Use yeast and carbohydrates to create air pressure. Feed the pressure into a turbine which creates movement (hooked up to a generator (magnets, commutator, etc.)). A small air tool could be used to test this power source without ever using any generator at all. Since this may not be as convenient, to store the power as pressure, the power may need to be converted into electricity, and stored in batteries too.

Problems:

 -Alcohol must be filtered so the yeast continues to grow. A high concentration of 
  alcohol means the yeast stops growing. But alcohol can also be used as a fuel 
  source. 

 -High carbon dioxide output.

 -sludgy. Food may sink and need to be filtered.

 -stink. Possible stink from certain foods. Not so bad smells when using  
  simple sugar fermentation, or non sulphur foods.

Advantages:

 -Yeast grows easily. Yeast only requires sugar, acid, minerals and small amounts of 
  protein (or plants, or grass, etc.) as its food source. 

 -Yeast can eat itself if it has too (recycling its own proteins and minerals)

 -Yeast is very controllable since it is alive but small and maintainable. Yeast 
  multiplies and multiplies. Yeast is not picky about its food as long as you give it 
  carbohydrates and obvious minerals like salt, trace minerals, etc. 

 -The ability to give it something simple in great abundance such as cane sugar, 
  grass, etc. should not be ignored. It's not like you have to feed yeast rare 
  protein from an endangered species in order for it to grow. 

 -no immediate sunlight required. Grow it in the dark, create power in the dark. 
  Sunlight is needed for the plants that it gets its food sources from, but plants 
  are like batteries: yeast doesn't need sunlight itself.

 -extensive knowledge exists for yeast growth already. Maybe humans could put yeast 
  to good use, instead of wasting efforts on "studying a highly technical subject in
  order to get thousands of people non-technically stupid drunk".