RESULTS OF THE NEW ICETOOLS INQUIRY ON OPERATOR S EXPERIENCE WITH TURBINE ICING

Transcription

1 RESULTS OF THE NEW ICETOOLS INQUIRY ON OPERATOR S EXPERIENCE WITH TURBINE ICING Michael Durstewitz Institut für Solare Energieversorgungstechnik e.v. (ISET) Division Information and Energy Economy Königstor 59, D Kassel Germany tel fax mdurstewitz@iset.uni-kassel.de ABSTRACT: One task within the FP5 project New Icetools has been the development and evaluation of a questionnaire regarding operators experience with iced wind turbines. This questionnaire was sent to turbine operators in European countries by mail, per download from the New Icetools web site and as well as an online form on the Internet. The questionnaire has been available in several languages (English, German, Swedish, Spanish) so that many operators at cold climate sites in Europe can be reached. The questionnaire shall collect specific information about e.g. cold climate sites, icing conditions, turbine equipment and consequences of icing events. Nearly 1 replies from operators were collected and evaluated with respect to icing cases, amount of ice upon blades, duration of icing, downtimes, observed losses etc. The presentation will include an introduction to the questionnaire as well as an analysis of operators replies. 1 INTRODUCTION A questionnaire, based on the existing layout of the WECO questionnaire, has been modified and adopted to the NEW ICETOOLS objectives by ISET and ENAIRGY. The structure of the questionnaire has been adapted according to the recommendations of project partners and the steering committee during the starting phase of the project. In order to reach a broad range of operators in Europe and other regions the questionnaire was translated into several languages in order to minimize language barriers and to increase the general acceptance. The questionnaire is available in English, German, Spanish and Swedish. 2 QUESTIONNAIRE DESIGN The whole questionnaire is prepared to fit onto one double sided A4 page. It is divided into different sections of interest with respect to general data (e.g. location, site description, observation date etc.), turbine specific information, meteorological conditions, service, downtime and accessibility information, info on ice throw and ice accretion, and contact information.

2 Wherever possible pre-defined categories e.g. the description of site, or the type of icing event were created in order to reduce the necessary effort in filling in the form and to simplify the evaluation of responses. The contents of the different query information blocks of the questionnaire are described below: 2.1 Contents The general data block of the New Icetools questionnaire is shown in Fig. 1. Here site specific figures are retrieved from the operators, e.g. name of site, elevation above sea level, geographic grid coordinates, description of site category. Furthermore the observation date, the duration of icing and turbine specific data are retrieved. Turbine specific data is type of turbine, rated power, hub height and rotor diameter. Finally the name of the observer and a phone number is asked for possible further inquiries and more detailed information. In the second data block relevant information about state and the reaction of the turbines under icing conditions is collected (Fig. 2). In this block the following items are asked for which were observed during an icing event: operation mode (idling, standstill, operation), the reaction of the wind turbines and the classification of the type of icing. Fig. 1: General data block Fig. 2: Data block with turbine specific information Meteorological data is asked for in data block 3 (Fig. 3). The standard set of weather data recorded by wind turbine control systems is usually wind speed, wind direction and ambient temperature. In most cases the exact meteorological conditions, e.g. air pressure and humidity, are not monitored on wind turbine sites. For additional information about the prevailing weather conditions operators have the opportunity to put their subjective weather

3 observations into their individual words. In order to estimate the accuracy of wind speed and wind direction by the turbines the participants are asked to indicate the design of the anemometer heating system. Fig. 3: Data block with meteorological information The last block on the first side of the questionnaire (Fig. 4) is related to accessibility aspects. In many cases wind turbine sites are located in remote areas and afar from main roads. Depending on the specific conditions of the landscape the wind turbines cannot be accessed easily or at all during winter time. This of cause effects the duration of turbine downtimes due to general failures and disruption of operation as a consequence of icing conditions as well. Any downtime caused by internal or external failures influence the economic viability of wind energy projects. If even minor failures which need to be removed by operator or service personnel cannot be removed for a longer period of time due to restricted site accessibility then financial losses will be unavoidable. Fig. 4: Data block with site accessibility information The second page of the New Icetools questionnaire contains two more data blocks. The first data block (Fig. 5) is a schematic sketch of a turbine site with concentric lines given the

