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01/10/2008
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Intense development is making fuel cells more viable, says Andrew Noakes, but there are still financial and environmental questions to answer
 As environmental concerns play an ever larger part in planning by car manufacturers and car buyers alike, alternatives to internal combustion engines are coming under ever greater scrutiny. Electric vehicles (EVs) have long been considered a potential replacement for internal combustion engined vehicles, at least in some applications – and given their advantages it's not difficult to see why.
EVs can be highly efficient, leading to potential reductions in energy use and a cut in costs. They cause no significant local emissions, giving an important air quality benefit in inner city use. Regenerative braking is easy to arrange, further improving energy usage, and noise pollution is low. Usually an EV does not have a multi-ratio gearbox, making it easy and relaxing to drive.
But it's not an entirely positive picture. The drawbacks are also well known, and currently rule out the widespread use of electric power for road vehicles. The main problems are limited performance, poor autonomous range, a slow rate of recharging and high long-term cost. The source of all these issues is the battery, which even in its latest lithium-ion guise is still a heavy, low energy density device with a limited service life.
Replacing the battery with a fuel cell stack potentially offers solutions to these problems, which is why virtually all major vehicle manufacturers are studying fuel cells in some detail.
In an electrochemical cell, electrodes are placed in contact with an electrolyte and separated by a membrane which is permeable to positive ions but impermeable to electrons. An electrical load across the electrodes causes positive ions to migrate through the membrane while electrons are forced to travel the long way, around the circuit. A fuel cell generates electricity through chemical reaction in a broadly similar manner, but instead of storing energy chemically the fuel cell derives its energy from a fuel and an oxidant supplied from outside the cell. Hydrogen is seen by many as the most promising fuel, although much research is being directed at fuel cells running on alternatives, while the most common source of oxidant is ambient air.
Automotive fuel cells are usually of the polymer electrolyte membrane (PEM) type, which offer rapid warm-up, small size and high power density in comparison with other fuel cell types. In a PEM cell, hydrogen is supplied to a porous carbon anode where a noble metal catalyst (usually platinum) is employed to encourage the hydrogen to split into protons and electrons at a relatively low temperature, around 80°C. Air is supplied to the cathode, where the protons combine with oxygen in the presence of the free electrons to form nothing more than water, so pure it is drinkable. The reaction also releases heat, which is removed using simple water-cooling channels within the fuel cell 'stack' and can be used for cabin heating or radiated to air.
The question marks arrive when the source of fuel – hydrogen – is examined in more detail. Currently, the usual way to produce bulk hydrogen is by reacting steam with natural gas (mostly methane) or other hydrocarbons at about 1,000°C to produce syngas, which is a mixture of hydrogen and carbon monoxide. But the process is energy intensive, and does not produce an ideal fuel: the CO in the syngas quickly 'poisons' the platinum catalyst in a fuel cell. Additional pre-processing is required to remove the CO, adding to the cost of the powertrain. Research is now being directed at platinum/ruthenium catalysts which are less susceptible to CO poisoning, allowing the use of syngas as a fuel.
Alternatively, electrolysis can be used to produce hydrogen from water, but again the drawback is the amount of energy consumed. As a result, only a tiny proportion of hydrogen produced commercially is made this way. Electrolysis would only provide an environmentally friendly source of hydrogen if the energy required was produced from renewable sources (which currently make up only a small proportion of world electricity generation). 'Biohydrogen' produced by algae is a greener alternative and schemes have also been suggested for production by fermentation of organic matter, but all these are a long way from commercial reality.
Another option is to carry an onboard processing unit to 'reform' hydrogen from gasoline, alcohols, natural gas or LPG. All of these fuels have a greater energy density than hydrogen and their use makes distribution of fuel and on-vehicle storage easier, but the extra processing equipment increases the cost of the powertrain. It also has the drawback of producing CO2 as a by-product, while the 'feedstock' is unlikely to be from a renewable resource.
