Oh really? Why? Is it to keep things at a nominal operating temperature in prep for the next day, or is it to continue generating electricity once the sun has gone down (or other reason)?
https://www.pe.com/2017/01/23/ivanpah-solar-plant-built-to-l...
There's a huge amount of right-wing disinformation about solar power so if you just Google for "Ivanpah natural gas" you'll get an outpouring of stupid blogs, but reading them will get you close to the truth.