research needs in the field of icing on wind turbines was carried out. This analysis ... One potential approach to build this bridge between science and industry.
Icing of Wind Turbines Vindforsk projects, a survey of the development and research needs Elforsk report 12:13
René Cattin
April 2012
Icing of Wind Turbines Vindforsk projects, a survey of the development and research needs Elforsk report 12:13
René Cattin
April 2012
ELFORSK
Preface
Vindforsk III is Swedish research program that is running in the period 20092012. The programme is divided into five activity areas: 1. The wind resource and external conditions. 2. Cost-effective plants 3. Operation and maintenance 4. Wind power in the power system 5. Standardization In the preparation of a final report at the end of the program, and preparation of a new program period, work with survey reports for different research areas is being carried out. One such research area is “Icing of wind turbines”. Work with a survey report for this area is carried out by René Cattin from Meteotest in Switzerland. The report contains descriptions of projects within the program; the status and trends of technology and research; and finally an analysis of research needs within the area. Conclusions and opinions in the report are those of the author. Vindforsk-III is funded by ABB, Arise windpower, AQ System, E.ON Elnät, E.ON Vind Sverige, EnergiNorge, Falkenberg Energi, Fortum, Fred. Olsen Renewables, Gothia Vind, Göteborg Energi, HS Kraft, Jämtkraft, Karlstads Energi, Luleå Energi, Mälarenergi, o2 Vindkompaniet, Rabbalshede Kraft, Skellefteå Kraft, Statkraft, Sena Renewable, Svenska kraftnät, Tekniska Verken i Linköping, Triventus, Wallenstam, Varberg Energi, Vattenfall Vindkraft, Vestas Northern Europe, Öresundskraft and the Swedish Energy Agency. Stockholm april 2012 Anders Björck Programme manager Vindforsk-III Electricity and heat production, Elforsk AB
ELFORSK
ELFORSK
Summary
In this study, a survey of existing research developments as well as of future research needs in the field of icing on wind turbines was carried out. This analysis was done for the following research fields:
•
Icing simulations (climatologies and forecasts)
•
Icing measurements
•
Effects of icing on wind turbines
•
De-icing and anti-icing
•
Health and Safety (ice throw and noise)
The following main conclusions could be drawn from the analysis:
•
There are a lot of promising research activities going on in the field on wind energy under icing conditions. However, one can identify a clear gap between the scientific activities coming mainly from the meteorological community and the industrial activities mainly driven by the manufacturers of wind turbines and components. There is a clear need to build a bridge between science and industry.
•
One potential approach to build this bridge between science and industry could be the establishment of dedicated test and research centers for wind turbines under icing conditions. Ideally, these test centers consist of state of the art wind turbines equipped with icing observation instruments as well as de-icing or anti icing systems. In addition, there should be facilities for carrying out meteorological measurements on a large scale (e.g. large towers upwind of the wind turbines).
•
These test centers should ideally be located at several locations in Europe and operated within a European collaboration (e.g. EU Framework Programme or COST Action).
•
There is a large need for more and better icing measurements, especially at wind turbine blades. Today's instruments are not reliable and accurate enough. As long as such instruments don't exist, large uncertainties remain in the understanding of icing on wind turbines in general and in the development of new wind turbine ice accretion models.
•
The research activities need to be extended to the full dimension of a wind turbine. Nowadays, many research activities are often focused on point measurements or simulations. In reality, icing of wind turbines is an issue which covers a vertical profile with a length of up to 120 m.
•
There is a need for more detailed investigations on ice throw of wind turbines and the development of validated ballistic models which are able to determine the risk of ice throw at any planned wind farm. Compared to other research activities it is a rather small issue. But if not addressed properly, it can quickly become a "show-stopper" for the development of wind energy in cold climate.
ELFORSK
ELFORSK
Innehåll 1
Projects within Vindforsk III
2
Concept of the study
6
3
Introduction to atmospheric icing
7
4
Trends and developments
1.1 1.2 1.3
4.1
4.2 4.3 4.4 4.5
5
5.2
11
Icing simulations ........................................................................... 11 4.1.1 Icing Climatologies ............................................................. 11 4.1.2 Icing Forecasts .................................................................. 16 4.1.3 Activities in the world ......................................................... 18 4.1.4 Activities in Sweden ............................................................ 19 Icing measurements ...................................................................... 20 4.2.1 Activities in the world ......................................................... 23 4.2.2 Activities in Sweden ............................................................ 23 Effects of icing on wind energy production......................................... 24 4.3.1 Activities in the world ......................................................... 26 4.3.2 Activities in Sweden ............................................................ 26 De-Icing and Anti-Icing .................................................................. 27 4.4.1 Activities in the world ......................................................... 29 4.4.2 Activities in Sweden ............................................................ 29 Safety issues ................................................................................ 30 4.5.1 Ice throw .......................................................................... 30 4.5.2 Noise ................................................................................ 33 4.5.3 Activities in the world ......................................................... 34 4.5.4 Activities in Sweden ............................................................ 34
Future research needs 5.1
1
About the Vindforsk programme 2009-2012 ........................................ 1 Vindforsk projects within the research area of icing of wind turbines ....... 1 Summaries of Vindforsk projects ....................................................... 3 1.3.1 V-313: Wind Energy in Cold Climate ....................................... 3 1.3.2 V-338: IEA RD&D Wind Task 19: Wind Energy in Cold Climates .. 4 1.3.3 V-359: Prestudy, Verification data for project V-313 .................. 5 1.3.4 V-363: Complementary measurements to V-313....................... 5
35
General needs............................................................................... 35 5.1.1 A bridge between science and industry .................................. 35 5.1.2 Extend the view to the full wind turbine ................................. 36 5.1.3 More and better measurements ............................................ 36 5.1.4 More research activities on ice throw ..................................... 37 Specific research topics .................................................................. 37 5.2.1 Icing simulations ................................................................ 37 5.2.2 Icing measurements ........................................................... 39 5.2.3 Effects of icing on wind energy production ............................. 40 5.2.4 De-icing and Anti-icing ........................................................ 40 5.2.5 Safety issues ..................................................................... 40
ELFORSK
1
Projects within Vindforsk III
1.1
About the Vindforsk programme 2009-2012
Vindforsk III is a co-financed research programme that provides funding for basic and applied wind energy research. The Swedish Energy Agency is financing 50 percent of the costs within the programme, and the other half is financed by energy companies and other companies with connection to wind power. The programme total budget is 80 million SEK over a four-year period. The overall objective of Vindforsk is to strengthen the conditions for building and operating wind power by: • •
• •
producing generalizable results concerning wind energy characteristics and opportunities Conducting research at the international forefront within a number of technology areas to preserve and strengthen the skills of existing research groups at universities and engineering consultants. strengthening the recruitment base for Swedish wind power industry making wind energy research visible and disseminate its results
The programme is divided into five activity areas: 1. 2. 3. 4. 5.
1.2
The wind resource and external conditions. Cost-effective plants Operation and maintenance Wind power in the power system Standardization
Vindforsk projects within the research area of icing of wind turbines
Specific conditions in Sweden such that wind power will be built in forested areas and in areas with icing of turbines and equipment has motivated the activity area “The wind resource and external conditions”. The objective of the research is to develop tools and models to enable more reliable estimates of production and design loads. One sub-research area within the activity area is wind power in cold climate. For this research area, the programme description states: The overall goal is to increase the ability to estimate the electricity production from the turbines in areas at risk of icing of blades and anemometers and low temperatures. The goal is increase this ability both during the planning phase of projects as well as for production forecasts during operation. The goal for the programme period 2009-2012 is to have sufficient knowledge to produce an icing mapping over Sweden with estimates of site-specific effects of icing and cold climate on the energy production.
1
From project applications to the programme, four projects with a total budget of around 13 Million SEK has been given funding within the cold climate area. Table 1 and 2 show the ongoing Vindforsk projects in the field of icing of wind turbines. Chapter 1.3 summarizes the projects and the results obtained so far. Project number
Project title
Project leader
Financing
V-313
Wind Energy in Cold climate
Hans Bergström, Uppsala University
SEK 8 000 000 cash funding and SEK 3 000 000 in kind
V-338
IEA RD&D Wind, Task 19
Göran Ronsten, WindRen AB
SEK 450 000 cash funding and SEK 450 000 in kind
V-359
Prestudy, Verification data for project V-313
Peter Schelander, Swedpower Poyry
SEK 225 000 cash funding and SEK 84 000 in kind
V-363
Complementary ice measurements to V-313
Hans Bergström, Uppsala University
SEK 753 000 cash funding and SEK 220 000 in kind
Table 1 Vindforsk projects within the area icing of wind turbines. Project number
Project title
V-313
Wind power cold climate
V-338
IEA RD&D Wind, Task 19
in
Important international cooperation
Planned Concrete use of results of the project
Within the Top-level Research Initiative project ICEWIND where Weathertech Scandinavia takes part in cooperation with several partners from the Nordic countries.
Will make it possible to determine an icing climatology over Sweden on a 1x1 km2 resolution, and using this to predict production loss due to icing.
2
V-359
Prestudy, Verification data for project V-313
A measurement plan for verification of input/output data in the transitional step between two numerical models. The concrete use of this plan will facilitate the improvement of methods used in coupling a NWPmodel with an ice accretion model, and potentially reduce the uncertainties involved in icing prediction.
V-363
Icing– complementary measurements to V-313
Increase the possibility to reach the goals in V-313 by new icing measurements.
Table 2 Expected results from the Vindforsk projects.
