Tech Planning Of Wind Farms

Planning of Wind Farms – An Overview A wind power project can basically be divided up into three phases:

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Planning, Erection, and Operation of the wind turbines.

In each phase, three important points have to be kept in mind: 1. 2. 3.

Technical aspects, Permits and legal aspects, and Economic aspects,

Planning of a wind farm Figure 1 shows the flow chart from planning to the completion of the project as well as the phases from the operation to the dismantling of the turbines. The planning phase is divided into three parts:

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Initial investigations, Site analysis, and Actual planning procedure.

During initial investigations, the basic feasibility of the project at the site selected is examined before moving on to additional formal steps of site analysis and planning that entail costs.

Figure 1: Flow chart of a wind power project [1]

1. Technical planning aspects Estimation of wind conditions An estimation of local wind conditions is especially crucial in the selection of the site. If the wind speeds are 10% smaller than expected, the energy yield will fall short by more than 30%, which can quickly cause economic problems. In addition to an evaluation of the wind speed based on general meteorological data, wind prediction also requires an analysis of the orography of the site selected, i.e. the structure of the terrain, the roughness of the surface, and the type and size of the terrain's boundaries. Furthermore, any individual obstacles - such as rows of trees, buildings, and any other wind turbines - must be registered accurately. Already at this stage, an experienced expert must be consulted to help determine how to continue and which methods will be used to accurately determine the potential of local wind energy production. Various methods are commonly used to measure, simulate, and evaluate wind conditions. Depending on local conditions and the quality of any wind and data available for the region - such as from measuring stations - a methodology will be chosen, and a decision will be made as to whether additional wind measurements are required to corroborate the initial findings.

Initial estimates of installed capacity and energy yield The space available and, above all, the access to the grid are decisive factors in determining how many turbines with which nominal power output should be used. Therefore, the local grid operator has to be asked for the maximum possible wind power feedable into the grid has to, how far away the next feeding point is, and what the voltage level is for a connection to the grid. For large wind parks (> 20 MW), it may make sense or even be necessary to set up you’re a separate transformer station. The number and nominal power output of the wind turbines to be installed can be determined from these two boundary conditions (the available space and great capacity). This estimation then serves as the basis for the first energy yield prediction. The expected yield is determined under consideration by wind direction sector using the frequency distribution of the wind speed determined for each wind direction sector and the performance characteristics of the wind turbines. This calculation is needed to find the optimal arrangement of the turbines within the wind farm so as to produce the greatest overall energy yield and minimize the inevitable effect that the wind turbines will have on each other (mutual shelter effect or "wind shade"). Figure 2 shows the layout of the wind farm based on such considerations.

Draft layout of a wind farm Of course, the ultimate goal in looking for the optimal arrangement of wind turbines on a given site is the highest possible energy yield of the entire wind farm over its service life. On the other hand, the conditions and costs of installation -- such as construction of power lines from the turbines to the transformer and interconnection stations or roads for assembly, maintenance, and service vehicles -- also play a role in the arrangement of the turbines. Today, a number proven of planning tools are developed -- such as software like WinPro, Windfarmer, etc. -- for providing an optimal layout of wind farms efficiently and quickly. Additional restrictions on the layout of the wind farm -- such as prescribed distances to buildings, environmental protection regulations, or maximum building heights -- are not only the result of technical considerations, but also of laws and regulations imposed by public officials. It is best to find out about what restrictions apply right away lest such conditions make time-consuming, expensive changes in planning necessary in order to get permits.

Figure 2: Wind farm layout with planned infrastructure [1]

General local frameworks Find out who owns the property, and sign leasing agreements (or at least preliminary agreements) with the property owners. The general local frameworks are studied mainly to ensure the practical implementation of the project. Find out:

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Whether the turbines can be set up stably, Whether the site can be reached by all of the vehicles required, and To what extent transmission lines are needed to feed power to the grid.

In addition, access must be ensured to all turbines for servicing, maintenance, and repairs over their entire service life. The soil must be studied for each wind turbine to ensure that the subsoil can handle the loads of the foundations. The shape of the foundation is based on these studies. Generally, flat gravity foundations are used, with pile foundations being used if the soil is too soft. Access and transport roads for construction should be investigated, as should the locally available space for cranes. Buildings (some of which may be of historic value), underpasses, overhead lines, traffic signs, train lines, antennas, curved alleys lined with trees, bodies of water, etc. may all represent obstacles towards the delivery of large system components, such as tower sections and rotor blades. Furthermore, such climatic conditions as seasonal heavy rains and storms must be taken into consideration. The location and type (such as voltage level) of grid access must be determined. In simple cases, a short stub cable can be used to connect the wind farm to the feeding point. The power cables may also sometimes be directly connected to the next transformer station where enough grid capacity is available; if none is available in the nearest station, a dedicated cell can be added for the farm. Very large projects often require their own transformer station so the wind farm can feed into high-voltage grids. The distances and types of cable routes used must be selected with respect to technical and economic aspects. If the cable routes are long, planning may be more intricate and require an environmental impact assessment.

2. Permit aspects

Local construction law determines what is required for a wind farm or a single wind turbine to be eligible for a permit. In addition, the environmental impact of the turbine generally has to be assessed as early as the planning phase in most countries. Generally, an environmental impact assessment is then conducted.

The ecological impact of wind turbines There must be a determination of whether the impact that the construction and operation of wind turbines will have on the environment is acceptable in terms of nature conservation. Additional reports may be required to assess the impact on flora and fauna, such as in areas where birds build nests or migrate. Local authorities check whether any considerable, undesirable effects on the environment are expected with consideration of the legally applicable criteria. If an environmental impact assessment has to be conducted, maps of biotopes, plants, and animal species -- such as the breeding grounds and habitats of migratory birds or bats -- are included along with an assessment of the visual impact of the wind farm. The following items are some of the indicators of how the wind turbines will affect humans and the environment:

• • • • •

A map of areas with rare birds, Other aspects of animal and plant protection, Noise impact reports, Reports on shadow casting caused by rotors, and Heritage and landscape conservation.

Noise pollution from wind turbines There are legal limits for noise pollution at different times of day in residential areas, industrial areas, rural areas, etc. Measurements of the noise pollution from wind turbines are part of the general testing to fulfill, for instance, IEC 61400-11. For each type of turbine, the sound power level, frequency (dominant individual tones), and pulses (low-frequency "rhythms") are determined. People generally find such characteristics, which are generally caused by the gears, generator, or inverter, to be especially annoying. The noise pollution on site can be forecast based on the findings of noise pollution measurements for a specific type of turbine. The level of noise pollution than a wind turbine produces is not constant but rather varies according to the current output, i.e. the wind speed. Generally, the noise (sound power level) generally increases by around one dB(A) for each meter per second of wind speed. However, the wind noise caused by trees and bushes also increases accordingly, usually even more so than the noise from the machine. The noise caused by turbines is also stronger in specific directions, which is taken into account in the measurements of independent testing institutes and in the wind farm planning software. The noise level is indicated in logarithms. An increase of 3.01 dB(A) indicates that the perceived noise level doubled. During planning, maps of isophones -- lines indicating a specific noise level -- are created for the planned site (see Figure 3). The overall noise level of all of the turbines is the product of the overlapping of the individual noise levels of each turbine. Thus, the areas with the highest levels of noise can be identified and measures taken to reduce these levels by changing the arrangement of the turbines and the types of turbines used in order to comply with legal limits.

