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التقرير
HYDROGEN AIRCRAFT FUEL RESEARCH PLANS
by Wolfgang Birkenstock The end is in sight. Even if opinions about the exact date may differ, the supply of fossil fuels is expected to be exhausted sometime in the next century. This is the latest when we will need alternative forms of energy at our disposal. They will have to be developed well enough not to just work in the laboratory but be used safely and reliably in operational service.
One possible energy source is hydrogen. It might be used in aviation as well. In view of the fact that today’s airliners have a service life of several decades, it seems only reasonable to look into the basic requirements for aircraft that are powered by hydrogen. This applies even if these aircraft are currently not economically competitive and will probably only be used in 20 to 30 years.
As early as 1956 tests took place in the USA with a modified B-57 Canberra. In 1988 a triple-jet hydrogen powered Tupolew Tu-154 flew in the former Soviet Union. Since the beginning of the nineties, Germany’s Dasa has been working on hydrogen propulsion systems within the framework of the 
German-Russian Project Cryoplane.An important future milestone will be a hydrogen-powered demonstrator based on the Dornier 328JET.
A very important consideration in using hydrogen as aircraft fuel, is the possibility of a significant reduction in harmful emissions. During the combustion of kerosene in today’s engines, carbon dioxide (CO2) and water (H2O) are produced. Additionally lesser amounts of sulphur dioxide (SO2), carbon monoxide (CO), nitrogen oxide (NOX) and unused hydrocarbons (HC) are also emitted. The last three substances are considered to be greenhouse gases.
However, if hydrogen is used, water is the principal product of combustion. Unfortunately the forming of nitrogen oxide cannot be avoided because the necessary nitrogen is present in the air. The emission of water, which acts as a greenhouse gas at altitudes of eleven to twelve km, is significantly higher than with a kerosene-fuelled engine.
However, according to Dasa’s Dr. Hans-Wilhelm Pohl and Dr. Hans-Günter Klug, project managers of Cryoplane, emissions are 20 to 30 percent less than with a comparative kerosene engine. Since the greenhouse effect of water depends very much on altitude, the harmful effects of water emissions can be reduced significantly by lowering the flight altitude. The downside of this is slightly higher drag and an increase in fuel consumption.
Hydrogen has one further advantage: In principle there is an unlimited supply of it, although it is fixed in the form of water. Energy has to be used to extract hydrogen from water to make it useable. If this problem is solved elegantly - for example with regenerative energies - a closed loop is created with its combustion: water-hydrogen-water.
How will a hydrogen-powered aircraft like the 328JET technology testbed differ from a conventional aircraft? The specific properties of hydrogen call for a few fundamental modifications. Compared with kerosene, hydrogen has a higher combustion value, a lower density and a wider combustion area.
While the combustion value for kerosene is around 42,800kJ/kg, it is 120,000kJ/kg for hydrogen. Hydrogen contains three times as much energy per kg, which in turn means that only a third as much fuel has to be carried to cover a certain range. This is how the aircraft’s maximum load is boosted. Dasa reports an increase of about five per cent with regional and short haul aircraft and around 20 per cent with long distance jets.
However, the significantly lower density of hydrogen causes problems with the fuel tanks. In order to reduce the space needed for the storage of hydrogen, there are plans to store the fuel in liquid form in its designated tanks at 20 degrees Kelvin (minus 253 Degrees Celsius). But even then is the specific volume of hydrogen twelve times larger than that of kerosene. If you take into account that only a third of the weight of kerosene has to be transported, a tank, which is four times bigger than that for a kerosene aircraft, is needed for a hydrogen airplane.
Dasa has explored various ways to accommodate this volume in the aircraft. An important consideration is, that the low storage temperature of hydrogen requires effective insulation.
In order to keep the weight of the insulation as low as possible, the ratio of tank surface to volume must be low. Spherical or cylindrical tanks are favourable. Conventional wing tanks are not feasible. For bigger airliners the best possible storage space is the fuselage above the passenger cabins. However, this is detrimental to the plane’s aerodynamics: The fuselage becomes stockier, and friction and pressure drag increases.
It is planned to fix two huge hydrogen tanks, supplied by MAN, underneath the wings of the 328JET testbed. Messer, formerly Messer Griesheim, is supplying the tank insulation. A so-called Super-Vacuum-Insulation will be used. It consists of a double-walled cell and additional interfacing to absorb radiation and minimize heat exchange.
Since the Fairchild Dornier 328 is a high-wing, it will not be practicable to house the tanks in the aircraft’s wings. Apart from this, one is trying to avoid expensive modifications to the demonstrator’s fuselage. Dr. Pohl informs us, "We want to realise this project with as little cost as possible."
Modifications to the engine are only minor, because the way the aggregate works basically does not change. The biggest change is that kerosene is being replaced with water. The combustion chamber can be shortened, because hydrogen gas burns spontaneously, and the fuel-air-mixture does not need to stay for long in the combustion chamber.
However, the combustion chamber needs to be redesigned to overcome the problems of nitrogen oxides in the emissions of hydrogen engines. The formation of nitrogen oxide is dependent on the peak temperatures reached in the combustion chamber and on the length of time that the gas mixture spends inside of the combustor. Without modification, a hydrogen engine may generate more nitrogen oxide than a kerosene engine.
Combustion chambers with pre-mixing seem to be the best solution: The homogenous fuel-air-mixture guarantees an even combustion. During trials at MTU in Munich, the use of a so called “Premixed Perforated Plate” reduced nitrogen oxide emissions by 95 per cent when compared with a modern kerosene turbofan. According to Dasa’s Dr. Jonny Ziemann this technology is a promising option for hydrogen fuelled engines. He is responsible for the engine aspect in the Cryoplane-Project.
