This massive storage battery is rechargeable indefinitely and involves no mined lithium or other chemistry.
Photo credit: K3-sport s.r.o.
By Gale A. Kirking, CFA, MBA
Editor-in-Chief at BlueGreen
It is a demanding but exhilarating hike up Dlouhé stráně mountain to the imposing reservoir of the pumped hydro energy storage (PHES) plant that Czechs polled in 2005 ranked among their country’s greatest wonders. The zig-zagging climb is around 10 kilometers (6.2 miles) and rises about 900 meters (m) from the valley-bottom village of Loučna nad Desnou.
Strong wind sweeps continuously across the levelled mountaintop (altitude 1,353 m a.s.l.), but today the water’s surface on the half-filled reservoir scarcely ripples. The exterior calm belies the fact that, beneath this 2.72 million m3 human-made basin, there is great power in this mountain. Set deep in the massif, some 511 m below, two turbines with combined nominal capacity of 870,000 horsepower generate 650 MW of electricity.
Reversing turbines deep in Dlouhé stráně mountain are driven by gravity-powered water from a reservoir at the peak. Photo credit: K3-sport s.r.o.
The Dlouhé stráně scheme is the most powerful hydroelectric plant in the Czech Republic and the largest “reverse turbine” power unit in Europe. Its upper reservoir is a giant, mountaintop storage battery, serving both to stockpile energy and to help “balance” the electricity load on the Czech Republic’s power grid when other generating sources are producing more or less energy than what consumers are drawing off the system. Most simply put, the Dlouhé stráně complex consists of two water reservoirs (one at the top of the mountain, one in the valley), two power-generating turbines buried in the lower mountain, which reverse to become massively powerful pumps, and two tubes, each 3.6 m in diameter and about 1,500 m long, connecting the top and bottom reservoirs via the turbine.
When excess electricity needs to be drawn off the grid (such as on a very sunny day or when energy demand is low), the turbine goes into pump mode and sends water streaming from the bottom reservoir to the top. Later, such as when the sun goes down and millions of Czechs are fixing dinner and watching television, the water will be allowed to rush down, spinning the turbines to produce electricity. This mammoth battery can be discharged and recharged indefinitely. Its water content rises and falls during a typical day by more than 20 m.
Only the reservoirs are visible today, nearly three decades after the construction’s completion. The mountain is quiet and its forest covering appears outwardly undisturbed.
Conceptually, PHES is as simple as an apple falling on Isaac Newton’s head, but its design is a creative engineer’s dream work. Image credit: Australian Renewable Energy Agency.
Hundreds of thousands of new solar panels necessitate more PHES
In addition to Dlouhé stráně, there are two smaller PHES schemes in the Czech Republic today. More such plants will be needed and built in the future, because the storage they provide is critical to solar-supplied grids and, in recent years, tens of thousands of new solar panels have been going up weekly across the Czech Republic.
The importance of such grid-level storage was demonstrated on Easter Monday in 2023. It was a sunny holiday, so people were out and about or at their weekend cottages. Electricity demand was therefore temporarily low. Because the country’s two nuclear and diverse other power plants were producing more than was being drawn off the system, the country’s electricity transmission company disconnected hundreds of solar plants from the grid. Such a waste! Had there been more PHES capacity available the electric company could have just turned on more pumps to send water to the tops of hills and mountains.
Why we are on top this mountain
But let me backtrack to explain what brought me to the top of Dlouhé stráně. In addition to the fact that I like to hike, it is because I have more than a passing interest in renewable energy and electricity storage.
I’ll be straight with you: I am a big proponent of solar energy development. Also, I am an unapologetically outspoken opponent of growing corn (in the United States) and oil rapeseed (in the European Union) to fuel our cars, trucks, and tractors. Moreover, I am unenthusiastic about the use of lithium batteries for grid-scale and even at-home (so-called “behind-the-meter”) energy storage. Electric car batteries today also require lithium, of course, but I do prefer them over burning corn and rapeseed oil (known as canola oil in the U.S.), the growing of which is inherently damaging to the soil and environment and not in the least sustainable.
I regard PHES as the most promising and important grid-scale energy storage technology in existence. Hands down.
