capturing grid power - energy...

32 IEEE power & energy magazine july/august 2009

Capturing Grid Power

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july/august 2009 IEEE power & energy magazine 33

Performance, Purpose, and Promise of Different Storage Technologies

By Bradford Roberts

MMAKING ELECTRICITY GRIDS “SMARTER” AND MODERNIZING THEM SO that they can accept large amounts of renewable energy resources are fairly universally accepted as steps necessary to achieve a clean and secure electric power industry. The best way to achieve this goal is a topic of debate among power system designers. Although energy storage in utility grids has existed for many decades, the impact of storage in future grids is receiving more attention than ever from system designers, grid operations and regulators. The amount of storage in a grid and its value is also a subject of debate. Understanding the leading storage technologies and how they can affect grid operations is an important fi rst step in this assessment.

Why Storage in the Grid?In April 2003, the U.S. Department of Energy convened a meeting of 65 senior executives representing the electric utility industry, equipment manufacturers, information technol-ogy providers, federal and state government agencies, interest groups, universities, and national laboratories. They gathered to discuss the future of the North American electrical system. The goal of the meeting was to establish “Grid 2030,” a national vision for elec-tricity’s second 100 years. From that meeting, energy storage emerged as one of the top fi ve concerns for the future grid. And since that meeting, more attention has been given to storage in the grid at all levels, from large-scale bulk-storage systems to small units at or near the point of load. Other nations are ahead of the United States with regard to bulk storages; they recognized the value to grid operations sooner. The future of electric grids will be impacted by a growing penetration of plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs), which will represent a new dimension for grid management; vast amounts of energy storage will be present in the grid in the form of millions of electric cars. Gigawatts to kilowatts, electricity storage devices will change the grid dramatically.

Spectrum of Electricity StorageIn industrialized countries, nearly every person depends on some form of energy storage everyday. Every electronic device depends on battery power to function properly; think of your cell phone or laptop computer. These storage energy devices continue to evolve as newer applications are introduced. One application that is having a great impact on potential utility grid applications is electric cars. The technologies that have worked in electronic devices are being scaled up for higher power use in cars and the electric grid. Figure 1 shows a storage technology chart published by the Electricity Storage Associa-tion (ESA) that shows various technologies in terms of total power (kW) and energy capacity (time).

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Power applications, such as uninterruptible power supply (UPS) backup for data centers and automotive starting batteries, represent the largest market for lead-acid batteries, whereas laptop batteries and power tools have fueled incredible growth for lithium-ion. For bulk energy storage in utility grids, pumped hydro

power plants dominate, with approximately 100 GW in service around the globe.

In general terms, power appli-cations would be storage systems rated for one hour or less, and energy applications would be for longer periods. The chart in Fig-ure 2 shows the positioning of en-ergy storage options by application (power level) and storage time.

Potential applications of each of these technologies are being found in the electric grid—in the transmission system for bulk storage, in the residential feeder ci rcuit for smaller systems. The location in the grid will vary based on the economics of the technology.

Wise Investments in the PastUtility system designers have seen the benefi ts of massive amounts of energy storage in the form of pumped hydro power plants.

A typical pumped hydro plant consists of two interconnected reservoirs (lakes), tunnels that convey water from one reservoir to another, valves, hydro machinery (a water pump-turbine), a motor-generator, transformers, a transmission switchyard, and a transmission connection (Figure 3). The product of the total volume of water and the differential height between

reservoirs is proportional to the amount of stored electricity. Thus, storing 1,000 MWh (deliverable in a system with an elevation change of 300 m) requires a water volume of about 1.4 million m3.

The earliest known use of pumped hydro technology was in Zurich, Switzerland, in 1882. For nearly a decade, a pump and tur-bine operated with a small reservoir as a hydromechanical storage sys-tem. Beginning in the early 1900s, several small hydroelectric pumped storage plants were constructed in Europe, mostly in Germany. The fi rst unit in North America was the Rocky River pumped storage plant, constructed in 1929 on the Housatonic River in Connecticut. Most of these early units were relatively expensive since they had

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figure 1. Electricity storage by technology.

figure 2. Storage technology application comparison.

