Wind turbine
Harvard researchers are developing a new type of storage battery for the renewable sector.
The team of engineers and chemists at the 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) programme to develop the battery. The grant is part of a $130-million funding effort by ARPA-E through its “Open 2012” programme, designed to support innovative energy technologies. Depending on performance, the award may be subject to renewal beyond a year.
Called a flow battery, the technology offers the prospect of cost-effective, grid-scale electrical energy storage based on eco-friendly small organic molecules, the team says. Because practical implementation is a core driver for the programme, 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.”
Flow batteries, a type of rechargeable fuel cell, 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, the team says. 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).
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.
“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.”
Cost and complexity limits the use of current batteries being looked at, the team says. For example, vanadium redox flow batteries have limited commercial head room because the high price of vanadium sets a floor on the cost per kilowatt-hour of storage and sodium-sulfur batteries operated with their components in a molten state, requiring the tanks to be kept at very high temperatures in hot houses, the researchers said.
Aziz believes 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, and they cycle very efficiently between the chemical states needed for energy storage.
“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.”
