Each way you move the energy costs you 50% in efficiency. Which is why pumped hydro has to have a 4x different in the price of energy in vs energy out to make it economically viable. That's why PG&E almost never uses their pumped storage. Only on days where the mid day price of power is very low does it make sense. And keep in mind that California is the ideal place for pumped storage. I seriously doubt that NZ has a 3x duck curve in its energy demand.
It's nowhere close to 50%. Round-trip (so that's after both ways) efficiency is about 70-80% for a pumped storage scheme. Buy 10MW to pump the water, and get back 7-8MW when you release it. Contrast that with a reality here in the UK where the gas dominated spot price this morning when I woke up was about £180 per MWh, yet yesterday afternoon solar and wind had it down to £25 per MWh, so you could buy 100MWh of energy for £2500 but sell it less than a day later and make 400% on your investment in under 24 hours despite the efficiency loss. Very silly to insist this can't be profitable.
For the cost and expense of building a pumped hydro plant though, you could just deploy batteries which will do the same thing for a much lower capital and management investment and vastly simplified engineering. And a higher round-trip efficiency.
LiFePO4 works to demand shift on a daily cycle just fine and scales better to solar input (where you need much higher power handling so you can charge it on limited sunlight - a pumped hydro system is limited to charging at about half its discharge rate).
I think that's the point about the lake Onslow project - its MASSIVE. So yes, expensive, but months of backup for the whole country would not be cheap even with batteries
The 1000 MW installed pump/generating capacity of the proposed Lake Onslow Scheme is not unusual by world standards. However, its energy storage capacity (up to 5 TWh) would appear to make it the world's largest pumped storage scheme by energy storage measure.
The unusual ratio between energy storage and installed capacity comes about from New Zealand's special hydro generation situation. Hydro is the dominant mode of power generation but national hydro storage capacity is relatively limited. This makes hydro electricity output vulnerable to extended dry periods. Developed to 5 TWh of storage, the large Lake Onslow Scheme would more that double New Zealand's total hydro storage capacity. The scheme would thus provide dry period insurance as well carrying out the usual short-term pumping and generating operations.
It would be an open-cycle system. The lower reservoir is the Clutha River, which is New Zealand's largest river by discharge measure.
It seems a long time ago now since I first proposed the possibility of pumped storage at Lake Onslow. But just confirming - having the equivalent amount of energy (4-5 TWh) in batteries would be impossibly expensive. Also, the batteries would need to be replaced from time to time.
