Solar, wind, hydro, and geothermal power have long been explored by humans as a means for harnessing the energy produced by the natural systems of the earth. Advances in these technologies have started to make each of them a viable alternative to fossil-fuel use. Collectively, these forms of renewable resources account for over 10 percent of the energy produced in the United States. As research and new technology is applied to each one, this percentage is sure to grow.

There are also many other new and intriguing forms of renewable resources that are currently being explored. Some of them are still in the research stages and are not presently making any significant contribution in terms of energy production. Yet they hold immense promise, and energy analysts throughout the world are keeping an eye on their development. Other types of renewable energy sources have gone beyond...