The 2017 hurricane season was one of the most active in history. Hurricanes Harvey, Irma, and Maria brought widespread destruction throughout the Caribbean. In addition to the emotional toll these severe storms had on people in the region, the disruption of critical infrastructure left many communities without such basic services as electricity for prolonged periods of time.
Over the past decades, electricity in the Caribbean has been primarily generated centrally by fuel oil or diesel-fired engines and distributed across the island by overhead lines. However, in recent years, electricity has been supplemented in homes, businesses, industries, government facilities, and utilities by solar photovoltaics (PV). In fact, over half of Caribbean electric utilities already own or operate solar PV as part of their generation mix. Over 225MW of solar is installed across rooftops, parking canopies, and large tracts of land. Solar PV is the most rapidly growing source of power for many Caribbean islands.
Despite the record sustained wind speeds of over 180 miles per hour, many solar PV systems in the Caribbean survived. Some solar installations in the British Virgin Islands, Turks and Caicos, Puerto Rico, and St. Eustatius faced tremendous wind yet continued producing power the following day.
Hurricane Maria aftermath in Puerto Rico. People attempt to remove broken power poles that landed on top a food truck in Vega Alta, Puerto Rico
In contrast, some PV systems in Puerto Rico, the US Virgin Islands, and Barbuda suffered major damage or complete failure with airborne solar modules, broken equipment, and twisted metal racking.
Recommendations for increasing resiliency
Generating energy with solar PV is a cost-effective and reliable solution for power generation in the Caribbean. Incorporation of the best available engineering, design, delivery, and operational practices can increase the reliability and survival rates from extreme wind loading. Given the variability in wind speed, wind direction, wind duration, topography, design, and construction, along with limited data, we cannot give an overarching statistical conclusion to explain survivorship versus failure. Instead, this guide combines recent field observations along with expert analysis to deliver actionable recommendations for increasing resiliency among retrofit and new construction solar PV installations.
Expert structural engineering teams were deployed to the Caribbean region in the fall of 2017 to investigate root causes of solar PV system failures in the wake of Hurricanes Irma and Maria. They uncovered several root causes of partial or full system failure through observation and determined several potential failures that could have occurred if other failures did not occur first (lurking failure modes). Some similarities of failed systems in the wake of Hurricanes Irma and Maria:
1. Top down or T clamp failure of modules
2. Undersized rack or rack not designed for wind load
3. Lack of lateral racking support (rack not properly designed for wind loading from the side)
4. Undersized bolts
5. Under torqued bolts
6. Lack of vibration-resistant connections
7. PV module design pressure to low for environment
8. Use of self-tapping screws instead of through bolting
Some common ground-mount PV attributes of surviving systems in the wake of Hurricanes Irma and Maria:
1. Dual post piers
2. Through bolting of solar modules (no top down or T clamps)
3. Lateral racking supports
4. Structural calculations on record
5. Owner’s engineer of record with QA/QC program
6. Vibration-resistant module bolted connections such as Nylocs.
The key output of the Rocky Mountain Institute recent report "Solar Under Storm" is a list of recommendations for building more resilient solar PV power plants.
• Specify high-load (up to 5,400 Pa uplift) PV modules, based on structural calculations; these are currently available from a number of Tier-1 module manufacturers.
• Require structural engineering in accordance with ASCE 7 and site conditions, with sealed calculations for wind forces, reactions, and attachment design (ground-mount foundation).
• Confirm with racking manufacturer that actual site conditions comply with their base condition assumptions from wind-tunnel testing.
• Specify bolt QA/QC process: there were several instances of inadequate torqueing of bolts in the investigation—a workmanship and oversight issue.
• Specify bolt hardware locking solution.
• Specify through bolting of modules as opposed to top-down or T clamps, or if top clamping is required, use clamps that hold modules individually or independently.
• Require structural engineer review of lateral loads due to racking and electrical hardware—often lateral loads are missed and recent failures have proven them to be a critical source of weakness (e.g., combiner boxes attached to end solar array posts caused increased loading and led to failure).
• Do not recommend trackers for projects in Category 4 or higher wind zones.
• Specify all hardware be sized based on 25 years (or project life) of corrosion.
• Do not recommend any self-tapping screws.
• Specify dual post fixed tilt ground mounts, which significantly reduce foundation failure risk.
Collaboration recommendations identify opportunities for increased resiliency, which require multiparty consideration and action but do not represent current industry standard actions.
• Collaborate with module suppliers for implementation of static and dynamic load tests representative of Category 5 hurricane winds.
• Collaborate with racking suppliers for full scale and connection test representative of Category 5 winds.
• Collaborate with equipment suppliers to document material origin and certificate of grade and coating consistent with assumptions used in engineering calculations. Perhaps the most opportune recommendation is for a regional and even international community of solar PV power plant stakeholders who have extreme wind exposure to regularly share lessons learned from new designs and extreme wind events. To that end, we formed a PV Resiliency working group on the online Caribbean Renewable Energy Community (CAREC), which is hosted by CARILEC, to connect, innovate, and collaborate. Join the working group at CAREC.
Photo at top by Kenneth Wilsey for FEMA - Humacao, Puerto Rico, Jan. 25, 2018
Written by Christopher Burgess, Director of Projects for the Islands Energy Program and Joseph Goodman, Principal with RMI's Electricity Practice. Excerpted from Rocky Mountain Institute report “Solar Under Storm”.