Positioning of Energy Storage Options

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a motor and pump on one shaft and a separate shaft with a generator and turbine. Subsequent developments through the middle of the 20th century typically used a tandem system with a single vertical shaft that had a motor-generator at the top, above a pump, and a turbine at the bottom. Whereas some of the earliest units used propellers, both the pump and the turbine in these later developments were usually of the Francis type, which uses fl ow inlet converted to axial fl ow outlet. Wicket gates, eventually under hydraulic control, regulated the power level. An advantage of the Fran-cis turbine shape is high effi ciency, but in this confi guration, it operates best with a very limited head range.

It was realized early on that a Francis turbine could also operate as a pump, but it was not used for both purposes until the Tennessee Valley Authority (TVA) and Allis-Chalmers constructed the Hiwassee Dam Unite 2 in 1956. This unit was a true reversible pump-turbine and, at 59.5 MW, it was larger than earlier installations. Developments in technology and materials over the next three decades improved overall effi ciency, reduced start-up issues, and allowed larger and larger units to be constructed.

The next major breakthrough, the variable speed design, was developed mainly in Japan. In most of the early designs, the only knob available to the operator was water fl ow, which was controlled by moving the wicker gates, but in this design, an adjustable-speed motor-generator allows the shaft rotation rate to change as well. By optimizing the two variables, the unit can be dispatched at optimum effi ciency over a large power range. The fi rst adjustable-speed system unit was constructed for use in Japan and became operational in 1990. Recently, an adjustable-speed system was constructed at Goldisthal in Thuringia, Ger-many. Two of the four 265-MW units at this plant are ad-justable speed.

Today, the global capacity of pumped hydro storage plants totals more than 95 GW, with approximately 20 GW operating in the United States. The original intent of these plants was to provide off-peak base loading for large coal and nuclear plants to optimize their overall performance and provide peaking energy each day. Their duty has since been expanded to include providing an-cillary service functions, such as frequency regulation in the generation mode. The newer adjustable-speed system design allows pumped hydro plants to provide ancillary service (frequency) capability in the “pumping” mode as well, which increases overall plant effi ciency. Filings with

the Federal Energy Regulatory Commission (FERC) have been made for additional pumped hydro facilities. These new plants represent 20 GW of new storage capacity that could be added to the U.S. grid.

Compressed Air Energy StorageCompressed air energy storage (CAES) is a peaking gas turbine power plant that consumes less than 40% of the gas used in a combined-cycle gas turbine (and 60% less gas than is used by a single-cycle gas turbine) to produce the same amount of electric output power. This is accomplished by blending compressed air to the input fuel to the turbine. By compressing air during off-peak periods when energy prices are very low, the plant’s output can produce electricity during peak periods at lower costs than conventional stand-alone gas turbines can achieve.

Making the CAES concept work depends on locat-ing plants near appropriate underground geological formations, such as mines, salt caverns, or depleted gas wells. The fi rst commercial CAES plant was a 290-MW unit built in Handorf, Germany, in 1978, and the second commercial site was a 110-MW unit built in McIntosh, Alabama, in 1991. These units are fast-acting plants and typically can be in service in 15 min when called upon for power. The plants used a fairly complex turbomachinery design integrated with a combined motor-generator and custom components.

Today, the Electric Power Research Institute (EPRI) has an advanced CAES program designed around a simpler

figure 3. Typical pumped hydroelectric storage plant.

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All of the energy storage technologies discussed are targeting ways to help the utility grid cope with balancing generation and load in the most optimal ways possible.


system using advanced turbine technology. Figure 4 shows a basic diagram of an advanced CAES design.

This proposed concept is targeted at plants in the 150–400 MW range with underground storage reservoirs of up to 10 hours of compressed air at 1,500 lbf/in2. Depend-ing on the reservoir size, multiple units could be deployed. The largest plant under consideration in the United States

would have an initial rating of 800 MW. In addition to these larger plants, EPRI has been studying an aboveground CAES alternative with high-pressure air stored in a series of large pipes. These smaller systems are target-ed at ratings of up to 15 MW for two hours.

Battery Energy StorageAdvancements in battery tech-nology over the last 20 years have been driven primarily by the use of batteries in consumer elect ronics and power tools. Only in the last ten years—with efforts to design better batteries for transportation—have pos-sible uses of battery technology for the power grid emerged. One

driver that has helped make potential utility applications possible is more efficient cost-effective power electron-ics. For battery technologies to be practically applied in the ac utility grid, reliable power conversion systems (PCSs) that convert battery dc power to ac were needed. These devices now exist and have many years of service experience, which makes a wide range of battery tech-

nologies practical for grid sup-port applications.

