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Characterizing Wind Turbine Inflow and Wakes Through Characterizing Wind Turbine Inflow and Wakes Daniel A. Pollak through comparison of SODAR and Met Tower SOARS® Summer 2011 Observations—A part of TWICS: The Turbine Science Research Mentors: Wake Inflow Characterization Study Julie Lundquist Writing and Communications Mentor: Abstract Wind offers an inexhaustible domestic Dave Hart energy source with minimal greenhouse gas Contributing Mentors: emissions. To maximize energy generation from Matthew Aitken, Cody Kirkpatrick, wind turbines it is essential to understand the Andy Clifton influence of inflow conditions and wakes on wind turbine energy production. In accordance with Acknowledgements The Significant this goal, the TWICS field campaign was conducted Opportunities in Atmospheric Research and in April and May 2011 at the National Renewable Science (SOARS) Program is managed by the Energy Lab’s (NREL) National Wind Technology University Corporation for Atmospheric Research Center (NWTC) in the complex terrain downwind (UCAR) with support from participating of Colorado’s Front Range mountains. TWICS universities. SOARS is funded by the National employed meteorological monitoring towers and Science Foundation, the National Oceanic and remote sensing systems to provide a three Atmospheric Administration (NOAA) Climate dimensional spatio-temporal illustration of the Program Office, the NOAA Oceans and Human inflow to and wake from a 2.3 MW turbine with a Health Initiative, the Center for Multi-Scale 100 meter rotor diameter. An important step in Modeling of Atmospheric Processes at Colorado analyzing the TWICS data was quantifying the State University, and the Cooperative Institute for performance of the different measurement Research in Environmental Sciences. SOARS is a devices that were used. This research compares partner project with Research Experience in Solid simultaneous measurements taken during TWICS Earth Science for Student (RESESS). by a Second Wind Triton Sodar and from the NREL M2 80 meter meteorological tower, which were located one kilometer apart. During the TWICS 1 Introduction campaign, we found strong linear correlations between wind speed measurements at 50 and 80 1.1 Motivation meters from the sodar and met tower. The high correlation suggests that flow is usually Harnessing the wind to power our homes, homogenous across the NWTC at time scales of businesses, and communities will spring the ten minutes, but that there are also occasional United States towards a cleaner energy future and periods of inhomogeneous flow. Wind speed will help reduce our nation’s carbon footprint. correlations were also found to vary with time of The wind resource in the United States is vast, day. This diurnal variation could represent free and is overhead, ready for use. In order to different conditions at the sodar and tower site encourage exploitation of this amazing resource, because of localized heating and turbulent mixing, the United States Department of Energy (DOE) has but may also be due to changes in sodar called for wind energy to generate 20% of performance as atmospheric stability changes America’s energy need by 2030. To answer this during the course of the day. Results from this call, there is an increased necessity to foster research will feed into future analysis of data strong understanding of the inflow characteristics collected during TWICS and help improve our of the atmosphere in all wind-prone landscapes understanding of turbine performance in the and the characteristics of the wakes that are atmospheric boundary layer. created by the wind turbines. In addition, it is important to understand the intricate interactions between the atmosphere and wind turbines, over all land types and terrain, and the effects that the by the wind industry. (Giordano 2010, Scott et al. subsequent wakes have on downstream turbines. 2010). This study will examine the performance The impetus for this study is based on the need to and consistency of SODAR observations in understand these concepts. complex terrain and in harsh weather conditions. Wind energy has grown rapidly in the (Borgeois et al 2007). United States over the last decade with more than 1,000 wind turbines currently in operation that 1.2 Basics can produce greater than 2.0 Mega-Watts (MW) of power. Additionally, half of new turbines Wind energy is, in essence, a type of solar installed in 2009 had hub heights ranging from 60- energy. Wind is caused by the uneven heating of 100 meters above ground level (AGL) and blades the Earth’s surface (and thus atmosphere) and is extending more than 40 meters out from the hub. modified by the various topography and (DOE Wind & Water Power Program 2010). As landforms that appear on Earth. This wind is a these turbines extend higher in to the atmosphere form of kinetic energy and can be harnessed it has become increasingly necessary to develop through wind turbines. These turbines turn the new ways to measure atmospheric conditions kinetic energy into mechanical energy which can around the turbine. In