The use of gas for generation at the world’s largest direct steam solar tower has never exceeded 5 percent, according to newly released data from NRG and confirmed by EIA and the California Energy Commission (CEC).
At the 377 MW Ivanpah CSP project, the use of natural gas is limited to 5 percent of generation, despite media reports that imply otherwise.
In one example, David Lamfrom, desert project manager of the National Parks Conservation Association, is quoted by the Press Enterprise as saying that he doubted that the project would have gone forward if it had been billed a hybrid plant: that “if it had been billed as a 75 percent renewable energy project, the BLM might have said ‘no.’”
This suggests to the reader that Ivanpah is only 75% renewable, and gets a quarter of its generation from gas. That is factually incorrect.
Adam Ward, spokesman for the CEC, confirmed that Ivanpah generates 5 percent or less of its megawatt hours from natural gas.
(Ivanpah must remain at least 95 percent renewable because it must meet the Renewable Portfolio Standard (RPS) guidelines under which it was certified.)
I asked NRG spokesman David Knox why the data on generation from natural gas didn’t show up at EIA in 2014. He told me that the plant was not able to report generation from natural gas until 2015, as the CEC had not yet decided on the methodology for calculating it.
Knox forwarded me NRG’s spreadsheet including newly released figures for January to August 2015 (indicated in red) which are not yet included on the EIA site, where you can check the monthly generation for Ivanpah Unit 1, Unit 2 and Unit 3.
EIA spokesman Jonathan Cogan told me that EIA has had delays getting the first months of data online, but expects to have it up later in the year. “When we finalize the 2015 data files, that will be incorporated into the browser,” he said.
Documents provided to me by NRG reveal new details about how many megawatt hours of generation came from natural gas in 2015.
Source: EIA and NRG
How the CEC decided on a method for counting natural gas generation in 2015
It turns out that it took quite a few meetings with the CEC to find a methodology on how to accurately determine how much gas contributed to power generated, after the project increased its use of natural gas in September of 2014 to 5 percent. Some of that now went towards generation, but how to know how much?
Knox said it was not that straightforward to determine how many megawatthours were generated by gas or by solar. Steam generated by the natural gas boiler mixes with the steam from the solar boiler at an intermediate stage in the steam turbine steam path – so it was hard to separate electric generation from natural gas alone.
Once the method of calculating how much gas was contributing to generation was worked out with the CEC, Ivanpah was able to report the generation from natural gas and from solar. When the generator breaker is closed, any gas that is consumed on-site is deemed to contribute towards electricity generation.
“So we meter the amount of gas consumed while the generator breaker is closed and use an agreed upon conversion factor to convert the energy from gas combustion into net MWh of electrical output,” explained Knox.
The Ivanpah concentrated solar tower project is the first-of-its-kind direct steam plant at utility-scale; 377 MW. Thousands of huge mirrors reflect sunlight onto a receiver where water is turned to steam by the heat of the focused sunlight.
Ivanpah has a PPA (Power Purchase Agreement) with PG&E for two of the three tower units comprising the plant, units 1 and 3.
In 2014, Ivanpah operators had asked the CEC to be permitted to raise gas use from a low initially planned 2 percent for parasitic overnight use to 5 percent.
The additional use was approved, and some generation was permitted from gas as a result of that, with the first morning steam created by burning gas, and during cloudy periods.
Ivanpah’s initial bad start caused the project to fall short of the 70% required output for the first two year look-back period, during a four year ramp-up to full generation.
In the first two-year look-back period, Ivanpah units 1 and 3 were generating on average 19% short of the contracted percentage. But by the second year, generation was only 3% below target and still improving.
PG&E asked the CPUC to let Ivanpah keep its PPA, as poor initial performance is fairly routine with new technology during a ramp-up period, and this 377 MW direct steam solar tower technology was without precedent when it began, other than small projects at 11 MW.
Current generation in the first quarter of 2016 has actually been above mature year requirements (the 100% level due in 2018) of 640,000 MWh for these two units.
Gas for keeping warm overnight helped speed morning start-up
A major cause of the very slow morning startups in early 2014 – just how cold the turbine gets by morning – had some very low tech solutions.
“The initial thinking of how warm the steam turbine would remain overnight was off quite a bit,” said Mitchell Samulelian, NRG’s vice president of operation for utility-scale renewable generation.
“A typical power plant steam turbine takes anywhere from four to 24 hours of warming to get hot enough to start up. But obviously if you take 24 hours to warm a solar plant, you are never going to make any money!”
The solution to that overnight cooling was to increase the ceiling steam heat – using gas, as well as to insulate the turbine. That has successfully kept the turbine at well above 750 Fahrenheit at night so that it wouldn’t cool off.
By keeping warm overnight, Ivanpah can now start up very quickly in the morning; going from 4 hours to under 25 minutes.
In a gas or coal power plant, this use of gas overnight for what is called “parasitic load” – providing energy needs onsite that is not for sale to the grid – is fairly routine.
“If a coal plant the size of Ivanpah at 400 MW is shut down overnight, and needed to run in the morning, this would also consume natural gas, because they would be using natural gas to make steam seals for the turbine in order to draw a vacuum and start up,” said Samuelian.
“So a lot of our fossil fuel plants have an auxiliary boiler that is for supplemental heating when the plant is off-line, no different than Ivanpah.”
The additional gas use (up to 5 percent of generated electricity) was approved for parasitic overnight use, and to create that first morning steam to speed morning start-up times, and during cloudy periods.
To stay under 5 percent the gas is closed off if necessary
“We can regulate how much gas we use,” Samuelian pointed out. “We can make a choice to actually not add any natural gas to stay online, if it’s a very cloudy day – versus just coming offline – if we think we’re getting close to our limit.”
And in 2016, gas has not even been as high as 5 percent.
“In these latest three to four months our gas use is in the 3 percent to 4 percent range,” said Samuelian.
Susan Kraemer is currently researching the transition to renewable energy for an upcoming book:
How We Learned to Run a Civilization on Infinite Energy:
Chronicling the engineering challenges overcome in launching a renewable energy economy, based on first hand accounts
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