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See the menu at left for discussions of individual storage technologies.
Each technology has some inherent limitations or disadvantages that make it practical or economical for only a limited range of applications. The capability of each technology for high power and high energy applications are indicated by the following symbols:
RatingsLarge -scale stationary applications of electric energy storage can be divided in three major functional categories:
Although some storage technologies can function in all application ranges, most options would not be economical to be applied in all three functional categories.
Size & WeightSize and weight of storage devices are important factors for certain applications. Metal-air batteries have the highest energy density in this chart. However, the electrically rechargeable types, such as zinc-air batteries, have a relatively small cycle life and are still in the development stage.
The energy density ranges reflect the differences among manufacturers, product models and the impact of packaging.
Capital CostsWhile capital cost is an important economic parameter, it should be realized that the total ownership cost (including the impact of equipment life and O&M costs) is a much more meaningful index for a complete economic analysis. For example, while the capital cost of lead-acid batteries is relatively low, they may not necessarily be the least expensive option for energy management (load leveling) due to their relatively short life for this type of application.
The battery costs in this chart have been adjusted to exclude the cost of power conversion electronics. The cost per unit energy has also been divided by the storage efficiency to obtain the cost per output (useful) energy.
Installation cost also varies with the type and size of the storage. The information in the chart and table here should only be used as a guide not as detailed data.
Life EfficiencyEfficiency and cycle life are two important parameters to consider along with other parameters before selecting a storage technology. Both of these parameters affect the overall storage cost. Low efficiency increases the effective energy cost as only a fraction of the stored energy could be utilized. Low cycle life also increases the total cost as the storage device needs to be replaced more often. The present values of these expenses need to be considered along with the capital cost and operating expenses to obtain a better picture of the total ownership cost for a storage technology.
Per-cycle CostPer-cycle cost can be the best way to evaluate the cost of storing energy in a frequent charge/discharge application, such as load leveling.
This chart shows the capital component of this cost, taking into account the impact of cycle life and efficiency. For a more complete per-cycle cost, one needs to also consider O&M, disposal, replacement and other ownership expenses, which may not be known for the emerging technologies.
It should be noted that per-cycle cost is not an appropriate criterion for peak shaving or energy arbitrage where the application is less frequent or the energy cost differential is large and volatile.
Description:
CAES is not a simple energy storage system like other batteries. It is a peaking gas turbine power plant that consumes less than 40% of the gas used in conventional gas turbine to produce the same amount of electric output power. This is because, unlike conventional gas turbines that consume about 2/3 of their input fuel to compress air at the time of generation, CAES pre-compresses air using the low cost electricity from the power grid at off-peak times and utilizes that energy later along with some gas fuel to generate electricity as needed. The compressed air is often stored in appropriate underground mines or caverns created inside salt rocks. It takes about 1.5 to 2 years to create such a cavern by dissolving salt.
Deployment Status:
The first commercial CAES was a 290 MW unit built in Hundorf, Germany in 1978. The second commercial CAES was a 110 MW unit built in McIntosh, Alabama in 1991. The construction took 30 months and cost $65M (about $591/kW). This unit comes on line within 14 minutes.
The third commercial CAES, the largest ever, is a 2700 MW plant that is planned for construction in Norton, Ohio. This 9-unit plant will compress air to 1500 psi in an existing limestone mine some 2200 feet under ground.
Developers / Suppliers:
CAES Development Company
Ridge Energy Storage
Dresser-Rand Company
Description:
Electrochemical capacitors (EC) store electrical 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 distance over which the charge separation occurs is just a few angstroms. The capacitance and energy density of these devices is thousands of times larger than electrolytic 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 significantly larger energy density than the symmetric ones and have lower leakage current.
Compared to lead-acid batteries, EC capacitors have lower energy density but they can be cycled tens of thousands of times and are much more powerful than batteries (fast charge and discharge capability).
Deployment Status: While the small electrochemical capacitors are well developed, the larger units with energy densities over 20 kWh/m3 are still under development.
Developers:
SAFT
NESS
ESMA
PowerCache (Maxwell)
ELIT
PowerSystem Co.
Description:
Most modern flywheel energy storage systems consist of a massive rotating cylinder (comprised of a rim attached to a shaft) that is substantially supported on a stator by magnetically levitated bearings that eliminate bearing wear and increase system life. To maintain efficiency, the flywheel system is operated in a low vacuum environment to reduce drag. The flywheel is connected to a motor/generator mounted onto the stator that, through some power electronics, interact with the utility grid. Some of the key features of flywheels are little maintenance, long life (20 years or 10s of thousands of deep cycles) and environmentally inert material. Flywheels can bridge the gap between short term ride-through and long term storage with excellent cyclic and load following characteristics.