Figure 3: Layout plan for a wind farm with isophones from the noise forecast [1]

If local residents cast doubt on these predicted noise levels, officials will measure the levels themselves. If they then impose conditions -- such as at night when the wind blows in a certain direction -- variable-speed turbines with adjustable pitch can be used to reduce the noise by slowing the speed of the rotors during operation. Turbines that run at a fixed speed and whose rotor blades cannot be pitched in and out of the wind do not offer this option. In the worst case, the operator might then have to switch off the turbines at night, which would reduce the energy yield considerably over long periods. Therefore, it pays to have a prudent forecast of noise levels and generous safety distances.

Shadow casting of wind turbines Another aspect that may affect the issuing of permits is whether the rotor blades cast a shadow on adjacent structures, creating a sort of "disco effect" as the sun light passes through the blades. This item should also be investigated for the "worst case" in planning; in other words, full sunlight without clouds should be assumed. The calculations are based on the local orbit of the sun across the seasons, the height of the hub, and the diameter of the rotors.

Figure 4 shows an example of such a forecast for two wind turbines in a wind farm. The limit defined by German law is a few hours of flickering per year. To ensure that this level is not surpassed, the wind turbine can be equipped with a special sensor that switches off the turbine automatically when a number of critical operating conditions are detected. Specifically, the turbine is switched off if the wind direction and speed are within a certain range when the sky is clear and the sun in a certain height.

Figure 4: Layout plan of a wind farm with isolines indicating shadow casting in hours per year [1]

3. Economic aspects The economic feasibility of a wind farm is decisive. If the wind farm is found not to be economically feasible, the project should not be begun at all. In calculating the economic feasibility over a period of around 20 years of operation, various cost items -- such as actual energy yield, repairs, etc. -- must be taken into consideration even though they cannot be forecast with certainty. On the one hand, investment costs are a crucial factor in determining feasibility; they mainly depend on the cost of the wind turbines themselves and on operating costs. On the other hand, the income generated by the electricity fed to the grid at a defined feed-in tariff is also decisive. Here, a long-term power purchase agreement (PPA) with fixed rates should be signed if possible. The financing for the project can only be considered ensured if the term of this PPA is long enough (generally at least 10 years). In assessing economic feasibility, the cost of operation (maintenance, repairs, insurance, etc.) and provisions for the dismantling of the wind turbines must be calculated prudently already in planning. Otherwise, it is not likely that investors and banks will be convinced of the project's financing, nor that the operation of the wind farm can be economically secured. However, if all of these steps in the planning of the wind power project are successful, the project can be implemented and put into operation.

Further links (listed by WWEA) • • •

Wind Power Planning Distance Education Course at Gotland University, Sweden [download here] "Developing Wind Power Projects" (2006), new book of Tore Wizelius Gotland University, Sweden http://shop.earthscan.co.uk/ProductDetails/mcs/productID/734 Gasch, Robert / Twele, Jochen: Wind Power Plants. Fundamentals, Design, Construction and Operation, Berlin: Solarpraxis 2002 http://www.fachbuch-erneuerbare-energien.de/gasch_engl.htm

[1] Source: abridged version of the chapter "Planung, Betrieb und Wirtschaftlichkeit von Windkraftanlagen" [The planning, operation, and economic feasibility of wind turbines], in Gasch/Twele (Eds.): Windkraftanlagen – Grundlagen und Entwurf, Planung und Betrieb, [Wind turbines -- principles, drafts, planning, and operation] Teubner-Verlag, 2005

Text Prof. Dr.-Ing. Dr.-Ing. Christoph Dipl.-Ing. Jan Germany www.windguard.de

and Jochen Heilmann, Deutsche Liersch, Deutsche

Twele, WindGuard WindGuard

FHTW Dynamics Dynamics

Figures: Berlin GmbH GmbH

Wind Measurement for Accurate Energy Predictions – An Overview The main tasks of a professional wind measurement, used for wind power generation applications, are wind measurement for accuracy and reliability of measuring needed for profit prognosis (predict), and wind measurement for the monitoring of already installed wind farms (verify). The following text summarises a number of important aspects, tips and instructions - based upon years of practical work - relating to wind measurement for wind power generation. This enables the user to meet the most important conditions for wind measurement. Ammonit highly recommends that inexperienced users contact professional wind consultants at the early stages of a project.

1. Prognosis (Predict)

1.1 General demand Before the installation of a wind farm, it is recommended to analyse the site professionally. The collected meteorological data should accurately describe the wind potential of the site. This is why the measuring systems should meet the highest quality demands concerning the accuracy and the reliability. For the evaluation of an energy prognosis based upon the meteorological data by a wind consultant, the following additional factors are important.

• •

•

The data has to be recorded for at least 12 months without gaps and must be checkable for plausibility. A crucial factor for precise measuring is the correct choice of sensors. For the wind measurement, only anemometers according to IEC 61400 – recommended: category - should be used. The anemometers must be calibrated by a specialised institute in accordance with national and international standards and regulations (MEASNET, details see below). The correct set-up of the measuring systems and – according to the requirements of the wind consultants – the provision of evidence of the installation.

The correct choice and positioning of sensors is of vital importance. Mistakes here have the consequence that the whole data material cannot be evaluated properly. If the wind consultant is relying upon data generated by an insufficient or incorrectly-installed wind measurement system, this could lead to big miscalculations and builds up the risk of economically non-viable operation of the planned investment. Small deviations could be caused simply by the use of non-calibrated anemometers, for example.

The following example shows the risk an investor takes when relying on data from a poorly installed measuring system. Assuming, for example, an average price of 0.08 €/kWh, one ends up losing Euro 50.000 in one year for a single 1500 kWh wind power generation unit. All due to the wrong prognosis. Over the total running time of a medium-sized wind farm, the loss adds up to several million Euro. In comparison with this, even the most highly developed measuring system is not expensive.

An example, what a wrong measurement can cause for energy prognosis: Correct Measurement 10 m = 4.4 m/s 30 m = 5.3 m/s

The result is a roughness length of 0.047 m or a wind speed in 85height of 6,15 m/s

A WEC with 1500 kW power and hub height of 85 m will produce on this site per year: 2.892 MWh

Possible deviations caused by

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No calibration Wrong fitting Skew wind

The result is a roughness length of 0.288 m or a wind speed in 85height of 6,73 m/s

By this small measurement mistake the prognosis for the same WEC is now: 3.544 MWh

10 m = 4.2 m/s 30 m = 5.5 m/s Overestimation of the energy prognosis:

Measurement error at 10 m = - 0,2 m/s ( - 4,5% ) Measurement error at 30 m = + 0,2 m/s (+ 3,8%)

= 22,5%

Measurement results can be made more accurate through higher initial levels of investment (especially an advanced calibrated anemometer). However, many mistakes can be avoided without additional costs. What is needed for this is the specialist knowledge.

1.2 Measuring sizes and heights Graph 1 Measurement heights

The measurement categories „wind speed“, „wind direction“, „atmospheric pressure”, temperature” and „relative humidity“ will be discussed in the following passages.

1.2.1 Wind speed The ideal approach would be to measure the wind speed at the hub height of the wind power generation station that is to be installed. Two arguments against this are that the exact hub height is most probably not yet known, since the final decision will be made on the basis of the measurement results, and secondly, that such as high measurement tower is very expensive and difficult to install. The alternative is to use two anemometers to measure the wind speeds at two lower heights. The height profile at this location is determined ("roughness length Z0") to calculate the wind speed at other heights. Since the calculation with a logarithmic formula represents only idealized wind circumstances, and the difference of the average speeds in different heights is small, this implies:

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The use of individually calibrated anemometers of category 1 The lower anemometer must be fitted high enough to avoid influences by obstacles (bushes, houses, etc.) The distance between the two anemometers should be at least 15 to 20 m

On a "simple" location (flat land, no obstacles), a measuring tower with calibrated anemometers at 10 and 30 m is sufficient. In more complex areas, the lower anemometer has to be fitted higher. In order to provide the minimum spacing, the measurement tower therefore must be higher. Here measuring at 20 and 40 m or even 30 and 50 m is needed.

