One of the unexpected pleasures is having a fondly cherished idea booted into touch. For me one such notion was that hydrogen powered planes are not practical. Hydrogen has always just seemed like a bad idea – think of the Hindenburg disaster – and for a hundred other reasons.
ONE OF THE BIG DRIVERS for hydrogen planes has been to reduce carbon emissions. But the aviation industry is not a big polluter – it contributes less than 3% of carbon emissions worldwide. Yet it has always been a huge target, from those who, like Chicken Little, think condensation trails mean the sky is falling. And also for the ‘flight shaming’ movement – which tries to make everyone who buys an airline ticket feel guilty.
Responding to the pressure of populism, the airline industry is super sensitive to flight shaming and has created huge and costly programmes such as CORSIA – the Carbon Offsetting and Reduction Scheme for International Aviation. But the real problem is that there is just no substitute for good old fossil fuels – especially for the all-important gravimetric efficiency, which is the amount of energy in fuel compared to its weight.
Fossil fuel’s energy-density advantage is all but impossible to beat, so CORSIA aims to have airlines offset their emissions using negative carbon emissions technology. One such is biofuels and Sustainable Aviation Fuel (SAF) but these are currently a publicity stunt as the growing and conversion of crops to liquid fuels is carbon-intensive. And besides – the world needs farmers to produce food – not JetA1.
Electric planes are still even more farfetched – especially for any airline wanting to fly sectors longer than 30 minutes. And for less than 30-minutes, people should take trains and busses if they are that worried about the environment.
And then the other day I got invited to a webinar organised by the Aeronautical Society of South Africa (AeSSA) on hydrogen powered flight. The presenter was the splendidly named Professor Pericles Pilides from Cranfield University.
Turns out both Airbus and Boeing are seriously looking at hydrogen power. Airbus says it will decide by 2025 whether there is a market for hydrogen-fuelled airliners and if so, it reckons the first hydrogen airliners will enter service in 2035. For Airbus the issue is not whether the technology is do-able but whether they can sell it to the airlines.
Back in 2008 Boeing built and operated the first aircraft ever to fly solely on hydrogen power. The fuel cells on the single-person plane were supplemented with power from lithium-ion batteries during takeoff and ascent. Four years later, the company unveiled the Phantom Eye, a liquid-hydrogen-powered unmanned aerial vehicle (UAV). It was designed to fly reconnaissance missions of up to four days at an altitude of 20,000 metres. Boeing was unable to sell the UAV to the military, however, and it is now a museum piece.
Boeing says that its more immediate focus is on sustainable aviation fuels and in its view there will be a mix of solutions, with hydrogen power more likely to fill the short-haul, smaller end of the sector.
Although Boeing has shown that hydrogen will work as aviation fuel, the big challenge will be to prove that an aircraft’s structure and fuel tanks can be built to operate as safely as today’s airliners. Like Airbus, Boeing estimates it will be two decades or more before a hydrogen powered Boeing airliner flies. But key bits such as the engines are already in development.
WEIGHT AND ENERGY DENSITY
Prof Pericles Pilides acknowledges that the biggest challenge is the extra weight required for hydrogen fuel tanks, be it for gaseous or liquid form. For liquid hydrogen, the challenge will be making lightweight vacuum-insulated tanks that maintain the fuel below its super-cold 20 degree Kelvin boiling point. Carrying fuel in gas form carries an even greater weight penalty, since the tanks must be huge.
Surprisingly, in terms of gravimetric efficiency, liquid hydrogen has 2.8 times the energy density of aviation fuel. But, when you factor in the weight of the hydrogen tanks, JetA1 has the advantage over hydrogen by a factor of 1.6. Whereas JetA1 constitutes about 78% of the combined weight of tank and fuel, liquid hydrogen accounts for just 18% of the total in current storage designs. Prof Pilides reckons that to compete with fossil fuels, the fuel weight fraction of hydrogen has to reach at least 28%.