Figure 5 shows the steady in-crease in the energy density of batteries since the fi rst lead-acid batteries were introduced in the mid-19th century.

A la rge va r iety of bat ter y types are being used for grid sup-port applications.

Sodium Sulfur BatteriesThe sodium sulfur (NaS) battery is a high-temperature battery sys-tem that consists of a liquid (mol-ten) sulfur positive electrode and a molten sodium negative electrode separated by a solid beta alumina ceramic electrolyte (Figure 6). The electrolyte allows only positive sodium ions to pass through it and combine with the sulfur to form sodium polysulfi des.

During discharge, positivesodium ions flow through the electrolyte and electrons f low in the external circuit of the

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figure 4. Advanced CAES one-line diagram.

Lead-Acid25–45 Wh/kg

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figure 5. Exponential improvement in battery performance.


battery, producing about 2 V. This process is reversible since charging causes sodium poly-sulfides to release the positive sodium ions back through the electrolyte to recombine as elemental sodium. The battery operates at about 300 °C. NaS battery cells are efficient (about 89%). This battery system is capable of six hours of dis-charge time on a daily basis.

NaS battery technology was originally developed in the 1960s for use in early electric cars, but was later abandoned for that ap-plication. NaS battery technol-ogy for large-scale applications was perfected in Japan. Current-ly, there are 190 battery systems in service in Japan, totaling more than 270 MW of capacity with stored energy suitable for six hours of daily peak shaving. The largest single NaS battery installation is a 34-MW, 245-MWh system for wind power stabilization in northern Japan (Figure 7). The battery will allow the output of the 51-MW wind farm to be 100% dispatchable during on-peak periods.

In the United States, utilities have deployed 9 MW of NaS batteries for peak shaving, backup power, fi rming wind capacity, and other applications.

Another high-temperature battery, which is based on sodium nickel chloride chemistry, is used for elec-tric transportation applications in Europe. Known as the Zebra battery, it is being considered for utility applica-tions as well.

Flow Battery TechnologyFlow batteries perform similarly to a hydrogen fuel cell. They employ electrolyte liquids flowing through a cell stack with ion exchange through a microporous mem-brane to generate an electrical charge. Several different chemistries have been developed for use in utility power applications. An advantage of flow battery designs is the ability to scale systems independently in terms of power and energy. More cell stacks allows for an increase in power rating; a greater volume of electrolytes translates to more runtime. Plus, flow batteries operate at ambient (rather than high) temperature levels.

Zinc-bromine fl ow batteries are being used for utility applications. The battery functions with a solution of zinc bromide salt dissolved in water and stored in two tanks. The battery is charged or discharged by pumping the elec-trolytes through a reactor cell. During the charging cycle, metallic zinc from the electrolyte solution is plated onto

the negative electrode surface of the reactor cell, as shown in Figure 8.

The bromide is converted to bromine at the positive surface of the electrode in the reactor cell and then is stored in the other electrolyte tank as a safe chemically complex oily liquid. To discharge the battery, the process is reversed,

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figure 6. NaS battery cell construction.

figure 7. A 34-MW, 245-MWh NaS battery installation.

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figure 8. Zinc-bromine flow battery diagram.


and the metallic zinc plated on the negative electrode is dis-solved in the electrolyte solution and available for the next charge cycle.

One of the advantages of flow batteries is that their construction is based on plastic components in the reactor stacks, piping, and tanks for holding the electrolytes. The result is that the batteries are relatively light in weight and have a longer life. The typical flow battery can be

used in any duty cycle and does not have self-discharge characteristics that can cause damage like other battery technologies can.

Flow battery manufacturers are using modular construction to create different system ratings and dura-tion times. Figure 9 shows a zinc-bromine fl ow battery pack-age with a rating of 500 kW for two hours. Other packages are being applied at utilities with system ratings of up to 2.8 MWh packaged in a 53-ft trailer.

Another type of fl ow battery is the vanadium redox battery (VRB). During the charge and discharge cycles, positive hydrogen ions are exchanged between the two electrolyte tanks through a hydrogen-ion permeable polymer membrane. Like the zinc-bromine battery, the VRB system’s power and energy ratings are independent of each other. Numerous other chemistries are being developed around the fl ow battery concept. New startup companies are expected to announce fl ow battery technologies in the next few years.

