Pumped Hydro Storage
This currently constitutes most of world-wide installed electrical storage which is currently about 3% of the global generation capacity. This system uses two reservoirs of water at different elevations. Water is pumped from the lower reservoir to the higher reservoir during off-peak time and then used to run a turbine at peak time. Typical discharge times range from hours to days and the efficiency is in the range if 70 to 85%.
In this system air is compressed in a natural underground structure or above ground vessels, and when needed it is mixed with natural gas and used in a gas turbine. The advantage of such a system is its large scale. The disadvantage is its efficiency which is (typically less than 50%.)
In Flywheel energy storage, the energy is stored in an accelerated rotor and maintained by keeping the rotor at a constant speed. The higher the speed, the higher the stored energy. Electricity is extracted by reducing the speed. The main advantage of such system is its long life, low maintenance, and high power density. The disadvantages are self-discharge due to air resistance and bearing losses. Efficiency can range from 20-85% depending of the the construction and type of bearings.
Electrochemical- Secondary batteries
Lead Acid batteries
This is presently the most used battery globally. They are used in UPS, stand alone systems with PV, wind power, and many other applications. Typical service life is 5-15 years and the cycle life ranges from 500-1500 cycles at 70-80% Depth of Discharge. The cyclic efficiency is 80-90%. They are reliable, easy to use and low cost. Some of its disadvantages are its life and its usable capacity at elevated temperatures.
Nickel Cadmium and Nickel Metal Hydride (Nicad, NiMH)
NiCADS have been in commercial use since 1915. They are reliable, robust and have higher energy density and cycle life compared with Lead Acid. Their disadvantage is their toxicity (Cadmium) and their cost.
NiMH Originally developed to replace NiCad, they have all the advantages of NiCad batteries, and have much higher energy density but cannot be made at higher capacity.
This has become the most important storage technology in the area of portable and mobile applications. They have higher cell voltage which means one Lithium Ion battery can replace three NiCads. Other advantages include high energy density, small size and weight, higher operating temperature, higher cyclic life (2000-5000), low self discharge, high efficiency typically 95-98%. Disadvantages include the need to monitoring system (BMS). This technology is still developing and their price is reducing.
This battery consists of liquid (molten) sulphur at the positive electrode and liquid (molten) sodium at the negative electrode. The battery temperature is kept at 300C to keep the electrodes molten. This battery has a cycle life of up to 4500 cycles and an efficiency of about 75%. The draw- back is the energy required to keep the batteries at high temperature. This energy comes from the battery itself.
In the Radox flow battery, two liquid electrolytes containing metal ions are pumped to opposite sides of the cell to form positive and negative terminals. Theoretically these batteries can be recharged within few minutes by pumping out the discharged electrolyte and replacing it with charged electrolyte. This battery has a low energy density and relies on pumps for its operation. Their advantage is long life, suitability for large scale storage. Their disadvantage is that they are complicated compared with other battery technologies, maintenance, and low energy density.
Supercapacitors have been known for 60 years. The technology fills the gap between conventional capacitors and batteries. Due to their almost unlimited cyclic life as well as high power capability as well as high storage capability compared with capacitors. This technology has a high development potential which can lead to much greater capacitance and energy storage capability.Advantages of this technology are high capacitance, long cyclic life, durability and reliability. Their disadvantage is that the capacity is low compared with batteries.
Superconductive Magnetic Coil
These devices store energy in the magnetic field created by flow of DC current is a superconducting coil kept at very low temperature (-200C). The main advantage of this device is very quick response time, high reliability, high overall efficiency, (depending on the refrigeration system) and high power output.
Thermal Energy Storage systems store heat in insulated storage tanks for later use. Thermal storage can be sub-divided into different technologies: Storage of sensible heat, storage of latent heat and thermo-chemical. For Electrical Storage systems it is mainly the sensible and latent heat storage which are applicable. A practical two tank system for a solar power plant uses one single molten salt as heat transfer fluid and storage medium. The molten salt is heated by solar radiation in the solar tower and then transported to the hot salt storage tank. To produce electricity, the hot salt passes through a steam generator which powers a turbine. Subsequently the cold salt is stored in a second tank before it is pumped to solar tower again.