A new kind of battery that stores energy from solar and wind power cheaply and cleanly has hit the market. It is by far the cheapest of a new generation of large, long-lived batteries that could make it possible to rely heavily on intermittent, renewable energy sources.

Aquion Energy, a company spun out of Carnegie Mellon University, recently delivered the first of its batteries to operators of small power grids, or "microgrids," that can operate independently of the centralized grid.

Microgrids, which typically use local energy sources such as wind, solar, and hydropower, could help hundreds of millions of people who live beyond conventional grids get reliable electricity. Batteries can store power from solar panels or wind turbines to provide round-the-clock power. Alternatively, diesel generators can be used.

Aquion's batteries use sodium ions from saltwater as their electrolyte. Electrical current moves through this brackish liquid from positive electrodes based on manganese oxide to negative ones based on carbon. The batteries are large and operate slowly, but they are also manufactured cheaply, using repurposed manufacturing equipment. Last week Aquion announced $34.6 million in funding to help it scale up production.