A capable and flexible technology

13 November 2020

Pumped storage hydropower has proven to be an ideal solution to the growing list of challenges faced by grid operators. As the transition to a clean energy future rapidly unfolds, this flexible technology will become even more important for a reliable, affordable and low carbon grid. Nicholas Troja and Samuel Law from the International Hydropower association explain more.

"Anything that can go wrong will go wrong". That old adage, Murphy’s law, must seem appropriate for many power grid operators in 2020. 

This year has tested the safe running and reliability of grids around the world like few others. Often termed ‘the biggest machine ever built,’ managing a power system, involving the coordination of complex and instantaneous interactions, is a formidable task at the best of times. 

With the impacts of the Covid-19 pandemic on top of extreme weather events, greater penetrations of variable renewables and increasingly aged thermal assets, the task has only become more demanding in many markets.

These challenges have brought into sharp focus the growing need for energy storage, such as that offered by pumped storage hydropower. 

Covid-19 continues to have an extraordinary impact on electricity markets. During the height of worldwide lockdowns, with large sections of the economy shutdown or greatly impaired, electricity demand declined by up to 30 per cent in some countries across Europe and in India.  

As Fatih Birol, Executive Director of the International Energy Agency (IEA) stated, the demand drop “fast forwarded some power systems 10 years into the future” regarding integrating higher percentages of variable renewable energy (VRE) which receive priority dispatch to the grid. Managing periods of such low demand can create “significant operational risks” for grid operators. In some markets, this has led to curtailing, or shutting down, wind and solar facilities to stabilise the grid.

During such periods, pumped storage hydropower, with its ability to both store and generate large quantities of energy over long periods, was the first port of call for those grid operators lucky enough to have such stations on hand. In Britain, its four pumped storage stations were hailed by the Financial Times newspaper as the “first line of defence in the battle to keep Britain’s lights on”. Able to increase system demand by pumping water back up to their upper reservoir, pumped storage is a more cost-effective way of managing the grid than paying operators to curtail variable supply. 

In August, the U.S. state of California experienced rolling blackouts for the first time since 2001 due to a combination of record heatwaves driving up demand, faltering gas-fired stations and a lack of dispatchable generation. As Stephen Berberich, President of the California Independent System Operator (CAISO) said, “we thought there would be adequate power to supply the demand…we were wrong” and the costs to the Californian economy will be significant.

These managed blackouts provide yet another wake-up call for policymakers on the need to appropriately plan for a zero-emissions future. With limited balancing resources such as pumped storage, California’s grid did not have the flexibility to shift sufficient generating capacity to the evenings when the sun had set yet the demand remained high. 

Given California’s aim of reaching 100 per cent clean electricity by 2045, mainly from wind and solar power which currently accounts for 20 per cent of generation, significant investment in flexible, low carbon balancing resources will be required. 

In response, California is betting big on batteries for short-duration storage, from sub-seconds to up to four hours, to manage intraday variations in net load. However, with those high levels of VRE on the grid, long-duration storage, which can discharge for 10 hours or more at rated power, will be needed to accommodate the seasonal patterns of VREs. It will do so by shifting generation over days, weeks and months of supply and demand imbalance. This is a story that rings true for many countries across the world with ambitious climate targets. 

Achieving California’s clean energy target is made even harder by the government’s decision to classify conventional hydropower stations greater than 30 MW as a non-renewable resource under its Renewables Portfolio Standard. This arbitrary classification is at odds with international consensus and penalises the state’s oldest source of affordable, flexible and low-carbon electricity.

Capabilities of pumped storage 

With a total installed capacity of nearly 160 GW, pumped storage currently accounts for over 94 per cent of both storage capacity and stored energy in grid scale applications globally. This has earned pumped storage its name as the world’s “water battery”. It is a mature and reliable technology capable of storing energy for daily or weekly cycles and up to months, as well as seasonal applications, depending on project scale and configurations.

