Fuel for thought

Hydrogen-powered civil aircraft are a possibility, according to new EU research. But will they ever be a reality? Helen Knight and Keith Nuthall report.

Car manufacturers, ever eager to display their environmentally friendly credentials, have long presented research into hydrogen fuel as evidence of a desire to clean up their act.

But doubts remain about the likelihood of hydrogen cars ever becoming a reality while petrol stays comparatively cheap, and the huge cost of establishing the necessary infrastructure looms dauntingly on the horizon.

Meanwhile, watching furtively from the wings is the aerospace industry. It is aware that replacing kerosene with hydrogen fuel would be a huge financial drain that could slow the sector’s growth, but also realises that international emissions agreements may eventually force it down that route.

So the industry is taking a cautious, wait-and-see approach – not actively developing hydrogen aircraft, but investigating the technological steps needed to produce them.

To this end, a European Union-funded project called Cryoplane carried out the conceptual groundwork for a civil aircraft powered by hydrogen liquefied at cryogenically low temperatures, and its final report has recently been published.

A consortium of 35 European partners were involved in the project. Led by Airbus Germany, it also included Airbus UK and Cranfield University’s school of mechanical engineering, which produced a system analysis of hydrogen as an aviation fuel, considering applicability, safety and environmental compatibility.

The research team also investigated medium and long-term scenarios for the switch from kerosene to hydrogen in aviation. A range of possible aircraft adaptations was considered, from business jets to large long-range aircraft such as the Airbus A380.

The key issue was modelling a liquid hydrogen fuel system, taking into account the fact that per unit of energy liquid hydrogen has four times the volume of the standard aviation fuel kerosene. As a result, fuel tanks have to be four times as large as on current models.

Modelling work showed that larger exterior surface areas would be required; and there would be a 25 per cent increase in aircraft empty weight, causing an increase of 25 per cent in energy consumption, even though, because of the light fuel, take-off weights would be lower. taking just the fuel into account, this would mean an overall operating increase of four to five per cent.

That said, Cryoplane researchers concluded that hydrogen-fuelled engines would be as powerful as kerosene engines, and that conventional turbo engines could be converted to run on hydrogen, although some further research is needed. They also decided that hydrogen-fuelled aircraft would not be less safe than current planes although regulations for airworthiness, ground handling and servicing would require adaptation.

Of course the big benefit of converting today’s aviation fleets to hydrogen would be environmental. As hydrogen can be produced from water, and produces water when burnt in air, its greenhouse gas emissions are substantially less than kerosene. There would be minor oxides of nitrogen emissions, and the water in the aircraft’s condensation trail would also act as a greenhouse gas at high altitudes, so the hydrogen-powered aircraft might have to cruise a few thousand feet lower than current levels to eliminate these contrails. However, water vapour stays in the upper atmosphere for only six months, while carbon dioxide hangs around for 140 years.

So the environmental benefits would be overwhelming, but could hydrogen aircraft be built?

A key question would be where to put the fuel tanks. Because of the need for additional bulk, researchers quickly realised that these would be too heavy for the wings. Different locations were developed, depending on the type of plane. With small regional aircraft and business planes tanks would be placed in the fuselage aft of the rear pressure bulkhead – the only feasible place considering the necessary centre of gravity. Even taking this into account, a wider fuselage was needed and a very large horizontal tail, which meant the aircraft suffered from increased trim drag and reduced maximum lift.

For regional aircraft of up to 100 seats and short/medium-range aircraft, the tanks would be placed both behind aft pressure bulkheads and on top of the fuselage, to ensure balance. These forward tanks would, however – because of space pressures – be situated on top of the fuselage, creating more drag. Because of the risk of explosions, a dry bay would have to be created, making this design even less aerodynamically efficient. However, there is a safety benefit; in a fire the liquid hydrogen in the top tank would boil off, evaporate and rise.

A two-tank solution was also adopted for long-range and very long-range aircraft such as the A380, with tanks placed in the fuselage aft of the rear pressure bulkhead and between the cabin and the cockpit, with the planes being large enough for a catwalk to link the two. Blocking off the cockpit completely from the cabin using the fuel tank could have an important additional benefit in deterring terrorists, and was considered by the industry after September 11. That said, structural concerns regarding the front tank being part of a pressure vessel have not been examined and would need careful study.

