At the threshold of the Sahara Desert near Ouarzazate, Morocco, some 500,000 parabolic mirrors run in neat rows across a valley, moving slowly in unison as the Sun sweeps overhead. This US$660-million solar-energy facility opened in February and will soon have company. Morocco has committed to generating 42% of its electricity from renewable sources by 2020. Across Africa, several nations are moving aggressively to develop their solar and wind capacity. The momentum has some experts…