As quickly as ethanol has risen in popularity as an alternative to fossil fuels in transportation, so have reasons against using it.  Critics site many potential problems inherent on relying on the "new" fuel source including:

• food- and water-scarcity issues that may result from converting farmlands to croplands;
  
• the vast amount of embodied energy in the product from farming, to harvesting, to processing 
• producing ethanol from switch grass requires 45 per cent more fossil energy than that created from the product! (according to this blogger's estimate)
  
• and, ethanol processing plants contribute bast amounts of air pollutants including sulfur dioxide, nitrogen oxides, and volatile organic compounts, according to Corpwatch.com
  

But even more compelling information was published in the August, 2007 issue of Environmental Design Update.

Using farmland to grow crops for ethanol or biomass fuel yields significantly less energy per acre than covering the land with ground-mounted PV arrays.  For example, one acre of land can grow enough biodiesel in one year to propel a car 5,400 miles; when covered with PV modules, the same land can produce enough electricity in one year to propel a car 817,280 miles - 150 times as far!

Now, what this blogger wants to know, is how much energy can we gather if we covered our cityscape rooftops with PVs, rather than our farmlands? 

Other resources:

Turning Crops to Ethanol Fuel: on the road to energy independance

Crunching the Numbers on Alternative Fuels

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