Schematic depicts the inner workings of a vanadium battery, now in use in a Utah plant, that can supply 250 kilowatts for eight hours.
VRB Power Systems
February 27, 2008, was a bad day for renewable energy. A cold front moved through West Texas, and the winds died in the evening just as electricity demand was peaking. Generation from wind power in the region rapidly plummeted from 1.7 gigawatts to only 300 megawatts (1 megawatt is enough to power about 250 average-size houses). The sudden loss of electricity supply forced grid operators to cut power to some offices and factories for several hours to prevent statewide blackouts.
By the next day everything was back to normal, but the Texas event highlights a huge, rarely discussed challenge to the adoption of wind and solar power on a large scale. Unlike fossil fuel plants, wind turbines and photovoltaic cells cannot be switched on and off at will: The wind blows when it blows and the sun shines when it shines, regardless of demand. Even though Texas relies on wind for just over 3 percent of its electricity, that is enough to inject uncertainty into the state’s power supplies. The problem is sure to grow more acute as states and utilities press for the expanded use of zero-carbon energy. Wind is the fastest-growing power source in the United States, solar is small but also building rapidly, and California is gearing up to source 20 percent of its power from renewables by 2017.
Experts reckon that when wind power provides a significant portion of the electricity supply (with “significant” defined as about 10 percent of grid capacity), some form of energy storage will be essential to keeping the grid stable. “Without storage, renewables will find it hard to make it big,” says Imre Gyuk, manager of energy systems research at the U.S. Department of Energy.
Fortunately, there is a promising solution on the horizon: an obscure piece of technology known as the vanadium redox flow battery. This unusual battery was invented more than 20 years ago by Maria Skyllas-Kazacos, a tenacious professor of electrochemistry at the University of New South Wales in Sydney, Australia. The vanadium battery has a marvelous advantage over lithium-ion and most other types of batteries. It can absorb and release huge amounts of electricity at the drop of a hat and do so over and over, making it ideal for smoothing out the flow from wind turbines and solar cells.
Skyllas-Kazacos’s invention, in short, could be the thing that saves renewable energy’s bacon.
To the engineers who maintain the electrical grid, one of the greatest virtues of a power supply is predictability, and that is exactly why renewable energy gives them the willies. Nuclear- and fossil fuel–powered plants produce electricity that is, in industry speak, “dispatchable”; that means it can be controlled from second to second to keep the grid balanced, so the amount of energy being put into the wires exactly matches demand. If the grid goes out of balance, power surges can damage transmission lines and equipment. Generators are therefore designed to protect themselves by going off-line if the grid becomes unstable. Sometimes this can amplify a small fluctuation into a cascading disaster, which is what happened in the northeastern United States and eastern Canada in August 2003, plunging 50 million people into a blackout. Unless the reliability of renewable energy sources can be improved, as these sources contribute more and more electricity to the grid, engineers will have an increasingly difficult time keeping the system balanced. This raises the specter of more blackouts, which nobody would tolerate. “We want to make renewables truly dispatchable so we can deliver given amounts of electricity at a given time,” Gyuk says.
The way to make renewables more reliable is to store the excess electricity generated during times of plenty (when there are high winds, for instance, or strong sun) and release it later to match the actual demand. Utilities have been using various storage techniques for decades. Hydroelectric plants, for instance, often draw on reservoirs to generate additional electricity at peak times, and then pump some of the water back uphill in off-peak periods. Compressed air is another, less common form of large-scale energy storage. It can be pumped into underground cavities and tapped later. These technologies have been suggested as ways of storing renewable energy, but both approaches rely on unusual geographical conditions.
“For most of us right now, the real key to effective storage is batteries,” says Jim Kelly, senior vice president of transmission and distribution at Southern California Edison. Specifically, what is needed is a battery that can store enough energy to pull an entire power station through a rough patch, can be charged and discharged over and over, and can release large amounts of electricity at a moment’s notice. Several promising battery technologies are already in early-stage commercialization, but the vanadium battery may have the edge in terms of scalability and economy.
We need a rechargeable battery that can store enough energy to pull a power station through a rough patch And release Electricity at a moment’s notice.
Like the battery in your cell phone or car, vanadium batteries are rechargeable, but chemically and structurally they go their own way. A vanadium battery consists of three main components: a stack where the electricity is generated and two tanks that hold liquid electrolytes. An electrolyte is any substance containing atoms or molecules that have positive or negative electrical charges. These charged atoms or molecules are known as ions, and the amount of charge on an ion is known as its oxidation state. In a battery, electrolytes are used as an energy storage medium. When two electrolytes, each containing ions with different oxidation states, are allowed to exchange charges, the result is an electric current. The technical term for this kind of charge exchange is a redox reaction, which is why the vanadium battery is formally known as the vanadium redox battery.
A traditional battery, such as the familiar AA dry cell, holds electrolytes in its own sealed container. But the vanadium battery is a flow system—that is, liquid electrolytes are pumped from external tanks into the stack, where the electricity-generating redox reaction takes place. Want to store more power? Use bigger tanks. The bigger the tanks, the more energy-rich electrolytes they can store. The downside is that flow batteries tend to be big. It takes a flow battery the size of a refrigerator, incorporating a 160-gallon tank of electrolytes, to store 20,000 *watt-hours of electricity, enough to power a full-size HDTV for about three days. This is because the energy density in the liquid electrolytes is relatively low compared with that of the chemicals in lithium-ion batteries. (Energy density is a measure of the amount of energy that can be extracted from a given volume or mass of a battery.) For this reason, flow batteries are unlikely to be found in mobile applications, like laptops or electric cars. In those cases the battery of choice remains lithium-ion, which has an energy density five times that of vanadium.
