Continuum Wind Flow Model: Introduction to Model Theory and Case Study Review

Abstract

When developing a wind farm, it is very important to accurately define the wind resource distribution across the project area such that an optimized turbine layout can be achieved. To estimate the wind resource distribution, typically, meteorological (met) towers are installed at various strategic locations and the wind speed and direction measured at these sites are used as inputs into wind flow models. Currently, linear and CFD models are most commonly used. Linear models can provide estimates quickly, with little training and at a low cost however, this type of model is well-known to deliver highly inaccurate estimates particularly in complex terrain. CFD models can provide more accurate estimates however they require significant computational time, an expert knowledge level and a much larger financial investment. Also, all commercially-available linear and CFD models are limited to using a single met site in the model creation. A new wind flow model, Continuum (patent pending), is introduced which is based on a simplified analysis of Navier-Stokes and utilizes data from all of the met sites simultaneously to develop site-calibrated models. The model coefficients, mUW and mDW, describe the sensitivity of the wind speed to changes in the upwind and downwind terrain exposure and are defined for downhill and uphill flow. The coefficients are a function of terrain complexity and, since terrain complexity can change across an area, the estimates are performed in a stepwise fashion where a path of nodes with a gradual change in complexity are found between each pair of sites. Also, coefficients are defined for each wind direction sector and estimates are performed on a sectorwise basis. The site-calibrated models are created by cross-predicting between each pair of met sites and, through a self-learning technique, the model coefficients that yield the minimum met cross-prediction error are found. A case study is presented where eleven met masts at a complex terrain site were modeled in Continuum. Using the site-calibrated model, the wind speeds were predicted at the met sites and excellent agreement was found between the estimated and actual wind speeds with a correlation coefficient of 0.96 and a RMSE of 0.90%. The largest wind speed estimate error was 1.6% and five of the eleven sites were modeled to within an error of 0.5%. In Continuum, a Round Robin analysis was performed using met subset sizes of 8, 9 and 10 mets where every possible combination of met sites were used to form a model which were then used to predict at the excluded met sites. The RMS error of the Round Robin predictions was ∼1.6% for all three subset sizes which confirmed the very good quality, high level of robustness and validity of the Continuum wind flow model.