Wind Energy - The Facts Part I

W I N D ENE RGY - THE FA CTS - WI ND RE S OURCE ES TI MATI ONThe European Wind Atlas employs meteorological35Table I.2.1: Wind atlases, Europedata from a selection of monitoring stations, andshows the distribution of wind speeds on a broad scale.It has been used extensively by developers and govern-Countryments in estimating the resource and regional varia-EU-27tions. It is possible to map wind speeds at a higherAustriaresolution, using, for instance, more detailed topo-Belgiumgraphical data and a larger sample size of meteoro-Bulgarialogical data, in order to show more local variations inCypruswind speed. This can be used by developers looking forCzech Republicsites in a particular country.There are many examples of national, regional andlocal wind atlases, for Europe and the rest of theworld, but they are far too numerous to list here.regarding its development potential, one of the firstquestions is always ‘Is there a wind atlas for thisarea?’.A review of national wind atlases for EuropeanOther WAsPapplication[1][14][1][2][3]Estonia[15][16, ]Italy[15]countries has been under taken for this edition ofLithuania[15]Wind Energy – The Facts, the results of which areLuxembourgshown in Table I.2.1. Where obtained and permissionMaltagranted, map reproductions are contained in AppendixThe NetherlandsA. The European Wind Atlas resulted in the develop-Polandment of a wind-mapping tool called WAsP, and this isPortugalused widely for both broad-scale wind mapping andRomaniamore site-specific applications. Table I.2.1 distin-Slovakia[1]guishes between the use of WAsP and ‘other’ windSlovenia[1]mapping methods.SpainSweden[8][9, 10]LatviaOFFSHOREOther modelDenmarkGermanyWhen investigating a particular region or countr yCoverage inthe EuropeanWind Atlas[18][19][20][11]UK[12]Other countriesWind atlases for offshore are covered in Chapter I.5(pages 107–124).Local Wind Resource Assessment andEnergy NorwayRussiaSwitzerlandThe previous section presented wind maps for Europeand considered the wind resource at a strategic level.[13][24][25]Sources: Mortensen et al. (2007); for sources 1–25, see the section at the end ofReferencesThe purpose of this section is to consider the resource1565 Part I.indd 352/17/2009 7:02:36 PM

36W IN D E N E R G Y - TH E FA C T S - T E CHNOLOGYassessment and modelling at a local, wind farm, level.first year of operation. It is noted that this example isTo the wind farm developer, the regional wind mapsfrom a wind farm constructed several years ago, soare valuable tools for site finding, but are not accu-the turbines are relatively modest in size comparedrate enough to justify the financing of the develop-with typical current norms. Also, some elements of thement. Here it will be shown that the single mostanalysis methods have altered a little – for example, aimportant characteristic of a site is its wind speed,more detailed definition of the wind farm loss factor isand that the per formance of a wind farm is verynow commonly used.sensitive to uncertainties and errors in the basic windspeed estimate.Figure I.2.2 represents a simplification of the process. In reality it will be necessary to also iterate theFor the majority of prospective wind farms, theturbine selection and layout design process, based ondeveloper must undertake a wind resource measure-environmental conditions such as turbine noise, com-ment and analysis programme. This must provide apliance with electrical grid requirements, commercialrobust prediction of the expected energy productionconsiderations associated with contracting for theover its lifetime. This section discusses the issuessupply of the turbines and detailed turbine loadingthat are pertinent to recording an appropriate set ofconsiderations.site wind data, and the methodologies that can beused to predict the expected long-term energy produc-THE IMPORTANCE OF THE WIND RESOURCEtion of a project. It is noted that a prediction of theenergy production of a wind farm is possible usingWind energy has the attractive attribute that the fuelmethods such as the wind atlas methodology withinis free and that this will remain the case for the proj-WAsP, using only off-site data from nearby meteoro-ect lifetime and beyond. The economics of a projectlogical stations. However, where the meteorologicalare thus crucially dependent on the site wind resource.stations used have only data from low elevations, suchAt the start of the project development process, theas 10 m height, and/or the stations are located farlong-term mean wind speed at the site is unknown. Tofrom the site, such analyses are generally used only toillustrate the importance of the long-term mean windassess the initial feasibility of wind farm sites. It isspeed, Table I.2.2 shows the energy production of aalso possible to make predictions of the wind speed at10 MW project for a range of long-term annual meana site using a numerical wind atlas methodology, basedwind speeds.on a data source such as the ‘reanalysis’ numericalIt can be seen that when the long-term meanweather model data sets. Again, such data are usuallywind speed is increased from 6 to 10 m/s, aboutused more for feasibility studies than final analyses.67 per cent, the energy production increases byThe text below describes an analysis where on-site134 per cent. This range of speeds would be typicalwind speed and direction measurements from a rela-of Bavaria at the low end and hilltop locations intively tall mast are available.Scotland or Ireland at the high end. As the capitalFigure I.2.2 provides an overview of the whole pro-cost is not strongly dependent on wind speed, thecess. The sections below describe this process stepsensitivity of the project economics to wind speed isby step. Appendix C provides a worked example of aclear. Table I.2.2 illustrates the impor tance of hav-real wind farm, for which these techniques were useding as accurate a definition of the site wind resourceto estimate long-term energy production forecast andas possible.compares this pre-construction production estimateThe sensitivity of energy yield to wind speed varia-with the actual production of the wind farm over thetion varies with the wind speed. For a low wind speed1565 Part I.indd 362/17/2009 7:02:37 PM

