A team of Harvard scientists and engineers has demonstrated a new type of battery that could fundamentally transform the way electricity is stored on the grid, making power from renewable energy sources such as wind and sun far more economical and reliable.
The mismatch between the availability of intermittent wind or sunshine and the variable demand is the biggest obstacle to using renewable sources for a large fraction of our electricity.
A cost-effective means of storing large amounts of electrical energy could solve this problem, offering a potentially viable means of storing surpluses from windfarms, for example.
The Harvard team has come up with a metal-free 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.
Rhubarb is an especially good source of quinones.
Flow batteries store energy in chemical fluids contained in external tanks, as with fuel cells, instead of within the battery container itself.
The two main components – the electrochemical conversion hardware through which the fluids are flowed (which sets the peak power capacity) and the chemical storage tanks (which 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.
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 maintain peak discharge power for less than an hour before they are drained, and are therefore ill-suited to store intermittent renewables.
To store 50 hours of energy from a 1-megawatt power capacity wind turbine (50 megawatt-hours), for example, a possible solution would be to buy traditional batteries with 50 megawatt-hours of energy storage, but they would come with 50 megawatts of power capacity.
Paying for 50 megawatts of power capacity when only 1 megawatt is necessary makes little economic sense.
For this reason, a growing number of engineers have focused their attention on flow-battery technology. But until now, flow batteries have relied on chemicals that are expensive or hard to maintain, driving up the cost of storing energy.
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 it sets a high floor on the cost per kilowatt-hour at any scale. Other flow batteries contain precious metal electrocatalysts, such as the platinum used in fuel cells.
The new flow battery developed by the Harvard team performs as well as vanadium flow batteries, with chemicals that are significantly less expensive, and with no precious-metal electrocatalyst.
Basically, the Harvard team’s battery marks a switch from using metal ions in various charge states, to organic molecules.
Quinones are abundant in crude oil as well as in green plants. The molecule the Harvard team used in its first quinone-based flow battery is almost identical to one found in rhubarb.
The quinones are dissolved in water, which prevents them from catching fire.
The next steps in the project will be to further test and optimise the system that has been demonstrated on the bench-top and bring it toward a commercial scale.
The Harvard team received funding from the US Department of Energy’s Advanced Research Projects Agency – Energy (ARPA-E) to develop the grid-scale battery, and plans to work with the agency to catalyse further technological and market breakthroughs over the next several years.