Oh yes, and I’m no fan of nuclear power, either. In the Czech Republic, where I primarily live (although I reside part of the time in my native U.S.), we have two nuclear power plants operating 6 reactors and there is never-ending talk of building more. My opinion is that lots more solar plus additional PHES would be a better solution than added nuclear.
I regard PHES as the most promising and important grid-scale energy storage technology in existence. Hands down. Conceptually, pumped water storage beautifully complements the virtually unlimited global capacity to generate massive quantities of solar-source energy. We as a society have barely begun tapping into that solar potential.
PHES: A well-kept secret in plain sight
Although it is a safe bet to say that most people have never heard of pumped hydro energy storage, PHES is in fact a well-kept secret hiding more or less in plain sight. Greater than 90% of global grid-level electricity storage today is in the form of pumped hydro. That’s according to a recent article in pv magazine written by professors Ricardo Rüther at Federal University of Catarina and Andrew Blakers at The Australian National University) https://www.pv-magazine.com/2024/10/08/energy-storage-is-a-solved-problem/). I’ll come back to them in a minute…
But first, let me admit that I got a little excited when a few months ago I read in the Czech newspapers that the ministries of environment and agriculture, respectively, had identified six and two sites particularly suitable for PHES development. To learn more, I reached out to Petr Hladík, who is Minister of the Environment in the Czech Republic.
“(W)e want to strengthen pumped storage electric plants. These can deliver electricity to the grid within 1 minute in case of an acute need.”
– Petr Hladík, Minister of the Environment
“In addition to developing battery energy storage,” Hladík told me, “we want to strengthen pumped storage electric plants. These can deliver electricity to the grid within 1 minute in case of an acute need. They are therefore one of the most advantageous types of electricity storage facilities. At the Ministry of Environment, we have been intensively identifying sites where pumped storage plants could be built. From a list of several dozen, we have prepared this list of six suitable sites that make sense in terms of capacity and, most importantly, that do not conflict with nature conservation.”
In order to minimize conflicts, the environmental minister noted, each of the listed six is a site where a lower water reservoir already exists, such as where a river is dammed to create an existing riverine hydropower plant. That way, only the upper reservoir, water flow, and additional turbine generators would need to be built. These sites, Hladík said, are “significantly more environmentally and aesthetically friendly, lower-cost, and faster to build than new reservoirs. With an installed capacity of 1,222 megawatts (MW), these six sites have the potential to double the country’s current capacity of pumped storage hydroelectric power plants.”
And what about repurposing coal mines?
There are several large open-pit brown coal mines in the Czech Republic targeted for closure. I asked Hladík if some of these might be used for PHES.
“That is possible,” he noted, “but there are other aspects that need to be examined, such as the conflict with nature protection in the Krušné hory (the “Ore Mountains” that straddle the Czech and German border) in connection with constructing an upper reservoir. Moreover, recreational use of the lakes created after mining is envisaged, so other conditions need to be examined and stipulations agreed to ensure that the two activities would be compatible.”
Click below to see a short video of how a former gold mine is being transformed to create a pumped hydro plant.
Then there’s the NIMBY (not-in-my-backyard) factor
Of course, presenting potential sites for PHES is one thing, building them is quite a number of steps further down the line. Whether it’s a proposal for windmills, solar farms, or any kind of infrastructure, there is always the NIMBY (not-in-my-backyard) factor.
Let’s take for example one of the six sites prioritized by Hladík’s ministry. In this case, the PHES would be built in South Bohemia at the existing Orlík water reservoir, where a hydroelectric dam across the Vltava River creates the country’s largest artificial lake. This answers the lower reservoir question. The upper reservoir would sit atop the nearby Pteč Hill. The two reservoirs would be connected by two pipes, each 4.5 m in diameter, via four turbines, each with an output of 110 MW. (By comparison the hydroelectric dam itself also has four turbines, each with a nominal capacity of just 91 MW.)
“This is an unpleasant surprise for us,” noted, David Kaiser, the mayor of Bukovany, to a reporter for the newspaper, Hospodářské noviny, upon learning of the proposal. His village of just over 100 residents lays at the foot of the 633-meter Pteč Hill. “We have never heard about the possibility of building a pumped hydroelectric power plant here,” Kaiser continued. “We certainly do not agree with that intention.”
