Wind Turbine Icing Physics and Anti-/De-Icing Technology

By Hui Hu, Linyue Gao and Yang Liu

Wind Turbine Icing Physics and Anti-/De-Icing Technology gives a comprehensive update of research on the underlying physics pertinent to wind turbine icing and the development of various effective and robust anti-/de-icing technology for wind turbine icing mitigation. The book introduces the most recent research results derived from both laboratory studies and field experiments. Specifically, the research results based on field measurement campaigns to quantify the characteristics of the ice structures accreted over the blades surfaces of utility-scale wind turbines by using a Supervisory Control and Data Acquisition (SCADA) system and an Unmanned-Aerial-Vehicle (UAV) equipped with a high-resolution digital camera are also introduced.

In addition, comprehensive lab experimental studies are explored, along with a suite of advanced flow diagnostic techniques, a detailed overview of the improvements, and the advantages and disadvantages of state-of-the-art ice mitigation strategies. This new addition to the Wind Energy Engineering series will be useful to all researchers and industry professionals who address icing issues through testing, research and industrial innovation.

• Covers detailed improvements and the advantages/disadvantages of state-of-the-art ice mitigation strategies
• Includes condition monitoring contents for lab-scale experiments and field tests
• Presents the potential of various bio-inspired icephobic coatings of wind turbine blades

• Language: English
• Publisher: Academic Press
• Release date: Aug 30, 2022
• ISBN: 9780323903257

Hui Hu

Dr. Hui Hu is the Martin C. Jischke Professor and Associate Dept. Chair of Aerospace Engineering at Iowa State University. He received his BS and MS degrees in Aerospace Engineering from Beijing University of Aeronautics and Astronautics (BUAA) in China, and a PhD degree in Mechanical Engineering from the University of Tokyo in Japan. His recent research interests include advanced flow diagnostics; wind turbine aerodynamics and rotorcraft aeromechanics; aircraft icing physics and anti-icing/de-icing technology; micro-flows and micro-scale heat transfer in microfluidics; film cooling and thermal management of gas turbines. Dr. Hu is an ASME Fellow and AIAA Associate Fellow, and is serving as an editor of Experimental Thermal and Fluid Science-Elsevier and an associate editor of ASME Journal of Fluid Engineering.

Book preview

Preface

Wind energy is one of the cleanest, carbon-free, renewable energy sources. To combat global warming, the United States has set goals for achieving a carbon pollution-free power sector by 2035 and net-zero emissions by 2050. Efficient utilization of wind energy is one of the main thrusts to achieve these goals. While winters are supposed to be the best season for wind energy harvesting due to the stronger wind and increased air density, wind turbine icing represents the most significant threat to the integrity and operation efficiency of wind turbines in cold climates. According to the statistics reported by the International Energy Agency (IEA), more than 30% of the wind turbines installed on the planet are operating in cold climates. More specifically, about 72%, 94%, and 19% of the turbines were reported to encounter various icing events in North America, Europe, and Asia, respectively. It has been found that even a light icing event, such as frost, could produce enough surface roughness on turbine blades to reduce their aerodynamic efficiency considerably, resulting in substantial power reduction of the wind turbines. Icing-induced power output losses are found to reach more than 20% of annual energy production on many wind farms with severe icing. In the case of extreme icing, it may not be possible to start wind turbines, with subsequent loss of all the possible power production for long periods of time. One notable example to highlight the importance of wind turbine icing protection is the massive turbine shutdown after a severe storm blasted Texas in February 2021. Frozen wind turbines were blamed as being partially responsible for the weeks-long blackout with millions of Texans being affected.

A better understanding of the underlying physics pertinent to wind turbine icing phenomena is essential for more accurate prediction of wind turbine icing events and development of effective and robust anti-/de-icing strategies to ensure safer and more efficient operations of wind turbines in cold climates. This book summarizes the research efforts conducted by the authors in recent years by carrying out comprehensive experimental campaigns to characterize the important microphysical processes pertinent to wind turbine phenomena by leveraging an unique Icing Research Tunnel available at Iowa State University (i.e., ISU-IRT) as well as by performing field studies to investigate ice accretion features on 50-m-long turbine blades and the icing-induced power losses to utility-scale, multimegawatt wind turbines.

