Improved site-assessments for building permits in Germany

Introduction

In Germany formal building regulations often request site-specific assessments and capacity checks of all Wind Turbine Generators (WTGs) including new and existing ones. Especially during the planning phase of repowering or extension projects this situation can be observed. When using the current technical methods and guidelines, the assessment cannot be completed for all WTGs, if the wind farm consists of different WTG types (produced by different manufacturers). Thus, in order to comply with the regulations, operators of WTGs are forced to curtail the power of many WTGs. This results in a significant loss of energy and reduced efficiency of WTGs in Germany. Recent surveys displayed that this results in a macro economical loss of approximately 10 million Euros per year. Considering the increasing amount of repowering projects, we estimate that the economic loss will increase significantly.

Approach

In this project a technical method was developed extending the existing methodology for site-specific assessments according to DIBt 2012. The technical method describes a simplified load estimation model, which would enable other expert engineers – not only the manufacturer of the WTG – to carry out capacity checks as well.

Main body of abstract

The simplified load estimation method (Equivalent Load Ratio, EqLR) allows evaluating site-specific turbulences (e.g. effective turbulence) and their impact on the fatigue loads of a wind turbine. No detailed simulation models of the considered WTGs are needed. Only wind parameters such as turbulences, air density, wind shear and Weibull parameters are being used. The model makes use of the same theoretical assumptions as the “Frandsen Model” and assumes a linear relation between turbulence level and short-term equivalent load (per wind speed). The accuracy of the model was evaluated by a joint-industry project including several turbine manufacturers (Siemens, Enercon, Vestas, Nordex, Senvion) and certification bodies (TÜV Nord, DNV GL, TÜV SÜD). It was concluded that the model provides conservative results if certain boundary conditions for turbulences and wind speeds are respected. The model was released by the manufacturers to be used for site-assessments and capacity checks in Germany by external consultants. By applying the load estimation model, many power curtailments of new and existing WTGs can be omitted. Further the complexity to obtain a building permit is reduced. Turbine manufacturers benefit from this approach as well as the high number for site-specific load simulations can be reduced significantly.

Conclusion

The presented methodology for site-specific assessments (and the evaluation for fatigue loads) has a large impact on the further development of onshore wind power in Germany. By omitting unnecessary power curtailments the efficiency of WTGs can be improved – levelized costs of energy can be reduced accordingly.


Learning objectives
The achievements of this project will enable operators of WTGs to avoid unnecessary power curtailments in many cases. Further, the process of obtaining a site-specific assessment and a building permit is accelerated.