However, putting this idea into practice has its pitfalls. Extreme lean or rich mixtures can cause self-ignition during premixing or a backlash from the combustion chamber into the pipes, in which the premixing is taking place.
Along with the conventional option of using a combustion chamber without premixing there is still the so-called Micromix procedure, which was developed at the Technical University in Aachen. Premixing is not required with this procedure. The big combustion chamber would be replaced with many small ones, such controlling inhomogenous mixtures. With this technology, NOX emissions can be reduced by up to 80 per cent as compared to an engine running on kerosene. The Aachen University will also modify the APU (Auxiliary Power Unit) for the 328JET to run on hydrogen.
Furthermore, there are fuel pumps, pipes and control valves in a hydrogen-powered engine. These components will have to be developed from scratch. This is Tupolev’s task. The Russian Dasa partner working for the Cyroplane Project, designs and supplies the fuel system. There is a slight increase in mechanical demands (hydrogen is stored in the tanks at 1.5 bar). Additionally, there are thermal requirements because of the extreme fluctuation in temperatures. If the aircraft was on the ground for some time, the hydrogen, which remained in the pipes, will have almost reached outside air temperature. The hydrogen is then present as gas. A few minutes later liquid fuel needs to be supplied to start the engines at a temperature of minus 252 Degrees Celsius.
Apart from this, the pumps need to be more powerful, since hydrogen is far less viscous. Regulation is more difficult because of compression effects: Kerosene in its liquid form has a constant density but the density of hydrogen changes not only during the change of state from liquid to gas. It changes tremendously.
The heat exchanger is a new development in this fuel system. It vaporises hydrogen, which is stored in tanks in liquid form, before it enters the combustion chamber. This process ensures even combustion. Otherwise, uncontrolled phase or density changes could occur. It still has not been decided exactly where the heat exchanger will be placed in the 328JET’s Pratt & Whitney Canada-PW306B Engines. It will probably be near the nozzle area. However, to save cost, the engine will not be modified significantly. With enough funds, the engine could be made a lot more efficient. This may be a future option. The engineers are considering warming up the hydrogen with air, which is fed into the engine, instead of installing the heat exchanger in the jet.
In order to be able to use hydrogen for civil aviation in a few decades, not only technical problems have to be solved. A whole new infrastructure has to be put in place: Airports have to be converted and production and availability have to be guaranteed on a bigger scale than today.
From a technical point of view the otherwise advantageous wide combustion range of hydrogen is a problem. The use of hydrogen in an aircraft may cause some passengers to remember the accident of the airship Hindenburg in 1937 in Lakehurst. This is why potential passengers will have to overcome their inhibitions.
“This is primarily a psychological problem”, says Dr Pohl. He refers to decades of experience with town gas, 50 per cent of which consists of hydrogen. In the atmosphere hydrogen disperses very quickly and cannot explode. Tests carried out by NASA have proved this.
The slight excess pressure in the tanks contributes to safety. In case of a leak it prevents air from entering the tanks and thus the formation of harmful gases. Instead hydrogen escapes and evaporates. In the case that hydrogen turns into gas in its tanks, which might happen when the aircraft has been on the ground for some time, a safety valve on the vertical tail will ensure a controlled release.
Whether or not a cleansing system will be necessary for the engine will be evident once development is under way. Once the engines are turned off, nitrogen could be blown through the engines to get rid of any remaining hydrogen. It is evident today that a lot of thought has to be given to safety before the new technology can be applied.
The original plan to have a hydrogen-powered aircraft airborne by the time the World Exhibition in Hannover takes place in the year 2000 must be revised. According to Dr. Pohl the sale of Dornier to Fairchild and the conversion from the turboprop-328 to jet engines were part of the problem. Conversion of the 328JET will be done mainly by Airbus Germany in Hamburg and Bremen.
There are still uncertainties as to who will modify the engine. Since Pratt & Whitney Canada supplies the engine for the 328JET, this company was also supposed to make the necessary modifications for the hydrogen demonstrator. However, the Canadians pulled out of the project a few months ago.
MTU stood in for Pratt & Whitney and has started to convert the P&W engines. Dasa’s Engineer Pohl assumes that the conversion of the demonstrator will finally be under way in the second half of this year, i.e. detailed planning and a start to the alterations can begin. To date, no conversion has taken place, and nothing has happened apart from a few test runs of the engines and a few rough ideas having been noted down.
The finances for this divided project have partially been secured. The 60 million DM for the first part, “aircraft and APU”, are funded by the Ministry of Economic Affairs and five federal states. For the second part, “Main Engines”, MTU is investing its own resources. Apart from this, Dasa is trying to secure public funds. It is still not known how Dasa will raise the remaining 50 million DM.
The technology carrier is intended to fly three years after the program is launched. To begin with only the APU will be powered by hydrogen, six months later hydrogen powered engines are supposed to follow. If everything goes according to plan this will happen in 2002. As early as the beginning of the nineties, Dasa tried to modify an Airbus-Airliner to run on the new fuel. Pohl says that “these plans are still current”. He is convinced that a hydrogen-powered Airbus will fly in 12 to 14 years.
الرابط
http://www.flug-revue.rotor.com/FRhe…09/FR9809k.htm
http://www.flug-revue.rotor.com/FRhe…09/FR9809k.htm
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