The following article is part 4 of a multi-part series.
II. Types of Energy Storage Systems
Throughout the next decade an increasing number of energy storage systems will mature and become commercially viable. Currently, pumped-hydro storage systems provide more than 127,000 MW of worldwide storage capacity while compressed air energy storage systems, battery technologies, and flywheels provide less than 860 MW of installed storage capacity. Each storage system is unique in both its storage technology and operational characteristics including the ability to perform multiple energy services.
A. Pumped Hydro
Pumped Hydro facilities are the most mature energy storage system. Pumped Hydro facilities consist of two adjacent reservoirs situated at different altitudes. During periods of lower priced electricity, water pumps remove water from the lower reservoir to the higher reservoir. During higher priced electric periods, water releases from the higher reservoir passing through hydraulic turbines that generate electricity, on its way down to the lower reservoir. Pumped Hydro’s potential for high power ratings (up to 4 GW) and long discharge durations (measured in hours) allow it to provide generation storage applications.
B. Compressed Air Energy Storage
Compressed air energy storage (“CAES”) systems store air underground (in caves or aquifers) or aboveground (in pipes or containers) and discharge the stored air through a combustion turbine generator creating electricity. Underground CAES systems have the potential for high power ratings (over 400 MW) and long discharge durations (measured in hours) that allow it to provide generation storage applications. Currently, aboveground CAES systems are still in development. Aboveground systems will provide CAES storage on a much smaller-scale (between 1 to 15 MW) and will target transmission and distribution support applications.
Battery storage systems utilize a large-scale rechargeable battery operating under the same basic principles as an automobile battery. Electrochemical batteries typically consist of two electrodes (anode and cathode) separated by an electrolyte material. A chemical reaction occurs when ions from the anode interact with ions from the cathode. The reaction creates an electric current. Battery storage systems utilize different technologies including: 1) lead-acid; 2) advanced lead-acid; 3) sodium-sulfur (NaS); 4) sodium nickel chloride (); and 5) lithium-ion (Li-ion).
Flow-cell batteries provide another form of battery storage. Flow-cell batteries store electrolytes in a separate container from the battery cell. Electrolyte containers pump electrolytes into the battery cell to create an electrochemical reaction, which then charges the battery. Adding more electrolyte tanks will increase output and discharge duration. Currently, vanadium redox batteries are the most mature flow-cell battery system.
Battery storage systems are one of the most versatile energy storage systems. Battery systems provide a wide range in power output (for both large and small scale applications), application specific output durations, portability, quick response time and dependability. The versatile operational performance of each battery technology allows it to provide end-user, generation, ancillary services, and transmission and distribution support storage applications.
Flywheels convert electric energy from the grid into kinetic energy through the acceleration of a rotor (flywheel) around a metal shaft. The flywheel decelerates to discharge stored energy into the grid. Flywheels are capable of rapidly discharging its stored energy and are therefore, ideal for ancillary service applications that require a rapid response to automated control signals (e.g., frequency response). Individual flywheels range in size from 150 kW to 1 MW and are capable of being interconnected to form large scale flywheel facilities ranging in size up to 20 MW.
Ultracapacitor storage systems consist of two oppositely charged metal plates separated by an insulator. Ultracapacitors store energy by increasing the electric charge accumulation on the plates and discharges energy by releasing charge accumulation off the plates. Ultracapacitor storage systems range in power ratings up to 15 MW. Ultracapacitors quick response time allows onsite systems to provide electric power during brief power interruptions. Advanced ultracapacitors provide even faster response time and are therefore, suitable for transmission and distribution applications such as voltage stability.  However, the cost of ultracapacitors may limit it to niche markets.
F. Superconducting Magnetic Energy Storage
Superconducting magnetic energy storage (“SMES”) systems store energy in a magnetic field created by the flow of a direct current in a coil of a cryogenically cooled superconducting wire. The system stores energy by increasing current through the wire and discharges energy by decreasing the current flow. SMES systems range in power ratings up to 10 MW. SMES systems discharge power almost instantaneously and are therefore, ideal for responses to brief occurrences of poor power quality. Utilities may use larger SMES systems for distribution network reliability and switching purposes.
G. Thermal Energy
Thermal energy systems most commonly store electricity through the use of cold thermal storage in water or ice tanks to reduce A/C compressor load during peak time periods. Ice-based thermal energy storage systems store energy by creating ice at night during off-peak hours and use the ice to create cool liquid that supplements AC compressor load during peak hours. Both end-users and electric utilities can utilize thermal storage systems. End-users can lower energy costs without sacrificing equipment performance by offsetting compressor load during peak time periods. Utilities can utilize thermal storage systems as a distributed energy storage resource to provide distribution system support.
 Electricity Energy Storage Technology Options: A White Paper Primer on Applications, Costs and Benefits, Prepared by Electric Power Research Institute, Rastler. D (Principal Investigator), December 2010.
 Ibid. .
 Ibid. .
 Energy Storage: The Missing Link in the Electricity Value Chain. Energy Storage Council. May 2002.
 Ibid. .
 Eyer, J.M., & Corey, G. Energy Storage for the Electricity Grid: Benefits and Market Potential Assessment. SANDIA National Laboratories Report # SAND2010-0815. February 2010.
 Ibid. .
 Zinc-bromine (Zn/Br) flow-cell systems are in the primary stages of field deployment and demonstration testing. Fe/Cr and Zn/Air flow cell systems are still undergoing further research and development. Ibid. .
 Electric Energy Storage: An Assessment of Potential Barriers and Opportunities. California Public Utilities Commission, Policy and Planning Division Staff White Paper. July 2010.
 Ibid. .
 Isser, S. Energy Storage White Paper, Submitted to the Renewable Technologies Working Group, ERCOT. November 2009 (updated January 2010).
 Ibid. .
 Consumer based thermal energy storage systems can also be heat based systems that reduce the load of heating equipment during peak time periods.
Written by Robert Clifford. Robert is a Boston-based attorney who represents clients before the Federal Energy Regulatory Commission and state public utility commissions.
Tags: batteries, compressed air energy storage, electrochemical batteries, energy storage systems, flow-cell batteries, flywheels, pumped-hydro storage systems, superconducting magnetic energy storage, thermal energy, ultracapacitors