The U.S. electric grid has several challenges to overcome for continuous delivery of uninterrupted power. Critical needs include the upgrade or replacement of aging electrical infrastructure, increased intelligence and communication between components on the electric grid, and increased reliability of power for electric customers. In addition, the increasing desire for renewable energy places challenges on grid operators as these sources of energy are intermittent. Accurate prediction of their availability is difficult, and curtailment of their output may be needed to keep grid frequency stable.

Energy storage has been identified as a means to address some of these issues. However, determining an economically viable and technically sound solution has proven elusive. American Electric Power (AEP) has researched several energy storage technologies to determine which technologies can help overcome certain electric grid challenges.

The First Large-Scale NaS Battery

In 2006, AEP deployed the first utility-grid scale sodium-sulfur (NaS)-based energy storage system in the United States, near Charleston, West Virginia. A 1-MW, 7.2-MWh NaS battery was deployed. In its first three years of operation, the battery provided approximately 1 MW of load leveling during hot summer days, improved the feeder load factor by 5% and reduced the oil temperature of the associated substation transformer. This energy storage system was successful in deferring the need to build a new substation.

Load leveling provides one of the largest cost benefits for energy storage systems on the distribution portion of the electric grid because it allows capital deferral of upgrading or replacing substation electrical equipment. However, this benefit may not be enough to completely cover the cost of deploying an energy storage system.

To fully rationalize the cost of deploying energy storage systems, benefits have to be realized from several applications of the technology. In addition to load leveling for capital deferral, these benefits include providing backup power, energy arbitrage, system frequency regulation, the reduction of system losses and the integration of renewable sources of energy. Most of these benefits compete for the same energy and power out of the battery, so analyses have to be performed to prioritize the benefits, and some energy allocation may be necessary for the applications chosen to be deployed.

Building Upon NaS Success

After successfully deploying the Charleston energy storage system, AEP decided to deploy more installations and added the ability to provide backup power as another application to increase the benefit provided by the NaS-based technology. In cooperation with S&C Electric Co., AEP deployed three 2-MW, 14.4-MWh systems, commissioned in 2009. These projects were partially funded by the U.S. Department of Energy (DOE) through Sandia National Labs. The NaS-based energy storage systems provide 480 V and are connected to a step-up transformer to attain 12.47-kV or 34.5-kV circuit voltage.

For AEP, the new energy storage systems provided load-leveling benefit to the substation as well as backup power to customers during certain faults on the electrical system. The maximum area that could be served by the battery was determined and broken into zones, creating flexibility in providing backup power to customers.

Each zone is segmented by intelligent reclosers or switches. During an outage, the last load information is recorded, and the battery comes on-line to energize as many customers as possible. Based on that last load information, the number of zones that can be energized by the energy storage system is determined, and the switches open to disconnect the zones that cannot be energized by the energy storage system. The entire area can be energized by the battery, but during heavy load conditions, fewer zones may be picked up to ensure the maximum number of customers receives backup power.

As an example, on Nov. 1, 2010, a motor vehicle accident occurred that disrupted the three-phase power out of the Milton, West Virginia substation. The battery successfully came on-line and restored power to the entire island, approximately 700 customers, until the damage was fixed and power was restored from the station one hour and 17 minutes later.

In addition to distribution system batteries, AEP, specifically its transmission group, commissioned a 4-MW, 25-MWh NaS-based energy storage system in Presidio, Texas, in 2010. This is the largest such NaS-based deployment in the United States to date.

The Next Wave of Energy Storage at AEP

The NaS-based energy storage systems have been successful and provided lessons that allowed AEP to continue improving energy storage systems to meet customer needs and enable the utility to operate the distribution system more efficiently. There will always be a need for substation-based energy storage systems that are easily transportable to help overcome sustained outages. However, AEP is demonstrating another approach where energy storage systems can be more flexible to provide greater benefits to customers while still supporting grid functions.

This new approach is called Community Energy Storage (CES) — small-scale, battery-based energy storage systems that are connected to the secondary of utility distribution transformers to provide backup power to customers. Several systems can be aggregated to provide grid-level benefits to the utility.

Certain features of CES will enable the systems to provide benefits to both customers and grid operators:

• Location. Energy storage systems that are located closer to customers increase the reliability of the systems to provide backup power. The closer those storage systems are located to customers, the less susceptible they are to weather conditions that may damage power lines when backup power is needed.

• Buffer renewable energy. Storage systems located closer to customers can more readily buffer small-scale, customer-owned renewable sources of energy, like wind and solar.

• Synergy with other industries. Today, utilities do not purchase a significant amount of battery-based energy storage systems. If utilities leverage technologies used by other industries, such as the automobile industry, there is greater opportunity to reduce cost by leveraging higher-purchased quantities than are presently available.

• Smaller size. These units will be easier to install, operate and maintain. Outages to smaller-sized units are less critical to the operation of the electric grid.