Power of the Sun

When completed, the Ivanpah Solar Electric Generating System (ISEGS) in California’s Mojave Desert will look striking and reduce carbon dioxide (CO2) emissions by more than 400,000 tons per year. The ISEGS innovations will also provide much-needed power during the peak hours of the day to more than 140,000 homes in California.

ISEGS is currently the largest solar plant under construction in the world. It is a 392-MW gross solar complex using more than 300,000 garage-door-size mirrors to focus the power of the sun on solar receivers atop power towers. The solar receivers are like traditional boilers, turned inside out. The project is located about five miles from the California-Nevada border on federal land managed by the Bureau of Land Management.

BrightSource Energy designed the proprietary solar thermal energy system, which generates power the same way as traditional power plants — by creating high-temperature steam to turn a turbine. However, instead of using fossil fuels or nuclear power to create the steam, the system uses the sun’s energy.

The ISEGS complex is comprised of three separate plants to be built in phases between 2010 and 2013, and will use BrightSource Energy’s Luz Power Tower solar thermal technology. The Ivanpah facility is the first BrightSource solar power plant to utilize this type of technology.

Partnering for a unique grade control technology solution

Bechtel, the San Francisco-based engineering, construction, and procurement firm, is responsible for managing the construction process of the ISEGS project. The firm has more than 2,000 employees working on the site.

“I worked with grade control technology in the past and felt comfortable soliciting input early in the process to see if there might be an efficient GPS machine control solution for building the solar fields,” states Mark Wagner, lead area superintendent with Bechtel. “We were looking at the precision placement of 173,500 pylons scattered over 3,600 acres, so traditional staking would be incredibly time consuming and very costly.”

It was clear that the ISEGS project would be more than a typical machine control system project, so Dave George, commercial manager-worldwide with Trimble brought in the local Trimble dealer and appropriate product and machine experts from within Trimble, including personnel from the U.S., New Zealand, and Holland. The project required a highly customized solution.

The three parties — Bechtel, Trimble, and the local Trimble dealer, Cashman Equipment Company — determined that an excavator-mounted vibratory hammer could work if the Trimble technology and custom attachment could work together. They travelled to vibratory hammer manufacturer, Hercules Machinery Company (HMC), headquarters in Fort Wayne, Indiana, to create a custom vibratory hammer that could be mounted to a long reach excavator, with the ability to traverse the ephemeral washes of the site and still meet the tight tolerances required for the pylon insertion.

Trimble experts recognized that determining the exact location points for the pylons with the machine — not with surveyors — was the most effective solution … a solution that was found in an unconventional source.

Reducing environmental impact is a priority

“We were looking at 173,500 holes that needed to be drilled and the same number of pylons that needed to be inserted,” Wagner says. “The rigs had to have the accuracy of a surveyor’s rod AND the ability to collect the pylon’s ‘as-built’ data.” The answer came from Trimble, who had experience with Trimble HYDROpro Construction Software, a software package for specialized waterway and coastal construction tasks that require precise point positioning.

HYDROpro Construction’s pile feature is used for precise pile positioning applications for bridge and wharf construction. Trimble engineers were able to modify this powerful construction software for use on the Mojave Desert site where water is scarce and the average annual rainfall is less than five inches.

There was three different rounds of testing before the custom approach was adopted for implementation. BrightSource has stringent requirements for its outcome and how the project would be completed.

“BrightSource had tested and rejected four other pile driving systems from other suppliers,” states Wagner. “We were happy to provide a creative solution that would work, while accommodating the stringent environmental concerns we needed to operate within.”

Because of environmental impact concerns regarding disruption of soil, plant and animal life, the site was not graded before the pylons were placed. The ISEGS site is under the jurisdiction of the California Bureau of Land Management and requires minimal environmental impact. The site’s three solar fields have alternating bands of “drive zones” and “no-drive zones.” The excavators are allowed to maneuver and operate in the 130-foot-wide, no-drive zones due to their innovative design.

The excavators are equipped with custom, shortened masts manufactured by HMC, so they can safely reach multiple pylons from one location within the no-drive zones. The excavators have wide tracks with triple grousers for low impact. Their long reach and multiple degrees of freedom of the mast mean that they don’t have to drive up to each pylon like a traditional pile-driving rig. Since the drill and the vibratory hammer are interchangeable on the mast, the same make and model excavators are used for both drilling and insertion. That means they travel in the same tracks with each pass through the solar field, reducing impact to the desert.

GPS makes for precise pylon placement

The Trimble HYDROpro Construction system is used strictly for its positioning and data collection capabilities, which saved Bechtel approximately 10- to 30-minutes of surveyor time per pylon, to lay out the holes, lay out the pylons, and collect the as-built data after the pylon was inserted.

“Our excavators with the HYDROpro systems provide precise XY location, as well as verticality and orientation,” states Wagner. “Trimble also helped us collect Z (sea level) elevation of the top of the pylon, which is fed into and collected by HYDROpro.”

The BrightSource Ivanpah Solar Electric Generating System has 14 excavators working on the site, and all are equipped with Trimble HYDROpro Systems. The Bechtel crews are averaging between 75 and 125 pylons per day, per machine.

Each excavator cab is outfitted with a Trimble Tablet handheld computer running HYDROpro System software. The machines have two receivers — a Trimble SPS852 GNSS Modular Site Positioning System Receiver and a Trimble SPS552H Heading Add-on GNSS Receiver, which is designed to calculate the precise heading of the machine, so that all the pylons can be oriented per specification.

Additionally, the Trimble system was instrumental because GPS was required by BrightSource for converting the latitude and longitude into an Earth Center, Earth Fixed location (ECEF). BrightSource uses a center of earth location in relation to the sun so that each mirror in its solar generating system is precisely aligned.

The environmental impact concerns are tremendous and govern how work is completed. At any given point, there are 60 to 90 biologists and botanists on site making sure that endangered plant life and wildlife are not harmed. Rare plants, snakes, jackrabbits, lizards, and the endangered desert tortoise, are moved offsite or into nurseries until the project is complete. If a desert tortoise wanders into a work area, everything must stop until a biologist can safely remove it from harm’s way.

BrightSource’s Ivanpah Solar Electric Generating System is on schedule and becoming a reality in California’s Mojave Desert. It is currently the largest concentrated solar plant under construction in the world. According to BrightSource, the ISEGS site will become a success story for solar electric generating facilities to be built all over the world.

To read the full story, click here to download the Spring 2013 issue of Sustainable Construction.