Hydrogen

Hydrogen is often advocated as an energy medium. Here are some relevant facts.

1. Hydrogen is the lightest of the elements with an atomic weight of 1.0. Liquid hydrogen has a density of 0.07. These facts give hydrogen both advantages and disadvantages. The advantage is that it stores approximately 2.6 times the energy per unit mass as gasoline, and the disadvantage is that it needs about 4 times the volume for a given amount of energy. A 15 gallon automobile gasoline tank contains 90 pounds of gasoline. The corresponding hydrogen tank would be 60 gallons, but the hydrogen would weigh only 34 pounds.

2. When hydrogen is burned in air the main product is water. Some nitrogen compounds may also be produced and may have to be controlled. Should greenhouse warming turn out to be an important problem, the key advantage of hydrogen is that carbon dioxide (CO₂) is not produced when hydrogen is burned.

3. Hydrogen is not available in significant quantities in nature in pure form. The main present way of getting hydrogen is steam methane reforming, and this will probably remain the most economical way as long as methane (natural gas) is available cheaply and in large quantities. When the price of methane goes up to more than three times its present price because of scarcity, hydrogen will be obtained by splitting water H₂O into hydrogen H₂ and oxygen O₂. The chemical reaction is written

   2H₂O + energy => 2H₂ + O₂.

   The main way of splitting water is by electrolysis. If fossil fuels, e.g. coal, oil or natural gas, are used to generate the electricity, there is no advantage over using the fossil fuels directly. Indeed you still get all the CO₂, and there is a considerable loss of energy. Therefore, the large scale use of hydrogen depends on using either nuclear or solar electricity. In both the nuclear and solar cases, there are possible technologies that don't use electricity as an intermediate form of energy. There is some hope that these processes may be somewhat more efficient than electrolysis. (There are also proposals to combine heat and electrolysis with some saving of energy.)

4. In either case, the law of conservation of energy tells us that all the energy to be obtained by burning the hydrogen must be supplied by the primary source, e.g. nuclear or solar. Of course, since these processes aren't 100 percent efficient, there is some loss of energy. Therefore, the use of hydrogen as an intermediate is justified only when there is some reason not to use the primary source directly.

5. If there is large scale use of solar energy, the energy is likely to be generated far from where it is used and at a different time. Hydrogen has been proposed as both a storage and transmission medium. It should work for these purposes. I don't know how hydrogen pipelines compare with high voltage electric transmission.

   Hydrogen can be transported by pipelines similar to those used to transport natural gas. There are some addtional problems, because hydrogen tends to leak more and can embrittle some metals used for pipelines. The existence of a 208 km hydrogen pipeline in Germany provides evidence that these difficulties can be overcome. (I read about this pipeline, but no-one confirmed that it exists. Maybe what I read was mistaken.)

   However, the technology of efficient long distance transport of electric energy may be improved enough to obviate the advantages of hydrogen except for vehicles.

Hydrogen as a motor fuel

Nuclear power requires heavy shielding to keep the neutrons away from people - too heavy for cars. It can be used in ships, and is used in American, British and Russian warships, especially submarines and aircraft carriers. The U.S. and Japan built commercial nuclear powered ships, one each (Savannah and Mutsu). (There were even proposals to use it in locomotives.) However, initial difficulties combined with anti-nuclear politics caused these projects to be abandoned and the ships mothballed. The Soviets built nuclear powered icebreakers, and these are in use. I think nuclear power will be revived for commercial ships when its political problems are overcome and the technology is further debugged.

Solar energy can't be used directly in cars except as a stunt. The current solar-powered cars are just religious exercises in the solar religion. The problem is that a solar array of a size that can be mounted on a car produces too little energy to give useful performance, and even that little isn't available at night or when it is very cloudy.

Hydrogen can be used as a fuel directly in an internal combustion engine not much different from the engines used with gasoline. The problem is that while hydrogen supplies three times the energy per pound of gasoline it has only one tenth the density when the hydrogen is in a liquid form and very much less when it is stored as a compressed gas. This means that hydrogen fuel tanks must be large.

Demonstrations of hydrogen powered vehicles have usually used compressed hydrogen gas. However, because of the low density, compressed hydrogen will not give a car as useful a range as gasoline. It may be even worse than using lead-acid batteries. Hydrogen can achieve a reasonable density adsorbed in metal hydrides, but then the weight of the metals makes the system very heavy.