W I N D ENE RGY - THE FA CTS - WI ND RE S OURCE ES TI MATI ON37Figure I.2.2: Overview of the energy prediction processSelect the siteChoose the met mast locationsFind reference stationInstall masts and instrumentsPurchase concurrentwind dataTake site measurementsCorrelate two sets of dataChoose turbineProvide estimate of long-term wind regimeMilestone 1Wind speed at the mast establishedMake topographical model of siteRun WAsP or other wind flow modelEvaluate and improve wind flow modelLay out turbines on the siteDefine environmental constraintsOptimise the layout for wakeand topographical effectswhile respectingenvironmental constraintsMilestone 2Gross output of the wind farmApply lossesCalculate net energyand uncertaintiesMilestone 3Net output of the windfarm established –task complete!Note: For some sites, no suitable reference station is available. In such cases, only site data is used and longer on-site data sets are desirable.Source: Garrad Hassan1565 Part I.indd 372/17/2009 7:02:37 PM

38W IN D E N E R G Y - TH E FA C T S - TE CHNOLOGYTable I.2.2: Sensitivity of wind farm energy production toannual mean wind speedWind speedWind speed normalised(m/s)to 6 m/s (%)Energyproductionof 10 MWwind sed normalisedto 6 m/sto 6 m/ssite (%)site (%)BEST PRACTICE FOR ACCURATE WINDSPEED MEASUREMENTSThe results shown here illustrate the importance ofhaving an accurate knowledge of the wind resource.A high-quality site wind speed measurement campaignis therefore of crucial importance in reducing the58311,15063100uncertainty in the predicted energy production of a610017,714100100proposed project. The goal for a wind measurement711724,534138102campaign is to provide information to allow the best813330,972175105possible estimate of the energy on the site to be pro-915036,656207110vided. The question of how many masts to use and how1016741,386234120tall they should be then arises.Note: *Assumes typical turbine performance, air density of 1.225 kg/m3, totallosses of 12 per cent and Rayleigh wind speed distribution.Number and Height of Meteorological MastsSource: Garrad HassanFor a small wind farm site, it is likely that one meteorological mast is sufficient to provide an accuratesite, the sensitivity is greater than for a high windassessment of the wind resource at the site. Forspeed site. For example, at a low wind speed site,medium wind farms, say in excess of 20 MW, locateda 1 per cent change in wind speed might result in ain complex terrain, it is likely that more than one mast2 per cent change in energy, whereas for a high windwill be required to give an adequate definition of thespeed site the difference might be only 1.5 per cent.wind resource across the site. For large projectsTable I.2.2 is in fact a simplification of the reality ofof around 100 MW, located in complex terrain, it isthe situation, where different specifications of turbineparticularly important to take great care in ‘designing’model would typically be selected for low and higha monitoring campaign to record the necessary datawind speed sites, but it serves to illustrate the impor-for a robust analysis in a cost-effective way.tance of wind speed to energy production.In simple terrain, where there is already a lot ofThe commercial value of a wind farm development isexperience and close neighbouring wind farms, thetherefore crucially dependent on the energy yield,performance of these wind farms can be used insteadwhich in turn is highly sensitive to the wind speed.of a measurement campaign. North Germany andA change of wind speed of a few per cent thus makesDenmark are obvious examples. A great many turbinesan enormous difference in financial terms for both debthave been sited in this way. However, great cautionand equity.must be exercised in extending this approach to moreIn summary, the single most important characteris-complex areas.tic of a wind farm site is the wind speed. Thus everyThe locations and specifications of the mast or mastseffort should be made to maximise the length, qualityneed to be considered on a site-specific basis, but gen-and geographical coverage across the wind farm siteerally, if there are significant numbers of turbines moreof the data collected. However, measurement is under-than 1 km from a meteorological mast in terrain thattaken at the very beginning of the project, and someis either complex or in which there is significant for-compromise is therefore inevitable.estry, it is likely that additional masts will be required.1565 Part I.indd 382/17/2009 7:02:37 PM

