M. Mikkonen.

Transcription

1 Wind study by using mobile sodar technology M. Mikkonen Oulu University of Applied Sciences, School of Engineering, Oulu, Finland Abstract In this paper is presented a concept of making wind study by using sodar technology. First is presented some theory about the sodar technology, the mobile unit, sodar instrument and the placement of the device. This includes practical example of the use of this technology. The example is a location where wind farm has been planned. Investors idea is to use their desired location where should be suitable winds for wind mill operation. Original data for suitable winds for the site has been taken from the wind atlas. For this location let s go find the best place for sodar at the site for the best data availability. Finally the the data gathered from this location is checked and make a conclusions will be presented. What s the benefits comparing it to the other ways to study wind at the site.. Finalizing the report by conclusions and some thought about the future of this technology. Keywords: wind study, sodar,, wind energy measuring, air traffic measuring, wind farm. 1 Introduction In this document is presented some basic knowledge of the sodar measurement. Document goes thru some theory for making the practical example of the use of sodar possible. Introduction of the unit that holds the sodar. Presenting the technology and operation of the sodar. Showing the principle of finding the right spot for making the measurements so that we get the best possible data availability from the device. Finally we use this information in practical example and make the final conclusion about it. of focus because of its mobility. Old and still useful way was to measure wind anomalies by building a met mast. For building the met mast you need a license for building, groundwork, foundation for holding the m tall mast, guy lines to make it steady. And all of this costs money, and a lot. Sodar makes the same measurements without all of this work and license handling. 2 Mobile sodar unit Mobile sodar device is a caravan size device what you can pull with your car. The instrument is placed in a trailer and it can be detached from it. Here is some features of the device: Power source: - Solar panels, Direct power grid, diesel generator. Working temperatures: C C Measurement specs: - Data availability >95% / m, >90% / m - Up to 1000m with availability of 50%-80% - Minimum 20m - Resolution 5m - Horizontal wind speed < 0,1 m/s - Vertical wind speed < 0,05 m/s - Wind direction 2-3 Other measurement capabilities: - Temperature from 2m height - Humidity from 2m height - Orientation,, pan and tilt of the device - Pressure Measuring wind by sodar (Sonic Detection and Ranging) is originally a Swedish invention. The technology itself is from the 1960 s. Original purpose for sodar was to measure wind anomalies (turbulence etc.) at the military air fields. From there the technology has been pushed further for civil use also. Now days the biggest market for this kind of wind study technology is wind farms. In the wind farm business this has had a lot Figure 1

2 During the winter, the operation is improved by a diesel heater which preheats the generator before it starts. Excess heat from the diesel heater is not wasted but used to melt any snow and ice in the space where the sodar antenna is placed. As a user, you do not need to worry about freezing instruments or lacking observations during the cold time of year. The diesel heater is an option that can be installed in the factory or retrofitted in the field. 2.1 Sodar The design of the antenna unit, which creates the sound pulses, is robust and has no moving parts (Figure2). and direction. Technology is based on monostatic technique. This means that the speaker that emits the sound pulses also acts as a microphone and listens to the reflected sound. In order to calculate the wind s three components, sodar is equipped with three separate speakers/microphones that emit sound in three directions. The geometry for the whole system is shown in figure 4. The sound that device emits spreads in a cone-shaped volume, which we call the measurement lobe. The center of the lobe inclines 15 to the vertical line, and has a width of 12º. The angle between the three measurement lobes is 120º. The measurement lobes are labelled A, B and C with A in the direction of the sodar units drawbar. The half angle of the cone is 6º. The sound used has a frequency of 3144 Hz. This corresponds to key 83 on a regular piano with 88 keys. Figure 2 Measures the wind s directional components, two horizontal and one vertical, by transmitting sound into the atmosphere. The sound is reflected by small temperature variations. These moves with the wind and the reflected sound have, due to the Doppler effect (the frequency of a transmitted sound is perceived differently if the sound source moves toward or away from the listener), a different frequency than the transmitted. The difference between the transmitted and received (reflected) frequency is used to calculate the wind speed Figure 3 Figure 4 3 Finding the location The results of a measurement with an sodar to 50 percent depends on the instrument and to 50 percent on how it is used. A large part of the 50 percent related to the usage consists of finding a suitable site for the instrument. Sometimes there is a need to measure on locations that are not ideal. In these cases it s recommended that the planning of the measurement campaign and installation is made together with the consultant that will analyze the measurement and that all involved are informed. It is also important to follow up the measurement and take action if any kind of disturbance is found. Echoes from objects in the surroundings, such as trees, buildings or a met mast, which reaches the sodar during the listening phase may cause a deviation in the measurement. Echoes that arrive before the sodar starts to listen or after the sodar has stopped listening do not affect the measurement. This means that there is a volume within which there should be no obstructions. The inner half sphere in

