Germany and California have both experienced negative electricity prices. The economics of this kind of battery could very well work with a low efficiency if it's cheap to build.
No, not really- negative prices are very short spikes. They aren't driven by fundamental oversupply, they're caused by fossil plants that are unable to ramp down their power output quickly enough to keep the grid in spec. The amount of storage required is very small, but the required power and speed are high.
The whole price advantage of thermal storage is in the amount of stored power. It's very cheap to build insulated tanks. It's expensive to build large turbines, pumps and heaters, the things you need for fast response. Thermal batteries are already kind of iffy compared to batteries (or rather, where batteries will be in 5+ years), so if you skew that price by several factors by going for high power, chemical batteries are definitely cheaper.
In a fully renewable grid, you'd need longer term storage like thermal batteries. 3 days is the most common figure. The electricity price would never go negative, even without batteries- renewables just turn off too quickly. You can just unplug solar panels if they're generating too much power.
Negative electricity prices are also driven by wind subsidies. Wind producers get 2.3 cents per kilowatt hour in tax credits. This means they make money even when the price of electricity is negative—-up to 2.3 cents a kilowatt hour.
Negative prices pretty much never happen in countries without subsidies.
It also isn't true that turning off any power station can't be done instantly. The station has to be able to cope with all its transmission lines suddenly being cut, which is a forced instant turnoff. It's more that the startup time is large, and they think that by losing money now, they'll still be running later when the power price is positive again.
3 days is silly. Extra capacity is very cheap, currently we have gas turbines filling this role but extra wind and solar never drop to zero and costs less than needing 5x as much storage.
Now as storage prices drop the equation changes, but not very quickly as their ineffecency means you still need extra capacity and that capacity reduces the need for storage.
Not just Germany and California. The central plains regions of Kansas/Oklahoma and parts of Texas are really high wind producers. We have negative prices from time to time and not just because of congestion. The marginal cost of energy is simply sometimes negative. This has an impact on all nodal prices in the market.
