The Electrochemical Society Interface • Fall 2010 49 L arge-scale stationary battery energy storage has been under development for several decades with the successful use of pumped hydroelectric storage as a model. Several large battery demonstration projects have been built and tested under a variety of electric utility grid applications. In addition, renewable energy sources such as wind and photovoltaics may require energy storage systems. While these applications are new and expanding, the shift toward an expanded role for battery energy storage in the de-regulated electricity market became evident by the late 1980s and early 1990s. Studies by Sandia National Laboratories identified opportunities for battery energy storage in the generation as well as on the transmission and distribution segments of the electric grid. Reports 1,2 from these studies describe battery storage application requirements and provide a preliminary estimate of potential costs and benefits of these applications for the U.S. electric grid. Applications fall into two broad categories: energy applications and power applications. Energy applications involve storage system discharge over periods of hours (typically one discharge cycle per day) with correspondingly long charging periods. Power applications involve comparatively short periods of discharge (seconds to minutes), short recharging periods, and often require many cycles per day.Detailed performance criteria for applications such as peak shaving and load leveling (energy applications) as well as frequency and voltage regulation, power quality, renewable generation smoothing and ramp rate control (power applications) are described elsewhere.2 Generally, the most important requirements have been the need for low cost, flexible designs, proven battery technologies, and reliable performance.While many battery technologies have been proposed and developed for electrical energy storage applications, only a handful have actually been used in fielded systems. Technologies that are used in fielded systems include leadacid, nickel/cadmium, sodium/sulfur, and vanadium-redox flow batteries. Cost effective energy storage systems have been identified 3 for utility, enduser, and renewable applications. Other battery technologies, such as the many lithium-ion batteries, are less matureand not yet well-developed for these applications.

Flywheels~~m. a~, A flywheel is an electromechanical storage system in which energy is stored in the kinetic energy of a rotating mass. Flywheel systems under development include those with steel flywheel rotors and resin/glass or resin/carbon-fiber composite rotors. The mechanics of energy storage in a flywheel system are common to both steel-and composite-rotor flywheels. In both systems, the momentum of the rotating rotor stores energy. The rotor contains a motor/generator that converts energy between electrical and mechanical forms. In both types of systems, the rotor operates in a vacuum and spins on bearings to reduce friction and increase efficiency. Steel-rotor systems rely mostly on the mass of the rotor to store energy while composite flywheels rely mostly on speed.

This paper describes the concept for augmenting the SEGIS Program (an industry-led effort to greatly enhance the utility of distributed PV systems) with energy storage in residential and small commercial applications (SEGIS-ES). The goal of SEGIS-ES is to develop electrical energy storage components and systems specifically designed and optimized for grid-tied PV applications. This report describes the scope of the proposed SEGIS-ES Program and why it will be necessary to integrate energy storage with PV systems as PVgenerated energy becomes more prevalent on the nation's utility grid. It also discusses the applications for which energy storage is most suited and for which it will provide the greatest economic and operational benefits to customers and utilities. Included is a detailed summary of the various storage technologies available, comparisons of their relative costs and development status, and a summary of key R&D needs for PV-storage systems. The report concludes with highlights of areas where further PV-specific R&D is needed and offers recommendations about how to proceed with their development.

The long-range goal of the Z-Pinch IFE program is to produce an economically-attractive power plant using

Researchers at Sandia National Laboratories and the U.S. Department of Energy's Solar Energy Technologies Program assessed status and needs related to optimizing the integration of electrical energy storage and gridconnected photovoltaic (PV) systems. At high levels of PV penetration on our electric grid, reliable and economical distributed energy storage will eliminate the need for backup utility generation capacity to offset the intermittent nature of PV generation. This paper summarizes the status of various storage technologies in the context of PV system integration, addressing applications, benefits, costs, and technology limitations. It then discusses further research and development needs, with an emphasis on new models, systems analysis tools, and even business models for high penetration of PV-storage systems on a national scale.