3 figure 5 represents the lowest measuring height while the outer represents the highest measuring height. The volume between the two half spheres must be free from 3.1 Fixed Echo Fixed echo an echo from an nonmoving object, for example a tree, a house wall or met mast. To avoid fixed echoes, it is important that a site visit is made prior to the installation and that the measurement data is carefully monitored after the installation. The goal of the installation must be to not have any disturbing objects in the volume between the two half spheres. Acoustic examination of the environment on a site is done by clapping hands together and listens for echoes. Estimate the time it takes for the echoes to reach you and try to determine what causes them. If possible, avoid places with too much echoes. It is not always possible to keep the volume between the half spheres completely free from objects. If that is the case, it is important not to point any of the lobes towards the object. Instead sodar should be positioned so that the lobes are directed on opposite sides of the object. Figure 6 interfering objects. The cones are the sodar measurement lobes. The inner half sphere represents the lowest measuring height while the outer represents the highest measuring height. The volume between the two half spheres must be free from interfering objects. It's easy to find the heights that could be affected by an object between the two half spheres. An example is shown in figure 7. Sodar is at A. At B is a met mast that is 100 meters high. The horizontal distance between the sodar and the mast is 120 meters. The distance between the sodar and the top of the mast can be calculated, using the Pythagorean theorem, to 156 meters. This means that it is the measurement levels between 120 meters and 155 meters which potentially could be affected by fixed echoes. Figure Terrain Figure 5 The vertical distance between the outer edge of the lobes and the ground at different distances from the sodar is shown in figure 6. Because of the small zenith angle, the sodar do not require a large open area in order to be deployed. This is of course beneficial when installations in the field are made. Steep terrain can sometimes affect the wind measurement and cause orographic deviation (a deviation in the measured wind speed due to the orography in the immediate surroundings of the sodar). The reason why the terrain can have an impact is that the sodar measures in three volumes and assumes that the flow through these is linear. All remote sensing instruments (sodar and lidar) that use Doppler

4 technology must make this assumption. The more curved the airflow, the less the validity of the assumption of linear flow and the greater the error can be. In general, noise from the surroundings is not a problem. Nevertheless it is still a good idea to take this into consideration when choosing your site. Sodars have been installed next to heavily trafficked roads, without this causing any disturbances. The quality number, defined as the signal-to-noise ratio (the relation between the echo that the sodar logs and the noise from the surroundings) times 10 and that is included in the data files from the sodar, can be used to examine if the noise affects the measurement. If it varies with the diurnal variation of the noise and drops below 21, which means that the data is rejected, the noise is a problem. 4 Data Figure 8 There are some simple rules of thumb that can be followed when assessing the suitability of the terrain for sodar measurements. When it concerns steep terrain you can define areas that are inappropriate for sodar placement by studying detailed maps before going to the project area. If the measuring height H is much greater than, or much smaller than the terrain s typical height variation Δ, then the deviation will be small. Figure 9 If H is at most 10% of Δ then the deviation will be small If H is greater than 4 Δ then the deviation will be small in the case of measuring on a hill. If you measure on a ridge the measuring height must be about 15 Δ. 3.3 Noise Sodar collects the data to its memory. Sodar has its own linux based pc unit for handling the data. Data is stored in specific cryptic form. Data is fetched from sodar in every day by the main server. From the server data is in readable form and can be analyzed. These are some of the features that the data includes: - Wind speed m/s - Wind direction - Turbulence intensity - Wind shear - Vertical velocity - Flow inclination - Temperature - Humidity Server performs specific filtering during the collection stage to ensure that data is of high quality. The purpose of the filters is to make data ready for further use without additional filtering. Despite the filters, deviating values can occur in the data. It is therefore important that the data is reviewed before it is used. The data availability of turbulence (standard deviation) is significant less than for wind speed. This is because different quality demands on the signal from the atmosphere are used if wind speed/wind direction or standard deviation is calculated. The sodar measures the vector wind speed unlike a cup anemometer which is measuring the scalar wind speed. The vector mean wind speed is always slightly lower than the mean scalar wind speed.