To accommodate the massively large hydrogen fuel tanks a potential solution is a blended wing– fuselage design. This will be like a flying LPG bottle with the hydrogen compressed to 350 bar. With fuel-cell technology it is expected that this blended wing could manage short-haul flights of 500 nautical miles.
Gaseous hydrogen would however occupy about twice as much space as tanks containing liquid hydrogen. The tanks would be made from combinations of existing composites and resins and the current best fuel-to-tank weight ratio for gaseous hydrogen is 11–12%.
The future is coming – liquid hydrogen tanks that have ratios greater than 50% are being tested. This has led to designs that look like the Airbus A380 – but with the entire top deck used for gaseous hydrogen.
FUEL CELLS OR COMBUSTION?
One of the great things about hydrogen is that modern jet engines can burn hydrogen with few modifications. But emissions are a surprising problem. I fondly imagined that the hydrogen would burn with the oxygen in the air and produce H2O – i.e. water. Although it would produce no carbon dioxide, due to the nitrogen in the air, burning hydrogen produces nitrogen oxides and water vapour. And Prof Pericles points out that water vapour at high altitudes becomes a greenhouse gas.
The solution is hydrogen-fuelled proton exchange membrane (PEM) fuel cells. These are emission-free if the hydrogen comes from carbon-free sources, and their exhaust water vapour can be condensed before release. PEMs, though, provide only half the 3.7 kW/kg power per unit weight of modern gas turbines burning conventional fuel, plus of course the weight of the fuel and the tank. It’s a big improvement from the 0.3 kW/kg of 15 years ago, and continued improvements can be expected.
Airbus has produced three concepts for hydrogen-fuelled airliners with capacities of up to 200 passengers and ranges of 2000 nautical miles or more. Each will be powered by a hybrid system of combustion turbines and fuel-cell-driven motors. In a turboelectric configuration, a hydrogen-fuelled gas turbine drives an electric generator, and the fan is driven by an electric motor.
The good news for general aviation is that modern PEM fuel cells can compete with piston aircraft engines in powering four- to six-passenger aircraft. But their energy-to-weight ratio is still far below that of turboprops and turbines. The fan of a turbofan provides about 80% of the engine’s total thrust, with the remainder delivered by combustion. It’s hoped that fuel-cell systems can be developed to deliver the entire thrust of a turbofan by electric power. Alternatively, fuel cells could be supplemented with battery power during takeoff and climb.
With the Hindenburg in mind, safety will be at the forefront of many passengers’ concerns, especially if they are sitting under huge gaseous hydrogen tanks.
Of course hydrogen-fuelled aircraft will need to meet the same levels of safety and integrity as those powered by JetA1, even if hydrogen-specific safety and regulatory standards don’t yet exist.
The questions are big: How do you ensure the structural integrity of the fuel tank? How do you inert a fuel tank with hydrogen? How do you refuel the plane?
Refuelling will require a whole new airport fuel storage and delivery infrastructure. However, one advantage is that hydrogen could be produced on site, eliminating distribution costs. A number of hydrogen suppliers have already lined up to provide fuel to airports should A or B get to fly on hydrogen.
SO HOW FAR INTO THE FUTURE IS HYDROGEN?
Most analysts expect the cost of fuel cells to follow a similar trajectory as photovoltaics, whose costs have fallen more than 80% in the past 10 years. And a high aviation demand should drive down the price of hydrogen from its current level of $12–$15/kg.
Prof Pilides expects there will be three waves of development as hydrogen technology matures. The first wave will begin with the commitment to mobilise U$100 billion for development costs over 15 years. This may sound like a lot, but is just 1% of the value of the European tourism industry.
In years 1 – 5 a JetA1 powered prototype will be flown to test the handling qualities of a plane with a massively pregnant fuselage. In years 4 – 9 a four-engine test bed will be flown with two engines on hydrogen and two on JetA1. In years 5-10 we should expect to see pure hydrogen prototypes flying. Then in year 11 cargo planes, and from year, 13 passenger planes.
So chances are, anyone younger than sixty will get to fly on a hydrogen powered passenger airliner.