Very. One of the first was in Scotland. Great to visit as you drive into the mountain and I think one of the Bond files had a scene filmed in it

https://en.wikipedia.org/wiki/Cruachan_Power_Station

Nope. It was filmed in Switzerland. https://en.wikipedia.org/wiki/Contra_Dam

Different films. "The World Is Not Enough" had scenes filmed at Cruachan, "GoldenEye" was at Contra.

Whoa! That sounds really cool so I looked it up. It's called "Pumped-storage hydroelectricity"

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

Yeah, and I believe that it's the only game in town for storing really large amounts of electricity.

Also interesting: https://www.quora.com/Can-electricity-be-stored

It's very efficient and it's a great solution overall; the only problem is there are only so many dams we can build, since each one requires drowning a valley.

Though you need less volume if it is only being used for overnight storage rather than long term rainfall storage. For example, to store enough energy to produce 1GW for 1 day (~10^5 seconds) with a head of 100m, use Pt=mgh to work out you need to store:

m = 10^9.10^5 / (10*100) = 10^11 kg of water. = 10^8 m^3

This is not a big reserviour, being 1/20th of the size of Warragamba dam's reserviour (in Sydney). One valley could provide 20GW for a day. Compare that with demand for where I live:

http://www.aemo.com.au/Electricity/Data/Price-and-Demand/Pri...

Peak demand for 7.5M people is 9GW, with an average closer to 7GW. One valley could provide overnight storage for over 20 million people, with first world demands (all of Australia). The limitation is transmission and generating/pumping capacity, rather than storage volume.

Note that's a winter peak you're looking at.

The highest summer peak appears to have been just above 13GW. http://www.wattclarity.com.au/2016/02/highest-electricity-de...

However your point still stands I believe.

But the summer peak is due to air conditioning during the day, when there's tons of "free" solar energy.

Solution: swimming pool on the roof.

http://www.fastcodesign.com/1672202/you-deserve-a-house-with...

The more sun, the more water in the pool. Win-win!

Seriously though has anyone crunched the numbers on rainwater storage for microgeneration in a domestic setting.

Could I efficiently use excess solar to pump water in to a house roof storage area and generate electric from that?

They crunch the numbers here: http://physics.ucsd.edu/do-the-math/2011/11/pump-up-the-stor...

The energy in one gallon of gasoline is equivalent to the potential energy stored in 13 tons of water one kilometer up

The Tesla Powerwall has 6.5 kwh = 23.4 M joules.

Gravity potential energy = mgh. Suppose pool is 10 meters up, then you need 239,000 kg of water = 63K gallons or 8400 cu ft. This is a huge pool: 29 ft. x 29 ft x 10 ft.

You could use the city water supply: borrow water through a turbine at night, and return it during the day :-)

Or have pool up very high, for example on a cliff overlooking the Mediterranean:

http://inhabitat.com/crazy-home-carved-into-a-coastal-cliff-...

The outflow from an Auckland homes storm water is not insignificant - an average roof would have something around 5 cubic metres of water come off it several times per year.

I remember a while ago on the show "Let's Get Inventing" (Saturday morning kids show with kids inventing stuff, think it was on TV2), they tried generating electricity from drainpipes, they couldn't get enough power to even power a servo squeezing the trigger on a spraybottle, they had to get a great big firehose from an firetruck to actually generate anything.

"Have you seen the swimming pool on the roof?"

;)

Flywheels are another option: https://en.wikipedia.org/wiki/Flywheel_energy_storage

Pumped storage is routinely used to store energy. It's not theoretical.

Has Flywheel storage every been seriously used in large-scale commercial applications? Someone mentions it on every energy-storage thread, and I'd like to know if the people suggesting it are just spinning our wheels.

FES is used in non-kill-zone habitable solutions like datacenters, bottoms of passenger trams/hybrid buses and electromagnetic aircraft catapults. The historical issue with FES is unscheduled, rapid disassembly, but carbon/kevlar composites have been shown to have greater integrity with least unsprung structural mass (because you want the most mass as close to the rim as possible to ensure highest moment of inertia.)

FES has 10x specific energy than supercapacitors or batteries with no memory and precise state of charge. Magnetic bearings and hard vacuums make them quite efficient.

The JET fusion reactor in Oxfordshire uses flywheels to provide very large pulses of electricity for short periods of time (a minute or two iirc). The reactor requires too much energy to run directly off the National Grid.

As I recall from a tour I went on of jet 30 years ago there are a large bank of flywheels which spin up over several minutes, these are then rapidly discharged to a bank of capacitors which are then used to power the experiment, I believe the duty cycle was 9 minutes. [1][2]

I believe some electric cars used capacitors to store regenerated electricity from breaking.

"More Ampères please Mister Woodbine." - Road to Welville

[1] - http://pia.sagepub.com/content/200/2/95.abstract

[2] - http://www.euro-fusionscipub.org/wp-content/uploads/2014/11/...

Sounds like they use it for high output rather than efficiency though. Still interesting that it has real world applications though. Shows that the technology has potential as well.

It's not even remotely on the same scale. There are three massive utility-level flywheel deployments according to Wikipedia that provide 15 minutes of output.

Edit: Seems the purpose is purely frequency regulation, rather than storage per se.

Better than nothing, but not that efficient. I took the tour of the somewhat mis-named Dinorwig Power Station (really a pumped storage facility) in Snowdonia, Wales. The figures they quoted there put it at about 75% efficiency. It's not really supposed to be for 24-hour power smoothing, more for rapid response to sudden demand; it's a lot faster to open a valve on a turbine than to stoke up a coal plant, and for some spikes (such as the example they quoted on the tour, when everyone puts the kettle on when Coronation Street finishes) you have a 30-second window or so. However, as demand has increased it's being used more and more, and the more it's used the more power we're wasting.

Interestingly, it does not use a dam. There's a natural lake at the bottom of a mountain and a man-made reservoir at the top, and the entire pumping facility is completely inside the mountain; it's not at all obvious that there's anything there at all from the outside. I think about that whenever people moan about the environmental impact, or flooding valleys.

Works great if you have the right geography.

I thought I read that some water districts run their pumps at reduced volume during peak hours and top them off before or after. It's a smaller volume but it's already a solved problem.

And it allows you to push some levers and dials as to whether it's cheaper this year to increase your water capacity or your power capacity. City planners love that kind of thing.

I grew up by one of these in NJ: https://en.wikipedia.org/wiki/Yards_Creek_Generating_Station

I remember parking at the lower reservoir and hiking to the upper one and being amazed at how big the water pipe is.