We have proved that this technology can perform the functions of storage that we were looking for to help us manage the variability of wind energy on our operating system. We are greatly encouraged by these results.

—Frank Novachek, Xcel Energy director of corporate planning

The preliminary test results indicate that the battery has the ability to:

• Effectively shift wind energy from off-peak to on-peak availability
• Reduce the need to compensate for the variability and limited predictability of wind generation resources
• Support the transmission grid system by providing voltage support, which contributes to system reliability
• Support regional electricity market by responding to real-time imbalances between generation and load

Results also indicate that this technology may be applicable for solar energy.

Testing will continue to determine the technology’s ability to facilitate integration of larger penetrations of wind energy on the grid. Phase II of the study will also assess the potential value of the various battery system functions and determine the potential cost effectiveness of the technology. A final report is expected in summer 2011.

The project is being conducted in Luverne, Minn., about 30 miles east of Sioux Falls, S.D. The battery installation is connected to a nearby 11-megawatt wind farm owned by Minwind Energy, LLC.

Collectively, the 20 50-kilowatt battery modules are roughly the size of two semi trailers and weigh approximately 80 tons. They are able to store about 7.2 megawatt-hours of electricity, with a charge/discharge capacity of one megawatt. Fully charged, the battery could power 500 homes for more than 7 hours.

Xcel Energy purchased the battery from NGK Insulators Ltd. The sodium-sulfur battery is commercially available and versions of this technology are in use elsewhere in the US and other parts of the world, but this is the first US application of the battery as a direct wind energy storage device.

At the beginning of this project, Xcel Energy evaluated multiple types of utility-scale electrochemical storage technologies. For the project, Xcel Energy wanted a battery that could satisfy the following functional requirements:

• Minimum power capacity of 1 MW
• Minimum discharge duration of 6 hours at the nominal power rating
• Minimum of 300 annual charge-discharge cycles
• Capable of providing frequency regulation and voltage support
• Modular and scalable in design.

In addition, the technology needed to be commercially available and have a sound track record. Xcel determined that NaS technology was the only battery available that met all the requirements.

In addition to NGK, partners in the project with Xcel Energy include: S&C Electric, the University of Minnesota, the National Renewable Energy Laboratory, the Great Plains Institute and Minwind Energy, LLC, and Gridpoint. Xcel Energy is testing emerging technology and energy storage devices as part of its overall Smart Grid strategy, which modernizes and upgrades the grid to allow for easier integration of renewable energy sources.

The project received a $1 million grant from Xcel Energy’s Renewable Development Fund.

Resources

• Sodium Sulfur Battery Energy Storage And Its Potential To Enable Further Integration of Wind (Wind-to-Battery Project)