Hydrogen was first used as a means of "powering" flight with the manned flight of a hydrogen balloon only ten days after the Montgolfiers' first manned hot-air balloon flight in 1782.

Despite achieving an excellent safety record - 50,000 passengers carried without a fatality - the use of hydrogen in air-passenger transport was brought to a spectacular end on May 6, 1937. On that day it took only 32s for the Hindenburg airship to be transformed into a pile of smoking debris.

Now, 60 years later, hydrogen is being reconsidered in civil aviation -but as a fuel rather than as buoyancy for "lighter-than-air" flight. Engineers from Daimler-Benz Aerospace (DASA), with German and Russian research partners, are carrying out work which may lead to the "Cryoplane", a commercially viable liquid- hydrogen (LH2)-powered aircraft.


Advantages of hydrogen

Hydrogen has much to commend it as an aviation fuel: nuclear power is too dangerous, electrical propulsion too heavy, and natural alcohols too low in energy/mass ratio. Hydrogen, on the other hand, can be generated from water, using renewable energy such as hydroelectric power. When liquefied and stored cryogenically below its boiling point of -253¹C, the resulting LH2 has nearly three times the energy to mass ratio of kerosene, and has been used extensively for many years as a rocket fuel. One litre of LH2 is equivalent to 800litres of hydrogen gas.

Economical, accessible reserves of crude oil are expected to run out between 2020 and 2040. Although transportation fuels can be produced from other fossil sources such as tar, shale and coal, this process involves higher production costs, making alternative technologies commercially attractive. Kerosene prices will escalate as supply diminishes, while the cost of LH2 will decrease as large-scale industrial products come on-stream. Although aviation is only a minor (3%) contributor to CO2 emissions, the Rio de Janeiro agreement to use taxation and regulation to reduce CO2 emissions will affect aviation as much as other industries.

"Burning kerosene produces both water and a lot of 'greenhouse' CO2 - gas that can stay around in the atmosphere for 100 years," says Dr Herman Klug of DASA. "The burning of LH2 produces no CO2, but a lot of water vapour that is also a 'greenhouse' gas whose effects vary with altitude. Close to the ground, water vapour dissipates within days, but it can persist for six months in the stratosphere."

He goes on to say that jet "contrails", composed of ice crystals, also contribute to the greenhouse effect. It is argued that the water resulting from use of LH2 fuel could produce more of these than kerosene. "On the other hand, the ice particles forming from LH2 exhaust will be bigger and have a supposedly reduced effect. A test aircraft will prove the point, but we can avoid the effect of a small fuel consumption penalty, by slightly reducing the cruising altitude of LH2-powered airliners."

LH2 combustion emits none of the secondary products associated with kerosene, except nitrogen oxides, but research suggests that emission of even these by-products can be reduced to one-third - and perhaps by one-tenth - of those from today's jet engines.

Hydrogen may, therefore, be an effective fuel, but anyone who has seen the dramatic film footage of the Hindenburg disaster will certainly question its safety. In the case of a fuel spillage, LH2 vapourises very quickly (kerosene hardly vapourises at all) and rises rapidly away from the spillage site, burning in an upwards direction that avoids the horrendous "fire carpets" which typify kerosene fires. Even if it catches fire, hydrogen has a large margin between flammability and explosion - leaking hydrogen will burn off rapidly but will not explode, and an explosive mixture can only be formed in a confined space. What happened to the Hindenburg proves the point: 200,000m3 (7 million ft3) of gaseous hydrogen burned, but did not explode.

Surprisingly, perhaps, hydrogen also burns with almost no heat radiation. It is anticipated that the aluminium fuselage of a modern airliner would be enough to withstand a fuel fire and protect the passengers within.

The commercial use of LH2 in aviation will place tremendous demands on technology for production, storage and distribution. LH2 boils at only 20¹C above absolute zero, requiring effective insulation. While it releases more energy per mass than kerosene, it requires four times the tank capacity, posing design problems for fuel storage on the ground and in the air.


Cold storage

Fuel stored in the wings of an aircraft near the centre of gravity avoids large changes in trim as the fuel is used. To keep LH2 fuel from boiling off uselessly into the atmosphere, however, it has to be kept cold, and that means the use of more insulation than can be accommodated in the wing space and larger tanks required to hold the greater volume of LH2 which must be carried. The solution offered by DASA is that fuel should be carried "piggy-back" fashion above the main cabin.

