Lithium ion batteries are capturing an increasing share of power grid support applications. Tesla recently claimed to have built more than a gigawatt-hour of electrical energy storage using its lithium ion Powerpack to help support renewable solar and wind energy production. But is lithium ion technology the best choice for stationary electric power grid support? Fans of redox flow batteries (RFB) would suggest otherwise.
Flow batteries (also called redox or reduction-oxidation batteries) use two different electrolytes that are each pumped through the two half cells. The cells are separated by a thin ion exchange membrane. Charging the battery causes a reduction reaction on one side of the membrane and an oxidation reaction on the other. A reduction reaction results in a gain of electrons while an oxidation results in a loss of electrons. In use during discharge, the electrolytes are continuously pumped from their tanks into the reaction cell and electrical energy is drawn from the electrodes. The only limit to the amount of energy that can be stored is the capacity of the electrolyte storage tanks.
“One of the primary reasons that DOE (Department of Energy) is interested in flow batteries is because the power and energy of those systems are separate,” Vincent Sprenkle, Manager of the Energy Storage group at Pacific Northwest National Laboratory (PNNL), told Design News. “What that gives you, from a grid perspective, is a high degree of flexibility. The other advantage is inherent safety. You are in an aqueous solution, so the fire hazards are not there as you would have with a pure organic electrolyte. We are doing water-soluble based systems. Also, in a megawatt hour system, you may only have kilowatt hours of it that are physically in contact with each other at any time,” he explained.
The Right Stuff
Choosing the materials for the reduction and oxidation reactions has a large effect on the flow battery’s characteristics. The flow battery concept was first used in 1884 with a zinc/chlorine battery that powered Charles Renard’s airship La France. More recently, redox flow batteries have been made from zinc bromide. This is the case for the ZCell, a 10 kilowatt-hour home energy storage system developed in Australia.
When the ZCell is charged, electrical current travels into the battery, causing zinc to be removed from the zinc bromide solution and to be electroplated onto a carbon-filled plastic electrode. The bromine gas that forms is reacted with other agents to form a thick oil. During discharge, the zinc is removed from the electrode and joins back up with bromine to make zinc bromide and an excess of electrons. These electrons travel by wire outside of the battery to power electrical devices before returning back to the opposite electrode of the flow battery.
PNNL has designed and is testing a modular 1kW/1kWh vanadium redox flow battery with an optimized stack design. The battery incorporates PNNL’s new electrolyte chemistry, delivering 80% increased power capacity and 90% increased efficiency with about half the operating cost of current vanadium redox flow batteries. (Image source: PNNL)
Another flow battery of interest was developed in the 1980s and uses an unusual property of the element vanadium. This metallic material can exist in four different oxidation states (2+,3+,4+, and 5+), depending upon the number of electrons around the vanadium nucleus. Energy is stored by providing extra electrons (during charging) to produce V2+, and V3+. During discharge, these electrons are removed to form V4+ and V5+.
The vanadium flow battery has two tanks, one containing the V2+ and V3+ cathodelyte solution, and the other tank containing the V4+ and V5+ anodelyte solution. These solutions are made up of vanadium dissolved in sulfuric acid. The tanks can store the cathodelyte and anodelyte almost indefinitely until the battery needs to generate electricity. At this point, the solutions are pumped into each side of a reaction cell that contains an ion-selective membrane.
During discharging, the V2+ oxidizes into V3+ in the negative side of the reaction cell and an electron is released, collected on the negative electrode, and conducted away by the external circuit. In the positive side of the reaction cell, V5+ accepts an electron from the external circuit, the reduction reaction creating V4+. Charge neutrality in the cell is maintained by exchanges of hydrogen ions (cations) through the membrane that separates the two sides of the reaction cell.
Vanadium is a common element used primarily in steel processing and as a catalyst. As with other flow batteries, the battery energy capacity is limited only by the size of the storage tanks that contain the cathodelyte and anodelyte solutions. The electrolyte liquids also provide good thermal regulation.