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01/11/2011
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Mercedes-Benz F125! concept revealed a novel storage solution for hydrogen. Andrew English discovers if this fuel still has a future as an alternative power solution
 A few months ago, Britain opened its first public hydrogen filling station at the Honda factory in Swindon. However, 'public filling station' is something of a misnomer here, since there's just one pump, the fuel is all contained in pressurised bottles behind a wire cage and you need to register an account before any sort of filling takes place.
Developing a robust and low-cost proton exchange membrane fuel cell, a safe and cost-effective hydrogen infrastructure and sources of renewable hydrogen fuel have all posed massive difficulties, but talk to any senior fuel-cell engineer and they'll tell you that on-board storage is the biggest stumbling block. This is in spite of the fact that, in 2009, Toyota, Ford, Renault/Nissan, Mercedes-Benz, Hyundai/Kia, General Motors/Opel and Honda signed a letter of understanding over the introduction of fuel-cell cars in Europe, which adopted gaseous hydrogen, pressurised at 700 bar, as the standard.
"We, at GM, firmly believe that 700 bar compressed hydrogen is the right solution for the early commercialisation of fuel-cell technology in the 2015 timeframe," says Dr Ulrich Eberle, from GM's European Alternative Propulsion Centre.
"Having gaseous storage like this, while it may not necessarily be the long-term future for on-board hydrogen storage, is a good thing, because it helps to develop an infrastructure and to get people
used to the idea of hydrogen,"
states Dr Valeska Ting, a postdoctoral researcher into nanoporous hydrogen storage at
the Department of Chemical Engineering, Bath University.
"If you want to move quickly, you have to pick technology you know about, so this does make sense," comments Dr Tim Mays, senior lecturer in chemical engineering at Bath and principal investigator of the Engineering and Physical Sciences Research Council's UK Sustainable Hydrogen Energy Consortium.
There are problems, however, with 700 bar storage, not the least of which is the expense, weight and circular shape of the advanced carbon composite and aluminium tanks, which rob cabin space, restrict range and raise costs. Honda was keen to adopt 350 bar as an industry standard. The FCX Clarity carries 4.1kg or 171L of gaseous hydrogen at 350 bar in a single tank, which gives a range of 386km. Honda is committed to the European letter of understanding, but two consecutive chief engineers on its fuel-cell project, Yozo Kami and Sachito Fujimoto, have publicly stated that compressing the hydrogen up to 700 bar reduces overall system efficiency.
This is at a vehicle level, although Stewart Dow, packaged energy manager for the Linde Group, says that compressing hydrogen from 350 to 700 bar isn't a significant factor in energy loss. "Compression uses energy," he says, "but it is not that significant. Zero to 200 bar uses the most energy. Then you are only talking about an order of three times to get it from there to 600 bar and our compression technology is fairly efficient."
The logistics of the hydrogen supply industry also mean the fuel is regularly transported as a liquid or 700 bar gas, and the output from steaming it out of natural gas is already pressurised at 12 bar and more out of some electrolysers. Yet, at a first principles level, there might be better storage solutions. As one scientist puts it: "These include squashing it, squashing it some more, liquefying it, soaking it up with a sponge, pouring it into a solution or freezing it onto something."
Mercedes-Benz F125! concept car, a fuel-cell hybrid, uses a hydrogen fuel tank integrated into the vehicle's structure by using metal-organic frameworks (MOF), which are crystalline compounds comprising metal ions.
In MOF storage, hydrogen gas molecules are transferred onto the MOR's surface (adsorbed) at ambient temperatures and relatively low pressures of 20 to 30 bar.
Mercedes claimed its MOF tank offers an inner surface of up to 10,000m²/g on which to store hydrogen and the F125! carries about 7.5kg of hydrogen, for a range in excess of 1000km.
"We are very excited by the Mercedes," says Mays, "especially the use of MOFs for storage, and we are extremely interested in finding out more about how the technology was practically applied."
While most literature on MOFs cites carboxylate-based compounds such as MOF 177 as the most promising, in energy density terms, other materials are starting to be taken seriously. "Some research groups in the UK are looking at porous polymers at the moment, which have different advantages, compared with the MOFs," he adds.
