MEASNET Procedure Evaluation of Site-Specific Wind Conditions Released Martin Strack; Deutsche WindGuard Consulting GmbH, Varel, on behalf of MEASNET EXTERNAL ARTICLE English The most important risks for a wind farm project are still associated with the wind conditions and the assumptions directly based upon these: the expected annual energy yield and the assessment of the stability of the wind turbine. Nevertheless, for the assessment of wind conditions at prospective wind turbine sites, no standard or internationally accepted procedures exist to define the methods, requirements and the way to handle the inherent risks, so that this can introduce certain arbitrariness in the determination of these key figures. For the processes and analyses based thereupon, however, high requirements are placed on the methods, but hardly some on the underlying input data: Even though the requirements for the assessment of structural integrity of the wind turbine are defined quite detailed by IEC standards, the requirements on the inputs to this assessment are hardly defined. A similar discrepancy exists for risk assessment and management performed on basis of available energy yield and uncertainty figures. To close this gap, an expert group within the MEASNET network was founded in order to come to a common understanding regarding the requirements on input data, methodologies and handling of uncertainties for site-specific assessment of wind conditions. This work has resulted in the preparation of a MEASNET Guideline Evaluation of Site-Specific Wind Conditions, which defines procedures and requirements with the aim to ensure reliable and high quality results with maximum traceability and comparability. In the following, the contents of the MEASNET guideline is presented and explained. Emphasis is given to aspects which distinguish the present guideline from the current state of the art, and which are particularly relevant for assessment of the risks of a project: the definition of rules for the assessment of suitability and completeness of wind measurement data, considerations on data integrity and risks arising from this, definition of a scope to be addressed to assess the parameters relevant for stability of a wind turbine, and assessment of uncertainties according to the state of the scientific knowledge. Background MEASNET is a network of measurement institutes, which has been established to harmonise wind energy related measurement and evaluation procedures. All MEASNET members are actively performing wind energy related measurements and evaluations. The MEASNET expert group Site Assessment was founded in 2004 with the aim to develop a MEAS- NET procedure for the task Evaluation of Site-Specific Wind 76 DEWI MAGAZIN NO. 36, FEBRUARY 2010
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Fig. 1: Evaluation process of site-specific wind conditions. Tab. 1: Definition of measurement campaign requirements for different terrain classes Conditions, in order to improve the traceability, quality and comparability of this task. This procedure covers the process of measurement as well as evaluation and transfer of site specific wind conditions, and shall provide the data basis for assessment of the suitability of wind turbine design according IEC for actual site parameters, i.e. the assessment of structural integrity, for which the demand for standardised procedures was and is rising. As the procedure includes all main critical aspects relevant for the subjects energy yield and risk assessment of wind energy projects, it has an important meaning also for these subjects. The expert group was aware, that the definition of a MEAS- NET guideline to ensure high requirements and comparability in the applied procedures is an ambitious goal, as the relevant tasks include evaluation, calculation and modelling procedures, where a wide variety of different methodologies exist and standardisation is difficult. On the other hand, the need for standardisation is obvious, as the procedure for assessment of the wind turbine structural integrity is defined in detail by respective IEC standards, which do closely define the methods to be applied without however covering the requirements on the input data for these, thus introducing certain arbitrariness in this process. To close this gap, and to come closer to an objective basis for such assessments, which determine important assumptions regarding the feasibility and risks of a wind energy project, the MEASNET procedure Evaluation of Site-Specific Wind Conditions was established. The MEASNET expert group consists of experts in the related topics from the following institutions and companies: Barlovento, Spain; CENER, Spain; CRES, Greece; Deutsche WindGuard, Germany (convener of the expert group from mid of 2009 on); DEWI, Germany; Risø DTU, Denmark; WIND-consult, Germany; WindTest KWK, Germany (convener of the expert group from 2004-2009); windtest grevenbroich gmbh, Germany. The involved experts and companies do represent an important share of expert knowledge and long-term application experience in the related topics, which is available in Europe, and are furthermore highly active in standardisation work of different IEC, IEA and MEASNET boards and further national committees (like