19 hours ago, MigL said:

The main discriminator for the pumped/reservoir approach, as compared to the large flywheel approach, seems to be the efficiency of the pumps compared to the efficiency of the flywheel bearings.

The main practical discriminator is scale.

Given an amenable geography, hydro comfortably works with GW. Flywheels work with MW.

Ditto weight-lifting.

20 hours ago, MigL said:

Does anyone have relevant 'ballpark' figures, and how they scale ?

The efficiency figures for pumped hydro are clear enough. Large water pumps have a hydraulic efficiency ~85%, and Francis turbines (typical design for large units) can reach ~95% efficiency. Pipe and distribution losses add a couple of per cent each giving the overall round trip efficiency of ~75% I quoted earlier. I don't believe solar evaporation losses are a thing in Wales.

But bear in mind that the context is trading up low cost wind energy on a stormy night in November to high value peak load GW at very short notice on Christmas Eve with perhaps a ten-fold increase in billable revenue - and the efficiency only needs to be reasonable.

Availability at required scale is the ultimate measure.

Lifespan would also be an important factor.

What is the lifespan of pumped hydro storage?

With a lifespan of 70-100 years, pumped hydro assets also last five times longer than current battery systems. There are many operating examples of pumped hydro projects in Australia and around the world used for water supply and electricity, including the Shoalhaven Scheme in southern NSW.

An interesting discussion about why weights aren't widely used (gravity is weak) that also includes several other methods, including flywheels.------

"You can use dead weights, but you need a huge amount of weight.

For example the biggest pumped hydroelectric system in the world (the Gianelli Hydroelectric Plant in California, USA) uses water stored in a reservoir about 9 miles long by 5 miles wide, lifted through a height of about 300 feet. Even then, it can only supply about 5% of California's electricity usage for less than 2 weeks before running dry - and given the current long term droughts in California, it can't even do that, because there would be no water available to refill it.

Trying to build devices like this for individual homes would be hopelessly uneconomical."

https://physics.stackexchange.com/questions/305563/why-dont-we-use-weights-to-store-energy

12 hours ago, npts2020 said:

An interesting discussion about why weights aren't widely used (gravity is weak) that also includes several other methods, including flywheels.------

"You can use dead weights, but you need a huge amount of weight.

For example the biggest pumped hydroelectric system in the world (the Gianelli Hydroelectric Plant in California, USA) uses water stored in a reservoir about 9 miles long by 5 miles wide, lifted through a height of about 300 feet. Even then, it can only supply about 5% of California's electricity usage for less than 2 weeks before running dry - and given the current long term droughts in California, it can't even do that, because there would be no water available to refill it.

Trying to build devices like this for individual homes would be hopelessly uneconomical."

https://physics.stackexchange.com/questions/305563/why-dont-we-use-weights-to-store-energy

First and foremost your comments suggest you don't understand the difference between weight and momentum.

Secondly to the west of California there is by far the largest water reservoir in the world, called the Pacific Ocean. There is no requirement for pumped storage to be freshwater.

Thirdly energy extraction from flowing water for single properties has been going on since time immemorial. Nor does that energy have to be electrical. In the 15th century the city of Exeter installed
a pumped water supply, driven by a water wheel pump.

12 hours ago, studiot said:

First and foremost your comments suggest you don't understand the difference between weight and momentum.

Secondly to the west of California there is by far the largest water reservoir in the world, called the Pacific Ocean. There is no requirement for pumped storage to be freshwater.

Thirdly energy extraction from flowing water for single properties has been going on since time immemorial. Nor does that energy have to be electrical. In the 15th century the city of Exeter installed
a pumped water supply, driven by a water wheel pump.

Actually, I understand the difference between weight and momentum fairly well (I think you are ascribing the first sentence of the quote, which didn't get highlighted for some reason, to me). Are you telling me that for momentum to be useful for energy that it doesn't require great amounts of weight (mass if you wish to be technical) for a flywheel as well? I know how big and heavy steam turbines are and they don't produce much energy once you cut the steam off. The point is there is no easy way to store energy mechanically and other methods seem to require a bit more technology, that's why this discussion is even going on at this point in time. Even amongst the different types of storage there are many ways of extracting it for reuse and all have pros and cons depending on their location and use.

23 hours ago, studiot said:

First and foremost your comments suggest you don't understand the difference between weight and momentum.

Not sure where momentum entered the discussion. The relevant variables are mass (or weight) and height. The limit of extractable energy is mgh if it’s a static source.

23 hours ago, studiot said:

Secondly to the west of California there is by far the largest water reservoir in the world, called the Pacific Ocean. There is no requirement for pumped storage to be freshwater.

No, but it needs to then be a dedicated saltwater system that draws from and drains to the ocean. People are going to be perturbed if saltwater drains into their freshwater sources, which means leaks are more problematic. Corrosion means it’s probably got a shorter lifetime and more expensive, and the pipes need to go somewhere

1 hour ago, swansont said:

Not sure where momentum entered the discussion. The relevant variables are mass (or weight) and height. The limit of extractable energy is mgh if it’s a static source.

On 2/9/2026 at 11:46 PM, npts2020 said:

including flywheels.------

1 hour ago, swansont said:

No, but it needs to then be a dedicated saltwater system that draws from and drains to the ocean. People are going to be perturbed if saltwater drains into their freshwater sources, which means leaks are more problematic. Corrosion means it’s probably got a shorter lifetime and more expensive, and the pipes need to go somewhere

A pumped storage system has two reservoirs, the ocean could be the second one.

Alternatively California already has a dozen desalination plants, so the spent seawater could be desalinated.

https://www.waterboards.ca.gov/water_issues/programs/ocean/desalination/docs/170105_desal_map_existing.pdf

I guess hydroelectric could be categorized as "cloud storage."

😗

4 hours ago, studiot said:

A pumped storage system has two reservoirs, the ocean could be the second one.

Alternatively California already has a dozen desalination plants, so the spent seawater could be desalinated.

https://www.waterboards.ca.gov/water_issues/programs/ocean/desalination/docs/170105_desal_map_existing.pdf

Yes pumped storage needs two reservoirs - where do you put the upper one?

Desalination plants generally process less than a million cubic meters per day while pumped storage plants are generally much larger than that. The biggest saltwater pumped-storage plant is a 240 MW tidal generation facility.

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

“the 240 MW Rance tidal power station in France can partially work as a pumped-storage station. When high tides occur at off-peak hours, the turbines can be used to pump more seawater into the reservoir than the high tide would have naturally brought in. It is the only large-scale power plant of its kind.”

So yes, you can do it, but basically nobody does at large scale.

This 1887 presidential address (to the ICE) about renewable electricity might interest by George Barclay Bruce might interest some

1. He specifically spoke about tapping into the "unemployed forces of nature," mentioning wind, streams, and tides, and converting them into electrical energy.

2. Energy Storage: He highlighted the necessity of learning how to store this energy efficiently, a concept essential to modern renewable energy systems.

3. Historical Significance: His 1887 address is considered the world's first recorded reference to the renewable generation and storage of electricity. 

Edited February 11Feb 11 by studiot