Choice of sensors Graph 2 Anemometer

Cup anemometers are the standard way of measuring wind speeds in wind energy measuring systems. These sensors have some problems when recording wind streams (inertia of the cups, "overspeeding" effect), but these are only of minor importance. The key features are A. B.

the linearity of the electronic signal and the insensitivity of the anemometer to turbulence and skew winds caused by tower or traverses.

Wind speed transmitters with large cups show much better attributes than anemometers with cups that are small in relation to the shaft. Opto-electronic transformers and AC-generators have proved to be the most suitable transducers. One of the reasons is that they are robust. But most opto-electronic transformers supply a much higher pulse-rate (at least 10 Hz per m/s), which is needed for the recording in short measuring-intervals or for the evaluation of turbulence. The consequences of the resulting wrong measurements have already been described. It is therefore highly recommended to use only anemometers of category 1 (according to IEC 61400), which are individually calibrated.

Anemometer calibration Anemometer producers guarantee a certain accuracy for their products, for example ± 0.3 to 0.5 m/s (or 3 to 5 % for speeds above 15 m/s). The measurements usually remain well within this tolerance range and this is

sufficient for all needs in weather forecast and in industrial processes. For a reliable prognosis in wind energy, this tolerance is not acceptable. When using a non-calibrated anemometer, it is necessary to reckon with a reduced accuracy for the predictions. This is a risk and we recommend using only anemometers which are individually calibrated. The anemometer should be calibrated by a specialised institute in accordance with national and international standards and regulations, which should also provide an official certificate of compliance (ISO 3966 1977, IEA-guidelines, uniform measuring process of the MEASNET-Group). The members of the MEASNET Group (www.measnet.com) are independent, international institutes which have specialised on applications in the field of wind energy. The development of standard measuring processes and the continuous flow of experience and information makes sure that the calibration of anemometers will be handled according to the strictest guidelines. The results of each anemometer calibration are presented in a calibration report, which describes precisely which aspects of the performance of the anemometer have been measured. In addition, the measurement equipment should be described in each report, including the reference tools and their last check-up. In really well-designed measuring projects, the anemometers will be calibrated a second time after use: this makes sure that there have been no changes while measuring. The repeat calibration is part of the standard offer from many wind experts.

1.2.2 Wind direction

Measurement height The monitoring of the wind direction is very important for the layout design of a wind farm. The wind direction on a location needs to be monitored only at one height. The wind vane should be fitted about 1.5 m below the top of the tower in order not to influence the top anemometer.

Choice of sensors For determination of the wind direction, potentiometric transmitters are increasing in use, because the resolution (1°) is excellent and they consume little power. It is important to keep in mind that the outgoing signal has to cover a full 360 degrees without gaps. Because they have only a very simple potentiometer, cheap wind vanes often show a big north-gap. These “low cost" sensors can also have only a limited safe-life, because the electro-mechanical material used in their construction is not durable enough.

1.2.3 Athmospheric pressure, temperature and relative humidity Measurement height The influence of athmospheric pressure, temperature and relative humidity on the energy output is of secondary importance.

The atmospheric pressure can be measured at any convenient height. The measured pressure can simply be projected. Since the equipment (pressure sensors) will require additional weather protection, it is recommended that it should be installed in the shelter box of the data logger. The temperature probe has to be supplied with a suitable shield against weather and solar radiation, and should be fitted at a height of at least 10 m to avoid the effects of heat radiation from the ground. Additionally, this height will protect the temperature probe. The relative humidity has no influence on the energy output, but it is helpful to know this parameter to estimate the danger of icing.

Choice of sensors As mentioned above, the quality of the sensors is important for the significance of data. For the temperature, mainly PT100 sensors are used. Often temperature probes are used in combination with air humidity sensors, because the additional cost is low. One should not forget to ensure that there should be a good weather and radiation shield for the temperature probe. It stops the sensor heating up even if the sun shines directly on it, without causing an air-jam.

2. Verification (Verify) Increasingly, measurement systems for recording meteorological data are used not only to investigate potential new sites, but also in the wind farm after it has subsequently been begun operations. Running in parallel to the ongoing operations, they provide reliable meteorological data which is needed for monitoring the performance and production of the wind power generators. It is important that meteorological data loggers offer the necessary functions as standard, so that can be used for all the applications without any other special equipment being needed. It is important to meet two requirements:

•

•

The operators of a wind farm would like to have a measuring system which is independent of the manufacturer of the wind-power generators, one which they can configure and operate themselves. Verification of the predictions and assurances for annual production for an installed wind farm can be provided by taking continuous meteorological measurements, which these may also indicate possible measures of optimisation. The manufacturers of the wind-power generators would like to be able to integrate the collected data in their SCADA system (Supervisory Controls and Data Acquisition), so that the measurements can be used for internal protocols or made available for telephone monitoring. The integration of the data in the company's own SCADA system improves the potential to react quickly to possible faults or to document the proper performance of machines.

Graph 3 Predict & verify

Frequently, two masts are installed at a location. One of these will later remain standing as a reference mast when the wind farm is in operation, and the other is erected directly on the proposed site of a wind power generator. This provides data for comparison purposes so that during the subsequent operation of the wind farm, the measurements on the reference mast can be converted to the wind potential at this turbine. This means that it is important to have measurement systems which can be precisely synchronised, for example by cascading via a data connection. Properly-installed professional equipment in the direct vicinity of the wind power generators is therefore also a necessary monitoring tool for the manufacturer of the plant.

3. Setting up the tower The most important rules for the best possible tower build-up: Graph 4 Setting up the tower

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•

• •

• •

All wind sensors must be fitted absolutely vertically. Even small deviations lead to skew winds and therefore to wrong measurements. Traverses keep the sensors as far away as possible from shaded or turbulent areas. However, the traverse must not start swinging. This can not only influence the measurement, but also lead to bearing damage of the transmitter. The top-anemometer is to be placed centrally on the top of the tower. It must be streamed on from all directions without obstruction. For the last piece (minimum 0.5 m) of the pillar, one should choose a diameter which is similar to the shaft of the transmitter and which corresponds to the set-up used during the calibration of the wind sensor in the wind channel. Next to the anemometer there should only be a thin lightning conductor. The lower anemometer(s) should be fitted on a vertical pipe attached to a traverse, so that the anemometer stays 30 to 60 cm over the traverse. A traverse directly under the anemometer can influence measuring! The fitting must be such that the transmitter lies at a 45° angle to the main wind direction, which is usually known approximately. With a cylindrical tower, the length of the traverse should be at least 7-times of the tower diameter. If a framework structure is used for the mast (width up to 30 cm), the traverse length should be around 1 m long. The wind vane should be fitted as high as possible on a traverse, but at least 1.5 m below the top anemometer. The traverse is to be fitted as described before. For fitting the vane you need a compass or a good map with a small scale in order to locate a prominent fixed point on the horizon. Mostly one has to screw the wind vane onto the tower while it is still lying on the ground. A good angle-measuring tool also helps. The lightning rod (thickness approx. 2 cm) must have a distance of 50 cm from the anemometer and must be free from vibrations. The lightning rod should be over the anemometer at a 60° angle. The best place for all cables is within the tower. The dead weight of free hanging cables over 50 m in length has to be secured with an additional rope. If fitting within the tower is impossible, you must fix the connections to tower and traverses at intervals of one metre. Be sure that no loose cables are flying in the wind. Also avoid contacts with sharp edges. Every little stress on the cable can lead to damage in the course of long-term operations!

