Image of what a solar roadway might look like in the future. 

Putting solar panels on a rooftop used to be a novel idea. Some even called it crazy as solar was such an expensive form of generating electricity. A lot has changed in just a few short years, as solar power is starting to go mainstream.

The same can be said for wind energy. What was once no more than an interesting idea is now a major source of growth for many utilities.

Just as solar and wind once represented the future of green energy, today, new technologies are beginning to emerge that represent the next stage of green energy. Like solar and wind a few years ago, these ideas might sound crazy now, but that doesn't mean they won't go mainstream, too.

Companies like Lockheed Martin (NYSE:LMT), for example, are working to pioneer ocean thermal energy in China. Meanwhile, other industrial giants are looking at new forms of clean energy, including General Electric (NYSE:GE), which is hoping to acquire Alstom, a company developing a technology to generate power from ocean tides. On top of that, General Electric is developing a solution that could turn 96% of America's nuclear waste into electricity.

Overall, in my research, I found nine crazy ideas that just might be the future of green energy. I've organized them in the slideshow below. While none of these ideas appear ready for the mainstream just yet, their day in the sun could come sooner than we realize.