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Know It All: Fuel Cells -- From Farts to Fuel

Published in Geare magazine, Issue #29, 2004

Electricity is frustrating. On the one hand it is by far the most versatile form of energy for filling the myriad of user requirements: light, heat, computing, running TV sets. And so on.

Equally, it ought to be the best form of energy for ground-based transportation (airplanes are a different matter). The internal combustion engine (ICE) we currently use for cars has several drawbacks, one of which is that it has to move in order to produce any torque, and has to be moving pretty fast (1,500 to 4000 rpm, depending on the engine design) to produce anywhere near its maximum torque. Since it is engine torque that produces acceleration, ICE cars must have gear boxes.

Electrical cars avoid all this. A suitably designed electrical car wouldn't even have a clutch! It will apply full torque right from a standing start. Imagine the saving there!

That's the good oil on electricity. But it has one huge problem. Electricity is hard to store.

Consider. One litre of petrol can provide about 37 megajoules of energy to an ICE. A fully charged car battery, however, only holds about 2.7 megajoules of electrical energy. To provide the same energy supply as a fifty litre petrol tank, a car would need to carry nearly 700 lead acid batteries! Actually, offsetting for the higher efficiency of electricity, this would come back to just 200 or so. Still an awful lot for a car to have to lug around.

But there is a technology that provides the efficiencies and conveniences of electrical cars, and the storage convenience of a chemical fuel.

That technology is the fuel cell. This uses hydrogen to generate electricity with great efficiency.

The fuel cell works like a regular battery. In a normal battery a chemical reaction takes place ... slowly. In chemical reactions the electrons of the constituent elements get shoved around, taking new configurations. In a battery, those electrons are piped from where they used to be, to where they need to be, via the wire that delivers electricity to the torch bulb, or the PDA, or whatever it is that the battery is running.

Eventually the chemical reaction is complete, the electrons stop flowing, and the battery is flat. With rechargeable batteries the reaction can be reversed, simply by pumping electricity back into the battery, which forces the flow of the electrons to reverse.

Fuel cells work by replenishing the chemicals, and allowing the compounds formed to be cleared from the cell. So instead of the pastes used in dry cells, or the acids used in wet cells, fuel cells use hydrogen gas and oxygen gas. The latter is derived from the air, so only the former needs to be carried.

Fuel cells are tremendously efficient. The real-world efficiency of an ICE is about 15%. That is, of those 37 megajoules of energy in a litre of petrol, only 5.5 end up pushing the car around. The rest end up as heat and noise. The real-world efficiency of a fuel cell/electric engine combination is approaching 50%.

That presumes that the fuel will be hydrogen. But hydrogen itself presents some storage difficulties, so an alternative is to use a hydrocarbon liquid -- say methanol -- to feed the fuel cell. This requires an additional processing step called 're-forming' to release the hydrogen, but even so the reforming/fuel cell/electric engine combination still has a real world efficiency of 36 per cent.

Another advantage of fuel cells is that a hydrogen fuel cell generates just one waste product: water. That makes it rather more environmentally friendly that ICEs.

So why isn't the world running on fuel cells? Why, for the usual reason of course. Cost.

Getting hydrogen and oxygen to combine in order to form water normally requires some kind of a jolt. They don't just do it spontaneously. The easiest way is to apply a flame to the hydrogen, which then ignites. But that just makes a different kind of ICE. Fuel cells get the reaction to occur at a much lower temperature (they operate at about 80 Celsius) by running the gases over a catalyst. This, unfortunately, is platinum, which as we know isn't especially cheap. There are also intricate manufacturing procedures to provide just the right texture on the various surfaces to allow maximum contact for the reactions to take place.

The net result of this is that fuel cells presently cost around $US3,000 per kilowatt of power output. So a fuel cell suitable for a car -- one able to deliver say 30 kilowatts -- now costs $US90,000.

Prices are falling though. Eventually electric cars will become a reality, thanks to the fuel cell.

© 2002-2009, Stephen Dawson