1.3
Summaries of Vindforsk projects
1.3.1 V-313: Wind Energy in Cold Climate The main goal of this project is to increase the knowledge on the creation of icing maps in a climatological sense (30 years or more). It will create the scientific basis to create an icing map over Sweden with a horizontal grid resolution of 1x1 km. The results of different numerical weather models (Arome, WRF, COAMPS) are compared to each other. The results are compared with icing measurements carried out in the O2 pilot project. Additionally, approaches to obtain long term icing climatologies are tested (long term correlation). The icing maps shall be used to calculate the production losses of wind turbines due to icing. The following main results are be obtained so far:
•
Periods with meteorological icing (growth of ice) are captured relatively well by the models. Periods with instrumental icing (persistence but also melting, sublimation and shedding of ice) are less good captured.
•
There are large differences between the simulated ice loads.
•
It is difficult to compare the model results with measurements as the measurements themselves contain a large uncertainty. Furthermore, relevant parameters such as liquid water content cannot be measured currently.
•
The horizontal resolution of the weather model is strongly relevant for an increase of the accuracy. Preliminary results indicate that 1x1 km is an optimum for Sweden.
3
•
It is not possible to run a weather model with 1x1 km for 30 years. The following alternatives were identified to create long term climatologies: create only 1- or 2-year climatologies of representative years or run the models with a coarser resolution and correlate with shorter high resolutions model runs.
•
There are significant differences in icing frequency between single years.
•
There are no results available on the production losses with respect to icing.
1.3.2 V-338: IEA RD&D Wind Task 19: Wind Energy in Cold Climates IEA RD&D Wind Task 191 is completing its third term, 2009-2011. The participating countries during this term have been Austria, Canada, Finland, Germany, Norway, Sweden, Switzerland and USA. VTT Technical Research Centre of Finland and Pöyry have acted as the operating agent of the Task. The main outcome of the Task during the period 2009-2011 is “Recommended practices for wind energy projects in cold climates”. The purpose of the report is to provide the best available recommendations on this topic, reduce the risks involved in undertaking projects in cold climates, and accelerate the growth of wind energy production in these areas. The document addresses many special issues that must be considered over the lifetime of a cold climate wind energy project. The importance of early project state, site measurements and project design is emphasized. The Task 19 working group paid especial attention to make the recommended practices usable to the wind energy project developers. Task 19 is preparing also a State-of-the-Art report which depicts the current top technology and methodology in the field of cold climate wind energy. Sweden is represented in IEA Wind Task 19 by Göran Ronsten. He participated in the regular meetings of IEA Task 19 (usually two meetings per year). Apart, the Swedish cold climate activities were presented at numerous national and international occasions. Some of these were:
•
European Wind Energy Conferences2
•
Winterwind conferences3
•
Ice & Rocks III4
•
IWAIS 2009 and 20115
•
1 2 3 4 5 6
Wind Turbine Blade Manufacture 20106
arcticwind.vtt.fi www.ewea.org winterwind.se www.seewind.org iwais2009.ch, iwais2011.cqu.edu.cn http://www2.amiplastics.com/Events/Resources/Programme/Wind Turbine Blade Manufacture 2010.pdf
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1.3.3 V-359: Prestudy, Verification data for project V-313 This project is dedicated to reduce the uncertainties of the icing simulations which are carried out in project V-313. This included a discussion of the modeling process itself with the strengths and weaknesses of the applied methodologies. Furthermore, sensitivity studies are carried out to determine the sensitivity of the models to different meteorological parameters. Finally, the importance of measurements for verification of the models and their availability is shown. A measurement plan will be elaborated The main results of this study are the following:
• A sensitivity analysis of the ice accretion model on a vertical cylinder to the
external meteorological conditions was carried out. One parameter was varied while all the others were kept constant and ice accretion was simulated. The study showed that the icing simulations are sensitive to meteorological parameters in the following order: 1. Wind speed, 2. Liquid Water Content LWC, 3. Droplet size/distribution, 4. Air temperature, 5. Air pressure
• Air temperature and air pressure are simulated well with numerical weather models. Wind speed simulations result in a much lower accuracy.
• There is a large uncertainty about the importance of the exact value of
Liquid Water Content LWC for icing simulations. There are hardly any measurements available which allow the compare on one hand the results of the simulations and on the other hand the spatial and temporal variations of this parameter.
• There are accurate devices available to measure air pressure, air temperature and wind speed.
• There exist methods to measure LWC and droplet size/distribution.
However, these instruments have a large uncertainty and some of them cannot be operated automatically.
• Icing is simulated on a vertical cylinder. There is a need for model for ice accretion on wind turbine blades.
1.3.4 V-363: Complementary measurements to V-313 This project is dedicated to provide complementary icing measurements for the project V-313. The following instruments are tested:
•
Goodrich 0872E3
•
Labkotec LID-3300IP
•
IceMonitor MK1
•
HoloOptics
In addition to these instruments, another approach is tested where three NRG IceFree anemometers are used: One is not heated, one is permanently heated and one is heated only when there is a significant difference between the readings of this anemometer and the permanently heated anemometer. First results of this projects are expected after the winter 2011/12.
5
2
Concept of the study
In this study, a survey of existing research developments as well as of future research needs in the field of icing on wind turbines was analyzed. This analysis was done for the following research fields:
•
Icing simulations (climatologies and forecasts)
•
Icing measurements
•
Effects of icing on wind turbines
•
De-icing and anti-icing
•
Health and Safety (ice throw and noise)
The study will approach the issue by the following concept:
•
The image of an ideal world is drawn for each field. This ideal is supposed to be the goal of the research efforts
•
The State of the Art is described for each field. Furthermore, the main activities and player in the world and in Sweden are listed
•
Proposals for future research efforts are derived from a comparison of the ideal world with the State of the Art for each field. Furthermore, some more general thoughts are given.
The following main sources of information have been used by the author for this study:
•
Information and contacts through IEA Wind Task 19 "Wind Energy in Cold Climate" where the author is the Swiss representative. IEA Task 19 has members from Norway, Sweden, Finland, Germany, Austria, USA and Canada, presumably Denmark and China will follow.
•
Personal contacts and experiences of the author collected during the last 10 years of activity in the field of icing on wind turbines.
•
Conference proceedings of the past Winterwind conferences as well as proceedings of the past IWAIS conferences (International Workshop on Atmospheric Icing on Structures), the BOREAS conference series, the DEWEK (German Wind Energy Conference) and EWEC (European Wind Energy conference).
6
3
Introduction to atmospheric icing7
Cold Climate (CC) areas are regions where icing events or periods with temperatures below the operational limits of standard wind turbines occur, which may impact project implementation, economics and safety. Areas where periods with temperatures below the operational limits of standard wind turbines occur are defined as Low Temperature Climate (LTC) whereas areas with icing events are defined as Icing Climate (IC). Although theoretically possible, active icing rarely occurs at temperatures below minus 25°C. In some areas wind turbines are only exposed to either icing or low temperature events. In some regions both low temperatures and icing events may take place. Therefore, a site can be in a Low Temperature Climate or in an Icing Climate or both while they are still all denoted Cold Climate sites. These definitions are further illustrated in Figure 1.
Fig. 1: Sketch of the definition of Cold Climate, Low Temperature Climate and Icing Climate. This report will deal only with wind energy in an Icing Climate (IC). Atmospheric icing is defined as the accretion of ice or snow on structures which are exposed to the atmosphere. In general, two different types of atmospheric icing that impact wind turbine development can be distinguished: in-cloud icing (rime ice or glaze) and precipitation icing (freezing rain or drizzle, wet snow). The different forms of atmospheric icing can be described as follows:
•
Rime Ice: Super cooled liquid water droplets from clouds or fog are transported by the wind. When they hit a surface, they freeze immediately. If the droplets are rather small, soft rime is formed, if the droplets are bigger, hard rime is formed. Its formation is asymmetrical (often needles), usually on the windward side of a structure. Its crystalline structure is rather irregular, surface uneven, and its form resembles glazed frost. Rime ice typically forms at temperatures from 0°C down to -20°C. The most severe rime icing occurs at exposed ridges
7
Text taken from Recommended Practices by IEA Task 19, arcticwind.vtt.fi
7
where moist air is lifted and wind speed is increased. Hard rime is opaque, usually white, ice formation which adheres firmly on surfaces making it very difficult to remove it. The density of hard rime ice ranges typically between 600 and 900 kg/m3 (ISO 124948). Soft rime is a fragile, snow-like formation consisting mainly of thin ice needles or flakes of ice. The growth of soft rime starts usually at a small point and grows triangularly into the windward direction. The density of soft rime is usually between 200 and 600 kg/m3 (ISO 12494), and it can be more easily removed.
•
Glaze: Glaze is caused by freezing rain, freezing drizzle or wet in-cloud icing and forms a smooth, transparent and homogenous ice layer with a strong adhesion on the structure. It usually occurs at temperatures between 0 and -6°C. Glaze is the type of ice having the highest density of around 900 kg/m3. Freezing rain or freezing drizzle occurs when warm air aloft melts snow crystals and forms rain droplets, which afterwards fall through a freezing air layer near the ground. Such temperature inversions may occur in connection with warm fronts or in valleys, where cold air may be trapped below warmer air aloft. Wet in-cloud icing occurs when the surface temperature is near 0°C. The water droplets which hit the surface do not freeze completely. A layer of liquid water forms which, due to wind and gravity, may flow around the object and freeze also on the leeward side.
•
Wet snow: Partly melted snow crystals with high liquid water content become sticky and are able to adhere on the surface of an object. Wet snow accretion therefore occurs when the air temperature is between 0 and +3°C. The typical density is 300 to 600 kg/m3. The wet snow will freeze when the wet snow accretion is followed by a temperature decrease.
There exists another phenomenon called sublimation, which means direct phase transition from water vapor into ice, producing hoarfrost. Although it is known to cause transmission losses through corona effects, hoarfrost is of low density, adhesion and strength, and therefore does not cause significant loads on structures. Therefore it will not be considered in this report. An icing event can be described with the following expressions9, applicable to all structures and instruments exposed to atmospheric icing:
•
Meteorological icing: Period during which the meteorological conditions for ice accretion are favorable (active ice formation)
•
Instrumental icing: Period during which the ice remains at a structure and/or an instrument or a wind turbine is disturbed by ice.
