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April 28, 2005

Fuel Cells

We all know that electric cars would be quieter and less polluting than those with combustion engines.  But we don’t all want to drive golf carts everywhere.  Production of practical electric cars has been stymied by limits of battery technology.  Gas-electric hybrids are a good thing but still burn at least two-thirds the gas of traditional cars to accomplish the same mission.

Fuel cells can be small and light enough to be the energy source for electric cars.  Since fuel cells produce only water as an exhaust, they are nonpolluting and don’t add greenhouse gasses to the atmosphere. This post is about hydrogen fuel cells and the energy generation and distribution systems necessary for their successful commercial use as a substitute for oil-based fuels in cars.

You may remember an experiment in high school chemistry in which the teacher passed an electric current through a test tube of water and split the water into it component elements, hydrogen (H2) and oxygen (O2). This process is called hydrolysis. The chemical equation you copied from the blackboard was:

2H20 + energy -> 2H2 + O2

The principal of a hydrogen fuel cell is to reverse that process.  Hydrogen gas and oxygen are combined to produce water and electrical energy.

2H2 + O2  -> 2H20 + energy

A combustion engine would combine the two elements in a fire and produce heat.  A fuel cell combines the two elements through a chemical process using a catalyst (usually platinum) and the energy is released in the form of electricity.  The low temperature catalytic reaction has a theoretically higher energy efficiency than a hot process.  To oversimplify, all the heat that your combustion engine radiates into the atmosphere and spills into the exhaust is wasted energy as far as moving the car. Much more detail on fuel cell design and operation is available in wikipedia.

The oxygen for this reaction in a fuel cell comes from the atmosphere just as the oxygen does for a combustion engine. But the hydrogen needs to be carried as fuel just as we carry gasoline today.  We will stop at  hydrogen stations and fill-up.  The important question for our economy and environment is where does the hydrogen we fill-up with come from!

Hydrogen can be extracted from methane, propane, coal, oil, natural gas and a variety of other sources.  However, we don’t reduce our dependence on fossil fuels if they are our source for hydrogen even though we can reduce the amount of CO2 released into the atmosphere by processing these hydrocarbon fuels centrally, putting the carbon back in the ground, and using only the hydrogen component in cars.

What will really change the world for the better is using electricity to produce hydrogen from water just as in the high school chemistry experiment.  I think the hydolysis will be done locally at or near the hydrogen stations we fill up from.  That means that there won’t have to be tankers full of hydrogen on the road or rails nor explosive pipelines to transport it to where it’s needed.  There just needs to be an expansion of our electric grid to bring the electricity to where its needed.  In this scenario, the hydrogen in a fuel cell is the mechanism for storing the electrical energy that goes into hydrolysis until that energy is needed to move a car.  The fuel cell is a glorified battery whose advantages are light weight and quick recharge.

Wait a minute though, you say, where is the electricity coming from that’s used to split the water in the first place?  That IS the important question.  Some of that electricity will come from wind, hydro, and solar sources.  Creating hydrogen through hydrolysis is a good way to store energy from a wind generator when the wind is blowing hard but the load on the system is low.  Hydropower can be efficiently produced with smaller dams and smaller storage lakes behind them if there is a way to use the off-peak surplus.  If all the hydrogen stations produce most of their hydrogen off-peak, the load on the generation and distribution systems for electricity smoothes out, capital costs per kilowatt go down, and “renewable” source become more efficient.

But a hydrogen economy is not a green panacea.  In order to generate enough electricity to make it the motive power for the world’s rapidly expanding car population, I believe that a significant amount of that electricity will have to come from nuclear power.  Nuclear power plants themselves, like hydropower and wind power sources, can be more efficiently designed if they can run at a constant energy output and if there is a market for off-peak production.  Use of electricity for hydrolysis is that market.  Fuel cells are the mechanism for using that electricity to replace fossil fuels for motive power.

I have blogged that growth of a car-driving middle class in India and China (and hopefully the rest of the world) forces us to reduce fossil fuel dependency quickly and resume the construction of nuclear plants in the US.

I’ve also blogged that China, with its lack of gasoline infrastructure, may lead the way to commercialization of the components of a hydrogen economy.

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» Hydrogen economy from Luca9200's blog
Tom Evslin has a great post on the Hydrogen Economy. [Read More]


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