One of the most challenging problems facing renewable power is the question of how to store energy from intermittent resources, that don't always blow or shine when the power is needed. The most common solutions include: batteries, pumped water storage, compressed air storage in tanks or underground, fly wheels, ultracapacitors, conversion to hydrogen for storage, and vanadium redox flow battery storage.1
1For an excellent overview of this technology see http://thefraserdomain.typepad.com/e...m_reflux_.html