One goal will be to provide reliable energy sources for developing countries while protecting the environment. The DOE predicts that oil and coal will provide a smaller portion of the world's energy, while natural gas, solar, hydroelectric, biomass, and geothermal will provide a larger percentage. The major source of renewable energy for electricity generation is hydroelectric power, but because all our potential sites have already been developed, this cannot be expanded much in the United States. Nuclear energy use will grow at a much slower rate than the other energy sources due to concerns such as cost, safety, and radioactive waste. The use of nuclear energy by the United States to generate electricity past the year 2020 is uncertain because no new nuclear power plants have been built in the last ten years.
Nuclear fusion is also a possible source of energy in the distant future.
It is predicted that commercial production of energy by fusion will not occur for at least 30 years.