Direct link for those who dislike jumping through hoops:
Biotechnology and Future Energy: BIO-FUEL.

I read through a very long and heated debate once about bio-fuels and if they are really a viable renewable resource. The contention: If you plant say 100,000 acres of bio-fuel plants, do you 'grow' enough fuel to allow you to harvest, refine, and replant 100,000 acres. If you can't then you aren't really creating any usable energy. I don't have a clue what the real answer is, but it is something to think about.
On a similar vein, I read an article long ago about one scientists argument that you extract the most energy from plants by burning them. High heat furnaces could burn the fuel to completion with a minimum of nasty gasses. Don't know the reality of that either. Part of the argument too was the carbon dioxide of combustion would be re-absorbed in the next crop of plants, yielding a Zero net on the environment.

The only problem (besides inefficiency) might be soil exhaustion. As I understand it, soil exhaustion might be a problem for some areas of the world (and US) in the near future, such that food supplies will tighten.

Otherwise, it could work as a renewable source. Its all solar energy anyway...

The claim was the ashes would go back as the fertilizer. When you think about it, plants are great storehouses of energy. With solarpanels & windmills, you need to store energy somewhere to keep the grid humming when the sun is down and the wind isn't blowing. Wow. Imagine the size of the batteries!