a significant obstacle.
These data underscore the complexity of the challenges ahead in terms of emissions from the aviation industry, where the balance between energy efficiency and growing demand for flights remains a significant obstacle. At this point, hydrogen-powered aircraft are emerging as an element to consider in the puzzle of sustainable aviation. With high energy density and zero polluting emissions, hydrogen is shaping up to be a key component in taking on current environmental challenges. As we explore this exciting territory, we will dive into a world where aviation does not just connect people, but it is also more and more respectful of our planet.
Basis of Hydrogen as a Fuel
Hydrogen is considered an energy vector due to its capacity to store and transport energy generated from other sources. Unlike fossil fuels, which are primary energy sources, hydrogen must be produced previously, generally through water electrolysis (a process that consumes energy). In terms of energy density, hydrogen has a major advantage: by mass, it offers nearly three times more energy than conventional jet fuel. However, its low volumetric density causes storage and distribution challenges. In terms of emissions, hydrogen combustion produces only water vapour, making it an appealing option for reducing greenhouse gas emissions in aviation, although its production currently relies mostly on non-renewable sources. The transition to hydrogen as an aviation fuel, therefore, depends critically on the sustainability and efficiency of its supply chain.
Design Challenges and Active Restrictions: The use of hydrogen as a fuel involves a variety of design challenges and active restrictions compared to aircraft powered by conventional kerosene. These differences are mainly due to hydrogen’s greater specific energy (energy by unit mass) and the unique storage challenges involved.
Hydrogen’s Energy Density and Weight: Although hydrogen’s energy density is approximately four times lower than kerosene’s, its energy per unit mass is almost three times higher. This means that for the same amount of energy needed to fly, the weight of the hydrogen necessary is only one third of the weight of the kerosene required.
Energy Use on Long-Distance Flights: Hydrogen and a lower take-off weight are expected to result in lower energy use for long-distance flights compared to kerosene. Nonetheless, this may not be true for medium and short-distance flights due to the complexity and energy efficiency of hydrogen storage.
Maintenance and Operating Costs: Due to their added complexity, hydrogen tanks and fuselages are expected to have higher maintenance costs and purchase prices. Integrating the tank into the fuselage adds further design challenges, which may result in a reduction of the direct operating cost for long-distance aircraft, assuming hydrogen and kerosene have similar costs per unit of energy.
Hydrogen’s Volumetric Density and Storage: Hydrogen has an extremely low volumetric energy density at ambient temperature and pressure. To reduce the volume required, hydrogen can be compressed as a gas or cooled into a liquid. For instance, the volume needed to store the energy that a Boeing 777-200ER carries in kerosene would be equivalent to approximately 500 fuselages if it is stored as hydrogen at ambient temperature and pressure.
Compressed Hydrogen Storage: Compressing hydrogen at high pressures is one way to increase its density for onboard storage. Compressed hydrogen tanks operate at ambient temperatures and require less active management than liquid hydrogen (LH2). Nevertheless, these tanks require high pressure-resistant tanks and have low gravimetric efficiencies.
Cryogenic Hydrogen (LH2) Storage: Cryogenic hydrogen storage offers advantages over storing the compressed gas, including a higher density and the possibility of storing it at pressures close to ambient. This requires, however, significant insulation and a careful design of the fuel system, especially for larger commercial transport aircraft, which are sensitive to weight.
Distribution and Infrastructure
The FlyZero Aerospace Technology Institute report underscores the critical importance of developing adequate infrastructure to produce and manage hydrogen at airports as an essential step in the transition toward hydrogen-powered aviation.
A vital component to this infrastructure is the logistics of delivering liquid hydrogen (LH2) to airports. This logistical challenge could be taken on by transporting LH2 to airports in specialised lorries or with especially designated pipelines. Once at the airport, the LH2 would require safe and efficient storage systems followed by a safe and properly protocolled transfer system to bring it to the aircraft.
In terms of safety, handling LH2 has its peculiarities. One positive aspect is that in the event of a spill, LH2 vaporises quickly, and the gaseous hydrogen left over rises and is dispersed into the atmosphere, thanks to its low density, significantly minimising the risk of fire. Although hydrogen fires reach higher temperatures compared to kerosene fires, hydrogen does not pool, eliminating the risk of prolonged fires on the ground, a hazard present with kerosene. As a result, the thermal radiation generated by a kerosene fire can be more dangerous over the long term than that of a hydrogen fire, despite the lower temperature of the former.
One option for supplying hydrogen to airports is receiving gaseous hydrogen (GH2) that would later be liquified on site. This configuration allows for greater flexibility in fuel management, as GH2 can be stored at more manageable temperatures and pressures prior to being converted into liquid hydrogen (LH2) for use in aircraft.
