Thursday, November 24, 2005

Fuel Cells - How do they work? part I

This is the second installment of my 3 part primer on FC technology. You may recall that i started the discussion with 'Why should i care?' which i feel should be the first question any researcher is asked (and a few times after you've started as well, no good inventing better buggy whips...).
If you're convinced that the idea has merit, a good place to start is to try and understand how they work. This gives you insight into their strengths and weaknesses which is neccesary if you are going to use them in the wild (i.e. electricity markets), which will be my 3rd posting on this topic.

Other people's work (linked here 'cause they have mostly done a better job than i'm going to)
How Stuff Works
Fuel Cell companies often have a 'how does it work' page, although they are of varying quality - Ballard, Ceramic Fuel Cells Ltd, Siemens to name just a couple.
Fuel Cell Today
The best mid-level (for tecchy's) overview of FC technology is the Fuel Cell handbook. This publication is produced by the US National Energy Technology Lab (a division of the DOE) with the intention of aiding the community as a byproduct of keeping up to date with developments. It's an excellent resource and they should be commended for distributing it free of charge.Fuel Cell Handbook 7th edition Nobember 2004

To understand fuel cells, you have to grapple with the concepts of thermodynamics.
To cut a long story short, chemical reactions (or systems in general) will always flow spontaneously in the direction that reduces the total energy of the system i.e. a balloon pumped up with air has a lot of energy stored in it (because of all those 'air' molecules being forced so close together) and will try and deflate as soon as it is able OR a piece of coal has a lot of energy stored in it (as bonds between atoms) and will try and exchange those particular bonds with others if there is air (oxygen) available (and a little heat to get the ball rolling).
Both of these examples show that when a pathway exists, chemical systems will always try and minimise their energy, it is exactly the same when water wants to flow downhill, it is just minimising the energy that it posseses (potential energy when it's raised above sea-level).
Fuel cells use the same energy potential as a normal burning process i.e. put a fuel and some oxygen together and if they are able to form new chemical bonds (i.e. combust) they will. This occurs every day in your car engine or a coal burning power station.
The big difference is the next step - whereas a car engine or a power station will use the heat/pressure that has been released as a by-product of the reaction to drive a piston or a turbine to do some work, a fuel cell gets the work done directly because the system wants to change (i could use TD terms here but i find the personal touch useful) and will drag electrons through a circuit to achieve its goal (and in the process do electrical work).
Check out some of the links above to see this in action, it'll make a lot more sense, just remember that the reason the reaction is occuring at all is because the system overall is attaining a lower total energy and has a pathway that will allow it.

This is the reason why fuel cells can be more efficient than combustion engines. Because a combustion engine has to 'push' something that we can then do some work with, it loses some of its total available energy doing this step and hence we can get less out of it to do some work. The fuel cell doesn't have any intermediary steps, it can drive a motor directly during its reaction because part of the reaction is moving electrons around.
It's almost as if the molecules use energy like money and engines have to spend it to get where they want to go and if you have to spend more of your money on say, train tickets, you have less at the end to spend on what you want. Fuel cells are smarter 'cause they make sure they're already where they want to be...

If you're interested in following this some more, the limit of efficiency of a combustion energy is defined by the Carnot Cycle whereas the efficiency of a fuel cell is proportional only to the amount of electrical energy you can extract relative to the amount of energy stored in the chemical bonds (propellar-hat moment: Efficiency = dG/dH).

Next step: how does a fuel cell 'drag the electrons through the circuit' as opposed to just burning?


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