Wind energy

Wind energy research at RETEG focuses on the complex interactions between wind turbines and the turbulent atmosphere. This interplay is characterized by turbine wake flows and their interaction with the turbulent atmospheric boundary layer, as well as the effects of turbulence on the performance of wind turbines.

Uncovering the Structure of Wind Farm Power Fluctuations

Supported by a grant from the National Science Foundation, work at RETEG has gone into advancing the understanding of how atmospheric turbulence translates into power fluctuations on the order of several minutes and faster. Important advances to come from this work are physical descriptions of the impact of rotor inertia on high-frequency oscillations and characteristic fluctuations related to many turbines operating in an atmospheric boundary layer flow with complex spatio-temporal correlations. Figure (1) shows power spectra of the power output for a wind farm, with modeled and measured high-frequency oscillations due to the advection of turbulent structures between rows of turbines. Equation 1 shows the derived model. Future work will focus on integrating this knowledge into improved grid operations.

Figure 1: Measured and modeled spectra of output power in aligned wind farm. a) five rows, Sx = 7; b) four rows, Sx = 10.

Ref: Tobin N., H. Zhu, and Chamorro, L.P., "Spectral behaviour of the turbulence-driven power fluctuations of wind turbines," J Turbul, 16(9): 832-846, 2015.

Ref: Liu H., Jin Y., Tobin, N. and Chamorro L.P., Uncovering the structure of power fluctuations of wind farms. Phys Rev E, Under review, 2017

Use of Windbreaks for Increasing Power Output of Wind Turbines and Wind Farms

Among the most important goals in wind energy research is improving the capacity factor of wind farms. By more consistently harvesting power from the available wind, reduced downtime allows for higher levels of wind integration into the grid. We take a novel approach to achieving this goal by employing windbreaks, which favorably divert wind into turbine rotors. For a single turbine, this has the potential to increase power output on the order of 10%, though an accumulating negative impact on subsequent rows means that power output is reduced deep within a wind farm. Ongoing work aims to identify cases where this approach could lead to increased wind-farm capacity factors.

Ref: Tobin, N., A.M. Hamed, and Chamorro L.P., “Fractional Flow Speed-up from Porous Windbreaks for Enhanced Wind-Turbine Power.“ Bound-Lay Meteorol, DOI 10.1007/s10546-016-0228-8, 2017

Laboratory and Field experiments

  • Laboratory experiments are carried out with instrumented miniature wind turbines of various sizes and geometries in the Talbot Laboratory open-loop wind tunnel (Fig. 3). High-resolution measurements of the power output and stresses on the turbines, which are fabricated at the University of Illinois Rapid-Prototyping Laboratory, are acquired via DAQ and force balances. Hotwire anemometry and PIV are used to characterize the flow-turbine interaction and turbulence within the turbine arrays.
  • Figure 3: (a) Miniature wind turbines in the Talbot Laboratory wind tunnel. (b) Downwind view of the wind tunnel test section.

  • Field experiments are carried out with a set of 16 heavily-instrumented small 1 kW wind turbines of 3.1 m rotor diameter. The turbines are customized to allow mobility and flexibility in the hub height. An array of CSAT3 sonic anemometers, multi-hole pitot tubes and other probes are used to obtain high-resolution measurements of the velocity and scalars of the incoming and wake flows (Fig. 4, Video 1).

    Figure 4: (a) Photograph of the small wind turbine and an array of CSAT3 sonic anemometers. (b) Close look of the array at sonic anemometers facing upwind.

    Video 1: Example of a small 3.2 m turbine operating in a flat field.
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    Our research group aims at providing fundamental insights on the role of turbulence in basic and applied problems of high interest, which can be divided in the following sub-areas:

    i) structure of the boundary layer over complex topographies;

    ii) wind & hydrokinetic energy technologies,

    iii) scalar transport over urban and natural environments,

    iv) flow-structure interaction; and

    v) instrumentation for turbulence measurements.

    We have developed a comprehensive research on these topics that are going to be sustained and expanded in the future. Our versatile experimental approach combines a set of state-of-the-art experimental techniques, including particle image velocimetry (PIV), computer vision, and our recently developed 3D particle tracking velocimetry (PTV). This framework allows us to study fluid dynamics from Eulerian and Lagrangian frame of references

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