•
Incubation time: Delay between the start of meteorological and the start of instrumental icing (dependent on the surface and the temperature of the structure)
8
ISO-12494: Atmospheric Icing on Structures Heimo et al, "Meteorological Measurements under icing conditions, IWAIS 2009
9
8
•
Recovery time: Delay between the end of meteorological and the end of instrumental icing (period during which the ice remains but is not actively formed)
Figure 2 illustrates how a wind measurement is affected by icing according to the definitions described above. When meteorological conditions for ice accretion are given (start of the meteorological icing), there is a certain delay – the incubation time - until ice accretion at the anemometer begins. By using anti-icing measures (coatings, warm surfaces etc.), the incubation time can be extended, in an ideal case until the end of meteorological icing avoiding icing of the instrument. As soon as there is ice on the sensor (start of the instrumental icing), the measurement is disturbed. Ice is accreted continuously on the sensor until the meteorological conditions for icing are not present anymore (end of the meteorological icing). But the ice will remain at the instrument for a certain time – the recovery time - until it melts or falls off (end of the instrumental icing). This delay can be much longer than the period of meteorological icing. Although the meteorological conditions for ice accretion are not present anymore, the readings of the instrument have to be discarded until the instrumental icing has ended. By using de-icing measures (heating, manual interference etc.), the recovery time can be shortened. In order to describe the icing characteristics of a site, the following simplifications apply:
•
Incubation time = 0, i.e. meteorological and instrumental icing start at the same time
•
The duration of meteorological and instrumental icing refers to an unheated structure, typically an fully unheated anemometer, an unheated wind turbine blade or a mounting boom
Fig. 2: Definition of Meteorological Icing and Instrumental Icing. Additional parameters to further describe the icing conditions at a site are:
•
Icing rate: Ice accumulation per time [g/hour]
•
Maximum ice load: Maximum ice mass accreted at a structure [kg/m]
•
Type of ice [rime, glace, wet snow]
9
At sites where there is adequate solar radiation during the winter months, the ice can melt away within a rather short time after the end of the meteorological icing. At northern sites, the ice can remain on a structure for a very long time after the meteorological icing. Such site specific characteristics can be described with the Performance Index. It is defined as the ratio between instrumental icing and meteorological icing: Performance Index = Instrumental Icing / Meteorological Icing
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4
Trends and developments
4.1
Icing simulations
4.1.1 Icing Climatologies The ideal world In an ideal world, there exist models which are able to deliver high resolution (25x25m), long term (> 10 years) icing climatologies for any planned wind park site. The models are able to deliver information on average icing frequency and its interannual variability, average and maximum ice loads as well as information on the typical duration of meteorological and instrumental icing in order to derive the typical performance index at the site. Furthermore, the models are able to deliver time series of ice loads for each wind turbine location of a planned wind park at least in hourly resolution. Finally, information on the uncertainty of the calculation is available for each time step. The effort to create this information is not larger than it is nowadays to perform an energy yield analysis for a wind park site. This information can then be used to determine the icing losses at the given site (see chapter 4.2, effects of icing), to exclude "suspicious" measurement data from the local measurements (chapter 4.2, icing measurements) and allow for an accurate cost/benefit analysis for different market available deicing or anti-icing systems (chapter 4.4). There exist long term icing maps for all countries which are affected by icing. State of the art The most common approach for icing simulations is to couple meteorological information with an ice accretion model. Today, there exist two different types of ice accretion models:
•
Model for simulation of ice accretion on a vertical, freely rotating cylinder (Makkonen10)
•
Models for simulation of ice accretion to two-dimensional (Turbice11, Lewice12) and three-dimensional (fensap-ice13) airfoils
There are two main sources for getting the meteorological information for an icing simulation: 10
11 12 13
Makkonen, L., 2000: “Models for the growth of rime, glaze,icicles and wet snow on structures”, Philosophical Transactions of the royal society, vol. 358, no. 1776, pp. 2913-2939 Makkonen et al, 2001, Modelling and prevention of ice accretion on wind turbines, Wind Engineering Volume 25, No. 1, 2001 icebox.grc.nasa.gov/design/lewice.html www.newmerical.com/index.php/products/ice3d-cfd-software/
11
•
Numerical weather models
•
Analysis of measurement data of weather stations
Today, Norway14, Finland15 and Switzerland16,17 have produced their own national icing atlas based on coupling of a numerical weather model with the Makkonen ice accretion formula. Ice accretion models One commonly used approach today is to model the ice accretion with the ice accretion model as formulated by Makkonen. The formula has its origin in the research of icing on overhead power lines where the conductors behave like rotating cylinders collecting ice. It references to ISO 1249435. The accretion model developed by Makkonen describes the ice accretion of incloud icing on a vertical, freely rotating cylinder with an initial diameter of 3 cm. Beside cloud droplets, also freezing drizzle is taken into consideration. Freezing drizzle is not very frequent, but if it occurs it gives a strong contribution to the ice load. The ice load accumulating on a cylindrical structure is simulated by using information about temperature, liquid water content (LWC) and wind speed, taken from the numerical weather model, as well as the volume number concentration of droplets. The latter is used to calculate the median volume droplet size (MVD). During ice accretion, the diameter of the cylinder is constantly increased. The accretion model calculates the liquid water mass flux that hits the cylindrical structure and the part of liquid water that contributes to icing. The latter is described by several coefficients:
•
Collision efficiency: ratio of droplets that actually hits the object, reduced because small particles are transported around the obstacle (Fig. 3).
•
Sticking efficiency: ratio of droplets that hit the object that is collected, reduced because some particles bounce from the surface.
•
Accretion efficiency: part of collected droplets that contributes to the rate of icing, reduced if the heat flux is too small to cause sufficient freezing.
One major shortcoming for wind energy applications is the fact that this model simulates icing for a vertical, freely rotating cylinder. This approach is dedicated to overhead power line icing and far from the characteristics of a moving wind turbine blade which has a different shape, different dimensions and is moving perpendicularly to the prevailing wind speed at different speeds between the tip to the root of the blade. Furthermore, a wind turbine blade crosses a vertical section of the atmosphere between 80 and 120 m length. There is currently no algorithm which is able to convert the ice load modeled on a cylinder into an ice load on a wind turbine blade.
14 15 16 17
www.nve.no/PageFiles/3912/kartbok3a_4143.pdf?epslanguage=no www.tuuliatlas.fi/icingatlas/index.html www.wind-data.ch/windkarte/vereisung.php?lng=en www.bfe.admin.ch/forschungwindenergie/02512/02744/index.html?lang=de& dossier_id=04846
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Fig. 3: Trajectories of droplets around a cylindrical structure. There exist similar models for freezing rain18 and wet snow19 on overhead power lines but these have not been applied for wind energy purposes. There exist dedicated numerical models for simulation of ice accretion to twodimensional airfoils such as TURBICE and LEWICE. In these models, constant external conditions have to be set for simulating an icing event. Rotational speed of the blade is taken into account. There exists at least one model which allows simulating ice accretion on a three-dimensional airfoil (fensapice). Also in this model, constant external conditions have to be set for simulating an icing event. This model has been thoroughly validated for aircraft applications. It has only rarely been used in wind energy applications20. To the knowledge of the author, none of these models has been coupled with the results of numerical weather models or with other time series of meteorological data during an icing event. Furthermore, only ice accretion is simulated, melting and sublimation effects are not simulated so far. Nowadays, there is no mechanical model which can parameterize the mechanical effects on the ice which is accreted to a structure based on meteorological conditions such as wind speed or mechanical influences such as vibrations in general or bending effects on wind turbine blades. However, such a model is needed to be able to accurately simulate a whole icing event on a wind turbine blade. It is the reason that in general, only short periods of ice accretion are simulated with the 2D and 3D models. Longer model runs would lead to unrealistic results as these mechanical effects are not considered and the external conditions are kept constant. Numerical weather models Numerical weather models are able to calculate the needed input parameters for the ice accretion models such as temperature, wind speed and LWC. An important aspect is the cloud microphysics which is applied in the numerical 18 19 20
Makkonen, L., 1998, “Modeling power line icing in freezing precipitation,” Atmospheric Research, Vol. 46, No. 1-2, p. 131–142. Makkonen, L., 1989. Estimation of wet snow accretion on structures. Cold Regions Science and Technology, 17(1): 83-88. Homola et al, 2011, Performance losses due to ice accretion for a 5MW wind turbine, Wind Energ. (2011) © 2011 John Wiley & Sons, Ltd.
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weather model. Nowadays, several options exist which are capable of predicting the LWC such as the Thompson21 or the Morrison22 scheme. A good and accurate simulation of MVD by numerical weather models is not yet available to the public. This parameter has to be estimated using literature. Another important parameter is the horizontal resolution of the numerical weather models. The icing rate is strongly dependent on small scale effects such as lifting of air or speed up effects. Therefore it is highly important to have an as good as possible representation of the terrain in order to achieve accurate icing forecasts (fig. 4). Today's computer resources allow running the numerical weather models at horizontal resolutions of typically 1-3 km, single test cases with even lower resolution have been run, too. The horizontal resolution needed for the representation of complex terrain or for the micro siting of wind parks (< 100 m) could not be reached so far.
