Allen,

Over the surface of the continental US, solar power density averages 198 MW per square km over a full year cycle. Assuming a run-of-the-mill 10% photovoltaic efficiency (as opposed to the 40% efficient bleeding edge stuff), we would only need 660,000 square km of solar cells to supply the entire world's 13 TW fossil fuel power needs. That's only 7% of the land area of the continental US.

Perhaps my numbers are off. I'd like to know which one(s) since my own research corroborates those numbers.

As long as the production of biofuels requires less energy input than it outputs, disregarding the solar energy needed for photosynthesis, then we can divert a portion of the output back into the production process and be carbon neutral. Theoretically. But the whole thing is just so inefficient at harnessing the sun (the ultimate source of energy in all schemes except nuclear) compared to photovoltaic or wind that it doesn't make sense to even consider it.

Andrew,

All estimates I've seen indicate that the earth receives more surface incident solar energy in one hour than the entire world consumes in fossil fuels per year. If you work out the numbers, and conservatively assume photovoltaic efficiency of 10% (satellites use 28% efficient cells, and 40% efficient cells have been developed in labs), then placing solar cells over just 7% of the continental US would completely replace the *world's* demand for fossil fuels.

Nuclear power plants on average only output 1GW. The world consumption of fossil fuels is currently equivalent to 13TW. To replace fossil fuels with nuclear would require us to build one nuclear plant per day for the next 30 years.

I agree with previous posters that biodiesel as a fossil fuel replacement is a sham. It is indeed carbon-neutral, but it's a very inefficient way to harness the sun.

Phat