Fuel cells take off

A fuel cell powered experimental light aircraft has taken to the air, thanks to the efforts of one of the world's leading aerospace companies. The aircraft is a small two-seat motor glider originally made by Diamond Aircraft in Austria. The fuel cell is made by Intelligent Energy, a company spun out of Loughborough University. Boeing's Environmentally Progressive Air Transport Technical Team based in Madrid, Spain has engineered it. The airframe has an empty weight of 770kg and was originally powered by a Rotax 914F 115hp engine. Taking into account the additional 100kg weight due to additional components, such as lithium-ion power batteries, it was decided that the aircraft would require 45kW of electrical power for take off and climb, half from the fuel cell and half from the batteries, and 18kW for cruise - all from the fuel cell. The motor is a Power Phase 75, described as an experimental brushless permanent magnet DC motor made by UQM Technologies. Motor efficiency during cruise at 2,000 rpm is 89% and 86% during climb at 1,500rpm. The system design includes a motor cooling radiator plus a condenser and intercooler for the fuel stacks. The water is recovered for cooling and humidification. The motor drives a variable pitch propeller, an MTV-1-A/175-05 made by MT Propeller. Its rotational speed is dictated by the propeller pitch controller. A mechanical adapter is used to transmit the propeller thrust to the airframe structure, rather than the motor, which is not designed to endure large axial loads. The fuel cells exist as two stacks electrically connected in series. Maximum dry weight is 93kg, excluding mounts and wiring harnesses. The system was extensively tested on the ground before taking to the air. Hydrogen is supplied at 200bar and compressed to 350bar for storage in the onboard tank. Refuelling was found to take from 10 to 30mins. The batteries were supplied by Saft Aviation and consist of 66 VL20P cells connected in series – 11 blocks with six cells in each. The cells have to provide 25kW to assist the fuel cell during 7mins take off and climb but are sized for 50kW for 5mins. Four flight missions were undertaken successfully. Apart from the maiden flight, take off duration was 400m, rate of climb was 100m/min and altitude 1000ft. Mission durations were more than 25min, at the end of which, hydrogen pressure was 100bar and battery charge was at more than 35%. Technical manager Nieves Lapeña Rey told us that one of the motivations for the project is to produce fewer emissions. Not only does the fuel cell emit no carbon dioxide, but it also produces no nitrogen oxides, as would be the case with a hydrogen fuelled internal combustion (IC) engine. In addition, the fuel cell is around 40% efficient, compared to the 15 to 20% efficiency expected from a similarly sized IC engine. Rey says: "We chose this particular airframe for the project because it is aerodynamically very efficient with a lift to drag ratio of 27. "This is only a prototype. There is a lot of room for technical development. It could be made commercial. It obviously depends on investment but you could have one in the next five years." But present fuel cells are still too expensive, even for aircraft. The US Department of Defence has a cost target for fuel cells of $100/kW although that current state of the art system is well above this. Nonetheless, industrial gas supplier, BOC, is developing a 300bar composite hydrogen cylinder and a proton exchange membrane fuel cell unit. It will have a 150W average output with more than 300W at peak. "We plan to launch both the small generator and cylinder this year," says Stewart Dow, packaged energy manager at BOC says. "We are convinced that this technology is suited to a wide range of applications and it will be priced to make it affordable for customers. "The key point is that the fuel cell generator is ideally suited for powering high efficiency technologies like compact fluorescent floodlighting. This type of lighting can be driven by our 150W generator set and yet provide the same illumination levels as a conventional 750W tungsten floodlight. But because of the inefficiencies of conventional generator set at low power, there would be little point using this high efficiency lighting technology, as the fuel consumption would hardly be reduced. "Water is its only exhaust emission, so it can be used in places where diesel or petrol generators would cause environmental concerns or where there is a risk of exposing users to carbon monoxide or nitrogen oxides. It's also worth emphasising that, because of the scalability and efficiency, fuel cells are a great way of generating a small amount of power and can already offer a commercially attractive alternative to existing off-grid generating technologies. "Furthermore, given the political urgency of tackling climate change, hydrogen technologies have a high priority, especially for portable power applications - and not just in the UK. Hydrogen is creating a great deal of interest in all kinds of application areas." We might also add that in a military scenario, a fuel cell powered system, especially one providing perimeter security, is infinitely quieter and more discreet than one powered by a internal combustion engine driven generator. Pointers * A fuel cell powered light aircraft has been successfully built and demonstrated by Boeing. * Fuel cells and hydrogen cylinders for ground based applications, remote from mains power supplies, are to be brought to market during the course of this year.