Compressed hydrogen tanks work at ambient temperatures, reducing the need for active management compared to liquid hydrogen (LH2). These tanks can spend long periods of time without having to be ventilated or filled, simplifying fuel management on aircraft. However, compressed hydrogen requires heavy pressure-resistant tanks to guarantee safety. These tanks have low gravimetric efficiencies, between 1% and 10%, although they could reach between 10% and 20% with advanced design and manufacturing techniques.
Tank Materials and Pressure: Storage tanks should be manufactured with materials able to withstand high pressures. To provide a simple comparison, the pressure in compressed hydrogen tanks can be up to 700 bar (approximately 700 times the atmospheric pressure at sea level). This pressure is necessary to keep the hydrogen in a densified state to facilitate storage and handling.
Losses due to Permeation: One challenge with hydrogen storage is permeability and embrittlement of the materials. Permeability occurs because hydrogen molecules are very small and some can pass through the walls of the tank. Reasonable permeation rates for LH2 tanks in launch vehicles allow for losses of approximately 0.25% of the tank volume during ascent and insertion into orbit.
Producing hydrogen right at the airport combines developing a hydrogen production infrastructure at the airport with the potential implementation of electrolysers that convert water into hydrogen and oxygen via electricity, preferably from renewable sources to optimise sustainability. This configuration involves not only creating the facilities to generate hydrogen, but also the systems necessary for its storage and distribution to aircraft. The on-site production option has major advantages, like cutting costs and emissions associated with transporting the hydrogen to the airport, and it also offers greater flexibility and control over fuel supply, critical aspects in airports, where reliability and efficiency are essential. Establishing this infrastructure, however, would require a significant investment.
Reducing the Carbon Footprint: When the electricity used to generate hydrogen comes from renewable sources, the process of locally producing hydrogen becomes more sustainable, making a significant contribution to reducing the carbon footprint. This sustainability is further strengthened if surplus renewable energy that would otherwise go to waste is stored in hydrogen form. This approach not only boosts the efficiency of the energy conversion process, but also maximises the use of the renewable resources available, effectively transforming surplus energy into a valuable asset for energy sustainability.
Efficiency in Flight and Comparative Performance
Propulsion Methods: Mainly two methods are used to convert hydrogen into thrust: combustion in gas turbines and hydrogen fuel cells. Gas turbines, which are currently used in commercial aircraft, can be adapted to burn hydrogen, modifying the combustion system and fuel supply.
Preference Based on Aircraft Type: Fuel cells tend to be the predominating option for small and short-haul aircraft, while hydrogen combustion is feasible in large and long-haul aircraft.
Gas Turbine Efficiency: Gas turbines with hydrogen have greater specific power than other propulsion methods, making them appropriate for larger, more powerful aircraft. One example is that the fuel cell propulsion system for an aircraft the size of a Boeing 737-800 is approximately three times heavier than the equivalent gas turbine system.
Development and Testing in Hydrogen Combustion Engines: Companies like CFM, Rolls-Royce and Pratt & Whitney are modifying existing turbofan engines to work with hydrogen, and these engines may be tested in aircraft demonstrators in the coming years.
Benefits of Hydrogen Combustion in Gas Turbines: Hydrogen combustion in gas turbines may lower the temperature of the hot gas entering the turbine, prolonging useful life and reducing maintenance frequency. Additionally, hydrogen stored as a liquid onboard an aircraft provides a great heat sink, allowing engine designers to explore creative ways to boost performance.
Thrust-Specific Fuel Consumption (TSFC): TSFC, a crucial metric in aircraft design indicates the fuel mass flow rate required per unit of thrust produced by the engine. Hydrogen provides nearly three times more energy as the same kerosene mass flow, rendering direct TSFC comparisons between fuels with different energy densities inappropriate.
Fuel Cell Operation and Design: Hydrogen fuel cells, especially those with proton-exchange membrane fuel cells (PEMFC) and solid oxide fuel cells (SOFC) have unique characteristics and challenges in their functioning and design. They include water usage, ohmic losses and limits in the rate at which reagents can reach reaction sites.
Thermal Management and Fuel Cell Design: Thermal management is a key challenge especially for low-temperature PEMFCs, which must dispel a significant amount of heat. Bipolar panels, which make up most of the volume and weight of the cell, are optimised to deliver reagents to electrodes, handle cooling and limit the weight.
PEMFC Operating Pressure and Performance: Increasing PEMFC operating pressure can improve their performance, but with decreasing energy efficiency.
To conclude our analysis on the integration of hydrogen in aviation, we would like to highlight how important a role it could play in the transition to more sustainable aviation. This approach would mean significant progress with a direct impact on the climate crisis. Although it is promising, the path to effective implementation of hydrogen in aviation is intricate, rife with technical and operational challenges that require attention and innovative solutions. Continuing research and development in this field is essential to striking up a balance between the benefits of air transport and environmental preservation.
You can learn more about this topic by reading our post “Hydrogen in the Future of Air Transport” (click here).