Overall engine configuration will remain unchanged, although some components - in particular the combustion chamber - will need to be redesigned to minimise nitrogen-oxide emissions. The cryogenic aero engine will have a considerably shorter combustion chamber than that of its kerosene counterpart, to suit the rapid-burning characteristics of hydrogen. Also included will be a heat exchanger which vaporises LH2 before injection into the combustion chamber.

"We have learned many lessons from the space industry. It is unlikely that extensive use will be made of space-based applications and solutions since design criteria are entirely different," explains Klug. The cryogenic tanks, pumps, pipes and valves of the proposed cryoplane require an enormous amount of analytical and experimental work in development and certification, as components and materials used in current cryogenic applications do not necessarily fulfil civil-aviation requirements for safety, service life and weight.

Creating the necessary fuel in the required amounts and the associated storage and transport infrastructure presents a problem.

Two projects are running for the evaluation of this aspect of LH2 manufacture and supply. The Europe Quebec Hydro-Hydrogen Pilot Project (EQHHPP) is co-ordinated by the Commission of the European Communities and the Government of Quebec. The 100mW pilot project is aimed at showing that clean, renewable energy (available in the form of hydroelectricity from Quebec) can be converted by electrolysis for shipment to Europe, storage and use for electricity/heat generation, vehicle and aviation propulsion.

Under the EQHHPP plan, gaseous hydrogen produced by electrolysis of water would be liquefied and transported in a new generation of container ship, now under design, which will carry five superinsulated barges each containing 3,000m3 of LH2. These will carry enough insulation to prevent any LH2 boil-off for 50 days. They could make 17 round trips a year, equivalent to a daily production of 45t of LH2.

On arrival, these barges can be towed individually on local waterways to the user's site. In 1990, the German Federal Institute for Material Research declared that LH2 would be no more dangerous than liquid propane or liquid nitrogen, and that it had no objection to the transport of LH2 in such ships.

There are no international regulations covering the transport of large quantities of hydrogen or its use as a fuel. French LH2 producer Air Liquide has more than 20 years' experience transporting LH2 from France to French Guiana for use in the Ariane space programme without apparent incident. Nevertheless, it has to ship LH2 to Japan from its Canadian plant via Oakland, California, because the Canadian province of British Columbia will not allow shipping to Japan direct from Vancouver.

New models for the transportation and use of LH2 need to be developed and standards harmonised. "It is usual for explosion simulation to consider TNT as a reference," says Patrick Sanglan of Air Liquide. "But this comparison is questionable for LH2 because of its behaviour. We need to lay down a whole new set of guidelines. In advance of that, we decided that the risk level should be equivalent to that for liquid nitrogen, even though we know that may lead to dramatically conservative results."


Large-scale conversions

The potential scale of the project to convert civil aviation to the use of LH2 is enormous. The first stage - conversion of domestic European widebodied aircraft - would involve some 500 aircraft and 70 airports. It also requires a quantum leap in technology to increase LH2 production from the present 20t per day to 6,000t. Using today's technology, that would consume the electrical output of ten large power stations. Patrick Sanglan of Air Liquide says that "the LH2 production of France would cover only 0.25% of its jet-fuel consumption".

A Tupolev Tu-155 has been in operation since 1988 as a flying testbed, with a starboard engine which can operate using kerosene, natural gas or hydrogen. Product cycles within aeronautics are extremely long: it can take 50 years from the development of a new aircraft to the phasing out of the last of the series. A radical development such as the introduction of new fuel could further prolong that process. Assuming that development targets are met, Daimler-Benz anticipates that the first series-produced LH2 Cryoplane will be a small regional aircraft, and the company is hard at work on a "demonstrator" programme . The intention is to modify a 30-seat Dornier 328, which they hope will fly in the year 2000, with a series version entering service in 2005.

In the future there may also be an LH2-powered "Follow Me" airport van and crew bus, as environmental awareness and legislation, coupled with increasing real and hidden gasoline costs, tilt the cost in favour of LH2 over the next few years.

Source: Flight International