As for the possible shapes of hydrogen planes, the report concludes that conventional designs adapted to cope with the larger tanks would be the best option. Of the unconventional configurations investigated, the twin boom system, in which the wings and tail are joined together using the hydrogen tanks, was regarded as the most promising. This design, also known as the joined-wing or Prandtl plane, would make a virtue of the desire to separate the tanks from the passenger cabin for safety reasons, and to prevent heat from the cabin raising the temperature of the liquid hydrogen, according to Prof John Fielding, head of the aerospace group at Cranfield University.

Prandtl planes have structural benefits as the tail and wings support each other, while the design creates a stiffer structure meaning weight can be reduced, said Fielding. Some researchers claim the design – also being investigated for kerosene-fuelled aircraft – offers aerodynamic benefits, as lift is provided by the tail as well as the wings. However, the report concludes that its use of large external tanks caused too much drag.

Finally, blended-wing-body designs were also investigated, but not deemed suitable by the report, because they ‘had a lot of unused volume’.

However, Prof Riti Singh, deputy head of the school of mechanical engineering at Cranfield, who co-ordinated the university’s work for the project, said dismissing the blended-wing design at this stage would be premature. This is because the most suitable design for a high-pressure vessel such as a liquid hydrogen fuel tank is a spherical shape, as this means the pressure inside the vessel is acting equally in every direction. As a result a spherical fuel tank will be lighter than an oblong design, which requires thicker walls to compensate for uneven pressure.

However, such a design would obviously have a larger diameter than an oblong fuel tank, making it difficult to place the tanks in the area above the passenger cabin of a conventional aircraft – the layout most favoured by Airbus.

Blended-wing-body aircraft, in contrast, effectively have one much deeper wing, making it easier to install the lighter spherical tanks. ‘Blended-wing-body aircraft create a fairly deep space as a feature of the design, and it is then easy to put in spherical tanks to keep the hydrogen at the necessary high pressure and temperature,’ said Singh. ‘There are some very fundamental reasons why blended-wing-bodies will be looked at again very seriously.’Our perspective is that you could do this with a conventional-type aircraft with the hydrogen tank at the top. But as the technology is not likely to be introduced for some time, it is quite probable that by then a blended-wing-body aircraft could be designed and introduced at the same time, and would be more effective.’

Regarding its operation, the team concluded that the fuel system would be more complex than for a conventional kerosene aircraft, as liquid hydrogen is virtually a boiling liquid, which can evaporate. It noted that Air Liquide has developed cryogenic tanks for the European Space Agency’s Ariane programmes, but said more work was required because aviation usage would require these tanks to have a much longer lifetime than in a space rocket. Tank insulation would be chosen from 22 types of foam or a customised super insulation material. These would have to be light and effective, and the tank would have to be tough and not break down as a result of small damage.

Obviously, the selection of an engine is a key issue and the report recommends three options, based largely on research carried out at Cranfield. The first and simplest option is to heat up the liquid hydrogen to a temperature suitable for burning using a heat exchanger within the exhaust of the gas turbine. This could be done without adding much weight or taking up much space within the aircraft.

However, according to Singh, the need to heat up the hydrogen before burning offers a valuable opportunity to improve the efficiency of the engine.

One of the problems with gas turbines is that they are more efficient when operating at higher temperatures, but the best materials, coatings and cooling technologies available today are only effective at temperatures of up to 1,650-1,850K. If the material used for the nozzle guideway into the turbine could be cooled using the liquid hydrogen, it would be better able to withstand higher combustion temperatures, making the engine more efficient.

A further option would be to use an inter-cooler between the engine’s two core compressors. Air coming out of the first compressor would be cooled by heating the hydrogen through the cooler, and once the hydrogen had been heated and turned into vapour it would continue into the combustion chamber. Meanwhile, the compressed air would now be at a lower temperature before going into the second compressor, reducing the necessary size of this compressor and the amount of work it must do to compress the gas, and therefore improving the overall engine cycle.

Performance requirements of engines would include high combustion efficiency, good durability, reliable ignition, acceptable combustor exit temperature for turbine durability, low pressure loss and low emissions. Engines would be mounted on wings, as with conventional kerosene planes.

