There is also a practical limit where wind energy takes too much energy out of weather systems, affecting climate, but that point is also far beyond the current world energy usage.
We're not running out of renewable energy anytime soon.
The Sahara desert is extremely sandy, and extremely windy, which makes it a pretty terrible, or at least prohibitively-expensive place to build a solar farm.
It is also extremely complicated and extremely expensive to transmit large amounts of power over long distances, particularly across national borders (everyone's on a different AC standard), particularly across many national borders.
China has many overland HVDC links, some thousands of km long and with huge capacities, to move renewable energy from inland areas to the coast.
The UK and Iceland are considering an undersea HVDC interconnector more than 1000 km long.
The distance between potential solar array sites in the northern Sahara and the electrical grids in Southern Europe are considerably less than this.
At an estimated cost of about 3 billion, for less than 1/3 the capacity of this power station.
Hinkley C's construction cost is £20.3b, and the cost to UK consumers over the plant's lifetime exceeds £50b.