Perhaps the most widely-accepted vision of what a greener future will look like is that of the "Hydrogen Economy." Everyone from ecofuturists like the Rocky Mountain Institute to petroconservatives like the Bush administration extol the virtues of hydrogen powered vehicles. We talk about hydrogen as a future fuel here on WorldChanging with some frequency; it remains our best bet for moving beyond greenhouse gas-emitting transportation technologies.
But the move to hydrogen is not without its challenges. We've mentioned that a few times here, but if you're interested in the future of transportation and energy, it's a subject worth understanding. Fortunately, the current issue of Physics Today has a lengthy and detailed article covering the current issues and possible developments in the efforts to build the hydrogen economy. (While not overly-complex, it is a Physics Today piece, so it assumes some comfort with scientific and technical terms.)
The challenges to the shift to hydrogen are in three fundamental areas:
The Physics Today article goes into detail about all of these issues. It also makes the point that these are not impossible challenges, and that smart people are doing good research into solutions. Despite the reality-check, none of this should be taken as a dismissal of the idea that our future will be fueled by hydrogen. H2 remains very attractive for a variety of reasons -- it has the potential to be environmentally very sound, it's not a limited natural resource found in unstable geographies, and hydrogen is quite literally the most abundant element in the universe. But it should put into perspective ambitious calls for the proliferation of hydrogen-fueled vehicles by the end of the decade or auto manufacturer claims that they're ignoring hybrids because they're building hydrogen cars. The hydrogen economy will happen, and happen in the next decade or two, but there are still obstacles to overcome.
I hate to be a cynic, but I've changed my mind about hydrogen. What's the point? Batteries are far less wasteful and will take less technological leaps to reach a commercially attractive goal. That Eliica car seems like a far better future to me than a hydrogen one.
For those who are interested in the Physics Today article, but don't wish to wade through the technobabble, I wrote up a summary of the piece:
- Fossil fuel use is unsustainable and associated with adverse health effects and global warming, but a suitable replacement for fossil fuels will not appear overnight. Our window of opportunity for a managed transition to alternative energy is increasingly short.
- One alternative is hydrogen which can both be burned in a combustion engine and used in fuel cell technology with water as it's only byproduct. Hydrogen is relatively abundant and so for practical purposes the only constraint on a transition to a hydrogen economy is technological and not political.
- Hydrogen does not occur in nature in a free form, but in compounds such as water that must be chemically transformed, thus making hydrogen, like electricity, a carrier of energy and not a source. Today hydrogen is produced from natural gas using steam reforming, a process that uses a fossil fuel to produce the energy and releases carbon into the atmosphere. The true benefits of a hydrogen economy can only be realized by using non-fossil fuel sources and renewable energy to produce hydrogen.
- A hydrogen economy is composed of three steps: Production, Storage and Use. Production, even using steam reforming, is still four times as expensive as gasoline. Storage can be pressurized gas or super-cooled liquid, but not yet in densities practical for driving a car. Use can be in the form of electricity from fuel cells, but these are currently ten times as expensive per kilowatt hour as gasoline.
- The challenge in the production of hydrogen is to find efficient and inexpensive ways to create hydrogen in sufficient quantities from non-fossil fuel sources. The most promising source is splitting water, a process which uses energy that is later recovered during oxidation, the process by which hydrogen is converted back into water. To eliminate fossil fuels the energy to split the water must come from renewable resources.
- Direct conversion of sunlight into hydrogen is currently a two step process using photovoltaics to produce electricity which is then used in electrolysis to split water molecules. One of the most exciting recent developments is the ability to combine these two steps into a single nanoscale process: Photon absorption creates a local electron-hole pair that electrochemically splits a neighboring water molecule. The challenge is finding robust semiconductor materials that meet the technical requirements, and so new strategies for nanostructured hybrid materials are needed to more efficiently use solar energy to split water.
- Water can be split in thermochemical cycles operating at high temperatures to facilitate the reaction kinetics, but identifying effective membrane materials that selectively pass hydrogen, oxygen, and water at high temperature remains a problem.
- Bio-inspired processes offer stunning opportunities for production of hydrogen. The natural world began forming its own hydrogen economy 3 billion years ago when it developed photosynthesis by which plants use hydrogen to manufacture the carbohydrates in their leaves and stalks and then emit oxygen to the atmosphere. Single cell organisms such as algae and many microbes produce hydrogen efficiently at ambient temperatures by molecular level processes. These natural mechanisms for producing hydrogen involve elaborate protein structures that have only recently been partially solved.
- Storing hydrogen in a high energy density form that flexibly links its production and eventual use is a key element of the hydrogen economy. Traditionally hydrogen has been stored as cylinders of super-cooled liquid or pressurized gas, but storing hydrogen as a liquid imposes severe energy costs because up to 40% of its energy content can be lost to liquefaction.
- The on-board storage of hydrogen for transportation use is a difficult challenge because both weight and volume are at a premium, although vehicles need store only about half of the energy that gasoline provides because the efficiency of fuel cells can be greater by a factor of two or more than that of internal combustion engines.
- Meeting the volume restrictions in cars or trucks requires using hydrogen stored at densities higher than its liquid density. It is where hydrogen is combined with light elements like lithium, nitrogen, and carbon that the most effective storage media can be found. Hydrocarbons like methanol and octane are notable as high volume density hydrogen storage compounds as well as high energy density fuels, and cycles that allow these fossil fuels to release and recapture their hydrogen are already in use in stationary chemical processing plants.
