One of the challenges with using energy from renewable sources, such as solar panels, is to figure out how to properly store the energy for later use, especially since renewable energy is often only produced intermittently. This is of particular interest to utility companies, who may have to integrate energy from both solar panels and fossil fuel-derived sources into a single grid.

The renewable energy storage market has therefore seen massive growth in recent years, and is predicted to increase 15.8 percent each year to a $10 billion dollar global industry by 2015, according to a report released on Wednesday by SBI Energy. While the past few years have seen global growth of the renewable energy storage industry by about 10.9 percent per year, the United States has been showing even more rapid increases; averaging a 14.4 percent jump each year from 2006 to 2010.

As it stands now, the United States invests about $0.6 billion each year in renewable energy storage, but the SBI Energy report expects this amount to drastically increase by about 26.6 percent each year until 2015, when the market is expected to be valued at about $2 billion.

One of the reasons for the projected acceleration of growth is that, while many states in the U.S. have goals for the percentage of electricity that must come from renewable sources, many of these projects are still in the early stages, and issues with how the energy will be stored are still being evaluated.

In California, utility companies have only recently begun to figure out how renewable energy will be stored. Funds from a $25 million federal grant, given to the Pacific Gas and Electric company, will be used to determine the viability of a 300 megawatt compressed-air storage system, and this amount will be matched by the California Public Utilities Commission. The storage system would use power during off-peak hours to compress air that would be kept underground, and released to produce electricity during peak-use hours. Different battery compositions and flywheels are other options being looked into by companies specializing in renewable energy storage.