“Personally, I will certainly be against this plan,” added Markéta Mazourová, mayor of Zbenice, another tiny village below Pteč. Mazourová told Hospodářské noviny’s reporter she feared the PHES project could interfere with the local springs that provide municipal drinking water.
Villages and villagers in South Bohemia probably have good reason to be nervous when people in big and shiny cars from the government and/or electric company show up with ideas about energy projects. Bukovany and Zbenice are just about 50 kilometers (ca 30 miles) down the Vltava from the village of Temelín, which sits nearly in the shadow of the country’s largest nuclear power plant. The building of that plant, conceived by the Communists in the 1970s, begun in 1981, and finally completed with gargantuan cost overruns in 2003, wiped a half-dozen villages off the map. It is still controversial, and yet many in the government would like to construct additional reactors at Temelín.
Obviously, there is a huge difference between a PHES facility and a nuclear power plant, but environmental matters and people’s concerns must be duly considered. Minister Hladík has noted that all of this is still at an early stage, an environmental impact assessment has yet to be made, and there is not yet a final decision on whether or not it will be built.
Height matters, so does time of day
To the non-engineer, it might seem there should be a simpler way. In the case of an existing hydroelectric dam, for example, why not just pump the water from below the dam back up over the top of the dam, let it collect in the reservoir, then run through the dam again according to need? Well, the elevation difference between the turbine and the upper reservoir is important. That difference is termed “head” and the higher the upper reservoir the better.
Head should ideally be in the range of 500–1,500 meters, I learned from Andrew Blakers, the aforementioned professor in Australia. A rule of thumb, he explains is that doubling the head doubles the energy and power produced but it does not double the cost. So, the higher the upper reservoir, the more bang for the invested buck (or euro or crown).
Another issue that comes to mind is that pumping all that water to the top of a mountain doesn’t come for free, either. How much energy is lost that way and what does that cost?
So, we’ll get to that, but the first thing to know is that cost isn’t everything. Electricity demand and its price are important, too, and this largely is about timing differences. Power is typically most expensive when the sun is going down, people are preparing dinner, and they’ve got the television switched on. It also can be pricey when folks are getting up in the morning before the sun and are switching on the coffeemaker, toaster, and microwave.
If we recall that sunny Easter Monday of 2023 in the Czech Republic, the price of electricity on the grid actually went negative that day. The transmission company stood ready to pay somebody, say in a neighboring country, just to take it. Now, that would not have been the case had the system been able to pump more water to the peaks of Dlouhé stráně and Pteč.
To get at the efficiency question, I read a summary of the feasibility study for an Australian PHES scheme known as Snowy 2.0 that is under construction. The designed so-called “cycle losses” are 24%, meaning that for every 100 Wh of electricity used to pump water up 76 Wh of transmissible energy is captured in the end. Professor Blakers points out that water at Snowy 2.0 flows through a tunnel 27 kilometers (17 miles) long, and that produces friction losses. Most PHES systems have much shorter tunnels and cycle losses of 15–20%. Moreover, he notes, batteries have cycle losses of 10–15%.
By the way, Snowy 2.0 (https://www.snowyhydro.com.au/snowy-20/about/) is designed to generate 2 gigawatts (GW) with all turbines running and store up to 350 gigawatt-hours (GWh) of electricity. That means it can generate flat out starting from a full reservoir for an entire week. This is equivalent to the usable fraction of energy storage in about 6 million electric vehicles. The cost to build Snowy 2.0 is expected to come in at around US$8 billion, which corresponds to about US$1,300 per electric vehicle battery equivalent – and it will last for 100 years of more. That makes this engineering megaproject three times as powerful as Dlouhé stráně and able to store about 95 times more electricity. It is not the biggest in the world, however. That bragging right goes to China’s just opened 3.6 GW Fengning PHES. The second-largest is the 3.0 GW PHES in the U.S. state of Virginia. An interesting scheme in the U.S. now under construction is the Lewis Ridge project created in a former open-pit coal mine in Kentucky. It promises to bring clean energy production and jobs to a region where the coal industry is declining.
