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In the old days, renewable energy was the best option available. Then it became a social statement. Now with fits and starts, it's creeping toward the mainstream.

To give CNET readers some perspective on renewable energy, we've assembled a slideshow on renewable energy through the ages, its present form, and some ideas as to where it's going.

The history of using the wind, sun, and water reaches back to antiquity. But what's remarkable about many renewable-energy technologies being pursued today--solar, wind, geothermal, and bioenergy--is that today's product ideas often have roots in research begun after the 1970s. There was a surge in renewable-energy activity following the OPEC-induced energy crisis but then much of it petered out as oil prices fell and environmental concerns subsided.

A few things are different than the 1970s, though. For starters, the global thirst for energy, particularly from developing countries, has mushroomed and is expected to keep growing rapidly. Today, there's the danger of climate change from a build-up of greenhouse gases in the atmosphere. And as always, countries are trying to improve their national security by reducing oil imports and promoting domestic energy sources.

As a result, today solar, wind, and biofuels are big, multibillion dollar industries, not just the ideological exercises of "back to the land" hippies. Led by China, countries around the world are trying to establish the technology and manufacturing base for these industries.

And as the prices come down and financing options become available, more consumers are going with solar power or small wind for both economic and environmental reasons.

Nobody has a clear picture on exactly how quickly renewable energy will grow in the future. A lot depends on the cost of fossil fuels, whether any big technology breakthroughs occur, and government policies since energy is a highly regulated industry.

But if the past and present teach us one thing, it's that innovation in energy is hard and change comes slowly to energy. That's no surprise given that expensive power plants and the fuel infrastructure took decades to build up.

Still, technology is one of the most important levers in making renewable energy cheaper and more accessible, the kind of innovation that's good for the planet.