W I N D ENE RGY - THE FA CTS - WI ND RE S OURCE ES TI MATI ON39In such circumstances, discussion with the analystfollowed, then a reasonable compromise is to ensureresponsible for assessing the wind resource at the sitethat masts are no less than 75 per cent of the hubis recommended at an early stage.height of the turbines.Turning now to the height of the masts, it is knownthat the wind speed generally increases with height,Specification of Monitoring Equipment andas illustrated in Figure I.2.3.Required SignalsFigure I.2.3 schematically shows the way in whichthe wind speed grows. This characteristic is calledA typical anemometry mast will have a number of ane-‘shear’ and the shape of this curve is known as themometers (devices to measure wind speed) installed‘wind shear profile’. Given the discussion above aboutat different heights on the mast, and one or two windthe importance of accurate wind speed measurements,vanes (devices to measure wind direction). These willit is clear that it will be important to measure the windbe connected to a data logger, at the base of a mast,speed as near to the hub height of the proposed turbinevia screened cables. It is unusual for there to be aas possible. If a hub height measurement is not made,power supply at a prospective wind farm site, so thethen it will be necessary to estimate the shear profile.whole anemometry system is usually battery operated.This can be done, but it creates uncertainties. Commer-Some systems have battery charging via a solar panelcial wind turbines typically have hub heights in theor small wind turbine. For some systems, particularly60–120 m range. The costs of meteorological mastsin cold climates, the measurement of the temperatureincrease with height. Tilt-up guyed masts may be usedis important to assist with the detection of icing ofup to heights of 60 or 80 m. Beyond such heights,the anemometers. In such circumstances, the usecranes are required to install masts, which increasesof heated or ‘ice-free’ anemometers is beneficial;costs. If ‘best practice’ of a hub height mast is nothowever, their use without an external power sourceis usually impractical. Measurement of the pressure atthe site is desirable but often not essential.Remote sensing techniques are now being used toFigure I.2.3: The atmospheric boundary layer shear profilemeasure the wind speeds at wind farm sites. The technology available for this is being developed rapidly, andZFree atmospherealthough the use of remote sensing devices on windfarms is not currently widespread, such devices areZG 2000 mGradient heightexpected to become more widely used in the near future.Remote sensing devices are essentially groundbased devices which can measure wind speeds at arange of heights without the need for a conventionalEkman layermast. There are two main sorts of devices:Planetary boundary layer1. Sodar (SOund Detection And Ranging), which emitsand receives sound and from this infers the windZs 50–100 mSurface layerspeed at different heights using the doppler shiftprinciple; and2. Lidar (LIght Detection And Ranging), which alsoSource: Garrad Hassanuses the doppler shift principle, but emits andreceives light from a laser.1565 Part I.indd 392/17/2009 7:02:38 PM

