Wednesday, November 23, 2005

Electrons vs. Molecules

Following on from a couple of comments to yesterday's posting concerning batteries competing with fuel cells, the basic issue is how to deploy non-petroleum energy sources for transportation. Over 90% of all energy for transportation, including planes, trains and automobiles, currently comes from oil. Hydrogen fuel cells and rechargeable batteries are only two possible alternatives for enabling primary energy sources such as natural gas, wind and solar power, or coal, to compete directly with petroleum products.

There are other alternative routes requiring greater changes in our current transportation systems, including converting to electric cars that pick up power from the roadway or from microwave transmission. (Sadly, the Bluetooth short-range wireless protocol has co-opted one of the best frequencies for the latter.) I think designers have generally assumed that a fuel cell constitutes a more modest and palatable change, because it simply replaces the internal combustion engine, while leaving the car it powers recognizably a car, able to use the same roads as today. Hybrids better that proposition by avoiding the need for entirely new fueling infrastructure. Pure battery cars fall somewhere in between; electricity is ubiquitous but not necessarily at the voltage and amperes required to recharge a battery car quickly enough to suit motorists. (That was the downfall of the Southern California experiment with GM's EV-1.)

So when we think about how to make hydrogen for fuel cells, we need to consider how else that energy source could be used. Nuclear power produces no greenhouse gas emissions, but is its best use making hydrogen for cars or backing down coal-fired power plants? The hydrogen for the first fuel cell cars will mostly come from natural gas, but is that gas better employed that way or should it be burned directly in internal combustion engines, as we see in more and more city bus and taxi fleets? These questions can be answered, but only by looking at entire energy systems, rather than the little slices in which we're typically most interested. This is referred to as "well-to-wheels" analysis, and it considers every step in the chain, from the energy invested to extract the primary energy source, be it coal, oil, gas or uranium, to its final use in a vehicle or other energy-using device. It can be done on the basis of both energy efficiency and greenhouse gas emissions.

Finally, it's important to remember that engineering analysis doesn't always exert the greatest influence on such decisions. Consumer choices, politics, relative returns on investment, and a number of other factors will largely determine the final selection of the successor to our current gasoline-driven systems, if in fact any one successor emerges. It's equally possible that we could see a diverse mix of future transportation systems relying on different technologies and energy sources, but sharing the roads together.

With that I'll wish my US readers a Happy Thanksgiving. New postings will resume on Monday, November 28.

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