|Fig. 1: Pumped Hydroelectric Storage Plant in Rönkhausen, Germany (Source: Wikimedia Commons)|
Much of the energy used today is stored in the form of hydrocarbons such as coal, natural gas, and petroleum. However, alternative methods of energy production, including nuclear power, photovoltaics, and wind power, are gaining market share and will likely continue to do so as humanity faces the threat of anthropogenic climate change. These energy generation mechanisms face several significant problems, but one of the largest is that of matching demand. Power demand is dependent on several factors, most notably the time of day; power demand surges during the day and in the evening, and ebbs at night. Coal and gas-fired power plants can be turned on and off quickly to match these fluctuations in energy usage, but photovoltaics only produce power when the sun is shining, wind farms only produce power when the wind is blowing, and it is difficult to modulate the amount of energy produced by a nuclear power plant. This mismatch between energy production and usage requires the introduction of large-scale energy storage if renewables and nuclear energy are going to play a significant role in our future energy consumption. Many forms of large-scale energy storage have been suggested, from new battery technologies to compressed air to supercapacitors, but these technologies all require further research and development and there is some doubt about their effectiveness. There is, however, a large-scale energy storage technology already in widespread use that could potentially store energy for a significant percentage of the world's population.
Pumped hydroelectric energy storage takes proven hydroelectric energy generation technology and runs the process in reverse to store energy. Excess energy is used to pump water uphill, and when demand exceeds supply the water is allowed to flow back downhill, turning turbines to generate electricity as it does so. Most facilities involve a closed system of two reservoirs, one at a higher altitude than the other, although some use a river as the lower source of water. Francis turbines are used both to pump the water to the higher reservoir and to generate electricity as it flows back to the lower one. The cycle is generally about 80% efficient, with losses due to water evaporation and engine non-idealities. The energy storage available from a given system can be calculated from
|E||=||ρ g h V η
where E is the energy storage capacity in Wh, η is the efficiency of the cycle, ρ is the density of the working fluid (for water, &rho =1000 kg/m3), g is the acceleration of gravity (9.81 m/s2), h is the altitude difference between the two reservoirs, and V is the volume of the upper reservoir. Below is an image of a typical system, the Tennessee Valley Authority pumped storage facility at Raccoon Mountain in Tennessee.
|Fig. 2: A typical pumped-hydroelectric storage facility. (Source: Wikimedia Commons)|
There is around 127 gigawatts of pumped storage capacity installed worldwide.  Of this, the United States has about 21.5 gigawatts of storage, in facilities that were built between the late 1960s and the early 1990s.  This capacity is enough to account for 25% of the daily surge in power demand in the US, strongly implying that expanded capacity could reasonably be used to store energy generated from renewable sources until is it needed at times of high demand.  Europe also has a significant capacity for using pumped hydroelectric storage to meet variable power demand. It is believed that converting the most appropriate of Norway's already-existing hydroelectric dams to pumped storage facilities could add at least 20 gigawatts to Europe's energy storage capacity, and other European countries could contribute significantly as well, [5,1]
Pumped hydroelectric storage is limited by the necessity of altitude differences between two large reservoirs of water, and is therefore most suited for implementation in mountainous areas. Another limitation is the presence of large quantities of water and the reduced efficiency when there is evaporation from the system, making it more difficult to implement pumped hydroelectric storage in arid regions. Several new ideas are being developed to mitigate these challenges to wider installation of pumped hydroelectric storage. Researchers at the Technical University of Denmark, working in conjunction with the architecture firm Gottlieb Paludan, have developed the idea of an artificial island outfitted wind turbines which generate energy used to pump seawater out of a deep reservoir. Seawater is then allowed to flow back into the reservoir, turning turbines to generate power, when energy is in demand.  Other seawater-based pumped hydroelectric storage plants, such as the Yanbaru project in Okinawa, Japan, are constructed to utilize coastal topography rather than artificial structures. Another new concept, introduced by the Californian startup Gravity Power, is to drill two shafts into the ground which are connected into a closed-loop system and filled with water. One shaft contains a piston that is raised by pumping the water in one direction to charge the system. The piston is then allowed to drop, pushing the water in the other direction and through a turbine, to generate energy. This system would require significantly less initial investment than building two traditional dammed reservoirs, and would be unobtrusive and unlimited by local topography.  It has also been suggested that former mines could be used as lower reservoirs of new pumped storage facilities, such as in the Summit Energy Storage Project in Ohio and the Mount Hope Hydro Project in New Jersey. [7,8] Finally, proposals to retrofit existing hydroelectric dams into pumped storage facilities are popular, as these facilities have already been paid for and are seen as integrated into the local ecosystem, avoiding both the economic and environmental arguments against building additional dams. 
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