Atmospheric stability and topography effects on wind turbine performance and wake properties in complex terrain
Introduction
The evaluation of wind turbine performance and wake properties is very important for wind energy resource assessment and power forecasting. Wind turbine performance curves such as the power curve and the thrust coefficient curve are the base inputs for wind energy estimation in wind farm micro-siting, while the wind turbine wakes influence the total power output of a wind farm significantly [1]. With the increased wind farms built in complex terrain, the uncertainty associated with atmospheric stability and topography is challenging the prediction of wind turbine performance and wake properties.
Recent studies showed that atmospheric stability can affect noticeably wind turbine performance and wake structure [[2], [3], [4], [5], [6], [7], [8]]. According to Wharton et al. [2], the power generated at a given wind speed is higher under stable conditions and lower under strongly unstable conditions with a 15% differences in average power output. In Sathe et al. [3], atmospheric stability was found to have significant effects on the tower and rotor loads: the differences in the calculated loads using diabatic wind conditions and neutral conditions are approximately 16% for the tower loads and 11% for the rotor loads. To better understand the effects of atmospheric stability on the wind-turbine wake structure, Zhang et al. [4] carried out the wind tunnel tests of downscaled wind turbine models with a simulated unstable boundary layer and a neutral boundary layer. A larger flow entrainment and a faster wake recovery rate were observed under unstable conditions due to its enhanced turbulence. In Abkar et al. [5], large-eddy simulation results showed that atmospheric stability has a significant effect on the spatial distribution of the mean velocity deficit and turbulence statistics in the wake region as well as on the wake meandering characteristics downwind of the turbine. Related work were also presented in Refs. [[6], [7], [8]] where the effects of atmospheric stability on the wind turbine performance and wake properties in complex terrain were seldom discussed.
This paper therefore presents an experimental study of effects of atmospheric stability and topography on wind turbine performance and wake properties in complex terrain. The experimental approach is based on a wind turbine wake experiment that provides wind turbine SCADA data as well as wind data from two masts. Measurements are categorized to reduce uncertainties in data and then classified into stable, neutral and unstable conditions based on the Obukhov length method [9]. To evaluate atmospheric stability effects on wind turbine performance, the wind turbine power is converted into an equivalent wind speed based on the manufacture power curve. To estimate the transfer parameters in the thrust measurements, fitting the measurements to the reference thrust coefficient curve from BEM calculations [10,11] is carried out. The effects of atmospheric stability and topography on wind turbine performance and wake properties are then analyzed.
The remainder of this paper is organized as follows. Section 2 describes the experimental setup of the study and Section 3 introduces the approaches of evaluating wind turbine performance curves and wake profiles. Results of data analysis are presented in Section 4 and discussed in Section 5. Finally, the effects of atmospheric stability and topography on the performance and wake properties of wind turbines in complex terrain are concluded in Section 6.
Section snippets
Experimental setup
The measurements are carried out in a wind farm in northwest China [12]. The inflow wind from south and east is dominated by complex terrain, while the inflow from north is dominated by a small slope terrain (Fig. 1). The experimental 2 MW wind turbine #14 with a rotor diameter of 93 m and a hub height of 67 m, is located near the south valley. Strain gauges are installed in two orthogonal directions at the button of the tower to measure the wind turbine thrust while the transfer parameters
Data processing
To reduce uncertainties in data, measurements are firstly categorized by considering the following facts: data synchronization and sensor errors, wake effects and operational conditions.
Atmospheric stability parameter and inlet wind speed profiles
In order to choose the suitable sonic anemometer for atmospheric stability classification, the relationship between the dimensionless wind shear and the stability parameter ζ is studied (Fig. 8). Results show that the standard deviation of the dimensionless wind shear is smaller for sonic anemometers at 30 m than at 70 m. For inflow mast M1 in relative flat terrain, the Bussinger-Dyer flux relationships show closer agreement with the measurements of the sonic anemometer at 30 m than at 70 m,
Discussion
Results show that the wind turbine performance and wake properties in complex terrain are significantly affected by atmospheric stability and topography. Effects of atmospheric stability on the power can be evaluated by studying the equivalent wind speed under different stability conditions. Compared with that under neutral conditions, the equivalent wind speed for a given mast wind speed increases by 2% under stable conditions and decreases by 5% under unstable conditions, yielding a power
Conclusions
This paper analyzes the wind farm measurement data to study the impacts of atmospheric stability on wind turbine performance and wake properties in complex terrain. Results show that atmospheric stability and topography have significant effects on wind turbine performance and the wake structure in complex terrain.
It is clear that atmospheric stability influences wind turbine performance mainly by changing the equivalent wind speed. Compared with neutral stability, the equivalent wind speed is
Acknowledgments
The following projects are acknowledged for financial support to this study: the Sino-Danish cooperation project “Wind farm layout optimization in complex terrain”(NO. 2014DFG62530) funded by the ministry of science and technology of China and (EUDP-Journal nr.: 64013-0405) funded by the Danish Energy Agency, the project “Research on operating characteristics and control strategies of wind farm and concentrated solar power combined generation system” (51507053) funded by National Natural
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2022, Renewable EnergyCitation Excerpt :Existing wake models are not designed for such farms, leading to inaccurate predictions, financial losses and energy security risks [6,7]. A deeper understanding of wind turbine wakes is required for developing wake models that can be applied in various conditions [5,8,9]. The above engineering wake models consider only the mean wake-deficit evolution, which may suffice to estimate the average wake-induced loss in power production.