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Electric Storage Revolution Coming Soon?
by Bob Shively, Enerdynamics President
For the last century, electric grids have been designed and operated with the paradigm that electricity cannot economically be stored except in very limited cases. This limitation means that generation output must match customer demand instantaneously across all minutes of the day, even as loads vary minute-by-minute. The result is the need to build and run expensive peaking generation, to maintain generation reserves at all times, and to occasionally interrupt curtailable customers. As variable generation such as wind and solar power attains higher penetration in our power systems, not only will load vary minute-by-minute, but supply will also vary. Fortunately, new storage technologies are developing rapidly and those currently being demonstrated on the grid appear likely to change the “no-storage” paradigm sooner than later.
Historically, pumped storage represented the only bulk storage technology in wide use. Pumped storage is a hydro technology where water pumped uphill with cheap off-peak power is reused to generate during more valuable peak hours. Currently, the U.S. has approximately 22,000 MW of pumped storage capacity representing 2% of overall U.S. generation capacity.
The downside to pumped storage is that it is a limited resource – it can only be built in water sheds with specific characteristics, and it can have significant environmental impacts. Recent research has focused on a number of alternate storage technologies including various types of electrochemical storage (commonly known as batteries), and non-electrochemical technologies such as thermal energy storage, flywheels, Compressed Air Energy Storage (CAES), very large capacitors, and Superconducting Magnetic Energy Storage (SMES). Following is a brief overview of how each technology works:
Batteries create electrical flow through chemical reactions. Batteries useful for the electric grid can be recharged by applying electrical flow that reverses the reaction that provides electricity. Batteries for grid use can be created from various chemical combinations including lead acid, nickel-cadmium (NiCad), lithium-ion (Li-ion), sodium/sulfur (Na/S), zinc/bromine (Zn/Br), vanadium-redox, and nickel-metal hydride (Ni-MH). In addition to stand-alone battery installations, they may be integrated in the grid by connecting electric vehicles.
Thermal energy storage is normally done at a customer location with a large cooling requirement. Cold water or ice is created using electric compressors during off-peak hours and is stored for cooling uses during peak hours.
Flywheels consist of a low-friction spinning cylinder attached to a shaft connected to a motor/generator. When electricity is stored, the motor converts electricity to kinetic energy stored in the spinning cylinder. When electricity is desired, the motion of the cylinder is used to turn the generator thus re-converting the kinetic energy to electricity.
CAES uses electricity to compress air in large underground cavities at high pressure. When electricity is desired, the compressed air is used to spin a combustion turbine that in turn spins a generator.
Capacitors store electricity as an electrostatic charge. Smaller capacitors have long been used as a means of supporting power quality on grids, but new much larger capacitors offer the opportunity to store larger amounts of power.
SMES consists of a coil of superconducting material that, when cooled below a critical temperature, allows power to circle through the coils with virtually no resistance. When electricity is desired, the power coils are reconnected allowing the power to flow onto the grid.
One advantage to the variety of technologies is that different technologies have different operational characteristics and thus provide a variety of potential benefits. As discussed earlier, pumped storage provides the capability of time-shifting, which is taking energy generated during off-peak hours and storing it for later use in peak hours. Other storage technologies such as thermal storage and CAES have the potential to provide similar benefits. As renewable technologies such as wind and solar gain further penetrations, these technologies also provide the potential for firming renewable outputs by storing power during high renewable generation and providing power during low renewable output.
Other storage technologies have operating characteristics that offer storage and/or supply over much shorter time intervals than the hours between peak and off-peak periods. These technologies offer the potential for very rapid charge/discharge cycles that range in seconds and minutes rather than hours. This offers benefits as resources for system regulation, spinning reserves, real-time balancing energy, and local voltage support. Technologies with potential in this category include batteries, electric vehicles, flywheels, capacitors, and SMES.
So what is holding back storage implementation on the grid? One constraint is the need for demonstration of operational and economic characteristics. Numerous demonstration projects are currently taking place to gain further knowledge in these areas. The second is the need to change market rules and operational practices to provide opportunities for owners of storage to monetize the benefits in a profitable manner.