The term "Hydrogen Economy" is familiar by now, but there are numerous attendant difficulties which may not be overcome, or not in time to circumvent the energy-crash caused by cheap oil running short, signalled by a massive and inexorable hike in oil prices, as is now well underway. Notwithstanding the economic minefield the "Oil Dearth Era" will set, there are intrinsic technical problems in producing and handing hydrogen per se, if it is to be used on a scale of substitution equivalent to that for oil.
I have written on this subject in previous postings at some length, but the following points are salient. Hydrogen is not a basic fuel as are oil, gas and coal, but it must be produced artificially by liberating it from other elements, such as carbon and oxygen with which it is normally combined in nature, in the form of methane (natural gas) and water. These are, however, all energy intensive processes and almost entirely require the use of fossil fuels or nuclear power to drive them. Most of the world's current 50 million tonnes or so of hydrogen, produced annually to make fertilizers and to crack hydrocarbons, comes from "synthesis gas", a mixture of CO and H2 formed by reacting fossil fuels with steam in a process called "reforming", and so both chemical feedstock and heat depend upon them; hardly a "green" process, since CO2 is incurred both from combustion and by chemical stripping of the carbon component.
The ideal would be to make clean hydrogen by the electrolysis of water using renewable electricity (wind, wave, solar, hydro), but we need to go a very long way before that can be done on a large scale, although some think that enough new nuclear power might be installed to make the necessary electricity. I am skeptical that this can be implemented quickly enough, if at all, in the vast dimension that is demanded.
Even if we can make enough hydrogen, there is the issue of how to store, handle and distribute it. In comparison with liquid hydrocarbon fuels, gaseous hydrogen at normal pressure is highly voluminous, and hence it is necessary to handle it either as an extremely volatile liquid (with a boiling point of -253 degrees C, and only 20 degrees above absolute zero), or under high pressures. Either arrangement would require special technology to maintain it safe over time and to prevent leaks, since hydrogen forms highly explosive mixtures with air over a range of concentrations, and there would in any case need to be built a completely new infrastructure for generation, handling and distribution, once again within 10 years or so, and we haven't started yet.
For onboard storage of hydrogen as a fuel in vehicles, a considerable proportion of the energy actually contained in the hydrogen would be required to liquefy (30 - 40%) or pressurise (20%) the material into a "fuel tank". A fuel/tank weight ratio of 6.5% has been proposed below which the hydrogen strategy is inviable and there are numerous suggestions of porous solids into which hydrogen might be packed to occupy a smaller volume, e.g. zeolites, in some cases allowing an energy density close to that of liquid hydrogen but at significantly higher temperatures then -253 degrees C. Nonetheless, cryogenic cooling is still required. As an alternative, it has been postulated that the hydrogen might be stored chemically in the form of methanol. Indeed, one litre of liquid hydrogen contains 70.8 g of hydrogen at -253 degrees C, while one litre of liquid methanol contains 98.8 g of hydrogen and that is at room temperature.
The "methanol economy" could achieve holy grail status as a CO2 emission remediation strategy, by providing the carbon component of CH3OH, thus both preventing it from being released into the atmosphere and providing a vital source of fuel. Actual carbon-capture from atmospheric air on a degree of real significance is the stuff of the future, but capturing CO2 from power stations is feasible, which could be reacted with H2:
CO2 + 3H2 ---> CH3OH + H2O.
We are still left with the problem of making hydrogen on a vast scale and the infrastructure to do so does not exist at all. It is possible that rather than using preformed H2, it might be produced in situ, in the form of electrons and protons, by electrolysing CO2 in aqueous (water) media, so overall the effect is equivalent:
CO2 + 6H+ + 6e- ---> CH3OH + H2O.
However, the latter is difficult, since the reduction of (electron addition to) CO2 at the cathode (negative electrode) occurs in competition with electron addition to protons (H+) making hydrogen atoms and hence H2, the production of which competes with CH3OH formation. CH3OH is not the only organic product of CO2 reduction (either by electrons or H2), but also formic acid HCO2H and formaldehyde H2CO), although George Olah and his team at the Loker Hydrocarbon Research Institute at USC (University of Southern California) have patented a means to convert the latter to methanol, in an overall reaction where HCOOH provides "hydrogen" to reduce H2CO:
HCOOH + H2CO ---> CH3OH + CO2.
It is thought that the methanol would ultimately be "burned" directly in "direct-methanol-fuel-cells", but these currently depend on scarce supplies of precious metals such as platinum, as indeed do hydrogen fuel cells, and that appears to be a drawback on the technology. However, methanol can be converted to mixtures of hydrocarbons by reacting it over zeolite catalysts, for either purpose of making fuel (methanol to gasoline (MTG) process; invented by Mobil in the '70's) or as a feedstock for e.g. making plastics (methanol to olefin (MTG) process. In principle, many organic chemicals including pharmaceuticals might be made from methanol.
Most methanol is currently produced from natural gas (as is hydrogen) and so feeding the methanol economy by this means would impose further demands on a reserve that is, after all finite, as is oil; hence using CO2 as the carbon source appears perfect. Much of the current state of play in the field is heavily guarded by patents, and so I have not been able to tie-down the best efficiency so far achieved for CO2 reduction and nor do I know whether it is more efficient to do this with pre-prepared H2 or by electrochemical methods. However, my impression is that the latter are quite some way off and the process should be seen as a means for storing H2 made independently.
According to one report, the overall energy efficiency incurred in reducing CO2 with H2 and handling the resulting CH3OH is about 20%, and that is before the "fuel" has actually been used in some way. Therefore, while there would be considerable advantages met in handling liquid methanol at room temperature rather than H2 (either as a cryogenic liquid or a highly compressed gas), in terms of energy efficiency I doubt methanol is better than hydrogen, for which a value of nearer 40 - 50% might be accounted in terms of its manufacture by water electrolysis and the subsequent handling processes. Nor can it be, in the sense that installing a gargantuan new electricity generating capacity of similar capacity is necessary to underpin it.
On safety grounds, convenience of handling, storage and distribution (for which the existing oil infrastructure could be adapted), and that methanol might be converted to the numerous products that we presently get from oil (which is becoming more expensive all the time), as well as providing a clean fuel, the strategy holds much appeal. What it is not though, is a limitless supply of synthetic "oil", since CO2-derived methanol depends on electricity from fossil fuels and uranium, and may prove no more than a means for temporarily extending the illusion that the carbon-driven Western lifestyle is sustainable, which it is not.
(1) "Beyond Oil and Gas: The Methanol Economy," G.A.Olah, A.Geoppert and G.K.Surya Prakash. Wiley-VCH, 2006.
(2) "Novel CO2 Electrochemical Reduction to Methanol for H2 Storage," T.Kobayashi and H.Takahashi, Energy and Fuels, 2004, 18, 285 - 286.
(3) "Beyond Oil and Gas: The Methanol Economy," G.A.Olah, Angew. Chem. Int. Ed., 2005, 44, 2636 - 2699.
(4) "Renewable hydrogen utilisation for the production of methanol," P.Galindo Cifra and O.Badr. https://aerade.cranfield.ac.uk/bitstream/1826/1449/1/Renewable+Hydrogen-Methanol.pdf