Fig. 4: Effect of horizontal grid resolution on terrain representation23. Numerical weather models with high horizontal resolution and sophisticated cloud microphysics today still require extensive computer resources. This does not allow for calculation of long term (>10 years) model runs in an economic way. One proposed approach is to correlate a short term (1 year) high resolution model run (e.g. 1 km) with a long term (10 years) model run at a coarser resolution (e.g. 5 km). This allows for temporal extrapolation of the high resolution icing data. However, reliable measurement data for validation of this extrapolation is rare and therefore the accuracy of this approach unknown. Icing simulations usually focus on one specific point (instrument position or position of the hub of the wind turbine). When using numerical weather models as input, mostly the ice accretion on a cylinder is chosen as an ice 21
22
23
G. Thompson, and V. Tatarskii, 2009: Impact of cloud microphysics on the development of trailing stratiform precipitation in a simulated squall line: Comparison of one- and two-moment schemes. Mon. Wea. Rev., 137, 991–1007. Morrison, H., and J. Pinto, 2005: Mesoscale modeling of springtime Arctic mixedphase stratiform clouds using a new two-moment bulk microphysics scheme. J. Atmos. Sci., 62, 3683–3704. http://seewind.org/files/element/file_download/ice_rocksiii_experiences_with_ sodar_lidar_cattin.pdf
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accretion model. Ice accretion models for wind turbines are not used for producing icing climatologies. There is currently no algorithm which can translate icing on a cylinder into icing on a wind turbine blade. In many cases, the frequency of icing and the ice loads increase with increasing height above ground. This is due to a higher probability of a structure being inside clouds (icing frequency) and surrounded by high water content (ice load). There is only rare information on the vertical profile of the LWC, the icing rate or ice loads. This is mainly due to a lack of suitable verification data. However, the blades of a wind turbine will cover a vertical profile of up to 120 m vertical distance. Fig. 5 shows the dependence of the ice load on the height above ground. In relation to the levels, a wind turbine is indicated in the graph too. The timing of icing episodes (especially the start and end of meteorological icing) can be predicted pretty well today. Problems occur for the end of instrumental icing (melting and sublimation, mechanically induced fall off of ice) as well as for simulation of the ice loads. Most of the validation work is done in a rather qualitative way, comparing ice loads visually on a graph. Detailed analysis on the quality of the simulation with regard to the differentiation between meteorological and instrumental icing, icing intensity, ice loads or the melting process have not been carried out. On main reason is the lack of available measurement data for validation. Wind speed and temperature simulations can easily and accurately be compared with measurements. The validation of icing simulations and especially of the modeled ice load is difficult as there is a lack of reliable and accurate instruments. Finally, LWC and MVD are very hard to measure and rarely measured at a site (chapter 4.2). It is suspected that the deviations in simulations of ice loads compared with measurements are often related to an inaccurate prediction of the wind speed, the LWC and the estimated MVD. In that sense there is important information missing for a proper validation of the forecasted values. There exist some case studies at selected sites but the availability of data and especially the relation with wind energy projects is too small. This leaves a high uncertainty on both, the measurements and the simulations of icing, especially with respect to wind turbines.
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Fig 5: Different Ice loads at different heights above ground, analysis from Sveg tower Sweden (graph modified by the author)24. The indicated wind turbine shows that different ice loads occur at different positions of the rotor. Analysis of meteorological data There exist numerous statistical methods to derive icing climatologies from observation data directly (e.g. cloud height, air temperature, satellite images). At some locations, these methods yield good results25,26. However, this approach is strongly dependent on the density and the quality of the measurements as well as on the complexity of the terrain which rules the transferability of the results away from the measurement point. Relative humidity has been found to be a unreliable parameter to be used for the characterization of icing conditions40,41. Climate change Climate change scenarios have not been considered so far for producing icing climatologies.
4.1.2 Icing Forecasts The ideal world In an ideal world, weather forecast systems are capable of providing accurate wind energy production forecasts for the next 72 hours which include potential production losses due to icing on the wind turbines. The calculation of the production includes the adaptation to the available de-icing or anti-icing 24 25 26
Ronsten et al, 2009, Measures needed for the successful development of wind energy in icing climates (from COST 727), EWEC 2009 Harstveit et al, 2009, Measurements of Cloud Water Content and Droplet Density; and Calculation of Cloud Water Gradients at Kuopio, Finland, IWAIS 2009 Bernstein et al, 2009 European Icing Frequency Derived From Surface Observations, IWAIS 2009
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systems in the wind park as well as to the operation strategy (immediate stop if iced blades, operation with iced blades allowed etc.). If a ramp in energy production is likely to occur due to icing, a dedicated warning message will be displayed. Probabilistic forecasts are delivered i.e. the uncertainty can be determined from ensemble forecasts. The information is used either for trading the energy on the spot market or for grid management (overhead power lines are also affected by icing!). On the very short term level (0-6h), icing forecasts are available and used for turbine control, i.e. for preventive operation of de-icing systems. These forecasts are strongly based on measurements on the wind turbine itself as well as on weather forecast information submitted to the SCADA system. State-of-the-Art With a large penetration of wind energy in the grid, wind energy production forecasts become important in order to keep the grid stable and to sell the energy at the spot market. According to the state-of-the-art workshop of COST Action ES100227,28 held in March 2011, the following main subjects need further research efforts in the field of wind energy production forecasts
•
Ramp forecasts (sudden drops or rises in production)
•
Complex terrain
•
Special external conditions such as icing
During light to moderate icing events, icing results in a reduced power production compared to the forecasted wind speeds. During strong icing events, the influence of icing on wind energy production can be seen as a special sort of a ramp. During the icing event, the production quickly drops or even comes to a full stop within short time. If this phenomenon occurs in a large wind park, there is suddenly much less energy available than expected which can result in problems to keep the grid stable. The basic background for the implementation of icing forecasts in wind energy production forecasts is the same as it is for icing climatologies (chapter 4.1.1), a numerical weather model is coupled with an ice accretion model (typically Makkonen) in order to simulate ice loads. This information is coupled with the forecasted wind speeds and transformed into energy production using a power curve with respect to icing. Therefore the same limitations are present as they are described in chapter 4.1.1. Today, there are hardly any operational production forecasts available which include icing. One main reason is that icing forecasts are critical in terms of availability at given times of the day and require extensive computer resources. With today's computers, it is not yet possible to achieve the needed performance for operational forecasts or at least, its additional investment does not cover the benefit. Furthermore, there is no independent
27 28
wire1002.ch/index.php?id=24 Giebel et al, 2011, The State of the Art in Short-Term Prediction of Wind Power, Anemos deliverable
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validation data available. Only few studies have been carried out but these clearly show the potential for such forecasts29. Icing forecasts are mainly available for aviation purposes where a much coarser spatial resolution and fewer requirements on the accuracy are required. With the horizontal grid sizes available today for operational numerical weather models, the details of local topography cannot be described sufficiently. Therefore, it is a common approach to perform statistical or dynamical post-processing in order to allow the elimination of systematic errors (e.g. bias) in the forecasts for parameters such as temperature or wind speed (e.g. Model Output Statistics MOS or Kalman Filter). Today, there is no post-processing of the output of the numerical weather models when performing icing simulations. Wind speed, temperature and LWC are usually taken from the direct model output DMO. As wind speed and LWC are sensitive input parameters to the ice accretion model (ice accretion but also sublimation effects), a statistical post-processing of wind speed forecasts might improve the results. In weather forecasting it is a common approach to apply ensemble forecasts, i.e. the same model is run with slightly different initial and/or boundary conditions. This results in a large number of model runs which gives important additional information on the uncertainty of the forecast. Due to the limitation in computer resources, icing simulations are today carried out in a deterministic way. No ensemble runs are being carried out so far. In that sense, information on the uncertainty is not available. Very short term forecasts (0-6h) for icing seem not to be existent at all at the moment.
4.1.3 Activities in the world The former COST Action 72730 performed a lot of activities in the field of icing simulation. First steps were carried out by the Norwegian Meteorological Institute which is also today very active in this field although they more focus on icing on overhead power lines. During COST Action 727, Meteotest from Switzerland formed a strong collaboration with Norwegian Meteorological Institute for further case studies. Finally, a broad study was carried out by the Norwegian Meteorological Institute carrying out icing simulations for six test sites in Europe (Fig. 6)31. At these sites, the simulations could be validated with measurement data. The icing simulations within COST Action 727 were all based on the numerical weather model WRF. Most intensive work on cloud microphysics of the WRF model has been carried out at the NCAR in the USA (Greg Thompson). Currently, the work is focused on a new version of the cloud microphysics scheme which is going to include predictions of the cloud droplet number concentration.
29 30 31
Oechslin et al, 2010, Wind power forecasting considering icing, DEWEK 2010 w3.cost.esf.org/index.php?id=205&action_number=727 Nygaard et al, 2009, Evaluation of icing simulations for all the "COST727 icing test stations" in Europe, IWAIS 2009
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Fig. 6: Test stations of COST Action 727. Meanwhile, Norway (by Kjeller Vindtekknik14), Switzerland (by Meteotest16,17) and Finland (by FMI/VTT15) have created icing atlases based on coupling of a numerical weather model with an ice accretion model on a vertical cylinder. Different weather models were used as input: WRF down to 1 km in Norway, COSMO 2.2 km in Switzerland and AROME 2.7 km in Finland. If it comes to simulations of blade icing, VTT Finland is an active player with the development of the Turbice11 model. Studies have also been carried out at the University of Narvik20. In the field of wind power forecasting considering icing, first studies were carried out in Switzerland (Meteotest)32. At the NCAR, USA, there is also a model development in progress33. Finally, the issue of icing forecasting is addressed in the Scandinavian research project ICEWIND34.
4.1.4 Activities in Sweden Sweden is very active in the field of icing simulations. There is the Vindforsk project V-313 dedicated to this issue as well as an O2 pilot project. The main players for icing simulations in Sweden are:
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SMHI
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Uppsala University
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Weathertech Scandinavia
•
Kjeller Vindteknikk
•
Leading Edge Atmospherics
32
imgi.uibk.ac.at/sekretariat/master_theses/Oechslin_Roger_2011_Master.pdf Haupt et al, 2011, A Wind Power Forecasting System to Optimize Power Integration, State of the Art workshop of COST Action ES1002, 2011 www.icewind.dk
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Most Swedish projects focus on simulations of icing using weather forecast model coupled with the ice accretion model on a vertical cylinder. There is hardly any work on wind turbine blade icing simulations.
