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| Engineers squeeze more hydrogen into vehicles | |
July 2008 |
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| By Tristan Honeywill |
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| There are still few signs of serious investment in a hydrogen refuelling infrastructure, but the industry is pressing ahead with efforts to improve the way vehicles will store the fuel. The European StorHy consortium’s 34 members have worked for four-and-a-half years on ways of making tanks lighter and more compact. The main aim is to improve energy density.
BMW prefers super-cold cryogenic liquid hydrogen storage. It wants its hydrogen car to be larger and to have a hydrogen IC engine. It drives better, but is less efficient and so needs much more than the 5kg of gas allowed by the tank – 10kg is closer. Toyota has taken an interest in solid, pressurised storage, but was not part of StorHy. Its high density makes it ideal for small vehicles with very tight packaging requirements. StorHy’s solid team focused on complex hydrides, working on a nanoscale catalyst and magnesium alanate, but this is still a young science and the materials too heavy. Hydrogen firms Air Liquide and Linde led StorHy’s pressure vessel project. The tanks consist of a metal or polymer liner in a fibre-reinforced composite structure with sensors integrated for safety and onboard diagnostics. Faber Cylinders, Comat Composite Materials and connector firm WEH worked on the materials. There are still doubts whether the carbon-fibre industry can make significant cost reductions for mass production, something that future projects must investigate. Dr Volker Strubel of Magna Steyr, who had the complex task of coordinating the consortium’s intellectual property, said: “State-of-the-art pressure vessels cost around €15,000 at the moment – rather expensive. For Daimler, the first application will be feasible at around €5,000, but the target price is around €2,000.” Magna Steyr led the project on the cryogenic storage tank system for BMW, producing a demonstrator that shows the technology is feasible in principle. “The current version is a big cylinder that’s difficult to package,” said Strubel. “The aim was to make it smaller and more formable. We reduced weight by a factor of almost three.”
BMW Group’s R&D director Professor Raymond Freymann estimated that, filled with 10kg of hydrogen, it could give a range of 500km. Work on integrating more auxiliaries and improving cost and reliability will continue. Strubel described the tank as “an ambitious technology for around the year 2015”. The cryogenic tank has maximum pressure of less than 10bar. Compared to pressures of 700bar, it needs much less high-quality carbon fibre to fulfil the mechanical and structural requirements. “Significant cost reductions will be possible as production scales up,” said Strubel. “We can integrate more parts, use less carbon fibre and can shift to a lightweight metal structure for the outer tank, but we could apply cheaper materials as well.”
The main issue is minimising the boil-off of hydrogen. The tank consists of a double vault with a high vacuum and super-insulation. Pressurised systems have a plastic liner and can accept a minimal amount of hydrogen permeation, but any leak from a cryogenic tank means a breach in the vacuum that increases boil-off. BMW has proposed a compressed cryogenic hydrogen tank, a mixture of the two approaches, as a possible next step for research. “We have to guarantee a high vacuum for 10 to 15 years, a massive challenge,” says Strubel. “It requires a special metallic crystalline structure galvanised coating. It’s high technology.” Crash safety work has so far been limited to simulations, the tank placed in the middle of the vehicle to protect it. A 700bar tank is a stiff structure and should be able to match diesel and gasoline tanks, but safety requirements for hydrogen could be stricter. The consortium also came up with some neat suggestions for refuelling. One of the ideas that emerged was a rack for the pressure vessel so that it slides in and out of the car with ease. Changing gas bottles could be an economic way of keeping small fleets of vehicles on the move. It could be useful while the industry waits for an infrastructure complete with public pumps to evolve. The StorHy consortium
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