Lithium-Ion BatteriesThe battery technology with the broadest base of applications today is the lithium-ion battery. This technol-ogy can be applied in a wide variety of shapes and sizes, allowing the battery to effi ciently fi ll the available space, such as a cell phone or laptop computer. In addition to their packaging fl exibility, these batteries are light in weight relative to aqueous battery technologies, such as lead-acid batteries. As previously shown in Figure 5, lithium-ion bat-teries have the highest power density of all batteries on the commercial market on a per-unit-of-volume basis. Safety issues with lithium-ion batteries in laptop computers have been a recent concern, but continued development of the technology for PHEV application has resulted in newer types of lithium-ion cells with more sophisticated cell management systems to improve performance and safety.

The leading lithium-ion cell design being applied in new PHEV designs is a combination of lithiated nickel, cobalt, and aluminum oxides, referred to as an NCA cell. The design’s life characteristics on fl oat and cycling duty have made NCA cells the primary choice for the next generation of PHEVs. Two lithium-ion designs that are starting to be used in higher-power utility grid applica-tions are lithium titanate and lithium iron phosphate.

Lithium TitanateThe lithium titanate approach uses manganese in the cathode and titanate anodes. This chemistry results in a

figure 9. Zinc-bromine flow battery system (500 kW). (Photo courtesy of Altairnano.)

Lithium-Ion Battery Storage System

figure 10. A 1,000 kWh lithium-ion battery system ap-plied in a utility frequency regulation application (photo courtesy of ZBB Energy Corporation).

The future of electric grids will be impacted by a growing penetration of plug-in hybrid electric vehicles and electric vehicles, which will represent a new dimension for grid management.


very stable design with fast-charge capability and good performance at lower temperatures. The batteries can be discharged to 0% and appear to have a relatively long life. Figure 10 shows a lithium-titanate battery in a utility power ancillary service application (frequency regulation).

Lithium Iron PhosphateThe lithium-ion battery using iron phosphate cathodes is a newer and safer technology. In this chemistry, it is much more diffi cult to release oxygen from the electrode, which reduces the risk of fi re in the battery cells. This design is more resistant to overcharge when operated in a range of up to 100% state of charge.

As mentioned previously, lithium-ion batteries are used in a wide variety of applications and will benefi t from economy of scale in production over the next decade. As shown in Figure 10, the ancillary services market appears to be the most available opportunity in utility power applications. As volume production increases, the future cost of lithium-ion battery systems will play a key role in how fast they penetrate utility power applications.

Lead-Acid BatteriesThe lead-acid battery is the oldest and most mature of all battery technologies. Because of the wide use of lead-acid batteries in a wide variety of applications, including automotive starting and UPS use, lead-acid batteries have the lowest cost of all battery technolo-gies. For utility power application, a 40-MWh lead-acid battery was installed in the Southern California gr id in 1988 to demonstrate the peak shaving capabilities of batter-ies in a grid application. The battery demonstrated the value of stored energy in the grid, but the lim-ited cycling capability of lead acid made the overall economics of the system unacceptable.

For backup power sources in large power plants, lead-acid battery plants are still used as “black start” sources in case of emergencies. Their long life and lower costs make them ideal for applications with low duty cycles.

Advanced Lead-Acid BatteriesThe high volume of production of lead-acid batteries offers a tremendous opportunity for expanded use of these batteries if their life could be signifi cantly extended in cycling applications. Adding carbon to the negative elec-trode seems to be the answer. Lead-acid batteries fail due to sulfation in the negative plate that increases as they are cycled more.

Adding as much as 40% of activated carbon to the negative electrode composition increases the battery’s life. Estimates of a cycling life improvement of up to 2,000 cycles represent a three to four times improvement over current lead-acid designs. This extended life coupled with the lower costs will lead storage developers to revisit lead-acid technology for grid applications.