4.2
Icing measurements
The ideal world In an ideal world, there exist instruments which are able to measure icing reliably and automatically and which are on the same level maintenance free as today's anemometers. These instruments can deliver information on duration of meteorological and instrumental icing, the icing intensity as well as on the total ice load. The price of such a system is similar to the cost of a high class anemometer. This information can be used in order to determine the influence of icing on the production of a planned wind park. The readings are also used to validate the results of icing simulations Furthermore there are instruments which can measure LWC and MVD automatically and for an affordable cost. These instruments are used to validate the output of the numerical weather models. There exist systems which are able to exactly measure the presence of ice, the ice load and the level of coverage of ice on a wind turbine blade (tip to root). This measurement is possible during operation and stand still of the wind turbine. This information is used to control the turbine and its anti-icing or de-icing systems. State of the Art The ISO issued in 2001 a standard ISO 1249435 for ice accretion on all kinds of structures, except for electric overhead line conductors. In this recommendation, a standard ice-measuring device is described as:
•
A smooth cylinder with a diameter of 30 mm placed with the axis vertical and rotating around the axis. The cylinder length should be a minimum length of 0.5 m, but, if heavy ice accretion is expected, the length should be 1 m.
•
The cylinder is placed 10 m above terrain.
There is one instrument which refers to this recommendation, the Icemonitor MK1. This instrument is often used as a reference when comparing icing simulations with measurement data. These recommendations can be applied for meteorological purposes where icing information is needed from a standstill structure at a given height above ground. However, they are not suitable to represent the icing conditions on wind turbine blades as the height above ground and the vertical cross-section covered by the rotor is much larger. Today there exist a variety of instruments which intend to measure icing in different ways. Some focus on meteorological icing (vibrating rods, IR beam 35
ISO 12494: Atmospheric icing of structures. ISO/TC 98/SC 3, 2000-07-20
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reflection, attenuation of ultrasonic signal), others directly measure the ice load (load cells) giving also information on the instrumental icing. All approaches follow feasible principles of measurement of icing. Many of the available ice detection instruments are in prototype state and therefore suffer from technical limitations. Some of the instruments need a lot of maintenance and are sensible for failures, especially when operated during harsh conditions. Other systems deliver readings which are suffering from noise or drifts and therefore hard to be interpreted. Finally, all instruments experience specific conditions under which they are not able to detect icing conditions or they produce false alarms (e.g. wet snow, soft rime, heavy icing etc.). In summary, it is nowadays not possible to fully rely on the readings of the existing instruments; they have to be interpreted together with other measurements or camera images. There is a clear need for better instruments. A large intercomparison of ice detectors was carried our during COST Action 727. Two instruments, the Goodrich Ice detector 0871LH136 and the Combitech IceMonitor37 were tested at five sites38 in Europe. Furthermore, at Guetsch mountain, five different ice detectors were examined; none of them was found to operate properly39. There are various indirect approaches to measure icing such as the use of temperature, relative humidity, visibility or cloud height. However, these approaches are strongly dependent on the characteristics of the site and the quality of data. Especially the relative humidity has been found to be a not very good indicator40,41. A further approach is to compare a heated and an unheated anemometer in order to identify the icing conditions. This has been proven to be a robust approach; however, it can deliver only information on instrumental icing. An ongoing study is adding a third anemometer which is heated only in intervals during periods of meteorological icing42. If successful, this approach might be able to deliver the needed information on meteorological icing, too. One of the most reliable ways to identify icing today is still by visual inspection. The use of automatic cameras has proven to be a good tool to identify icing on standstill structures. However, the image analysis has to be done manually today, there don't exist automatic algorithms to determine the presence of ice in camera images. IR lens in combination with IR light has been proven to be able to deliver good images also during night conditions. The measurement of LWC and MVD in order to simulate icing indirectly through an ice accretion model is nowadays a difficult and expensive task. 36 37 38 39 40 41 42
Cattin et al, 2009, A test of the Goodrich 0871LH1 ice detector at the Guetsch station, IWAIS 2009 Wareing et al, 2009, Test Site data on icing monitors and conductor ice loads. IWAIS 2009 Wareing et al, 2009, European Test Sites, IWAIS 2009 Cattin et al, 2007, Alpine Test Site Guetsch - Meteorological measurements and wind turbine performance analysis, IWAIS 2007 Rast et al, 2009, Icing Indices: a good solution?, IWAIS 2009 Cattin et al, 2009 Four years of monitoring a wind turbine under icing conditions, IWAIS 2009 Vindforsk V-363
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However, this is an important information when the output of numerical weather models has to be validated. There exist instruments43 which are able to do this on a thermal (heated wires), optical (laser scanning44) or mechanical basis (Rotating Multi Cylinder45,46). The main drawbacks are, that these instruments are either very expensive or not able to measure the LWC and MVD automatically. Furthermore, the uncertainties can be very large depending on the approach. A good but old (1996) data set for comparisons exists from the TV tower at Kivenlahti in Finland47,48 a second one from Ylläs mountain in Finland. Here a rotating multicylinder was used for manual measurements. There exist approaches to measure icing directly on the blades of a wind turbine. The most common and straightforward approach to detect icing on wind turbine blades is through a deviation from the effective production compared to the production according to the power curve. This approach has been proven to be very robust and efficient to detect iced blades during operation. However, it is not applicable when the turbine is not operating or when the wind speeds are very low. Other systems identify the ice on the blades through changes of the "Eigen frequency" of the blades or changes in blade mass. The sensors are either accelerators (IGUS) or strain gauges (MOOG/Insensys) which are installed directly inside the blades. The manufacturers claim that these systems are working, however there exist hardly any independent test cases from R&D projects. Some of these systems depend on access to the current pitch data of the wind turbine which might not be accessible depending on the manufacturer. Some of these systems are also able to measure the ice load. Here again, an independent study from an R&D project is not available. Another approach to identify icing on the blades of a wind turbine is again the use of automatic cameras. First approaches mounted the camera on the spinner of the rotor so that the camera was rotating and always pointing to the same blade49. The disadvantage of this approach was that the camera is strongly exposed to the weather (icing) and the the view angle towards the blade is very flat. This results in the effect that especially with long rotor blades, it is hard if not impossible to see the tip of the blades. A different approach is to mount the camera on the nacelle pointing side wards and catching the blade by motion detection when it moves through the 43 44 45 46 47 48
49
Ide, 1999, Comparison of liquid water content measurement techniques in an icing wind tunnel, NASA/TM-1999-209643 / ARL-TR-2134 www.dropletmeasurement.com and www.gerberscience.com Howe, 1991, Rotating multicylinder method for the measurement of cloud liquidwater content and droplet size, CRREL report AD-A234-780 Makkonen, 1992: Analysis of Rotating Multicylinder Data in Measuring CloudDroplet Size and Liquid Water Content. J. Atmos. Oceanic Technol., 9, 258–263. Thompson et al, 2009, Using the Weather Research and Forecasting (WRF) Model to Predict Ground/Structural Icing, IWAIS 2009 Nygaard et al, 2011, Prediction of In-Cloud Icing Conditions at Ground Level Using the WRF Model, Journal Of Applied Meteorology And Climatology, Volume: 50, Issue: 12, Pages: 2445 - 2459 Ronsten, 2005, Visualisation of the elastic blade motion of an operating wind turbine, FOI-R-1398—SE, ISSN 1650-1942
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image. This approach has the advantage that it is not necessary to put a hole in to the spinner, the camera is less exposed (view angle approximately perpendicular to the wind direction) and the view angle towards the blade is more steep, i.e. it is possible to see the whole blade. The disadvantages are that the video detection cannot capture 100% of the situations, the front of the blade cannot be seen properly and the system randomly catches a blade so it is not always the same blade which is captured. Again, there doesn't exist an automatic algorithm to detect ice from the camera images, the analysis has to be done manually. One problem which is related to all camera approaches to detect icing on the blades is the illumination during the night. Strong headlights disturb the neighborhood. Stroboscopic light is hard to trigger and again disturbs the neighborhood. Best results were achieved so far by using very strong IR headlights and a camera with a lens sensitive for IR light. However, the headlight must be over dimensioned strongly in order to achieve useful images. Today there is no clear standardization of icing measurements available. The main reason is the lack of reliable and accurate instruments.
4.2.1 Activities in the world During COST Action 727, many ice detector tests were carried out in Europe. However, The Action has ended in 2009 and most of the tests have been stopped. The Kanagawa institute in Japan has a long tradition in testing ice detectors in their icing wind tunnel. Meanwhile, also VTT Finland offers an icing wind tunnel. In Switzerland, many tests and developments for observing icing with automatic cameras have been carried out by Meteotest at the test sites on Guetsch50 and St. Brais. At Riviere-au-Renard, Québec, Canada, a large test site dedicated to icing on wind turbine is currently being set up51. At this site, numerous icing measurements will be performed. There are numerous systems for blade ice detection in operation around the world. However, they are usually not part of R&D projects. In that sense, only little information is available on the performance of these systems.
