The experimental battery's negative electrode incorporates microscopic carbon spheres, resulting in charging and discharge up to three times faster than conventional lithium-ion batteries, according to Noriko Yoshizawa, chief researcher at the National Institute of Advanced Industrial Science and Technology. Those quick charge/discharge cycles will enable the batteries to match fluctuations in individual wind turbine power output, she says. "The carbon sphere battery that we are developing is characterised by similar performance when charging and discharging at low current over long periods and doing so at large current over short periods," she says. Yoshizawa is unable to provide information on the capacity of the device, but states: "Charge/discharge will be in the several-MWh class, as a module."
Government studies have reportedly found that lithium-ion batteries are more efficient and have more potential for size reduction than sodium sulphur batteries, which are to be used in conjunction with a wind plant of GE Energy turbines in Japan (Windpower Monthly, June 2007). On the other hand, scaling-up lithium-ion batteries for greater capacity was found to be difficult. This has led the national institute to collaborate with Tokai Carbon to study the physical properties of the carbon spheres, seeking increased storage capacity and lower production costs before a sales date of 2010 at the earliest.
The target retail price for the battery is ´20,000 ($169) for each kilowatt-hour of energy storage capacity, says Yoshizawa. Japan's government is pushing battery storage at wind power plants as a means of reducing the variations in power flows into grid while also growing the knowledge base for batteries for use in plug-in automobiles. Wind project developers complain about the added cost of these storage measures, arguing that slight increases in the existing generating reserves on the network would be a far more effective and cheaper method of achieving the same end.