Organization

While the book is composed of 8 chapters, the content of each chapter is highlighted as follows:

Chapter 1 provides a general introduction to the statistics and status of wind turbines operating in cold climate sites. Cold climate sites refer to the regions where wind turbines would experience low-temperature climate or icing climate, or both. According to the International Energy Agency (IEA) Ice Classification Guideline, icing climate sites are usually categorized into five classes based on the severity and frequency of meteorological icing, instrumental icing, and their potential impacts on turbine annual energy production. While typical performances of wind turbines operating at cold climate sites of different icing classes are introduced for the scenarios with the wind turbines equipped with and without ice protection systems, the recommendations and standards that are commonly used to guide the design/operation of wind turbines under different icing conditions are also presented in this chapter.

Chapter 2 introduces the fundamental physics of the dynamic icing process on wind turbine blades under various icing conditions. The key parameters influencing the wind turbine icing process are discussed at first. Two stages of the dynamic icing process on turbine blades are illustrated: (1) the droplet impinging process as the airborne water droplets are intercepted by the rotating turbine blades and (2) the solidification (i.e., freezing) process of the collected water mass on the blade surfaces. While different approaches used to evaluate the droplet trajectories, water collection efficiency, unsteady heat transfer, and transient water transport process pertinent to wind turbine icing phenomena are presented, formation mechanisms of the different ice structures/types that may accrete on turbine blades are elucidated. The major icing test facilities and their key features along with the primary scaling parameters typically used in the similarity analysis of wind turbine icing experiments are also introduced in this chapter.

Chapter 3 focuses on the measurement techniques used to quantify the ice structures accreted on wind turbine blades. Ice shape documentation is known to play an important role in the follow-on computational or experimental aerodynamic studies, ice accretion modeling, wind turbine icing prediction tool development and validation, and anti-/de-icing design criteria formulation. The technical basis and implementations of various ice shape documentation methods, ranging from the simple pencil tracing method for extracting 2D ice profile, to mold and casting method, and more advanced nonintrusive laser-based 3D scanning method for ice shape quantification, are also introduced in this chapter.

Chapter 4 introduces the state-of-the-art ice detection techniques developed for wind turbines operating in cold climates and the risk matrix commonly used to evaluate wind turbine icing events based on field measurements. The ice detection techniques are usually categorized into direct detection and indirect detection methods, depending on the types of weather/operational sensors used for the measurements. The ice-induced risks for wind turbines include the icing influence on turbine operation status, power production, structural behavior, and ice throw and fall behaviors. Special natural icing environments and the potential sea-icing risks for offshore wind turbines are discussed briefly. The guidelines for better risk management of wind turbines operating in cold and wet environments are also given in this chapter.

Chapter 5 summarizes the technical basis of conventional icing mitigation methods. The working mechanisms and implementation restrictions of the various icing mitigation techniques, such as control-based, anti-/de-icing fluids, and mechanical and thermal approaches, are summarized, compared, and analyzed. The icing mitigation methods summarized here can be batch-manufactured/applied on the existing wind turbines. Most of the technologies can be centrally controlled for large-scale wind farms/sites with dozens or even hundreds of wind turbines.

Chapter 6 explores the applications of various anti-/de-icing surface coatings/materials for wind turbine mitigation. The state-of-the-air anti-/de-icing coatings/materials can be generally divided into three categories: (1) superhydrophobic surfaces with micro-/nano-scale textures, (2) slippery liquid-infused porous surfaces (SLIPS) with a layer of liquid lubricant (which is immiscible with water) sandwiched between ice and solid substrate materials, and (3) soft materials/surfaces with ultra-low ice adhesion strength and good mechanical durability. While these anti-/de-icing coatings/materials have shown excellent performances in preventing ice formation under static icing conditions, there remain challenges when they are applied onto blade surfaces of wind turbines and exposed to dynamic impact icing conditions. Another challenge that limits the use of these anti-/de-icing coatings for wind turbine icing mitigation is their poor durability under harsh icing conditions. To overcome these challenges, a novel hybrid anti-/de-icing strategy has been developed by combining minimum surface heating near blade leading-edge and durable hydro-/ice-phobic coatings to achieve effective icing control with minimized energy input for wind turbine icing mitigation.

