Improvements to the Simplified Loads Methodology in IEC 61400-2: November 22, 2021 - November 21, 2022

David Wood, Brent Summerville, Brent Summerville (NREL Technical Monitor)

Research output: NRELSubcontract Report


The "Simplified Loads Model" (SLM) of IEC 61400-2 provides a simple methodology to assess the structural integrity of a small wind turbine (SWT). The SLM is unique to the small wind turbine standard. It was included to allow SWT manufacturers with limited resources to undertake integrity checks at a reasonable cost in time and resources and avoid the expense of detailed aeroelastic simulations. Unfortunately, the SLM has gained the reputation of being overly conservative and this has reduced its value to the SWT community and its use in SWT design and certification. Conservatism in design standards is needed but excessive conservatism is not. The aim of this report is to address the principal areas of excess conservatism and recommend changes to the SLM that preserve its simplicity but reduce the excess. The changes for the ultimate loads are consistent with their treatment in aeroelastic modelling for certification and with related codes for wind loading on structures. The recommendations for a new fatigue design load case are also based on aeroelastic simulations, in this case of five SWTs of varying configurations with rated power from 2.4 to 50 kW. It is also pointed out the design load case for yawed operation omits an important term. The recommended inclusion of this term would make the SLM slightly more conservative for this case.
Original languageAmerican English
Number of pages26
StatePublished - 2022

Bibliographical note

Work performed by the University of Calgary, Calgary, Canada

NREL Publication Number

  • NREL/SR-5000-83708


  • Simplified Loads Model
  • small wind standard
  • small wind turbines


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