Pumped hydro storage has been in use for over a century now for storing excess electricity produced during off-peak hours and supplying it during peak demand hours. It acts as a giant battery and plays a crucial role in balancing electricity supply and demand on power grids. With increasing reliance on variable renewable energy sources like solar and wind that are dependent on the sun and wind, pumped hydro storage is gaining renewed focus as a means to store excess renewable energy.
What is pumped hydro storage?
Pumped hydro storage involves using surplus electricity to pump water from a lower elevation reservoir to a higher elevation reservoir. When electricity is needed, the stored water is released through turbines back into the lower reservoir, generating electricity. Essentially, it works just like conventional hydroelectric plants but in a reversible manner. During low demand hours, electricity is used to pump water uphill which stores the energy as gravitational potential energy of water. During periods of high demand, the same water is allowed to flow back down through turbines, converting the stored potential energy back into electricity.
Major components of a Pumped Hydro Storage plant include two water reservoirs - an upper and lower reservoir, two penstocks or pipelines to carry water between the reservoirs, a pump-turbine unit and a generator. The pump-turbine allows the same machine to work both as a pump as well as a turbine.
Benefits of pumped hydro storage
Pumped hydro storage has numerous benefits which make it ideal for large scale electricity storage and complementing renewable energy sources:
Efficiency- Pumped hydro storage systems achieve over 80% round-trip efficiency compared to other competing technologies like lithium-ion batteries which are around 60-70%. This means more of the stored energy can be recovered and supplied to the grid.
Large capacity- Pumped hydro plants can store huge amounts of potential energy in the form of water held in reservoirs. This allows them to shift tens or even hundreds of megawatt-hours of electricity.
Proven technology- Pumped hydro storage utilizes well established hydropower technology that has been operating reliably and economically over decades. This reduces technological and commercial risks.
Long project life- With proper maintenance, pumped hydro facilities can operate for 50-100 years or longer, far exceeding other storage technologies.
Frequency regulation- The fast response times of pumped hydro, ranging from seconds to minutes, allow them to provide important frequency regulation services and stabilize power grids with variable renewable output.
Challenges with development
However, pumped hydro storage also faces some challenges that have slowed its development in recent decades:
Site specificity- Pumped hydro storage requires specific topographical conditions of being near two water reservoirs at different elevations. Suitable sites are limited and new projects are difficult to site.
High upfront costs- Developing new large scale pumped hydro facilities involves considerable capital costs for civil works required to construct the dams and reservoirs. Projects costs range between $0.5-3 billion.
Long lead times- It can take 5-10 years or longer to plan, obtain approvals and construct a new pumped hydro plant due to feasibility studies, environmental clearances and major civil engineering works.
Environmental impacts- Large dams and disruption of aquatic ecosystems are environmental impacts that require careful mitigation planning. Public opposition can delay or obstruct projects.
Role of pumped hydro with renewables
However, the inherent strengths of pumped hydro storage such as multi-day storage, flexibility, cost-effectiveness and ability to shift huge amounts of electricity make it almost irreplaceable in energy systems with very high shares of fluctuating renewables. Studies show that at 55-60% renewable penetration levels, additional bulk storage like pumped hydro storage is necessary for reliable renewable integration.
Most countries with leading shares of wind and solar in their electricity mix like Denmark, Ireland, Portugal and Spain have already established pumped hydro reservoirs to utilize their existing hydropower infrastructure for energy storage.
Moving forward, new technological developments offer solutions to make more pumped hydro capacity available. Closed-loop designs recycle water within tunnels or man-made caverns without releasing it into natural water bodies to reduce environmental effects. Advanced industrial-scale designs integrated within wind and solar plants can fast-track new projects through modular prefabricated components. Underwater compressed air energy storage is a variant that stores energy in compressed air chambers beneath the lower reservoir.
With the cost of variable renewable power falling and higher shares inevitable in future, pumped hydro storage will play a vital balancing role by providing clean bulk electricity storage for durations ranging from hours to multiple days at utility scales. It remains indispensable for stable grid operations with very high penetrations of renewable energy well into the coming decades.
Despite some drawbacks, pumped hydro storage continues to be one of the most important large-scale electricity storage technologies. Its flexibility, longevity, proven performance and ability to store massive amounts of renewable energy economically make it a technology that will continue advancing renewable integration for power systems worldwide. With new project designs and storage concepts emerging, its strategic importance for clean energy transition is set to grow even further.
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