Unless it means something different in Germany than it does in North America, generally speaking a "Think Tank" is code for Political Lobby Group. If you think what comes out of the mouth of such a thing is unbiased, you are crazy.
Good for them for producing what is no doubt a lot of solar energy. I doubt whatever figures they list are meaningful in anyway.
However the point is that renewable energy, on its own is not enough. If on any given day, your power needs fluctuate between 40-50GW, you NEED 50GW of CONSISTENT power from somewhere. Its fantastic that solar can produce 20GW (which I doubt anyway), however if tomorrow it produces 4GW, you now have a deficit of 16GW. Which means blackouts, or total electrical grid collapse as it all cascades into one huge fail. You have to replace that 16GW from someplace... sure you can buy it from your neighbor (which is what Germany does), but realistically what does that do, put the coal and nukes just across your border where you get almost the amount of risk plus none of the control? Renewables are great if you have storage, however that more less takes the form of huge water reservoirs which A) you have a limited supply, B) have their own environmental impact, and C) horribly inefficient.
Barring some magic electrical storage technology, you need both base power, and/or power you can spin up very quickly. Nukes are good because they are always on, constant power. Coal, gas, oil, are also always on, and also you have the ability to shut down generation, or turn it on again as need arises relatively. If the sun isn't shining, solar isn't much help, and if the wind isn't blowing neither will that. Unless your country's name rhymes with Riceland geothermal is pointlessly small potential. Biomass is too small to ever be that useful. Hydro is the only one that is a bit different. Barring some sort of huge long drought, it is pretty great. The only problem with it, is that it is a finite resource. Only certain areas have potential, and once you exploit them all there is no more to be had. Also some countries have more/less area to which to exploit. Tidal power is interesting, but so far no one has really been able to harness it effectively. Only a handful of sites exist around the world, and they are largely experimental. Same goes for current and wave type generators.
Anyway most successful systems employ a mix of generation types, including renewables. Problem is once the ratio gets out of wack, you are going to run into trouble, The only other way to do it is to massively over produce, however that is wasteful, and inefficient, and thus likely very very expensive.