Renewable energy; that's the straight forward one; energy sources like wind, solar and water are renewable. Unless something pretty darn crazy happens, the rivers are going to keep flowing, the sun shining and the wind blowing. The energy can be completely clean - it doesn't have to involve us ruining the planet by mining coal, or burning fossil fuels; it's there and all we need to do is harness it. Renewable energy is the perfect example of clean energy.
Unfortunately, clean energy isn't just these wonderful renewable sources. 'Clean energy' is any energy source that has been decided to be, well; not too terrible. (Or better than it was...) Nuclear power is considered clean energy. Heck you can even have clean coal burning -- though how clean is it really?
This video is a little old, but it gives you a good look at what a has been called 'clean' in the past.
My advice? If anyone starts going on about 'clean energy' take another quick look into what they are really talking about!
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