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.

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.

No abstract

AS A CONCEPT, ENERGY STORAGE HAS IMMEDIATE APPEAL. IT transfers energy through time, from generation to consumption, just as energy transmission transfers energy from one place to another. Indeed, in the early days of electricity, the Leyden jar and Volta's columnar apparatus were very much cutting-edge technology. However, after the invention of the dynamo, ac generation, and the spectacular spread of continental electric grids, the storage of electricity became relatively unimportant-with one exception. The lead-acid battery became an essential ingredient of the automobile. Due to large production volume, this battery has become relatively inexpensive and fairly reliable. More recently, energy storage has also become a critical component of electronic equipment such as portable computers and mobile phones. In this market, where cost is less important, the need for greater energy density, zero maintenance, and long cycle life has allowed advanced batteries such as lithium-ion (Li-ion) and nickel-metal-hydride batteries to find wide application.By contrast, energy storage (other than pumped hydro) has found very little application in utility or large-scale industrial applications. A large 20-MW/15-min storage facility in Puerto Rico kept the entire island grid stable for several years. Because the battery was designed primarily as an energy source for spinning reserve but was used as a rapidly fluctuating power source performing frequency regulation, it reached end-of-life prematurely and did not meet predicted life expectations. However, recognizing its vital importance to the island, the facility has recently been repowered with 20 MW of tubular plate lead-acid batteries. An ©DIGI TAL VISIO N, LTD. even bigger 27-MW/15-min facility was commissioned in Fairbanks, Alaska, in 2003. It utilizes nickel-cadmium batteries and does an excellent job providing voltage support for the long transmission line from Anchorage. Even when not discharging, it can supply about 10 Mvar. Similarly successful is a 250-kW/8-h vanadium redox facility that was put into service on a long distribution line in Utah. In Wisconsin, a system of six 1-MW/1-s superconducting magnetic energy storage (SMES) devices was effectively used to inject power into a collapse-prone transmission loop. A number of demonstrations in the 100-kW to 1-MW range have been field-tested with the U.S. Department of Energy's support and in cooperation with major utilities. Technologies include lead acid, zinc bromine (ZnBr), Li ion, and sodium sulfur (NaS). In addition, a considerable number of megawatt-sized lead-acid uninterrupted power supply (UPS) systems are in operation throughout the United States for financial institutions, server farms, airports, and the like. Japan, on the other hand, boasts over 70-MW installed capacity for NaS technology alone-the largest facilities at 8 MW/8 h.Meanwhile, analytical studies have shown the applicability of energy storage for voltage support and frequency stability, for peak shaving, renewables firming, transmission upgrad...

The first U.S. demonstration of the NGK sodium/sulfur battery technology was launched in August 2002 when a prototype system was installed at a commercial office building in Gahanna, Ohio. American Electric Power served as the host utility that provided the office space and technical support throughout the project. The system was used to both reduce demand peaks (peak-shaving operation) and to mitigate grid power disturbances (power quality operation) at the demonstration site. This report documents the results of the demonstration, provides an economic analysis of a commercial sodium/sulfur battery energy storage system at a typical site, and describes a side-by-side demonstration of the capabilities of the sodium/sulfur battery system, a lead-acid battery system, and a flywheel-based energy storage system in a power quality application.4