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Comment by programminggeek | Hacker News Reader

This is probably a bad idea, but couldn't you store energy basically having wind/solar power water pumps to pull water from one location to another? Something like 2 lakes side by side with a dam in the middle pulling energy off the transfer between when energy is needed and then water being pumped back in the reservoir when energy is being claimed from wind/solar?

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This is addressed in the "Renewable Energy - Without the Hot Air" Report

http://www.inference.phy.cam.ac.uk/withouthotair/c26/page_18...

basically the author says that what you describe is the best option right now but there are only so many places you can put those facilities because they require very specific geography.

That being said building lakes and dams is really bad for the river eco system and really expensive to buy land out from under people to flood it.

I really should get around to reading that. His info theory book/lectures was/were very good. http://www.inference.phy.cam.ac.uk/itprnn/book.html

If you haven't seen already you might be interested in http://lightsailenergy.com/tech.html; they're working on compressed-air-and-water-vapour storage (also DaniFong posts on HN sometimes)

At this point, it's surprising we don't just cut off the flow of existing hydro when we have other sources peaking... e.g. it's what you describe but no need for upward pumping.

We do this to some extent, but the reality is we do need to ensure rivers continue to flow for a number of reasons (I guess).

Something like this?

http://en.wikipedia.org/wiki/Helms_Pumped_Storage_Plant

Yes, basically that, but using solar/wind to power the pumps, turning the lake into a giant battery to store the energy from the solar/wind power, which seems to be one of the major drawbacks of wind/solar.

That's an interesting and seemingly obvious approach, so I'm curious why it's not really discussed.

I'm guessing that at current wind/solar capacity, any fluctuations can be absorbed by just slowing down the flow out of the reservoir, rather than ever having it reverse.

You also need a downhill source of water, which means any dam that's the last one downstream can't really use this approach.

You could maybe even get away with an entirely closed-loop system, but if you look at the scale of the problem, you need either a huge pair of reservoirs (lake-sized) or a huge elevation difference (mountain-sized), or preferably both. It doesn't appear economical to build e.g. a tank on a tower.

One of the major reasons pumped storage isn't used more is because the energy loss associated with pumping and storing the water. I can't find the exact amount right now but from a lecture I watched some time back[1], pumped storage is a few orders of magnitude more expensive than using conventional fossil fuel generated electricity per kWh.

It's just not very cost effective right now.

[1] http://www.youtube.com/watch?v=MoAFiL1UdSg

The orders of magnitude doesn't pass the sniff test. If it were true, utilities would not be building pumped storage capacity.

I can certainly believe the capital requirements are much higher for pumped storage than for natural gas peakers.

It takes a huge space. 1 kWh means moving a metric ton (a cubic meter) of water 400m of elevation.

Round trip efficiency isn't great, but also not terrible. It beats the fuel-cell/electrolysis cycle, but loses to batteries. It can be profitable though because energy rates can reach near zero (and even occasionally slightly negative) at night.

99% of installed energy storage is pumped hydro.

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