- Nanostructured materials offer a host of promising routes for storing hydrogen at high capacity in compounds that have fast recycling. The capture and release cycle is a complex process that involves molecular dissociation, diffusion, chemical bonding, and van der Waals attraction. Each of these steps can be optimized in a specific nanoscale environment and nanoscale architectures offer unexplored options for effectively controlling reactivity and bonding to meet the desired storage requirements.
- A major attraction of hydrogen as a fuel is its natural compatibility with fuel cells. The higher efficiency of fuel cells, currently 60% compared to 22% for gasoline or 45% for diesel internal combustion engines, would dramatically improve the efficiency of future energy use. Coupling fuel cells to electric motors, which are more than 90% efficient, converts the chemical energy of hydrogen to mechanical work without heat as an intermediary.
- Although fuel cells are more efficient, there are also good reasons for burning hydrogen in combustion engines for transportation. Jet engines and internal combustion engines can be rather easily modified to run on hydrogen instead of hydrocarbons. Internal combustion engines run as much as 25% more efficiently on hydrogen compared to gasoline and produce no carbon emissions.
- A host of fundamental performance problems remain to be solved before hydrogen in fuel cells can compete with gasoline. The heart of the fuel cell is the ionic conducting membrane that transmits protons or oxygen ions between electrodes while electrons go through an external load to do their electrical work. Each of these half-reactions at work in that kind of circuit requires catalysts interacting with electrons, ions, and gases traveling in different media. Designing nanoscale architectures for these triple percolation networks that effectively coordinate the interaction of reactants with nanostructured catalysts is a major opportunity for improving fuel cell performance.
- It is now becoming possible to understand these reactions at the atomic level using sophisticated surface structure and spectroscopy tools, and when combined with equally powerful and impressive computational quantum chemistry using density functional theory, are opening a new chapter in atomic level understanding of the catalytic process.
- Will the hydrogen economy succeed? Historical precedents suggest that it might. New energy sources and carriers have flourished when coupled with new energy converters. Coal became king as fuel for the steam engine to power the industrial revolution, it transformed the face of land transportation from horse and buggy to rail, and on the sea from sail to steamship. Oil fueled the internal combustion engine to provide automobiles and trucks that crisscross continents, and later the jet engine to conquer the skies.
- Hydrogen has its own natural energy conversion partner, the fuel cell. Together they interface intimately with the broad base of electrical technology already in place, and they can expand to propel cars, locomotives and ships, power consumer electronics, and generate neighborhood heat and light. Bringing hydrogen and fuel cells to that level of impact is a fascinating challenge and opportunity for basic science, spanning chemistry, physics, biology, and materials.
End of summary.
Aircraft are going to be one of the most difficult things to convert to elemental hydrogen fuel. Not because hydrogen hasn't got lightness going for it, but because the cryogenic insulation required to carry it is going to be a very tough retrofit if it's feasible at all. On top of that, even liquid H2 is very bulky for its energy content; an aircraft which carries a flight's worth of kerosene completely inside the wings is going to need some good-sized external tanks to make the same flight with H2, and the change in flying characteristics is going to require a lot of money for recertification of the modified aircraft. In short, it will not be easy, cheap or quick.
If it does happen, there are all kinds of neat little hacks for the engine designers. It would be a fairly small matter to pump the liquid fuel to high pressure, heat it with the outlet gas of the turbine core (after all the expansion work is recovered) and then expand the fuel through a turbine before admitting it to the combustion chambers. This would increase efficiency twice: once by recuperating exhaust heat, and a second time by adding another element which generates net power without significant losses.
Interesting idea, E-P -- has that concept been tested?
Although the linked article mentions the use of hydrogen as aircraft fuel in passing, I think you're absolutely right that air travel is going to be the hardest thing to shift away from fossil fuels.
One correction to the summary above - the issue with fuel cell cost from the article was that they were about 100 times (not 10 times) more expensive per kilowatt (not per kilowatt-hour) than a typical internal combustion engine (not than the fuel itself). That's a huge hurdle for the R&D process.
The actual quote:
the production cost of prototype fuel cells remains high: $3000 per kilowatt of power produced for prototype fuel cells (mass production could reduce this cost by a factor of 10 or more), compared with $30 per kilowatt for gasoline engines.
hey! check it out :D
Sunlight to Fuel Hydrogen Future http://www.wired.com/news/technology/0,1282,65936,00.html - Solar power these days comes from cells that turn light into electricity, but researchers are now working on materials that can crank out hydrogen.
The fact is they will make hydrogen work.
In the end for some uses like jets what they will do is rather obvious to an oil guy but your missing it entirely.... they will simply combine hydrogen and carbon to make ... jet fuel. They already do this on a grand scale as it is to make various low emmission fuel blends anyway.
Ah, but where do you get the carbon? Either you can make your energy-capture system do the work a la green plants (which reduces its efficiency) or you can supply it from some other source, which will probably not be renewable.
There may be a future in systems which cycle carbon as a hydrogen carrier and reclaim it, but the difficulties of making open-loop systems work are probably going to drive efforts toward hydrogen, batteries, or other systems with no emissions.
The easy place to get the carbon would be methane. Doesnt realy matter over the long run as sooner or later energy storage will be such that even airplanes can go electric.