EMPOWERING OFFSHORE WINDFARMS BY RELIABLE MEASUREMENTS

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1 EMPOWERING OFFSHORE WINDFARMS BY RELIABLE MEASUREMENTS Joerg Bendfeld University of Paderborn Fakultät Elektrotechnik, Mathematik und Informatik Lehrstuhl für Elektrische Energietechnik Pohlweg 55 D Paderborn Karl Navratil University of Paderborn Fakultät Elektrotechnik, Mathematik und Informatik Lehrstuhl für Elektrische Energietechnik Pohlweg 55 D Paderborn ABSTRACT For the estimation of the average annual wind speed and thus the energy production of offshore wind farms it is imperative to know the exact wind conditions at each project site. Suitable prognosis-methods are missing so far. Methods used onshore cannot easily be transferred. The only available approach to achieve exact information about the wind conditions is a met mast. An Offshore met mast has to deliver data for many different purposes: 1. In the first stage to get a project bankable the data for the realistic prediction of annual energy production is important. 2. The second aim is the identification of seasonal time slots for maintenance of the offshore wind farm. 3. The third aim will be the identification of the turbulences for load-calculation, or the direct load measurement. 4. During the operation of the wind farm the performance has to be monitored. For the dimensioning of the metmast it is recommendable to measure wind speeds within the entire rotor operating range of the future wind plant. With planned hub heights of approximately 100 yd and a rotor diameter of approximately 140 yd the measuring range extends from 30 yd up to 170 yd above sea level. Most promising are the new (for wind energy purposes) remote sensing techniques: For example Lidar. Lidar is based on laser doppler scatterometry 1. Introduction Offshore wind should be discussed in the context of the challenge of climate change. It is widely accepted that developed countries by 2050 will have to make very significant cuts in their greenhouse gas emissions, around 80%, to limit the climate change to a manageable level. Because of this the electricity generation system needs to become carbon neutral in the next three to four decades. In that context the winds over the seas are a huge resource of clean energy. Offshore wind can deliver a massive contribution to a low greenhouse gas power system.

2 Since no wind turbines are installed in U.S. waters, there is a shortage of critical data on the environmental and siting effects of turbines and on the installation, operations, and maintenance of these turbines. This lack of data drives up the costs of financing offshore wind projects to the point where financing charges account for roughly half of the cost of offshore wind energy. (1) Also the bankability of projects without reliable annual energy production prediction is at least questionable. 2. Met Mast For the use of wind energy purposes the yield error resulting from the prognosis error is essential. Since the energy in the wind power is commensurate proportional with the wind speed to the power of three, according to the principles of error propagation the yield error can be determined up to approximately 30 % in the worst case. Such error rates can have significant dramatic effects on the efficiency of projects economics and constitute a decisive factor for the final project realization. Therefore these prognosis models are not suitable for offshore locations. Until now the only reliable method to receive exact yield prognoses is the construction of a met mast in the respective area capturing collecting dependable actual measurement data for future yields. In order to get the most reliable data for an offshore wind power-project, a met mast is the only way. The met mast has to meet certain basic standards: Dimensioning suitable for its purpose must be chosen. High data availability and economic data transfer have to be given. The measuring platform and thus energy supply should be easily maintainable that a maximum of two maintenance procedures within a year will be sufficient. Figure 1: Example of an existing met mast

