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Seawater power station
Seawater Power Stations, or Multipurpose seawater power stations (MP-SPS), are experimental power stations which serve multiple purposes. Typical MP-SPS designs include components of wave or tidal power alongside pumped hydroelectric energy storage. MP-SPS concept designs have also included on-site geothermal[verification needed], solar, and/or wind power, as well as desalination equipment. These power stations aim to take advantage of multiple renewable/alternative resources that are present near the shoreline.
MP-SPS are optimized for active continental margins where there is a rapid elevation gain right off the shore. This rapid elevation gain allows for more pumped hydroelectric storage capacity, because the higher the seawater is pumped, the faster it will flow, increasing downstream turbine output. Space requirements are also paramount when designing these systems: pumped hydro storage capacity is also dependent on the size of the upper reservoir (higher volumes of seawater can produce more energy). The lower reservoir must be protected as well, so as to keep marine life from entering the system.
MP-SPS generating capacity is determined by the wave, tidal, geothermal, solar, and/or wind resource in the area. Designs should maximize local resources while minimizing impact on the surrounding environment. Typically, wave or tidal energy resources are abundant on coastlines, but these types of power systems tend to have greater environmental impacts compared to wind or solar systems. Solar and wind resources are often strong on coastlines, although they face intermittency issues that can be amplified by the rapid shifts in weather patterns that can occur near coastlines. [verification needed]
Spacing requirements can be challenging for the terrestrial power systems (sun and wind). This can be minimized by building wind turbines in a staggered manner to allow more turbines to fit into a smaller land area, and by using solar (conventional photovoltaics, solar thermal, or concentrated solar) to cover the upper reservoir. [verification needed]
Another design consideration is the addition of a desalination component to the station. There are several methods of desalination, but the method employed should consider the resources available. If there is significant thermal energy available (via geothermal vents or solar radiation) then the power station would work best with a thermal-based desalination method such as flash distillation. Otherwise, electrical generation can be used to operate reverse osmosis systems, or other similar membrane-based methods of desalination. The byproduct of desalination will be fresh water and valuable salts and other minerals, which can be sold.[verification needed]
Because these stations are directly on the coastal margin where there is significant sea spray, it is important that the equipment be corrosion resistant for longevity and reliability.
The first modern power station to use seawater for pumper hydroelectric energy storage is the Okinawa Yanbaru Seawater Pumped Storage Power Station W, rated at a maximum capacity of 30 MW. This station used local utility over generation to pump seawater into the upper reservoir (150 meters above sea level) during off-peak hours, later releasing the seawater during intervals of peak demand. The station was built in 1999 and functioned according to design. It was eventually dismantled in 2016 because local electrical demand had not grown enough to make the station economical. 
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