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Solar power project weathers the storm

A 23-MW solar power project in Florida required some extra solid construction due to the hurricane rating and flood zone designation of the area where it was built-all of which prepared the project to stand up well when Hurricane Irma hit this past fall.

By Tony Kryzanowski

It could be called a solar array on steroids because of its robust construction, but it turns out that the extra money invested to bulk up the $40 million, 23-megawatt (MW) Big Bend Solar Project owned by Tampa Electric Company (TECO) in Florida was a good investment, given how well the array withstood high winds during the recent hurricane season.

Hurricane Irma hit the Tampa Bay area in early September 2017 and knocked out power to 425,000 TECO customers, representing 60 percent of the company's customers. But the Big Bend Solar Project site sustained no damage.

"It was great news to hear that it had made it through the hurricanes unscathed," says Lauren Busby Ahsler, who was the structural engineering manager and also the project engineering head for SunLink, who designed the mounting system for this project, located near the Big Bend Power Station at Apollo Beach.

"We were confident in our design and that we had followed the building code to the appropriate levels for what occurred," says Busby Ahsler.

The project, constructed by Burns and McDonnell, is located on 106 acres and consists of 202,000 frameless, thin-film, PV panels supplied by First Solar, installed in 1772 rows and supported by 14,000 direct-driven, corrosion-protected steel piles. Since it is a tracking solar system, it is estimated that the array will be able to achieve a 20 percent energy production gain. SMA provided the inverters for the project.

The Big Bend Solar Project is the first utility-scale solar project undertaken by TECO and is the largest solar array constructed in west-central Florida. Construction was completed in March 2017, and it provides power to about 3300 homes. Since 2000, TECO has invested about $50 million in solar power projects and has recently announced major new investments.

"In the next four years, we will add six million solar panels in 10 projects for a total of 600 megawatts of solar generation-enough electricity to power 100,000 homes," says Cherie Jacobs, spokesperson for TECO. "When complete, nearly seven percent of our energy generation will come from the sun, which is the highest percentage of any utility in Florida. Our investment will top $850 million for these projects."

 
  

While it passed a major performance hurdle having survived the recent hurricane season in Florida, the project represented what might be described as almost the perfect storm of engineering design challenges. A typical solar project might present one or two major challenges, but Busby Ahsler says it is rare for a project to present as many as those on the Big Bend Solar Project, rating it as one of the most difficult projects from an engineering perspective that she has ever been involved in. Some of the challenges were known during the early design stage. Others only became known as the project progressed.

"We really had to do some serious engineering to make this project a reality," says Busby Ahsler. "It was really and truly an engineering feat, given all the circumstances. This one was a challenge just to figure out how to make it happen. From that perspective, it was really gratifying for us to be able to deliver."

Jacobs says that TECO was aware of the hurricane rating and flood zone designation of the area. This was factored into the cost estimates and the project request for proposals (RFP) specifications.

SunLink learned a lot from this project in terms of general robustness that can be engineered into future solar projects located in similar environments and facing similar climatic challenges.

Frequent flooding based on maps provided by FEMA was a concern with the Big Bend site, given its location next to Apollo Beach. To address these concerns, SunLink designed its racking system to four different heights. It sits 2' 6" off the ground at its lowest point and 10' 3" off the ground at its highest point, because the site decreases in elevation as it approaches the Gulf Coast. Busby Ahsler says it was like designing four different projects at one time. Adding to the stress was a short window of only six months from the 23-MW project being sold to delivery. Luckily, the project developer was able to avoid any hurricanes.

 
 A challenge with the Big Bend solar project was the potential wind speeds that the solar array could encounter-SunLink designed a robust enough module support system that could withstand those wind speeds. Most U.S. solar sites are designed for a maximum 105-mph wind load, and Big Bend was designed to 135 mph.
  

Another issue was the corrosiveness of the soil, which wasn't fully understood until SunLink reviewed the geotechnical reports in detail.

"We had galvanization and an epoxy coating on the posts to give them additional levels of corrosion protection because of the corrosiveness of the soil and the likelihood of flooding that brings in saltwater," says Busby Ahsler.

A third issue was the potential wind speeds that the solar array could encounter, and the need by SunLink to design a robust enough module support system that could withstand those wind speeds. Most U.S. solar sites are designed for about a maximum 105 mph wind load. This one had to be designed to 135 mph.

"We certainly knew that it was in a hurricane zone, and we knew that most of our competitors were not designing the type of modules for locations with wind speeds that high," says Busby Ahsler. "From my experience, they seem to top out at 130 miles per hour, and this is 135. So it was at a higher wind speed than most people are designing those modules for."

The Big Bend support structure required thicker steel components. There were also modifications to the rail design to support the PV modules, which in itself provided a challenge being that they were frameless, thin-film, PV modules.

"Because they don't have a frame, there is no structural support provided by the module frame itself," says Busby Ahsler. "So in designing our rails, we needed to support the full length of all of those modules, rather than being able to count on the frame for supporting a portion of it."

 
Frequent flooding, based on information in maps provided by FEMA, was a concern with the Big Bend solar project site, given its location next to Apollo Beach. To address these concerns, SunLink designed its racking system to four different heights. 
  

SunLink conducted considerable in-house load testing on its module and rail assembly to ensure that it could handle hurricane force winds. It designed the support structure with longer rails and four support points as specified by First Solar for these modules, and with the highest wind load capacity they have ever designed into a module support structure of this type.

Certain aspects of the structural design also had to be considered together to provide a workable solution. For example, the array at its highest point had to be designed for higher wind pressures because of that elevation, and engineering work was required on the foundation elements of this portion of the array to ensure that it was strong enough to withstand both the flooding and the wind load challenges. Overall, the array was equipped with typical, wide-flange driven posts but they were driven deeper, as the soil consistency was not as solid as on other solar project sites.

"There were a lot of things learned from this project," says Busby Ahsler. "I think there are a lot of challenges to look for and plan ahead for and try to spot further into the future. I think we do a better job of researching flooding risk at our sites now that we've gone through the design for a site that has flooding. I think we understand what design changes are required to accommodate that."

The same would apply for potential future corrosive environments and designing for areas that typically experience higher wind speeds.

"We've established good testing processes to allow us to use First Solar up to these higher wind speeds, and that continues to be incorporated into our product offerings," she says.

The Big Bend experience demonstrates to developers planning solar projects in coastal zones with higher wind speeds that they should expect potentially higher capital costs, to bulk up and customize solar array structures, so that their investments survive over the long haul in this environment. But at the same time, having renewable power alternatives designed to withstand local weather and environmental conditions can also add to the resiliency of the power supply infrastructure of power companies doing business in these areas. This might represent a niche marketing opportunity for solar developers with the tools, experience, and project success to deliver projects like this in future.

That potential benefit was not lost on TECO when they proceeded with the Big Bend Solar Project.

"Solar has been determined to be a component of system reliability," says TECO's Cherie Jacobs.

 


March/April 2018