You ask a very good question. Are biofuels just new ways of using fossil fuels or are they genuine new energy sources?
In principle, biofuels are genuine new energy sources, since the free energy to form their carbon compounds from CO2
comes from sunlight. They're essentially a chemical form of solar energy, as opposed to an electrical form.
In practice, however, many biofuels are grown using conventional farming techniques which involve very large inputs of fossil fuel energy. Corn ethanol in the U.S., for example, currently uses so much fossil fuel that the energy output only slightly exceeds the fossil energy input.
It seems that biofuels are unlikely to provide a very large fraction of total energy needs for a population as large as the earth currently has. There's a wonderful free book by David MacKay (http://www.withouthotair.com/
) describing the basic science of different terms in the energy budget. He focuses on the U.K., with a high population density and relatively little sunlight, so various solar sources including biofuels would be more significant elsewhere.
(published on 01/18/12)