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Comment by stingraycharles | Hacker News Reader

Gravity seems much more natural to me than heat; eg pumping water up a dam and releasing it again when you need the energy.

It strikes me that heat has the problem that it always loses energy in its “stable state” because the surrounding environment absorbs the heat, and gravity doesn’t have this problem.

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The issue of finding a location that has dramatic elevation change, a basin capable of storing vast amounts of water, and a suitable source/sink for pumping make pumped hydro difficult to deploy. Then there are additional logistics challenges such as environmental damage and proximity to human settlements for maintenance and engineering teams (prior challenges mean you don't really get to select the locations).

Heat sinks meanwhile can be built wherever you have a big rock by drilling some holes. Additionally, gravity definitely does lose stored energy in its "stable state", through evaporation and water entering the water table. Losses depend on geology and local climate, but it's not negligible.

Not to say that pumped hydro is a bad technology, it's just got it's own challenges and uses. It's most applicable in the form of electrical grid storage. But specifically on the scale of keeping towns and cities warm, heatsinks outperform almost across the board.

And, if you inundate an area of wilderness to create reservoir you have to also count the lost carbon capture of the growing plants, and the significant methane emissions of the now dead and rotting plant matter under the water. In dry deserts this may be negligible, but the mountains where people most want to install hydro projects are generally very forested.

The "vast basin" part of that is an exaggeration.

Look at https://www.whitepinepumpedstorage.com/ and the sizes of the upper and lower reservoirs there. This is to be a 8 GWh, 1 GW facility.

"vast" is a pretty non specific word so I can maybe see why you would think that? I feel like it's pretty obvious from context I'm not talking about an inland sea here. The quoted 5000 "acre-feet" in that project is a considerable amount of water for a man-made structure!

Either way, the details page[1] supports all of my above points. It even comments on the page how rare it is to find a suitable site like the one they've chosen.

[1]https://www.whitepinepumpedstorage.com/project-details

> This is to be a 8 GWh, 1 GW facility

That is big for time-shifting daily energy usage, but way too small for seasonal energy storage

Right. PHES is not a good fit for seasonal storage.

Pumped hydro is great, but the potential energy from gravity is actually really low. Stacking blocks will probably never work, and pumped hydro only works due to scale and existing geography. It’s also not modular and can’t be co-located with generation or loads unless the geography works out.

Terrament is working on a modular gravity storage solution that uses deep mine shafts to gain 20x more height than stacking blocks above ground. So you don’t need water or mountains. And since gravity storage uses ballast that is really just dumb weight, it could even be economical to make that ballast a secondary storage like thermal storage.

Pumped hydro is great, but only in certain areas where you already have the elevation gain available.

Compressed air storage is another one that's pretty good, but it's only particularly good if you can store it in underground caverns or unused mines, so it's also geography dependent.

For longer term storage, producing hydrogen can be a good one.

And batteries are actually fast becoming competitive with some of these options from the other end, generally better for shorter term storage but getting better at longer term. Even shorter term, flywheels can be a good option.

There's room for several different types of grid-scale energy storage, based on how efficient they are at different energy storage periods and numbers of charge-discharge cycles, and also in some cases on local conditions, like the availability of terrain and water sources for pumped hydro.

There's a good paper here which shows what the most efficient energy storage systems are for various combinations of length of storage (hours per discharge) against number of discharges per year, and for each one shows the current cost, and a predicted cost based on trends in technological advancements: https://www.sciencedirect.com/science/article/pii/S254243511...

There's a lot of the chart that is dominated by pumped hydro currently, but plenty of other storage technologies that are more cost effective on different timescales and numbers of discharges. But it looks like it's predicted for prices of battery storage and hydrogen storage to fall relative to the others, causing a different predicted landscape in 20 years.

And some of these, like pumped hydro, are dependent on geographic features, or access to certain resources, so even when one dominates overall, there can be others that dominate in particular geographical regions.

With water you lose some due to evaporation.

With a mineshaft gravity storage you could actually try to spin up a convective loop powered by geothermal that dries the sand over time, allowing you to lift dry sand when charging and abseil heavier wet sand when discharging.

That was my first thought too. But lakes lose ~20% / year to evaporation, and with the use of shade balls that is cut by ~90%, so we are at 2% / year - which is about the same as very efficient daily loss from heat storage.

If the heat is being turned back to electricity, a heat sink is needed and if this is done by evaporation the water loss will greatly exceed that of natural evaporation from a PHES facility.

It's not the evaporation per se that matters in the pumped hydro, it is the evaporation loss of water you already invested in pumping, so it is just an efficiency loss. The vapor loss for the heat sink is indirect, you usually just calculate the turbine efficiency. I don't know if the (generally much smaller) pumping requirements for turbines is already included in their efficiency calculations, but it would need to be of course.

Either way, it just goes to show further that pumped hydro would be more efficient, when and where it is feasible.

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