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High-precision solar tracking

Concentrated solar thermal power is gaining traction thanks to the massive ground-breaking Ivanpah project in California, which required some high-precision customized solar tracker gearboxes.

By Tony Kryzanowski

As the U.S. celebrates 50 years since the lunar landing of Apollo 11, one is left wondering if in another 50 years the country will be celebrating the utility-scale launch of the massive $2.2 billion Ivanpah concentrated solar thermal power generation complex in the Mojave Desert as a game changer in renewable power generation.

The 377-megawatt (MW) power generation plant, which is located at Ivanpah Dry Lake, California, about 65 kilometers southwest of Las Vegas, features proprietary concentrated solar thermal power technology deployed by BrightSource Energy and is the largest operational concentrated solar thermal power plant in the world.

This accomplishment at Ivanpah has not gone unnoticed, as there are a number of massive concentrated solar thermal power plants currently under construction throughout the world—generally in some of the hottest and driest locales—that are about to eclipse the size of this groundbreaking installation.

The 580-MW Noor-Ouorzazate solar thermal complex on the edge of the Sahara Desert in Morocco is expected to be completely on line next year. BrightSource is in partnership with Shanghai Electric to build a 100-MW solar thermal project in Dubai and has two additional concentrated solar thermal plants in different project phases. One was commissioned in 2018 and another is under development. These are a 121-MW project in Ashalim, Israel, and a 135-MW project with power storage capabilities in the sparsely-populated Qinghai province in northwest China. This is one of many expected concentrated solar thermal power sites to be rolled out as part of China's 1.35-gigawatt (GW) Concentrated Solar Power Commercial Demonstration Pilot Project.

Concentrated solar thermal power generation essentially replaces fossil fuel with concentrated solar thermal energy. A large array of mirrors called 'heliostats' reflect the sun's rays to the top of a boiler tower, which then superheats water or a combination of water and molten salt, reaching temperatures of up to 1,000 degrees Fahrenheit, producing steam to power electricity generating turbines. A heliostat mirror area can range from 25 square meters to 98 square meters.

 
  

As with the lunar landing, it took considerable American ingenuity to reach the point where the project is now providing its contracted power production to its customers, Pacific Gas & Electric and Southern California Edison. The project provides enough power for 140,000 homes, and over its 30-year lifecycle, which will amount to the equivalent of taking 2.1 million cars off the road.

"As the first solar thermal tower facility constructed at this scale, seeing the project through to completion presented a number of challenges—permitting, financing, and engineering—that required innovative solutions and new ways of approaching power plant development," says BrightSource Energy.

The Ivanpah project was financed through a combination of equity and debt and is owned by BrightSource Energy, NRG Energy (which operates the facility), and Internet search giant Google.

The U.S. Department of Energy (DOE) provided a conditional loan guarantee of $1.6 billion for the project, and it was built by Bechtel, the engineering, procurement, and construction contractor (EPC) on the project.

The physical installation has an out-of-this-world appearance, taking in about 14.2 square kilometers. Built in three phases starting in 2010, it consists of 173,500 software-controlled heliostats to track the sun and reflect that sunlight to three 2200-ton boilers atop three boiler towers 137-meters tall. They feature Luz Power Tower 550 technology to heat water to 550 degrees Celsius, providing steam to Siemens SST-900 industrial steam turbines. It began producing power in December 2013.

 
  

In addition to being a testament to American ingenuity, the project also demonstrates how renewable energy development is helping to drive growth in the American economy. Tracking the sun, the heliostats are equipped with Cone Drive solar tracker drives to accurately reflect sunlight to the receivers atop the towers. This Traverse City, Michigan-based company has been making precision motion control technology since 1925, and in 1931, began production of its double-enveloping worm gear technology. The company is known for its impact on the war effort, as during the 1940s many American guns were equipped with Cone Drive gearing in all branches of the armed forces, from aircraft to submarines.

In 2016, the company launched Harmonic Solutions, described as the ultimate in high-precision gearing, representing a significant move into the robotics industry. Before moving into the renewable energy market in 2010, Cone Drive was already selling its products into 25 different market sectors.

Last September, the company was acquired by the Timken Company of North Canton, Ohio, a world leader in engineered bearing and power transmission products. Cone Drive's precision gearboxes were already being used extensively in many power transmission configurations.

