Reducing the uncertainty of Wind Resource Assessments through a new complex flow scoring system
Creators: name: Barber, Sarah
name: Clifton, Andrew
Publisher: name: Zenodo
Year: 2021
Subjects: Wind resource assessment
Complex terrain
Project risk

description: For a given Wind Resource Assessment (WRA), the modeller is faced with a difficult choice of simulation tools and calculation steps with varying accuracies and costs. The most cost-effective choice is expected to be a compromise between performance and cost. This choice is dependent on the characteristics of the site. To help with this choice, wind energy sites are usually classified as "complex" or "not complex" to help assess whether the flow modeling tool WAsP can be applied with high confidence or not. This is done by calculating the average RIX and DRIX values (Mortensen et al., 2008) at the site using WAsP. RIX values &gt; 0 are taken as the start of “complex terrain” (Mortensen et al., 2011). For terrain above this threshold, other, more complete flow models are generally recommended (EMD International, 2021). However, this strict definition makes it difficult for wind energy project planners to confidently assess the uncertainties associated with their WRAs. A classification on an analogue scale or with a number of different categories related to the flow complexity – and more importantly the consequences of this classification – would be help modellers choose the most appropriate tool for their site. In this work, a first investigation into a new complex flow scoring system is carried out for four different sites. This is done by asking the modellers carrying out WRAs at these sites to fill out a survey to estimate the relative value of certain parameters on a scale of 0-100. These parameters are defined based on previous work on terrain classification related to lidar measurements (Clifton, 2015). This splits complex flow into the following categories: (a) Complex terrain (e.g. from IEC 61400-12-1 (IEC, 2017)); (b) Surface roughness (e.g. forested land, changes in ground cover); (c) Presence of obstacles (e.g. buildings, towers and wind turbines); (d) Local meteorology (e.g. low-level jets, divergent flows and fronts). The parameters used for this work should not be too difficult or time-consuming to calculate, and the site classification given in IEC 61400-12-1 and 61400-12-2, for example, are too complex for this application. A full list of parameters can be found in S. Barber et al. (2020) and on Figure 2.<br> In this study, these parameters are all weighted equally and then averaged in order to get a single flow complexity score. The results show that although a statistically significant number of samples is not yet available, the results are promising. It is interesting that the sites with higher complexity scores also have a higher variation from the average. The individual scores provide more insight into the suitability of the method. It can be seen that the variation in answers between modellers varies between question. The answers to the questions “For the validation/calibration met mast, in how many 30 degree sectors is there a positive slope steeper than 30 degrees less than 250 m away from the validation position in any direction?” seem to be particularly inconsistent. Further investigation lead to the conclusion that the participants did not all read the question properly. Further studies are underway in order to improve the method and associate the scores with concrete recommendations for WRAs.