Solar Under Storm Part II: Designing Hurricane-Resilient PV Systems

The last three years have seen major hurricanes wreak havoc on island nations. Hurricanes Harvey, Irma, Maria, and Dorian caused catastrophic damage to homes, infrastructure, and economies throughout the Caribbean basin. At the same time, the use of solar photovoltaic (PV) power is increasing on Caribbean islands—helping reduce costs and improve electric grids. And while some solar installations were destroyed by the hurricane force winds, many remained intact. In 2017, Rocky Mountain Institute sent expert structural engineering teams to the Caribbean region to investigate survivals and failures among ground-mounted PV systems. RMI’s Solar Under Storm report published in 2018 discussed the root causes of PV system failures from hurricanes and described recommendations for building more resilient solar PV power plants.

RMI and partners recently returned to the Caribbean to look at the same occurrence among roof-mounted PV systems. Why did some of the roof-mounted PV systems survive hurricanes virtually unscathed while others suffered extensive damage? RMI’s latest report, Solar Under Storm Part II: Select Best Practices for Resilient Roof-Mount PV Systems with Hurricane Exposure, developed and written with the Clinton Climate Initiative and FCX Solar, answers that question, and proves that rooftop solar PV can be made resilient at little additional cost.

Failures Are Only Half the Story

As buildings and roofs vary widely, it’s difficult to have one overarching conclusion to explain the diversity of outcomes hurricanes have on roof-mounted PV systems. In order to help guide solar installers, utilities, grid planners, and even individuals installing their own PV systems, Solar Under Storm Part II contains field observations, over 500 pictures, and expert analysis to recommend ways to increase resilience among solar PV rooftop installations.

It is important to not only identify why some systems fail in hurricane-force winds. It is also crucial to develop cost-effective ways to avoid these failures. To do this, RMI looked at both the expected material cost to implement a mitigation action to avoid a certain failure and the impact that mitigation action will have on total cost of ownership. The total cost of ownership can be decreased in two ways: reducing the total economic damage and reducing the frequency of individual failures. By reducing the total economic damage, less money is spent on replacing parts. And reducing the frequency of failures increases the amount of time the PV system is up and running by minimizing the time spent fixing minor issues, especially cascading issues that could lead to major, expensive failures if not immediately addressed.

The report lays out the different ways the team noticed the PV systems failing, the potential mitigation action, and if that action would be low, medium, or high cost on a per watt basis.

Recommended Practices

The recommendations for building more resilient rooftop solar PV are organized into two categories: specifications and collaboration. Specifications include which types of fasteners to use, identifying a project QA/QC process, where and how to attach the modules to the roof, specifying high-load PV modules, and more. For example, many systems use top-down clips to join together groups of modules. RMI observed these clips failing in 96 percent of the systems, meaning if one module gets loose and comes off the roof, the adjacent ones can easily become dislodged as well. Our recommendations include using top-down clips that only retain one module or specify that each module be through bolted to the roof.

The importance of collaboration cannot be stressed enough. All parties involved need to talk to each other. These recommendations include suggestions such as designers and installers collaborating with module and racking suppliers for more efficient solutions to solve wind load challenges, and installers and modules suppliers collaborating to ensure local availability of specified modules, racking, and fasteners. It’s one thing to specify a specific rooftop solution that will withstand hurricane force winds, but all of that equipment must be locally available and at a reasonable price point to ensure the resilience of the system.

The Cost for Resiliency

Despite what one might think, the additional cost to implement the recommendations in both Solar Under Storm reports is relatively small. RMI estimates that in general, commercial projects would incur an increase of approximately 5 percent in engineering, procurement, and construction costs to implement the recommended best practices. On a typical 20 kilowatt pitched roof installation, that translates to an extra $2,200. This is money well spent upfront for those projects that are planned to provide clean renewable energy for 25 years.

Resilient Energy in the Face of Hurricanes

Generating energy with solar PV is a cost-effective and reliable solution for power generation in the Caribbean. Currently, there are over 570 megawatts of solar energy installed across rooftops, parking canopies, and large tracts of land throughout the Caribbean islands. Yet the increasing frequency and severity of storms poses a threat to these solar PV systems. However, incorporating the recommendations in the two Solar Under Storm reports, can increase the reliability and survivability of these systems, keeping the lights on in homes and schools, water pumps and sewage systems functional, and the medical equipment running in hospitals.

Solar Under Storm Part II provides a set of best practices regarding specifications of equipment and procedures that can be implemented at a fraction of additional cost along with a framework for continued collaboration. This report will help solar professionals deliver clean, reliable energy that can keep providing power even in the face of hurricanes and other extreme storms.