Solar Structures

Engineers designing utility-scale ground mount racking systems frequently have the option of turning to a wind loading standard such as ASCE 7-22 for design of the system, but it is common for the codified methods to be too crude or conservative to allow for a competitive design. In some countries, the applicable code or standard may not have specific procedures for solar structures, in which case trying to approximate wind loads from the generic structures can be extremely inaccurate. As a result, it is most common for wind loads to be determined from a wind tunnel test of a scale model of the system being designed.

The typical wind tunnel model type for determining wind loads is a rigid pressure model in which surface pressures are measured on the top and bottom surfaces of the PV modules. The models are typically arranged in a generic array that is big enough to capture edge effects as well as the lower loads in the interior part of the array. The effect of self-excitation and buffeting from vortex shedding from upwind rows is also captured.

With high-frequency pressure measurements, the wind tunnel data can then be analyzed to produce static load cases pertaining to the specific components of the system being designed. Determining what load effects are important is achieved through conversation with the structural engineer.

Beyond the static loads, the vibration of the system can induce inertial loads that significantly increase the total loads that the structure must resist. These dynamic loads are extremely sensitive to the high-energy frequency ranges caused by vortex shedding.

When the vortex shedding frequency is close to the system’s natural frequency, the inertial loads can be extreme. Designing the system based only on the static loads would result in a lighter more flexible structure, which would be more sensitive to dynamic effects. Ignoring or underestimating these dynamic loads can lead to catastrophic failure.

With wind tunnel pressure model data, it is most common to account for the potential for dynamic (inertial) loads through combination of the static load excitation and the modes of vibration provided by the structural engineer. Dynamic loads vary with mode shape, natural frequency, and damping ratio.

It is critical that the potential for dynamic loads be considered in concert with the static loads and not after decisions have been made about material gauges. Dynamic loads can significantly increase total loads above the assumed static value.