One of the main criticisms of wind and solar power is their nature as intermittent sources of energy. If the wind stops blowing on the plains of North Dakota, or clouds cover the desert in California, blackouts could occur across the country if these renewable energies were primary sources of power.

So the challenge for investors is finding who will supply the technology to smooth out those peaks and valleys. Some new technologies are now emerging that have the potential to play a big role in the future of renewables.

Battery storage
A123 Systems (Nasdaq: AONE) and Ener1 (Nasdaq: HEV) have been testing battery storage in locations around the world with some success. Ener1 has a $40 million deal with the Russian Federal Grid Company, and A123 Systems is on its second deal with AES Energy Gener in Chile for a total of 32 MW.

Inverter manufacturer Satcon Technology (Nasdaq: SATC) has even talked about adding battery storage to their inverters to kill two birds with one stone.

The problem for battery storage is scale. Batteries may be able to charge quickly and release energy quickly, but right now they can't store mass amounts of energy for the grid -- something renewable power needs.

Pumped hydro
Store energy by pumping water uphill and release energy by capturing the energy of it running downhill. Simple, cost effective -- and so far, our best answer to energy storage. There are 21 GW of pumped hydro in the U.S. and 38 GW in Europe, but this isn't a solution for all problems. Wind-heavy areas like North Dakota and offshore aren't exactly conducive to this, nor are deserts where solar power is most effective.

Compressed air storage
Maybe the more immediate answer is compressed air storage. Think of this as a huge compressed air tank that is filled during off-peak hours and released as needed to provide electricity. There are two sites online in Alabama and Germany, with Pacific Gas and Electric (NYSE: PCG) and New York State Electric and Gas looking to get in the game.

The PG&E site in Kern County is 300 MW so the potential size of compressed air energy storage is much larger than battery storage right now.

Bottom line
Energy storage roadblock that may decide how quickly renewable energy becomes adopted on a wider scale. With GW of projects in the works at solar leaders First Solar (Nasdaq: FSLR) and SunPower (Nasdaq: SPWRA), the next challenge may be out of their hands. Southern California can only handle so much solar power before the lights start flickering when a cloud passes overhead. Energy storage is the key to make sure that doesn't happen.

There are no sure-fire solutions, but batteries are sure to play a role in smoothing out energy supply. It may not be on a GW scale, but even on a much smaller scale, A123 and Ener1 are working to play a role in making the grid both cleaner and smarter.