The choice of using solid steel versus composite rims is based on the system cost, weight, size, and performance trades of using dense steel (200 to 375 m/s tip speed) vs. a much lighter but stronger composite that can achieve much higher rim velocities (600to 1000 m/s tip speed). Actual delivered energy depends on the speed range of the flywheel as it cannot deliver its rated power at very low speeds. For example, over 3:1 speed range, a flywheel will deliver ~90% of its stored energy to the electric load.
Development / Deployment Status:
While high-power flywheels are developed and deployed for aerospace and UPS applications, there is an effort, pioneered by Beacon Power, to optimize low cost commercial flywheel designs for long duration operation (up to several hours). 2kW / 6kWh systems are in telecom service today. Megawatts for minutes or hours can be stored using a flywheel farm approach. Forty 25kW / 25 kWh wheels can store 1MW for 1 hour efficiently in a small footprint.
The stored energy can be approximated by:
<INSERT FORMULA IMAGE>
where w is the rotational velocity (rad/sec), I the moment of inertia for the thin rim cylinder, m is the cylinder mass. and v is linear rim velocity.
Developers / Suppliers:
Beacon
Active Power, Inc.
AFS Trinity Power
Piller Gmb
Urenco Power Technologies Limited
Description:
Lead-acid is one of the oldest and most developed battery technologies. It is a low cost and popular storage choice for power quality, UPS and some spinning reserve applications. Its application for energy management, however, has been very limited due to its short cycle life. The amount of energy (kWh) that a lead-acid battery can deliver is not fixed and depends on its rate of discharge.
Lead-acid batteries, nevertheless, have been used in a few commercial and large-scale energy management applications. The largest one is a 40 MWh system in Chino, California, built in 1988. The table below lists and compares the lead-acid storage systems that are larger than 1MWh.
Developers / Suppliers:
GNB Industrial Power/Exide
Delco
East Penn
Teledyne
Optima Batteries
Winston Salem
JCI Battery Group
Trojan
Crown Battery
Description:
The cathode in these batteries is a lithiated metal oxide (LiCoO2, LiMO2, etc.) and the anode is made of graphitic carbon with a layer structure. The electrolyte is made up of lithium salts (such as LiPF6) dissolved in organic carbonates.
When the battery is being charged, the Lithium atoms in the cathode become ions and migrate through the electrolyte toward the carbon anode where the combine with external electrons and are deposited between carbon layers as lithium atoms. This process is reversed during discharge.
The main advantages of Li-ion batteries, compared to other advanced batteries, are:
Deployment Status:
While Li-ion batteries took over 50% of small portable market in a few years, there are some challenges for making large-scale Li-ion batteries. The main hurdle is the high cost (above $600/kWh) due to special packaging and internal overcharge protection circuits.
Several companies are working to reduce the manufacturing cost of Li-ion batteries to capture large energy markets (multi-kW, kWh sizes for residential & commercial markets). The auto industry is the driver behind this development.
Developers / Suppliers:
SAFT
HITACHI
Description:
Metal-air batteries are the most compact and, potentially, the least expensive batteries available. They are also environmentally benign. The main disadvantage, however, is that electrical recharging of these batteries is very difficult and inefficient. Although many manufacturers offer refuelable units where the consumed metal is mechanically replaced and processed separately, not many developers offer an electrically rechargeable battery. Rechargeable metal air batteries that are under development have a life of only a few hundred cycles and an efficiency about 50%.
The anodes in these batteries are commonly available metals with high energy density like aluminum or zinc that release electrons when oxidized. The cathodes or air electrodes are often made of a porous carbon structure or a metal mesh covered with proper catalysts. The electrolytes are often a good OH- ion conductor such as KOH. The electrolyte may be in liquid form or a solid polymer membrane saturated with KOH.
Deployment Status:
While the high energy density and low cost of metal-air batteries may make them ideal for many primary battery applications, the electrical rechargeability feature of these batteries needs to be developed further before they can compete with other rechargeable battery technologies.
