You are using long-term in an extremely vague way.
Pumped hydro is not a solution for seasonal storage or yearly storage. Seasonal variation can be a problem in higher latitudes.
For example we have a serious problem in New Zealand where our existing "green" hydro lakes are sometimes low and our economy is affected: creating national power crises during dry years. We use coal-burning Huntley and peakers to somewhat cover occasional low hydro generation.
Unfortunately our existing generators also have regulatory capture, and they prevent generating competition (e.g. new solar farms) through rather dirty tactics (according to the insider I spoke with).
Apparently much of our hydro generation is equivalent to “run-of-river” which requires the river to flow. Although the lakes themselves are large, they don't have enough capacity to cover a dry year.
NZ had planned a pumped hydro, but it was expensive: planned cost of 16 billion compared against total NZ export income of ~100 billion. https://www.rnz.co.nz/news/national/503816/govt-confirms-it-... So completely uneconomic risk (plus other problems like NIMBY).
The way I understand it from people working in the field, is that the issue isn't really that lakes/rivers are running dry, but more that most hydropower is produced on the South Island while most consumption is on the North Island and the connection is woefully inadequate (typical NZ problem, never invest in infrastructure just keep bandaiding it). That said installing wind and solar (both of which are plentiful) would do a lot towards alleviating the problems, but there are the issues of the existing generators lobbying and the political climate, where the conservative party still has a significant number of politicians who think climate change isn't real.
Long term storage is definitely the weak point of moving to 100% carbon free electricity. Unfortunately geothermal does not cover this need. If we want to cover a dankelflaute with geothermal, we basically need enough geothermal to cover ~100% of our power needs. Pumped hydro is the best answer we have at the moment, even if it isn't a great answer.
What will likely happen is that people will decide that "99% is good enough", and use fossil generators to cover dankelflautes,
I would guess dankelflaute is mostly irrelevant in New Zealand because our hydro lakes (assuming rivers are flowing) can smooth out generation shortfalls shorter than say a month (kinda equivalent to pumped hydro or batteries). A 3 day dankelflaute as given in the Wikipedia example would likely not matter in New Zealand. Plus our weather is variable, and generation is spread out.
New Zealand's hydro storage is less than a sixth of the country's total yearly electricity use. Hydro generation typically accounts for over 50% of our annual electricity generation, making us reliant on river flows. The maximum storage (full lakes) is around 5 TWh.
The lakes can't smooth out over long periods.
About 20% of New Zealand electricity comes from geothermal. I would guess political and environmental issues would dominate costs for increasing geothermal generation (plus I would assume we've taken all the low hanging fruit).
The size is orders of magnitude smaller than the depth of natural geothermal wells, and the temperature much higher. Even so, they are aiming for heat loss of < 1%/month, entirely adequate for seasonal energy storage.
> Although the lakes themselves are large, they don't have enough capacity to cover a dry year.
It was shocking to me to drive by many of the California lakes/reservoirs that were overfull in the spring of 2019 only to hear that they were basically running dry two years later, and realize that as substantial a water storage system as they are, they're not multi-year scale against the required water supply.
You are using long-term in an extremely vague way.
Pumped hydro is not a solution for seasonal storage or yearly storage. Seasonal variation can be a problem in higher latitudes.
For example we have a serious problem in New Zealand where our existing "green" hydro lakes are sometimes low and our economy is affected: creating national power crises during dry years. We use coal-burning Huntley and peakers to somewhat cover occasional low hydro generation.
Unfortunately our existing generators also have regulatory capture, and they prevent generating competition (e.g. new solar farms) through rather dirty tactics (according to the insider I spoke with).
Apparently much of our hydro generation is equivalent to “run-of-river” which requires the river to flow. Although the lakes themselves are large, they don't have enough capacity to cover a dry year.
NZ had planned a pumped hydro, but it was expensive: planned cost of 16 billion compared against total NZ export income of ~100 billion. https://www.rnz.co.nz/news/national/503816/govt-confirms-it-... So completely uneconomic risk (plus other problems like NIMBY).