Improved Understanding of Vertical Mixing in the Lower Atmospheric Boundary Layer in the Presence of Wind Turbines via Numerical Simulations and Measurements

在过去的几十年里，风能在美国和世界范围内一直在稳步增长，预计在不久的将来还会继续增长。风能有无数的好处：它是可再生的和丰富的，不会产生任何有害的污染物或温室气体排放，而且技术如此先进，往往在经济上与其他传统能源竞争。然而，人们越来越担心风力涡轮机在下边界层可能产生的不良影响，特别是对近地表温度的影响。对于这些影响可能是什么，文献中存在高度分歧。数值模拟表明：白天和夜间变暖；夜间变暖，白天变冷；变暖或变冷取决于大气稳定性；变冷；没有显著影响。这些相互矛盾的发现突显了风力涡轮机和大气之间反馈的内在复杂性。然而，只有一种机制被广泛接受(但从未试验过)来解释风力涡轮机如何影响下边界层，即风力涡轮机尾流中产生的湍流增强了垂直混合。尾迹是风力涡轮机的顺风向羽状体积，其特点是风速较低，湍流较大，而不受干扰的上风气流。垂直混合增强将表现为地面附近垂直动量通量的增加(因此风切变增加)和湍流动能(TKE)的增加。少数几个测量风力涡轮机近地表性质变化的观测活动存在缺陷或没有定论，也没有直接测量垂直动量通量的变化。因此，目前还没有关于增强垂直混合机制的直接证据。此外，增强的垂直混合并不能解释所有观察到的影响。智力优势：本项目提出的研究假设是，垂直混合一般不会增强，但更有可能的是，在风力涡轮机表面下风附近减少垂直混合。由于风力机在叶片所跨越的直径为D的转子盘周围区域提取动能，垂直风廓线被极大地改变，但所产生的地面附近的风切变通常不改变或减小，而不是增强，在转子下方。因此，近地表的热量、水汽和动量通量，以及最终的温度，通常不直接受到尾迹湍流的影响，而是受到减弱的、不增强的风切变、大气稳定性和无扰动流动特性的综合影响。为了验证这一假设，将采用观测和数值相结合的方法。这项名为Vertex for垂直增强混合的测量活动将在刘易斯的特拉华大学Gamesa 2兆瓦风力涡轮机附近进行，使用12-15个地面通量塔、一个60米长的气象塔和2个扫描激光雷达。Vertex将量化由于风力涡轮机而导致的动量和热通量、气温和地面附近TKE的变化。风力涡轮机和大气流动之间的相互作用将用至少两个最先进的数值代码来模拟，这些代码可以将湍流尾迹的细节解析到几米级的分辨率。来自Vertex的数据将用于验证模拟。还将评估对各种流入和大气稳定性条件的敏感性。广泛的影响：这项研究将提供一个了解风力涡轮机是否以及如何以任何显著方式改变近地表特性。这一理解很重要，因为如果这种变化被认为过于戏剧性，它要么会加强风能的部署，要么会减缓风能的部署。研究成果将被传达给：1)通过参加会议和同行评审的论文向科学界；2)通过有针对性的外联努力，包括在每年的特拉华大学海岸日和新闻发布会上展出；以及3)通过新开发的关于“大气中的能源”的研究生和本科生课程。

Wind energy has been growing steadily in the U.S. and worldwide in the past decades and it is projected to continue its growth in the near future. Wind energy has countless benefits: it is renewable and abundant, does not produce any harmful pollutant or greenhouse gas emissions, and is so technologically advanced that it is often economically competitive with other traditional energy sources. However, concerns are rising about possible undesirable effects of wind turbines in the lower boundary layer, especially on near-surface temperature. The literature is highly divided about what these effects could be. Numerical simulations have shown: warming during day and night; warming at night and cooling during the day; warming or cooling depending on atmospheric stability; cooling; and no significant effects. These contradictory findings highlight the inherent complexity of the feedbacks between wind turbines and atmosphere.Only one mechanism, however, has been widely accepted (but never tested) to explain how wind turbines affect the lower boundary layer, namely that turbulence generated in wind turbine wakes enhances vertical mixing. Wakes are plume-like volumes downwind of wind turbines that are characterized by lower wind speeds and higher turbulence than the undisturbed upwind flow. Enhanced vertical mixing would manifest as increased vertical momentum fluxes (thus increased wind shear) and increased turbulent kinetic energy (TKE) near the ground. The few observational campaigns that have measured changes in near-surface properties by wind turbines have been flawed or inconclusive and have not measured directly vertical momentum flux changes. Therefore, no direct evidence of the enhanced vertical mixing mechanism is available today. In addition, enhanced vertical mixing does not explain all the observed effects.Intellectual Merit:The research hypothesis put forward in this project is that vertical mixing is generally not enhanced, but, more likely, reduced near the surface downwind of wind turbines. The vertical wind profile is greatly altered due to the extraction of kinetic energy by wind turbines in the region around the rotor disk of diameter D spanned by the turbine blades, but the resulting wind shear near the ground is generally unaltered or reduced, not enhanced, below the rotor. Near-surface heat, moisture, and momentum fluxes, and eventually temperature, are therefore generally not directly affected by wake turbulence, but rather by a combination of the reduced, not enhanced, wind shear, atmospheric stability, and undisturbed flow properties.To test this hypothesis, a combined observational and numerical approach will be carried out. The measurement campaign, named VERTEX for VERTical Enhanced MiXing, will be conducted near the University of Delaware Gamesa 2-MW wind turbine in Lewes, using 12-15 surface flux towers, a 60-m meteorological towers, and 2 scanning lidars. VERTEX will quantify changes in momentum and heat fluxes, air temperature, and TKE near the ground due to the wind turbine. The interactions between the wind turbine and the atmospheric flow will be simulated with at least two state-of-the-art numerical codes that can resolve the details of the turbulent wake down to a few meter resolution. Data from VERTEX will be used to validate the simulations. The sensitivity to a variety of inflow and atmospheric stability conditions will also be assessed.Broader Impacts:This research will provide an understanding if and how wind turbines can alter near-surface properties in any significant way. This understanding is important because it will either enhance wind energy deployment or slow it if such changes are considered too dramatic. Research findings will be communicated to: 1) the scientific community via participation to conferences and peer-reviewed papers; 2) the general public via targeted outreach efforts, including an exhibit at the annual University of Delaware's Coast Day and press releases; and 3) university students via a newly developed graduate and undergraduate course on "Energy in the atmosphere".