Tuesday, October 25, 2005

Running on Metal

The two big problems with hydrogen power for cars are these:

1. Where does the hydrogen come from?

2. How do you store hydrogen in the car?

A group of Israelis at a company called Engineuity believe they have the answer.

From IsraCast:

The Hydrogen car Engineuity is working on will use metals such as Magnesium or Aluminum which will come in the form of a long coil. The gas tank in conventional vehicles will be replaced by a device called a Metal-Steam combustor that will separate Hydrogen out of heated water. The basic idea behind the technology is relatively simple: the tip of the metal coil is inserted into the Metal-Steam combustor together with water where it will be heated to very high temperatures. The metal atoms will bond to the Oxygen from the water, creating metal oxide. As a result, the Hydrogen molecules are free, and will be sent into the engine alongside the steam.

Now this is very clever, solving both of the core problems with one not-so-exotic technique. In this solution, the hydrogen comes from water — no problem there, we’ve got plenty of water all over the place, especially if the system can use salt water. And what could be easier or safer to store than water? One of the major problems facing most hydrogen storage ideas is “hydrogen density": how much hydrogen can you pack into a given volume?

The approach to hydrogen storage I’ve seen most often discussed is to use metal hydrides, especially magnesium. Such a system could hold about 8% of its weight in hydrogen. A practical car would need something like 40 pounds of hydrogen in a full tank. With a magnesium hydride storage system, that means you’d need 500 pounds of “fuel”. Much worse, though, is this: the magnesium hydride system is a very exotic, still unproven technology. It’s prone to contamination problems and longevity problems. And nobody is really sure what would happen in the event of an accident.

For this Israeli system, the hydrogen is about 11% of the weight of the water — so you’d need 360 pounds of water to hold 40 pounds of hydrogen. In addition, you’d need 480 pounds of magnesium, or 360 pounds of aluminum, to “soak up” all that oxygen in the water. So in total you’d need 720 to 840 pounds of “fuel” — more than the magnesium hydride system, but not exotic and very safe.

The numbers I quote above are not in the IsraCast article; I derived these from first principles using just the assumption that 40 pounds of hydrogen is roughly the same energy content as a tank of gasoline in a modern car. I’m not assuming any radical improvements in overall efficiency, or any radical changes in what drivers find desirable in a car. In other words, my starting point is to duplicate today’s car, but with hydrogen technology.

The IsraCast article paints a much more optimistic picture, without backing it up. For example, they say that to refuel the car would typically require 220 pounds of metal. Tey also claim that this should cost no more than filling your tank with gasoline — I don’t know where they buy their aluminum or magnesium, but in these parts you won’t be buying it for anything like $0.25/pound (if you believe their 220 pound claim), much less $0.10/pound (if you believe my numbers). This looks expensive to me, unless someone comes up with a way to radically reduce the cost of producing the metal.

Which brings me to one last point: while this clever Israeli technique neatly solves the core car problems, it doesn’t address another one: where does the energy come from to run all these cars? In their system, the energy input goes into refining the metal, either from ore or from reclaimed metal oxides produced by the cars. Refining metal is a very energy-intensive process, and that energy has to come from somewhere. In favor of their system is that it allows that energy consumption to be very centralized, making it possible to consider things like nuclear power or massive hydroelectric facilities. But however it’s done, it would have to be with power production that doesn’t currently exist — and to make any sense at all, it would have to be power production that didn’t use hydrocarbons. A big challenge there…

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