> "The share of hydropower in electricity generation from renewable energies has fallen over the years and is currently still around 8 percent. This proportion will continue to fall in the future, as the potential for hydropower use in Germany has been largely exploited..."
That's primary hydro. As I already told you, that's not pumped hydro. You don't seem to be understanding this point, so let me explain in more detail.
Primary hydro involves exploiting natural water flows, extracting energy from water that falls at higher elevations as it flows down to lower elevations. It is limited by these natural water flows, and the need to be on the rivers in which the water is flowing.
Pumped hydro, on the other hand, creates its own water flow. It does not need to exploit natural precipitation. It can recycle the water it uses, getting much more energy flow per unit of water (until the water seeps or evaporates away, but that is slow.) It can be placed in locations that aren't on rivers. As an example, consider this PHES project in an arid part of the Great Basin in Nevada:
https://www.whitepinepumpedstorage.com/
Notice the graphics with two reservoirs, constructed on flat areas by surrounding them with earthen walls, sitting in the middle of a desert without any permanent rivers in sight.
The potential for this sort of pumped hydro is enormous anywhere there is sufficient vertical relief. In Australia, for example, the potential is some 100x what would be needed for a 100% RE grid. They've built a global PHES geographical database for finding places around the world (where they have data; Russia is excluded for example).
http://re100.eng.anu.edu.au/global/
Go play with this; the opportunities are vast.
PHES is not suitable for flat terrain, so it can't be a complete solution in general, but to call it maxed out is very wrong.
> Actually: they are not. All the detailed models show this exact problem, such as the paper I linked.
Let's look at a review paper.
https://ieeexplore.ieee.org/document/9837910
"With every iteration in the research and with every technological breakthrough in these areas, 100% RE systems become increasingly viable. Even former critics must admit that adding e-fuels through PtX makes 100% RE possible at costs similar to fossil fuels. These critics are still questioning whether 100% RE is the cheapest solution but no longer claim it would be unfeasible or prohibitively expensive."
> For Germany, they say we need 750 GW of Wind/Solar + 500 GWh of batteries + 150 GW of gas plants + the electrolysers to produce the gas. In addition they also assume a massive increase in efficiency and thus reduction in consumption.
Battery storage is approaching $100/kWh (not cells, the whole turnkey system) in China now. So, 500 GWh would be $50B. If the batteries have a lifespan of 20 years that's $2.5B/year (+ interest). Can you not afford this? That's less than half of what Germany spends on pizza.
Also, consider how many GWh of storage in cars would be needed if all the cars in Germany were BEVs (I get about 3 TWh).