Bill Gates is at it again. Through his investments in a group called Breakthrough Energy Ventures (BEV), Gates is exploring new ways to store renewable energy. While many innovative companies are creating ways to generate energy, BEV is focused on technologies that will allow enough energy storage to supply the major power-grids with clean energy even during windless days, cloudy weather, and nighttime.

One of the more promising ways to store energy is through the creation of long-duration storage systems. Short-duration devices like lithium-ion batteries are fine for laptops, mobile phones and electric cars. But cheaper and longer-duration systems are needed for the electrical power-grid.

A BEV-backed startup known as Form Energy is poised to meet that demand. The company has teamed up with Minnesota-based co-op Great River Energy to build a new battery that can discharge for 150 hours. Storage for this length of time is far better than conventional batteries and will help wind and solar energy sources to dominate the US energy landscape in a few years. So, how does it work?

Flow batteries are based on the chemistry that produces electricity when two specialized liquids flow next to each other, separated only by a thin membrane. Flow batteries are also known as reduction-oxidation (redox) flow batteries, due to the ionic exchange (accompanied by a flow of electric current) that occurs in the membrane as the fluids pass by one another.

To story energy in liquid form, the redox flow battery needs a positive and a negative chemical stored in separate tanks. The chemicals are pumped in and out of a chamber where they exchange ions across a membrane – flowing one way to charge and the other to discharge. The energy capacity of these redox batteries is a function of the electrolyte volume (amount of liquid electrolyte), while the power is a function of the surface area of the electrodes.

As an indication of the importance of this promising technology, last year Form Energy won a $3.7 million grant from the Energy Department’s cutting edge clean tech funding office, Advanced Research Projects Agency – Enery (ARPA-E). This grant was to develop a “long-duration energy storage system that takes advantage of the low cost and high abundance of sulfur in a water-based solution.”

One of the benefits of aqueous sulfur flow batteries is their lower chemical cost among other rechargeable batteries like lithium ion. Perhaps that’s why the energy portfolio of the Great River’s co-op is betting on the new tech to store energy from new wind farms over an option to build a massive coal power plant.

Ionic exchange creates current in flow batteries. (Image Source: Flow-Battery-Wikimedia)