FGW, BWE, AEE, VDI). A particular great value of the expert group is a comprehensive and open exchange of knowledge and experiences in the process of defining and improving the MEASNET procedure. Scope of the MEASNET Procedure The results of the Evaluation of site-specific wind conditions carried out according to the defined MEASNET procedure shall provide a traceable basis for the assessment of the certification body concerning the conformity of the design parameters with the site-specific conditions, according to IEC 61400-1 ([1] or [2]). Implicitly, the derived conditions will have an influence on the installation, operation and maintenance (O&M), loading and durability as well as the performance and energy yield of wind turbines (WT), that are to be installed at the site. According to IEC 61400-1, the site-specific conditions can broken down into wind conditions, other environmental conditions, soil conditions and electrical conditions [1], [2]. The MEASNET guideline focuses on the site-specific wind conditions and ambient conditions as far as they affect the wind flow. The process of site assessment encompasses the gathering (measurement), processing (evaluation) and interpretation of meteorological data. For each of these steps, the scope of and the requirements on the work are described in the guideline. Fig. 1 illustrates the main components of the evaluation process, as described within the guideline. The sketched process may be divided into two phases: 78 DEWI MAGAZIN NO. 36, FEBRUARY 2010
Fig. 2: Example of a simple terrain site, as understood in the guideline. Such sites have only minor relief leading to negligible orographic effects on wind conditions. These sites are, therefore, mainly influenced by roughness conditions. Fig. 3: Example of a complex terrain site, as understood in the guideline. Such sites are characterised by orographic features with terrain slopes greater than 0.3 (approx. 17 ), which have a dominant influence on wind conditions. Measurement: On-site measurement of wind conditions and documentation thereof. Data Evaluation and Extrapolation and preparation of Derived Results, including documentation of the data and results. Each of the above site assessment phases may be performed and reported separately, provided that all requirements of the specific phases are fulfilled and correctly referenced; thus, the current MEASNET procedure covers the measurement, for the purpose of assessment of site-specific wind conditions, as well as the evaluation and transfer of such measurements. As the IEC standards and additional recommendations typically applied for site-specific wind measurements are intended primarily for other purposes (e.g. power curve measurements [3]), they do not handle many relevant aspects; it is, therefore, expected that the current MEASNET procedure will play an important role in the standardisation of wind measurements for assessment of site specific wind conditions. Representativeness of Wind Measurements The quality of a wind measurement campaign and the accuracy of assessments based on such campaign are determined by different factors. One is the proper design of the measurement itself, including the quality and calibration of the equipment etc. These important issues are handled in different standards and recommendations (e.g. in [3]). Another determining factor in the application of measured wind data is the representativeness of the data for its designated purpose. This property can depend on the distance (vertical and horizontal) between the measurement location and the application position, the complexity of the terrain and other flow-determining factors. Generally, it depends on the differences between conditions at the two sites and the capability of the applied wind field calculation models to resolve these differences. However, such parameters are rather soft factors, which are difficult to assess objectively, especially given the lack of metrics for such properties as differences in site conditions or terrain complexity. It is, therefore, difficult to develop a common understanding and to standardise these parameters. On the other hand, such common understanding and a certain level of standardisation of these soft factors is absolutely necessary, in order to standardise procedures or establish reasonable minimum requirements. This is especially important, as generally those, who rely on the results of the evaluation of wind data, are not aware of the site specific conditions and particularities and thus can not assess the situation in this respect: this might be the case for the certification body which assesses the structural integrity of the turbine, i.e. the suitability for the determined site conditions; and this is true for the banker or investor who has to base important decisions on the evaluation. After evaluating different models to describe the complexity of a site, which turned out not to be appropriate, the expert group decided to set up a definition of such requirements on base of classes of terrain complexity, which are determined by the main characteristics of the site based on two classes (Tab. 1 and Fig. 2 and 3), and which set up key points for the site specific assessment of the representativeness of wind measurements