06/09/2022
The aviation sector has long been under pressure to find ways of reducing emissions of CO2. This has involved research into alternatives to fossil fuels; an earlier article had looked at the role electric propulsion has to play in replacing fossil fuels. However, another alternative fuel for aviation is hydrogen.
Research into hydrogen as an aircraft fuel is not new in itself. April 1988 marked the maiden flight of the Tupolev TU-155 – an experimental aircraft powered by liquid hydrogen. However, the collapse of the Soviet Union saw the end of theTu-155 after only 100 flights.
With the growing emphasis on ways of combating climate change, in recent years there has been renewed interest and research into how hydrogen can help contribute in reducing aviation’s CO2 emissions.
Why hydrogen and how might hydrogen be used?
From an emissions perspective, the use of hydrogen as a fuel avoids the CO2 emissions associated with fossil fuels, such as kerosene.
Hydrogen has the potential to serve as a fuel source for aircraft in one or both of the following ways:
- Feeding hydrogen to an aircraft engine for combustion. The burning of hydrogen does not generate CO2 emissions.
- Using hydrogen in a fuel cell to generate electrical energy for use in powering an electric motor to drive a propulsion unit. As well as avoiding the generation of CO2, this would also avoid NOx emissions.
The Specific Energy vs Energy Density conundrum:
At first glance, the fact that hydrogen has a specific energy nearly three times that of kerosene – 120 MJ/kg compared to 43 MJ/kg – makes it appear an excellent candidate to replace kerosene as a fuel for burning in a jet engine. However, specific energy is only half the story.
Things are more complicated when looking at the energy density (i.e. the energy per unit volume). Liquid hydrogen and kerosene have respective energy densities of 8.5 MJ/L and 35 MJ/L. Put simply, for a given amount of energy output when combusted, hydrogen will require a larger storage volume than conventional aviation fuels; in other words, larger and heavier fuel tanks will be required. As hydrogen requires a temperature of -252.87 degrees Celsius to be in liquid form, the storage tanks must also be well insulated. So, the volume and mass of the vessels required to store liquid hydrogen can work against the advantages provided by hydrogen’s favourable specific energy. This has the potential to leave an aircraft designer facing some uncomfortable choices when looking to design an aircraft capable of completing a flight of a given range:
1: Reduce the payload (passengers and/or cargo) to provide enough space to store sufficient liquid hydrogen to cover the given range . However, this risks making operation of the aircraft commercially uncompetitive.
2: Increase the aircraft size to compensate for an increase in the size and mass of fuel tanks needed to hold sufficient liquid hydrogen to cover the given range, in an attempt to maintain payload capacity. However, without other changes in aircraft design, simply increasing aircraft size results in a corresponding increase in aircraft mass, with more fuel and even larger (and heavier) fuel tanks required for the aircraft to cover a given distance. So, this can quickly become a self-defeating strategy.
Things are more problematic when considering gaseous hydrogen, for which the energy density is pressure dependent. Gaseous hydrogen has a lower energy density (compared to liquid hydrogen) of 4.5 MJ/L, based on storage at a pressure of 69 MPa (690 bar) at 25 degrees Celsius. The storage of gaseous hydrogen under such high pressure requires the use of storage vessels designed to sustain such high pressures.
Current developments:
The lack of CO2 emissions associated with the use of hydrogen has driven research into how a propulsion system of an aircraft can be powered using hydrogen fuel.
Airbus have a stated ambition “to develop the world’s first zero-emission commercial aircraft by 2035”. In February 2022, they launched their ZEROe demonstrator, employing an Airbus A380 aircraft as a platform for testing different forms of hydrogen combustion technology. Current plans are for the aircraft to carry four tanks containing liquid hydrogen, as well as a hydrogen combustion engine mounted along the rear fuselage. The liquid hydrogen would be converted into its gaseous form prior to combustion in the engine. The engine will be developed by CFM International, based upon a GE Passport turbofan engine with a modified control system, fuel system and combustor.
Airbus are also looking at three different concept aircraft to be powered using liquid hydrogen, all using hybrid-hydrogen engines. Two of the concepts resemble the conventional cylindrical fuselage + pair of wings design associated with modern aircraft – one employing a turbofan engine to provide a range of over 2000 nautical miles for < 200 passengers, the other using a turboprop engine with a range of over 1000 nautical miles for <100 passengers. The third of the concepts is more radical in design, employing a blended-wing body. The hybrid engines are envisaged to use hydrogen powered fuel cells to create electricity to drive a motor in the engine, in addition to being capable of burning hydrogen to generate thrust.
