Renewable energy sources
The are a number of ways to produce renewable energy. The most common ways are using wind mills attached to turbines to generate electricity, and using solar cells to directly generate electricity from the sun. We discuss wind energy here, and solar energy here. Recently concentrating solar enrgy using mirrors has been attempted on a large scale. Solar energy is discussed here.
Neither of these has provided practical sources of energy for transportation as yet. Renewable energy for transportation has come from plants in the form of liquids or gas. The developed world has concentrated its efforts on producing renewable liquid energy to replace gasoline and diesel fuels. These have largely consisted of the alcohol ethanol (distilled rom corn in the US and sugar cane in Brazil), and bio-diesels from vegetable oils from soybeans, rape seed and palm oils.
In the developing world quite a different approach has been popular. This is the generation of biogas from human and animal wastes by having them decay in the absence of oxygen (this is called anaerobic decay). We explain this process in detail here. Often this has been done on a small scale (private homes and farms) in India and China. But recently, Sweden, Austria and Germany have been developing anaerobic digestion production facilities on a large scale. There is limited activity in the US, but as we shall see this has great potential.
Another source of biogas (which contains about 60% methane gas - the main component of natural gas) can be the anaerobic digestion of fast growing algae and micro flowers, as well as switchgrasses, etc.