Nickel-Cadmium BatteriesAs shown in Figure 5, which depicts the exponential growth in the power density of batteries, nickel-cadmium (Ni-Cad) batteries represented a substantial increase in battery power in the middle of the last century. The Ni-Cad battery quickly gained a reputation as a rugged, durable stored energy source with good cycling capability and a broad discharge range. Ni-Cad batteries have been applied in a variety of backup power applications and were cho-sen to provide “spinning reserve” for a transmission proj-

ect in Alaska. This project involves a 26-MW Ni-Cad battery rated for 15 min, which represents the largest bat-tery in a utility application in North America. The project was featured in the March/April 2005 issue of IEEE Power & Energy Magazine. Ni-Cad batteries are still being used for util-ity applications, such as power ramp rate control for “smoothing” wind farm power variability in areas with weak power grids (such as island power systems).

Flywheel Energy StorageSpinning a weighted mass on the end of the shaft of an electrical motor or generator to provide “ride-through” energy during short input power sags or outages is a concept that has been around for decades. Slow-speed (up to

figure 11. A 100-kWh high-speed flywheel assembly (photo courtesy of Beacon Power Corporation).

As countries around the world continue to increase their renewable energy portfolio, the participation of storage in the success formula needs attention.


8,000 r/min) steel fl ywheels have been used as “battery substitutes” in the UPS market for many years. These devices are practical for ride-through times of up to 30 s. Achieving longer storage times at high power levels requires signifi cant changes to the fl ywheel design and choice of materials. In the simplest terms, the amount of energy that can be stored kinetically in a fl ywheel is a function of the cube of rotational speed. Higher speeds translate to higher energy storage densities.

Modern fl ywheel energy storage systems considered for utility power applications consist of a massive rotating cylinder, as shown in Figure 11.

The cylinder is “weighted” with most of the mass located on the outer edge to increase the moment of inertia and maxi-mize the amount of energy stored. Flywheels of this design can be operated in a vacuum and supported on magnetically levitated bearings. This assembly is considered the “stator” of a motor-generator, with the outer shell acting as the gener-ator portion of the device. Typical operating speeds are up to 60,000 r/min. Actual delivered energy depends on the speed range of the fl ywheel. For example, above a 3:1 speed range, a fl ywheel will deliver up to 90% of its stored energy to an external load.

Currently, high-speed fl ywheel systems rated 1,000 kW (15 min) or larger are being deployed in the U.S. grid for frequency regulation use. At least three independent system operators (ISOs) have opened their markets for fast-response systems, such as fl ywheels and battery-powered systems.

Electrochemical CapacitorsCommonly called “supercapacitors,” electrochemical capacitors look and perform similar to lithium-ion batteries. They store energy in the two series capacitors of the electric double layer (EDL), which is formed between each of the electrodes and the electrolyte ions. The dis-tance over which the charge separation occurs is just a few angstroms. The extremely large surface area makes the capacitance and energy density of these devices thou-sands of times larger than those of conventional electro-lytic capacitors.

The electrodes are often made with porous carbon material. The electrolyte is either aqueous or organic. The aqueous capacitors have a lower energy density due to a lower cell voltage, but are less expensive and work in a wider temperature range. The asymmetrical capacitors that

use metal for one of the electrodes have a signifi cantly larger energy density than the symmetric ones do and also have a lower leakage current.

Compared with lead-acid batteries, electrochemical capacitors have lower energy density, but they can be cycled hundreds of thousands of times and are much more powerful than batteries (fast charge and discharge capability).

Supercapacitors have been applied for blade-pitch control devices for individual wind turbine generators to control the rate at which power increases and decreases with changes in wind velocity. This functionality is desirable if wind turbines are connected to weak utility power grids.

New Battery TechnologyWith the growing interest in energy storage for greater use in transportation and renewable energy, research activities are increasing in private industry, universities, and national laboratories. In North America, the U.S. Congress mandated increased funding for research and development (R&D) in energy storage. Major universities, including the Massachusetts Institute of Technology (MIT), are working to design new storage technologies. MIT is investigating ways to create very large-scale batteries capable of storing enormous amounts of power in the utility grid.

