While most utility companies see the advantage to clean, renewable energy sources, such as solar and wind, they also know that the sun only shines and the wind only blows for some of the time. Plants generating more traditional kinds of power – such as coal, nuclear and natural gas – can be turned on and off as needed. They can be shut down during low-demand periods and only take a few minutes to restart. Renewable energy, though, must be stored for use during peak hours.

Batteries seem like the obvious solution. After all, they power many everyday devices and have been in use for over 200 years. But such chemical storage is cost-prohibitive on the large scale that solar and wind energy farms require.

To solve the problem, companies are developing alternate solutions, including: bulk gravitational storage (water pumped back up behind hydro-electric dams); bulk mechanical storage (compressed air inside underground caverns); mechanical storage (via flywheels); and thermal storage.

Most renewable energy generators will probably use a combination of these methods, depending on the requirements of a given location. And the fact that scientists continue to develop cost- and energy-efficient solutions seems to promise more options to come.