Posters

Abstract

Modelling wake interactions between wind farms offshore is of increasing importance as sites become more crowded. The wake loss prediction methods which are familiar for single farm contexts and short range wakes - fast-running ‘engineering' methods - are outside their range of applicability, and few alternatives are available. Moreover, atmospheric boundary layer (ABL) characteristics may play a strong role and such features are absent from most rapid models and only accessible using CFD methods. The present work attempts to address these challenges by using a linearised, rapid model (RB Smith, Wind Energy 2009) which already embodies ABL height and inversion strength and to which sensitivity to surface layer stability has been added, as well as bulk horizontal dissipation. This is new work, the first time this type of model has been applied to farm wake loss directly, which has been achieved via Monin-Obhukhov theory and the tall wind profiles of Gryning et al (Boundary-Layer Meteorology, 2007), as will be described. Observations of long range wakes and how they vary with stability are scant, however a case study has been made using the SAR observations of Djath et al (J. Renewable Sustainable Energy 2018) for the Alpha Ventus Wind Farm. Agreement between modelled and observed farm wake length will be shown to be very good for neutral surface layer stability i.e. approximately 20 km wake length. For non-neutral cases qualitative agreement will be shown but with poorer agreement quantitively as conditions diverge from neutral. The present case study offers encouragement that the modelling approach adopted has useful predictive skill for long range wake losses that may be further improved given further development and validation. The model used also has a role to play in modelling blockage and gravity wave effects as will be described. Development will progress but confident application relies on successful case studies. Attempting this case study has clearly demonstrated a need for improved measurement and characterisation of atmospheric boundary layer conditions. The present work has relied on a number of assumptions and would have been better served had more measurements and/or validated models for atmospheric behaviour been available: in particular, ABL height and surface layer stability being available from measurements or weather models validated for this purpose. These points will be explained and recommendations made on how to make progress on these topics.