A Harvard University team of scientists and engineers has demonstrated a new type of battery technology that could fundamentally transform the way electricity is stored for the electric grid.  The new technology could make power production from renewable energy sources such as wind and solar far more economical and reliable.

The problem is between the availability of intermittent wind or sunshine and the variability of demand.  A cost-effective means of storing large amounts of electrical energy would end the biggest obstacle to getting a large fraction of electricity from renewable sources.

For example, to cover a couple days at full power for a 1-megawatt wind turbine a battery set would have to be sized at about 50 megawatt-hours.  The current solution is to buy traditional batteries with 50 megawatt-hours of energy storage, but they’d come with 50 megawatts of power capacity.  Buying 50 megawatts of power capacity when only 1-megawatt hour of power capacity is needed is terribly expensive and makes no economic sense.

The batteries we are familiar with are solid-electrode batteries, such as those commonly found in cars and electronic devices, with the power conversion hardware and energy capacity are packaged together in one unit.  It’s an eminently satisfactory solution for those uses.

A prototype flow battery in Aziz's lab at Harvard School of Engineering and Applied Sciences. Image Credit: Eliza Grinnell, SEAS. Click image for the largest view.

A prototype flow battery in Aziz’s lab at Harvard School of Engineering and Applied Sciences. Image Credit: Eliza Grinnell, SEAS. Click image for the largest view.

The Harvard team offers a new chemistry for the “Flow Battery” concept.  Flow batteries store energy in electrolyte chemical fluids contained in external tanks that are instead packaged in the battery container itself. The two main components, the electrochemical conversion hardware of electrodes through which the electrolyte fluids flow sets the peak power capacity, and the chemical storage tanks that set the energy capacity may be independently sized. Thus the amount of energy that can be stored is limited only by the size of the tanks. The design permits larger amounts of energy to be stored at lower cost than with traditional batteries.

The wind turbine noted above would only need a 1-megawatt power capacity hardware unit with 50-megawatt hours of electrolyte.  That poses a very different economic solution.

The Harvard team’s new solution is a metal free electrolyte solution for a flow battery that relies on the electrochemistry of naturally abundant, inexpensive, small organic (carbon-based) molecules called quinones, which are similar to molecules that store energy in plants and animals.  The team’s paper will publish in Nature today.

The battery was designed, built, and tested in the laboratory of Michael J. Aziz, Gene and Tracy Sykes Professor of Materials and Energy Technologies at the Harvard School of Engineering and Applied Sciences (SEAS). Roy G. Gordon, Thomas Dudley Cabot Professor of Chemistry and Professor of Materials Science, led the work on the synthesis and chemical screening of molecules. Alán Aspuru-Guzik, Professor of Chemistry and Chemical Biology, used his pioneering high-throughput molecular screening methods to calculate the properties of more than 10,000 quinone molecules in search of the best candidates for the battery.

Until now flow batteries have relied on chemicals that are expensive or difficult to maintain, driving up the energy storage costs.  The active components of electrolytes in most flow batteries have been metals.  Vanadium is used in the most commercially advanced flow battery technology now in development, but vanadium’s price sets a high starting point on the cost per kilowatt-hour at any size. Other flow batteries contain precious metal electrocatalysts such as the platinum used in fuel cells.

Harvard’s new flow battery already performs as well as vanadium flow batteries, with chemicals that are significantly less expensive, and with no precious metal electrocatalyst.

Professor Gordon said, “The whole world of electricity storage has been using metal ions in various charge states but there is a limited number that you can put into solution and use to store energy, and none of them can economically store massive amounts of renewable energy. With organic molecules, we introduce a vast new set of possibilities. Some of them will be terrible and some will be really good. With these quinones we have the first ones that look really good.”

Professor Aspuru-Guzik noted that the project is very well aligned with the White House Materials Genome Initiative. “This project illustrates what the synergy of high-throughput quantum chemistry and experimental insight can do,” he said. “In a very quick time period, our team honed in to the right molecule. Computational screening, together with experimentation, can lead to discovery of new materials in many application domains.”

Quinones are abundant in crude oil as well as in green plants. The molecule that the Harvard team used in its first quinone-based flow battery is almost identical to one found in rhubarb. The quinones are in a water-based solution, which prevents them from catching fire.

Co-lead author Michael Marshak, a postdoctoral fellow at SEAS and in the Department of Chemistry and Chemical Biology explained to back up a commercial wind turbine a large storage tank would be needed, possibly located in a below-grade basement.  Or if you had a whole field of turbines or large solar farm, you could imagine a few very large storage tanks.

Marshak also noted the same technology could also have applications at the consumer level. “Imagine a device the size of a home heating oil tank sitting in your basement. It would store a day’s worth of sunshine from the solar panels on the roof of your house, potentially providing enough to power your household from late afternoon, through the night, into the next morning, without burning any fossil fuels,” he said.

Professor Aziz, who led the team, said the next steps in the project will be to further test and optimize the system that has been demonstrated on the bench top and bring it toward a commercial scale. “So far, we’ve seen no sign of degradation after more than 100 cycles, but commercial applications require thousands of cycles,” he said. He also expects to achieve significant improvements in the underlying chemistry of the battery system. “I think the chemistry we have right now might be the best that’s out there for stationary storage and quite possibly cheap enough to make it in the marketplace,” he said. “But we have ideas that could lead to huge improvements.”

By the end of the three-year development period, Connecticut-based Sustainable Innovations, LLC, a collaborator on the project, expects to deploy demonstration versions of the organic flow battery contained in a unit the size of a horse trailer. The portable, scaled-up storage system could be hooked up to solar panels on the roof of a commercial building, and electricity from the solar panels could either directly supply the needs of the building or go into storage and come out of storage when there’s a need. Sustainable Innovations anticipates playing a key role in the product’s commercialization by leveraging its ultra-low cost electrochemical cell design and system architecture already under development for energy storage applications.

Now for the personal angle, Professor Aziz said, “You could theoretically put this on any node on the grid. If the market price fluctuates enough, you could put a storage device there and buy electricity to store it when the price is low and then sell it back when the price is high. In addition, you might be able to avoid the permitting and gas supply problems of having to build a gas-fired power plant just to meet the occasional needs of a growing peak demand.”

“The intermittent renewables storage problem is the biggest barrier to getting most of our power from the sun and the wind,” Aziz said. “A safe and economical flow battery could play a huge role in our transition off fossil fuels to renewable electricity. I’m excited that we have a good shot at it.”

It all surely sounds good.  There is a huge reduction in electro conversion hardware investment, a massive change in the cost of energy storage and even a hint of arbitrage on grid rates.  What is not to like?  Intermittent production is still, intermittent.  More capital costs are required for a generating system that even now is non competitive without major incentives and subsidies as well as increased consumer rates.

The story is far from over.  But the effort, ingenuity and innovation at Harvard are of the highest order.  Congratulations are earned, deserved and given. Lets hope to see some real world based economic forecasts soon.


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