Pump Up that Seawater! A Remix to Pumped-Storage Hydro

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: wastedenergy.net)

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.

When engineers in Japan first began designing the system, they were faced with many challenges due to the system’s reliance on salt water and its unique interaction with the ocean. Special technologies were innovated and employed to deal with these barriers.

First, to prevent leakage of salt water into the surrounding environment, engineers included a rubber lining on the upper reservoir made from ethylene propylene diene monomer. In case the rubber lining fails, drainage pipes are equipped with seawater detectors and pressure gauges to alert system operators to salt water infiltration. If salt water leakage is detected, the pump system can be used to pump the salt water up through the drainage pipes back into the upper reservoir.

Corrosion of the equipment caused by salt water is also a concern. In response, fiberglass reinforced plastic (FRP) was used for the penstock – the pipe that carries water from the intake to the turbine – which prevents corrosion from seawater as well as the adhesion of marine organisms. These factors would otherwise decrease the system’s longevity and lower its efficiency. The generating turbine is made from austenite stainless steel, found to be the most anticorrosive of the stainless steel options tested.

Lastly, because this pumped-storage hydro plant interacts directly with the ocean, engineers needed to find a way to limit its impact on marine life. First, they built the discharge outlet in the immediate area with the least coral development. Then, to reduce the velocity of water reentering the ocean, and therefore the impact on the local ecosystem, engineers built a breakwater of concrete blocks around the outlet. This limits the discharge velocity to approximately 10 centimeters per second.

Although the plant on Okinawa Island is the only operating seawater pumped-storage hydro plant in the world, many similar systems have been proposed recently. In Glinsk, Ireland there is a proposal for a 480 MW seawater pumped-storage hydro plant. This plant would be able to accept approximately one-third of the excess electricity generated by the 5,000 MW of wind turbines expected to be in operation by 2020 according to Ireland’s energy plan.

In Lanai, Hawaii there is a proposal for a 300 MW seawater pumped-storage hydro plant. This project would be largely used to facilitate greater renewable penetration in Hawaii, as it would be able to store the excess electricity from 400 MW of wind that is proposed to come online in the future.

The benefits of pumped-storage hydro, in general, are great. Most importantly, it serves as a relatively cheap way to store excess electricity. This is especially important for renewable energies like wind, as often the wind blows late at night when power demand is low. Because these systems store energy, they also lower the need for peaking plants which are deployed to meet power needs during periods of high demand and often generate expensive electricity from fossil fuels.

Seawater pumped-storage hydro offers many additional exciting benefits. To begin, it requires little fresh water compared to a typical pumped-storage hydro plant, which is very important for many parts of the world where fresh water resources are scarce. Moreover, because only one reservoir needs to be created or exploited on land, seawater systems lead to less land-use change. This has climate implications as creating artificial reservoirs often leads to the destruction of carbon sinks through deforestation. It also permits greater flexibility in siting these plants, as only one suitable site for a reservoir needs to be found. This means that storage plants can be built closer to power generation plants, including renewables like solar and wind.

A majority of population centers are located on the coast, which makes seawater pumped-storage hydro systems a practical solution to the energy storage problem for many areas. Likewise, some of the world’s greatest renewable energy potentials – especially wind – are located on or just off the coast, which means that seawater pumped-storage hydro systems could be a great tool to integrate these resources into a reliable electricity system.

Overall, seawater pumped-storage hydro appears to be an exciting technology that increases the reach of an already successful technology. Especially for a region like the Caribbean, where Worldwatch is currently working with three governments to produce Sustainable Energy Roadmaps, it could be a practical solution to consider. With limited supplies of fresh water, tremendous renewable energy potential on and just off its coasts, and limited energy storage capability, seawater pumped-storage hydro could provide the Caribbean with tremendous benefits.

Matt Lucky is a Sustainable Energy Fellow at the Worldwatch Institute.

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