addition to limitations of then be transformed into electricity through a building meteorological towers that reach the full generator. Turbines come in various sizes, but it is height of modern utility-scale turbines, the data the large utility-sized turbines (over 1 megaWatt from conventional anemometers mounted on (MW)) that could help provide bulk power to the these towers only provide data for one specific power grid, propelling the United States towards point and do not show the spatial variability and its 20% wind energy by 2030 goal. (US-DOE-EERE reality of wind speed, wind direction and 2011). turbulence (Lundquist et al. 2009). As a result, development and use of remote sensing 1.3 TWICS technologies for this purpose have become increasingly important. The two remote sensing The Turbine Wake Inflow and systems used are the SOund Detection and Characteristic Study, or TWICS, was designed to Ranging (SODAR) and the LIght detection and help answer the DOE 20% by 2030 call by ranging (LIDAR). Similar to a weather radar, the providing more information on how the SODAR and LIDAR send out radiation and can atmosphere inflow and wind turbine interact, and determine characteristics of the air through the the characterization of the turbulent structure of speed and time in which the pulses of radiation wind turbine wakes, especially over complex return via the Doppler effect. Instead of emitting terrain. The insight gathered by TWICS can radio waves as weather radars do, the SODAR and improve wind farm site placement and design, LIDAR utilize sound and light waves, respectively. create more robust turbines, and provide These remote sensing techniques allow important input into mesoscale wind models. This for concise measurement of the volume that in turn will lead to more energy efficient wind intersects the turbine inflow or wake. The farms and will increase production and lower the scanning lidar makes it possible to capture these cost of this wind-generated energy. The goals of spatial temporal characteristics of turbine wakes. the project were to 1) assess the structure and (Lundquist et al. 2009; Harris and Hand, 2006). To variability of wakes from multi-MW turbines, 2) to increase confidence from the data of these assess how well mesoscale models perform in remote sensing systems, it is important to prediction turbine inflow in complex terrain, and determine the reliability and accuracy of their 3) To assess how well turbine wake models measurements, especially if they are to replace represent observed structure and variability of the conventional met tower systems that remain wakes (Lundquist et al 2009). the only widely accepted form of measurements The TWICS campaign took place at the many of these areas have great potential for wind National Wind Technology Center (NWTC), part of power generation. the National Renewable Energy Laboratory In 2008, a study sponsored by the (NREL), just south of Boulder, Colorado. The European Union was launched in a hilly location in turbine of interest was a 2.3 MegaWatt (MW) Bosnia-Herzegovina to increase knowledge of the wind turbine which stands 80 meters high and has turbulence intensity and inflow wind blades that extend approximately 50 meters from characteristics in complex terrain. This study took the hub (Figure 1). The TWICS team gathered data wind measurements carried out with a 30 meter from NREL’s M2 met tower, SecondWind’s Triton met tower, a SODAR and a LIDAR and is much like Sodar, NOAA’s (National Oceanic and Atmospheric the present study. (Borgeois et al. 2009) Administration) high resolution Doppler lidar The Bosnia-Herzegovina site, Maligrad, (HRDL) and CU’s (University of Colorado) was selected because it is known for its variability Windcube lidar. All were located upwind of the in wind conditions and its widely fluctuating turbine (Figure 2). During the months of April and meteorological conditions. The main prevailing May 2011, data were taken during times of high- wind that drives these conditions is a katabatic magnitude prevailing wind events,[which here is wind called the Bora wind (in Bosnia and Croatia) 292 degrees] called herein intensive observing and has characteristics of high winds and large periods (IOPs). Four IOPs occurred during the turbulence intensities. The two motivations of the Spring 2011 study. Before diving into the study were as follows: (1) To assess the intricacies of TWICS, a description of the project’s performance of the SODAR and LIDAR under two foci will be provided in addition to results various and intense wind and meteorological from previous studies. conditions and to (2) analyze the suitability of this site for a future wind farm and determine the 1.4 Characterizing Atmospheric Inflow wind turbine design that would best fit under the local wind conditions (Borgeois et al. 2009). There are two key concepts that with Data were taken from the 30 meter met better understanding can help to create more tower, SODAR and LIDAR for 18 days in November energy efficient wind farms that are located and December of 2007.
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