Smaller players are also exploring the use of hydrogen in aviation propulsion. One such example is ZeroAvia, who were only founded in 2018 but in September 2020 completed a test flight at Cranfield, UK of a modified Piper M-class 6 seater aircraft powered by a hydrogen fuel cell. The development of this aircraft was part of a project – HyFlyer – supported by the UK government to provide a zero emissions fuel cell powertrain for use in aviation. Going forwards, the HyFlyer programme is seeking to develop the first certified hydrogen-electric powertrains for use on aircraft carrying up to 19 passengers.
Time for a radical change in aircraft design?
One way of addressing the challenge of storing enough hydrogen on a passenger-carrying commercial aircraft to provide sufficient range and profitability might be to employ an aircraft configuration other the conventional cylindrical tube + pair of wings design. For example, the use of a blended-wing body configuration may be more aerodynamically efficient than current designs, whilst also providing more options for the storage of hydrogen fuel.
However, more radical departures in aircraft configuration are also being considered, which may lend themselves to the use of non-fossil fuel powered powertrains. One example of a new concept design of commercial passenger-carrying aircraft is the “The Flying-V”, developed by Delft University of Technology (TU Delft) of the Netherlands. The aircraft shape literally defines the shape of a “V”, with the passenger cabin extending from the tip of the “V” aft-wards along each leg of the “V”. The design concept envisages an aircraft with a wing span of 65 metres, plus room for around 314 passengers and 160m3 of space for cargo. The design is characterised by the lack of a tail plane. The outermost ends of the wings include winglets containing rudders for yaw control.
The V-shaped configuration of the aircraft and the use of an oval-shaped cabin may provide a more aerodynamically efficient design than conventional aircraft designs, thereby reducing the amount of fuel required for a given range. Compared to an Airbus A350, the wing area is claimed to be about twice as large, with fuel consumption estimated to be 20% lower for a given range.
To date, a flight test of a scale model has been undertaken using two 4kW electric ducted-fan engines. However, at first glance, the aerodynamic efficiencies of the design concept could make it a suitable candidate for use with a hydrogen powered propulsion system.
Patent applications:
Patent applications can often provide an indicator as to what technical innovations may be around the corner.
The following application provides an example of a hybrid-hydrogen engine. The application describes a hydrogen source feeding fuel to one or both of a fuel cell and a gas turbine engine, with both the fuel cell and gas turbine engine coupled to a propulsion unit. In one operating mode, the fuel cell uses hydrogen in a redox reaction to generate electricity to electrically drive the propulsion unit. In a second operation mode, the gas turbine engine burns hydrogen to drive the propulsion unit using mechanical power.
How and where to store hydrogen fuel on an aircraft is also an issue. This application addresses that problem by having a number of cylindrical hydrogen storage tanks aligned along the length of the outside of the aircraft fuselage, with an aerodynamically-shaped fairing enclosing the tanks.
Generating hydrogen without CO2 emissions:
Although the combustion of hydrogen to generate mechanical energy or its use in a fuel cell to generate electrical energy may be free of CO2 emissions, what can sometimes be forgotten are the emissions associated with producing the hydrogen fuel in the first place.
The majority of hydrogen used in industry around the world is produced using fossil fuels, such as natural gas. For example, steam-methane reforming is a commonly used process in hydrogen production, with high temperature steam used to produce hydrogen from methane (CH4); however, CO2 is released as a by-product.
CO2 emissions can be drastically reduced by generating hydrogen through electrolysis powered by electricity. From an emissions-perspective, it is best if the electricity used is from renewable sources. However, the worldwide capacity for producing hydrogen by electrolysis can currently only meet a small fraction of the total demand for hydrogen production. The IEA’s 2021 Global Hydrogen Review highlighted the scale of the challenge in increasing global capacity for generating hydrogen in a greener manner. The IEA review stated that in 2020, hydrogen demand was around 90 Mt; nearly all was produced using fossil fuels, with consequent CO2 emissions of around 900 Mt. Even when projecting to 2030, the IEA report estimates that worldwide production of hydrogen from use of electrolysers would amount to only 8 Mt – still far less than the 90 Mt demand for hydrogen in 2020.
So, a challenge for countries across the world will be to find new ways of scaling up production of hydrogen in an environmentally friendly manner.
___________________
The challenges in producing and using hydrogen to replace fossil fuels in aviation will provide opportunities for innovation and the development of valuable new intellectual property (IP). We at Reddie & Grose are very much looking forward to working with clients in protecting their IP to help them achieve their commercial and environmental goals.
This article is for general information only. Its content is not a statement of the law on any subject and does not constitute advice. Please contact Reddie & Grose LLP for advice before taking any action in reliance on it.
