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NEWA Meso-Micro Challenge for Wind Resource Assessment

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Background 

This challenge is organized in the context of the New European Wind Atlas (NEWA) project, whose overarching goal is to produce a seamless high resolution wind atlas for Europe. The wind atlas methodology will be based on a mesoscale to microscale (meso-micro) model-chain, validated with dedicated experiments as well as other observational databases from public and private sources. Wind resource assessment is related to the development of wind farms and implies the prediction of long-term wind statistics, notably the annual energy prediction (AEP).

In the development of meso-micro methodologies for wind resource assessment there is a tradeoff to be made between modeling fidelity and its associated cost to yield the required accuracy for the intended use (Figure 1). Accuracy is a qualitative concept that is used here to define the closeness of agreement between the predicted quantity of interest and the true value in the real word. Considering wind resource assessment applications, accuracy should gradually improve from the early-stage prospecting phase to the project financing phase, i.e. from planning to bankable accuracy. This process will hopefully remove the bias and reduce the uncertainty of the assessment to desired financial limits. This typically implies using off-the-shelf wind atlas products during early planning phase to design tools of increasing fidelity as the project matures. The required fidelity will depend on the complexity of the site as indicated in Figure 1 and is capped by the maximum allocated cost in terms of computing time.    

Figure 1: Illustration of the process of improved accuracy ΔU from planning to bankable thresholds against the maximum allocated computing time cost for different site/flow complexities.

Hierarchy of Meso-Micro Methodologies 

A hierarchy of meso-micro methodologies for wind resource assessment is illustrated in Figure 2, ranging from the Global Wind Atlas lower-end of modeling fidelity, where the WAsP downscaling method is used directly from global reanalysis data without mesoscale modeling, to the dynamic coupling of a mesoscale model with a Computational Fluid Dynamics (CFD) model based on large-eddy simulation (LES) in the higher-end of modeling fidelity. The Weather Research and Forecasting (WRF) mesoscale model will be used in NEWA to produce the wind atlas and, therefore, it is explicitly mentioned in Figure 1, also as the most popular choice for a mesoscale model. Between these limits, a hierarchy of methodologies is established depending on the type of coupling and the type of CFD model. Since the application demands statistical quantities of interest, we shall leave dynamical coupling methods out of the design tools range. In the context of NEWA, wind farm design tools will be based on statistical downscaling methodologies that combine WRF outputs with steady or unsteady Reynolds-Averaged Navier Stokes (RANS) microscale CFD model simulations that include thermal stratification.

Similarly, uncertainty quantification can also have different levels of fidelity depending on how rigorous is the analysis; from ad-hoc engineering methods to formal UQ probabilistic methods.

Figure 2: Hierarchy of meso-micro methodologies for wind resource assessment classified in terms of the type of coupling and typical intended use.

Objectives 

The objectives of this challenge are:

  • To determine the applicability range of meso-micro methodologies for wind resource assessment within the NEWA validation domain.
  • To establish open-access practices for the assessment of these methodologies to improve the traceability of the state-of-the-art as additional datasets are incorporated in the validation domain.
  • To identify knowledge-gaps that will feed plans for future targeted experiments and validation activities.
  • To engage with lead users of the NEWA model-chain whose first release will be provided open-access in the fall 2017. 

Data 

The ultimate goal is to incorporate as many sites as possible in the challenge, at least during the duration of the NEWA project (until April 2019). This will constitute the "NEWA validation domain" as illustrated in Figure 3, overlapping with the application domain. 

Figure 3: Illustration of the NEWA validation domain overlapping with the wind energy application domain, both mapped in terms of the range of operating conditions versus the flow complexity to be modeled.

The intended validation domain spans modeling conditions supported by high-quality data, for which the methodologies are well understood, such that there is good agreement between observations and simulations.

The application range not covered by NEWA shall be filled with future experiments and industry data. By supporting an open-access database and assessment process it will be possible to sustain a more traceable development of wind resource assessment technology. 

Acknowledgements 

This challenge was launched with the support from NEWA (FP7-ENERGY.2013.10.1.2, European Commission's grant agreement number 618122) and MesoWake (FP7-PEOPLE-2013-IOF, European Commission’s grant agreement number 624562) EU projects. The benchmark are coordinated within the International Energy Agency IEA-Wind Task 31 "Wakebench".