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Toward a More Stable Grid

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Success from a new grid-scale flywheel energy storage facility.

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Matt Hoff October 2011

Maintaining a stabilized energy grid is difficult because of the unpredictability of intermittent energy sources. For power providers, grid stabilization has been a rising concern in recent years, especially because of the increasing use of intermittent energy sources such as wind turbines. Maintaining a stabilized energy grid is difficult because of the unpredictability of these intermittent energy sources. If wind turbines, for example, are supplying 5% of the overall power for the electric grid and the turbines stop moving because the air grows still, the grid has to find a way to kick into overdrive to compensate for this sudden decrease in energy. It is not as easy as it sounds.

Philip LeGoy, senior consultant of power plant design for Ireland's Electricity Supply Board, says his country is regularly obtaining 25% of its electricity from wind. "If I have 1,000MW of wind power," he says, "it is a detrimental thing when it goes off-line."

When the amount of energy provided by wind turbines falls short of the amount of electricity needed to operate a city's power grid, the grid has to work harder to balance the discrepancy between the energy needed and the energy being produced. Demand for energy fluctuates rapidly between high need and low need, and power plants often cannot throttle power output to accommodate sudden increases in demand without suffering major repercussions.

Poor grid balance and stabilization can result in high network losses, equipment overloading, unacceptable voltage and frequency levels, voltage instability, and even outages. These repercussions are indeed detrimental, to say the least. In order to successfully harness, maintain, and distribute vast amounts of power, the active and reactive power balance in a system must be controlled.

One solution to the grid stabilization issue is to build energy storage plants that connect to the electrical grid. Storage plants help stabilizes the grid by quickly providing additional power during periods of high demand (when the intermittent energy source ceases to contribute electricity, for example). In order to accomplish this, the plants store energy during periods of low demand.

While elaborate storage systems have been developed over the years, these older technologies require a net surplus of power in order to generate and distribute the additional electricity. This is counterproductive. Older systems continue to place strenuous demands on power generation plants, causing additional and unnecessary degeneration. They also contribute to the production of harmful greenhouse gases.

Recently, a milestone in environmentally-friendly, renewable energy storage was reached when Beacon Power announced the opening of the world's first grid-scale, 20MW flywheel energy storage facility, on June 21, 2011. "We are very proud to have reached this technical and commercial milestone in building and operating the first grid-scale flywheel-based storage plant in the world," says Bill Capp, president and CEO of Beacon Power, which worked with Oztek Corp. for power control solutions to bring this vision to fruition.

Oztek, which has been developing and manufacturing advanced inverter, DC/DC, and motor drive controls since 1997, worked to develop vital software and essential control hardware technologies used to interface the large arrays of giant flywheels to the power-grid.

The energy storage plant utilizes 200 flywheels, each weighing more than 2,800 lb, to store up to 20MW of power that can then be transferred to the grid during times of high demand. In order to store energy, the flywheels rely on mechanical inertia. The flywheel is accelerated by an electric motor that doubles as a generator upon reversal, slowing down the disc and producing electricity. Since friction must be minimized in order to prolong the storage time, the flywheel is suspended in a vacuum, employing a sensorless permanent magnet motor drive. Oztek designed and supplied the sensorless motor drive as well as the grid tie inverter controller.

The grid tie inverter provides the interface between the flywheel and New York City's main power grid. Unlike systems implemented in years past, the flywheel system is clean and energy efficient. A very small percentage of the power is lost as heat during transfer, and the system is able to respond to large demand changes in seconds.

"Oztek is extremely proud of its contributions to this milestone in sustainable, utility-scale frequency regulation services," says Dave Zendzian, CTO of Oztek. "Developing hardware and software solutions to control the 2,800 lb flywheels, as well as operating hundreds of power inverters in parallel, has provided no shortage of technical challenges. Due to the high-power nature of the installation, many of the algorithms employed in the controllers needed to be designed using simulation coupled with design verification on smaller-scale hardware platforms. As such, there are always risks and challenges when you attempt to bring up the full-scale system. To see the system up and running at full capacity is very satisfying for all of us."

Since New York City has a reputation for being one of the most energy-efficient cities in the world, it is sensible to unveil and deploy the new technology there.

Stephen G. Whitley, president and CEO of the New York Independent System Operator, agrees. "New York's competitive marketplace for electricity provides fertile ground for energy innovations such as Beacon Power's flywheel system. It is great to see pioneering technology bringing new solutions to meet New York's energy needs."

Beacon's Stephentown energy storage plant is a new and highly effective answer to an old, stubborn problem, and Oztek's sensorless magnet motor drive and grid tie inverter controls are essential components of this new power storage technology. Soon it will be common for intermittent energy sources to drain into highly efficient power reservoirs, and companies like Beacon and Oztek are leading the way. Thanks to their ongoing efforts, power providers will save on maintenance costs, and consumers will save on electricity. The environment, too, will reap the rewards.

Oztek Corp.
Merrimack, NH
oztekcorp.com

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