Solar power and wind power are usually the first topics that come to mind when the discussion turns to renewable energy sources.

The global solar footprint has grown almost a hundredfold, from 175 megawatts in 2000 to 15 gigawatts in 2010. But despite the growth of those sectors, wind and solar still account only for a tiny fraction of global electrical generation, which is dominated by coal, natural gas and nuclear power.

Conventional hydroelectric power currently provides about seven percent of the electricity in the United States. But the lack of new sites and tight environmental regulations ensure that there won't be a lot of growth in large hydro -- never mind the efforts to dismantle existing dams to restore fisheries.

But dams are not the only way to harness hydro power.

Startups are innovating and looking to use wave power, tidal power and power from river currents to drive turbines and create electricity. Water is 800 times more dense than air and tides can be predicted decades in advance. But what if you took Neptune's fury out of the equation? The microhydro firm Hydrovolts has a turbine for irrigation ditches that could turn farms into power plants. Beyond that, there are osmotic pressure gradients and ocean thermal technologies that, at first glance, seem to border on magic.

There is also enormous power below our feet: geothermal energy from naturally occurring underground steam sources. While most geothermal wells are now in the western U.S., hot dry rock (HDR) geothermal power could expand the footprint because it doesn't rely on natural underground reserves. A recent MIT study estimated the available portion of HDR power to “exceed 200,000 exajoules, or 2,000 times the annual consumption of primary energy in the United States in 2005."

Unfortunately, it's not fully tested, and HDR has the potential to create some seismic activity.

New technologies in energystorageare also emerging -- fuel cells, flow batteries, phase-change materials, flywheels, pumped hydro, and compressed air energy storage (CAES) -- as are new types of nuclear reactors. Most of these technologies are in the experimental stage. Which ones will make it to maturity?

Read more on this topic in a joint effort by General Electric Ecomagination and Greentech Media, and join the conversation here.