Written by Mohamed Alazab
Edited by Malak Manie
An overview of energy storage technologies
Energy storage is not new. Batteries have been used since the early 1800s, and pumped-storage hydropower has been operating in the United States since the 1920s. But the demand for a more dynamic and cleaner grid has led to a significant increase in the construction of new energy storage projects, and to the development of new or better energy storage solutions. In the 20th century grid, electrical power was largely generated by burning fossil fuel, where, when less power was required, less fuel was burned. Concerns with air pollution, energy imports, and global warming have spawned the growth of renewable energy technologies such as solar and wind power. Wind power is uncontrolled and may be generated at a time when no additional power is needed. Solar power varies with cloud cover and is only available during daylight hours at best, while demand often peaks after sunset. For these reasons, interest in storing power from these intermittent sources grows as the renewable energy industry begins to generate a larger fraction of overall energy consumption. Energy storage is also valued for its rapid response – most storage technologies can begin discharging power to the grid very quickly, while fossil fuel sources tend to take longer to ramp up. This rapid response is important for ensuring the stability of the grid when unexpected increases in demand occur.
Energy storage technologies
There are many ways of storing energy, such as mechanical, chemical, thermal electrochemical, etc.; each has its own strengths and weaknesses.
Batteries, like those in a flashlight or cell phone, can be used to store energy on a large scale. Batteries can be located anywhere so they are often seen as storage for distribution: when a battery facility is located near consumers to provide power stability, or end-use, like batteries in electric vehicles.
There are many different types of batteries that have large-scale energy storage potential, including Sodium-sulfur, Solid State Batteries, Lithium-ion batteries, and Lead-acid batteries. Advancements in battery technologies have been made largely due to the expanding electric vehicle (EV) industry. As more developments are made with EVs, battery cost should continue to decline. Electric vehicles could also have an impact on energy storage through vehicle-to-grid technologies, in which their batteries can be connected to the grid and discharge power for others to use.
Thermal energy storage facilities use temperature to store energy. When energy needs to be stored, rocks, salts, water, or other materials are heated and kept in insulated environments. When energy needs to be generated, the thermal energy is released by pumping cold water onto the hot rocks, salts, or hot water to produce steam, which spins turbines. Thermal energy storage can also be used to heat and cool buildings instead of generating electricity. For example, thermal storage can be used to make ice overnight to cool a building during the day. Thermal efficiency can range from 50 percent to 90 percent depending on the type of thermal energy used.
Thermal storage is also used for electricity generation by using power from the sun, even when the sun is not shining. Concentrating solar plants can capture heat from the sun and store the energy in water, molten salts, or other fluids. This stored energy is later used to generate electricity, enabling the use of solar energy even after sunset.
Plants like these are currently operating or proposed in California, Arizona, and Nevada. For example, the proposed Rice Solar Energy Project in Blythe, California will use a molten salt storage system with a concentrating solar tower to provide power for approximately 68,000 homes each year.
Thermal storage technologies also exist for end-use energy storage. One method is freezing water at night using off-peak electricity, then releasing the stored cold energy from the ice to help with air conditioning during the day.
For example, Ice Energy’s Ice Bear system creates a block of ice at night, and then uses the ice during the day to condense the air conditioning system’s refrigerant. In this way, the Ice Bear system shifts the building’s electricity consumption from the daytime peak to off-peak times when the electricity is less expensive. Additionally, the Bonneville Power Administration is conducting a pilot program on storing excess wind generation in residential water heaters.
Hydrogen fuel cells, which generate electricity by combining hydrogen and oxygen, have appealing characteristics: they are reliable and quiet (with no moving parts), have a small footprint and high energy density, and release no emissions (when running on pure hydrogen, their only byproduct is water). The process can also be reversed, making it useful for energy storage: electrolysis of water produces oxygen and hydrogen. Fuel cell facilities can, therefore, produce hydrogen when electricity is cheap, and later use that hydrogen to generate electricity when it is needed (in most cases, the hydrogen is produced in one location, and used in another). Hydrogen can also be produced by reforming biogas, ethanol, or hydrocarbons, a cheaper method that emits carbon pollution. Although hydrogen fuel cells remain expensive (primarily because of their need for platinum, an expensive metal), they are being used as primary and backup power for many critical facilities (telecom relays, data centers, credit card processing, etc).
Pumped hydroelectric storage facilities store energy in the form of water in an upper reservoir, pumped from another reservoir at a lower elevation. During periods of high electricity demand, power is generated by releasing the stored water through turbines in the same manner as a conventional hydropower station. During periods of low demand (usually nights or weekends when electricity is at a lower cost), the upper reservoir is recharged by using lower-cost electricity from the grid to pump the water back to the upper reservoir.
Reversible pump-turbine/motor-generator assemblies can act as both pumps and turbines. Pumped storage stations are unlike traditional hydroelectric stations in that they are a net consumer of electricity, due to hydraulic and electrical losses incurred in the cycle of pumping from lower to upper reservoirs. However, these plants are typically highly efficient (round-trip efficiencies reaching greater than 80%), and can prove very beneficial in terms of balancing load within the overall power system. Pumped-storage facilities can be very economical due to peak and off-peak price differentials and their potential to provide critical ancillary grid services.
Compressed Air Energy Storage (CAES)
With compressed air storage, air is pumped into an underground hole, most likely a salt cavern, during off-peak hours when electricity is cheaper. When energy is needed, the air from the underground cave is released back up into the facility, where it is heated, and the resulting expansion turns an electricity generator. This heating process usually uses natural gas, which releases carbon; however, CAES triples the energy output of facilities using natural gas alone. CAES can achieve up to 70 percent energy efficiency when the heat from the air pressure is retained, otherwise, the efficiency lies between 42 and 55 percent
Energy storage in Egypt
In Egypt, the high dam with its reservoirs is operated to provide electricity at times of peak demand. Water is stored in the reservoir during periods of low demand and released when demand is high. The net effect is like pumped storage, but without the pumping loss. Batteries are used in wind and solar farms across the country. some of the techniques that we discuss above can be applied in Egypt like thermal energy storage that is used in some countries by integrated it with concentration solar power systems (csp), Pump storage hydropower. And as the renewable energy market increase here in Egypt, we have to localize the industries of energy storage technology, and this will facilitate the process of integrating renewable energy into the grid.
In the end, we need to stress on some points: energy storage improves reliability and resilience and can smooth out the delivery of variable or intermittent energy resources such as wind and solar energy, by storing excess energy when the wind is blowing, and the sun is shining and delivering it when the opposite is happening. Energy storage can reduce the cost to provide frequency regulation and spinning reserve services, as well as offset the costs to consumers by storing low-cost energy and using it later, during peak periods at higher electricity rates.
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