by Keith D. Foote
Flow batteries store their electricity in external tanks, similar to fuel cells, instead of within the battery itself. The two basic components, the electrochemical conversion hardware, which the fluids flow through (this sets the peak power capacity), and the chemical storage tanks (these set the energy capacity), can be sized to fit the circumstances. This means the amount of energy stored is limited only by the size of the tanks. Larger amounts of energy can be stored for less cost than with traditional tanks.
A large number of technologies have been considered as ways to store excess renewable energy. Some areas pump water uphill, and then later run it downhill through a generator. Other ideas, such as conventional auto batteries, are simply too expensive, and come with safety concerns.
Quinone flow batteries are new in that they do not use metal ions, but rely on carbon-based, organic molecules, called quinones. Some quinones are responsible for the transport of electrons in animals, plants, and microorganisms. These quinones are naturally abundant and inexpensive. The quinones used in the flow batteries are quite similar to the one used by rhubarb, and can be cheaply synthesized from crude oil.
The quinone molecules could reduce the cost of energy storage materials by 66 percent. These batteries can store large amounts of energy, solving the problem of intermittent power from sources such as wind and solar. This advance in battery technology has the potential to make renewable energy available 24/7. These batteries are efficient enough to consider using at power plants to make the grid more efficient.
In Japan, flow batteries using metal ions have been used for decades as a way to help manage their power grid, but these flow batteries are expensive to build and maintain, and are not terribly cost efficient. They are used in Japan as an extreme solution to spread out the fluctuations of their power grid. The Japanese are paying about $700 per kilowatt-hour for this flow battery system. They are planning to move to a new vanadium ion system, but it will not be significantly less expensive.
Batteries can be rated based on their construction costs. Modern metal ion batteries use vanadium, which currently costs $80 per kilowatt-hour. When the cost of the other battery materials are considered, it is estimated new metal ion flow batteries will cost well over $350-700 per kilowatt-hour. Quinone flow batteries bring the cost down to roughly $27 per kilowatt-hour. This is a significant savings and makes the batteries much more cost effective, and much more functional in terms of renewable energy.
Currently, researchers are only using quinones for the negative side of the battery. The positive side contains bromine, which is quite corrosive, and toxic. At the negative side, each quinone molecule gives up two electrons and two protons. The electrons travel through the negative electrode to meet up with the separated protons at the positive electrode. The protons pass through a thin membrane separating the water/quinones from the bromine solution with its positive electrode.
Quinone flow batteries can be used to make the power grid more efficient. Currently, if power companies relied on wind and solar power, they could only supply about 20% of the needed electricity. This is, in large, part because the sun and wind are intermittent. Quinine flow batteries can store large amounts of energy for those times when the sun isn't shining and the wind isn't blowing.
Quinine flow batteries have great potential for storing renewable energy. These batteries, just as when they are used in the grid, can store energy for later use. When there is no sun or wind, the homeowner can draw on energy stored in the flow batteries. As this battery technology advances, it is completely conceivable a home could receive all of its energy from renewable sources.
Storing electricity in the flow batteries electrolytes provides benefits unavailable in traditional batteries. Unlike most battery technologies, such as lead-acid batteries, the electrodes are not a part of the electrochemical reaction. Because of this, the electrodes do not break down from the continuous cycling activity. As a consequence, flow batteries routinely deliver more 100% discharge cycles than other battery technologies.
One weakness of quinone flow batteries is their use of bromine. Bromine is very caustic, and consequently dangerous. Remember, the positive and negative fluids are not contained within the battery, but are stored in exposed containers. Fear may present a barrier to implementing quinone flow batteries on a large scale. The public perception of bromine as a dangerous chemical is based on fact, not illusion. The not-in-my-backyard issue may hinder the use of high tech batteries on the grid. If adequate safety measures are not developed, it will also hinder the use of these batteries in peoples homes.
Quinone flow batteries are the first step in the creation of a totally organic battery. It is hoped that this battery, and its descendants, will reduce pollution, and destruction of the environment. By embracing alternative energy sources, we steadily lower pollution (such as our exposure to lead). Researchers are continuing their quest to replace bromine with an equally functional, but less dangerous, solution.