5 4.1 Data filters Filters are applied on data at two occasions in the process of collecting data. The first stage is when the echo received by the sodar is undergoing signal processing. This is done in the computer inside the sodar trailer. The second filtering stage is when data is transferred from the sodar to the user. The transfer can be made in the following ways: 1. The user use a modem and the software, to call the sodar. The filters are a part of sodars software. 2. The sodar is automatically transferring data to a server owned by the company every ten minutes. The filters are installed on the server. Figure 10 5 Wind measurement at the site Example site is located at the top of the hill. Hill is 63m high from the sea level. Size of the area where the wind mill should be located is 75000m2 = 7,5 hectare. Figure 11 Simplified description of the process from transmitted sound pulses to readable measurement values. Grey blocks indicate processes performed in the sodar while the blue blocks indicate processes performed on the user's computer or on company s server if the system has the web solution. 4.1 Data spectrum Peaks in the spectrum, which lie to the right of the centre line, should be interpreted as the wind blowing towards the measurement lobe. This in turn means that the frequency of the echo is higher than that transmitted. Similarly, peaks in the spectrum to the left of the centre line mean that it is blowing away from the measurement lobe; the frequency of the echo is lower than the transmitted sound. In calm conditions or when the wind blows perpendicular to the measurement lobe, the difference in frequency between the transmitted sound and the reflected echo is very small, the peak in the spectrum in such cases will be on or very close to the centre line. Figure 12 There are five possibilities for placing the sodar and the places are marked with red color numbers in the map. The planned wind mill locations are marked with blue circles at the map. Sodar location number 1: Top of the hill. Can be suitable if there is no obstacles like trees in nearby sodar unit. Sodar location number 2: Between the hill and the ridge. This is not suitable for measurements. Curved air flow from west to east can give error in data. Sodar location number 3: Top of the hill. Can be suitable if there is no obstacles like trees in nearby sodar unit. Sodar location number 4: Half ridge. Curved air flow from the north or south gives error in data. Sodar location number 5: Half ridge. Curved air flow

6 from the south or north gives error in data. The chosen location is number Data from the site For this site the collected data is from 6 days, 23hours and 50 minutes. Figure 13 From the data we get average wind speeds from three heights: - 50m, 5.14 m/s Avg - 100m, 7.18 m/s Avg - 150m, 9.01 m/s Avg It is also important to know the possible orientation of the wind mill. Figure 14 Orientation is not so dependent of the measured height. It seems that in this location the orientation moves between south to north west. We need to also inspect the data availability in the measured range. 2.8 Conclusions The sodar is definitely fast way to kick start wind farm projects. It s a shame that in most of the cases in Finland, the sodar isn t yet trusted like the met mast. Mostly they are both been used for collecting the data. First measurements are made with sodar as a standalone instrument. If the sodar gets good enough measurements the met mast will be built in the project area. Then the sodar is been used as a correlation data collector at the nearby surrounding of the met mast. In this example case we choice the right location for the sodar. After we made some measurements and got some promising readings. For making profitable power with wind mill according to our measurements. We could built the wind mill between 100m-150m (163m-213m from sea level), depending about the size of power wanted. The final conclusion is hard to make with this amount of data. For better analysis we would make the measurements at least 3months time, if it would be possible 1 year is preferred. Also data availability at 150m should be better. The reason for this poor availability should be investigated and if necessary the the sodar unit should be moved also to other possible measuring locations in the site. References [1] ww.toragon.se [2] [3] [4] Tuulisampo Oy [5] en.wikipedia.org/wiki/sodar [6]en.wikipedia.org/wiki/Radio_acoustic_sounding_syst em [7] [8]Remote wind speed sensing for site assessment and normal year correction.pdf Figure 15 The graph indicates that the availability has been good from 50m to 110m >95% and from 50m to 150 >80%.