The sun shines bright over the southwestern U.S., where you’ll find a majority of the nation’s ground-mounted solar installations. However, utilities in other regions such as the Northeast and Midwest are also embracing the benefits of generating energy from the sun. While it is valuable for those areas to invest in renewable technology, additional challenges exist.
One major concern for solar farm owners is corrosion of steel components. Solar panels are generally mounted on steel pile foundations that are embedded in the soil. Since they are made of steel, some subsurface corrosion will occur over time, but owners can take steps to plan for the amount of deterioration that will take place once the piles are put into the ground.
During the early stages of the design process, geotechnical engineers will conduct a series of tests to gather information on the physical properties of the soil at the planned project site, including any risks that could contribute to corrosion. These tests, which can establish the presence of salts, electrical resistivity, redox potential and soil moisture retention potential, are provided to the corrosion consultant, project manager and/or structural engineers. The information is used to perform calculations to get a corrosion rate, or the speed at which the foundation piles will deteriorate specific to an environment.
Limiting Foundation Pile Corrosion
A combination of the corrosion rate, the project owner’s goals and the desired design life of the solar installation assists engineers with decisions on how to prevent foundation pile corrosion, or how to design the project to allow for corrosion but limit the amount. While there currently is no official standard for corrosion design, the primary way the industry is mitigating corrosion is by using galvanization and sacrificial steel. The galvanizing layer will deteriorate before the underlying carbon steel it’s protecting. Corrosion calculations will account for the expected metal loss over the installation’s desired design life and will help determine the recommended thickness of the sacrificial steel.
Another option to mitigate corrosion is to recommend a corrosive-resistant coating on the piles, such as an epoxy. The coating can help prohibit surface corrosion damage; however, some localized corrosion can still occur. Also, as the piles are driven into the soil the epoxy coating could be damaged, which could expose portions of the pile to external elements.
Although rare in solar installations, sacrificial anodes can be installed to drive corrosion to another point in the system. The anode is made of a metal alloy, usually zinc, aluminum or magnesium, that has a more active voltage than the metal it’s protecting. The sacrificial anode corrodes instead of the steel pile.
Active corrosion protection is another option and involves using an impressed current system — a small amount of electricity from the generating solar station — to direct the chemical reaction (corrosion) elsewhere, away from the piles.
Concurrent with corrosion modeling, an in situ corrosion pile test program could be used to validate the corrosion design. The test program would spare additional (not part of the structural system) foundations to be installed throughout the plant and removed over time to be measured or weighed for galvanizing or steel loss. These measurements would either validate the corrosion design or indicate that mitigation measures must be taken to protect the foundations to the project design life.
Determining Corrosion Rates
A few approaches can be taken to calculate the corrosion rate of metal. The U.S. Department of Transportation’s Federal Highway Administration (FHWA) uses data collected from decades of infrastructure projects to publish information about corrosion and coatings. The research FHWA provides also typically matches recommendations provided by other industry resources and corrosion consultants.
Selecting the Right Project Path
Multiple soil conditions can impact the steel/subsurface interface of foundation piles. Due to the lack of standards and the varying nature of soil corrosion, the risks to pile foundations should be considered in all aspects of design. It’s up to project owners to use the information to decide if they want to take a conservative or middle-of-the-road approach in measuring risk against corrosion or if they want to limit their focus on corrosion mitigation efforts altogether. Professionals who have studied steel pile corrosion at solar installations are key resources that can assist in projects of any size.
Buckeye Partners partnered with Burns & McDonnell on a new 164-MW solar energy project in Texas. Once completed, it will generate enough energy to power 28,000 homes annually.
