Battery Storage: Smoothing Out the Wrinkles of Renewable Energy

On April 14, 2011, Duke Energy announced that it will be incorporating a huge battery storage system at its new 153-megawatt (MW) Notrees Windpower facility in West Texas. The project is the largest wind-farm battery storage installation announced to date.


Xtreme Power Dynamic Power ResourceThe announcement highlights the advances energy companies are making toward incorporating renewable power sources into the electric grid in the U.S. Duke will use a 36 MW-capacity Dynamic Power Resource (DPR) from Texas-based Xtreme Power to provide large-scale storage at the facility (photo used with permission).

Growing Importance of Renewables in the Energy Mix

Along with federal incentives, the Renewable Portfolio Standards (RPSs) enacted in recent years across the U.S. at the state level are pushing electric utilities to explore better ways to incorporate renewable power sources like wind and solar into the electric grid. The Pew Center on Global Climate Change says that 31 of the states already have renewable or alternative energy portfolio standards in place. Seven other states have set goals for renewables. In April 2011, California passed a law requiring that 33 percent of its power come from renewable sources by 2020.

According to the U.S. Energy Information Administration’s “Annual Energy Outlook 2011,” electricity generated by renewable sources is projected to grow from about 10 percent of the total mix in 2009 to about 14 percent of the total in 2035. Total electricity generation is projected to increase from about 3 trillion kilowatt hours (kWh) to about 5 trillion during that same period, with about 12 percent of that new capacity coming from wind power and another 12 percent from other renewables.

Renewable energy sources such as wind and solar create a challenge for electric utilities in that they derive from natural sources that can’t be controlled. The wind blows when it blows, and the sun shines when it shines. But to deliver electricity reliably and affordably, power companies need to be able to predict to a reasonable degree the generation capacity that will be available during any given time period. Although a grid can incorporate some percentage of renewable sources without much problem, the adoption of increasingly demanding RPSs in the U.S market is moving utilities to develop systems that can store power and release it when it is needed — thus ‘smoothing out the wrinkles’ of renewable energy generation.

In fact, storage technologies hold promise for helping power companies deal with the peak-demand issues they already faced before the advent of RPS. To meet electricity demand during peak times, power companies often have to bring expensive gas-fired generators online or purchase power at a premium from other companies. Effective storage systems would give utilities a way to save up excess power during lower-consumption periods, then release it during peak consumption — a strategy referred to as “time-shifting.”

Besides battery storage systems, the industry is testing pumped hydro — using excess generation during a given period to pump water uphill, where it can be held and released later to drive turbine generators. The industry is also testing compressed air energy storage (CAES), in which excess capacity is used to pump air into metal tanks or underground caverns, from which it can be released later to drive turbines. Other technologies being developed include systems based on capacitors and flywheels.

If cost-effective storage technologies can be developed, they hold considerable potential, according to “Electric Power Industry Needs for Grid-Scale Storage Applications,” a report from Sandia National Laboratories. Stored energy can be released on-demand without requiring the “spinning reserves” currently employed to meet demand spikes. The authors of the report write:

The use of an energy storage technology has the potential to be far faster than regulation by a gas or steam turbine. This faster response time can minimize momentary electricity interruptions, which are more costly than sustained interruptions. These storage technologies can vary output rapidly, changing from no output to full output within seconds. To optimize efficiency and response time, energy storage technologies used for area and frequency regulation must be able to communicate with the grid quickly and efficiently.

While promising, storage technologies such as batteries, capacitors, and flywheels, suffer from insufficient technical progress, according to the report. Their high cost and complexity “are major obstacles to production scale-up and integration of storage devices at grid scale,” and their limited storage duration and energy capacity are “too short to meet the current needs of the electric power industry.” At this stage, interoperability is also a problem, they write:

Without sufficient power electronics, control systems, and other communication systems, even the most advanced storage technology will be unable to be reliable and secure when integrated into the grid.

Is Wind Power Really That Hard to Integrate?

Interestingly, the American Wind Energy Association (AWEA) says that the variability of wind energy is not as great a problem as is claimed by opponents.  The organization asserts that the variability of a well-designed wind farm isn’t really that large and that the power industry’s existing practices allow for the incorporation of significant wind generation.

The association’s “Energy Storage Factsheet” points out that power companies are already able to employ traditional methods to create flexibility of supply, such as demand response, flexible generation, hydro, pumped hydro, and gas storage:

Wind integration studies in the U.S. have consistently found that using these existing sources of system flexibility to accommodate the variability added by wind energy costs less than $0.005 per kilowatt-hour (kWh) of wind energy, or approximately 10 percent of the typical wholesale value of wind energy … most regions have more than enough existing, low-cost flexible resources to accommodate wind energy providing 20 percent or more of a region’s electricity.

While the association’s position is that power companies already have enough flexibility in their grids to accommodate wind energy, it acknowledges that energy storage technologies might become more affordable, feasible, and useful over time.

Wind Energy in Texas

In spite of the technical and cost challenges, the potential is great enough and the stakes are high enough to prompt power companies to invest in development of storage solutions, as can be seen in Duke’s Notrees project in Texas.

Texas is the top U.S. state in total installed wind capacity, according to AWEA, with 10,085 MW installed as of the end of 2010. The association says about Texas that “if it were a country, it would rank sixth in the world in wind capacity.” (The total U.S. utility-scale wind energy capacity was 40,181 MW at the end of 2010.)

The Notrees facility is a project of Duke Energy Renewables, a company business unit dedicated to solar and wind projects. The renewables organization now operates a portfolio of nine wind farms and four solar farms, with a total generating capacity of about 1,000 MW, according to the company. The Notrees project is partially funded by a grant from the U.S. Department of Energy (DoE).

Xtreme Power, the company providing the battery systems for the Notrees project, is based in Kyle, Texas. The company recently completed an installation of a 15 MW Dynamic Power Resource (DPR) storage system for the Kahuku Wind project on the island of Oahu in Hawaii. An announcement from Xtreme Power says that its DPR system “digitally smoothes the wind farm’s output to ±1 MW per minute by rapidly absorbing or releasing power as needed so that Hawaiian Electric Company’s … grid system and local customers are shielded from disturbances due to varying winds at the Kahuku Wind wind farm.”

Drawing of Xtreme Power Dynamic Power ResourceIn the drawing of a DPR reproduced here (used with permission from Xtreme), the equipment shown in blue represents the system’s power electronics positioned at the front of the DPR unit. Power storage cells are positioned behind in parallel racks, shown here in red and black.

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