40W IN D E N E R G Y - T H E FA C TS - TE CHNOLOGYSodar has been used for assessing wind farm sites forRecommendations provided by the Internationalsome years, particularly in the US and Germany. It isElectrotechnical Committee (IEC), the Internationaloften used in combination with conventional anemom-Energy Agency (IEA) and the International Network foretry and, historically, the results have been used toHarmonised and Recognised Wind Energy Measurementprovide more information to better understand the(MEASNET), provide substantial detail on minimumpatterns of the wind regime at a site, rather than nec-technical requirements for anemometers, wind vanesessarily using the data in a direct, quantitative way.and data loggers. It is strongly recommended that any-Recently, some wind energy-specific Sodar productsone intending to make ‘bankable’ wind measurementshave come onto the market and experience is currentlyshould refer to these documents. Historically, a nota-being gained from these new devices.ble deviation from best practice, as defined in the IECLidar devices have made an entry into the windand IEA documents, is the use of anemometers thatmarket over the last one to two years, and two mainhave not been individually calibrated for the assessmentcommercial models are currently available. Publishedof the wind resource at the site. Each sensor will havepapers on the devices show they are capable of achiev-a slightly different operational characteristic, as a resulting impressive accuracy levels, and it is expected thatof variations in manufacturing tolerances. The use oftheir use in wind energy applications will increase.individually calibrated anemometers has a direct impactThe clear merit of remote sensing devices is thatthey do not need a mast. However, Lidar devices, inon reducing the uncertainty in the predicted windspeed at a site and is therefore to be recommended.particular, are relatively expensive to purchase andOver the past decade, perhaps the most significantboth devices draw significantly more power than con-shortcoming of wind speed measurements at prospec-ventional anemometry, so for remote sites a local,tive wind farm sites has been the poor mountingoff-grid power supply solution would be needed.arrangement of the sensors. There is an increasingIt is recommended that in-house or external expertsbody of measured data which has demonstrated thatare used to help make an informed decision about whenif the separation of anemometers from the meteoro-and how to use remote sensing devices at potentiallogical mast, booms and other sensors is not suffi-wind farm sites.cient, then the wind speed recorded by the sensor isSignals that would typically be recorded for each sen-not the true wind speed. Instead, it is a wind speedsor, with a ten-minute averaging period, are as follows:that is influenced by the presence of the other objects. mean wind speed;The effect of the mast structure on the flow field maximum three-second gust wind speed;around the mast top is illustrated in Figure I.2.4. The true standard deviation of wind speed;figure shows that there is a complicated flow pattern, mean wind direction;which must be accommodated when mounting the mean temperature; andanemometry. logger battery voltage.These results have been predicted using a commercial computational fluid dynamics (CFD) code. It isIn recent years, it has become standard practice toimportant to be aware of the potential influence of thedownload data remotely, via either modem or a satellitesupport structure on the measured data. Detailedlink. This approach has made managing large quantitiesguidance is provided in the International Energyof data from masts, on a range of prospective sites,Agency’s Annex XI (1999), on specific separationsignificantly more efficient than manual downloading. Itdistances which are required to reduce the influencealso has the potential to improve data coverage rates.of the support structure on the measurement to1565 Part I.indd 402/17/2009 7:02:38 PM

W I N D ENE RGY - THE FA CTS - WI ND RE S OURCE ES TI MATI ON41Figure I.2.4: Predicted wind speed distribution around and above a meteorological mast using CFDWind into the pageWind left to Source: Garrad Hassanacceptable levels. Illustrative examples that demon-requirement is for data to cover one year, so that anystrate good and poor mounting arrangements are pre-seasonal variation can be properly captured. In additionsented in Figure I.2.5.to specifying and installing appropriate equipment,If the guidance presented above is followed, a high-vigilance is required in the regular downloading andquality set

were common in the 1980s and 1990s, when wind energy penetration was relatively low, but have been overtaken somewhat by events, as penetrations of wind energy are now substantial in many European countries. Wind Atlases ONSHORE Figure I.2.1 shows the onshore wind energy resource as computed on a broad scale for the European Wind Atlas.