Designing PV Systems for Hurricane Conditions

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HURRICANE MARIA, PUERTO RICO – 2017:

Atlantic hurricanesare some of the most destructiveforces on the face of the Earth. Earlier this year, Hurricane Mariaproduced wind gusts approaching 200 miles per hour (MPH) before making landfall in Puerto Rico. The results of these devastating winds were experienced by the operators of many solar facilities on the island, including one of the largest PV systems in the Caribbean, the Humacao Solar Farm. This is an installation that previouslygeneratedalmost half of all solar production in Puerto Rico.

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1: Humacao Solar Farm – Puerto Rico; 5 Days after landfall by Hurricane Maria – 2017


As can be seen in the satellite photos taken after the storm passed, many portions of this facility were entirely destroyed, including some array blocks currently under construction.

This site, and many projects like it in the region would have been designed to the minimum requirements of the ASCE Building code, which calls for design loads to be calculated considering gusts of up to 180 MPH. This specification is much lower than maximum windspeed that can be produced by a large hurricane. This is not a mistake of the building code, but ratheran intentional situation to allow for economical designs in non-essential buildings. But this situation, and the damage sustained during the 2017 hurricane season, highlights the divide between minimum building code requirements and worst-case storm conditions. Even for systems which are designed to all applicable building codes, this alone does not alleviate the risk of severe damage or total destruction from hurricanes. Owners and developers throughout the Atlantic, Gulf Coast, and Caribbean regions must consider not only the code, but many other factors when designing their PV projects to resist the incredible force of nature.

It is no coincidence that the expansion of solar power has collided with the threat of hurricanes more often in recent years. The areas of the world that are regularly affected by the most ferocious of these storms are the same zones which enjoy abundant sunlight, making them ideal locations for solar power production. As we have seen in the aftermath of Hurricane Maria in Puerto Rico, many energy companies are scrambling to position themselves in a position to rebuild the power infrastructure on the island. Many are doing so in a way that incorporates solar power in potentially higher densities than has ever been used in the Tropics before. This rapid re-development certainly presents a unique opportunity to explore how an optimal power grid can be built to embrace the latest generation of renewable energy production, but it also sets up the potential for millions of new solar panels to be installed directly in the path of the strongest windstorms on Earth. 

The direct risks posed to solar systems from hurricanes are usually associated with the extreme wind forces, which can tear panels off of racking and pull foundations out of the ground. Additionally, windborne debris can shatter glass and damage other electrical equipment as well. Rooftop PV systems are particularly at risk, due to their minimized weight and increased exposure to magnified wind forces. Wind speed, direction, and intensity are all highly variable, (even within a single storm) which presents a unique challenge to engineers designing products and structures to resist these storms.

ENGINEERING MITIGATION MEASURES:

In the US (including Puerto Rico and the US Virgin Islands) engineers are required to comply with at least the minimum design windspeeds identified in the ASCE 7 building code. The primary design factors of this building code rely on the wind hazard maps, that describe distinct areas of the US with prescribed design wind speeds.  It is also important to note that the “sustained” windspeed figures quoted by meteorologists and government agencies are not directly comparable to the windspeeds used in the building code. Sustained windspeed is measured over a 1-minute duration, and seeks to provide a measure of baseline wind intensity for meteorological analysis, while the building code employs a gust measurementmade over a 3-second duration, representingthemaximum wind forcethat can affect structures within the path of the storm. While there is no uniformly identified conversion factor to translate between these two scales, empirical data suggests that 3-second gustscan be up to 30% higher than sustained windspeeds.

In addition to predicting wind hazard maps, the ASCE 7-10 Code also provides increased “risk categories” which can be invoked to increase the design windresistance strength of a structure. The highestof these risk categories are usually reserved for essential facilities such as government buildings and emergency services, but can also include toxic material storage facilities or tall buildings in urban environments. Most solar power plants are considered as Category I structures (the lowest risk category found in the code).

There is also a sometimes-overlooked provision of the Code that requires any power system that is used as primary or backup power source for an essential facility, such as a hospital or military base, to be designed to the same risk category as the building that it supports.  Similarly, if a rooftop solar system is installed directly on a building that would fall into a higher risk category (even if it is not part of an emergency power backup) it is considered part of that structure, and should be designed to match the risk category of the building usage below it. In these cases, engineers must consider how the current usage of the associated structure fits within the applicable building codes. Simply applying the same design windspeed that was originally used to construct the building (especially if the building was constructed prior to 2010) is likely to produce an error in the calculations and underestimate the true wind forces on the structure. As can be seen in the satellite photo of the Humacao Solar Farm after Maria, a large amount of the damage to these sites comes from PV modules being ripped off of racking supports by wind forces.

The Engineer of Record on a project has the responsibility and authority to set the design windspeed and other criteria at a safe level. One of the simplest ways that an engineer can design their project to be more survivable in a hurricane is to increase the risk category in their structural design parameters. Even in some of the most at-risk locations, a typical Category I structure is only required to resist 160 MPH winds, which could be expected from a Category 3 storm (approximately 120 - 130 MPH sustained winds). If all components of this projects were instead designed as a Risk Category III structure, its design would be necessarily strengthened to resist forces from winds of up to 200 MPH, which is equivalent to a direct hit from a borderline Category 4 / Category 5 storm.

