>Gas is burned at night to keep the system primed and to heat water used in the tower boilers. This allows electricity production to start up more quickly when the sun comes up each morning, plant operators said. Gas also is burned during periods of intermittent cloud cover.

https://www.pe.com/2017/01/23/ivanpah-solar-plant-built-to-l...

The steam cycle has to be up and running before sunrise so they can meet their generating obligations, so they have to ramp up with natural gas while it is still dark. The facility is also capable of operating as a dispatchable "peaker" power plant.

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.