Pumped storage operates by storing electricity in the form of gravitational potential energy through pumping water from a lower to an upper reservoir (see figure 1). The result of this simple solution is a very high round-trip efficiency of 80 per cent, which compares favourably to other storage technologies.

Figure 1: Illustration of a closed-loop (off-river) pumped storage station and how it can be used support VRE.

Pumped storage tends to have high energy-to-power ratios and is well suited to provide long discharge durations at very low energy storage costs. Across different timescales, pumped storage can serve multiple functions (see figure 2). For example, at shorter discharge durations, it is suitable for ancillary services such as frequency balancing and back-up reserve. 

With four to eight hours of discharge, it can provide daily shifting for day-night energy arbitrage. For longer durations over 10 hours, it can accommodate multi-day supply profile changes, reduce energy curtailment, replace peak generation capacity and provide transmission benefits.

The majority of today’s pumped storage stations were built some forty years ago. Yet, they are still providing vital services to our power systems today. With occasional refurbishment, these long-term assets can last for many decades to come. 

Despite being a mature technology, the resurgence of interest in pumped storage has brought forth numerous new R&D initiatives. One prominent example is the European Commission’s four-year XFLEX HYDRO project, which aims to develop new technological solutions to enhance hydropower's flexibility. Latest innovations, such as variable speed turbines and smart digital operating systems, will be tested on a range of pumped storage demonstration sites.

While often thought of as geographically constrained, recent studies have identified vast technical potential for pumped storage development worldwide. Research by the Australian National University highlighted over 600,000 potential sites for low-impact off-river pumped storage development, including locations in California. There is also growing interest in retrofitting pumped storage at disused mines, underground caverns, non-powered dams and reservoir hydropower stations.

Figure 2: The plot above visualises (logarithmic scale used) the estimated discharge durations relative to installed capacity and energy storage capacity for some 250 pumped storage stations currently in operation, based on information from IHA’s Pumped Storage Tracking Tool. The vast majority of pumped storage stations have a discharge duration longer than 6 hours, and some are capable of seasonal storage.                              

A clean, affordable and secure transition 

California is a pioneer in the energy transition. Though many opponents of wind and solar have unfortunately used the blackouts as an example of why their rapid roll-out is a threat to a secure, reliable grid. As noted earlier, the blackouts were not due to too much VRE capacity being on the grid, but a lack of integrated planning to support an evolving electricity mix with sufficient dispatchable generation and storage. 

The IEA recently stated that, dispatchable pumped storage, along with conventional hydropower, is the often overlooked workhorse of flexibility. However, its development, like many energy storage technologies, is currently being hampered by the lack of appropriate regulatory frameworks and market signals to reward its contribution to the grid. Outside China, year-on-year installed capacity growth has been anaemic at just 1.5 per cent since 2014 (see figure 3). 

Figure 3: Global pumped storage installed capacity by region. Note that 2019 recorded the lowest growth in pumped storage capacity for over a decade, with only 304 MW added. Source: IHA’s database.

Given the technology’s long lead times, investment decisions are needed urgently to ensure that pumped storage, in conjunction with other low-carbon flexibility options, are available to grid operators without needing to rely on carbon-intensive gas-fired generation as a backup. This is especially important as VRE penetration reaches increasingly high levels not yet experienced on a regular basis. 

IHA is continuing to work across the hydropower sector and is seeking to learn lessons from other sectors to support the development and deployment of pumped storage. Together with national authorities and multilateral development banks, we are developing a new global initiative to shape and enhance the role of the technology in future power systems. 



Nick Troja is a Senior Hydropower Sector Analyst at IHA. His work focuses on building and sharing knowledge on global hydropower, including identifying trends in project financing, policies and market dynamics. Email: [email protected]

Samuel Law is a Hydropower Sector Analyst. His work focuses on building and sharing knowledge on sustainable hydropower development, working on topics such as clean energy systems, green financing mechanisms and regional hydropower development. Email: [email protected]


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