4 distance from the wind turbine in radii between 5 and 4 m. The operators are asked to indicate if, where and which kind of ice fragments were found in the surrounding of the wind turbine. In order to distinguish different ice masses thrown from the wind turbines the New Icetools project has agreed on three different ice mass categories. These categories have been chosen in a way which allows an estimation of mass without technical equipment e.g. a scale. The reason for this is that the typical observers of ice on wind turbines do not carry a scale in their pockets when visiting the wind turbines. Moreover, the mass boundaries are related to goods of day to day life (grocery products e.g. piece of chocolate (1g), butter (25g), bread (1g)) which we think are well known to almost everybody. Thus the upper limit of category 1 ice fragments is set to 25g, the upper limit of category 2 is 1g. Fragments associated to category 3 are all pieces with a mass above 1g. Additionally a sketch of a rotor blade in the same data block is supposed to be used for reporting on the shape of ice accretion observed on the blades. Finally the operators are prompted to indicate whether blade heating elements are available and in operation during the time when icing was observed. Fig. 5: Data block with information mass and distance of ice fragments

5 The last block of the questionnaire provides contact information to all participants of the survey. This is of cause a subject to be changed regarding to the different areas of responsibility inside the New Icetools project. At present the project has assigned four different contact addresses for Austria, Finland, Germany and Sweden. Fig. 6: Data block with contact information 2.2 Dissemination The dissemination of the questionnaire to manufacturers, operators, developers associations and the general public was established by using different ways of distribution, e.g. via general mail, electronic mail, personal and telephone contacts, announcements in conference presentations, in printed articles magazines, etc. All language versions of the questionnaires are available on Iset s New Icetools Homepage ( for download in PDF versions. Furthermore an internet based online form of the questionnaire has been developed in order to establish an online platform for the collection of information. The online form of the questionnaire is offered in an English and a German version. For convenient evaluation of the questionnaires a data base has been prepared at ISET. 3 ANALYSIS OF OPERATOR REPLIES The emission of questionnaires was initiated in winter 22/23 and repeated in the following winter season 23/24. In order to draw the attention of the target group to this topic announcements in magazines, at congresses and fairs were launched additionally as mentioned above. Until December 24 a total number of 95 replies from 8 different locations and 168 turbines were registered in the New Icetools database. 11 turbines (~6.5%) of the turbines are equipped with blade heating devices. All communication opportunities have been used. The quality of data and information of the returned questionnaires varies in a wide range between very good and poor. The information received cannot be treated as representative! 3.1 Origin of reports All reports were received with information from sites in Austria, Germany, Finland and Sweden. No reports were received from other countries.

6 # of reports AT DE FI SE country Fig. 7: Replies by participating countries 3.2 Records by observation date Around fifty percent of reported dates of icing events were observed during the project phase between 22 and 24, a few records were observation before the starting time of New Icetools. 45 reports do not mention any exact date of the observation. The data fields are left blank or have rather vague or general answers e.g. winter, don t know etc.. # of reports n.a. Year Fig. 8: Replies by year 3.3 Records by site category and country The sites of wind turbine installations are distinguished into four different landscape categories: coastal, lowland, low mountain range and alpine sites. The distribution for the complete set of data grouped by reporting countries is given below. Reports from alpine site were received only from Austria where the majority of data is reported from the site category

7 lowland. In Germany and Finland most icing events were reported from the category low mountain sites. The Swedish data refer to the coastal and lowland categories. For about 3% of all collected data the site categories were not specified in the questionnaires. 1% 9% 8% 7% 6% 5% 4% 3% 2% 1% % AT DE FI SE total alpine low mountain lowland coastal n.a. Fig. 9: Replies by site category and country 3.4 Duration of icing event versus site elevation The duration of reported icing events in dependence on the site elevation above sea level ranges between and 2 days. Up to an elevation below ~3m a.s.l. in most cases only short periods of icing between and 3 days were reported. However, some exceptions with longer downtime periods between 7 and 1 days were registered as well. For elevations higher than 3m a.s.l. the cold climate influenced downtimes increase up to 2 days. The figure presents data points of individual reports. Red dots indicate turbines sites where the turbines are equipped with blade heating or de-icing systems. 25 duration of icing event [d] elevation a.s.l. [m] Fig. 1: Replies by duration of icing event versus site elevation

