Why is Solar energy classed as a renewable source of Energy?

Well, technically it isn't, but it might as well be. By the time the Sun eventually runs out of fuel in a few billion years, Earth will be completely uninhabitable because the temperature will be above the boiling point of water.

(This has nothing to do with "global warming". The Sun is very slowly getting hotter as it gets older and in a billion years or so the average temperature on Earth will have increased to the point where life as we know it will no longer be possible on the surface.)