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May 4, 2004

Hydrogen Economy?

The Engineering Realities

I posted a piece a short while ago on Fuel Cells, pointing out that, like batteries, they constitute an energy storage medium, not a source, and that the problems to be overcome for them to compete as a viable means of vehicle propulsion are a lot more fundamental and challenging than the current over-hype would lead voters and potential investors to believe.

On a more general note, Dr. Robert E. Uhrig, distinguished professor of nuclear engineering emeritus at the University of Tennessee and distinguished scientist emeritus of the nuclear engineering and technology division at Oak Ridge National Laboratory, has written an article entitled "Engineering Challenges of the Hydrogen Economy" in the Spring 2004 issue of The Bent of Tau Beta Pi that puts a realistic perspective on claims being popularized of hydrogen as a potential major resource for powering the nation.

One of the first questions Dr. Uhrig raises is, why are even the advocates talking about more than a quarter of a century being required for this to be achieved? It seems inordinately long compared to the rapid acceptance of other leading technologies once the time was right. A look at some of the numbers involved gives an indication of the true scale of what is being implied.

As mentioned above, hydrogen is an energy carrier, not a source. It does not occur naturally but must be extracted from other substances such as water. This inevitably requires more energy to accomplish than the energy content of the hydrogen produced. The same is true of electricity, of course. Such carriers are justified only when special benefits are associated with their use. For electricity, it is convenience and ease of distribution. In the case of hydrogen it would, according to the scenarios presented, be the replacement of fossil fuels and portability to rival gasoline. However, the amount required to replace the oil used by the United States alone works out at over seven times the present worldwide production for all purposes, including fertilizers and feedstock for the chemicals industry. Producing this amount of hydrogen by water electrolysis would require more than doubling the present US electrical generating capacity (930 large nuclear plants!). Steam methane reforming, the other major method of hydrogen production, would require more natural gas to replace transportation fuels alone than the amount currently used for all purposes. The most efficient method would probably be the high-temperature cracking of water in conjunction with a sulfur-iodine catalyst, but this is generally envisioned as using nuclear energy, unlikely to be realized on any significant scale while the present atmosphere of ignorance and superstition surrounding the subject persists.

The difficulties don't end there. Once hydrogen is produced, it has to be distributed to where it is to be used. Typically, this is envisaged as involving clusters of hydrogen production plants located around the country, with a pipeline network distributing to local centers, analogous to the electrical grid. Such system would require something like 90,000 miles of primary pipelines, plus an additional 725,000 miles of smaller local distribution pipes. Alternatively, a distributed array of millions of small production units has been proposed, but this would require doubling the size of the electrical grid to provide the necessary power, as well as doubling the generating capacity. If the power needs in turn were also be met by distributed local generating units, the requirement would be for around one and a half million 1MWe windmills or 170 million 10KWe photovoltaic units. Whatever method is used, the dispensing of fuel to vehicles at service stations would involve connection to valves delivering at pressures of over 5,000 psi, with attendant problems of complexity, safety, and cost. The bottom line is that it isn't going to happen anytime soon, and it's by no means obvious that a sound case exists for wanting it to happen at all.

Click here for the full PDF article, with all the details and figures.

What makes gasoline so effective as a fuel is the amount of energy that hydrocarbon molecules are able to pack into a small volume, resulting in lots of miles in a tank of gas. Both H and C are abundant. The key to accessing them efficiently is a highly concentrated energy source. If microbes floating around in the oceans (or processes in the atmosphere of Jupiter, asteroids, or wherever hydrocarbons are eventually shown to originate from) can figure out how to fit them together in such a way as to lock up lots of energy, I believe that engineers and professors of chemical engineering are capable of doing at least as well. My guess is that we'll be manufacturing our own synthetics at nuclear plants, and producing electricity as a by-product, long before the last barrel of the natural stuff is pumped out of the ground.

 
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