A renewable grid (excluding nuclear and hydro) is definitely not a straight forward engineering problem because energy sources such as wind and solar are intermittent. That requires the grid to be backfilled by a non intermittent source of energy, and/or storage of energy from intermittent sources (using for instance a lot of batteries.) That storage can get complicated at scale, especially with inverters feeding directly into the grid. (Batteries are DC, the grid is AC). Lots of complex mathematics to prevent surges and brownouts. Nuclear + renewables would be a great solution but almost no one wants to build nuclear power plants.