Another storage medium that has recently reappeared on the radar is cryogenic hydrogen storage as a liquid. BMW employed this technology using 8kg (165 litres) of cryogenic hydrogen, stored in a Dewar flask at minus 253°C. The German company has been developing a hydrogen-burning V12 engine since the early 1990s. Here, too, the cost and bulk of the flask tanks have been huge issues, as have the evaporation of the fuel and the almost 55% loss of calorific value, turning it from a gas into a liquid. The latest development is to compress the liquid in the flask to increase fuel density, but Mays remains cautious of this approach. "The hydrogen density is high as a liquid, but you still have the problems of boil-off and the loss of energy in turning hydrogen into a liquid and keeping it in this state," he says.
While the US Department of Energy has set 2015 targets for potential hydrogen storage systems of a capacity of 40 g H2 per litre, less than 10 minutes to refuel, 1,000 lifetime refuelling cycles and an operating temperature range of 30 to 50°C, hydrides are lagging behind these, particularly in respect of the heat required to release the fuel.
Researchers in the UK are also concentrating on improving the release of hydrogen from metal hydrides – for example, via alloying and using catalysts. "It might be that there is no single solution for storage and different storage technologies will serve different niche applications" says Nuno Bimbo, a PhD student at Bath, working with Mays and Ting.
"For instance, we could use metal or complex hydrides for storing hydrogen in trucks and porous materials to store hydrogen in light duty vehicles."
But are the rules fair? Some scientists have started to question the DoE targets themselves. Yet General Motors now compares new storage technologies not with DoE targets, but with existing 700 bar gaseous storage system capacities of 0.023kg of hydrogen per litre, 0.048kg of H² per kg and a refilling time of three minutes. "Maybe the DoE targets are too tough," says Bath University's Mays.
"They might be tough, but there are other considerations," comments an unnamed source at Honda. "America is crucial to the success of fuel cells. California is the only place in the world where you can lease a fuel-cell car. It's the DoE's train set and, whatever we think of it, we're going to have to play on it."
Sodium alumate storage and sodium borohydride storage mediums, while never that promising, are some of the most high-profile victims of the DoE's 'stop/go' funding targets. Might the same fate await the similar storage medium of ammonia? This compound of nitrogen and hydrogen is hydrogen rich and can be reformed on board or burnt in conventional ICE engines. There's already a massive supply infrastructure for fertilisers and it can be stored as a liquid or inert salt tablets. "We are looking seriously at this technology," says Kwon Tae Cho, a senior research engineer with Hyundai.
"Ammonia needs to be treated carefully and still needs breaking down to produce hydrogen," Mays says. "We've also seen urea from animal urine suggested as a possible fuel or hydrogen source. While these ideas have their difficulties, it's good to think out of the box like this."
Porous organic polymers from a consortium of British universities and activated charcoal made more viable with new analysis methods from the Massachusetts's Institute of Technology are this year's models. While some question America's commitment to fuel-cell research under energy secretary Steven Chu, who has systematically reduced DoE funding for hydrogen research from the levels at which it has been over the last decade, other sources of funding are opening. Described as 'The Crown Jewels' by Markus Bachmeier, Linde's head of hydrogen solutions, the EU's Joint Technology Initiative on Hydrogen and Fuel Cells, and Germany's National Hydrogen and Fuel Cell Technology Innovation Programme (NIP), have provided extensive funding for infrastructure and storage research. "They've been most helpful," he says.
"There's been a variety of funding since 2000," says Mays. "For example, through the UK Sustainable Hydrogen Energy Consortium, there's £6 million across various projects and probably a total of more than £30 million on other hydrogen research – this is generous and it is a great start, but there could always be more." In fact, a group of UK researchers has recently bid for a major funding change that may lead to an effective national hydrogen and fuel cell research programme.
"Hydrogen has been oversold in the past," states Mays. "It was over-hyped throughout the 1980s and 1990s, and Steven Chu found himself continually signing cheques with modest results, as he saw it. We are in a period of recovery from all that. But climate change and energy security were not the issues then that they are now. While I don't want to re-oversell hydrogen, we are seeing some promising and realistic developments in storage and I am quietly confident."
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