Only 90,000 square kilometers! Is that all?
If you have no trouble building a solar plant the size of INDIANA, go right ahead.
Look: 90,000 square kilometers may be only one percent of the Sahara's area, but it is still a fucking big area.
Obviously, they wouldn't build one huge solar thermal plant; it'd be a conglomeration of hundreds of smaller plants. But even so, you're talking about covering a square patch of desert 300 km on a side with solar reflectors.
Problem the first:
Steerable mirrors--thousands of them--all of which must be kept clean (in a desert!) to maximize reflectivity and ensure peak efficiency.
Problem the second:
...but you need to build two--at least two--of these plants, one on either side of the planet, because unless you're planning to stop the Earth's rotation there is no way a solar power plant in the Sahara can supply power 24 hours per day. Otherwise you'll need coal/oil/nuclear plants to provide power at night; and if you have those already, why bother with the solar plant? Particularly when it's far away?
Problem the third:
The Sahara Desert is surrounded on three sides by insane muslim radical nations. What do you do when they blow up your power plant? I'm just saying that the security of your power source is questionable.
To be sure, the article isn't discussing such a grandiose operation; it's talking about a real-world plan to provide 15% of Europe's power by 2050. But even so, I think the problems I outlined above are still serious issues.