and the requirements following from this. These key points will improve the way of assessment for many cases, where no detailed analysis of the situation is available, but will generally not replace a detailed expert analysis to define a measurement campaign which will provide the required data with sufficient accuracy and in the most efficient manner. For the measurement height, a commonly used rule of thumb was assumed appropriate, according to which the height of the top wind speed measurement level shall be at least 2/3 of the planned hub height. Higher measurement heights will reduce the uncertainty of the vertical extrapolation of the wind conditions and hence are generally recommended, and might also be required in special cases. For the definition of the number of required wind DEWI MAGAZIN NO. 36, FEBRUARY 2010 79
measurement masts the assumption is made, that the wind conditions measured at a mast can be extrapolated with tolerable uncertainty using the defined wind field modelling approaches within a certain radius around the mast ( representativeness radius). All wind turbines shall be located in the representativeness radius of at least one mast. In order to define the representativeness radius for different terrain types, two exemplary terrain classes are defined. The simple terrain class (Fig. 2) describes flat terrain with no noticeable terrain elevation variations, where the wind conditions are mainly influenced by terrain roughness conditions. The complex terrain class (Fig. 3) corresponds to a site with considerable orographic variation (relief) and significant slopes. In Tab. 1 the representativeness radii, e.g. the maximum distance of any wind turbine from the nearest measurement mast, are defined for each of these classes. For less complex yet hilly terrain, the representativeness radius should be obtained by interpolation between the shown classes. For even more complex, mountainous sites, smaller values for the representativeness radius have to be assumed. These should be determined on the basis of a sitespecific analysis. The indicated representativeness radii are valid for homogenous roughness conditions. In the case of nonhomogeneous roughness conditions within the wind farm area or near-by surroundings which create the risk that the mast measurements are influenced by a surface roughness value different than that observed by the wind turbine the representativeness radius may not be applicable in all directions and a site-specific analysis concerning an appropriate measurement layout shall be carried out. Data Integrity The way of handling measurement data and the derived results in wind energy context, as basis for commercial decisions, is quite special when compared to usual requirements established for commercial processes or financial transactions. A short analogy might be helpful to demonstrate this. Provided, that a customer would enter a book shop with a hand written paper containing 4 groups of 4 numerical digits, stating that this was his credit card number, would he be able to purchase a book for 30 euros? Even if he would show his credit card where the number is visible, could this transaction happen without reading the card s magnetic stripe and checking the correctness of the credit card number? The situation in wind energy is different. Commercial decisions and huge financial transactions can be performed on basis of data for which the correctness and integrity are neither proven nor verifiable. The hand written paper is, unfortunately, a fitting analogy characterising the wind data in many situations. Of course, many wind energy experts and consultants spend Ice & Rocks III Windfarm Havøygavlen (www.statoilhydro.com) Zadar / Croatia 5 th + 6 th of May 2010 Conference on Wind energy in complex terrain The follow-up event of the successful conference series Ice and Rocks (Oberzeiring 2005 and Andermatt 2007) is organised in the course of the FP6 Project SEEWIND. Conference fee is 360 (net) and includes attendance fee, coffee & lunch breaks, social event with buffet and live music in the evening of 5th of May, as well as a site visit to WF Ravne on Pag. Programme and Registration: Starting from 15 th of November 2009 www.seewind.org 80 DEWI MAGAZIN NO. 36, FEBRUARY 2010
a great deal of effort and have sound methods of performing tests and data plausibility checks. It must, however, be emphasised that it is simply impossible to completely verify or guarantee the correctness of past data processing steps by such tests. Consequently, such guarantee is excluded in disclaimers within all such reports. As it is typically simply not possible to assess uncertainties or risks arising from this gap, this aspect is usually ignored for risk assessment of a project. Without insinuating that data is intentionally manipulated or altered, the expert group decided that the subject of measurement data integrity should not be ignored in the evaluation of measurement data and their uncertainty assessment. As technically established and widely accepted solutions exist, which guarantee data traceability to national standards, in the form of measurement procedures accredited according to the ISO/IEC 17025, the application of the MEASNET guideline to is primarily relating to