One of the biggest challenges with using renewable energy for electricity generation—specifically wind and solar power—is intermittency. The wind doesn’t always blow and the sun doesn’t always shine. Affordable, reliable, and deployable storage is seen as the holy grail of renewable energy integration, and recent advances in storage technology are getting closer to finding it.

The current electricity grid has virtually no storage—pumped hydropower is the most prevalent, but is largely location dependent. As higher levels of solar and wind energy are added to the grid, however, storage will become increasingly fundamental to ensuring that the power supply remains stable and demand is met. Utilities and businesses around the globe are starting to use large-scale batteries to complement their renewable energy generation: in Texas, for example, Duke Energy installed a 36 megawatt lead-acid storage system to balance its wind power.

Storage system ratings

Credit: Energy Storage Association

Storage technologies not only provide utilities with grid reliability for renewable integration, but also offer additional benefits such as ancillary services, ramp rate control, frequency regulation, and peak shaving, which can lower costs and improve the performance of the transmission system. Power system operators have always had to match electricity demand with supply, and energy storage is an additional tool in their grid-management toolbox.

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In a previous blog, I discussed the value of pumped-storage hydro systems, especially when it comes to integrating intermittent renewable energies like wind and solar into a power system. However, traditional pumped-storage hydro systems require two reservoirs of fresh water (one upper and one lower), which are not always available at locations that might otherwise benefit from an energy storage system. An exciting technology that tackles this problem – requiring only one on-land reservoir – and that has gained recent momentum is seawater pumped-storage hydro.

An aerial view of the seawater pumped-storage hydro system on Okinawa Island (Source:

Seawater pumped-storage hydro works similarly to traditional systems. Excess electricity from fossil fuel, nuclear, or renewable energy power plants is used during periods of low power demand to pump water uphill to be stored in reservoirs as potential energy. Then, when demand peaks the reservoirs are opened, allowing water to pass through hydroelectric turbines to generate the electricity needed to meet power demand. The main difference for seawater pumped-storage is that instead of having a lake, river, or some other source of fresh water serve as the lower reservoir, these systems pump salt water uphill from the ocean to a land reservoir above. This lowers the system’s fresh water footprint and greatly expands the potential for pumped-storage hydro worldwide because seawater pumped-storage is much less site-specific than traditional systems.

There is currently one seawater pumped-storage hydro system operating in the world, on the northern coast of Okinawa Island, Japan. The system began operation in 1999 and has the potential to generate up to 30 megawatts (MW) of power. The hydropower plant has a total head – the vertical distance, or drop, between the intake of the plant and the turbine – of 136 meters and the upper reservoir is located just 600 meters from the coast.

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