4.2.2 Activities in Sweden Sweden is very active in the field of icing measurements. There is the Vindforsk project V-313 dedicated to this issue as well as an O2 Vindkompaniet’s pilot project. Most of the measurements are carried out with the Icemonitor MK1 and the Holooptics T41 ice sensors which are both produced in Sweden. In some cases, also the Labko and the Goodrich ice 50 51
www.meteotest.ch/cost727/index.html www.eolien.qc.ca/?id=430&titre=Site_nordique_experimental_en_ eolien_CORUS__SNEEC_&em=57
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detector are used. These measurements are all point measurements. In some cases, high towers are used and icing is measured at several heights. However, analysis of vertical profiles has not been presented very often. Many of these instruments are monitored with automatic cameras. This facilitates the interpretation of the measurement data and increases the quality of the measurements. Both Swedish ice detectors still have potential for improvement (robustness, accuracy). However, it seems that there have been no significant developments during the last years (since COST Action 727). Blade ice measurements seem to be performed at pilot wind turbines but little information on results is available.
4.3
Effects of icing on wind energy production
The ideal world In an ideal world, the effect of icing on wind energy production is exactly known. This means that the process of an icing event and its influence on wind turbine blades is fully understood and there exist specific power curves for different ice loads and icing intensities. Wind power production of a wind turbine can be exactly predicted as a function of temperature, wind speed, ice loads on the blade (or LWC and MVD) and icing intensity. Furthermore, the effect of different anti-icing or de-icing systems is also known and can be included in the calculation depending on the icing frequency and the ratio between the meteorological and instrumental icing. There exists a site classification which allows putting a given site into an icing class depending on the icing characteristics of the site. This classification is validated and used as a standard. Wind turbines are certified with respect to this site classification i.e. the planner knows exactly how a given turbine will perform in each icing class. During planning process, it is possible to include icing effects in the energy yield calculation accurately (similar e.g. to the calculation of the park effect). State of the Art Today it is obvious that icing affects the wind energy production. A lot of simulations (icing wind tunnels and numerical models) have been carried out in order to quantify the production loss depending on the ice load on the blade52,53,54. Already small amounts of ice can cause reduced power production. It also seems that the inner parts of the blade (closer to the root) are less sensitive to production loss caused by icing. Usually, only small ice loads have been simulated. 52 53 54
Homola et al, 2011, Performance losses due to ice accretion for a 5MW wind turbine, Wind Energ. (2011) © 2011 John Wiley & Sons, Ltd. Seifert et al, 1997, Aerodynamics of iced airfoils and their influence on loads and power productions, EWEC 1997 Barber at al, 2009, The Effect of Ice Shapes on Wind Turbine Performance, IWAIS 2009
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Based on these experiments, modified power curves for iced wind turbines were developed. There also exist specific 3D power curves which include the power production as a function of wind speed and ice load. These power curves are able to describe the modification of the power curve due to icing in the right order of magnitude. There also exist comparisons of the predicted icing losses to the real power production. However, these comparisons show that there still remains a rather large uncertainty on the effective loss in power production. One main problem in this field is the clear lack of trustable validation data, especially on the real ice loads on the blades and its development during an icing event. In many cases, the ice load measurements are taken from a point measurement on the nacelle of the wind turbine which is not representative for the blades neither during the meteorological icing (ice build-up) nor during the recovery time (ice melting, sublimation). There is no clear information of the effective degradation of the blade due to the ice. Icing intensity as well as the difference between meteorological and instrumental icing are hardly separated in the calculations. Another problem lies in the difficulty to access production data because of restrictions due to confidentiality reasons. There will be a site classification presented by IEA Task 19 in their recommended practices report in 2012 which will indicate production losses depending on the icing frequency. But these numbers have a rather high uncertainty range and have not been validated very thoroughly due to a lack of available production data. Due to the lack of commercially available de-icing and anti-icing systems, it is nowadays hard to quantify the effect of such systems on the wind power production. Some studies from Switzerland indicate that the hot air heating system of ENERCON is mainly active during periods with meteorological icing. In most cases, undisturbed operation can then be resumed after the end of the meteorological icing whereas a turbine without such system would remain disturbed until the end of instrumental icing57. Heating during operation of the wind turbine represents a fully new situation again which has not been examined yet. There is little experience with the effect of such systems on the incubation and the recovery time of the blade during icing events.
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Fig. 6: Example of a 3D power curve as a function of wind speed and ice load (modified by the author)55.
4.3.1 Activities in the world In Finland, VTT is an active player developing the turbice model and carrying out wind tunnel tests. In Norway, active work is performed by Kjeller Vindteknikk and the University of Narvik. The latter is also operating a test site. In Switzerland, extensive monitoring of the power production of ENERCON wind turbines with hot air heating system were carried out at the test sites on Guetsch and St. Brais.
4.3.2 Activities in Sweden The evaluation of the effect of icing on wind energy production is less in the focus of the Swedish research activities. There seem to be excellent data bases from a lot of measurements and production data (e.g. Vindforsk V-313 and O2 Vindkompaniet’s pilot project). However, the analysis of icing events and the effect on the power curve is done in a rather general way by showing time series of production data versus wind speed as well as long term empirical power curves. No studies could be found which clearly focus on a more detailed look at specific icing events.
55
Byrkjedal, 2011, "Detailed national mapping of icing - Influence from icing on wind energy production and the benefits of forecasting to improve profitability", "Wind energy aerodynamics - icing and de-icing of WT blades" KTH Stockholm and Chalmers Gothenburg
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4.4
De-Icing and Anti-Icing
The ideal world In an ideal world, the planer of a wind park can chose between an anti-icing system which completely prevents ice to be built up on the entire wind turbine or a de-icing system which is capable of fast and efficiently removing the ice from the wind turbine during icing events. The performance and the costs of these systems are well known so that the choice of the system depending on the site specific icing characteristics can be easily made. The additional cost of such a system can be compensated by additional production within 2-3 years of operation. The system has the same level of maintenance as the other parts of the wind turbine. State of the Art De-icing refers to removal of ice from the blades. It corresponds to a shortening of the recovery time during an icing event. Anti-icing refers to the prevention or at least delay of ice buildup on the blades. It corresponds to an increase of the incubation time during an icing event. One thing which is common for this research field is that most of this work is done internally by manufacturers and therefore, only very little information about the technical specifications and the performance of these systems is available to the public. Up to now there exists only one system which is sold on a large basis and has been tested in independent R&D projects: the hot air heating de-icing system of ENERCON. The other manufacturers all claim to work on a system. Some also present internal case studies. The main research trends are towards Hot Air Heatings, Electro-Thermal Systems and Anti-Freeze Coatings. The most recent activities of the wind turbine manufacturers can be found in the proceedings of Winterwind 20123. De-icing The hot air de-icing system by ENERCON works on the following principle56: Hot air produced by an electric fan heater located in the root of the rotor blade is propelled over the ribs inside the rotor blade and along the front of the blade all the way to the tip. From there, the air circulates back via the center rib in the direction of the fan creating a continuous flow of air. Inside the blade, the hot air heats the laminate to above 0°C temperatures causing the ice and snow to melt. The system has been tested thoroughly at the site St. Brais in Switzerland with good results57,58. For an ENERCON E-82 and E-70, the energy consumption of the rotor blade heating system is roughly 85 kW. In the case of rated wind, the wind turbine would still produce approx. 96% energy even in heating mode. Electro-Thermal systems consist usually of heating surfaces which are installed on the leading edge of the wind turbine blades. Today, many prototypes exist and are being tested on wind turbines. 56 57 58
ENERCON Windblatt 2011/01 Cattin et al, 2011, Vereisung WEA St. Brais, BFE Schlussbericht (german) Cattin et al, 2010, Wind turbine blade heating, does it pay?, DEWEK 2010
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The electrical heating uses an electrical resistance embedded inside the membrane or laminated on the surface. Electrically heated foils can be heating wires or carbon fibers. Heating elements cover the leading edge area of the blade. The ice detector and blade surface temperature are used to control the operation of the heating system. Additional temperature sensors are installed to protect the blade from permanent damage induced by overheating. Apart from the technical challenges related to Electro-Thermal systems for removal of ice, the control of such a system is another important field. The right timing for switching on and off the heating systems has to be identified during operation of the wind turbine. This aims at finding the most economical way to optimize the cost/benefit in terms of the used energy for heating versus additional production. Currently, it is not possible to predict icing turbine-specifically in order to start up de-icing systems before there is ice on the blades. Only after the detection of ice on the blades, the process of removing the ice can be started. A preventive de-icing, which would then be anti-icing is not available today. There are not yet any information of long term effects of heating systems on the blade structure of large wind turbines. As de-icing systems mostly focus on the leading edge of a rotor blade, there is a probability for secondary icing, i.e. the ice is melted but re-freezes on the unheated parts of the rotor blades. Anti-icing Over recent years, much emphasis has been placed on the development of coatings that act as passive anti-ice surfaces59. Most products that are available are hydrophobic coatings that prevent the water from settling on the surface and subsequently reduce the ice formation. On first thought, (super-) hydrophobic coatings should improve the anti-icing behavior due to the reduced wetting. Nevertheless, it has been shown in comprehensive studies performed by the Fraunhofer IFAM that the hydrophobicity is only one of the determining factors for predicting anti-ice properties – and furthermore, a hydrophobic coating is not necessarily an anti-ice coating. A systematic study showed that with increasing the hydrophobicity of a coating with polysiloxane additives, an increase of ice formation occurs at angles of more than 140°. Up to now no general coating type could be observed with outstanding results. A further coating concept is chemical freezing point depression which is based on the leaching of depressors out of the paint matrix. Due to the leaching aspect, this effect is only temporary and is only suitable for technical applications that require ice free surfaces for a short time. Tests of such coatings have shown promising results. Another concept is the use of biochemical technologies for the prevention of ice formation on technical surfaces. This biochemical method is used by organisms in polar and sub-polar regions and is the reason for the survival of, for example, fish, amphibians, plants and insects. It involves coupling antifreeze proteins to coatings. It is based on substances with constitutive 59
Rehfeld et al, 2009, Progress in anti-ice technologies –coating concepts and evaluation, IWAIS 2009
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properties that cause freezing point depression due to the configuration and conformation of the molecules. Such molecules selectively depress the freezing point, but have no effect on the melting point of ice. This leads to a temperature difference between melting point and freezing point and is called “thermal hysteresis”. These molecules are described as thermal hysteresis proteins or (more commonly) anti-freeze proteins (AFPs). Another approach is the development of coatings that contain hydrophilic centres in a hydrophobic environment. This allows water molecules to adhere to certain sites, yet the hydrophobic surroundings of these sites promote the removal of ice crystals. Most of these approaches are tested in labs, there are only few field tests. Field tests are mainly available for hydrophobic coatings. Biochemical technologies are still in the labs. One big advantage is, that the whole surface of the blade can be kept icefree with this method (no secondary icing). Furthermore, no control system is needed. As a side effect, such coatings could also be used for meteorological instruments, e.g. ice free anemometers.