measurement accredited according to ISO/IEC 17025. If such traceability is not ensured, this is seen as deviation to the defined procedure, which must be commented and considered in the uncertainty assessment of the work. In order to assist in the handling of such deviations, different data integrity situations have been characterised, which also define the method of handling the additional resulting uncertainty. By classifying work on unsecured data explicitly as a deviation from the MEASNET guideline and as an exceptional application, the intention is particularly to rule out hand written paper cases, where it is impossible for the expert to assess the data correctness and integrity. As such especially problematic cases can be easily avoided by following certain rules even without noticeably increasing the effort spent on the measurement campaign it is expected that the proposed assessment scheme and explicit emphasis of the data integrity topic within the MEASNET guideline will push the requirement to improve the state of the art in this subject, so that such cases will become increasingly rare. Relevance of the MEASNET Guideline for the Practise The current MEASNET guideline makes a detailed definition of the scope of the work to be handled, concerning the contents and requirements of the data evaluation steps, the scope of parameters and derived results to be determined as well as the way to document the results. Also site parameters and related procedures, which are relevant for site assessment but whose methodology or requirements for application are not agreed standard, like extreme wind speed or flow inclination along a site, are handled. This comprehensive scope, in itself, might enrich the state of the art and improve the comparability of results obtained from such procedures. The MEASNET guideline is freely available for download at the MEASNET homepage, with the aim to help the general improvement of the state of the art. On the other hand, the experts are aware that even the detailed definition of such a procedure and the clear statement of minimum requirements is not necessarily a guarantee for correct and appropriate results. Experience shows that many critical aspects of the work cannot be determined sufficiently, that each definition may have deviating interpretations, and that such complex procedure requires the ongoing practise and re-examination with direct feedback to the guideline document. Hence, the aim of the expert group is not reached by issuing the guideline document and by performing an ongoing re-editing and improvement of the document itself, but additionally a process ensuring and verifying the correct implementation of the guideline in the daily work has been started. This process, which is an essential characteristic of MEASNET procedures and reason for its success in different subjects, comprises a mutual assessment of the participants by application of an assessment procedure, as well as ongoing mutual checks of the capabilities and the correct implementation of the procedure by means of regular round-robin tests. From experiences with internal as well as external round robin tests, the expert group members are aware about the difficulties and required effort to define and perform feasible and significant mutual assessment and round robin tests in this subject. However, based on experiences gained from other standardisation and quality assurance efforts, such measures are seen as prerequisite to assure ongoing quality and comparability in the application of the defined procedure. Summary The MEASNET expert group Site Assessment has issued the first release of the MEASNET procedure Evaluation of site-specific wind conditions, defining a procedure and requirements on input data, methodologies and handling of uncertainties, to be applied for site-specific assessment of wind conditions as well as assessments of site suitability, energy yield and project risks based thereupon. Aspects which distinguish the present guideline from the current state of the art, are particularly the definition of rules for assessing the suitability and completeness of wind measurement data, considerations regarding data integrity and risks arising from this, definition of a scope for assessing parameters relevant for wind turbine stability, assessment of uncertainties according to the state of the scientific knowledge and a mutual assessment and control scheme implemented by the expert group, which will introduce the procedure in the current work. As the MEASNET network has established de-facto standards for other wind energy relevant technical topics, which are accepted world-wide and which are especially relevant for the assessment of risks and bankability of projects, it is expected, that the proposed guideline may also have a considerable impact in this context. The guideline is freely available for download at www.measnet.com. References: [1] IEC: IEC61400-1 Wind turbine generator systems - Part 1: Safety Requirements, 2 nd Ed., 1998. [2] IEC: IEC61400-1 Wind turbines - Part 1: Design Requirements, 3 rd Ed., 2005. [3] IEC: IEC61400-12-1 Wind turbines - Part 12-1: Power performance measurements of electricity producing wind turbines, 1 st Ed., 2005. DEWI MAGAZIN NO. 36, FEBRUARY 2010 81