8 3.5 Latitude, site elevation and icing duration This figure gives a rough overview on the origin of reports by means of site elevation and angular distance north from the earth's equator measured in degrees. Heading from south to north one can also imagine the relief of the landscape from alpine sites in Austria, low mountain, plain and coastal areas in Germany up to the fell regions in Finish Lapland. The testified turbine downtime from these sites are indicated by different colours for different duration reports Tauernwindpark, AT elvation asl [m] Black forest, DE Hessian mountains, DE Olostunturi, FI downtime [d] unknown latidude [deg N] Fig. 11: Replies by latitude, site elevation and icing duration 3.6 Icing type and site category About 5% of the replies included explicit information on the type of ice which had affected the wind turbines. These answers have been grouped by site category. Although the absolute numbers of replies are different within each site category the normalized figures in per cent are quite similar for the site categories coast and lowlands reporting both glaze ice (freezing rain) around 6%, hard rime (sleet) at ~3 45% and only a few or no incidents at all with soft rime. Reports from the low mountains site category show that glaze and soft rime have been observed to ~25% each, while hard rime is reported for about 5% of the observations. Due to the small amount of data from alpine sites the uncertainty of this categorized result is very high. 3.7 Reaction of turbine Most reports (~45%) of icing incidents do not specify a certain reaction of the wind turbine. No reaction of the turbine has been reported in 3%. Emergency shutdowns of the wind turbine refer to 2% of the reports. An emergency shutdown due to ice can be initiated by the control system e.g. by a signal of the vibration sensor triggered by a mass unbalance of the rotor, by a significant mismatch in the relation of measured wind speed and power output, or, due to a permanent constant deviation of the longitudinal axis of the wind turbine and the measured wind direction on the nacelle. The latter case leads to an continuous yawing of the wind turbine which finally will activate the twist sensor in order to protect a damage of the cable.

9 1% 9% 8% 7% 6% 5% 4% 3% 2% 1% % coast glaze, freezing rain hard rime, sleet soft rime lowlands low mountains alpine total Fig. 12: Replies by icing type and site category unknown / not specified no reaction fault report without shutdown emergency shutdown damage of turbine components % 1% 2% 3% 4% 5% Fig. 13: Replies by reaction of turbine 3.8 Ambient temperature versus site elevation The distribution of the ambient temperature, site elevation and frequency of reports clearly indicates that most icing incidents have been reported for temperatures above 5 C. The size of the circles in the figure (small or big) indicate the number of reported incidents.

10 25 2 elev.asl / m T / C 5 Fig. 14: Replies by ambient temperature versus site elevation 3.9 Ambient temperature versus wind speed Observations of weather conditions e.g. wind speed and ambient temperature during icing incidents show that in many cases the wind conditions are sufficient for turbine operation. For medium to long term downtime periods caused by icing with simultaneous high wind speed conditions the loss of energy can accumulate to significant shares if the turbines do not operate at all or with reduced efficiency v / m/s T / C Fig. 15: Replies by ambient temperature versus wind speed 3.1 Distance and frequency versus turbine height The analysis of the estimated mass and distance of ice fragments dropped or thrown away from wind turbines versus maximum height of the turbines (hub height + radius) shows that in

11 most cases the estimated distance to the wind turbine was within a formerly recommended safety margin of 1.5 x (hub height + radius). This margin is indicated by the solid lines in the diagram. However, two events have been reported where the estimated distance to the wind turbine is out of this limits. The analysis of these particular reports show that high wind speeds were reported during these events. Explanations for these irregular distances of iced peaces could be that either very light (feathering) fragments have been observed which are blown away like leaves in a strong wind or that the distance to the wind turbine has been overestimated. 2 H + R / m distance to WT / m Fig. 16: Replies by distance and frequency versus turbine height 4 CONCLUSION About 1 replies from operators regarding experience with icing conditions have been evaluated. The analysis of these reports gives a good overview about icing conditions, downtime of turbines etc. in different European countries and at various locations. However, it has to be mentioned that the two winter seasons during the project duration have been comparatively mild which might be one reason for an only moderate participation in this specific survey. REFERENCES M. Durstewitz, G. Kury, NEW ICETOOLS: Wind Turbines in icing environment: Improvement of tools for siting, certification and operation Questionnaire And Market Analyses, Final Report WP5, Contract No: NNE , Dec. 24