The Solar Photovoltaic (PV) industry is experiencing phenomenal growth. Wind loads for ground-mounted PV power plants are often developed by using static pressure coefficients from wind tunnel studies in calculation methods found in ASCE 7. Structural failures of utility scale PV plants are rare events, but some failures have been observed in code-compliant structures.
Many wind loading codes and standards define flexible structures as slender structures that have a fundamental natural frequency less than 1 Hz. This paper demonstrates that this is not a suitable threshold for small structures like ground-mounted arrays of photovoltaic panels because structures this small can experience both self-excitation and buffeting from upwind panels at frequencies well above this value during both serviceability and design wind events.
This paper focuses on dynamic effects of wind for large-scale (often referred to as “utility scale”) solar photovoltaic power plants, and can be applied to most ground-mounted PV systems with repetitive rows of solar panels. This topic has relevance increasing in time as the solar industry scales in size and deployment, while continuously striving to drive down cost.
Solar market trends have been studied and the results published by GTM Research (a division of Greentech Media) and the Solar Energy Industries Association (SEIA). In Figure 1, from U.S. Solar Market Insight 2014 Year-in- Review, the blue bars show the phenomenal growth of the U.S. solar industry from 2005 through 2014. Market forecasts for the next two years are for 12 GigaWatts (GWdc) of installed capacity by the end of 2016. The Federal Investment Tax Credit (ITC) has been a driving force in attracting investors to kick-start the growth of the solar industry in the U.S. As the ITC and other incentive programs are expected to sunset, the solar industry is keenly focused on driving down
the installed cost of PV systems, with a goal of grid parity without incentives. The descending line in Figure 1 shows the trend in decrease of system price from 2005 to 2014.
Most of the reduction of system price has been a sharp decline in the cost of the power-producing PV modules (panels) themselves. As the cost of modules has decreased dramatically, a great deal of emphasis has been placed on soft cost (the cost of engineering and permitting) and Balance of System (BOS) cost, including the cost of the rack mounting system and foundation (but excluding inverters).
As design engineers have strived to drive down the cost of the rack systems, many manufacturers have engaged wind consultants to model their systems in boundary layer wind tunnels. The products of these studies include more-accurate wind pressure coefficients to be used with procedures in ASCE 7. Economy of design has commonly included optimizing a reduction of steel, with a resulting trend toward structures that are more flexible. Structural failures have been observed in code-compliant ground-mounted rack systems during wind events at wind speeds significantly less than design wind speed. Recent research has been focused on determining the cause of failure in otherwise code-compliant structures and improving estimation of wind loads.