Crescent Dunes CSP: Cutting Edge Engineering For Solar At Night

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Innovating at the lab bench scale is very different from actually building completely new energy technologies that must be pioneered from the ground up, at full scale, in the real world.

Ten hours of solar thermal energy storage now supply Las Vegas with solar power from midday to midnight.

Cresent Dunes solar after dark

SolarReserve’s 110 MW Crescent Dunes Concentrated Solar Power (CSP) plant just came online in Tonopah, Nevada. Its technology was literally invented by rocket scientists, and demonstrated at “lab scale” at the 10 MW Solar Two pilot project with DOE funding.

But part of commercializing a breakthrough technology at full scale is negotiating the difference between technology design and boots-on-the-ground engineering for manufacturing.

As first-of-a-kind, Crescent Dunes was almost handmade

“Molten salt power towers are very new, and especially ones the size of Crescent Dunes. This is the largest one out there with integrated storage,” Dave Morse, Operations Director for Automation at Delta told Cleantechnica in a Skype interview.

“As the first-of-a-kind, Crescent Dunes was almost handmade” added Kevin Smith, CEO of SolarReserve.

SolarReserve created the algorithms controlling the collector field and the pointing of the 10,347 billboard-sized tracking mirror heliostats.

As the go-to firm for industrial automation, Delta’s motion control software supports these algorithms with local control at each heliostat so that each one precisely reflects and concentrates sunlight onto the large 100-foot receiver at the top of the 540-foot tower.

Delta makes high-speed control systems and servo motors for extremely precise motion control for a wide range of applications.

“Most of the engineers in the world know that that’s what we do and they come to us with their motion-control ideas,” Morse said.

With breakthrough technologies there is no help out here. You are on a path nobody has been on; there’s no rule to follow

The thing with breakthrough technologies is that “there is no help out here,” he pointed out. “There is really nothing you can look up; because you actually are trying to invent the wheel. You are on a path nobody has been on; there’s no rule to follow.”

The way trackers are deployed in PV, it is relatively easy to point a tracker at a moving object like the sun.

Similarly, tracking in trough CSP (as opposed to tower) is easy; the rows of parabolic mirrors concentrate sunlight onto a pipe running along right in front of each row. A heat transfer fluid (HTF) in the pipe is heated. It is easy to point the parabolic reflection at the nearby pipe.

“But it is much harder to point a reflection of the sun at something somewhere else; as we did at Crescent Dunes aiming at the distant tower,” he explained.

“It is basically trigonometry that you’re doing, every second of the day; trying to make sure that you hold that position.”

“As the sun is moving, the angle changes. The sun constantly moves, but what you’re trying to point it at doesn’t. It’s very difficult to actually say, okay; where is the sun. It is basically trigonometry that you’re doing, every second of the day; trying to make sure that you hold that position.”

Each heliostat includes its own on-board computer to do the math for its own unique position in relation to the tower all day (and through the seasons as the sun’s angle in the sky changes).

And that is, if all else proceeds according to plan. But in the real world, perfection in meeting a first-of-a-kind spec is not guaranteed.

“If a mirror manufacturer couldn’t hold a previously agreed-upon specification for the high reflectivity required, it would be our jobs at Delta to compensate,” said Morse.

This greatly increased the precision in pointing that Delta had to achieve.

The precision had to be plus or minus a milliradian

“With higher reflectivity, if the precision was plus or minus half a degree it would be okay. But with a lower reflectivity, the precision had to be plus or minus a milliradian,” said Morse.

A milliradian is a much more precise angular measure than a degree. It is 1/6400 of a degree.

“The bottom line is, it’s challenging to manufacture today’s technology to the level that we would expect, because we’re at the cutting edge. Our job is figuring out how to design cutting edge technology for mass production, and that’s really the key difference between what we can dream up in the lab and what’s real.”

These challenges are what made working on Crescent Dunes exciting. “Tower CSP is far more interesting to us. Anyone can do trough. It’s very simple, and we do quote on it, and we do have business in it, but it’s not our focus,” said Morse.

During the process of constructing the 110 MW tower project, both Delta’s and SolarReserve’s engineers were discovering ways to do things better. “Part of our ongoing activities is advanced R&D, including cost analysis,” Smith said.

Crescent Dunes commissioning
The Crescent Dunes heliostat field cost $300 million, almost half of Crescent Dunes’ total cost. For its next project, SolarReserve believes it can get that down to $200 million, a 30 percent reduction, with improvements in pointing accuracy, and with less steel and thinner glass.

Software also had to compensate for any leaning of an individual pedestal supporting its heliostat due to slight settling of the concrete or the backlash

For Delta, accustomed to controlled indoor settings, bringing these kinds of precision controls outdoors bought unexpected real-world challenges. The software had to compensate for any leaning of an individual pedestal supporting its heliostat due to slight settling of the concrete or the backlash.

“Even walking around the five square kilometer site is difficult with equipment in tow. The heliostats are very large and tall, which required us to use a 4-wheel drive scissor lift trucks to gain access to the motors which are higher than four meters in the air,” said Morse. “Our vehicle got stuck on several occasions.”

CSP requires high direct normal irradiance (DNI), with none of the atmospheric losses caused by scattering or absorption from floating particles and gas molecules found around industrial or agricultural zones.

The clear high desert of Tonopah, the stargazing capital in America, with unparalleled views of the Milky Way, makes for ideal DNI nearly 365 days a year. Site manager Brian Painter at SolarReserve recently suggested powering half of Nevada or sending power to California, by building ten more tower CSP projects near Tonopah!

Before permitting, SolarReserve had monitored the site for its weather patterns over several years, and output models account for the relatively seldom dust events.

However, the occasional sand storm sand created its own challenges.

“We had several days where testing was completely halted due to sand storms, as visibility was down to 30 meters; so on those days, we couldn’t see the sun spot on the tower to conduct any calibration,” Morse said. “Even though they were perfectly blue sky days, Sand Devils would sprout and cover the team in sand, or worse, blow sand inside the control cabinets while working on them.”

How molten salt tower technology makes solar power at night:

Highly focused sunlight is reflected off Crescent Dunes’ 10,347 heliostats – aiming thousands of “suns” onto the tower receiver.

Molten salt flowing through piping in the receiver absorbs the heat from this concentrated sunlight – heating from so-called “cold” at 550°F, up to over 1,050°F “hot” – then flows down piping inside the tower into a thermal storage tank.

The tank holds enough in storage to dispatch up to ten hours of electricity at nameplate capacity every day, or 1,100 MWh a day.

To generate electricity, whether the sun is shining or not, stored molten salt is sent through a heat exchanger to boil water for high-quality superheated steam driving a steam turbine.

Storage in molten salt makes a thirty-year supply of solar, dispatchable day or night

Initially heating the solid salts from room temperature up to their molten state took about a month, but it can then be kept within this range of temperatures while circulating in operation daily for thirty years.

If not used, this “stored sunlight” can stay available for months, but SolarReserve’s contract with NV Energy requires each day’s storage be used either that night, or be dispatched on request by the utility as needed.

Coming up, Delta is now moving into the same booming CSP markets overseas as SolarReserve; South Africa, China, Chile, and Australia.

“SolarReserve is an amazing partner and they did everything right,” Morse added. “For those working through these challenges, it is a great sense of accomplishment and we are proud to be part of the Crescent Dunes project.”

Image Credits: SolarReserve

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