Energy-storage devices come in all shapes and sizes. Performance characteristics vary, as do costs and benefits. It is important to select the right device for the application. For example, as intermittent wind and solar are increasingly added to the generation mix, bulk storage is becoming a must-have for the grid. At the same time, distribution utilities require fast response times and flexibility, as well as storage capabilities. For them, dynamic energy storage makes it possible to level the load profile, stabilize voltage fluctuations, address voltage irregularities and provide frequency regulation.

Traditional lead-acid batteries have serious limitations, including slow charging times, low energy density and end-of-life environmental concerns. They are also sensitive to temperature fluctuations and have relatively short lives and high maintenance requirements.

Capacitors have millisecond response times but a small capacity. They might replace batteries in small-scale applications but are not yet cost effective for large-scale purposes.

Superconducting magnetic energy-storage (SMES) devices are fast acting and have a mid-range capacity, but are prohibitively expensive for large-scale applications.

Thermal storage is cost effective and has hours of capability, but a slower response time makes it unsuitable for energy or reliability products that require a response of five minutes or faster.

As with everything in the energy industry, one size does not fit all. Vendors are now combining existing technologies with innovative concepts to provide utilities with access to advanced technologies in manageable doses.

Battery Energy-Storage System

ABB (Zurich, Switzerland) and Saft Groupe, SA (Bagnolet, France) developed a unique energy-storage system by combining power electronics, in the form of an ABB converter, with Saft's nickel-cadmium (NiCd) batteries. The two companies refer to the unique combination as BESS, for battery energy-storage system.

Brian Scott, lead engineer for flexible ac transmission systems at ABB Grid Systems, said, “In 2003, ABB installed the BESS system at the Golden Valley Electric Association in Fairbanks, Alaska.” The system is rated for 26 MW for 15 minutes or 40 MW for seven minutes. “This is long enough to start up and bring local generation on-line,” Scott added.

According to Scott, BESS increased reliability and voltage support in the grid during unexpected system disturbances; this is especially the case with BESS in remote, islanded or weak systems, such as the one in Fairbanks.

Lithium-Ion Storage

Last year, Saft Groupe, SA and ABB teamed up again for a collaboration project with EDF Energy Networks UK (London, England). This project moved a little further up the technology ladder by combining Saft's first high-voltage lithium-ion battery system with ABB's SVC Light device, a voltage-source converter technology that is actually a static compensator device.

The result: They developed a device with voltage control, active power-flow control and dynamic energy-storage capability. EDF Energy Network will install the device on its 11-kV distribution system in Hemsby, Norfolk, U.K., in 2009. The device will supply 200 kWh (600 kW for 15 minutes or longer) to the system, improving voltage and power control as well as grid stability.

NaS Batteries

Sodium-sulfur (NaS) batteries are being installed at a fast pace on utility systems worldwide. One area gaining interest is the distributed energy storage system (DESS) application collaboration between NGK Insulators Ltd. (Nagoya, Japan), S&C Electric Co. (Chicago, Illinois) and American Electric Power (Columbus, Ohio). Leveraging the experience these companies have gained from previous projects together, the collaboration combines NGK's high-efficiency, long-life-cycle NaS battery technology with the power conversion and IntelliTEAM II (IT-II) technology of S&C.

An American Electric Power facility in Milton, West Virginia, was selected for one of three projects. The storage system was designed to improve reliability for a radial feeder by controlling the feeder in basic elements (teams) and by providing an alternate source, the DESS.

Jim Sember, vice president of the Power Quality Products Division for S&C, explained, “Teams are groups of IT-II devices. The devices talk to each other using UtiliNet spread-spectrum radios. Each team knows (in real time) how much load is being served. When a fault takes place, it is isolated and IT-II restores power to the unfaulted sections of the feeder by supplying the load from the DESS, creating an islanded grid section.

“The system will only pick up as much load as the NaS battery has capacity to carry at the time of the fault,” said Sember. “The system has the intelligence to monitor the load feeder and the capacity of the NaS battery. It will shrink the island to fit the capacity of the NaS battery as the outage begins to deplete the capacity or the load increases.”

Flow Batteries

A completely new class of battery, also known as regenerative fuel cells or redox flow systems, is now available. Flow batteries have features that make them especially attractive for utility-scale applications. Their operational principle differs from traditional batteries. Rather than store energy in both the electrolyte and the electrodes, flow batteries store and release energy using a reversible reaction between two electrolyte solutions separated by an ion-permeable membrane. Both electrolytes are stored separately in bulk-storage tanks. The size of the tanks defines the energy capacity of the storage system.

The power rating of a flow battery is determined by its cell stack. As a result, the power and energy ratings are decoupled, which gives the system designer an extra degree of freedom.

