Energy Storage Technology
Posted June 11, 2008on:
No storage technology is suitable for all applications. Each technology stores energy in a different form, giving it inherent properties that tailor it for one role rather than another. To rank the technologies for each application on technical grounds, they are evaluated on five issues.
Real power is the MW output of generation facilities, and is used for commodity power sales and peak shaving strategies.
Reactive power maintains the electric field of AC equipment and is required for the proper operation of the grid; it is measured in megavars (MVAR).
The length of time a storage facility can discharge energy. Generally, longer endurances tend to be real power, with shorter times, reactive power.
Some applications, like grid support, require discharges to commence less than a second after beginning; others, like power sales, can be scheduled allowing for a reaction time of a few minutes.
Some applications require that the storage facility be housed inside, taking up valuable floor space and requiring additional space-conditioning costs.
Pumped-hydro storage is the oldest and largest of all of the commercially available energy storage technologies, with facilities up to 1000 MW. Pumped storage projects differ from conventional hydroelectric projects in that they normally pump water from a lower reservoir to an upper reservoir when demand for electricity is low.
Pumped-hydro facilities consist of two large reservoirs; one located at a low level and the other situated at a higher elevation. During off-peak hours, water is pumped from the lower to the upper reservoir, where it is stored. To generate electricity, the water is then released back down to the lower reservoir, passing through hydraulic turbines and generating electrical power.
For example, in the summer water is released during the day for generating power to satisfy the high demand for electricity for air conditioning. At night, when demand decreases, the water is pumped back to the upper reservoir for use the next day.
Compressed air energy storage (CAES) systems use off-peak power to pressurize air into an underground reservoir (salt cavern, abandoned hard rock mine, or aquifer) which is then released during peak daytime hours to be used in a gas turbine for power production. Facilities are sized in the range of several hundred megawatts. In a gas turbine, roughly two thirds of the energy produced is used to pressurize the air. The idea is to use low-cost power from an off-peak baseload facility in place of the more expensive gas turbine-produced power to compress the air for combustion. Since CAES facilities have no need for air compressors tied to the turbines, they can produce two to three times as much power as conventional gas turbines for the same amount of fuel.
Flow batteries – also known as regenerative fuel cells – are capable of storing and releasing energy through a reversible electrochemical reaction between two salt solutions (electrolytes). These systems are excellent at storing real power (MW), but poor at delivering reactive power (MVAR) quickly.
Designs exist around the use of zinc bromide (ZnBr), vanadium bromide (VBr), and sodium bromide (NaBr) as the electrolytes. Charging of the facility occurs when electrical energy from the grid is converted into potential chemical energy. Release of the potential energy occurs within an electrochemical cell, with a separate compartment for each electrolyte, physically separated by an ion-exchange membrane.
The technology is a closed loop cycle, so there is no discharge of the regenerative electrolyte solutions from the facility. The scale of the facility is based primarily on the size of the electrolytic tanks.
A number of battery technologies exist for use as utility-scale energy storage facilities. Primarily, these installations have been lead-acid, but other battery technologies such as sodium sulphur (NaS) and lithiumion are quickly becoming commercially available.
All batteries are electrochemical cells. They are composed of two electrodes separated by an electrolyte. During discharge, ions from the anode (first electrode) are released into the solution and oxides are deposited on the cathode (second electrode). Reversing the electrical charge through the system recharges the battery. When the cell is being recharged, the chemical reactions are reversed, restoring the battery to its original condition.
Superconducting magnetic energy storage (SMES)
Superconducting Magnetic Energy Storage (SMES) systems store energy in the magnetic field created by the flow of direct current in a coil of cryogenically cooled, superconducting material. A SMES system includes a superconducting coil, a power conditioning system, a cryogenically cooled refrigerator and a cryostat/vacuum vessel.
A flywheel energy storage system works by accelerating a rotor to a very high speed and maintaining the energy in the system as inertial energy. Advanced composite materials are used for the rotor to lower its weight while allowing for the extremely high speeds; energy is stored in the rotor in proportion to its momentum, but the square of the angular momentum. The flywheel releases the energy by reversing the process and using the motor as a generator. As the flywheel releases its stored energy, the flywheel’s rotor slows until it is discharged.
Not generally thought of as one of the new, high-tech energy storage technologies, thermal energy storage systems already exist in widely used applications. Thermal systems can either be ice-based (for peak-shaving commercial and industrial cooling loads), or heliostat-based (mirror-based) using molten salt for electric power production (still in the development phase).