I am an engineer that works on hydro control systems - It's worth noting that a hydro turbine, depending on it's construction, has a certain turndown ratio - Th

I am an engineer that works on hydro control systems - It's worth noting that a hydro turbine, depending on it's construction, has a certain turndown ratio - That ratio of it's full scale output to it's minimum output. At the turn of the century designers of these turbines did not particularly care about operating at a lot of different power output levels, they cared about maximizing power output at a particular point, and importantly, maximizing efficiency at that point. These units typically have a turndown ratio very close to one. Consequently many hydro turbines are capable of a relatively narrow operating range, and are not suitable for fast grid support of the type that would be required as the primary source of solar backup. They're on-off type resources in many cases. These turbines can be retrofitted to variable geometry style turbines with much wider operating bands but these upgrades are slow and expensive. Long story short - hydro is good as a 'base load' but we desperately need storage. Pumped storage, battery storage, compression storage. And we need massive transmission system upgrades (in the US at least). Nothing else will lead to a reliable grid based on renewables in the US.