The contract to supply solar tracker gearboxes to the Ivanpah concentrated solar thermal project was the company's first significant project in the solar market and the largest production order in its 85-year history, creating 20 new jobs.

For the Ivanpah project, they supplied 95,616 of their Azimuth Drive 89 gearboxes, which precisely rotate the heliostats from east to west.

Jacob Randall, director of Global Strategic Markets at Cone Drive, says that given the high volume of solar trackers required for the Ivanpah project, it significantly changed the company's focus from high-mix, low-volume production of highly precise gear devices to high volumes of duplicate high-precision devices, customized as required for each solar thermal project.

 
 Tracking the sun, the heliostats on the Ivanpah project are equipped with Cone Drive solar tracker drives to accurately reflect sunlight to the receivers atop the towers. For the Ivanpah project, Cone Drive, which is now owned by the Timken Company, supplied 95,616 of their Azimuth Drive 89 gearboxes, which precisely rotate the heliostats from east to west.
  

Randall describes the Cone Drive gearboxes as "mission critical" to the Ivanpah project, as each unit is required to consistently rotate and accurately position the 173,500 heliostats every day over a 30-year period to reflect a beam of sunlight over a distance of 400 meters to a one-square-meter target on the boiler towers. The company's double-enveloping worm gear technology was a good fit for this project because of its precision and torsional stiffness.

"The mirrors out in those fields can range anywhere from 25 square meters to up to 98 square meters, so they are like sails sitting out in the middle of a desert," says Randall. "When you get those high wind loads, it can be detrimental to the actual structure itself."

So it is extremely critical that the drive is very stiff, which is what the Cone Drive double-enveloping worm gear design provides.

The major advantages of their gear design versus a single-enveloping gear design is that there are three to eight teeth engaged versus one or two teeth engaged. It has a higher load capacity, lower contact stress, 20 to 30 percent greater torque, and 300 percent shock load capacity versus only 200 percent on a single-enveloping design.

 
  

"You are trying to get the boiler at the top of the tower to a certain temperature," says Randall. "The hotter you get it, the more efficient the system operates. If some of those mirrors are not focused on that tower, then you are not producing the energy required to heat up the media inside of the system, and so essentially, you are having to put out more of those heliostats in the field to make sure you can reach and achieve that temperature."

He says that it is critical to aim and concentrate every single heliostat in all types of weather conditions—and that none of their competitors can provide a double-enveloping worm gear.

The company has also diversified into solar tracking drives for photovoltaic (PV) systems. Solar tracker drives for both sectors are designed based on the company's long history of gearing design, specifically the double-enveloping worm gear design.

Other suppliers on the Ivanpah project included Riley Power, a subsidiary of Babcock Power, for the three boilers; Gestamp Renewables for the pre-fabricated steel frame for the heliostats; SPX for the air-cooled condensers; Prospect Steel for the structural steel, joists, and furnishings for the towers; and Las Vegas Paving for the civil work. Elementary Energy provided the equipment for the power generating system.

Cone Drive by Timken continues to be a major solar tracking drive supplier to BrightSource Energy, supplying 52,000 units for the Ashalim project in Israel and 70,370 units for BrightSource Energy's project in Dubai.

One ongoing issue with the Ivanpah project is bird mortality when they fly in the path of the concentrated sunlight reflecting to the boiler towers. BrightSource continues to investigate and deploy a variety of proven and innovative bird deterrence measures, some commonly used in other industrial applications, to mitigate the number of bird fatalities.

Among this technology's biggest challenges is cost. Ironically, one of concentrated solar thermal's biggest competitors is solar photovoltaic (PV) technology. Since 2010, when the Ivanpah project broke ground, there has been a significant drop in the cost of solar PV hardware, making it an economical option in many situations, even in head-to-head competition with fossil fuel-generated power. Concentrated solar thermal must also compete with other cheap power-generating sources, like natural gas.

While there is no doubt that concentrated solar thermal power generation technology works, it is not the perfect system for every scenario, and it has some environmental challenges to overcome. It is now at the point of finding its place within the entire pantheon of available renewable power options being developed around the world. As is true with any potential game-changing technology, entrepreneurs around the world are finding ways and places where its deployment makes economic sense.

 


Fall 2019