Ivanpah, the world’s largest power tower Concentrating Solar Power (CSP) project, seems to attract a lot of controversy.

In such novel technology built at such scale, it is perhaps not surprising that there be some confusion. Ivanpah has 377MW net capacity. The first power tower in the US had 10MW capacity. Power tower Concentrated Solar Power (CSP) technology makes electricity by essentially using sunlight to boil water for the steam used to drive a turbine.
Query: "I had the opportunity to fly over the site a couple of weeks ago. I noticed not all the heliostats are aimed at the receiver. Isn’t that inefficient?"

Why aren’t all of the heliostats aimed at the receiver?

"That was never the plan," responds NRG Energy spokesman Jeff Holland for NRG Energy, which is the operating partner in Ivanpah. "To focus all 174,000 would never be necessary and so that would never be a consideration. It has to do with how each heliostat is positioned. Depending on the month of the year, where the sun is in the sky and the amount of clouds or haze, that affects which mirrors are activated."

"If it’s daylight, we are focusing a percentage of the heliostats and mirrors onto the boilers to make the required amount of electricity." But that percentage varies, he says.

For example, in the evening, shadows from mountains cover some heliostats on the west side. At that time, the sun, which is in the west, is reflected off heliostats far over on the east side of the tower.

"We built that amount of mirrors to account for all the different seasonal changes and weather changes that are possible out on site. So there is always a good amount of them that are not in service. But each one of them, eventually, throughout the year will be used. It’s just we don’t need 100% of them 100% of the time."
This power plan smooths generation over the day, not peaking at midday like PV.

"On a good sun day you will have 20% of the heliostats in the peak of the day that do not need to be in operation, because they were designed to compensate for low DNI at the beginning and end of the day," explains Gustavo Buhacoff, who managed commissioning and startup for BrightSource, and who as Director for Operations and Maintenance, is responsible for changing the algorithms controlling the heliostat positioning.

"We only ever have 100% aimed early in the morning after we start ramping up, and towards the end of the day, because we designed this project so that we can produce the peak capacity longer when we don’t have the ideal insolation."

"I saw some heliostats shining at the camera but the heliostat next to it is not, indicating there might be a lot of heliostats with operating problems like broken trackers."

Can the tracker motors break?

"Motors can break," Buhacoff agrees. "But maybe what he saw was part of our aiming policy we talked about earlier to reduce the solar flux."

As described to CSP Today previously, the standby heliostats have now been ‘un’ focused outwards into a much larger ‘donut’ around the receivers of much more diffused solar flux that extends further away from the receivers, not only to reduce glare, but also reducing solar flux dangers to birds.

During the first six months, 321 birds died at the site, about a third of which had been affected by solar flux. The original plan had been to aim standby heliostats to points near the receiver. But that has been found to create unnecessary solar flux, from heliostats in standby.
So, in addition to introducing bird deterrent technologies, in July the heliostat algorithms were changed. The aim of the heliostats looks random, but is not.

"To reduce the impact to avian mortality here, we now aim some in different directions so that the concentration is reduced," explains Buhacoff. "We changed the way we control the mirrors. The heliostats that are not in use are pointed further away from the tower to reduce the flux concentration. So that is actually done on purpose."

This applies only to heliostats in standby.
"The things that we are doing now in both glare and avian mitigation plans have to do with repositioning heliostats that are already at rest and not focused on the boilers," says Holland.

This softer focus also reduced the glare. Some glare complaints had surfaced during the long period of heliostat commissioning, when mirrors were not yet under computerised control.

Since Ivanpah is really the first utility-scale power tower in the US, the glare issue has probably come to the fore for the first time.

"Not one article has ever indicated the amount of power they actually have generating… Most solar projects will publish the power level to inform everyone they are meeting the specifications. When they don’t, you assume they aren’t generating close to the specified power level."
Why no generation numbers? Perhaps nobody asked. It wouldn’t be the first time!

So how is Ivanpah really doing?

"As far as plant operations, I think we’ve produced about 200,000 MWh to date at Ivanpah in the first 175 days of operations," Holland offers readily. "And that’s been a bit in line with our thinking."

NREL projects 1,079,232 MWh annual generation from Ivanpah. But that is once the startup adjustments are out of the data. As by far the largest power tower to come on line so far – engineers have been testing components, which lowers initial generation numbers.

"We did have planned outages for testing, so even though we were technically online, there were still some testing items for some of the equipment that we wanted to make sure was completely synced up after full operations commenced," he explains. "And so they would do various testing on heliostat fields, on focusing of the mirrors, heat intensity, things like that."

Holland believes Ivanpah will be generating along the lines of the NREL projection within a few years. His attitude is unworried. It is to be expected that there will be bumps along the way, in such trailblazing technology. So there is not some big secret being hidden. And NRGEnergy is committed over the long term.

"While moving the needle against climate change comes with new lessons learned, NRG Energy believes that renewable, clean energy is vital to sustaining future generations," says Holland.

"Our commitment to Ivanpah and renewable energy is grounded in the understanding that companies like ours should be leading the race for a clean energy future to reduce the impact of climate change for future generations."

By Susan Kraemer,