At each of the three towers at Ivanpah, heat from mirrors concentrating sunlight is focused up to a solar receiver. Water transfers the heat to a steam cycle power block at each base.
PG&E has moved to cut short its 25 year contract with the Ivanpah tower solar project, which comprises three Concentrated Solar Power (CSP) units, each with a central tower supporting its solar receiver, for a total plant capacity of 392 MW. PG&E contracted for the generation from two of the tower units, SCE for that of the third.
PG&E would like to end the contract it signed in 2010 for the generation from the two units because without thermal energy storage, Ivanpah is only generating during sun hours – which photovoltaic solar (PV) can now do at lower cost than any other energy technology – and so it is not worth the high price typical of a first mover technology.
So why didn’t PG&E require Ivanpah to include thermal energy storage?
In 2010, my contact at PG&E’s renewables contracting division told me that they wouldn’t need to contract for solar thermal energy storage from Ivanpah. They’d also signed other contracts with SolarReserve for Tower CSP with thermal storage. (Those contracts fell through, as they didn’t get financing in time.) But mostly, PG&E was confident they had plenty of energy storage in water at Hetch Hetchy Dam.
Ivanpah was the first US commercial-scale Tower CSP project contracted for in 2010 and the last in the world to lack thermal energy storage. There’s a reason Ivanpah was the last without storage. By not storing its heat, it can’t dispatch solar energy on demand.
Why thermal energy storage matters
Dispatchable solar is the advantage of this thermal solar technology. Every subsequent Tower CSP plant built in Chile, Dubai, Morocco, and India includes up to 15 hours hours of thermal energy storage. All 30 Chinese CSP plants underway in China since first commercial tranche of 13 has included at least eight hours of thermal energy storage daily.
From the first commercial CSP projects in China, thermal energy storage has been included per CSTA
Because it includes ten hours of thermal energy storage, Crescent Dunes, contracted by Nevada Energy at the same “first sample” price as its California neighbor, still has value by providing solar at night to Las Vegas, and despite some “first sample” technical hurdles, later fixed. So because of its thermal energy storage, it has been worth it for NV Energy to stick with it as a purely solar source for night generation.
In Tower CSP thousands of heliostats (mirrors) encircling a central tower focus and concentrate sunlight up to a solar receiver. (How Parabolic Trough CSP differs from Tower CSP) The solar flux heats a transfer fluid like molten salts to carry down and store the heat till it is needed to run a steam power cycle like a coal plant (but by making the steam by heated sunlight instead of burning coal).
BrightSource tried a different heat transfer fluid.
Ivanpah couldn’t include thermal energy storage because it transfers its heat into water
However, water can not transfer or store heat at the kinds of temperatures or for the very long duration that molten salts and newer thermal energy storage media can. So choosing water for heat transfer meant forgoing thermal energy storage. But without it, Ivanpah could not produce enough heat in the dark before sunrise to warm up the power cycle for the day and eventually had to resort to using some fossil fuels for this small task. If Ivanpah had included storage, pre-heating the power cycle would have been easily accomplished using a small amount of the stored solar energy.
So water was Ivanpah’s mistake
By trying to be a squeaky clean water-based solar thermal power source, its generation was never optimal. And because water can’t store the temperature required for thermal energy storage, they did not have the selling point of this technology, its cheap and durable thermal energy storage, a kind of battery that can withstand daily cycling for decades.
Water was behind the miscalculation by PG&E, too. Given climate change, the expectation that there would be more droughts in the future was quite well known in 2010 when PG&E originally contracted for Ivanpah’s expected generation. Democrats like Barbara Boxer brought it up with both California utilities in Senate hearings on renewables planning. So relying on pumped hydro at Hetch Hetchy Dam for energy storage was a mistake.
So these two miscalculations regarding water by both parties to the contract has resulted in PG&E looking to shorten its pioneering contract with Ivanpah by about half.
Brightsource Innovates a Solar-cured Coating for DEWA Tower CSP
How a Bank Resurrected SolarReserve’s Redstone CSP with ACWA Power
Completed! Redstone Tower CSP project in South Africa is online