SITE DESIGN PRACTICES:

While these design techniques can allow the structures and foundations employed in the solar project to resist the direct wind forces of a storm, they do little to address the hazard posed by windborne debris. Many of the existing rows of the Humacao Solar Farm retained almost all their modules on the racking, but these rows are still at risk for breakage due to debris either from outside the facility, or from nearby modules being blown off their mounts and becoming projectiles themselves. In CEA’s experience there are no module manufacturer warranties which cover damage caused by hurricane-strength windstorms.However, there are some tests performed by tier-one module manufacturers for hailstorms can be comparable to some of the impacts sustained during a hurricane.

The typical hailstorm simulated is a series of impacts from a one-inch (25 mm) diameter ice ball traveling at approximately 50 MPH (23 m/s).While the type of impacts sustained during a hurricane are not fully represented by the hailstone test, they can be similar in some ways. In areas of the US coastal plain that are covered predominantly by open grassland, there would be very few potential projectiles comparable to the size and mass of a hailstone to impact solar modules, leading to lower risk. But sites that are closer to forests or commercial development may be more at risk from impacts of larger, more massive objects. Debris smaller than the 1-inch hailstone (such as leaves, nails, or small shards of other material) should have little impact to solar facilities since their low mass cannot produce the necessary impulse to crack the tempered glass of a high quality solar module, but larger items, like wooden boards, metal structure, or falling trees could create significant damage to modules and structures. One existing design practice that may already be responsible for preventinga majority of large impact damage on existing arrays is theshading setback requirement employed in most ground-mount systems. This setback usually provides a clear space equal to twice the height of any nearby objects and structures.  The presence of this open distance has the potential to provide passive protection fromdebris falling or blowing in from outside of the array. However, shading setbacks are usually only considered along the South, East, and West boundaries of a site. In order to provide protection against the circling winds in a hurricane, this same setback must be applied to all sides of the property.

Similarly, having a robust perimeter fence can help to catch most debris that would be large enough to damage a typical solar panel. Fencing designs would also have to consider the possibility of the fence itself becoming dislodged and impacting the array. This hazard may only affect the first few rows of modules, but the chances of an electrical fault could bemuch higher if large fence sections make contact with the array.

HISTORICAL AND SEASONAL STORM VARIATIONS:

2017 has been one of the most hyperactive Atlantic hurricane seasons on record. While it may be too soon to determine the exact causes of this increase in hurricane activity (and property damage), he data presented by most of the world's leading climate scientists have made us keenly aware of the increasing scale and severity of climate change. Climate change is predicted to affect not only air temperatures, but also (and possibly more critically) sea temperatures and sea levels. The weather patterns which create Atlantic hurricanes, and especially the conditions which allow them to rapidly strengthen, are highly dependent on energy in the form of heat. When a hurricane is forming, it relies on a cycle of strengthening processes to grow and rotate. Each of these steps are highly dependent on the presence of warm water and humid air streams to feed the storm. During the formation of Hurricane Harvey in August, the waters in the Gulf of Mexico were at least 1.5°C higher than historical norms, causing additional heat and moisture to be continually pumped in allowing it to grow to become the wettest single storm system in US history.

In addition to the increased rainfall and wind velocity, rising sea levels and coastal subsidence have been shown to increase storm surges along the Gulf Coast by as much as 6 inches. Many coastal areas having very flat land slopes, where land rising less than 12 inches in elevation per mile of distance from the ocean. A change of 6 inches in storm surge in these areas can caused thousands of additional acres of dry land, and countless homes and businesses, to be flooded during a large storm.

IN CONCLUSION:

Recent history has shown that major hurricanes can strike almost anywhere in the Caribbean, and along the Atlantic and Gulf Coasts of the United States. These intense storms can causeincredible damage, even hundreds of miles inland. In order for owners, operators, and designers of solar power systems to ensure their facilities can withstand these forces, they should take into account not only the minimum design loads from the building code, but also additional practical considerations that can greatly reduce their risk of major damage from these storms.

The Structural Engineers and Developers for new solar projects should consider the integration of higher Risk Categories in their design specifications. Sometimes this is already required by the project siting and usage, but even when not required, it should be explored as an additional risk-mitigation measure.

The Designers and Civil Engineers for new projects should continue to utilize the advent of clear spacing around ground-mounted solar sites, making sure to extend this protection to all sides and directions. They should also consider the strength and stability of the project fencing as a primary safety feature, preventing debris from impacting arrays during hurricane landfall, and other intense windstorms.

Finally, the solar industry at large, in cooperation with climate scientists from around the world, should continue to explore the changing conditions we face. There are still so many unknowns in this field. The answers this research can provide may help us finally understand where are how these major storms will intersect with the development of our global power systems, and how we can avoid and prevent the unnecessary destruction of our precious energy resources.

Author: Mr. Nicholas Hudson, Project Manager, Solar Engineer - Clean Energy Associates [CEA]

Article Link: Designing PV Systems for Hurricane Conditions

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