4.4.1 Activities in the world De-Icing To the authors' knowledge, Enercon, Siemens, WinWind, Vestas, Repower and Nordex actively work on de-icing systems. Furthermore, there are two independant suppliers of electro-thermal systems (EcoTemp, Kelly Aerospace) A lot of these activities are carried out by the manufacturers themselves. Therefore, only little information is available. In Switzerland, extensive monitoring of the Enercon blade heating systems was carried out at the test sites Guetsch and St. Brais. Anti-Icing Several reserach activities in the field of anti-icing are currently carried out. Main players are the following:
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Fraunhofer IFAM, Germany
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Zürcher Hochschule für angewandte Wissenschaften ZHAW, Switzerland
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Renewable Energy Technology Centre (joint-venture between Repower and Suzlon), Germany
4.4.2 Activities in Sweden De-Icing Sweden is a favorite location for manufacturers to test their de-icing systems. To the authores knowledge, Enercon, Vestas and WinWind carry out tests in Swedish wind parks. The project MAVIC (Measurements Analysis Wind Power for Icy Climates) observes the performance of wind turbines equipped with a de-icing system at
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three wind farms in Northern Sweden: Uljabuouda (WinWind wind turbines) and Jokkmokksliden The Vindpilot project evaluates the performance of Enercon wind turbines at the wind farms Dragaliden and Gabrielsbergetin in Northern Sweden. Anti-Icing Top-Nano, YKI, Institute for Surface Chemistry is one player in the field of anti-icing research which is located in Sweden. The work is focused on:
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Use of superhydrophobic surfaces
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Use of surfaces exposing chemical groups that are classified as waterstructure breakers
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To understand on a molecular level why or why not these concepts work
4.5
Safety issues
4.5.1 Ice throw The ideal world In an ideal world, ice throw is an integral and widely accepted part of the environmental analysis of a planned wind turbine similar to noise or shadowing studies. There exist models which can, based on measured site-specific data on frequency of icing and ice loads exactly simulate risk zones around each planned wind turbine. These models can produce such results for both, ice fall (standstill turbine) and ice throw (operating turbine). These models have been validated thoroughly by empirical studies. These models include valid assumptions on the typical rotor position when ice is thrown off as well as on the behavior of ice fragments when falling through the air. They also include different algorithms to include the changes in ice throw because of the presence of anti-icing or de-icing systems. Furthermore, there is a dedicated risk analysis model which allows the calculation of the risk of death for each point around the planned wind turbine based on the ice throw variability but also the probability of presence of persons in this perimeter. This information allows adjusting the siting of wind turbines according to the risk of ice throw similar to noise and shadowing.
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State-of-the-Art The most often approach to identify risk zone for ice throw and ice falls are the following formulas60: for an operating turbine
for a turbine at standstill d = maximum falling distance of ice fragments [m] D = Rotor diameter [m] The major drawback of the formulas is the fact, that the dependency of the ice throw risk on the wind statistics under typical icing conditions is neglected. Ballistic models were developed which can simulate the trajectories of ice fragments which are either thrown away from a rotating blade or dropped off a blade at standstill. Monte Carlo simulations have been carried out feeding such models with a large number of ice fragments for constant external situations (constant wind speed and direction)61,62,63,64. This method can be used to determine risk zones around a wind turbine (Fig. 8). Similar simulations which respect the orientation of the wind turbine and include real time series of wind speed and direction have also been carried out. Open questions are the preferred blade position for an ice fragment to be thrown off the blade (upward or downward motion of the blade), its flying characteristics and its typical shape. Here again mechanical models are needed to simulate the sticking of the ice to the blade during rotation. Additionally, the effect of breaking ice while flying through the air has not yet been examined. Finally there is a clear lack of validation data for the simulation results.
60 61 62 63 64
Seifert et al, 2003, Risk analysis of ice throw from wind turbines, Boreas VI, Pyhätunturi, Finland, 2003. Battisti et al, 2006, "Ice Risk Assessment for wind turbine rotors equipped with de-icing systems": Finnish Meteorological Institute, Boreas VII, Saarisalka, 2006 Battisti et al, 2006, Sea ice and icing risk for offshore wind turbines, Owemes 2006, Citaveccia, Italy Biswas et al, 2011, A model of ice throw trajectories from wind turbines. Wind Energ.. doi: 10.1002/we.519 igwindkraft.at/redsystem/mmedia/2011.10.27/1319728756.pdf
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Fig 8: Example of an Ice throw simulation (Source: Meteotest) Today there exist only rare systematic empirical studies on the distribution of ice throw of wind turbines. Maybe the best known study is the Guetsch study65,66. In 2004, a 600 kW Enercon E-40 wind turbine with integrated blade heating was installed on Gütsch mountain, Switzerland, at 2'300 m asl. As the wind turbine is located close to ski slopes, ice throw was considered as an important safety issue. During four winters between 2005 and 2009, the area around the wind turbine was inspected after every icing event for ice fragments that had fallen off the blades. Distance from and direction relative to the turbine as well as size and weight of the recovered fragments were mapped and, together with photos, collected in a data base. In total, more than 220 ice fragments could be recorded and analyzed giving a detailed view on the distribution of ice throw around the wind turbine. This study is still unique in the world and therefore referenced many times (Fig. 9). It showed a clear dependency of the ice throw on the prevailing wind conditions during icing events and gave indications on preferred rotor positions for ice throw. However, as the wind turbine at the Guetsch is rather small compared to nowadays standard wind turbines, the question was often raised, how these results can be transferred to larger wind turbines. In addition, no difference between ice fall and ice throw as made in the study.
65 66
Cattin et al, 2012, Ice throw reloaded – studies at Guetsch and St. Brais (winterwind.se/download/25/) Cattin et al, 2007, Wind turbine ice throw studies in the Swiss Alps, EWEC 2007
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Fig. 9: Distribution of ice fragments from the Guetsch ice throw study41. An earlier experimental study carried out during the New Icetools project collected information on ice throw provided by the operators67. The main problem was, that the approach was not systematic and in some cases subjective. Furthermore, the number of fragments per turbine was rather too low for a systematic analysis. This study delivered a good impression on the presence and the possible range of ice throw of wind turbines.
4.5.2 Noise The ideal world In an ideal world, the effect of iced blades on the noise emissions is clearly known depending on the ice load attached to a blade. The inclusion of icing is a standard add-on in market available tools for determining the noise impact of a planned wind park. State of the Art Today there are hardly any studies on the influence of icing on the noise emissions of a wind turbine. It is mainly unknown how this works. An experimental noise measurement was carried out in Switzerland68. There was a tendency towards higher noise emission with iced blades. But these results are highly uncertain as it turned out, that a noise measurement under icing conditions is a tricky task as the microphones get iced too (Fig. 10)
67 68
Durstewitz, 2005, Results of the New Icetools inquiry on operator's experience with turbine icing, Boreas VII, Saariselkä, Finnland Cattin et al, 2011, Vereisung WEA St. Brais, BFE Schlussbericht (german)
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Fig. 10: Iced microphone of a noise measurement (St. Brais, Switzerland)
4.5.3 Activities in the world Ice throw In Switzerland, Meteotest carried out the Guetsch study from 2005 to 200941. Currently, the study is extended by a similar experiment carried out at larger wind turbines at the St. Brais site in Switzerland. On basis of the Guetsch study, the ballistic model SWIM (Simple Wind turbine Ice throw Model) was developed. In Austria, there are currently observations on ice throw and ice fall going on69. Austria has one of the strictest regulations regarding ice throw and ice fall: It is not allowed to operate wind turbines with iced blades and there needs to be a manual inspection before a turbine may be restarted. The goal is to complement the experiment with fall studies (pieces thrown out of helicopters) and to end up with a risk assessment model. Ballistic models were developed and applied in Italy and Canada61,62,63, too (see chapter 4.5.1). Noise In Switzerland, some basic tests were carried out in order to measure the noise increase due to icing. Currently, there are no activities.
4.5.4 Activities in Sweden To the author's knowledge, there is no dedicated project on-going in the field of ice throw. The same goes for noise studies with respect to icing.
69
Krenn & Cattin, 2011, The Alps – windy but also icy, Winterwind 2011
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5
Future research needs
5.1
General needs
5.1.1 A bridge between science and industry There are a lot of promising research activities going on in the field of wind energy under icing conditions. However, one can identify a clear gap between the scientific approach coming mainly from the meteorological community and the industrial approach mainly driven by the manufacturers of wind turbines and components. The meteorological community is strongly focused on providing information about the meteorological conditions which lead to icing and its occurrence, frequency and intensity. However, these activities are strongly dependent on reliable measurement data and practical experience of operating wind turbines in order to validate the work. The manufacturers' community on the other hand is strongly characterized by engineering as well as confidentiality matters. However, the engineers are strongly dependent on scientific basic knowledge about icing. It seems that in many cases, the industrial developers don’t make too much use of the available knowledge in the meteorological community. Vice versa, the industry is reluctant in providing the scientists with valuable operational data and experiences which would help them to further develop their algorithms, models and their knowledge and understanding of icing in general. As a conclusion it seems to be very important to focus the future research activities on building a bridge between these two communities. As long as there is no suitable and validated understanding of the meteorology and the processes of icing on wind turbines, the manufacturers will struggle to develop efficient and reliable solutions. On the other hand, without operational data and experiences, the scientists will not be able to further develop their understanding of the icing processes the uncertainties remain large. One potential approach to build this bridge between science and industry could be the establishment of dedicated test and research centers for wind turbines under icing conditions. Ideally, these test centers consist of state of the art wind turbines equipped with icing observation instruments as well as de-icing or anti icing systems. In addition, there should be facilities for carrying out meteorological measurements. Another important fact is that many studies often touch only small parts of the icing problem and furthermore the projects are often limited in time. In order to create a long term and representative research, it would again be favorable to have dedicated test centers where all aspects can be examined over a long time.
