How Efficient Is Pumped Hydro Storage?

Pumped hydro storage is a type of hydroelectric energy storage that uses water power to store and generate electricity. It consists of an upper reservoir and a lower reservoir. During times of low energy demand, excess electricity is used to pump water from the lower reservoir up to the upper reservoir. When energy demand is high, water is released from the upper reservoir and flows down through turbines to generate electricity. The height difference between the reservoirs creates hydraulic head, allowing the water to drive the turbines.

Pumped hydro storage technology first emerged in the 1890s in Switzerland and Italy. It expanded through the 20th century as grid-scale energy storage became increasingly important. The first large-scale pumped hydro storage plants were built in the 1930s. Today it is the largest-capacity and most widespread form of grid energy storage, accounting for around 95% of all storage capacity worldwide. There are over 300 pumped hydro plants worldwide, with capacities ranging from under 100 MW to over 3,000 MW.

The main uses of pumped hydro storage are for load balancing, supply stability, and to store excess renewable energy. It helps smooth out fluctuations between electricity supply and demand. It provides long-duration storage to shift excess renewable generation to times of higher demand. Overall, it increases reliability and utilization of generation assets to balance grids.

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How It Works

Pumped hydro storage works by using electricity to pump water from a lower reservoir to an upper reservoir. When electricity demand is low, excess generation capacity is used to pump the water to the upper reservoir, where it is stored. When electricity demand is high, water is released from the upper reservoir through turbines to generate electricity.

There are two main types of reservoirs used for pumped hydro storage. Closed-loop systems use reservoirs that are not connected to a natural body of water, like rivers or lakes. These systems pump water between the two reservoirs to store and generate electricity. Open-loop systems make use of existing bodies of water, like reservoirs behind hydroelectric dams, and pump water between the main reservoir and upstream storage reservoirs.

Pumped hydro is a mature and proven technology that accounts for around 95% of all energy storage capacity worldwide. It provides an efficient way to store large amounts of energy and help balance electricity supply and demand.

Sources:

https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity

https://www.ge.com/renewableenergy/hydro-power/hydro-pumped-storage

Efficiency Metrics

There are several common ways to measure the efficiency of pumped hydro storage:

Energy ratio – The ratio between energy output and energy input during a complete pump-generate cycle. A higher energy ratio indicates greater efficiency. Energy ratios for pumped hydro facilities typically range from 70-87% according to NREL [1].

Roundtrip efficiency – Similar to energy ratio, this measures the amount of electricity generated compared to the amount used for pumping, accounting for generation, transmission, and pump losses over a full cycle. Roundtrip efficiency levels are also in the 70-87% range [1].

Capacity factor – The ratio of a plant’s actual power output over a period of time compared to its potential output if operated at full nameplate capacity. Capacity factors for pumped hydro storage can vary greatly depending on how the plant is utilized.

Real-World Efficiency Levels

Efficiency rates for modern pumped hydro storage plants typically range from 70-85%, with some of the most advanced facilities achieving round-trip efficiency over 80% (Blakers, 2021). This means that 80-85% of the electrical energy used to pump the water uphill can be regained when that water flows back down to generate electricity.

There are several factors that impact the real-world efficiency of a pumped hydro storage plant:

The height differential between the upper and lower reservoirs – larger height differences allow for more potential energy storage.
Friction and turbulence losses in the piping and hydraulic systems.

Efficiency ratings of the pump/turbines and electrical components.
Evaporation losses from the surface of the reservoirs.

Ongoing improvements in pump/turbine designs, variable speed drives, and control systems have enabled higher round-trip efficiencies in modern pumped hydro plants (GreenBuildingAdvisor, 2022). However, the maximum potential efficiency is ultimately limited by the physics of pumping water uphill and letting it fall back down.

Efficiency vs. Other Storage

Compared to other energy storage technologies like lithium ion batteries, flywheels, and compressed air storage, pumped hydro has both advantages and disadvantages when it comes to efficiency. Lithium ion batteries can achieve roundtrip efficiencies over 90%, higher than the 70-85% seen with pumped hydro. However, batteries have shorter lifetimes and degrade with use, while pumped hydro facilities can operate for decades.

Flywheels can also reach up to 90% roundtrip efficiency, but they have much shorter discharge times measured in minutes or hours. Pumped hydro can store huge amounts of energy for days or weeks at a time. Compressed air energy storage, or CAES, has efficiencies of around 70-75%, comparable to pumped hydro. However, the storage capacity of CAES is limited compared to the potential gigawatt-hours of energy that can be stored via pumped hydro.

While no technology is perfect, pumped hydro offers a unique combination of high capacity, low self-discharge, and reasonable roundtrip efficiency. The decades-long lifespan of pumped hydro plants also makes them attractive long-term energy storage assets.

