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Here’s a thoughtful piece by Elon Musk, chairman of Tesla Motors. Of course we know exactly where Tesla’s economic interest lies — even so, consider his analysis framework. Musk doesn’t attempt what is really required to compare solar to biofuels on a life-cycle energy efficiency basis, considering all inputs such as capital, land area, water, fertilizer and environmental impact. E.g., he doesn’t look at the energy inputs required to manufacture, deploy and decommission solar. But market prices will perform most of that balancing if the right price signals are in place, such as a revenue-neutral carbon price. An excerpt:

Photosynthesis vs. Photovoltaics



Another way to think about the problem is that plants are essentially just a very inefficient way to convert sunlight into stored chemical energy. Crops typically have a net efficiency of about 1/2% or so, compared with commercially available photovoltaics at 20%. That means you need about 40 times more land area for crops than you do for photovoltaics to capture the same energy. Complicating the issue is that crops require arable land, which will apply great pressure to what little remains of unfarmed wilderness areas around the world. In contrast, photovoltaics can usually be installed on your home or business rooftop, efficiently delivering energy right where it is consumed and taking up no extra land at all.

If you want to use plants most effectively as an energy source for transportation, the best way is to burn them whole (no processing needed!) in a combined cycle biomass electric generator at 60% efficiency and use the output to charge electric vehicles. That requires no technology breakthroughs, uses the full energy content of the plant, and is far more efficient than refining a small part of the plant or even most of the plant, using cellulosic technology, into ethanol to power the 20% efficient internal combustion engines of cars.

[From a discussion I had with Prof. Nate Lewis at CalTech]

The fastest growing crop, switchgrass, stores energy at a yearly averaged rate of 1 W/m2, for a peak solar efficiency of less than 0.5% (220 W/m2 mean latitude yearly averaged insolation). However, you would be lucky to get 0.2% after considering energy inputs and outputs. Wang and the Argonne GREET model are somewhat more optimistic, achieving about 0.3% net for taking the fastest growing crops and just burning them. Making ethanol is another story altogether, and if not negative, is less than 0.1% at best and more like 0.01% from current corn technology and maybe 0.1% from cellulosic if cellulosic is ever actually developed to work at commercial scale. There is an excellent paper by Pimentel, a professor at Cornell, and Patzek, a prof at Berkeley, in Natural Resources Research (Vol. 14:1, 65-76) on the energy yields of a variety of crops, including corn, switch grass, wood, soybeans and sunflowers.

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