1.3.

4 ENERGY STORAGE
Technology Description
Advanced storage technologies
under active development
include processes that are
mechanical (flywheels,
pneumatic), electrochemical
(advanced batteries, reversible
fuel cells, hydrogen,
ultracapacitors), and purely
electrical (superconducting
magnetic storage). Energy
storage devices are added to the
utility grid to improve
productivity, increase reliability
or defer equipment upgrades.
Energy storage devices must be
charged and recharged with
electricity generated elsewhere.
Because the storage efficiency
(output compared to input
energy) is less than 100%, on a
kilowatt-per-kilowatt basis, A 5-MVA battery energy-storage system for power quality and peak shaving.
energy storage does not directly
decrease CO2 production. The exception to this rule is the use of advanced energy storage in conjunction with
intermittent renewable energy sources, such as photovoltaics and wind, that produce no direct CO2. Energy
storage allows these intermittent resources to be dispatchable.
Energy-storage devices do positively affect CO2 production on an industrial output basis by providing high-
quality power, maximizing industrial productivity. New battery technologies, including sodium sulfur and flow
batteries, significantly improve the energy and power densities for stationary battery storage as compared to
traditional flooded lead-acid batteries.
System Concepts
• Stationary applications: The efficiency of a typical steam-power plant falls from about 38% at peak load to
28%-31% at night. Utilities and customers could store electrical energy at off-peak times, allowing power
plants to operate near peak efficiency. The stored energy could be used during high-demand periods
displacing low-efficiency peaking generators. CO2 emissions would be reduced if the efficiency of the
energy storage were greater than 85%. Energy storage also can be used to alleviate the pressure on highly
loaded components in the grid (transmission lines, transformers, etc.) These components are typically only
loaded heavily for a small portion of the day. The storage system would be placed downstream from the
heavily loaded component. This would reduce electrical losses of overloaded systems. Equipment
upgrades also would be postponed, allowing the most efficient use of capital by utility companies. For
intermittent renewables, advnaced energy storage technology would improve their applicability.
• Power quality: The operation of modern, computerized manufacturing depends directly on the quality of
power the plant receives. Any voltage sag or momentary interruption can trip off a manufacturing line and
electronic equipment. Industries that are particularly sensitive are semiconductor manufacturing, plastics
and paper manufacturing, electronic retailers, and financial services such as banking, stock brokerages, and
credit card-processing centers. If an interruption occurs that disrupts these processes, product is often lost,
plant cleanup can be required, equipment can be damaged, and transactions can be lost. Any loss must be
made up decreasing the overall efficiency of the operation, thereby increasing the amount of CO2
production required for each unit of output. Energy-storage value is usually measured economically with

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 the cost of power-quality losses, which is estimated in excess of $1.5 B/year in the United States alone.
Industry is also installing energy-storage systems to purchase relatively cheap off-peak power for use
during on-peak times. This use dovetails very nicely with the utilities’ interest in minimizing the load on
highly loaded sections of the electric grid. Many energy-storage systems offer multiple benefits. (An
example is shown in the photo.) This 5-MVA, 3.5-MWh valve-regulated lead-acid battery system is
installed at a lead recycling plant in the Los Angeles, California, area. The system provides power-quality
protection for the plant’s pollution-control equipment, preventing an environmental release in the event of a
loss of power. The system carries the critical plant loads while an orderly shutdown occurs. The battery
system also in discharged daily during the afternoon peak (and recharged nightly), reducing the plant’s
energy costs.
Representative Technologies
For utilities, the most mature storage technology is pumped hydro; however, it requires topography with
significant differences in elevation, so it’s only practical in certain locations. Compressed-air energy storage
uses off-peak electricity to force air into underground caverns or dedicated tanks, and releases the air to drive
turbines to generate on-peak electricity; this, too, is location specific. Batteries, both conventional and
advanced, are commonly used for energy-storage systems. Advanced flowing electrolyte batteries offer the
promise of longer lifetimes and easier scalability to large, multi-MW systems. Superconducting magnetic
energy storage (SMES) is largely focused on high-power, short-duration applications such as power quality and
transmission system stability. Ultracapacitors have very high power density but currently have relatively low
total energy capacity and are also applicable for high-power, short-duration applications. Flywheels are now
commercially viable in power quality and UPS applications, and emerging for high power, high-energy
applications.
Technology Status - Utilities
Technology Efficiency Energy density Power density Sizes Comments
[%] [W-h/kg] [kW/kg] [MW-h] m
Pumped hydro 75 0.27/100 m low 5,000-20,000 37 existing in U.S.
Compressed gas 70 0 low 250-2,200 1 U.S., 1 German
SMES 90+ 0 high 20 MW high-power applications
Batteries 70–84 30-50 0.2-0.4 17-40 Most common device
Flywheels 90+ 15-30 1-3 0.1-20 kWh US & foreign development
Ultracapacitors 90+ 2-10 high 0.1-0.5 kWh High-power density

System Components
Each energy-storage system consists of four major components: the storage device (battery, flywheel, etc.); a
power-conversion system; a control system for the storage system, possibly tied in with a utility SCADA
(Supervisory Control And Data Acquisition) system or industrial facility control system; and interconnection
hardware connecting the storage system to the grid. All common energy-storage devices are DC devices
(battery) or produce a varying output (flywheels) requiring a power conversion system to connect it to the AC
grid. The control system must manage the charging and discharging of the system, monitor the state of health
of the various components and interface with the local environment at a minimum to receive on/off signals.
Interconnection hardware allows for the safe connection between the storage system and the local grid.
Current Research, Development, and Demonstration
RD&D Goals
• Research program goals in this area focus on energy-storage technologies with high reliability and
affordable costs. For capital cost this is interpreted to mean less than or equal to those of some of lower
cost new power generation options ($400–$600/kW). Battery storage systems range from $300-$2000/kW.
For operating cost, this figure would range from compressed gas energy storage, which can cost as little as
$1 to $5/kWh, to pumped hydro storage, which can range between $10 and $45/kWh.
RD&D Challenges
• The major hurdles for all storage technologies are cost reduction and developing methods of accurately
identifying all the potential value streams from a given installation. Advanced batteries need field
experience and manufacturing increases to bring down costs. Flywheels need further development of fail-

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 safe designs and/or lightweight containment. Magnetic bearings could reduce parasitic loads and make
flywheels attractive for small uninterruptible power supplies and possibly larger systems using multiple
individual units. Ultracapacitor development requires improved large modules to deliver the required larger
energies. Advanced higher-power batteries with greater energy storage and longer cycle life are necessary
for economic large-scale utility and industrial applications.
RD&D Activities
• The Japanese are investing heavily in high-temperature, sodium-sulfur batteries for utility load-leveling
applications. They also are pursuing large-scale vanadium reduction-oxidation battery chemistries. The
British are developing a utility-scale flow battery system based on sodium bromine/sodium bromide
chemistry. DOE’s Energy Storage Systems Program works on improved and advanced electrical energy
storage for stationary (utility, customer-side, and renewables) applications. It focuses on three areas:
system integration using near-term components including field evaluations, advanced component
development, and systems analysis. This work is being done in collaboration with a number of universities
and industrial partners.
Commercialization and Deployment Activities
• For utilities, only pumped hydro has made a significant penetration with approximately 37 GW.
• Approximately 150 MW of utility peak-shaving batteries are in service in Japan.
• Two 10-MW flow battery systems are under construction – one in the United Kingdom and the other in the
United States.
• Megawatt-scale power quality systems are cost effective and entering the marketplace today.

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