Thermal StorageAll of the energy storage technologies discussed are targeting ways to help the utility grid cope with balanc-ing generation and load in the most optimal ways possible. Traditionally, utility grids have been designed to deal with the highest load peaks that typically occur less than a few hours per day for only a few days per year. Just like batter-ies and peaking generators, any storage device that helps meet this objective should be considered in utility system planning. Thermal storage devices that can be deployed at the residential and commercial level should be given more attention. Modular ice storage systems can generate ice during off-peak power periods to power air-conditioning systems for several hours each day during the peak after-noon load times. Similarly, in cold climates, modular heat storage systems can capture electric power during off-peak periods and use that energy to store heat in a ceramic heatsink to be dispatched during higher peak periods in the winter. As more utilities consider real-time pricing of energy based on actual cost, all forms of energy storage

Utility system designers have seen the benefits of massive amounts of energy storage in the form of pumped hydro power plants.


will provide more value and contribute to lowering the overall peak demand.

This concept is not limited to small applications. In Europe, a very large thermal storage system (up to 10,000 MWh) is being proposed.

What About Hydrogen?The development of hydrogen-based fuel cells as clean energy sources continues around the world. In the transportation arena, PHEVs appear to be developing a commanding lead over fuel cell-powered vehicles as the clean energy choice. Proponents of a hydrogen economy argue that large wind farms could be used to power hydrogen-processing facilities and that pipelines—in lieu of large electrical transmission lines—could carry bulk hydrogen—as the energy source—to major population centers. Like today’s large natural gas pipeline networks that store gas conveniently in the system to match cus-tomer demand, hydrogen would be stored as necessary to match the demand for fuel cells for electricity and hydro-gen-powered cars.

Critics question the overall effi ciencies of creating large quantities of hydrogen to power fuel cells to create electricity. Large-scale adoption of hydrogen would require a signifi cant paradigm shift in the overall energy delivery strategy in major world markets. Today, changes of this magnitude do not appear possible in any of the world’s major utility markets.

ConclusionsEducation about the value of energy storage in operating electric power grids has been lacking for a long time. During the 2003 conference aimed at establishing a vision for the future smart electric grid, storage was identifi ed as playing a vital role in managing new and more complex networks. Since that time, more attention has been given to the benefi ts storage can provide. The infrastructure stimulus bill passed by the U.S. Congress provided increased funding for storage in the electric grid and signifi cant monies to advance storage devices for PHEVs.

As countries around the world continue to increase their renewable energy portfolio—namely, wind power—the par ticipation of storage in the success formula needs attention. The November/December 2007 “wind inte gration” issue of IEEE Power & Energy Magazine contained only very minor references to storage’s ability

to add value to wind resources by reducing the impact of wind availability. Like wind power, storage can benefi t from fi nancial stimulus to support its growth and demonstrate its value in actual performance. The United States, Japan, and Germany currently benefi t from having fairly large amounts of storage (pumped hydro) in their grids. Recognizing the value of storage in dealing with the variability of renewable re-sources is essential to harnessing the maximum potential of wind and solar power. Fortunately, storage systems used in grid applications will benefi t from the huge investment in electric-based transportation. In fact, the growth of EVs to 50 million units (15-kW capacity average) by 2030 would dwarf the installed capacity of major renewable energy sources. The real technology challenge will be making all of the new electric power resources func-tion in a fully integrated “smart grid.”

For Further ReadingDOE Electricity Advisory Committee. (2008). Bottling electricity: Storage as a strategic tool for managing vari-ability and capacity concerns in the modern grid [Online]. Available: www.doe.energy.gov/eac

D. Rastler, “New demand for energy storage,” Elect. Per-spect., vol. 33, no. 5, pp. 30–47, Sept./Oct. 2008.

(2008, Nov.). Utility scale energy storage grinds into gear. Climate Change Bus. J. [Online]. Available: www. climatechangebusiness.com

B. Lee and D. Gushee. (2008, June). Massive electric-ity storage. AICHe White Paper [Online]. Available: www.aiche.org

C. Vartanian, “The coming convergence, renew-ables, smart grid and storage,” IEEE Energy 2030, Nov. 2008.

Parliamentary Offi ce of Science and Technology. (2008, Apr.). Electricity storage [Online]. no. 306. Avail-able: www.parliament.uk/parliamentary-offices/post/pubs2008.cfm

J. McDowall. (2008). Understanding lithium-ion tech-nology, BATTCON 2008 [Online]. Available: http://www.battcon,com/Archive Papers.htm#McDowall2008

BiographyBradford Roberts is the power quality systems director for S&C Electric Company and chair of the Electricity Storage Association. p&e

One of the advantages of flow batteries is that their construction is based on plastic components in the reactor stacks, piping, and tanks for holding the electrolytes.

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