I received an informative and interesting email (actually, I received several, but I’m posting this one) in response to my Let the Hydrogen Economy Evolve series on RPPI.org. The author, a mechanical engineer and retired university professor, highlights some of the engineering practicalities that I did not emphasize in my writing. His remarks:
The “Hydrogen economy” and/or the “Hydrogen car” discussion seems seldom to address some important practical engineering considerations:
1) Thermodynamic feasibility [which means, ultimately, economic viability]
1) Speaking as a retired thermodynamics instructor I can assure you that there is no way that spending heat energy to make hydrogen to be burned to make heat energy can be anything but a losing proposition. If environmental impact is the only criterion for acceptability, a case can possibly be made that large industrial power plants create less undesirable gaseous products than do small (e.g., automotive) ones. But thermodynamically, and hence – with certainty – economically, this is a loser. Nuclear electrical power plants could win hands down in the environment-only sweepstakes as the prime power source, but this seems to be politically unacceptable in the US at present, for nontechnical reasons.
2) The cost of distributing Hydrogen as a fuel for automotive use would be staggering. The problem is that Hydrogen has such a low density that, to transport it in gaseous form would require extremely high pressure confinement and/or containers of incredible proportions. At 1 atmosphere pressure and 20 C, hydrogen gas has a density of about 90 grams per cubic meter. Gasoline has a density of about 750 kilograms per cubic meter, about 8000 times higher. So even if only 1/3 the mass of hydrogen were needed to replace a unit mass of gasoline, the volume to be transported would be 2700 times greater. Transport in liquid phase seems completely impractical, as it would require Hydrogen confinement at -260 C. Transport at high pressure … say at 270 atmospheres, or 4000 psi … would still require moving 10 times the volume of the energy equivalent in gasoline. And that brings up the final, but by no means the least, consideration: safety.
3) Can you imagine the nations roadways with fuel trucks of 10 times the volume of today’s tankers [or equivalently, 10 times as many of them] loaded with Hydrogen at up to 4000 psi? A modest impact with another vehicle, an overturning, and a rupture of the tank would lead to an incredible fireball of burning gas that would make the Hindenburg disaster look like a picnic bonfire. And how about filling an automobile tank to a pressure of about 4000 psi from a high pressure hose. It takes little imagination to conjure up a minor misfit of couplings, a slight leak of odorless, invisible hydrogen, and a static spark to create a filling-station disaster. And how does one fight a hydrogen fire? Deprivation of Oxygen is the only possibility, and that takes specialized equipment. Re-equip all the fire stations before thinking of going to a Hydrogen car.
In short, the fact that the automobile exhaust pipe would emit only water vapor due to combustion seems to be the Mesmerizing fact in this discussion. But that is only a small part of the problem of changing any significant part of the nation’s automotive fleet to Hydrogen power. And I don’t want to drive on a highway that kills 50,000 people a year with the added hazard of high-pressure Hydrogen tanks aboard the vehicles!