Curtailment is sort of free but it's also not. Particularly for a renewable source that needs it's uptime to be as much as possible in order to make financial sense.
" Cheaper to build capacity that can be ramped up and down " ? Not so sure about that one ?
I'm not suggesting that the storage has enough capacity for the whole period that the particular generator is offline. However, storage that takes the immeadiate intermittency away for frequency management and also helps with either end of the duck curve is crucial I think.
You're example of SA isn't quite right I reckon. Depending on the type of storage you have available to you, I think this could be managed in a modern grid. It might almost mean paying people to take energy and store it in their cars or houses but I think going forward that will happen.
There is already plenty of modelling going on to predict and manage wind intermittency. It might take a decade or two to get it perfectly right but that's ok. We have enough fossil backup to get us there.
