Credit: SolarReserve

By Susan Kraemer

SolarReserve has now experienced almost four months with no avian mortality at its 110 MW Crescent Dunes CSP project in Nevada, after changing the algorithm for aiming heliostats in standby position, Smith said.

Crescent Dunes is currently in its commissioning phase. It is the second utility-scale power tower to be built in the US, after Brightsource’s 377 MW Ivanpah plant, and the first to include storage.

In January 2015, 115 birds were accidentally killed at Crescent Dunes when heliostats were in standby. SolarReserve halted testing that day, put a new standby algorithm into effect, and resumed testing.

SolarReserve changed the tight ‘donut’ of intense solar flux above the receiver, to a diffused ‘pancake’ of flux points, spread over the entire heliostat field, in which no more than four ‘suns’ transect any one position in the air.

"Since we fixed the problem, some days we've been in standby for five, six, seven hours in our plant commissioning process and had zero fatalities since the end of January,” Smith said.

Standby period key
The low daytime standby use on CSP power towers with storage also reduces the period in which birds are at risk, Smith said.

"The critical period for birds isn't when we are pointing at the tower; it is when we are in standby,” he said.

According to Tim Conner, VP of Engineering and Technology at SolarReserve, and ex-Rocketdyne engineer, the receivers on the SolarReserve plant seem to create a deterrent for birds.

“Birds appear to avoid the glare, the brightness of the receiver. When it is in operation it’s very bright white. You don't want to look at it. Birds do the same thing. They tend to fly around it.”

The Crescent Dunes project uses a molten salt storage system, where heliostats focus on the receiver all day.

Solar flux heats only solid objects at a focal point where multiple reflections transect, and is the basis of power tower technology. When focused on a receiver, only the receiver is heated, not air.

Because Crescent Dunes constantly heats molten salt, it goes into standby typically only at startup. All day the molten salt gets reheated, not only in full sun, but during partial cloud cover.

“Once we go into operation, we’ll maintain the focus of the heliostats on the receiver for pretty much all day, regardless of the cloud cover,” said Smith.

The need for standby cannot be entirely removed, and this requirement will depend on the thermal sensitivity of the power tower designs of different projects, Craig Turchi, Senior CSP Engineer at National Renewable Energy Laboratory, said.

“Depending on the cloud duration, any tower, with or without storage, will need to move some heliostats to standby during cloud passage to avoid thermal shock to the receiver when the sun re-emerges,” he said.

“Changes on the receiver will still vary with the sun and thermal sensitivity will depend on the receiver design.”

On the Crescent Dunes plant, SolarReserve changes the molten salts flow rate to control the temperature in the receiver and absorb thermal shock.

Steam-based storage

Steam-based storage reduces standby needs on direct steam power towers, according to former Abengoa Solar COO Scott Frier.

Steam driven plants such as Brightsource’s Ivanpah facility must adjust the number of heliostats on the sun when clouds pass over, to prevent damage to the steam turbines when proper steam condition can’t be maintained. With no storage, a direct steam power tower is an on/off system.

“Steam storage gives similar benefits to molten salt storage in that there are not typically many situations the plant might need to be placed in the standby mode,” Frier said.

The lower mass of steam means it cools faster than salt, but it is also a simpler to reheat it. Frier said.

Steam acts more like a storage buffer, as it is not as efficient or robust as salt, he said.
Steam-storage power tower projects include Abengoa's 50 MW Khi Solar One plant in South Africa, which has entered the commissioning phase.

BrightSource Energy has stared the permitting procedure for two 135 MW power towers with steam storage at Qinghai in China.

Storage of any type appears to add benefits to avian populations, but further research is needed, NREL’s Craig Turchi said.

NREL has recently released its Review of Avian Mitigation and Monitoring Information at Existing Utility-Scale Solar Facilities.

“Yes, we believe the greatest threat occurs during standby, but we cannot say that there is no threat when the heliostats are on the receiver,” Turchi said.

“I don’t believe we have sufficient data to state anything definitive…This is a major question, because we can make changes to the operating protocols to minimise the airspace experiencing high flux during standby, but there is less flexibility to change anything when on-sun.”

The Khi Solar One and Crescent Dunes plants will add further data to this research when both towers come online this year.