Our climate is also affected by other substances emitted by aircraft, as a result of burning kerosene jet fuel. Significant amounts of NOX, contrails and particulate matter have a specific warming effect of their own. Released at high altitudes, aviation emissions have 2–4 times the impact of comparable ground source emissions.
It’s therefore unsurprising that the race is on for low- and zero-emission flight. Both established aircraft manufacturing giants like Airbus and startups such as ZeroAvia are already taking significant steps towards sustainable aviation. Alongside an increased push for the use of Sustainable Aviation Fuels (SAF), new propulsion systems based on battery-electric cells, hydrogen-electric cells, and liquid hydrogen combustion are all at various stages of development.
The promise of hydrogen fuel cells
While there are many potential routes towards low- and zero-emission flight, in the past two years interest has swung firmly in favour of an energy carrier that will be an important part of our future energy mix: hydrogen.
Whilst combusting hydrogen is one option, hydrogen fuel cells are a particularly promising propulsion idea. They generate electricity through an electrochemical reaction which combines hydrogen with oxygen to create water, releasing electrical energy that can be harnessed for propulsion.
A report by the Clean Sky 2 Joint Undertaking and Fuel Cells and Hydrogen Joint Undertaking, released in 2020, estimated that hydrogen fuel-cell propulsion could reduce climate impact by 75 to 90%.
In fact, fuel cells that use pure hydrogen generated from renewables are completely carbon-free in use. And unlike batteries that need to be recharged, fuel cells can continue to generate electricity as long as a fuel source (hydrogen) is provided. Individual fuel cells can also be ‘stacked’ to form larger systems capable of producing more power, thereby allowing scalability.
And this is all starting to feel real. Late in 2020 – in a landmark moment for low-carbon flight – ZeroAvia flew a modified six-seater Piper M-Class aircraft powered by a hydrogen fuel cell from its R&D base at Cranfield airport in the UK.
Hydrogen fuel cell flight: progress and challenges
Exciting though this area is, there remain significant engineering challenges to overcome with hydrogen fuel cells, especially if they are to work with larger aircraft – particularly around the size and weight of a fuel cell system.
A major contributor to fuel cell weight is the need for thermal management.
“Hydrogen fuel cell systems are only about 50% efficient,” explains Kathryn Evans, Applied Technologies Aerospace Lead at Reaction Engines. ““For every kilowatt of useful energy that is produced for propulsion, there is a kilowatt of waste heat. That’s a lot of heat to get rid of – heat which can damage the system and create safety risks if not removed effectively.”
The problem is that most heat exchangers are far too bulky for flight – both from a weight and volume perspective. They were designed for ground-based applications. Fuel cell heat can be managed on small flights, but today’s commercial passenger jets would need heat exchangers so large they would be unable to take off.
“Both weight and space are already at a premium when designing any aircraft,” continues Evans. “The only way we will make hydrogen fuel cell planes fly beyond short distances or with small payloads is if we find a way to remove waste heat quickly and efficiently without weighing too much and without creating too much aircraft drag.”
Reimagining heat exchange
The key to solving this problem is to find ways to quickly and efficiently cool the fuel cell with systems that are practical to deploy onto planes – in other words, they don’t add lots of weight or aircraft drag, or take up too much space.
The way to do this is to optimise how we transfer heat.
A standard fuel cell cooling system will use a coolant fluid, which flows through the cell and picks up the heat from the cell. This fluid then flows out of the fuel cell via piping and into tubes within a heat exchanger. These tubes are housed in a duct, with ambient air flowing over them, which is cooler than the heated fluid. The heat from the fluid is transferred to the air, and the now-cooled fluid is recirculated to be used to cool the fuel cell again.
Most heat exchangers transfer heat away from the fluid at about 50-70% thermal efficiency. The rate at which heat is transferred between the two mediums is determined by the surface area of the tubes between the air and the fuel cell coolant fluid, their relative temperatures, and the tube materials and thickness. The more efficient this is, the quicker you can cool the air, and the less cooling fluid you need to carry with you, and the lighter the plane.
Improving heat transfer can be done by increasing the surface area, making the tubes much smaller, and packing more of them in, in a way that is designed to work with the heat flow through the chamber. Thinner tube walls also allow a faster transfer. When you get down to millimetre scale tubes, heat transfer becomes extremely efficient – 90% or higher.
Designing heat transfer technology at a scale that can sit on board a plane is tricky, but it can be done by taking advantage of new materials, fabrication techniques and understanding of thermodynamics. And this is a non-negotiable – mainstream hydrogen fuel cell flight will not be possible without this technology.
Reaction Engines – applying rocket science to get zero-emission flight off the ground
At Reaction Engines, we’re known for our rocket technology. At the heart of this is a cutting-edge microtube heat exchanger that can cool air from 1,000oC to -150oC in 1/20th of a second.
Our heat exchanger incorporates thousands of millimetre-scale tubes – made from hair-width materials and arranged in a spiral design. The result is incredibly high levels of heat transfer – as high as 99% effective cooling – within a much smaller and lighter device.
By applying this technology to fuel cells, we can solve the thermal management problem that could otherwise keep hydrogen fuel cell planes grounded. “It has been recognised by our partners as a genuinely ‘enabling technology’,” explains Evans. “It will be critical to getting hydrogen fuel cell planes off the ground.”
Reaction Engines welcomes interest and discussions from companies across the aerospace supply chain with an interest in hydrogen fuel cell powered flight.