Storing energy remains the missing link for many clean power technologies, but DOE researchers and startup companies are racing to fill the gap.

Without a way to save electricity and heat for later use, intermittent renewable energy will struggle to close price and performance differences with fossil fuels. But stored energy has advantages. It can respond to increased power demands faster than a turbine can spool up, and it can also save excess power and then supply it when needed.

In the drive to cut greenhouse gas emissions, the Department of Energy is betting that energy storage, particularly on the grid, will be a big part of the solution.

At a Senate hearing yesterday on DOE's budget, Energy Secretary Ernest Moniz noted that the agency has requested increases in funding for energy storage research and development through its Office of Electricity and through its Energy Efficiency and Renewable Energy division. "It's very important both for the technology, but also for system integration," he said.

DOE presented some of its energy storage projects this week at the annual summit of the Advanced Research Projects Agency-Energy. "The most important technology we can develop right now is storage," said SunEdison CEO Ahmad Chatila at the summit.

One approach is the aluminum-air battery. The device essentially reverses the smelting process used to produce aluminum. It uses cheap materials and has a higher energy density than lithium-ion cells. However, the battery requires a mechanical recharge, meaning users have to swap out the aluminum plates to fill it up.

Developers Phinergy and Alcoa envision the design as a range extender for electric vehicles, providing more than 1,000 miles of range on a charge, or as an emergency power solution, since the cells have a theoretically infinite shelf life.

Safer, bigger 'flow' batteries
Another strategy for storing energy is flow batteries. ARPA-E is funding no fewer than 17 flow battery projects, with chemistries including iron, zinc and potassium.

Flow batteries are particularly suited for grid-level electricity storage because they can be scaled up easily. They produce energy by pumping liquid electrolytes through a cell, and increasing their capacity is simply a matter of increasing the size of the storage tanks for the electrolytes.

Because the active components are separated, flow batteries are safer on large scales than conventional batteries. Flow batteries also charge quickly and last for thousands of cycles, but needed materials such as energy-dense electrolytes and ion exchange membranes remain expensive.

Companies including ITN Energy Systems and Teledyne Scientific & Imaging are working to increase energy densities in flow batteries using sustainable materials, all while driving down costs.

But electricity isn't the only bankable form of energy; researchers are also looking for better ways to store and then later dispatch heat.

A team at the University of California, Los Angeles, is developing a method to store heat from concentrating solar power plants that uses supercritical fluids, which are fluids heated to high temperatures and under high pressures.

Most concentrating solar power plants with a storage component use molten salt to save up heat and release it at night to boil water and spin a turbine.

Using a supercritical fluid, these power plants can cut their energy storage costs up to 40 percent by using a cheaper, simpler, more reliable design. This lets a solar power plant provide power that is more consistent and makes it more competitive with baseload electricity producers like coal and nuclear power plants.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500