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:
Large -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 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.
While 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.
Efficiency 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.
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.
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%.
Integrated ZnBr energy storage systems are now available in a range if sizes:
On transportable trailers (storage systems including power electronics) with unit capacities of up to 1MW/3MWh for utility-scale applications. As a building block, these units can be paralleled and expanded for much larger applications
As 5kW/20kWh Community Energy Storage (CES)
systems that are now being deployed by electricity utilities.
Vanadium Redox Batteries store 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 Vanadium Redox Batteries are independent of each other.
Updated April 2009
Vanadium Redox Batteries were pioneered in the Australian University of New South Wales (UNSW) in early 1980’s. The Australian Pinnacle Vanadium Redox Batteries bought the basic patents in 1998 and licensed them to Sumitomo Electric Industries (SEI) and VRB Power Systems. Vanadium Redox Batteries storage up to 500kW, 10 hrs (5MWh) have been installed in Japan by SEI. Vanadium Redox Batteries have also been applied for power quality applications (3MW, 1.5 sec., SEI).
VRB Power Systems, Inc.
Sumitomo Electric Industries, Ltd.
Cellennium Company Limited
Compressed Air Energy Storage (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.
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.
CAES Development Company
Ridge Energy Storage
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.
GNB Industrial Power/Exide
JCI Battery Group
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.
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.
AER Energy Resources
Chem TekMetallic Power
Electric FuelPower Zinc
Zoxy Energy Systems