For the other components of a measuring station (shelter box with data logger, solar-power supply and installation of a remote data transmission), the general rule is: Fit them as high as possible, but still within reach for access and maintenance. Experience shows that the solar panel and the GSM-antenna are at special risk of theft and vandalism on stations with free access. Try, therefore, to make your measuring

system look less attractive. A GSM-antenna for example still works as well if an old, grey plastic-pipe is put over it. And a station which works with a little, plain solar panel, is less interesting. Make sure after installation that the tower is absolutely vertical. If you cannot climb up the tower, you must test the orientation at the lower part with a suitable measuring tool (for example an inclinometer) and then check the tower from all sides for any bends. The human eye is, according to experience, able to notice even small deviations. Electronic tilt sensors fitted to the top of the tower are a good help for installation. The inclination of the tower can be controlled at all times, and if remote data transmission is used, the operator of the station will also be warned of any impending danger.

3.1 Avoidable mistakes Here are some of the most common mistakes that you should avoid when installing anemometers on the measurement tower:

Graph 5 Wrong traverses

Wrong traverses Close to the tower and traverses there is always turbulence and shading, which can have a negative effect on the measurements. Wind transmitters installed on traverses should therefore not be fitted directly on the boom. In relation to the diameter of the tower, the boom must be as short as possible. At the same time, the traverse must remain stable, so that it does not oscillate.

Shaded by wind vane Graph 6 Shaded by wind vane

The anemometer should be streamed upon from all directions without obstacles. The important top anemometer can be fitted in an ideal position, since it is situated over the tower. However, this advantage is often ruined by fitting a wind vane right next to it!

Marion Ammonit Germany www.ammonit.de

Gesellschaft

für

Messtechnik

Große mbH

Management of Meteorological Variables and Wind Mapping

INTRODUCTION Once established the feasibility for the installation of wind farms in a determined country or region from the political, legal and economic point of view, the next step is to deeply study the geographic region about which the wind potential is intended to be analyzed. In order to determine which region is the most adequate to start the studies, what must be known is the distribution of the atmospheric capes in the area, the direction or main directions of the winds and the local conditions of the area, as the obstacles (buildings, trees, etc.), the ground surface and the orography of the area. In the present conditions of the Argentine Republic, such study must be based on reports or previous analyses which could be available or which have to be done, on reports by the National Meteorological Service and satellite images, among others. It is important to say that nowadays there is not a unique wind map of the Argentine Republic, which would be the basic instrument for any kind of consult or preliminary study. The analysis of this generic information with which we count on at present only gives a generic data and it must be considered as such. It is also advisable to count on the local assessment of a meteorologist, who can give a better interpretation as regards historic values of the region, wind tendencies, pressure, average temperature and humidity, as well as data concerning the meteorological phenomena which usually take place in the area (such as rain and/or snow storms, extreme temperatures, strong winds, etc.) Once this preliminary study has been done a series of processes must be followed so as to know the area in detail and which, finally, allow to get a scientific and meteorologically sustained answer to the question How much wind can we expect in the determined area? So as to answer the previous question, it must be taken into account that the wind conditions for an area are defined by the wind profiles of this area, the average wind speed and direction, the wind speed distribution and direction, and the wind daylight and seasonal patterns.

DEVELOPMENT It must be considered that to determine the period in which the measurement in the area will be done, the lasting of that period depends on the kind of project that is intended to be carried out. If the intention is to develop a complete wind map of a region, it must be considered the measurement taking for at least 10 years (i.e., long-term); otherwise, if it is a preliminary analysis of the wind resource, it must be considered the realization of the measurement, at least, for a year in its initial phase (i.e., short and medium-term). After this, the quantity and setting of the anemometers to be installed must be defined, taking into account 2 that it is advisable the average surface monitored by each anemometer to be of 2500 km . Technically, it is recommended the use of head anemometers, calibrated every 6 months in a certified wind tunnel. Once calibrated and installed the anemometers in the region, it is advisable that they work, for their optimal performance, during nearly 4 weeks before the measurement starts, without their data to be considered for the study which has to be done. These data, must only be taken into account so as to be able to study the correct equipment calibration and the acquisition process of data, as well as the correct functioning of the electronic equipments of meteorological measurement and data store. Calibrating the anemometers every 6 months and doing a bimonthly follow-up of each of them in the field during the data collection period, will minimize the error introduction or the loss of data. The errors in this kind of wind study must be understood as a very complex factor and can lead to the complete failure of the whole wind project. Taking into account that the energy found in the wind is proportional to the cubic wind 3 speed (E~V ) and that, according to Betz’s law, theoretically the 59.3% of the wind energy can be extracted, the measurement that are done on the “wind resource” must be very accurate and free of possible errors.

Considering that the wind which is further from the surface has more speed and less turbulence, due to the fact that it is not affected by obstacles from the land, it is advisable to carry out the measurement as high as the standards allow. In general, it is advisable to carry out in the same spot two measurements at different heights, at 10 and at 30 meters, though measurements have been done with meteorological masts placed even at 50 or 100 meters high. Once the measurement network of meteorological data has been installed on the land, the analysis of the air turbulence must be carried out. This analysis must be done measurement the vertical movement of the air (through the use of ultrasound anemometers), as well as the air temperature. It is important to take into account the density of the air in the region, due to the fact that the air density in warmer regions lowers, and in colder regions it increases. It must be considered that it is better for the production of wind energy regions in which the air density is as high as possible, i.e., regions with cold temperatures (example: Argentina Patagonic region). 3 Air Density in Normal Conditions of Pressure and Temperature: 1.225 Kg. /m .

Normal Conditions of Pressure and Temperature: 1013 hP and 20˚C (NCPT)

Data Collection and Processing Considering that the meteorological data obtained is the “raw material” of the project and having used anemometric stations properly calibrated, installed and verified, the processing phase must begin, having a clear idea of what information must be obtained when this phase ends. Because of that, and before the data processing, it is important to define adequate policies and protocols which allow to manipulate the information and to process it with highest levels of security and efficiency:

• • •

It is advisable to establish the frequency for the data collection in the electronic equipments of the anemometers in 1 Hz. The meteorological values obtained must be averaged every 10 minutes (some anemometers allow to do this average internally). The policies of getting, collecting and transmitting the data coming from the anemometers installed in the measurement field must be defined.

It will be essential to do height extrapolations, which allow to estimate the winds that finally will be used for the wind electric energy production[5]. To extrapolate the winds there are two different equations:

1. Hellmann’s equation: This equation allows to extrapolate the winds at a second height (h2) though it is usually used only as an approximation.

v = wind speed h = height above the ground [m] α = Hellmann’s Exponent (For example: in Germany α=0.16)

2. Logarithmic Profile equation: This equation must be applied only on average values, not on individual values, and must be used with measurements that imply long-term periods.

v = wind speed [m/s] h = height from the ground [m] d = thickness of the moving cape [m] Z0 = ground surface

So as to be conservative, in the analysis and interpretation of any of these equations, an error margin of +/10% must be considered.

The data processing in the first phase implies to obtain : 1. Weibull’s Curve.

f = density of frequency v = wind speed (center of class) [m/s] A = scale parameter [m/s] c = shape parameter (note: c is k)

There is a relationship between Weibull’s parameters and the mean wind speed:

Increasing parameter c of Weibull with the height (empiric) Weibull c2 = c1 + 0.008 (h2 – h1)

2. Study of wind drafts 3. Maximum and minimum wind speeds 4. Compass card of the region

EVALUATION OF THE WIND RESOURCE What is a wind map: It is a representation of the magnitude and the direction of the winds of a region in graphic form, using cartography with a scale and determined symbolism.

