To harvest the full yield available from intermittent sources of energy like wind and solar, Cheap storage is needed. A lot of research dollars are going into building a variety of storage options for renewable energy to extend their contribution to the grid. We have too much wind at night and too much solar in the day: but seldom overlapping in any one region.

Utilities are looking into storing energy in compressed-air in caves, in gravity; by pumping water up - to let it drop when needed - or in rolling batteries; by loading up extra juice at night into electric cars - to be dispatchable back to the grid again at peak with interactive vehicle-to-grid technology.
PG&E is one public utility scouting for caves suitable for compressed-air storage capable of storing 3,000 megawatt-hours (or 300 megawatts for ten hours). There are already a few compressed-air facilities in the world where off-peak electricity is used to pump air underground for storage. During peak-demand times, the air is released and pushed through a turbine to make electricity. Utility-scale battery storage systems only deliver 1 or 2 megawatts for a few hours.But underground limestone caverns aren’t always right where you need them; at least with the right geological attributes that make them safe as depositories. You need the portability and scalability of a battery and the economy and capacity of a geological feature.

One of the options is compressed-air storage; till now only possible in underground caverns. But SustainX Energy Solutions; a Dartmouth College start-up that got $4 million in VC funding from Polaris Venture Partners and Rockport Capital this year is working on compressing and storing air in cheap off-the-shelf shipping containers. Over the next two years the company  will try to develop a way to cram 4 megawatt-hours worth of stored energy into each 40-foot long container and to reduce the energy that it currently takes to compress and release air by about 70%.

The SustainX energy storage system utilizes a proprietary energy conversion system to store electrical energy in the form of compressed air. The result is a high efficiency, long lifetime, low maintenance energy storage solution. The technology allows for competitive energy densities at low specific energy cost. Unlike conventional batteries and other chemical-based energy storage systems, their solution is a clean technology with minimal ecological impact. It is innovative storage/conversion system is easily scalable and is design  to be modular, fitting compactly into shippable containers that can be delivered via truck, ship and rail.