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DanArmak comments on Stupid Questions May 2015 - Less Wrong Discussion
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Actually, this scheme is currently employed by utilities, albeit usually not with seawater. The technique is called pumped storage hydro. Pumped storage hydro accounts for the vast majority of grid energy storage world-wide. Pumped storage hydro is used to by power companies to achieve various goals, e.g.:
flatten out load variations (as you suggested elsewhere in this thread)
provide "instant-on" reserve generation for voltage and frequency support
level out the fluctuating output of intermittent energy sources such as wind and solar (as you suggested above)
Wikipedia states that round-trip efficiency of pumped storage hydro can range between 70% and 87%, making it an economical solution in many cases.
A couple of obstacles to using pumped storage hydro are:
Certain topological/geographic features are needed to make PSH viable
Social and ecological concerns
Yes! Thank you.
I wonder why there aren't more of them, or bigger ones. The only seaside-cliff one listed on Wikipedia is the Okinawa Yanbaru station, completed in 1999, which only provides 30 MW.
Apparently the cost/demand situation isn't favorable.
Yes, it seems like using a seaside cliff would have several advantages over a freshwater solution, not the least of which is an unlimited water supply in the lower reservoir.
I guess the problem is scale, after all. I'm quite bad at physical calculations, so the below may be wrong.
Even a small hydroelectric dam generates gigawatts of power. Assuming a 30 meter tall cliff, each cubic meter of water generates 294 kJ when descending. To produce 1 GW of power, we would need 1,000,000/294=3400 cubic meters of water descending every second (watt = joule/second).
If we build a lake at the top, 10 meters deep and 1 kilometer on a side, it would contain 10 million cubic meters of water. If we run it at 1GW, it would be emptied after 49 minutes. Not very useful, after all.
It makes me really appreciate the scale of natural phenomena like Niagara Falls.
Apart from what g_pepper has correctly pointed out regarding size/power of hydro plants...
With the right terrain, this is pretty trivial, all you need is a relatively small dam wall closing off a small ravine between mountains... here is a nice example:
http://www.iwb.ch/media/de/picdb/2012/366/nant_de_drance_stausee_vieux.jpg
http://www.iwb.ch/media/de/picdb/2012/367/nant_de_drance_stauseen_vieu.jpg
Actually, multi-gigawatt hydro plant is a large hydro plant, e.g. Hoover Dam has a capacity of 2GW. A medium sized hydro plant might have a capacity of around 200 MW, e.g. Martin Dam in Alabama has a capacity of 182 MW.
Your point is well taken however; scale issues will probably prevent pumped storage hydro from being the one-and-only solution to intermittent energy sources. Just for comparison, the reservoir created by the above-mentioned Martin Dam covers 40,000 acres!
However, pumped storage hydro can still be a useful and economical part of the solution. Other components would be natural gas powered combustion turbines which can be brought online quickly as needed, and a mix of renewable sources. To this latter point, some areas tend to be windier at night than during the day; this suggests that a mix of wind and solar and wind might be a useful combination.
Still, it is hard to imagine that we'll be getting away from fossil and nuclear any time soon; renewables can help reduce the amount of fossil fuels that we consume, but won't (for now) be able to eliminate the need for fossil. Pumped storage hydro can be a valuable part of the solution by smoothing over irregularities in the supply and demand while reducing the use of natural gas powered generation.