Kinds of data needed The data which is needed to draw the wind map of a region are of varied source and, depending on the method applied to do the job, they will have to be of different kind, having each method their compulsory data entry well defined. In this way, at the moment of developing the wind map which data is available must be reveled and in what way they can be used to apply which method. Nevertheless, and not considering the method to be applied, the data necessary for the mapping can be summarized in the following list:

• • • • •

Anemometric measurement or surface measurement. Orographic data. Topographic data. Data of land use/natural coverage. Satellite images.

The data measured on surface is of vital importance, due to the fact that it can be used to obtain the wind map of a region as well as to validate the results obtained through other methods which do not use measurements as entrance. On the other side, the surface data is still the most accurate at the moment of doing the project.

PARTS OF A WIND MAP Data The most frequent data represented on the map for a determined height are the mean wind speed (measured in m/s), the mean wind direction (expressed in arrows or characteristic symbols of plotting in meteorology), the mean energy density (measured in W/m2), the frequencies distribution, the compasses cards, the Weibull (A y k) parameters, the studies on wind drafts and the studies of turbulence, among others. Besides, the results must present not only the average historic data, but also the seasonal regimens and the daylight and night cycles of the resource. There is another data which is used as entrance for the wind map models, but which can become very useful to be used and represented as a summary of the outcome. These are the ground surface map, the land use and vegetal coverage map, and topographic maps. All these data will be represented at different heights, being nowadays the most common 30 and 50 m; though there are also atlases which represent the information at 10, 25, 30, 50, 75, 80, 100, 125 and 200 m. Really, once the calculus have been done and knowing how the wind profile behaves for an area, the values can be easily extrapolated in height through methods as the ones mentioned above. Wind classes definition: Whatever the data represented on the wind map is, the objective is always the same: to reveal the wind potential in an area.

One of the data which is usually represented is the quantity of energy than can be obtained from a region. This is measured in W/m2 and there is a table of equivalencies between the wind speed and power, which is used in the USA, called Wind Class [1].

Models

In order to build wind maps data and models are needed. The models will be all those processes (programs, algorithms, methods) which allow to draw the wind behavior and distribution in an area or given region. Once determined which ones will be the models to use and collected all the necessary data to feed the model, both things combine to become a wind map. In some cases, the models can be combined between themselves to get a more accurate result. Kinds of climatic scales and their models: The models can be of macro scale, known as synoptic scale (more than 2000 km); meso scale (2 to 2000 km) or micro scale (up to the 2 km). The most commonly used for the wind resource evaluation are those of meso and micro scale, both of them can be used separated or in combination. In general, the most common experiences are those in which both models are used together. Generally, the models used –independently of the scale- can be of numerical or statistic type. In the case of the numerical models, are based in a group of more or less complex equations which model the physics reality of the climatic phenomena. On the other side there are statistic models, which are characterized for applying principles of statistics and probabilities to solve the problem of how winds behave. Some of these methods are based on principles of traditional statistics and others use modern techniques of artificial intelligence, for example. General numerical models: the numerical models can be classified in three different categories according to the way in how they model the reality (accuracy with which their equations model the physics behavior of the winds).

• • •

Solving the fundamental equation models. Simplified physics models. Statistics analysis models.

Solving the fundamental equation models These are models which solve the general equation of the flux movement of Navier-Stokes [3]. They include the description of the topography, of effects of the surface ground, they allow to model complex thermal effects and use geographic information, through the GIS systems. These are called meso scale models. They allow the atmospheric representation or simulation in greater detail, at the same time they allow the modeling of a wider area than the rest of the numerical methods. These consider all –or almost all- the important meteorological phenomena. On the other side, they do not depend on data measured on surface. Known examples: KAMM (Karlsruhe Atmospheric Mesoscale Model, from the homonym university in Germany), MM5 (Mesoscale Model version 5 of NCAR/Penn - National Center for Atmospheric Research/ Pennsylvania), ETA (generated model every 12 hours created by the NCEP - National Center for Environmental Prediction and used by the National Meteorological Service of the Argentine republic) and MatMeso, among others. At the same time, this kind of model requires the use of other methods so as to achieve a greater resolution and surface measurement if it is wished to validate that the outcome of the method is correct in all the cases.

Classification of the meso scale phenomena(Fujita, 1986)

• • •

Alfa Mesoscale (a): they have a dimension of between 200 and 2000 km with phenomena which can last between 6 hours and 2 days, as small hurricanes and weak anticyclones. Beta Mesoscale (b), which counts with sizes of between 20 and 200 Km. lasting between 30 minutes and 6 hours; there can be fields of local winds, mountain winds, breezes from the continent and the sea, connective complexes of meso scale and big electric storms. Gamma Mesoscale (c) of an estimated size of between 2 and 20 km, lasting between 3 and 30 minutes, representing phenomena like most of the electric storms and big size tornados.

In order to achieve a collection of wind resource data using a meso scale method the following steps must be followed: first, wind data and measures must be collected in height. In general, measurements of radio sound are used, though the measurements on surface can be considered to calibrate the model and estimate errors. The model is executed to simulate the winds of 10 to 15 years and, depending on the power of the calculus available and the region to be modeled, the resulting grid ca be between 1 and 5 Km. It is also possible to obtain a greater accuracy if a micro scale model is executed or one which allows a greater resolution within each point of the grid, for example the WAsP or WindMAP. After the execution of the model, the map of the wind resource is traced. In this map the data mentioned above can be represented.

Simplified physics models They use a more reduced group of equations and –due to this- they model a smaller quantity of climatic phenomena. They are used to trace wind maps in low or medium complexity surfaces, getting maps equally useful and accurate, but requiring a lot lower potential of calculus. The advantages of this kind of methods are that they function with anemometric seasonal data of surface, with no need of height data and, besides, they are ideal for low complexity surfaces. As a counterpart, their disadvantages are that they do not model the reality completely, they can only represent some aspects of the wind behavior and other meteorological variables. Then, they are not capable of modeling complex meteorological phenomena, but very important ones, as the breeze from the sea or the continent, or the wind produced by thermal effects, like the mountain winds; they do not take into account the splitting of the air flux produced by the irregular surface. It depends on anemometric measurements on surface, what implies that if the measurements are not enough or they are done in a wrong way, the model will generate an incorrect result. The anemometric non reliable measurements can not be used without using correction techniques which can introduce new errors in the calculus.

Models based on GIS These kind of models are based on completely different functioning principles. For their functioning they use wind measurements in height which are extrapolated to low altitude. Moreover, they are based on the GIS (Geographical Information System) technology for the collection of data and the drawing of the part of the region to be analyzed. In 1995 the NREL - National Renewable Energy Laboratory started to develop a new method of wind mapping based on the GIS technology. The model is called WRAM (Wind Resource Assessment Model). It produces maps of great quality and was used to develop the wind maps of several union states (North Dakota, South Dakota and Vermont; part of Minnesota, Iowa and Nebraska); apart from several international atlases like Dominican Republic, Mongolia, Philippines and regions of Chile, China, Indonesia y Mexico. This method needs of wind values previously calculated and, really, it is no other thing than a method of representation, more than of calculus.

Models from the point of view of the principles

From the point of view of the physic-mathematics principles, the numericalal models are classified in:

• •

Based on the Jackson-Hunt’s theory Based on the uniform mass model

In the first case, these models tend to satisfy the Navier-Stokes’ equations [3]. Their basic characteristic is the description of two fundamental principles: the mass conservation and the moment conservation. Due to this, this kind of model is very sophisticated and has a very good output: an error among the 8 and the 10%. In the case of the models based on the mass uniform model, they only describe (different from the previous ones) the mass conservation. They are less sophisticated and has a similar output –under determined conditions- to the most complex models. Examples of this kind of model are the WindMAP and the WAsP. It can be deduced from the description of both models that the mass conservation principle is the most important determinant of the wind variation, always referring to surfaces of low or moderated complexity.

