In absolute terms, wind is the second fastest growing energy source in the United States, behind natural gas. Worldwide, it is adding new capacity more than six times as fast as nuclear power, and grew by the equivalent of about 104 natural gas–fired plants (enough to serve 5.2 million U.S. homes)—in 2005 and 2006, according to the Worldwatch Institute. As of December 2006, total worldwide wind capacity stood at about 74 gigawatts, according to the Global Wind Energy Council in Brussels.
Wind power is touted for its lack of significant greenhouse gas emissions, water and air pollution, or radioactivity; anything that can increase wind power’s contribution toward meeting electricity needs will only further attenuate emissions of greenhouse gases and other pollutants.
Some experts are now looking to vanadium redox-flow batteries (VRBs) to provide the boost that wind power needs if it is to reach the next tier of capacity. Already these units are modulating wind power in several significant installations around the world…
Inside the VRB
Whereas a conventional battery stores chemical energy within an electrolyte solution, a VRB contains two different electrolyte solutions, each in a separate tank. In a charged VRB, one electrolyte is positively charged, and one is negatively charged. In order for the battery to provide power, the electrolytes flow through a fuel cell stack on opposite sides of a proton exchange membrane. Their opposite charges create a gradient that powers an external current.
Several characteristics unique to VRBs enable them to sustain utility-scale storage and power at potentially competitive prices. First, unlike conventional batteries, power output is independent from energy storage capacity—output depends on the size of the fuel cell stack, while the energy storage capacity depends on the size of the electrolyte tanks. Neither constrains the other, although the ratio of storage to power determines how long the batteries can run without recharging. Power can flow undiminished as long as there is fresh electrolyte to circulate through the stack.
VRBs, unlike many of their conventional counterparts, can be fully discharged without reducing life expectancy. In contrast, discharging a lead-acid battery more than 20–30% erodes longevity. Even under the best circumstances, lead-acid batteries are good for little more than 1,000 charge–discharge cycles. But a VRB in Sapporo has undergone around 14,000 cycles, says Dennis Witmer, director of the Arctic Energy Technology Development Laboratory at the University of Alaska, Fairbanks. The limiting factors are the proton exchange membrane and the pumps, both of which can be replaced. Discharged electrolyte can be replenished by running a current through the battery.
VRBs are far greener than other batteries, as they lack potentially toxic metals as lead, cadmium, zinc, and nickel, which can contaminate the environment at all phases of the conventional battery life cycle. VRBs’ most toxic component is the sulfuric acid of the electrolyte, which is only one-third as acidic as that in a lead-acid battery. But unlike lead-acid batteries, the electrolytes in a VRB function indefinitely, eliminating the disposal problem.
Vanadium itself has very low toxicity, and the batteries are designed to contain electrolyte spills. “We have the best environmental footprint of any storage technology,” says Simon Clarke, executive vice president for corporate development at VRB Power Systems.
For now, VRBs are not a viable option for cars. The energy density of gasoline equals 13,000 watt-hours per kilogram, while the typical VRB is still not much better than a lead-acid battery—about 40 watt-hours per kilogram, says Dan Lewis, research director of the Economic Research Council in London. Lithium ion batteries, as used in the latest generation of hybrid vehicles, have an energy density of about 200 watt-hours per kilogram.
Other types of flow batteries under development, such as those using vanadium bromide, could double the density of storage, but probably would still be inadequate for cars. Developing a sufficiently energy-dense flow battery would solve the problem of how long it takes to recharge currently available batteries for electric vehicles. Hypothetical automotive flow batteries could be replenished in minutes by replacing discharged electrolyte with freshly charged electrolyte.
As a plus, VRBs are not resource limited. The USGS’s estimate of the world vanadium resource is far greater than would be necessary to supply storage for total world electricity production.
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