3 The required energy should be supplied by means of on-board converter systems. Here a system of wind generators, solar stations and diesel generators is recommendable. The main focus of the measurements is on the evaluation of the wind scheme and other meteorological parameters at different heights. Even oceanographic data is of high interest for several reasons. Mast If the prevailing wind direction distribution is nearly accurate known the design of the Mast can be made according to this facts. If the distribution is broad or the prevailing wind direction distribution isn t known a general Layout with three arms is recommended (Figure 2 and Figure 4). But if the prevailing wind direction distribution is rather narrow a general layout with two arms can be sufficient (Figure 3). For maintenance reasons the longer booms (length more than 5m ) should be hinged. But this implies more platforms to reach the end of the booms to change the instrumentation. (2) prevailing wind direction Figure 3: Topview of a metmast with two booms A series of identical conventional and reliable cup anemometers should be arranged on booms. The anemometers measure the wind speed at several different height levels. The lowest level should be at the lower tip-height of the planned wind energy converter, the highest level should be the hub height of the planned converter is at yd. Mast The registration of the wind direction is conducted by means of conventional wind vanes. Furthermore, the wind is measured three-dimensionally by two ultrasonic anemometers. Data concerning atmospheric humidity, air temperature and air pressure and temperature difference complete these figures. 3. Future aspects Figure 2: Topview of a metmast with three booms The next interesting aspects concern the conditions above the hub height up to the upper tip height ( yd above the sea level). Met masts with a height of more than 100 yd will be very expensive. The information should be achieved by remote sensing technologies. The remote sensing instruments used for wind energy today are Sodar and Lidar

4 Sodars (Sound detection and ranging / based on acoustic energy reflections from turbulent fluctuations) isn t very reliable offshore because of the noises from the waves and the low ambient turbulence offshore. LIDARs (Light detection and ranging / based on laser energy reflections from atmospheric particles) was tested offshore but isn t a standard measuring device. 4. Lidar technology In order to reduce costs associated with the siting of tall masts, the wind energy industry needs methods such as Lidar for remotely obtaining accurate wind profiles. Anemometer Wind vane Measuring Range Lidar However, widespread acceptance by the industry requires that this technique be extensively validated. Figure 4: Lidar measurement vs. classic measurement In wind energy it is common to use the 10 minute average horizontal wind speed and direction at a specific height. A conically scanning lidar estimates the horizontal wind speed and direction over a large area. At a given hub height of around 95yd the scanning circle will be up to 100 yd at a given cone angle of 30. There is a big difference in the measuring principle between the classic and the Lidar measurement. The classic measurement is a point measurement and the Lidar method uses a lot (from 4 up 50) spatial measurements with interpolation. Figure 4: Lidar

5 Lidar Figure 5: Conical scanning area Both methods work and deliver very useful data, but the Lidar method isn t accepted by all institutions. In order to improve the trust hybrid masts were erected offshore. The new met masts are equipped with classic measuring devices and Lidar devices. By adjusting the Lidar very careful in the measuring tower a high quality measurement a be achieved. 1. Model improvements In order to improve model calculations it is indispensable to employ under water or on water measuring technology. Here under water temperature measuring is used in the first place. The use of an oceanography sensor ADP or a Buoy with currents is highly reasonable for the capture of many oceanographic parameters. ADPs work by sending out beams of acoustic energy and listening for the backscattered energy along these beams from small bubbles or particulates in the water. The Doppler shift of the backscattered energy gives a measure of the fluid velocity towards or away from the ADCP. Figure 5: Integrated Lidar The oceanographic data such as the current, the significant wave height, the Wave Peak Period and the water temperature are recorded by means of an ADP (Acoustic Doppler Current Profiler) with additional wave measuring devices or by Buoy with currents. (see 3) The oceanographic measurements are to contribute in advance to the solution to various questions. By means of measurements of currents and wave parameters the scour formation can be analyzed. Moreover, highly relevant data to the foundations design can be provided. 2. Conclusion Met masts are the most reliable way to obtain unimpeachable information about the offshore Climatic conditions. Offshore wind farms with a rated power of up to 1 GW need a good data basis. This data is completed by oceanographic data. By the use of Lidar in future cost reduction can be achieved.

6 3. References (1) Jacques Beaudry Losique, A National Offshore Wind Strategy: Creating an Offshore Wind Energy Industry in the United States. February 2011 (2) J.Bendfeld; M. Splett; J.Voss; A. Higgen; J. Krieger Design of a Multi Purpose Offshore Metmast American Wind Energy Association, WINDPOWER 2007 Conference and Exhibition, Los Angeles, USA (3) Bendfeld, Cost effective wave and current measurements for ocean energy, World Renewable Energy Forum Denver, Colorado USA