Biofuels are a complex matter and Giampietro and Mayumi use almost 300 pages to eviscerate it in all its aspects. The main point of their analysis is based on fundamental physics: the efficiency of photosynthesis is low and the result is that the areas needed for cultivation are large. If we are thinking of amounts of biofuels comparable to the present needs for transportation, the task is simply unthinkable: there would be no space left for food production. As the authors flatly state at page 128 of the book, "Full substitution of fossil energy with agro-biofuels is impossible."If we wanted to produce enough solar panels, windmills, wave-motion generators, nuclear facilites, hydroelectric plants, ethanol factories or hydrogen plants to keep our economy at the present enormous scale, we need fossil fuels. This becomes even more imperative if we need to keep the economy growing in perpetuity. This simply cannot be done.
The large area needed is only one of the problems with biofuels. More in general, agriculture is a good technology for producing food, but it is terribly expensive in terms of the resources it requires. It needs land, water, fertilizers, pesticides, mechanical work; all supplies that normally come from fossil fuels. Taking all that into account, the EROEI (energy return for energy invested) of biofuels is generally low; unless the invested energy is supplied by low cost human labor, as it is the case for Brazilian sugar cane. Apart from Brazil, the need of an energy subsidy in the form of fossil fuels makes biofuels unable to deliver their promise of being a "sustainable" technology. They can't help us in reducing our dependency on fossil fuels nor in reducing the emission of greenhouse gases in the atmosphere.
It's worth noting that nothing we do with fossil fuels cannot be done some other way - from biofuels, to celluose-based plastics to hydroelectric electricity; even factories can be powered by the sun. The problem is entirely one of scale. If we don't deal with scale, we aren't really dealing with the problem at all. That's why Peak Oil and economic reform are joined at the hip.
Cassandra's Legacy: Why biofuels are not a good idea.
ADDENDUM: Do The Math blog comes to a similar conclusion (emphasis in original):
Another way to highlight how daunting a full-scale embrace of biofuels would be, consider that global oil consumption amounts to 6 TW of power (30 billion barrels per year, or 1000 barrels per second, at 6 GJ per barrel). This is about 12 times the human metabolic dietary intake—largely derived from agricultural lands. We’re not about to give up eating, so in the simplest analysis, we would have to find an additional cropland approximately ten times the area of our current cropland.
For scale, Earth’s land totals about 140 million square kilometers. About 50 million are classified as agricultural (includes permanent grazing land), and 13 million as arable. On what planet would we find enough land for sufficient biofuel crops?