The Ivanpah Solar Power Facility (the plant being shut down) apparently uses quite a significant amount of natural gas to operate (more than anticipated), and seems to be more polluting than a normal NG power plant, though it seems to generate a bit more power for the amount of pollution generated. Per wikipedia:

The plant burns natural gas each morning to commence operation. The Wall Street Journal reported, “Instead of ramping up the plant each day before sunrise by burning one hour’s worth of natural gas to generate steam, Ivanpah needs more than four times that much.”[38] On August 27, 2014, the State of California approved Ivanpah to increase its annual natural gas consumption from 328,000,000 cubic feet (9,300,000 m3) of natural gas, as previously approved, to 525,000,000 cubic feet (14,900,000 m3).[39] In 2014, the plant burned 868×109 British thermal units (254 GWh) of natural gas emitting 46,084 metric tons of carbon dioxide, which is nearly twice the pollution threshold at which power plants and factories in California are required to participate in the state’s cap and trade program to reduce carbon emissions.[40] If that fuel had been used in a Combined Cycle Gas Turbine (CCGT) plant, it would have generated about 124 GWh of electrical energy.[41] The facility used that gas plus solar energy to produce 419 GWh of electrical energy (more than three times that of the referenced CCGT plant), all the while operating at well below its expected output. In 2015, the facility showed higher production numbers, with Q1 increases of 170% over the same >time period in 2014.[42]

The facility uses three Rentech Type-D water tube boilers and three night time preservation boilers. The California Energy Resources Conservation and Development Commission approved for each a stack “130 feet (40 m) high and 60 inches (1.5 m) in diameter” and consumption of 242,500 cu ft/h (6,870 m3/h) of fuel.

The wikipedia article also mentions it has no energy storage capabilities:

For the first plant, the largest-ever fully solar-powered steam turbine generator set was ordered, with a 123 MW Siemens SST-900 single-casing reheat turbine.[23] Siemens also supplied instrumentation and control systems.[24] The plants use BrightSource Energy’s “Luz Power Tower 550” (LPT 550) technology[25] which heats the steam to 550 °C directly in the receivers.[26] The plants have no storage.

However, the similar Cresent Dunes molten salt solar array, does have energy storage, and can store 1,100 MW·he.