Primus Power has raised enough money from venture investors to build a commercial-size flow battery to store energy on the power grid, the start-up said today.

The company received $11 million from I2BF Global Ventures and DBL Investors, which was spun out of a JPMorgan fund to pursue "double bottom line" investments that earn money and address social and environmental issues. Existing investors Kleiner, Perkins, Caufield & Byers and Chrysalix Energy Venture Capital also invested in this round.

Primus Power has developed a flow battery to provide backup power for the power grid or store solar or wind energy. A flow battery uses liquid electrolytes that flow from one vessel to another past a solid electrode, causing a chemical reaction that induces a flow of electricity. The process is reversed to store energy.

Flow batteries can discharge and charge quickly and can store a relatively large amount of energy per volume, according to the Energy Storage Association. However, critics say they are complex and difficult to maintain. Primus Power said that its battery has a high power density, addressing one of the traditional shortcomings of the technology. Greentech Media reported that Primus Power's flow battery has a zinc bromine system.

Primus Power said that its flow battery is low cost and can be used for many different types of applications. In 2009, it was awarded $14 million as part of the $47 million Department of Energy project to build a 25 megawatt, 75 megawatt-hour storage system to "firm" wind energy. It also received a research grant from the Department of Energy's ARPA-E program to develop a long-lasting electrode for flow batteries with high power density that is well suited for storing wind and solar energy.

There's been a surge in interest and experimentation around grid storage over the past few years, aided by government-funded projects and research. Energy storage is considered a key technology for making intermittent wind and solar more reliable. Costs remain a hurdle to adoption as does utility regulation, which is generally geared around power plants.