For those of you interested in renewables and aircraft, here is the short version:

it's possible, but the barriers to alternative-fuel planes are much higher than for alternative-fuel cars, trucks, buses, trains, and ships. They will very likely be the last mode of transportation to make the transition.

The slightly longer version goes like this:

To get the obvious out of the way, you can't run an airliner on batteries - they're too heavy. Nor is hybrid technology remotely useful on an aircraft.

Next cab off the rank is biofuels. There's no reason why you couldn't run a turbofan on the right mix of them. But putting different fuels into aircraft engines is a very big deal; regulators understandably get very nervous as to the effect on reliability and demand huge amounts of testing to do so. General Aviation airplanes are the very last vehiles to run on leaded fuel for just this reason. And given that biofuels can't go anywhere near meeting demands for automotive fuel, there's absolutely no sense in burning them in aircraft right now. But, if, say, the production of biodiesel from algae starts to supply significant fractions of our transport fuels, it's not inconceivable that they'd be used in airliners as well.

Finally, hydrogen. Hydrogen can be burned in conventional aircraft engines (with some modifications as described with the biofuels), but from an environmental perspective this doesn't make any sense unless the hydrogen is produced in a greenhouse-friendly manner, which is a huge challenge. But let's assume hydrogen is widely and cheaply available - would you use it for plane fuel? It is also incredibly hard to store efficiently and safely; it has a nasty habit of causing the tanks it is stored in to go brittle, something you probably don't want to happen to your planes wing tanks; and yet, your container for holding the hydrogen has to not only be safe but be lightweight and not add bulk to the aircraft. The ultimate promise is hydrogen fuel cells, which when combined with electric motors would form a much more efficient propulsion system than conventional jets. But hydrogen fuel cells are incredibly expensive, and they are also very heavy, which is kind of a problem for aircraft!

So it's not just airline intransigence that ties them to fossil fuels at this stage. It's very real engineering issues.

One point that may not be obvious with regards to wind generation and nuclear capacity is the "capacity factor"; basically, the proportion of the energy it actually provides compared to what it would if it produced at maximum performance 100% of the time. While wind generators need relatively little maintenance and rarely break down, they rely on the vagaries of the wind. Nuclear power, by contrast, can run 24 hours a day until it needs maintenance, either unscheduled or scheduled. According to the American Wind Energy Association, the typical capacity factor for wind power is somewhere between 25 and 40%, whereas nuclear power stations are achieving about 90% capacity factor (according to the NEI). So over a year, 1 megawatt of nuclear capacity results in about 3 times more power being generated than the equivalent megawatt of wind capacity. Coal has about a 70% capacity factor, apparently.

It is more realistic to quote the kilowatt-hours generated using the various technologies, in my view, but you can hardly blame the wind industry for picking the figure most flattering to them.

The claim that that solar energy can't replace fossil fuel electricity generation because it would take up too much land ignores the vast areas (though small compared with the total landmass of the United States) already covered with buildings. For most typical homes, there is more than enough room on the roof for a solar array that would supply all their electricity needs; the barrier to the installation is not the area required, it is the cost of the panels.

The argument is far more accurate in the context of biofuels. To give an illustration, one of the most efficient crops for the production of biodiesel is rapeseed. To replace its oil usage with rapeseed-derived biodiesel, the US would need to plant rapeseed on *ten times* the land it currently grows wheat on - more than 20% of the US's *total* area, much of which is of course not arable land. You can do similar calculations for things like cellulosic ethanol - it's just not enough to replace oil any time soon.

To sum up, solar is not capacity limited to any serious extent even with present technology; it's a cost issue. Present-day biofuel technology has very serious issues with potential capacity.

On a number of occasions on this blog, you've suggested that governments should pass specific regulations to, say, ban a particular source of carbon emissions.

Why make things hard? What's wrong with simply charging for carbon emissions (cap-and-trade schemes seem to be most popular with economists), and let the market sort out the best way to reduce emissions, be it geosequestration, wind, solar, biofuels, nuclear (if you want), energy conservation, or whatever combination of the above is the cheapest way to do so.

On the specific merits of nuclear versus coal; it's very simple, even absent global warming issues. Look at the body counts, particularly using Western examples (Chernobyl was a shockingly badly designed plant run by idiots). If you want to include Chernobyl, I'll raise you hundreds of thousands of Chinese coal miners.