[Manuel_D]

> You can cycle 40GWh when it's full. Then you can get 350GWh out of it. Then put 240GWh back into it and cycle it a few times, then get another 350GWh out of it again in a month or two. The last 110 fills itself.

But doing so would reduce the usable storage of other facilities using the same body of water. This is explained here:

> At the extreme, the water stored in Talbingo/Jounama could be reduced to 28 GL. This would allow 160 GL of Tantangara water to be accommodated in Talbingo. This equates to a recyclable energy storage capacity for Snowy 2.0 of about 235 GWh (160/239x350). In this case the energy capacity of Tumut 3 is reduced from 60 GWh to 10 GWh, so the net energy storage is 185 GWh (235-50).

How much can Snowy 2 store without adversely impacting other storage facilities? 40 GWh.

Cycling 240 GWh of energy would almost entirely eliminate Tumut 3's storage capacity, and yield a net increase in storage capacity increase of only 185 GWh. 240 GWh is only correct if we ignore the capacity reduction of Tumut 3. And of course, I doubt Tumut 3's operators would agree to this scenario without being bought out because it'd destroy their ability to turn a profit and have a chilling effect on future hydro projects.

[Schroedingersat]

Why would Tumut 3 be trying to be full in a scenario where the energy is needed? They'd run their turbines and sell energy. The downstream dams dispatch their dispatchable energy and you leave enough water in the middle two that the maximum can be pumped back upstream. The extra dispatchable energy is an upside. It's like having a battery that can't be charged past 70% but fills itself the rest of the way.

So yeah, bad faith. And now you've had it pointed out twice it's just lying.

[Manuel_D]

> Why would Tumut 3 be trying to be full in a scenario where the energy is needed? They'd run their turbines and sell energy.

The issue is that the maximum cyclical storage capacity is determined by the minimum of both the upper and lower reservoirs. Snowy 2's lower reservoir is Tumut 3's upper reservoir. And Tumut 3's lower reservoir is barely 1/10th the size of Snowy 3's upper reservoir. That's the bottleneck.

If your point is that we should just accept the fact that Tumut 3 can't be run at full capacity if Snow 2 is deployed, then yes that's correct.

> The downstream dams dispatch their dispatchable energy and you leave enough water in the middle two that the maximum can be pumped back upstream

Right: in order for Snowy 2 to avoid losing any water, then Tumut 3's upper reservoir (which, remember is Snowy 2's lower reservoir) has to start empty in order to accommodate the water from Snowy 2. And then Tumut 3 can't drain this water when prices are high because Snowy 2 needs it re-charge its upper reservoir when electricity prices are low. In order to run Snow 2 at maximum cyclic capacity, Tumut 3 has to essentially become totally subservient to it.

Imagine I have 3 cups: 30 Liter cup flows to/from a 15 liter cup, to a 5 liter cup. I only have 20 liters of actual cyclic storage capacity, not 50. The 15 and 5 liter cups have to start empty in order to catch the water flowing down from the 30 liter cup. If the 15 and 5 liter cup started full, they'd overflow and lose water.

So if Snowy 2 is running at max cyclic capacity, Tumut 3 can only store and release the water that can fit in its lower reservoir (the 5 liter cup). That's why running snowy 2 at max cyclic capacity would completely shaft Tumut 3.

> The extra dispatchable energy is an upside. It's like having a battery that can't be charged past 70% but fills itself the rest of the way.

But that metaphorical battery fills itself very slowly. It's not cyclic capacity and thus isn't nearly as useful.

Imagine you have company A that sells a battery that stores 1 GWh and you can charge and discharge it at a rate of 200 MW and charge it at a rate of 200 MW. Company B sells a battery that stores 10 GWh for the same price that can also discharge at a rate of 200 MW, but it's super sensitive to charging and can only be charged at a rate of 1 MW - it'll take a month and a half to get back to 10 GWh.

Which of these batteries is more useful? The first one, by a massive margin.

[Schroedingersat]

The system you described has a capacity of 35L (that's how much it can pour through both pipes and still be ready to cycle) and a cyclable capacity of 20L. Only someone deliberately trying to misconstrue the role of seasonal storage would characterise it as 5L. You also carefully ignored the upstream turbines which aren't two way.

> But that metaphorical battery fills itself very slowly. It's not cyclic capacity and thus isn't nearly as useful.

It's seasonal storage. The fastest it can empty or fill is a week. A renewable grid doesn't ever require it to run at max power until it is empty and then fill at max power until it is full. That's a failure mode of a grid with large centralised production that has major unplanned outages like nuclear plants.

Is a load balancing or grid forming battery more useful? Yes. Can snowy 2 form a buffer for 350GWh of energy consumption in any realistic scenario? Also yes.

[Manuel_D]

> You also carefully ignored the upstream turbines which aren't two way.

What about them? Those aren't pumped hydro storage plants, they're just normal dams. There's no pump: you can't supply them with electricity to pump water back into the reservoir.

Cyclable capacity is the only type of capacity anyone cares about. Again imagine I sell someone a battery claiming it has 10 GWh of capacity. they drain 10 KWh, and then they try to charge it back up but it stops at only 3 KWh. They call tech support and I say "well, sir, the battery only has 3 KWh of cyclable capacity." I guarantee you >99% of people would think they were cheated. Saying that the battery has a capacity of 10 KWh is highly misleading; it's only true in a pedantic sense.

The whole point of Australia's storage plans is to even out solar energy's daily output. The plan is to pump the water into the upper reservoir during the day, and release it at night. The requires cyclical storage. The trickle of water that precipitation puts into the upper reservoir is negligible.

[Schroedingersat]

> The whole point of Australia's storage plans is to even out solar energy's daily output.

...which it can do by curtailing or releasing the dispatchable energy in tumut 2 if tumut 3 needs to adjust

also the 'trickle' is an entire watershed, not surface precipitation

In all practical senses, over the time scales for which seasonal storage is required, snowy 2 adds 240-350GWh of load shifting. Your sleight of hand doesn't work I already know where the ball is.

[Manuel_D]

A solar heavy grid mostly depends on cyclic storage, not seasonal storage. The non-cyclical storage potential is acceptable for the kind of storage that isn't needed.

Looping back to my battery analogy. The extra 7 Kwh of non-cyclical storage could come in hand if you needed to use it for an extended period of time if the power goes out. But it's not useful if you need to use it every day. Australia, California, and plenty of other energy markets need cyclical storage that is used every day/night cycle to smooth out the duck curve[1].

If you had clarified that most of Snowy 2's storage capacity is not suitable for cyclical storage from the outset, this whole tangent could have been averted. Cyclical storage is the kind of storage that it's in demand, so it's important not to present non-cyclical storage that has a very limited recharge rate as equivalent to a lithium battery.

1. https://en.wikipedia.org/wiki/Duck_curve