Investigating wind variations within the atmospheric boundary layer : a Maltese case study using LiDAR

Abstract

Conventional wind monitoring masts, with sensors at multiple levels, have typically formed an essential part of wind measurement campaigns to gauge wind parameters and climatological behaviour at a potential wind turbine site. As bigger, megawatt-class wind turbines are being introduced into the market, monitoring masts need to be taller to reach the machines’ hub height. However, this results in costlier masts and in particular, makes offshore measurements more challenging. Therefore, remote sensing technologies, such as Light Detecting and Ranging (LiDAR), are becoming increasingly popular due to the precise nature in which they capture wind flow data at altitudes beyond those attainable with traditional monitoring masts. In July 2022 a LiDAR wind measurement system was set up atop the White Tower at l- Aħrax limits of Mellieħa, Malta. Over the course of one calendar year (12 months), spanning from 1 September 2022 to 31 August 2023, data was collected by this LiDAR unit at 10 different heights ranging from 11 m to 191 m above the unit’s window (20 m and 200 m above ground level, respectively). As a result, data collected enabled a more detailed analysis, besides that of wind speed, wind direction, standard deviation of wind speed, turbulence intensity, and temperature. The variation of wind speed shear and wind direction shear were assessed independently and in relation with other collected wind parameters on monthly and cumulative 12-month basis. The objective of this study was to assess the variation of wind within the atmospheric boundary layer at a coastal location in Malta. This study ensued by collecting raw data at the coastal location namely wind speed, wind direction, turbulence intensity, and atmospheric temperature. Whilst the raw data was assessed, their variations, namely wind speed shear and wind direction shear also underwent analysis. The horizontal wind speed at the White Tower measurement was observed to increase with elevation with an overall average of 6.33 ms-1 at 100 m. The diurnal average horizontal wind speed pattern shows that this is low during the night and higher during the day. Furthermore, the vector average wind direction varied from 280.09° (west) at 20 m to 290.73° (west-northwest) at 200 m. In addition, the vector average wind direction at 100 m at the White Tower site was 288.07° (west-northwest). The wind shear exponent yielded was 0.0468 which is significantly lower than the 1/7 (0.14) value generally adopted by the power law. Moreover, the average wind shear exponent increased in magnitude as lower heights were eliminated from the wind shear value derived using the power with values being in line with that observed in literature. Furthermore, the diurnal pattern revealed that the shear exponent decreases during the daytime and increases at nighttime. On the other hand, the variation of the average shear exponent during warm months is observed to be higher during the night whilst negative shear exponent values were noted during daytime. Monthly variations of the average wind direction shear showed no distinct pattern in the difference between the pairs of heights assessed i.e., 200 m & 20 m, 200 m & 100 m, and 100 m & 20 m respective pairs. However, the diurnal variation of the average change in wind direction fluctuates during the warm months, whereas a relatively constant average change in wind direction with minor fluctuations is observed during the cool months. Average wind direction changes are also greater during warm months than those observed during cool months. The turbulence intensity is noted to decrease with higher elevation ranging from 13.1% at 20 m to 9.2% at 200 m. Moreover, the diurnal variation in average turbulence intensity demonstrates fluctuations over the 24-hour period, with higher values during the day and lower values during the night. At the 100 m measurement height, turbulence intensity was highest at low wind speeds and decreased as wind speed increases.