Flow batteries: For when the wind don’t blow and the sun don’t shine

prototype flow battery
The Harvard prototype organic flow battery….Credit: SEAS

A team of scientists and engineers at Harvard tackled the problem of storing electricity from short term or irregular energy sources, such as wind mills or solar panels by looking to improve on a type of battery technology known as a flow battery. As published in the journal Nature [08 January 2014, paywalled, A metal-free organic–inorganic aqueous flow battery], the researchers believe they may have found a way to make flow batteries commercially viable for mass energy storage (that is, for grid-level applications).

While we all know about batteries, including the buzzwords such as ‘rechargeable’ and ‘lithium ion,’ people are generally not very familiar with the underlying technology (much less the physics), so that types of batteries such as a flow battery are almost unknown. Most common batteries, such as those used in the home, store the electrolyte (which conducts the electricity) and the electrode (which stores the electric energy) in the same container. In a flow battery, the two kinds of electrolytes store oppositely charged electric energy and are in separate containers. An outside source such as windmill or solar panel charges each electrolyte. When electricity is needed, the electrolytes are pumped into a container with a membrane that separates the electrolytes but allows an energy exchange that provides the electrical current.

This approach has some important advantages: Unlike a standard battery that, at best, can hold only a few hours charge, a flow battery’s charge is limited mainly by the size of the containers for the electrolytes. With a reasonable amount of efficiency, a flow battery can store at least two days’ supply of electricity – enough to build up a charge from wind or solar sources and store it during low wind or nighttime. Because the system exchanges ions through a membrane instead of between electrodes, it doesn’t suffer from a slow degradation of the electrode’s ability to take a charge. In a similar fashion, the large size of the electrolyte containers means that for practical purposes, the electrolyte is almost infinitely rechargeable.

Because of these advantages, flow batteries have been in use for several decades and are important for research. Why then aren’t they ubiquitous? In a word, cost. Most flow battery designs use metallic-based electrolytes, typically vanadium or platinum, which are relatively expensive. Other factors, such as insulation, membrane replacement and charge control sensors contribute to making flow battery energy cost about US$700 per kilowatt-hour. To be more widely practical, the cost needs to be about US$100 per kilowatt-hour (U.S. Dept. of Energy).

To tackle the cost problem, the Harvard researchers decided to explore a new area of materials for electricity storage – organic molecules. Specifically, they chose quinones, which are known for their affinity for electrical charge (electrophilic) and come in a vast number of configurations (hydrogen peroxide and Vitamin K are just two examples). The researchers tested hundreds of quinones and settled on AQDS (if you really want to know, that’s 9,10-anthraquinone-2,7-disulphonic acid). AQDS is found naturally in rhubarb and is easily extracted from crude oil. It mixes with water for storage in tanks. This makes it inexpensive, costing about US$27 per kilowatt-hour compared to about US$80 for metallic materials. In the current configuration, the Harvard flow battery uses quinones on one side of the charge and uses a bromine mixture on the other side. Unfortunately, bromine is both toxic and corrosive, so the researchers are hoping to develop a variation of quinone (or another organic molecule) to replace it.

It’s important to understand that at this stage of research, the Harvard flow battery is a small prototype (as in the picture above). It has successfully been recharged 100 times, but needs to demonstrate thousands of recharges. It, of course, be much bigger and demonstrate reliability over a period of years before utility companies will be seriously engaged. Nevertheless, this version of the flow battery seems to have an angle with organic molecules that has a lot of room for tuning and improvement (like replacing the bromine electrolyte). It already has a rapid recharge capacity and a good record for holding the charge. The Harvard team is already working with a company (Sustainable Innovations, LLC Connecticut) to find commercial applications, probably for storage of home solar energy.

If this approach to battery technology proves to be commercially scalable and reliable, it could solve one of the major problems of renewable energy sources – the storage of energy when the sources are not available. That could have enormous impact on the spread of wind and solar power use.

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