Corn ethanol and wind are two renewable energy sources being pursued in the United States. A research paper on "Energy Sprawl" released in August 2009 indicates that energy efficiency is as important a strategy in energy policy as pursuing land-intensive renewable energy sources like biofuels. Photo: brooklyn/flickr
A key challenge for renewable energy like biofuels, solar, and wind becomes the large amount of land needed to produce electricity and liquid fuels at large scale.
Fossil fuels coal, oil, and natural gas are energy dense because they contain concentrated ancient sunlight. Created from carbon-rich plant residues laid down millions of years ago, fossil fuels carry via those plants the energy of millions of years of sunlight densely packed into energy-rich rocks, liquid, and gas.
On the other hand, renewables depend on current sunlight -- today’s sun and wind, this year’s growing season -- to produce their power. Consequently the energy density of renewables is much lower than fossil fuels.