According to a recent study by SolarPower Europe, new solar photovoltaic (PV) capacity installed worldwide in 2016 reached more than 76 gigawatts. This was largely due to dramatic growth led by the United States and China, with both countries almost doubling the amount of solar added from 2015. Globally there is now more than 305 GW of solar capacity, and with the increase of solar projects around the world, different climates, topography and other geographic considerations are making design for the environment increasingly important to ensure the integrity of the system over its lifetime.
PV plants are subject to a multitude of threats from the physical environment over the span of their lifetime. These threats must be taken into consideration during structural design in order to mitigate the risk to the system. With PV plants now being installed all over the world, the applicable risks will vary greatly depending on the geographic location. Therefore, the design of each plant must be thoroughly evaluated for its unique set of potential environmental hazards.
The first decision to make in determining the environmental loads on a plant is to identify the appropriate risk category for the plant. Building codes around the world use this categorization to determine the probability of occurrence of the design loads, or in other words, how extreme the design loads are. More critical solar project facilities can be designed for a higher risk category, and therefore, for loads with a lower probability of occurrence and a larger magnitude. For example, a rooftop system at a hospital that is required to remain operational after a design level event will be designed to higher loading conditions than a utility-scale facility behind a fence that is not supplying critical power to the grid.
Evaluating Different Types of Environmental Loads
When designing a PV plant, the environmental loads that usually first come to mind are wind and snow loads. However, certain architectures or geographical locations will necessitate consideration of seismic, ice or thermal loads as well.
Due to the large surface area and low weight of PV arrays, wind loads are often the most critical environmental forces. Consideration must be taken for both the potential wind magnitude at the site as well as surrounding topographic features that may amplify or weaken the wind as it approaches the PV plant. In many designs, wind tunnel test data is used to most accurately predict the resulting loads on a particular architecture. More accurate prediction of the wind loads allows for optimized structures that still meet the desired levels of performance and reliability.
Snow is another common critical design load on many PV plants. Rooftop and fixed-tilt ground-mounted systems are particularly vulnerable due to the relatively shallow slope of the modules. In addition to the snow’s weight on the tables, the designer must consider snow accumulation between tables, drifting and the potential need for an elevated system to allow for snow shedding. Tracker systems allow for some mitigation of the snow loads by tracking to maximum tilt to shed a large portion of the snow.
Seismic loads are less discussed in the solar industry, but they can be a significant concern in locations with high snow load and moderate to high seismic risk. Since the snow weight is considered part of the seismic mass, the resulting loads can be significant as snow accumulates (force = mass x acceleration). Many PV plants use wide flange post sections which have significantly lower strengths in one direction than the other. As earthquakes act in all directions, this is a vulnerability that needs to be expressly accounted for in design.
Ice, temperature and flooding are additional environmental risks that need to be examined for PV plants in some locations. Ice can add weight to a table and can be a concern for freezing up moving parts. Temperature becomes a concern with systems like trackers where there are long continuous members such as torque tubes. Thermal swings such as those found in desert climates can cause enough expansion or contraction of the steel along a row to result in non-trivial forces on the system. Finally, systems in floodplains must take the expected flood levels into account in the architecture and evaluate whether to design an elevated system.
Designing for these potential load effects helps to mitigate the risk to a PV plant from environmental hazards, thus increasing the overall reliability of the system throughout its lifetime and providing better value to the plant owner.
Lauren Busby Ahsler is a structural engineering manager at SunLink. She works on project engineering as well as tracker product development for SunLink.
— Solar Builder magazine