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Comment by rplnt | Hacker News Reader

As for storage, shouldn't there be plenty of hydroelectric dams that could run "backwards"? Or is there an interconnection issue you mention?

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For a hydroelectric dam to be able to "run backwards", it needs a lower reservoir. Many dams have only an upper reservoir.

Simply turning them off would be running backwards since water would build up behind them.

It’s not about building up capacity alone: you want to take power off the grid at negative energy price times.

You are taking power off the grid: The power otherwise produced by the hydro plant.

Of course with a lower reservoir, you can go into negative territory instead of "just" taking the plant down to 0.

No, you need to consume what other methods, e.g. wind, are producing when there isn't the demand to use it.

There is always demand. The issue is that sometimes demand exceeds supply. Taking away supply has the same net effect as increasing demand.

EDIT: with the one caveat of transmission capacity, which certainly can be an issue.

I don't think you're quite getting this. There are times where there is excess demand, even when removing these stores. What's so hard to understand?

Prices go negative in the US at times and in Germany more often. That is exactly when something like this would be used: to store that capacity.

The point is that you have a given supply and demand. As long as they are in the right positions in the grid that transmission capacity is not the limiting factor, then you can fix that by reducing supply, or by increasing demand - apart from the issue of transmission those options are equivalent from the point of view of bringing the supply under control. They may not be equivalent in terms of cost, but that is a separate issue.

The original point was that you do not need to have a reservoir to pump from to take advantage of the fast start/stop capability of a hydro plant: Closing the sluice gates and/or bypassing the turbines is sufficient to reduce supply by the generating capacity of the plant.

Having a reservoir as the potential to significantly increase the reduction in excess supply you can handle, that is true, but the lack of that option does not change the benefit of shutting off generating capacity.

Hydro plants have been used to adjust grid supply for as long as they have existed in this way. Not least because with grid prices adjusting dynamically, as long as the reservoir is not full, most hydro plants yield better economic result if they respond to falling prices by letting the reservoir water levels build back up.

> Having a reservoir as the potential to significantly increase the reduction in excess supply you can handle, that is true, but the lack of that option does not change the benefit of shutting off generating capacity.

The point is to not turn off generation capacity that we don't control the start of, like wind. Why should we not generate if the wind is blowing at night?

What I commented on, and what you quoted above, was about hydro plants, not wind. With hydro plants, as long as the reservoir has capacity the reservoir acts like a battery, as hydro plants tend to be built with turbine capacity sufficient for a flow large enough to drain the reservoir. As such, unlike with wind, nothing is lost if you close the sluice gates or a while - the reservoir just builds up and you can generate more when needed.

That's the entire point: That hydro plants can act as storage even if there's no lower reservoir to pump from. Which means they're good fast response plants exactly so that types of plants where turning them off wastes potential energy, like wind farms, can keep running.

The original claim I took issue with, was that a hydro plant can't serve this purpose without a second reservoir, which is not true - a second reservoir can be used to increase total effective storage capacity, and to make the plant increase demand rather than just decrease supply, but a hydro plant on itself can improve grid responsiveness, given enough transmission capacity.

This whole discussion has been around the use of pumped hydro plants to store the output of other sources during times of low demand.

Yes and no. The two comments leading up to my first comment was one suggesting that you could achieve a similar effect by simply closing a damn, followed by one implying that did not take power out of the grid. My first comment then stated:

> You are taking power off the grid: The power otherwise produced by the hydro plant. > > Of course with a lower reservoir, you can go into negative territory instead of "just" taking the plant down to 0.

In other words, my point from the beginning was that there is - absent transmissions limitations - no conceptual difference in the outcome of stopping a damn and pumping into a reservoir, as I've gone into at great length in the subsequent comments.

Nope. Adding power to the grid is +X MW. Removing power to the grid is -X MW. Not producing any power is 0 MW.

That's true when looking at an individual plant, but is not directly relevant when you're looking at the grid as a whole (though it may be an issue when you have transmission limitations), where the problem is that at some point you have a supply of +2X MW, and demand is only X MW, and you need to deal with the X MW of oversupply. If one of the suppliers can then turn off X MW of supply rapidly, that has the same effect as if you add X MW of demand.

At a hydro plant you have multiple options: You can bypass the turbines; you can close the dam, as long as it's not near capacity; if you have a lower reservoir you can close the dam and pump water up to the upper reservoir. All of these help reduce the excess supply, but pumping up does so by increasing demand and reducing supply, and the other two options reduce supply.

You can of course do pumped storage without normally producing electricity too - in places where the only convenient location for a reservoir is not "in line" with a water source with sufficient fall height, you can still pump up to a convenient location, in which case the plant is basically "only" storage.

Unfortunately the capacity of these dams in most areas is far from enough to compensate all fluctuation in renewable energy production.

You not only need variable volume reservoirs on both ends, but you also want to have very high drop, because liars from seepage and evaporation scale with volume but not with height. Low-drop pumped storage might be useful for very short term storage, but usually you'd want something that is efficient over a wider range of usages.

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