Types of energy storage

Energy storage is important for maintaining reliable supply, reducing wastage and lowering costs.

The method of energy storage depends on the source of energy. In this article, Masoud Abshar, Managing Director, Magellan Power explains types of electrical energy storage.

Pumped hydro storage

This constitutes the majority of installed electrical storage around the world, 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 times 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 of 70-85%.

Compressed air

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 it is large scale. The disadvantage is its efficiency is typically less than 50%.

Flywheel storage

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 energy stored. Electricity is extracted by reducing the speed. The main advantages of such a system are: 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 on the construction and type of bearings.

Electrochemical energy storage (secondary batteries)

Lead-acid batteries

This is presently the most commonly used battery globally. These type of batteries 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) batteries

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. Originally developed to replace NiCad, NiMh batteries have all the advantages of NiCad batteries and have much higher energy density but cannot be made at higher capacity.

Li-Ion batteries

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%.

Sodium sulfur (NaS)

This battery consists of liquid (molten) sulfur at the positive electrode and liquid (molten) sodium at the negative electrode. The battery temperature is kept at 300°C to keep the electrodes molten. This battery has a cycle life of up to 4500 cycles and an efficiency of about 75%. The drawback is the energy required to keep the batteries at high temperature. This energy comes from the battery itself.

Flow battery

In the Redox 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 a few minutes by pumping out the discharged electrolyte and replacing it with charged electrolyte. These batteries have a low energy density and rely on pumps for its operation. They have a long life and are suitable for large-scale storage. Disadvantages include: complicated compared to other battery technologies, maintenance, and low energy density.

Electrical supercapacitor

Supercapacitors have been around for 60 years. The technology fills the gap between conventional capacitors and batteries, and has a high development potential which can lead to much greater capacitance and energy storage capability. Supercapacitors have almost unlimited cyclic life, high power capability as well as high storage capability compared with conventional capacitors. Advantages of this technology are high capacitance, long cyclic life, durability and reliability. Their disadvantage is that the capacity is low compared with batteries.

Magnetic coil

These devices store energy in the magnetic field created by flow of DC current in a superconducting coil kept at a low temperature (-200°C). The main advantage of this device is quick response time, high reliability, high overall efficiency (depending on the refrigeration system) and high power output.

Thermal energy storage

Thermal energy storage systems store heat in insulated storage tanks for later use. Thermal storage can be subdivided into different technologies: storage of sensible heat, storage of latent heat and thermo-chemical storage. For electrical storage systems it is mainly the sensible and latent heat storage that 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 the solar tower again