Chapter 7 reports a novel anti-/de-icing method that utilizes the thermal effects induced by Dielectric-Barrie-Discharge (DBD) plasma actuation to prevent/remove dynamic ice accretion on the blade surface of wind turbines. While the fundamental mechanisms of thermal energy generation in DBD plasma discharges are introduced at first, the differences in the working mechanisms between the plasma-based surface heating approach and conventional resistive electrical heating methods are compared for wind turbine anti-/de-icing applications. A duty-cycle modulation concept is presented to further enhance the plasma-induced thermal effects for improved anti-/de-icing performance. A comprehensive experimental campaign is also conducted to explore/optimize the design paradigm for the development of novel plasma-based anti-/de-icing strategies tailored specifically for wind turbine icing mitigation.

Chapter 8 provides an overview of the past and ongoing atmospheric icing-related studies globally, including, but not limited to, icing physics, ice detection, ice prediction, and ice mitigation techniques. Inspired by the atmospheric icing investigations, recommendations are made for the safer and more effective operation of wind turbines in cold and wet environments. Some suggestions are also provided for the future research needs. Particularly, the emerging collaboration needs from academic, industrial, and governmental institutions are highlighted to address issues associated with wind turbine icing.

Chapter 1: Introduction

Abstract

This chapter provides a general picture of the status of wind turbines in cold climate regions. Cold climate sites include the regions that experience low-temperature climate or icing climate, or both. Based on the severity and frequency of meteorological icing, instrumental icing, and their potential impact on turbine annual energy production, icing climate sites can be further categorized into five classes according to International Energy Agency Ice Classification guideline. Wind turbines equipped with and without ice protection systems exhibit different performances in those five ice classes. In addition, the recommendations and standards that are commonly used to guide the design and operation of wind turbines in cold regions are summarized.

Keywords

Cold climate; Icing climate; Low-temperature climate; Wind turbine icing; Ice protection system

1.1: Cold climate

1.1.1: Cold climate, low-temperature climate, and icing climate

Cold climate (CC) sites refer to the areas experiencing frequent atmospheric icing or periods with temperatures below the wind turbine operational limits clarified in the International Electrotechnical Commission (IEC) standard IEC 61400-1 ed3, according to International Energy Agency (IEA) Task 19 (IEA, 2016). Cold climate sites can be classified into low-temperature climate (LTC) sites and icing climate (IC) sites.

As illustrated in Fig. 1.1, low-temperature climate sites have at least one of the following characteristics according to Det Norske Veritas and Germanischer Lloyd (DNV GL, also known as DNV after 2021) Recommended Practices (DNV GL, 2016).

•The annual air temperature of the site is below 0°C.

•The air temperature is below −   20°C on more than 9 days/year (hourly value) based on more than 10-year historical data.

Fig. 1.1

Fig. 1.1 Definition of cold climate, low-temperature climate, and icing climate. No permission required.

According to IEA Recommended Practices (IEA, 2016), icing climate sites exhibit at least one of the following characteristics.

•Instrumental icing occurs more than 1% of the year.

•Meteorological icing happens more than 0.5% of the year.

Fig. 1.2 shows phases of an icing event in terms of meteorological icing and instrumental icing. Meteorological icing refers to the period during which the meteorological conditions allow ice accretion, including air temperature, wind speed, liquid water content, and droplet distribution (IEA, 2016). In the field measurements, relative humidity is commonly used to present the combined effects of liquid water content and droplet distribution. Meteorological icing is considered as the prerequisite of instrumental icing, that is, the period during which ice structures are present/detectable/visible at a structure and/or a meteorological instrument, such as cup and vane anemometers mounted on the top of turbine nacelle to measure the hub-height wind speed and wind direction. For wind turbines, there is a need to highlight the differences of the icing processes over the turbine rotor and other instruments and devices in some cases. Rotor icing is defined as the period during which ice is present at the turbine rotor. Otherwise, we can use instrumental icing to present all the icing processes over wind turbine structures and sensors. Incubation shows the time between the start of meteorological icing and the start of instrumental/rotor icing, relying on the surface properties and structure surface temperatures. Accretion refers to the period of ice accumulation. Persistence indicates the period during which the ice remains persistent with no detectable growth or ablation, while ablation corresponds to the period during which ice is eliminated through melting, erosion, sublimation, and shedding. Such persistence/ablation leads to the delay between the end of meteorological icing and the end of instrumental icing. According to the definition given in IEA (2016), typically, incubation and persistence/ablation time for rotor icing are shorter than instrumental icing due to their differences in dimension, shape, flow velocity, and vibrations. In addition, the duration of rotor icing significantly differs for a wind turbine under operation in contrast to a turbine at standstill or idling conditions.

