In a world facing the real possibility of disastrous global warming, a fuel that does not produce carbon dioxide would appear to be a real godsend. Carbon dioxide is the ubiquitous by-product of all other combustion processes and the most important greenhouse gas responsible for that warming. Hydrogen is a potentially attractive replacement for both coal and oil as a fuel source because it produces no pollutants when it is burned. Only water is formed.
Although it will most likely play a role as a fuel in a renewable energy society, I believe that at the present time it is a mistake to push the use of hydrogen as a substitute for non-renewable carbon based fuels. Let me explain why.
It must be emphasized that hydrogen is made from natural gas because this is the least expensive way to make itconsiderably less expensive, for example, than of using electrolysis of water using electricity at off-peak rates. It is unrealistic to assume that, at least for the near term, hydrogen would be made in any quantity from anything but methane. We are left with the likelihood that the "hydrogen economy", like today's "hydrocarbon economy", would be based on a non-renewable resource.
Of course, it is possible to break apart water and obtain hydrogen in other ways. The formation of hydrogen and oxygen from water using electricity is the one that is most often touted. If the electricity is provided by PV panels, we are talking about using a renewable energy resource, sunlight, to provide hydrogen in a non-polluting way. Such a proposal, when first heard, sounds attractive.
However, a little further examination indicates that is not a good answer.
The biggest problem is the prodigious amount of electrical energy that would be required to replace even a portion of the hydrocarbon fuels we now use. Wilson Clark, in his classic book, Energy For Survival, makes his point very clear. "The amounts of hydrogen that would be required in a hydrogen economy are enormous. For instance, according to Dr. Gregory, to produce enough hydrogen to fully substitute for the natural gas produced in the United States at the present time  --i.e., 70 trillion cubic feet of hydrogen-- would require more than 1 million megawatt of electric power to produce. Total electric generating capacity in the United States is only 360,000 megawatts. To meet the projected hydrogen requirements for natural gas alone would call for a fourfold increase in generating capacity, which would mean building 1,000 additional 1,000-megawatt power stations! This does not provide for increased electric power demand for other purposes, nor does it take into account the generation of hydrogen for transport fuel or as an additive in chemical and industrial processes."
By way of comparison, world production of photovoltaic generating capacity was about 50 megawatts (peak sun) last year. Even if this capacity were to be increased a 100-fold and all of it used to produce hydrogen, we would still be making a fraction of 1% of what would be needed to replace the natural gas consumed in the U.S. In addition...
Finally, why photovoltaics? As pointed out earlier, photovoltaics is not a good choice for generating vast amounts of electricity. It is much more suitable for smaller scale applications where grid power is not available. Although it will probably be used to generate utility power as well, utilities have never considered using it in any other capacity than for peaking power. In addition, these systems presently produce electricity at a cost of from $.25 to $.75 per kilowatt hour (20 year life cycle cost). Even were the cost to be cut in half, which is what we expect to happen during the next decade, we are talking about a much more expensive kind of electricity than could be produced by other renewable sources, such as the LUZ concentrating solar thermal facility that is presently supplying peaking power to the Los Angeles basin at about $.08 per kilowatt hour.
If these questions are answered primarily by, "because photovoltaics is renewable and non-polluting, and the burning of hydrogen produces no pollutants", I suggest that a much more thorough analysis of the situation needs to be carried out.