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.

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).

https://en.wikipedia.org/wiki/Dunkelflaute

> Unfortunately geothermal does not cover this need.

Artificial geothermal could though.

https://www.orcasciences.com/articles/standard-thermal

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.

Which is honestly fine, we would be in such an amazing place if we got to 99%

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