Probably the biggest problem with wind and solar is they
are intermittent - they don't generate power all the time.
The solution is energy storage, but that's
expensive. Many institutions are searching for ways to make it
The cheapest solution right now is pumped hydro:
use excess wind and solar energy to pump water up a hill to a storage lake, then
when energy is needed let the water run back down through turbines and
generators. The problem is you need a hill (preferably a big one) next to
a water source (preferably a big one). That severely limits where you can
use pumped hydro.
Here's an idea which can expand the number of places
where pumped hydro can be used: