Pumped hydro storage is one type of hydro electric power generation and storage used by utilities for load balancing. This method of energy storing is in water form, pumped from a lower elevated reservoir to a higher elevation reservoir. The pumps are run by low cost off peak electric power. The stored water is released during periods of high electrical demand through traditional turbines to generate electric power. This system adds electric power during peak demand periods when electricity rates are highest. The largest capacity form of grid energy storage available is the pumped storage. It accounts for more than 99% of storage capacity worldwide.
Most facilities use the elevation differences between bodies of water or artificial reservoirs. Pure pumped storage plants shifts the water in between reservoirs. A combination of this and conventional hydro electric plants is called the pump back approach. It uses natural stream flow. Conventional hydro electric plants do not use pumped storage.
Considering the evaporation losses from the exposed water surface and the conversion losses, an estimated 70% to 80% of the electric energy used to pump the water up to the higher reservoir can be recovered. This method is the most cost effective means in storing large amounts of electric energy in an operating basis. Critical decision aspects include capital cost and the appropriate geographical locations (i.e., proximity to demand and transmission capacity). It is a requirement for low energy density pumped storage systems to have large bodies of water or large variations in height.
One effective way to store a great amount of energy is through a large body of water situated on a hill. This occurs naturally in some places while it is man made in other places. This is very cost effective because it flattens out load difference on the power grid, allowing thermal power stations like coal fired plants, nuclear plants and renewable energy power plants to provide base load power to continue operating during peak efficiency. Capital costs for purpose built hydro storage however are relatively high. Thermal plants are less able to respond to sudden demand in electric power, while for pumped storage plants like any other hydro electric plants can respond to load changes even in just a few seconds.
Italy and Switzerland were the first two countries to use pumped storage in the 1890s. The United States first use pumped storage in 1930 by the Connecticut Electric and Power Company.
The five largest operational pumped-storage plants in the world now are the following:
1. Bath County Station- Virginia, United States (with a 3,003 megawatt capacity)
2. Guangdong Power Station - China (with a 2,400 megawatt capacity)
3. Okutataragi Hydroelectric Power Station - Japan (with a 1,932 megawatt capacity)
4. Ludington Power Plant - Michigan, United States (with a 1,872 megawatt capacity)
5. Tianhuangping Power Station- China (with a 1,836 megawatt capacity)
Pumped storages' new use is to level fluctuating outputs of intermittent power sources. Pumped storages supply a load at times of high electricity output and during low demand of electricity, making it possible for additional system peak capacity. Pumped storage systems help in controlling electrical network frequency and generate reserve generation aside from energy management.
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