**6.2 Pumped hydroelectric storage (PHES)**

Pumped hydroelectric energy storage (PHES) is the most established storage innovation in the world today. The International Energy Agency (IEA) estimates that PHES installments make an increase in capacity of 26 GW, and energy storage capability of PHES will overtake battery storage globally by 2050. The current storage volume of PHES plants is measured at 9000 GWh, although the battery capacity is just 7 GWh [44]. By 2023, electricity production from PHES has expanded by one quarter to 146 TWh; however, the estimated operating hours of the PHES were considerably unpredictable, and the spectrum of capacity factors was broad due to the uncertainty of market conditions. Storage consist of a reservoir, specifically upper and lower [18], Besides the power stored behind hydropower dams, 97% of all energy stored for electrical usage is currently in the form of pumped hydropower storage [18]. Where current surplus energy or costs are low at the moment, water is pumped through pipelines from the pumping station in the lower reservoir to the upper reservoir, as seen in **Figure 4**. At the same time, as pumped hydro storage is introduced into the grid or the capability of the water reservoirs is more advantageous, the hydropower facility can provide most of the load necessities, obviating the need to build huge top-load fuel mills [46].

Pumped storage hydropower (PSH) systems account for more than 94% of the global energy storage capacity. Water-spinning turbine, as water flows down from a higher tank to a lower reservoir, generates electricity in PSH. This energy storage system (ESS) may provide large storage capacity at a low cost, fulfilling the needs of bigger electrical networks. The difficulty with pumped hydro storage systems is that they take years to create and need significant investments.