Graphics of the combined models

The graphic shows the steps to develop a wind map using a meso scale method and one of micro scale together, as the Wind Atlas style.

EXAMPLES OF THE MOST KNOWN MODELS • • • • • •

KAMM (Karlsruhe Atmospheric Mesoscale Model) [meso scale] [4] Wind Atlas Analysis and Application Program (WAsP) [Simplified] [4] MesoMAP [meso scale] [7] WRAM Method [GIS] [8] WindMAP [Simplified] [7] WindSCAPE [Mix]: Raptor [micro] + TAPM [meso]

WIND MAP EXAMPLE: EUROPE

WIND MAP EXAMPLE: EUROPE OFFSHORE

WIND MAP EXAMPLE: DENMARK

CONCLUSIONS One must be conservative in the interpretation not only of the data obtained as a consequence of the measurements done in the field by the measurement equipments but also with the extrapolations which are done, in height as well as on the surface. It is convenient to estimate between a 10% and a 20% less in the obtained data, and with those values to do the calculus and estimations. Finally, the report of the “wind potential” of the region must present a technical and meteorologically sustained detail of the following information:

• • • •

The preliminary analysis of the region. (In this case it is advisable to count with the wind map of the country) Equipment installation process. Wind Mapping. Results and final report.

It does not matter what kind of wind project will be started, the evaluation phase of the potential of a region is one of the most important ones. According to its result the feasibility or not of a future project will be determined; also which is the best place within a region to establish a new wind complex. The wind maps (atlas, resource evaluations, or whatever name they are assigned) are fundamental instruments to start any work of planning the installation of a wind farm. But all of them depend, at the same time, on fundamental incomes which will allow their creation: the meteorological data, of whatever kind they are.

REFERENCES 1. 2. 3. 4.

5. 6. 7.

8. 9.

[1]. Wind Energy Danish Assoc. WindPower.org. Wind class standard definitions “Wind Class”. 11/Feb/2004, [2]. Brower, M., B. Bailey, and J. Zack. The New US Wind Resource Atlas [cdrom]. In: European Wind Energy Conference & Exhibition 2003. [Madrid], European Wind Energy Association, 2003. [3]. Cambridge University Press. Foundations of Fluid Mechanics. Navier-Stokes Equations [on line]. 14/Aug/2004. [4]. Frank, H. P., O. Rathmann, N. G. Mortensen, and L. Landberg. The Numericalal Wind Atlas: The KAMM/WasP Method. [Roskilde, Denmark]: Information Service Department, RisØ National Laboratory, June 2001. [5]. Gasch, R., and J. Twele. Wind Power Plants. Fundamentals, Design, Construction and Operation. [Berlin, Germany]: Solarpraxis AG, 2002. [6]. Manwell, J. F., J. G. McGowen, and A. L. Rogers. Wind Energy Explained: Theory, Design and Application. [West Sussex, England]: John Wiley & Sons Ltd, 2002. [7]. Brazil Mining and Energy Ministry. Mapas do Potencial Eólico Anual [cdrom]. In: Atlas Do Potencial Eólico Brasileiro. [Brasilia, Federative Republic of Brazil], e-dea Technologies/ Christianne Steil, 2001. [8]. Nielsen, J., S. Innis, and K. Pollock. Renewable Energy Atlas of the West. [9]. RisØ National Laboratory. Wind Energy Department. Wind Resource Atlas for Denmark. [Denmark]: 23/Jan/2004.

Eng. Eng. Research Argentine www.argentinaeolica.org.ar

Luis Alejandro & Wind

Mariano J. Development Energy

Faiella Gesino Area Association

Siting of Wind Farms: Basic Aspects When searching the internet for the definition of the word “layout” I came across following: Layout in word processing and desktop publishing refers to the arrangement of text and graphics. The layout of a document can determine which points are emphasised and whether the document is aesthetically pleasing. While no computer program can substitute for a professional layout artist, a powerful desktop publishing tool can make it easier to lay out professional looking documents (source: www.webopedia.com) In principle the same is valid for wind farm planning: The term layout in wind industry is used for choosing optimal locations for wind turbines. Tools like flow models help to identify the best positions, but cannot replace the engineer making the final decision by balancing interests. So what is that engineering experience, what factors influence the decision?

Jessica Rautenstrauch, wind energy consultant from Anemos, Germany, at work. © Paul Langrock (www.unendlich-viel-energie.de)

Wind resource The wind resource is the most obvious factor to concentrate on when choosing a wind turbine location. We have a wide range of options to determine the wind resource of the site. The quality of the tools varies significantly and so does their price. Common sense is a good starting point. Nature itself helps to guide us to suitable sites. Flagging of trees – permanent flagging and not the temporary bending in the wind – shows us the prevailing wind direction and is a good indicator for the strength of the wind. However because of the uncertainty involved, using common sense as the only tool is of course insufficient. For any bankable estimate of the energy yield on-site wind speed measurements are required. The number of measurement masts required for a specific site depends next to the size of the project mainly on the complexity of the terrain. The measurement height should be minimum 2/3 of the expected future hub height. An increase in measurement height beyond this leads to a reduction of the uncertainty in the energy estimate. The measurement period must be one year or more to avoid any seasonal bias. Since the wind speed varies also inter-annually typically up to +/-12% a long-term correction is highly recommended. The measured wind regime is extrapolated across the site to derive a resource map of the site using different flow models /4, 5/. A wind map like the one in Graph 1 can then be used to identify the windiest locations. However additionally technical constraints should be taken into account when developing a layout /3/. A number of site specific wind load parameters can be extracted from the wind speed measurement. They are used to optimize the technical suitability of the chosen layout and the wind turbine type for the site specific wind regime.

Graph 1: Example Wind Resource Map. The colours denote the energy content of the wind, red high and blue low energy content.

Technical restrictions Wind turbines are designed for specific conditions. During the construction and design phase assumptions are made about the wind climate that the wind turbines will be exposed to. In rough terms: For very complex sites with high wind speeds “heavy-duty” versions of wind turbines are available, which are sturdier but also more costly. Low wind speed sites in flat terrain do not put so high demands on the on the wind turbine structure, hence the construction can be more light-weight and hence cheaper. The different turbines have been classified by the IEC, class 1 being the highest wind speed class. The following table is a simplified summary of the IEC classification /1/.

IEC class

I

II

III

IV

Vave (m/s) annual average wind speed at hub height

10

8.5

7.5

5

Vref (m/s) 50-year maximum 10-minute wind speed

50

42.5

37.5

30

Table 1: IEC classes

But not only the wind speed but also other parameters play a role and have to be checked, when developing a layout for a specific turbine. One of the most important parameters is the turbulence intensity. Turbulence intensity quantifies how much the wind varies typically within 10 minutes. Because the fatigue loads of a number of major components in a wind turbine are mainly caused by turbulence, the knowledge of how turbulent a site is of crucial importance. We have to distinguish between two different sources of turbulence. Turbulence is generated by terrain features – which is referred to as ambient turbulence intensity - as well as by neighbouring wind turbines – which referred to as induced turbulence (Figure 1). Sources of ambient turbulence are for example forests, hills, cliffs or thermal effects. Thus ambient turbulence can be reduced by avoiding critical terrain features. But the wake-induced turbulence has far more impact than the ambient turbulence intensity /2/. Decreasing the spacing increases the turbulence induced by the wakes of neighbouring wind turbines meaning that there are limits to how close you can space the turbines. As a general rule the distance between wind turbines in prevailing wind direction should be a minimum of the equivalent of five rotor diameters. The spacing inside a row perpendicular to the main wind direction should be a minimum of three rotor diameters.

