Offshore wind profiling using light detection and ranging measurements

Abstract

The advantages and limitations of the ZephIR®, a continuous-wave, focused light detection and ranging (LiDAR) wind profiler, to observe offshore winds and turbulence characteristics were tested during a 6 month campaign at the transformer/platform of Horns Rev, the world's largest wind farm. The LiDAR system is a ground-based sensing technique which avoids the use of high and costly meteorological masts. Three different inflow conditions were selected to perform LiDAR wind profiling. Comparisons of LiDAR mean wind speeds against cup anemometers from different masts showed high correlations for the open sea sectors and good agreement with their longitudinal turbulence characteristics. Cup anemometer mean wind speed profiles were extended with LiDAR profiles up to 161 m on each inflow sector. The extension resulted in a good profile match for the three surrounding masts. These extended profiles, averaged over all observed stabilities and surface roughness lengths, were compared to the logarithmic profile. The observed deviations were relatively small. Offshore wind farm wakes were also observed from LiDAR measurements where the wind speed deficits were detected at all LiDAR heights. Profile-derived friction velocities and roughness lengths were compared to Charnock's sea roughness model. These average values were found to be close to the model, although the scatter of the individual estimations of sea roughness length was large.where ū is the mean wind speed at height z; u * is the friction velocity; k is the von Karman constant (∼0.4); z o is the aerodynamic roughness length, and y M is the universal stability function. This last term depends on both the height and the Obukhov length L.For wind resource assessments, equation (1) has been used to extrapolate wind speed measurements normally performed at low heights, e.g. 10 m. The vertical wind profi le is determined from equation (1) by estimating u * and L, e.g. from measurements of turbulence fl uxes. The surface roughness length z o can be estimated in relation to the land cover or by using roughness models for the sea state. 2 Equation (1) has been validated from experiments at heights up to 32 m (e.g. the Kansas experiment 3 ) and 50-80 m in Gryning et al. 4 Beyond these levels, deviations have been reported in different boundary layer studies. The height of the boundary layer is introduced in Gryning et al. 4 as a length scale to correct vertical wind profi les measured up to 250 m. The inversion height is estimated in Lange et al. 5 from air density differences to correct offshore wind profi les at Rødsand (Denmark). These attempts to extend vertical profi les are important for the development of the wind energy because the knowledge of the wind resource at high levels in the atmosphere is still immature.Conventional techniques (e.g. cup and sonic anemometers) have been extensively used to observe winds and turbulence. They have reached a limit in the vertical range which is similar to the current turbine's hub height. This is due mainly to the costs of er...

Hub Height Ocean Winds over the North Sea Observed by the NORSEWInD Lidar Array: Measuring Techniques, Quality Control and Data Management

“…These platforms included large gas and oil drilling rigs with tall derrick structures (Beatrice, Siri, Taqa, ORP), smaller unmanned production platforms (Jacky, Schooner, Babbage), wind farm transformer stations (Horns Rev 2 in the North Sea, Denmark) and a platform mounted meteorological mast (Fino3 in the North Sea, Germany). One lidar was deployed on the coast of the island of Utsira, see [8] for details. Common to all installations was the risk of flow distortion around the structures or influence from the surrounding landscape on the observed wind profile.…”

“…Secondly the marine atmospheric boundary layer and its temporal behavior at higher levels are poorly known. There is a need for improved parameterizations of the marine vertical wind profiles in order to improve modeling of offshore winds for wind energy [2] and to experimentally evaluate atmospheric wind resource model predictions against offshore winds [3].…”

“…The early experiments at DTU Wind Energy (formerly Risø) with a focused continuous wave (cw) Doppler wind lidar took place onshore over flat terrain at Høvsøre near the tall meteorological mast in 2003 [4]. This was followed by an experimental deployment offshore on the Fino-1 platform in 2005 [5], the Nysted 1 offshore wind farm transformer platform in 2006 [6] and the Horns Rev 1 offshore wind farm transformer platform in 2007 [2]. At all three offshore sites meteorological masts were located nearby and the concurrent meteorological observations were used for comparisons to the lidar observations.…”

“…The research objectives of the NORSEWiND project included systematic analysis of the marine wind shear observed from the lidars [8] and investigation of the flow distortion around the offshore platforms [9]. It was important to investigate the platforms' influence on the free stream wind speed profiles at hub height.…”

Wind Farm Wake: The Horns Rev Photo Case

“…Furthermore, the turbulence intensity is observed to be ~17% at 70 m, ~16% at 40 m and ~18% at 30 m which also indicates unstable conditions. The observations from 70 m are from a top-mounted cup anemometer whereas the observations from 40 m and 30 m are from boom-mounted cup anemometers at the northwest booms (see [10] for details). Thus the wind speed and turbulence intensity data may be influenced by the mast and booms.…”