Despite the "green" movement that has taken over the country in the last few years, more than 80 percent of the world's energy supply still comes from fossil fuels. Solar panels and wind turbines haven't been able to make much of a dent in coal and petroleum's dominance. But a new two-part study published in the journal Energy Policy claims it's possible and affordable for the world to achieve 100 percent renewable energy by 2030. The renewable sources of energy the researchers' calculations focused on included wind power, solar power, waves and geothermal energy, even as some question if solar power is worth the expense.