They are washing the mirrors.

The Ivanpah plant draws water for the boilers atop its towers, and for washing its many thousands of mirrors, from underground wells at the site. The water will be recycled; an on-site treatment plant will filter out wastewater sludge, which a waste hauler will remove and dispose of, according to the company.

Very expensive way to produce energy. They may try to reduce the cost by opening a restaurant that serves roast birds of various kind that fly into the solar oven.