Sources:

https://iopscience.iop.org/article/10.1088/2516-1083/abeb5b

https://www.eesi.org/papers/view/energy-storage-2019

Improving Efficiency

Pumped hydro facilities aim for higher efficiencies to maximize the energy returned from the water pumped uphill. Here are some ways to improve the efficiency of pumped hydro storage:

Turbine and pump upgrades: Replacing older turbines and pumps with newer, more efficient models can increase the energy conversion efficiency during both generation and pumping cycles. Variable speed pumps that can adjust based on electricity rates can also improve efficiency.

Lowering friction losses: Friction of water in pipes and tunnels leads to lost energy. Improving cabling and pipelines with smoother materials reduces such losses. Keeping water velocities low also minimizes friction.

Advanced controls: Sophisticated control systems and variable frequency drives better optimize the entire pumped hydro process, leading to gains in efficiency.

Innovative designs: New pumped storage configurations like underground reservoirs, underwater reservoirs, and closed loop systems aim to increase efficiency through improved engineering. These may see greater adoption going forward.

Overall, both operational improvements and adoption of better technologies can increase the efficiency of pumped hydro facilities. With more real-world operating experience and R&D, efficiency levels are expected to continue rising.

Economic Viability

The economic viability of pumped hydro storage depends heavily on the costs to build and operate compared to revenue from electricity sales. Construction costs for pumped hydro can range from $1,000 to $3,000 per kW of capacity, with fixed operating costs around $20-30 per kW-year. This is generally cheaper than batteries on a per kWh storage basis, but batteries benefit from modularity and faster deployment times. Pumped hydro plants take 3-7 years to build while battery farms can be installed in months.

The profitability of pumped hydro is also closely tied to wholesale electricity prices, which fluctuate based on factors like demand, fuel costs, and availability of renewable energy. When electricity prices are high during peak hours, pumped storage can generate more revenue by storing off-peak energy and releasing it when prices spike. However, long periods of low electricity prices between peaks can reduce financial returns. Government incentives and capacity payments may improve the economics, but market conditions have a major impact.

Overall, pumped hydro storage requires extensive planning and favorable electricity pricing to achieve a reasonable return on investment. The long lifespan of plants (50+ years) helps improve the lifetime economics, but upfront capital requirements are high. Careful analysis of projected electricity costs and demand patterns is essential in determining project feasibility.

[1] https://www.sciencedirect.com/science/article/pii/S0196890413008236

[2] https://sustainenergyres.springeropen.com/articles/10.1186/s40807-018-0048-1

Environmental Impact

Pumped hydro storage can have significant effects on land usage and wildlife habitats. Traditional pumped hydro requires large reservoirs that can alter landscapes and disrupt local ecosystems. However, closed-loop pumped hydro has a smaller footprint and reduced environmental impacts.

According to a recent life cycle assessment, the land occupation of closed-loop pumped hydro was 0.43 m2-yrs/kWh compared to 18.8 m2-yrs/kWh for conventional hydroelectric dams. The study concluded closed-loop systems have lower impacts on land usage and human health versus other storage technologies like lithium-ion batteries (Simon 2023).

Pumped hydro facilities also contribute to greenhouse gas emissions indirectly through energy losses and construction, but at much lower levels than fossil fuel plants. One analysis found the life cycle GHG emissions of pumped hydro storage to be just 4.6 g CO2 eq/kWh, compared to over 1000 g CO2 eq/kWh for a natural gas power plant (Oyler 2020). Overall, pumped hydro can enable greater adoption of renewable energy and lower carbon electricity systems.

Future Outlook

With the growth of renewables like wind and solar power, the need for energy storage is expected to rise significantly in the coming years. Pumped hydro storage is well-positioned to meet a large portion of this demand due to its proven technology, cost-effectiveness, and ability to provide utility-scale long duration storage. According to the National Hydropower Association, pumped hydro storage capacity is projected to nearly double globally by 2040. Several major new projects have been proposed in regions like the western United States and Australia.

One potential challenge for pumped hydro is finding suitable sites with the right geographic features. However, recent innovations like offshore and underground pumped hydro may unlock new suitable sites and help drive continued growth. Overall, pumped hydro is likely to remain the dominant form of grid-scale energy storage for the foreseeable future given its technical and economic advantages.

Conclusion

In summary, pumped hydro storage can be a highly efficient form of energy storage, with real-world roundtrip efficiencies typically ranging from 70-85%. The key factors determining efficiency are turbine-generator performance, hydraulic losses, evaporation losses, and the pumping height difference. Overall, pumped hydro provides greater efficiency than battery storage, but lower efficiency levels than compressed air storage. However, pumped hydro benefits from a longer lifetime and the ability to store large capacities of energy.

Looking forward, pumped hydro will continue serving as the predominant form of grid-scale energy storage globally. Efficiency improvements are possible through upgrades to existing facilities and optimized design of new pumped hydro plants. However, the greatest barrier to expansion is siting challenges and high capital costs. With sufficient investment and planning, pumped hydro can play an important role in enabling higher penetrations of renewable energy and providing grid stability services.