Potential PHES locations number in the thousands
Although the basic principles of PHES are universal and simple as the mythical apple falling on Isaac Newton’s head, the field of PHES is a paradise for creative and visionary engineers. There are numerous possibilities for how pumped storage can be designed and built, and each scheme is in fact unique. It turns out that there are, from an engineering viewpoint, thousands of places where PHES can be built.
Professor Blakers leads a team at The Australian National University that, using geographic information system (GIS) technology, has surveyed the entire globe and identified about 820,000 “potentially feasible” sites around the planet where PHES might be built. Together, those sites have storage potential of 86 million gigawatt-hours (GWh). Data on those sites are publicly available through the group’s Pumped Hydro Energy Storage Atlases project (https://re100.eng.anu.edu.au/pumped_hydro_atlas/).
Some potentially feasible greenfield sites around the planet where PHES might be built. From the Pumped Hydro Energy Storage Atlases, maintained by the 100% Renewable Energy Group at The Australian National University.
Now, only a tiny fraction of those potential sites would ever be developed, but that’s okay because 86 million GWh is a huge amount of electricity. Working with data from the International Energy Agency, I calculated that’s about 1,000 times the daily electricity output generated for the entire planet as of 2023. The Australian team says it’s about 200 times greater than what would be needed to support a global electricity system 100% based on renewable energy.
More good news, is that PHES need not be built solely on greenfield sites. Blakers’ team classifies PHES sites globally into four types: 1) Greenfield sites would require two new reservoirs and a (system) build from scratch. 2) Bluefield sites are those utilizing at least one existing reservoir. 3) Brownfield sites exist where mining locations can be repurposed for PHES reservoirs. 4) Ocean sites are places where the ocean can be used as the lower reservoir.
Extraordinarily good PHES sites at extraordinarily low capital costs
“This is equivalent to the effective storage in about 2,000 billion electric vehicles,” Rüther and Blakers write in their recent pv magazine article of the Atlases-listed sites, “which is far more storage than the world will ever need… Because there is a large surplus of sites in most regions, only the very best sites need ever be developed. Importantly, PHES are capital-intensive investments but have far longer expected lifetimes than batteries. Extraordinarily good PHES sites can be found in most regions of the world, with extraordinarily low capital costs.”
In Europe, Rüther and Blakers point out, the Balkan countries have huge PHES potential and could in theory provide far more storage than the European Union as a whole would ever need.
An emeritus professor of renewable energy engineering, Blakers has been working on solar energy and related technologies since 1979. He felt already more than 4 decades ago that solar energy would one day be the main source of energy for the world. This, he notes, has turned out to be precisely the case.
“There’s been continuous, rapid, exponential growth for 40 or 50 years, and now solar is the dominant form of new generation capacity,” Blakers relates. “We are showing that very large scale and rapid deployment of solar and wind does not lead to technical problems but instead leads to very low cost, very stable electricity systems. And when we electrify transport, heating and industry we can readily get rid of all fossil fuels over the next 15 years.”
It comes down to engineering
My explorations of PHES have reconfirmed to my satisfaction that solving renewable energy is largely a large set of routine engineering problems. Of course, there are important environmental, political, and social issues to resolve. The biggest hurdles are in people’s minds and public attitudes, because human beings have difficulty accepting change and understanding that tradeoffs can and need to be made.
Ultimately, we need more solar. We need smarter grids to connect and integrate all points of the electrical system from the outlet in the kitchen to the turbine at Dlouhé stráně and all those in between. We need wise public energy policy that is not constrained by uninformed thinking and by politicians inciting public fear of the new and misunderstood only to swell their own power, bank accounts, and control.
In my opinion, because we care about global warming, the environment, and one another, we need to speak out against foolishness like growing food crops to replace fossil fuels, to advocate for more solar and other truly sustainable energy sources, and to call for more investment in pumped hydro energy storage.
Gale A. Kirking is the founder of BlueGreen, a communications and consulting boutique created to help businesses and other organizations in formulating sustainability documents, designing and creating SDGs communications, and maximizing their sustainability impacts. He is a former journalist, a Chartered Financial Analyst, and an investment professional with a strong orientation to responsible investing. A United States citizen, Kirking has run a specialty communications business in Central Europe for more than 20 years. Unless otherwise explicitly indicated, the opinions expressed in this essay are his own.