Fuel Cells 2
Last time I talked about how fuel cells might change the face of motoring by getting rid of the internal combustion engine - and at the same time, they could wean us off our addiction to oil by bringing us the Hydrogen Economy. A fuel cell acts like a battery and delivers electricity as long as you keep on pumping the fuel into it. This electricity can then feed electric motors that then directly power the wheels.
Over the last few thousand years, we've changed the fuel that we burn from wood to coal, and then to oil. The trouble is that while we burn about 8 billion tonnes of hydrocarbon fuels each year, the burning usually happens with a crude, wasteful and dirty flame. When a fuel burns, everything happens very violently and rapidly.
Now here is the critical bit about burning - the fuel is consumed as it gives up electrons, and at the same time, the oxygen is consumed as it accepts the electrons.
A fuel cell splits and slows down these two separate processes in a steady controlled fashion. This makes it easy to tap off the electrons as electricity.
Fuel cells have another big advantage. They burn hydrogen. When you burn hydrogen and oxygen, you get water. But when you burn hydrocarbons, the "hydro" part gives you water (that's mostly OK), but the "carbon" part gives you all kinds of nasties, including carbon dioxide, carbon monoxide and even small particles of carbon that can get deep into your lungs.
A final advantage of fuel cells is that they are much more efficient than your regular car internal combustion engine. Part of the reason is that if you want to grab the energy of the expanding hot gases of the burning petrol-air mixture inside your engine's cylinders, you need really complicated moving parts. These moving parts chew up energy. But a fuel cell has no moving parts - it just turns hydrogen and oxygen directly into electricity.
The classic fuel cell has two electrodes, called an anode and a cathode. They're both made of expensive metals called catalysts.
You feed hydrogen gas to the anode. The platinum catalyst breaks it down into hydrogen ions and electrons. They then move off in different directions.
The electrons travel through a copper wire as electricity, then into your motor or electrical device, and finally into the cathode. The hydrogen ions take a different path and travel through the fuel cell until they too end up at the cathode.
So the electrons and the hydrogen ions have ended up at the cathode - but to wrap up the electricity and the chemistry equations, you need to feed oxygen to the cathode. This oxygen can come either from compressed or liquid oxygen, or far more easily, from the air that we breathe. The oxygen and the electrons and the hydrogen ions all combine to make water.
So now your fuel cell has turned hydrogen and oxygen into electricity. It has no moving parts, is completely silent and can reach efficiencies up to 40%. This is much better than your noisy car engine which might reach efficiencies of maybe 20 - 30%. The only waste products of a fuel cell are heat and water.
Fuel cells in your car can do more than make your car travel down the road - they can even feed electricity into the electrical grid when you park your car at night, or any time when you're not using your car. Just look at the numbers. Your house might use up power at a maximum rate of 15 kilowatts. But a car engine will generate anything between 50 and 250 kilowatts. If 5% of the cars in any city or town were used to feed electricity into the grid rather than drive around the streets, they could feed the entire electrical needs of that area.
A fuel cell needs fuel. There's lots of oxygen in the atmosphere. But hydrogen is much harder to come by.
There are two main choices in getting your hydrogen. You can either carry it, or you can make it as you drive along.
Carrying hydrogen means you carry it either as a very cold liquid, or a compressed gas. To cool it takes up about half the energy in the hydrogen. Regardless, you need expensive special tanks which either take up a lot of room or are heavy, or both.
If you want to make hydrogen as you drive along, you need a machine called a "reformer". You feed into it a chemical which contains hydrogen, and it throws away everything except for the hydrogen. If your chemical has long hydrocarbon chains (like oil), you need to run it at high temperatures, so you get a whole bunch of messy by-products and it takes a lot of effort. Petrol reforming needs temperatures of 800°C to 1,000°C. So there's a tendency to use short hydrocarbon molecules like extremely pure sulphur-free petrol, or methanol. Methanol is much easier to reform, but it takes more energy to refine methanol from crude oil than it does to refine petrol.
Theory is one thing, but a working fuel cell car is another - and that's what I'll talk about, next time...
Published 07 December 2006
© 2024 Karl S. Kruszelnicki Pty Ltd