The most practical way I know of using hydrogen as a motor fuel is to accept the difficulties of handling liquid hydrogen and solve them. There are two.

1. The low density. A hydrogen fuel tank will have three times the size of a gasoline tank. Also it must be insulated, and this will add to its bulk. This seems entirely bearable.

2. Safety problems. Liquid hydrogen is cold enough to freeze air, and accidents have occured from pressure build-up following plugged valves. Some say these problems can't be overcome, but I side with those who think they can be overcome. In a collision the hydrogen tank may rupture, as can a gasoline tank. Limited accident experience suggests that the danger is somewhat less with hydrogen.

3. Since the insulation can't be perfect, the hydrogen will gradually evaporate, typically 1.7 percent per day. This is too fast for a car to sit for months between uses. A tank of compressed hydrogen holding enough to get to a hydrogen station would solve this. If the engine is flexible enough to burn gasoline as well as hydrogen, a half gallon gasoline tank would suffice. Some automobile companies, e.g. BMW, have experimented with vehicles powered by liquid hydrogen. However, hydrogen cannot come into common use until the political obstacles to nuclear expansion are overcome or the technological obstacles to large scale solar energy are overcome. It is unlikely to be used as long as gasoline remains so cheap, i.e. as long as oil remains cheap and fear of global warming does not prevent its use. We hydrogen enthusiasts will just have to wait.

   Here's what Pimentel (1996, p. 211-212) has to say.

   In terms of energy contained, 9.5 kg of hydrogen is equivalent to 25kg of gasoline ( Peschka 1987). Storing 25 kg of gasoline requires a tank with a mass of 17 kg, whereas the storage of 9.5 kg of hydrogen requires 55kg, (Peschka 1987). Part of the reason for this difference is that the volume of hydrogen fuel is about 4 times greater for the same energy content of gasoline. Although the hydrogen storage vessel is large, hydrogen burns 1.33 times more efficiently than gasoline in automobiles ( Bockris and Wass 1988). In tests a BMW 745i liquid-hydrogen test vehicle with a 75 kg tank and the energy equivalent of 40 liters of gasoline had a cruising range in traffic of 400 km, or a fuel efficiency of 10 km per liter ( Winter 1986).

   At present, commercial hydrogen is more expensive than gasoline. Assuming $0.05 per kwh of electricity from a nuclear power plant during low demand, hydrogen would cost $0.09 per kwh ( Bockris and Wass 1988). This is the equivalent of $0.67 per liter of gasoline. Gasoline sells at the pump in the United States for about $0.30 per liter. However, estimates of the real cost of burning a liter of gasoline range from $1.06 to $1.32 when production, pollution, and other external costs are included (Worldwatch Institute 1989). Therefore, based on these calculations hydrogen fuel may eventually become competitive.

   The references above are copied from Pimentel (1996, p. 211). I plan to look them up, and this may change what I say.

   The above comparison between current costs of gasoline and hydrogen power for cars seems to be somewhat biased in favor of hydrogen. Taxes seem to be included in gasoline cost and not in hydrogen estimates, but roads will still have to be maintained when hydrogen is used as a fuel. Also I would conjecture that the Worldwatch estimate of the "real cost" of burning a liter of gasoline is exaggerated.

   For me the decisive point is that the costs of a automobile transportation system using hydrogen produced from water using nuclear energy are low enough so that people worldwide who use automobiles will not give up the freedom they provide, regardless of efforts to get people to settle for public transportation or low range cars of one kind or another. This doesn't say that adequate batteries won't be developed to make electric cars better than liquid-hydrogen internal combustion powered cars. Maybe they will, but we won't settle for less mobility than hydrogen can provide.

   Mazda has developed a hydrogen powered Wankel car. They are trying to get California to declare it zero emission with some prospect of success. However, if California chickens out of the zero emission demand (as it should and probably will), I'll bet the Mazda will not be offered any time soon.

West Virginia University has a Hydrogen Review page. It mentions several ways of using hydrogen for motor vehicles. It would seem to me from their numbers that liquid hydrogen is the winner - as stated above. However, the page makes no comparisons at all.