Pumped-storage hydroelectricity (PSH), or pumped hydroelectric energy storage (PHES), is a type of used by for. A PSH system stores energy in the form of of water, pumped from a lower elevation to a higher elevation. Low-cost surplus off-peak electric power is typically used to run the pumps. During periods of high electrical demand, the stored w. Pumped-storage hydroelectricity (PSH), or pumped hydroelectric energy storage (PHES), is a type of used by for. A PSH system stores energy in the form of of water, pumped from a lower elevation to a higher elevation. Low-cost surplus off-peak electric power is typically used to run the pumps. During periods of high electrical demand, the stored water is released through to produce electric power. Pumped-storage hydroelectricity allows energy from (such as,, and other renewables) or excess electricity from continuous base-load sources (such as coal or nuclear) to be saved for periods of higher demand. The reservoirs used with pumped storage can be quite small, when contrasted with the lakes of conventional hydroelectric plants of similar power capacity, and generating periods are often less than half a day. The of PSH varies between 70% and 80%. A pumped-storage hydroelectricity generally consists of two water reservoirs at different heights, connected with each other. At times of low electrical demand, excess generation capacity is used to pump water into the upper reservoir. When there is higher demand, water is released back into the lower reservoir through a, generating electricity. Pumped storage plants usually use reversible turbine/generator assemblies, which can act both as a pump and as a turbine generator (usually designs). Variable speed operation further optimizes the round trip efficiency in pumped hydro storage plants. In micro-PSH applications, a group of pumps and (PAT) could be implemented respectively for pumping and generating phases. The same pump could be used in both modes by changing rotational direction and speed: the operation point in pumping usually differs from the operation point in PAT mode. In closed-loop systems, pure pumped-storage plants store water in an upper reservoir with no natural inflows, while pump-back plants utilize a combination of pumped storage and conventional with an upper reservoir that is replenished in part by natural inflows from a stream or river. Plants that do not use pumped storage are referred to as conventional hydroelectric plants; conventional hydroelectric plants that have significant storage capacity may be able to play a similar role in the as pumped storage if appropriately equipped. Taking into account conversion losses and evaporation losses from the exposed water surface, of 70–80% or more can be achieved. This technique is currently the most cost-effective means of storing large amounts of electrical energy, but capital costs and the necessity of appropriate geography are critical decision factors in selecting pumped-storage plant sites. The relatively low energy density of pumped storage systems requires either large flows and/or large differences in height between reservoirs. The only way to store a significant amount of energy is by having a large body of water located relatively near, but as high as possible above, a second body of water. In some places this occurs naturally, in others one or both bodies of water were man-made. Projects in which both reservoirs are artificial and in which no natural inflows are involved with either reservoir are referred to as "closed loop" systems. These systems may be economical because they flatten out load variations on the power grid, permitting such as and that provide base-load electricity to continue operating at peak efficiency, while reducing the need for "peaking" power plants that use the same fuels as many base-load thermal plants, gas and oil, but have been designed for flexibility rather than maximal efficiency. Hence pumped storage systems are crucial when. Capital costs for pumped-storage plants are relatively high, although this is somewhat mitigated by their proven long service life of decades - and in some cases over a century, which is three to five times longer than utility-scale batteries. When, pumped hydro operators may earn twice - when "buying" the electricity to pump the water to the upper reservoir at negative and again when selling the electricity at a later time when prices are high. Along with energy management, pumped storage systems help stabilize and provide reserve generation. Thermal plants are much less able to respond to sudden changes in electrical demand that potentially cause frequency and instability. Pumped storage plants, like other hydroelectric plants, can respond to load changes within seconds. The most important use for pumped storage has traditionally been to balance baseload powerplants, but they may also be used to the fluctuating output of. Pumped storage provides a load at times of high electricity output and low electricity demand, enabling addition. The main requirement for PSH is hilly country. The global greenfield pumped hydro atlas lists more than 800,000 potential sites around the world with combined storage of 86 million GWh (equivalent to the effective storage in about 2 trillion electric vehicle batteries), which is about 100 times more than needed to support 100% renewable electricity. Most are closed-loop systems away from rivers. Areas of natural beauty and new dams on rivers can be avoided because of the very large number of potential sites. Some projects utilise existing reservoirs (dubbed "bluefield") such as the 350 Gigawatt-hour Snowy 2.0 scheme under construction in Australia. Some recently proposed projects propose to take advantage of such as disused mines such as the Kidston project under construction in Australia.