Researchers also looked into other uses of the hydrogen other than propelling the plane. These could include using hydrogen-powered environmental control systems to replace conventional hot bleed air usage and air-conditioners that usually heat and cool cabins.

However, a more detailed analysis of its advantages is required. The team also considered whether the auxiliary power unit could be a continually operating hydrogen fuel cell. In this case, ‘it might be possible to eliminate the option to use an engine for system power and allow the engine to be downscaled,’ the report states.

Boeing is already investigating the use of fuel cells as auxiliary power units at its European Research and Technology Centre in Madrid.

Of course there are also safety concerns. No one involved in the development of hydrogen planes will want to see another Hindenburg-style disaster involving the fuel, which could spell commercial doom for a nascent technology. Cryoplane concludes that the overall safety level would be ‘not worse’ than for kerosene aircraft. However, there are safety issues that need further investigation and these include the key area of managing large-scale releases of liquid and gas hydrogen, for whatever reason. For instance, work is required to examine the likelihood that sufficiently large clouds of gas might detonate. Research is also required into the ability of valves and other connections to deal with the low temperature of liquid hydrogen.

Another key issue is economics. The simple fact is that, under today’s economic conditions, hydrogen is not yet economically competitive. The report’s scenarios for the production, transportation and storage of hydrogen fuel envisaged that liquid hydrogen would not be used in commercial transport operations until 2015. Looking at competition with kerosene, the consortium was even more pessimistic, saying that if hydrogen were developed as an alternative fuel, the prices of these two energy sources would not be equivalent until 2040. It added that if efforts to limit greenhouse gases were effectively stalled for a long time by some political initiative, the date might shift much further into the future.

‘To design a conventional gas turbine from scratch today would be $2bn (£1.1bn), so to design a totally new type of gas turbine would be even more, and to design a completely new airframe would add another $5-10bn (£2.75bn-£5.5bn),’ said Singh.

‘There isn’t an economic case to shift from conventional aircraft – the only case is environmental. These are very large risks, so there is no likelihood that the airframe manufacturers will move in that direction, other than to carry out studies, unless the political decision is taken, and taken internationally,’ he said.

Any international political drive towards hydrogen aircraft is only likely to come about as a result of grass-roots pressure, he added. And even though grass-roots pressure helped bring about the introduction of unleaded petrol for cars, there are no show-stopping technological barriers to the introduction of hydrogen aircraft. ‘The technical challenges are understood. Known design principles can be applied in new components,’ said the report.

Of course further R&D is needed, but the report claimed that hydrogen planes could be flying within 15-20 years. ‘Hydrogen could be a suitable alternative fuel for future aviation. Based on renewable energy sources it offers the chance to continue the long-term growth of aviation without damaging the atmosphere,’ it concluded.

The case against hydrogen fuel cells

NASA recently carried out an investigation into a blended-wing aircraft powered by a number of distributed hydrogen fuel cells.

Powering aircraft with fuel cells would eliminate carbon dioxide emissions and increase fuel efficiency. However, significant advances in fuel cell technologies would be needed simply to make such an aircraft possible, according to the report, which was compiled by experts at NASA’s Langley and Glenn research centres.

Indeed, even with projected improvements in the technology within 25-30 years, a fuel cell-based system would be much heavier than conventional aircraft propulsion systems.

A large aircraft requires 10s of MW of propulsive power, while the highest electric motor power output possible with current compact fuel cell technology is around 1MW peak, and 671kW continuous.

Each of these fuel cells also weighs around 7,076lb, while a conventional engine – of which aircraft have just two – weighs around 8,600lb.

This would mean significant advances in airframe technology would be required to offset this weight increase – something in the order of a 15 per cent reduction in drag and 30 per cent reduction in airframe weight.

But despite these issues, there could be a case for hybrid aircraft powered by fuel cells during their cruise mode, according to Cranfield University’s Prof Riti Singh.

Fuel cells might not be suitable for take-off and acceleration, but once the aircraft is already in the air, and has reached its cruising speed, it could be feasible to use the technology to overcome air resistance and therefore keep the plane at this speed, he said. ‘On the flight from the UK to Singapore the take-off and climb is just 25 minutes, with the rest of the 11.5 hours cruising. So it could be an interesting and important issue,’ he said.