Fig. 1.2

Fig. 1.2 Comparison of meteorological icing and instrumental icing. No permission required.

According to DNV GL Recommended Practices (DNV GL, 2016), the occurrence of low-temperature climate and icing climate follows the trend shown in Fig. 1.3 concerning ambient temperature. Icing climate weathers tend to occur between −   20°C and +   3°C. When the temperature is extremely low, even below −   20°C, it is unlikely for wind turbines to accumulate ice structures on their surfaces due to the decreased humidity, that is, liquid water content. Such trend is also reported by Battisti (2013). The probability of ice formation reaches the highest point at about −   5°C and drops dramatically as the temperature further decreases. When the temperature is lower than −   20°C, the probability of ice formation is lower than 15%.

Fig. 1.3

Fig. 1.3 Low-temperature climate and icing climate with respect to ambient temperature. Data from IEA. (2016). Wind energy in cold climates available technologies—Report. 1–119.

While the total wind power installation is approaching 800 GW globally in 2022, over 30% of wind turbines are installed in cold climate regions, that is, the largest unconventional market. According to the statistics conducted by Battisti (2013), approximately 72%, 94%, and 19% of the wind power installations tend to encounter icing events (including both light icing and moderate to heavy icing) in the cold climate regions in North America, Europe, and Asia, respectively. Such trend suggests that the icing issue plays a dominate role in Europe and North America, while low-temperature issue is more prevailing in Asian wind sites.

1.1.2: Low-temperature climate map

Low-temperature climate areas do not automatically mean icing conditions, but when building wind turbines to low-temperature areas, special low-temperature adaptations to turbines should be considered. The low-temperature map in Fig. 1.4 is calculated using the Modern-Era Retrospective analysis for Research and Applications (MERRA) reanalysis data from 1979 to 2013 at 60 m above ground level (agl) by VTT Technical Research Centre of Finland Ltd. (Rissanen & Lehtomäki, 2015). For the calculation, temperature data with a resolution of 1 h is used, and based on the definition mentioned in Section 1.1.1, those sites in LTC are highlighted in purple (gray in print version) in Fig. 1.4.

Fig. 1.4

Fig. 1.4 Low-temperature climate map by VTT WIceAtlas ( http://virtual.vtt.fi/virtual/wiceatla/ ). The sites with low-temperature climates are highlighted in purple ( gray in print version). From Rissanen, S., & Lehtomäki, V. (2015). Wind Power Icing Atlas (WIceAtlas) & icing map of the world. Winterwind. http://windren.se/WW2015/WW2015_44_533_Rissanen_VTT_Icing_atlas_world.pdf. The map is from website: http://virtual.vtt.fi/virtual/wiceatla/.

1.1.3: Icing climate map

According to IEA Ice Classification, based on the frequency of instrumental and meteorological icing, the icing climate sites can be further classified into five classes, as listed in Table 1.1. The sites belonging to IEA Ice Class 1 are usually considered to have low icing risks, while the sites classified to IEA Ice Class 2 may tend to have moderate icing risks in winter. Those sites in IEA Ice Classes 3–5 are suggested to be inclined to have high icing risks in winter. Major two simplifications are adopted in describing the duration of meteorological icing and instrumental icing in this IEA Ice Classification. First, incubation time is assumed to be zero, that is, meteorological icing and instrumental icing start at the same time. Second, the duration of meteorological and instrumental icing is for unheated structures, for example, wind turbines equipped with no ice protection systems