Greener storage for green energy

Storing energy from these intermittent sources has aroused interest, yet practical economics and basic chemistry have limited the wider use of green energy. Storage, to be viable, cannot add much to the price of renewable electricity without making it unacceptably expensive. Fossil fuels remain the world's chief energy source due to their relatively low cost.

To give renewals a fighting chance, a team led by engineers and chemists at Harvard University will use a one-year, $600,000 innovation grant from the U.S. Department of Energy's Advanced Research Projects Agency-Energy (ARPA-E) program to develop a new type of storage battery. The grant may be subject to renewal beyond a year, depending on performance. The award is part of a $130-million funding effort by ARPA-E through its "OPEN 2012" program, designed to support innovative energy technologies.

Called a flow battery, the technology offers the prospect of cost-effective, grid-scale electrical energy storage based on eco-friendly small organic molecules. Because practical implementation is a core driver for the program, the researchers are collaborating with Sustainable Innovations, LLC, a commercial electrochemical system developer.

"Storage of very large amounts of energy is required if we are to generate a major portion of our electricity from intermittent renewable sources such as wind turbines and photovoltaics," says lead investigator Michael Aziz, Gene and Tracy Sykes Professor of Materials and Energy Technologies at the Harvard School of Engineering and Applied Sciences (SEAS). "Currently no cost-effective solution exists to this large-scale storage problem. Flow batteries may make stationary storage viable in the marketplace, and that will enable wind and solar to displace a lot more fossil fuel."

A type of highly rechargable fuel cell, flow batteries are suitable for storing large amounts of electrical energy in the form of liquid chemicals, which are flowed past the electrochemical conversion hardware and stored externally in inexpensive tanks that can be arbitrarily large. This permits the designer to independently size the electrochemical conversion hardware (which sets the peak power capacity) and the chemical storage tanks (which set the energy capacity).

By contrast, in solid-electrode batteries, such as those commonly found in cars and mobile devices, the power conversion hardware and energy capacity are packaged together in one unit, and cannot be decoupled. Consequently they can maintain peak discharge power for less than an hour before being drained. Studies indicate that 1 to 2 days (the cycle of day/night) are required for rendering renewables like wind and solar dispatchable through the current electrical grid.

To store 50 hours of energy from a 1-megawatt wind turbine (50 megawatt-hours), for example, a possible solution would be to buy solid-electrode batteries with 50 megawatt-hours of energy storage. The effective result, paying for 50 megawatts of power capacity when only 1 megawatt is necessary, however, makes little economic sense.

"Not only are existing solid-state batteries impractical for storing intermittent wind and solar energy, but flow batteries currently under development have their own set of limitations," says Aziz. "The chemicals used for storage in flow batteries can be expensive or difficult to maintain."

For example, vanadium redox flow batteries -- the type of chemistry receiving the most attention -- have limited commercial head room because the high price of vanadium sets a floor on the cost per kilowatt-hour of storage. Sodium-sulfur batteries operate with their components in a molten state, requiring the tanks to be kept at very high temperatures in hot houses. Both cost and complexity limit their use.

Aziz believes that using a particular class of small organic molecules may be the key. These molecules, which his team has already been working on, are found in plants and can be synthesized artificially for very low cost. They are also non-toxic and can be stored at room temperature. Furthermore, they cycle very efficiently between the chemical states needed for energy storage.

As an expert in materials science and a developer of high-performance flow cells, Aziz will focus his efforts on molecular and electrode electrochemistry and flow cell development. Joining him will be Roy Gordon, Thomas Dudley Cabot Professor of Chemistry and Professor of Materials Science, who will be responsible for the chemical screening and synthesis of molecules and of practical electrocatalytic and protective coatings. Alán Aspuru-Guzik, an Associate Professor in the Department of Chemistry and Chemical Biology, will use his pioneering high-throughput molecular screening methods to identify optimal molecules. Trent M. Molter, President and CEO of Sustainable Innovations, LLC, will provide expertise on implementing these innovations into commercial electrochemical systems.

"We think our particular approach could have advantages over other flow batteries, such as higher power density, high efficiency, inexpensive chemicals, and a safer type of energy storage," says Aziz. "The success of this program would render intermittent renewables like wind and photovoltaics dispatchable at will, and thereby permit them to supply a large fraction of our electricity needs."

Aziz foresees using next-generation flow batteries for local energy storage, such as in the basement of a house or office outfitted with rooftop solar panels or, at a larger scale, directly integrated into wind and solar farms. The technology could even out-compete lead-acid batteries for solar energy storage in remote areas without access to a grid.

"While not eliminating fossil fuels, flow battery storage potentially eliminates a barrier to doing so within the existing energy system and market," says Aziz. "The best engineering and chemistry alone are not enough to solve our energy challenges. Compatibility with current infrastructure is almost always essential, and economic viability is always essential. Flow batteries may play a huge role in our transition off of fossil fuels and I am very excited that Harvard has the opportunity to develop a potential game-changer."