Figure 1: Shadowing in wind farm

If a layout is too close the resulting fatigue loads might be too high. In order to then ensure the lifetime of the main components wind sector management might have to be applied, meaning that some wind turbines might have to be switched off when they are operating in the wake of the neighbouring wind turbine. Another parameter which has to be checked when developing a layout is the flow inclination, velocity tilt or in-flow angle. When wind turbines are to be placed on steep slopes or cliffs the wind might hit the rotor not perpendicular but at an angle. This angle is related to the terrain slope. With increasing height above ground level the effect of the terrain slope is normally reduced such that the terrain slope is only of indicative use to estimate the velocity tilt. A large in-flow angle will not only reduce the energy production but will also lead to an increased level of fatigue of some of the mayor components.

Figure 2: Distorted wind profile at steep slope (left) and behind a forest (right)

Furthermore a steep slope might cause a negative gradient across some parts of the rotor (Figure 2). Normally the wind speed increases with increasing height. In flat terrain the wind speed increases logarithmically with height. In complex terrain the wind profile is not a simple increase and additionally a separation of the flow might occur, leading to heavily increased turbulence. The resulting wind speed gradients across the rotor lead to high fatigue loads particularly on the yaw system. Obstacles like forest can have a similar effect on the wind profile and should be thus avoided.

Planning constraints Next to the wind resource and technical considerations a good layout should also take planning constraints into account. The visual impact is course the most obvious. A layout that follows the shape of the terrain rather than straight rows of wind turbines appears to be less intrusive. Noise is another important parameter to take into account. Next to noise also the impact due to flicker at the nearest inhabited houses should be estimated. The accepted levels vary from country to country. Electro-magnetic interference can cause problems. Hence placing wind turbines in a transmission corridor should be avoided. Some areas on site might have to be excluded from development due to other factors related to fauna, flora and archaeology.

Jessica Rautenstrauch, wind energy consultant from Anemos, Germany, at work. © Paul Langrock (www.unendlich-viel-energie.de)

Summary A large number of parameters have to be taken into account when developing a layout. Some work can be done using tools, but in the end the balance between financial, technical and planning constraints can be best done by an experiences engineer.

Literature • • • • •

/1/ IEC 61400-1, Ed.2 – Wind Turbine Generator Systems – Part 1: Safety Requirements, FDIS 998 /2/ S. Frandsen, St.; L. Thøgersen, L.;: Integrated Fatigue Loading for Wind Turbines in Wind Farms by Combining Ambient Turbulence and Wakes; Wind Engineering, Vol. 23 No. 6, 1999 /3/ K. Kaiser, W. Langreder: Site Specific Wind Parameter and their Effect on Mechanical Loads, Proceedings EWEC, Copenhagen, 2001 /4/ E.rik L. Petersen, N. G. Mortensen, L. Landberg, J. Højstrup and H. Frank: (, Wind Power Meteorology Part I: Climate and Turbulence, Wind Energy, 1, 25-45 (1998), Risø-I-1206, 1997 /5/ E. L. Petersen, N. G. Mortensen, L. Landberg, J. Højstrup and H. Frank: Wind Power Meteorology Part II: Siting and Models, Wind Energy, 1, 55-72 (1998)

Wiebke Suzlon Energy: www.suzlon.com

Langreder

Wind Farm Siting and Layout Design The development of a wind farm is typically initiated by a land owner, a developer or the proactive planning of a local community. In each case the process starts with finding a site that is suitable for a wind farm development.

1. SITE FINDING Finding a wind farm site is a juggling act where many, often conflicting, issues need to be considered and balanced before a decision to develop a potential site is taken. At the most basic level, after establishing that the output from a wind power project can be sold at an acceptable price, the following are the acid tests for any potential development:

• • • • •

1.Is grid connection likely to be cost effective for the desired size of development? 2.Is the wind resource adequate? 3.Will the project be able to obtain all the permits necessary for the wind farm to be built? 4.Is access to the site and construction of the wind farm likely to be cost effective? 5.Can the rights to the land be secured?

A good expectation that the answer to all of the above tests will be positive is a pre-requisite for making the investment necessary to realise a project, although inevitably early decisions need to be made on incomplete information. Each of the issues is considered in a little more detail below:

Grid connection When looking for a site, proximity to a medium voltage grid is a good initial indicator that an appropriate connection is practical. The next stage of the process is to hold discussions with the appropriate electrical authority. The results of such discussions will usually indicate a cap, or a series of caps on the maximum installed capacity at a potential site which are associated with progressively more costly grid connection scenarios. Some detailed analysis by the electrical authority, at the cost of the developer, may be

necessary before even approximate figures are available. The presence of only a high voltage line close to a small or medium wind farm may not be helpful as the cost of connection to such a grid may be prohibitive.

Wind Resource It is difficult to generalise how best to assess the wind resource at a potential wind farm site when no site wind data are available, as different countries have markedly different wind regimes. Some general rules, for which there are many notable exceptions, are listed below:

• • • •

Good exposure, particularly in the prevailing wind direction, will substantially improve the resource at a site. The rate at which wind speed reduces away from the areas of a site with the best exposure should not be underestimated. An “ideal” hill would have smooth slopes of approximately 17 degrees gradient. Steeper slopes do not give substantial additional enhancement of the wind flow but can cause separation of the flow which complicates the wind conditions at a site. Low vegetation at and around a site retards the wind flow less than tall vegetation. However, a site with good exposure and small trees is likely to prove better than a site with poor exposure and no trees.

Building permits Key issues will vary between regions and countries but common sense indicates that areas with special designations are best avoided. Low visibility from key areas of habitation or recreation is also desirable. If there are dwellings within a few hundred meters of the wind farm site noise or shadow flicker may prove an insuperable problem in some countries. Turbines can interfere with electromagnetic telecommunications signals. The presence of a telecommunications mast at a site or such signals which cross a site may therefore complicate the process of obtaining a building permit. A check for television communications should also be made which may not be apparent from visual inspection.

Access The distance to the nearest road access and the complexity of the terrain will substantially influence the capital cost of the project.

Land availability Land availability varies from country to country but a potential site where there are relatively few landowners and landowners who can give exclusive rights to the developer is the ideal situation. The problem of site finding lends itself well to a thorough and detailed Geographical Information System (GIS) based approach where wind atlases, an electrical grid map, roads, environmental designations and other criteria can all be input and the optimal sites defined. In practice, however, a more pragmatic approach may well prove more appropriate.

2. WIND FARM LAYOUT DESIGN The wind farm layout is typically designed using a professional wind farm design package. Such tools allows for an effective iteration and optimisation of the key parameters for the layout.

Preliminary layout design Once a site has been identified and the decision has been taken to invest in its development the wind farm design procedure commences. This is inevitably an iterative process. The first task is to define the constraints on the development:

• • • • • • • • •

Maximum installed capacity (due to grid connection or Power Purchase Agreement terms) Site boundary Set backs from roads, dwellings, overhead lines, ownership boundaries etc. Environmental constraints Location of noise and shadow flicker sensitive dwellings, if any, and assessment criteria Location of visually sensitive viewpoints, if any, and assessment criteria Turbine minimum spacings as defined by the turbine supplier. Constraints associated with communications signals such as microwave link corridors, if any. Local regulations that limit the turbine type permissible for the development.

These constraints may change as discussions and negotiations progress with various parties. For the purpose of defining the preliminary layout it is necessary to define approximately what sizes of turbine are under consideration for the development, as the installed capacity achievable with different sizes of turbine may vary significantly. The selection of a specific turbine model is often best left to the more detailed design phase when the commercial terms of the various suppliers are known.

