In an earlier post “Power the World with Photovoltaics,” we demonstrate that the entire energy requirements of the human race could be fulfilled by a photovoltaic array 143,872 square miles in size. Insofar as this is only about one-quarter of one percent of the earth’s surface, or 668 square feet per person, there is no shortage of available space for photovoltaics.
With biofuel, however, there is a question as to whether or not there is enough land available to grow biofuel and also preserve farms and wilderness. For example, some of the best biofuel crops – biodiesel from jatropha and bioethanol from sugar cane – are able to produce about 6,000 barrels of fuel per square mile per year. This equates to about 55 million Btu’s of energy per square mile per year.
This means that using the best biofuel crops we’ve got today, to produce enough fuel to fulfill entire energy requirements of the human race (400 quadrillion Btu’s per year) we would need to devote 10.8 million square miles to growing biofuel. Considering there is only about 5.5 million square miles of arable farmland on the entire planet, this is not possible.
Biofuel is an important source of fuel, and in some parts of the world growing biofuel makes compelling economic sense, but today at least, biofuel doesn’t show nearly the potential of photovoltaics to efficiently turn sunshine into energy to power human civilization.
There is an interesting analysis “Widescale Biodiesel Production from Algae,” authored by Michael Briggs at the University of New Hampshire. He cites studies that indicate biofuel may soon be economically derived from algae. But he makes a huge assumption – stating that algae farms could yield “5,000 gallons per acre-year.” This equates to a yield over 10 times that of the best biofuel crops we’ve got.
Comparing various forms of solar energy boils down to how efficiently they convert sunlight into usable power. Since raw sunlight provides 100 watts per square foot, a photovoltaic array that produces 10 watts per square foot has an efficency of 10%. Some photovoltaic arrays can do much better than that, with efficiencies reported as high as 20% on high-quality production panels.
You can make the same calculaton with today’s best biofuel crops – diesel fuel extracted from jatropha and ethanol distilled from sugar cane both produce about 55 million Btu’s per square mile per year. Since raw sunshine provides about 41 trillion Btu’s per square mile per year, their efficiency is a paltry .13%.
If the promise of biodiesel extracted from algae is realized, you will see a ten-fold improvement in biofuel efficiency, to 1.3% (based on 5,000 gallons per acre-year). At that level of efficiency – less than one-tenth that of today’s photovoltaics, but cheaper to implement – it might be feasible for biofuel to become a realistic alternative to petroleum.