Despite the unanswered questions over the production of hydrogen, vehicle manufacturers are pressing ahead with the design of fuel cell development vehicles. Ford, GM, Volkswagen, Toyota and Nissan/Renault have all demonstrated research vehicles recently. Daimler has probably the largest fleet of fuel cell test vehicles, with a total test mileage now approaching four million kilometres. It has been running fuel cell Mercedes-Benz A-classes since 2002, and a fleet of 60 have been undergoing customer trials in Germany, the US, Japan and Singapore since 2004. Following a trial of fuel cell buses in 10 European cities, Daimler has built 33 Citaro fuel cell buses which are now in use in Amsterdam, Barcelona, Hamburg, London, Luxembourg, Madrid, Reykjavik, Beijing and Perth.
Recently the company has announced a test programme with fuel cell B-Class cars which will begin in 2010. The B-Class incorporates ideas first seen in the F600 HyGenius fuel cell concept, announced at the Tokyo show in 2005 and recently demonstrated to EAD at a test circuit near Seville (see panel). The F600's fuel cell stack is smaller, lighter, more powerful and more economical than before, and has better low-temperature performance. The B-Class F-Cell promises a higher peak output (100kW/136PS) from a 40% smaller stack, with similar fuel consumption. Using a hydrogen tank pressurised to 700 bar, a range of up to 400km is claimed.
Daimler is aiming for a series production version around 2012-2015, and also intends to offer a fuel cell Citaro bus using a cluster of two B-class systems.
Honda has also recently hit the headlines with fuel cell vehicles, delivering the first of up to 200 FCX Clarity fuel cell cars to specially selected customers in southern California. All the cars will be leased for up to three years at a cost of $600 per month. It's safe to assume that Honda is subsidising the cost by a considerable amount.
The FCX is based around Honda's new 'V Flow' PEM fuel cell stack, which has a maximum output of 100kW (136ps). Hydrogen and air flow vertically within the new stack and gravity aids drainage of the water by-product, improving output and delivering better cold-start performance. Both hydrogen and air flow through wave-shaped channels which weave around horizontal cooling channels – a layout said to spread hydrogen and air more effectively over the electrodes, giving a 10% improvement in output over straight channels. The efficient cooling which results has allowed Honda to reduce the number of coolant channels within the stack, giving a 20% reduction in stack length and a 30% weight reduction. The FCX fuel cell stack is said to generate 50% more output per unit volume and 67% more per unit mass than Honda's previous system.
As with other fuel cell vehicles, excess output and energy recaptured during braking are stored in a lithium-ion battery and used to supplement the stack output when needed. FCX is fitted with a 171?litre hydrogen tank pressurised at up to 345?bar, which allows a range of up to 430km. The FCX is said to offer two to three times the fuel economy of a gasoline vehicle or 1.5 times that of a hybrid of comparable size and performance.
But as well as demonstrating that fuel cell vehicles can be a viable alternative to IC-engined vehicles and are nearing production readiness – in performance if not yet perhaps in cost – the scheme also shows up one of the biggest fuel cell problems. Southern California is about the only place on earth where Honda could have sensibly offered the FCX, because it has by far the greatest density of hydrogen filling stations thanks to the California Fuel Cell Partnership. The CFCP decided as long ago as 2001 that hydrogen was the way forward for fuel cell vehicles, and supported the building of hydrogen filling stations. Twenty-six stations are now operational on the west coast of the US, 15 of them in the Long Beach/Santa Ana area (and four more are planned). Elsewhere the hydrogen infrastructure is practically non-existent.
Inevitably it's a chicken-and-egg problem: investment in an infrastructure for hydrogen distribution seems hard to justify unless there are enough hydrogen vehicles on the roads to form a viable market. But without the filling stations, the vehicles will never sell. In the medium term it may be that BMW's approach – building dual-fuel vehicles which can burn gasoline or hydrogen, so they can cope with gaps in the hydrogen filling network while it grows – might be an important step on the road to a hydrogen infrastructure for road vehicles. Until that infrastructure arrives, fuel cell cars will never be able to replace IC-engined vehicles, however great their apparent advantages might be.
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Author
Andrew Noakes
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