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Sub-Surface Pumped Hydroelectric Storage

Pumped storage hydroelectricity (PSH), or pumped hydroelectric energy storage (PHES), is a type of hydroelectric energy storage which is used by electric power systems for load balancing. This system stores energy in a form of water gravitational potential energy, pumped from a reservoir of a lower elevation to a reservoir of a higher elevation. Lower cost surplus off peak electric power is normally used for running the pumps. During high electrical demand periods, the stored water is released to flow through turbines so as to produce electric power. Despite the efficiency losses of the pumping process making the plant an energy net consumer, the system generates profits by increasing revenue through selling more electrical power during peak demand periods, as electricity prices are much higher. Sub-Surface Pumped Hydroelectric Storage uses below ground reservoirs for water transfer.  

What is different about Sub-Surface Pumped Hydroelectric Storage?

Pumped hydroelectric storage (PHS) system typically contains an upper and lower reservoir.  Sometimes rivers can be used as the lower reservoir; massive lakes or even an ocean.  An interesting concept that is considered is to locate one or both reservoirs sub surface (below ground). The underground excavation process, the cost for materials handling required time and construction risk could make the economics of such system complicated, so existing subsurface structures (like abandoned mines, caverns, and man created storage reservoirs) can be utilized to offset or minimize  the setup costs The idea is to divert and gravity feed water from a source above the ground into an underground powerhouse, where the water would flow through the pump turbine.  The generated power is delivered to the power grid during the periods of peak demand. After flowing through the turbines, the water is then stored temporarily in the lower reservoir before being pumped back to the original source (higher reservoir) using the off peak (lower cost) power.  

Use of Sub-Surface Pumped Hydroelectric Storage in Solar Energy and its efficiency

Pumped storage hydroelectricity allows intermittent energy sources (similar to solar or wind energy) and other renewable energy sources, or the excess electrical energy from the continuous base load sources (like coal or nuclear) to be efficiently saved for later periods of higher demand. The reservoirs used for pumped storage systems are quite small in comparison to conventional hydroelectric dams which have an equal power capacity. The PSH systems round-trip energy efficiency  varies in the range of 70%–80. The main disadvantage of PSH system is the geographic nature of the site required, as it needs both height and availability of water. Appropriate sites could be in mountainous regions, which potentially are areas of outstanding natural beauty that would be expensive to execute a similar project.  
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