Specification of anemometry The wind resource at the site is the key parameter in determining its economic viability. To assess the energy for a project it is necessary to obtain data on the local wind regime. Typically this means installing anemometry equipment at the site. The preliminary layout allows the wind measurements to be made in appropriate locations. As a general rule the mast should be at least two thirds of the hub height of the turbines. A useful rule in complex terrain is that no turbine is located more than 1 km from the closest mast. In very severe terrain, the closest mast should be within 500m, but for wind farms located in simple terrain a much lower density of masts over the site may be appropriate. For large developments that require several masts there may be advantages in initially installing just one mast on the site. Once it is confirmed that the wind resource is reasonable, other masts can be installed to confirm the variation in wind speed over the site area. Provided the original mast remains as a constant reference other masts can be moved after, say, six months of operation to reduce the total number of masts required.+

Detailed layout design A key element of the layout design is the minimum turbine spacing used. In order to ensure that the turbines are not being used outside their design conditions, the minimum acceptable turbine spacing should be obtained from the turbine supplier and adhered to. The appropriate spacing for turbines is strongly

dependent on the nature of the terrain and the wind rose at a site. If turbines are spaced closer than 5 rotor diameters in a frequent wind direction it is likely that unacceptably high wake losses will result. For areas with predominantly uni-directional wind roses, such as the San Gorgonio Pass in California, greater distances between turbines in the prevailing wind direction and tighter spacings perpendicular to the prevailing wind direction will prove to be more productive. Tight spacings require approval by the turbine supplier if warranty arrangements are not to be affected. With the wind farm constraints defined, the layout of the wind farm can be optimised. This process is also called wind farm “micrositing”. The aim of such a process is to maximise the energy production of the wind farm whilst minimising the infrastructure and operating costs and meeting all constraints. For most projects the economics are substantially more sensitive to changes in energy production than infrastructure costs. It is therefore appropriate to use the energy production as the dominant layout design parameter. The detailed design of the wind farm is facilitated by the use of commercially available wind farm design tools. Once an appropriate analysis of the wind regime at the site has been undertaken, a model is set up which can be used to design the layout, predict the energy production of the wind farm as well as being used to address economic and planning related issues. For large wind farms it is often difficult to manually derive the most productive layout. For such sites a computational optimisation using a wind farm design tool may identify a layout for which substantial gains in predicted energy production are achieved. Even a 1 % gain in energy production from improved micrositing could easily represent an increase in annual revenue of $50,000 to $100,000 for a 50 MW wind farm. The computational optimisation process will usually involve many thousands of iterations and can include noise and visual constraints. Wind farm design tools conveniently allow many permutations on wind farm size, turbine type, hub height and layout to be considered quickly and efficiently increasing the likelihood that an optimal project results. Financial models may be linked to the tool so that returns from different options can be directly calculated, further streamlining the development decision making process. In many countries the visual influence of a wind farm on the landscape is an important issue. The use of computational design tools allows the Zone of Visual Influence (ZVI), or visibility footprint, to be calculated to identify from where the wind farm will be visible. The tools may also be used to provide visualisations, to facilitate the production of photomontages and to predict the noise and shadow flicker which results from a proposed development. These are often key aspects of the Environmental Assessment for a project. Figure 1 shows an initial preliminary layout of a wind farm consisting of 26 turbines that meets all site specific constraints. There are two noise sensitive dwellings west of the proposed wind farm with a defined noise limit that are marked with dwelling icons. The solid black line represents the site boundary in which the turbines can be placed.

GH WindFarmer Figure 1

The layout of the wind farm after the optimisation is shown in Figure 2. Compared with the initial layout the predicted energy production has increased by approximately 3 %. In the upper section of Figure 2 the optimised layout of the wind farm superimposed with the noise levels predicted for this layout can be seen. A rendered visualisation of the wind farm appearance from a viewpoint southeast of the wind farm is shown at the bottom.

GH WindFarmer Figure 2

Conclusion The design of a wind farm is a compromise between high energy yield, easy access, easy permitting and commercial viability. Careful consideration of a very large number of factors is typically required to reach the best designs and consequently dedicated software tools are used by the majority of wind farm developers.

For more information, see: • •

www.garradhassan.com www.garradhassan.com/products/ghwindfarmer/index.php

Wolfgang Andrew Garrad Hassan www.garradhassan.com

and

Partners

Schlez Tindal Ltd

Repowering of wind turbines The expression “Repowering” refers to power plant in general and includes all measures which improve the efficiency and capacity by means of retrofit to the latest technology. Considering a coal power plant,

repowering could mean to install a new steam generator or a new turbine. Possible modifications on wind turbines are limited, thus repowering affects the whole plant in general and essentially the entire wind farm. In short, aim of the repowering is to use the existing renewable energy resources on site more efficiently, respectively in a technically adapted or improved manner. Progressing technology provides the option for operators to improve the profitability of their site, or to cope with new technical or legal conditions. Frequently planned and often locally supported is to restore the landscape. The reduction of the number of plant is linked with a significant growth of hub heights and a reduction of the rotational speed.

The actual repowering of larger capacities and entire wind farms in Germany started since the amendment of the Renewable Energy Law in 2004 provides financial incentives for repowering projects. Since Germany runs short on productive sites onshore, the government and the wind energy industry place their hopes on repowering onshore before the offshore market will be opened up in a few years.

Animation (in German language only) "Repowering Movie with friendly permission of the German Wind Energy Association BWE"

Presently, the individual capacity of a wind turbine has advanced enough to fulfil the initially formulated aim of doubling the capacity and reducing the number of plant by 50 . The 2 MW turbine class and upward emits considerably less noise and complies with the actual grid code to feed in larger capacities. The history of repowering started in California with the scrapping of the first and second generation of wind turbines of the oldest plant and resale of usable plant until 1993. Reasons mentioned were technical progress, reduction of the plant density, and restoring of the landscape near tourist routes. A change in the legal framework in Denmark ended in that until 2002 approximately 1800 wind turbines were replaced and mostly scrapped. In special cases used wind turbines lend themselves for rebuilding on a new site after a thorough general overhaul and in most cases a new foundation. It is recommendable to assess the mechanical wear and to repair or to replace the affected components before being installed on the new site. Based on good operating conditions and experience with maintenance the second hand plant could allow more than 10 years of operation. Since 2003 the second-hand market experiences steady growth with plants from Denmark, the Netherlands and Germany mostly being shipped to the Balkans and Eastern Europe.

Further information: • • • •

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German Ministry of Environment: http://www.erneuerbare-energien.de/inhalt/36356/ Gesetz zur Neuregelung des Rechts der Erneuerbaren Energien im Strombereich http://217.160.60.235/BGBL/bgbl1f/bgbl104s1918.pdf (in German) German Wind Energy Association: http://www.wind-energie.de/de/themen/repowering REPOWERING - Potentials and framework conditions in Germany and Denmark and new markets for second-hand wind turbines (Proceedings of a BWE/WWEAWorkshop at HusumWind 2005) CD-ROM is available at: http://www.wwindea.org/home/index.php?option=com_content&task=blogcategory&id=20&Itemi d=41 K. Rehfeldt: Analysis about the Potential of Repowering in Germany, in: WWEA (Ed.), Wind Energy International 2005/2006, 201-203. Book is available at: http://www.wwindea.org/home/index.php?option=com_content&task=blogcategory&id=20&Itemi d=41 World Wind Energy Association: Repowering Working Group http://www.wwindea.org/home/index.php?option=com_content&task=blogcategory&id=22&Itemi d=53

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