Many different electrolyte combinations have been proposed for use in flow batteries. The three primary types are vanadium redox, polysulfide bromide and zinc bromine.

Flow batteries can be extremely expensive and require a long period of time from the conceptual idea to the commissioning of the equipment. In addition, there are not a lot of manufacturers producing these devices. VRB Power Systems (Vancouver, British Columbia, Canada) is one of the few developing large-scale systems, and ZBB Energy Corp. (Menomonee Falls, Wisconsin) is manufacturing the zinc-bromine battery.

Advanced Flywheels

Flywheels store energy in the form of inertia of a spinning disk on a metal shaft. Electricity is used to spin a disk (storing energy). Flywheels can be a large slow-moving disk, a small fast-moving disk or a combination of a large disk spinning very fast. When power is needed, the disk is coupled with a form of generation and electricity is produced.

Today's flywheels take advantage of ultralow friction-bearing assemblies, vacuums, superconductivity and composite materials for rotors. They are used in applications requiring fast discharge times for voltage and frequency stabilization (power quality). They can provide ride-through power for power disturbances such as voltage sags and surges. They also can bridge the gap between a power outage and the time required to switch to long-term storage or generator power with excellent load-following characteristics.

There are several advantages attributed to flywheels over batteries. Flywheels have a longer life cycle, fewer maintenance requirements, higher cycling efficiency and shorter recharge times. Beacon Power (Tyngsboro, Massachusetts) has ganged several of its flywheel devices together to provide larger amounts of power to the system, which shows promise for future utility-scale flywheel installations.

Advanced Capacitors

There are three types of capacitors: electrostatic, electrolytic and electrochemical. All three capacitors have the term “ultracapacitor” attributed to them, correctly or incorrectly. They also are referred to as supercapacitors, electrochemical capacitors and nanotechnology capacitors. Typically, these advanced capacitors are used for fast-response, short-duration applications, such as backup power during brief outages.

The advanced capacitor resembles a regular capacitor with the exception that it offers very high capacitance in a small package. It also offers fast charge and discharge, combined with its extremely long life of approximately 500,000 cycles. This makes the advanced capacitor a very attractive replacement for a number of small-scale power-quality applications.

Compared to batteries, advanced capacitors have a longer life.

SMES Systems

What about a technology that stores electricity directly without needing to convert it to some other form of energy before it is stored? It would seem that taking electricity directly from the grid and storing it with no conversion would be more efficient. Well it is, but the technologies are still under development. Ultracapacitors and SMES devices fill this niche, but they also are also in the development stages when it comes to utility-scale devices.

SMES devices use a coil of superconducting wire, which allows a current to flow through it with virtually no loss. The current creates a magnetic field used to store the electric energy. When the stored energy is needed, the SMES is discharged almost instantaneously, using special switches to tap the circulating current and release it to serve a load.

Increasing the size of the windings can increase the amount of stored energy. Larger coils present their own challenge, because the associated increase in magnetic field becomes more difficult to contain. Technical improvements and a better knowledge of dealing with and controlling cryogenic systems have allowed SMES to penetrate the market and compete with more-common storage systems. The dynamic performance of SMES devices is far superior to most other storage technologies.

Response times are very fast (in milliseconds), and energy can be transferred very quickly with this technology. SMES devices are most suitable for high-value/low-energy applications, where the storage requirement is for less than a few seconds, with power requirements up to 1 MW or 5 MW. Large-scale SMES projects have been proposed but are still being developed.

Distributed Thermal Storage

Thermal storage is not new. It has been around for a long time, but has improved significantly in both large-scale and distributed applications over the last decade. For distribution, distributed thermal solutions are now available that enable the storage units to be placed directly on individual buildings, while being controlled and aggregated by the utility to act in unison, and offer an alternative to new peak power generation.

For example, Ice Energy (Windsor, Colorado) has developed a small-scale thermal-storage solution that effectively eliminates the electric consumption of a building's refrigerant-based HVAC system for a minimum of six hours each day, usually during peak hours. These units can be remotely controlled and dispatched in an aggregated fashion so that the ice-make and ice-melt cycles can be matched specifically to the load profile of the feeder or substation.

Thermal-storage systems, like those from Ice Energy, are commercially available today and have shown to be cost effective for utilities with significant thermally driven peak load.

Putting It All Together

Not all energy-storage technologies are created equal. The type of energy storage required is dependent on the ratio of charge/discharge power rating. It varies depending on the application and favors different technologies. Energy costs, efficiencies, densities and environmental concerns determine the type of technology to be used.

Even though the grid operates effectively, it is not efficient without storage. Cost-effective ways of storing electrical energy can help make the grid more efficient and reliable, transforming it into a whole whose sum is truly more than that of its parts.