The behemoth Ivanpah solar power plant built with federal subsidies to combat climate change is using increasing amounts of natural gas, a greenhouse-gas-emitting fuel, state and federal data show.

The most recent numbers from the California Air Resources Board show that in 2015, the plant’s second year of operation, carbon emissions from Ivanpah’s gas use jumped by 48.4 percent to 68,676 metric tons.

That’s more than twice the pollution threshold for power plants or factories in California to be required to participate in the state’s cap-and-trade program to reduce carbon emissions.

Carbon emissions data for 2016 won’t be available until the end of this year, but data made public by the U.S. Energy Information Administration show that natural gas consumption at the plant increased by about 7 percent during the first three quarters of 2016 when compared to the same period in 2015.

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The Ivanpah plant was at the center of the Obama administration’s push to reduce America’s carbon footprint by using millions of taxpayer dollars to promote green energy, but little was said about the plant’s own carbon emissions before it began operating at the end of 2013.

The Ivanpah plant works primarily by having 352,000 mirrors focus heat from the sun onto three boilers mounted on towers, each taller than the length of a football field. Water in these boilers is turned into steam that then turns turbines to generate electricity.

But burning natural gas in smaller, auxiliary boilers also is part of the process.

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.

A spokesman for the plant’s operator, Houston-based NRG Energy, said natural gas use is increasing as expected while the plant boosts its overall electricity production.

“Ivanpah does use more gas when we operate more,” said David Knox, the NRG spokesman, in an email. “The reason for this is that the more the units run, the more we use the auxiliary boilers to support that increased operation.”

Approved in 2010, the Ivanpah plant was at the center of the Obama administration’s push to bring alternative-energy projects to public lands. It received $1.6 billion in loan guarantees and $600 million in federal tax credits.

Given the high-level of public investment, the plant’s natural gas use should have been better publicly disclosed before the project was approved, said David Lamfrom, California desert manager for the National Parks Conservation Association.

Lamfrom had opposed the project because it consumed about 5.6 square miles of mostly undisturbed public lands that was home to the desert tortoise, a species threatened with extinction. He also said it was too close – three miles – from the Mojave National Preserve, which is part of the national park system.

The Ivanpah plant would have had a tougher time winning approvals had people known the extent that it relies on a fossil fuel, said Lamfrom, describing it as a hybrid facility.

“The bottom line is the public didn’t expect this project to consume this much natural gas,” Lamfrom said in a telephone interview. “We did not have full knowledge that this was what we were signing up for.”

Despite its gas use, Ivanpah still qualifies under state rules as an alternative energy source for its utility consumers, Southern California Edison and Pacific Gas & Electric, because no more than 5 percent of electricity produced at the plant stems from its day-time burning of natural gas, according to the California Energy Commission.

This is important because Edison, PG&E and the other utilities in California must get 33 percent of their electricity from renewable sources by 2020, and 50 percent by 2030.

Less than 25 percent of the gas burned at the plant counts toward this 5 percent limit, according to data from the U.S. Energy Information Administration. That’s because most of the gas is burned at night for maintenance purposes when the plant is not producing electricity.

These rules may be favorable to the plant operator, but do not make a lot of sense, said Sadrul Ula, managing director of the Winston Chung Global Energy Center at UC Riverside, who is familiar with the technology. All gas burned at the plant – day or night – should be counted when evaluating the plant because all of it is necessary for Ivanpah to function, he said by telephone.

“If they did not burn the gas during the night, the solar energy production would be much lower,” Ula said.

Ula said natural gas use is increasing as the overall production increases because the plant’s three tower units are spending less time out service. Thus, the gas use is expected to continue to increase until the plant reaches its optimal production level.

He noted that solar tower plants that use molten-salt energy storage systems have no carbon emissions, pointing to the smaller Crescent Dunes solar tower project near Tonopah, Nev., as an example.

Ivanpah’s electricity production got off to a slow start, generating only 45 percent of its annual production goal, during its first year in 2014, federal data shows. Plant operators have said they always expected it to take four years for Ivanpah to reach its goal of 940,000 megawatt hours a year.

That’s enough electricity to power about 140,000 California homes – a goal made public by the U.S. Department of Energy.

Last year, the lower electricity output meant the plant was not meeting its contractual agreement to provide alternative energy to Pacific Gas & Electric, a Bay Area utility.

In response, the California Public Utilities Commission approved what is called a forbearance agreement between PG&E and Ivanpah’s owners. Under the agreement, the plant’s owners paid an undisclosed sum and were given until July 31, 2016, to improve its production.

By the end of June, the plant was meeting the production terms with PG&E, according to an NRG quarterly report to its stockholders. Those terms are confidential.

But the Ivanpah plant still is not meeting its annual production goal. During the 12 months that ended Sept. 30, the plant’s electricity was producing about three fourths of the goal, according to federal data.

A U.S. Energy Department fact sheet said that Ivanpah is expected to prevent up to 500,000 metric tons of carbon emissions each year by producing solar energy.

It was not clear, however, if that figure took into account Ivanpah’s carbon emissions from burning natural gas.

Contact the writer: 951-368-9471 or ddanelski@scng.com