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Gravity Energy Storage

2 points by ncmncm ↗ HN
Seemingly everyone who talks about storage for intermittent renewable energy goes straight for batteries. But today most utility storage uses gravity, because gravity storage is cheaper. That is unlikely to change. (There will be reasons to use a mix of other storage in many places, too, even when it costs more.)

There are many different kinds of gravity storage, most dead simple [*]. Transmission lines mean it doesn't all need to be right nearby. Most scale up or down as needed.

Pumped hydro, unlike hydro power generation, doesn't need a watershed. Any hilltop depression will do, or a box canyon with a dam, or even a flat hilltop with a dike. Sea water is fine, if your turbine and pump are OK with it. But most existing pumped hydro uses a hydro-power reservoir.

The world is absolutely perforated with disused vertical mine shafts. Hang a 10,000t weight from a motor / generator winch; raise to store energy, lower to extract it. It starts up instantly on demand.

Anchor a bag to the sea floor, with a hose down to it. Pump air in, inflating it, and let it out through a turbine. The deeper it is, the more energy it stores per unit of air pumped in. Valves switch your pump and turbine among as many hoses as you have. Startup takes only as long as to spin up your turbine.

Run a cable from a float down through a sea-floor pulley and up to a motor / generator winch on shore. Store energy by cranking the float down toward the pulley. The winch can be shared among as many cable reels as you have, with a simple clutch/brake. Startup is instant. A 10m diameter float exerts >500t; 20m >4000t, 30m >14000t.

If it's so easy, why don't we have a lot of this already? Building much storage without enough non-fossil generating capacity to charge it from would be stupid; the money is better spent building out generating capacity. So, after we have lots, then build storage. But storage can usefully be charged from existing nukes or geo, for load-following; we mostly use pumped hydro for that now.

[*] except Energy Vault (NRGV), a $2B market-cap investment scam. Lately they are promoting a condominium for concrete blocks, with elevators. I am not joking!

9 comments

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ggm
People are obsessed with losses. Systemic losses across the pump in and output phase. I think this is typically allowing a second order effect to override the first order benefit, but there you are: "the losses are too high" is a low bar hall pass out.

What they forget is that the energy input in question is often otherwise being wasted. System loss against throwing it away is not such a bad sum.

There is also a return on capital side of this: spend $100m on Tesla batteries, earn $70m on frequency stability services a year, you're in positive return inside a year on your capex. Invest the same certifying a new pumped hydro, you've barely scratched the surface and you can't earn that sweet FCAS income.

You're not wrong. I will say anything with seawater is a massive turn off. No engineer likes salt water. But that's a minor issue to your underlying point: we're missing cheap, low capital effective storage and gravity based systems because of (in my opinion) second order cost and loss arguments.

Don't hate on the cement block condo btw. I believe the more ugly "crane and bucket model" is in test.

ncmncm
Cement block condo is super-expensive, but could in principle work (for a short time) if anybody was dumb enough to buy one. The crane thing won't even work if there is any wind at all. But there is always wind. (Maybe they could build a dome over it?)

Losses are almost irrelevant when top-line generation is so cheap. So, even though round-trip efficiency of hydrogen is low, it doesn't matter; you just add a few more panels. I.e., losses increase capex but not opex.

aurizon
Here in Canada we have a high ratio of uneducated fools in government, as is the case when elections are like a grade 10 popularity contest. Governmental elected officials, by and large, have near zero technical minds. What this means is the can not tell the difference between a good idea and a bad one. They flock to the confidence men, they have seen the enemy and he is us, (Pogo) Most of your gravity methods are workable. Deep sea compressed air looks like an efficiency loser. The towers and cement weights seem like winners, but concrete is both brittle and expensive. Make it with cemented earth(5% cement 95% earth/sand) and it is cheaper - but fragile. Make a steel container - ($$) Your mine will hold a weight shaped like a shaft - as long as you want. What about mine water tables? 95% of Canada's mines need 24/7 pumping = flood to near surface all year. Some desert areas might work as deep hole, but there are many many deserts underlaid by aquifers down a few hundred feet = limit of use of gravity. We do not have much already = stupid politicians, if it not done in my term so I can keep the con going, I will not support it. Look at US bridges that fall down a lot.
ncmncm
Mine water is not a problem. With an iron weight, water just reduces total MWh capacity by up to 13%. So, you just add 13% more iron.

The crane and blocks thing obviously will not work. Multiple YT vids explain why, if not clear.

Efficiency is not especially important when generation capacity is cheap. Often something else matters more.

Government people can get along by hiring a engineering consultant to lay out costs, and picking the cheapest alternative.

aurizon
A weight that sinks = granite is a lot cheaper than iron. There is an ecology of governments and consultants designed to weed out those that do not bribe.
ncmncm
Scrap iron is very cheap. But if you use granite, the flooded penalty is 20% instead of 13%. Not a big deal. Could use lead, instead, flooded penalty 9%.

The choice of material for the weight is just about which is cheapest.

aurizon
True, local rock is cheapest - but the structure that you lift has to with stand tens of thousands of haul-up/lower-down cycles = standard blocks with enduring quality. Scrap iron does not pack well, unless melted into standard blocks = $200/tonne or more. Crushed granite and various sizes of graded rock (the MacAdam principal) with close to 5% of voids to which you add 5-7% portland cement, as well as a few rebars and cemented in attachment lugs is what these block are made of. They will have a rebar pattern that prevents fragmentation in there are any cracks over time. AS the cement cures, the cement re=hydrates and creates a void free very strong in compression block. With machine pick-up and place-down that carefully avoids crack-making impacts they can survive tens of thousands of trips - which will probably be 1-2 times/day - as power is needed. A water filled mineshaft would work, but most vertical shafts are 8-10 feet wide and perhaps 16-20 long, as they were usually made with 2 elevators - 1 goes up as 1 goes down. For this application they could fit the shaft. Water might erode and places with bad mix. These shafts can be several thousand feet deep - varies. These built on land pyramids are far wider/deeper = lower height needed for same storage as a deep mine limited in width. Most large mines now use 'trackless mining' = a long spiral highway 30 feet wide and 20 feet high going down at an angle, with rock hauled up by electric carts(huge ones) https://www.google.com/search?q=trackless+mining+wiki&rlz=1C...

I have not seen how one of these surface ones operate, especially with wind/winter and how many of these cement blocks they use get cracked/dropped.

Can you imagine an attachment failure at the worst time knocking down multiple blocks? Recovering from that would take days/weeks and the actual occurrence might halt things for a while while regulators sniffed into everything, esp if there were any lives lost?

ncmncm
We have a name for stacked blocks without mortar, after an earthquake or even a heavy storm.

That word is "ruins". They may be picturesque, in the distance.

aurizon
yes, in the UK an old country 'pile' often well kept - some not...