Developers:
EVionyx
AER Energy Resources
Metallic Power
Chem Tek
Power Zinc
Electric Fuel
Alupower
Aluminum Power
Zoxy Energy Systems
Description:
A NAS battery consists of liquid (molten) sulfur at the positive electrode and liquid (molten) sodium at the negative electrode as active materials separated by a solid beta alumina ceramic electrolyte. The electrolyte allows only the positive sodium ions to go through it and combine with the sulfur to form sodium polysulfides. 2Na + 4S = Na2S4 During discharge, as positive Na+ ions flow through the electrolyte and electrons flow in the external circuit of the battery producing about 2 volts. This process is reversible as charging causes sodium polysulfides to release the positive sodium ions back through the electrolyte to recombine as elemental sodium. The battery is kept at about 300 degrees C to allow this process. NAS battery cells are efficient ( about 89%).
Deployment Status:
NAS battery technology has been demonstrated at over 190 sites in Japan totaling more than 270 MW with stored energy suitable for 6 hours daily peak shaving. The largest NAS installation is a 34 MW, 245 MWh unit for wind stabilization in Northern Japan. U.S. utilities have deployed 9 MW for peak shaving, backup power, firming wind capacity and other applications; and project development is in-progress for an equal amount.
The demand for NAS batteries as an effective means of stabilizing renewable energy output and providing ancillary services is expanding. Several projects are under development in Europe, as well as in Japan and the US. Annual production capacity: 90MW, 150 MW planned in 2010
Developers / Suppliers:
NGK Insulators, Ltd.
Description:
Conventional pumped hydro uses two water reservoirs, separated vertically. During off peak hours water is pumped from the lower reservoir to the upper reservoir. When required, the water flow is reversed to generate electricity. Some high dam hydro plants have a storage capability and can be dispatched as a pumped hydro. Underground pumped storage, using flooded mine shafts or other cavities, are also technically possible. Open sea can also be used as the lower reservoir. A seawater pumped hydro plant was first built in Japan in 1999 (Yanbaru, 30 MW).
Pumped hydro was first used in Italy and Switzerland in the 1890's. By 1933 reversible pump-turbines with motor-generators were available. Adjustable speed machines are now being used to improve efficiency. Pumped hydro is available at almost any scale with discharge times ranging from several hours to a few days. Their efficiency is in the 70% to 85% range.
There is over 90 GW of pumped storage in operation world wide, which is about 3 % of global generation capacity. Pumped storage plants are characterized by long construction times and high capital expenditure.
Pumped storage is the most widespread energy storage system in use on power networks. Its main applications are for energy management, frequency control and provision of reserve.
Developers / Suppliers:
MWH
Description:VRB stores energy by employing vanadium redox couples (V2+/V3+ in the negative and V4+/V5+ in the positive half-cells). These are stored in mild sulfuric acid solutions (electrolytes).
During the charge/ discharge cycles, H+ ions are exchanged between the two electrolyte tanks through the hydrogen-ion permeable polymer membrane. The cell voltage is 1.4-1.6 volts. The net efficiency of this battery can be as high as 85%. Like other flow batteries, the power and energy ratings of VRB are independent of each other.
Deployment Status: 
VRB was pioneered in the Australian University of New South Wales (UNSW) in early 1980's. The Australian Pinnacle VRB bought the basic patents in 1998 and licensed them to Sumitomo Electric Industries (SEI) and VRB Power Systems. VRB storages up to 500kW, 10 hrs (5MWh) have been installed in Japan by SEI. VRBs have also been applied for power quality applications (3MW, 1.5 sec., SEI).Prudent Energy Inc.
Current Developers / Suppliers:
Past Developers/Suppliers:
VRB Power Systems, Inc. Reliable Power
Pinnacle
Sumitomo Electric Industries, Ltd.
Cellennium Company Limited
Description:
In each cell of a ZnBr battery, two different electrolytes flow past carbon-plastic composite electrodes in two compartments separated by a microporous polyolefin membrane.
During discharge, Zn and Br combine into zinc bromide, generating 1.8 volts across each cell. This will increase the Zn2+ and Br- ion density in both electrolyte tanks. During charge, metallic zinc will be deposited (plated) as a thin film on one side of the carbon-plastic composite electrode. Meanwhile, bromine evolves as a dilute solution on the other side of the membrane, reacting with other agents (organic amines) to make thick bromine oil that sinks down to the bottom of the electrolytic tank. It is allowed to mix with the rest of the electrolyte during discharge. The net efficiency of this battery is about 75%.
Deployment Status:
The ZnBr battery was developed by Exxon in the early 1970's.
Integrated ZnBr energy storage systems are now available in a range if sizes:
As 5kW/20kWh Community Energy Storage (CES) systems that are now being deployed by electricity utilities. 
Developers / Suppliers:
ZBB Energy Corp
Premium Power Corporation
RedFlow Technologies Ltd