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The test sites need to be equipped with comprehensive measurement systems on large towers (up to the tip of a typical wind turbine) as well as with operating wind turbines. On the wind turbines, there needs to be complete information on the ice accretion at the blades (where, how much) as well as on the power production. The exact ice coverage at all positions of the blade has to be known under all conditions as well as the operational parameters such as rotational speed, pitch angle and power production. With the presence of such test centers, new models or instruments can be tested at a traceable site and the results can be compared to other references. These test centers should ideally be located at several locations in Europe and operated within a European collaboration (e.g. EU Framework Programme).
5.1.2 Extend the view to the full wind turbine A further important research issue is to extend the activities on the full dimension of a wind turbine. Nowadays, many research activities are often focused on specific points (one measurement height at met towers or one blade section of a wind turbine, ice accretion on a vertical cylinder etc.). In reality, icing of wind turbines is an issue which covers a vertical profile with a length of up to 120 m where on the one hand, differences in the meteorological conditions can occur, on the other hand, depending on the position on the blade, rotational speed and blade shape may look very different. In order to move ahead with the icing research, new measurement and modeling tools should be developed which are able to provide this "full image" of icing on wind turbines.
5.1.3 More and better measurements There is a big limitation when it comes to measurements of the relevant parameters of icing on wind turbines:
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Icing measurements at points
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Icing measurements on wind turbines blades
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Measurements of meteorological parameters, especially Liquid Water Content LWC and Droplet Size Distribution
The measurement of the parameters listed above is either not possible or the readings have a large uncertainty. Furthermore, there is a lack of measurement of vertical profiles (whole blade area) instead of point measurements at single heights. This has the following consequences:
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The output of the numerical weather models cannot be verified (e.g. LWC, droplet distribution missing).
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Ice accretion models cannot be validated and improved (especially wind turbine ice accretion models)
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Mechanical icing models cannot be developed
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The effect of icing on the power curve cannot be put in relation to ice loads and icing intensities.
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The monitoring of the performance of de-icing systems is not possible
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The development and validation of reliable control systems for de-icing systems is difficult
As long as it is not possible to measure the relevant parameters with good accuracy, all further developments will be slowed down as they are dependent on guessing instead of comparing with real measurement values. Furthermore, the external meteorological conditions also have to be available at different height levels ideally upwind of the wind turbine for the prevailing wind direction during icing conditions. This measured data has to be collected and analyzed in detail for all phases and characteristics of an icing event (meteorological and instrumental icing, incubation and recovery time, icing intensity, melting, sublimation effects etc.) in order to be able to clearly understand the wind turbine under icing conditions.
5.1.4 More research activities on ice throw An important field is the ice throw issue because it is a very sensitive topic. Compared to the other ongoing and needed research activities it is a rather small issue. But if not addressed properly, it can quickly become a "showstopper" for the development of wind energy in cold climate. As long as there is only little detailed information on the frequency of ice throw, on typical distances and weights etc, it is very difficult to collect arguments to name and mitigate the risk. Therefore there is a need for detailed investigations on ice throw of wind turbines and the development of validated models which are able to determine the risk of ice throw at any planned wind farm. In the following chapters, more specific proposals for research topics are listed
5.2
Specific research topics
5.2.1 Icing simulations The full link between the external meteorological conditions and a wind turbine is to a large part unclear and needs to be further investigated. Figure 12 illustrates the situation. Most research efforts have focused on the simulation of icing on a vertical, freely rotating cylinder with a diameter of 3 cm and a length of 50 cm (left) or on a 2D blade section. The external meteorological conditions (mainly temperature, wind speed, LWC and MVD) are approximately the same in both cases. In the latter case they are kept constant. The cylinder itself doesn't move apart from the free rotation around the vertical axis. The reality is much different as it can be seen in Fig 13. There are different external meteorological conditions over the whole rotor (vertical profiles of temperature, wind speed and LWC). The blades of a wind turbine cover a vertical section of 80 to 120 m. MVD can be assumed to remain stable for the whole section. Furthermore, the rotor blade is not an ideal cylinder and is not rotating around the vertical axis. The dimension of
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the shape of the rotor blade is changing between tip and root of the blade. Finally, the rotor blade is moving which results in different relative wind speeds and different exposure of the rotor blade towards the external meteorological conditions. Additionally, mechanical effects apply on the blade.
Fig. 12: Left: Icing on a vertical cylinder. Right: Icing on a 2D blade section. In both cases, the external meteorological conditions are in approximation the same.
Figure 13: There are differences in the vertical profile of the external meteorological conditions for a wind turbine. Furthermore, depending on the position on the blade (tip to roof), different speeds and dimensions of the blade appear.
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In addition, the following research ideas are proposed: • Development of a wind turbine ice accretion model which can be used with real time series of the external meteorological conditions (real icing events) instead of constant conditions. The model should cover the whole blade of an operating wind turbine.
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More focus on the simulation of the different phases of an icing event (especially the end of an icing event with melting and sublimation)
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Development of a mechanical model for icing (shedding)
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Coupling of the existing blade ice accretion models with the output of numerical weather models.
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Intelligent combination of measurements and forecasts for a nowcasting Æ preventive operation of de-icing systems
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Evaluation of the vertical profile of icing and its relevance for wind turbine icing.
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Procedures for downscaling of the results of the numerical weather models to a higher horizontal resolution for better terrain representation, especially for LWC (statistical or dynamic downscaling, e.g. CFD-models)
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Develop innovative methods for long term correlation of shorter model runs with high resolution models in order to be able to obtain climatological icing maps
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Development of new cloud microphysics for numerical weather models which are capable of predicting the cloud droplet distribution accurately.
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Evaluate the possibilities for the simulation of freezing rain events on wind turbines
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Evaluate climate change scenarios with respect to icing.
5.2.2 Icing measurements There is a strong and urgent need for new and reliable icing measurement instruments. This includes point measurements but also icing measurements at the blades of wind turbines:
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Development, test and standardization of new ice detectors instruments for point measurements but also for detailed measurements of icing on wind turbine blades.
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Development and tests of new automatic instrument for measurements of Liquid Water Content and Droplet Size Distribution, e.g. an automatic Rotating Multicylinder system
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Development of an instrument classification (parameters measured) and test and calibration procedures (e.g. for wind tunnels)
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Development of an improved trigger for blade detection with automatic cameras. Improve the methods for taking images during the night.
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•
Automatic image analysis tools for automatic ice detection in camera images (image processing)
5.2.3 Effects of icing on wind energy production In order to be able to better understand the effects of icing on wind energy production, it seems most important to get access to production data of wind turbines as well as to measurement or observation data of ice accretion at the blades of the same turbine. With this information, the following activities can be carried out:
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Carry out detailed case studies of icing events and their effect on wind turbines which take into account ice accretion, icing intensity, icing persistence, melting and sublimation
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Deeper validation of power curves under icing conditions
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Development of reliable power curves as a function of ice load but also of icing intensity or type of ice (hard rime, soft rime, glaze)
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Include the effect of de-icing and anti-icing systems on energy production
5.2.4 De-icing and Anti-icing It is crucial to be able to understand the effects of icing on wind turbines and to develop reliable anti-icing and de-icing systems and to control them efficiently i.e. the ice accretion on a heated blade or a blade treated with an anti-icing surface. In addition, much more experimental data is needed in order to understand and further develop these systems. The following actions might contribute to this goal:
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Independent validations of de-icing and anti-icing systems within research projects
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Anti-Icing coatings to be tested in the field
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Development of intelligent turbine control systems for de-icing systems
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Evaluate the influence of secondary icing
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Develop now-casting models for preventive operation of de-icing systems (e.g. forecast icing 1h ahead based on measurements and weather models)
5.2.5 Safety issues Ice throw There is a clear need to carry out more experimental studies in order to better understand the characteristics of ice throw. Special emphasis has to be put on
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The exact time when ice is thrown off the wind turbine
40
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Distinction between ice throw and ice fall
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Difference between unheated and heated blades
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Preferred blade position for ice throw (e.g. upward or downward motion of the blade)
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Correlation between wind direction and ice throw direction
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Trajectories of ice fragments, behavior in the air, “sailing” effects
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Ice throw for different ice fragments (sizes, weights, ice types)
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Mechanical models for ice on blades
Based on this information, ballistic ice throw models need to be improved and validated. This will allow an accurate calculation of the ice throw risk under site specific icing and wind conditions. In addition to the systematic experiments, event oriented experiments should be carried out. This implies a permanent observation of the wind turbine (human or with cameras) in order to receive ice throw recordings for the following scenarios:
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Ice fall of a standstill wind turbine
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Heating of an iced wind turbine
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Start of iced wind turbine
Noise Today, there is hardly any knowledge on the effect of iced blades on the noise emissions of a wind turbine. The following questions remain unanswered:
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How can noise measurements be carried out (icing of microphones)?
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What’s the increase in dB when a wind turbine is operated with iced blades?
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How can increased noise due to icing be included in standard tools for noise assessments?
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