Abstract
Abstract. Conventional monostatic wind lidar (light detection and ranging) systems are well-established wind speed remote sensing devices in the field of wind energy that provide reliable measurement results for flat terrain and homogeneous wind fields. These conventional wind lidar systems use a common transmitting and receiving unit and become unacceptably inaccurate as the wind fields become increasingly inhomogeneous due to their spatial and temporal averaging procedure (large measurement volume) that is inherent to the monostatic measurement principle. The new three-component fiber laser-based wind lidar sensor developed by the Physikalisch-Technische Bundesanstalt (PTB) uses one transmitting unit (fiber laser) and three receiving units to measure the velocity vector of single aerosols in a spatially highly resolved measurement volume (with diameter d and length l) in heights from 5 m (d=300 µm, l=2 mm) to 250 m (d=14 mm, l=4 m) with a resolution of about 0.1 m s−1. Detailed comparison measurements with a 135 m high wind met mast and a conventional lidar system have proven that the high spatial and temporal resolution of the new, so-called bistatic lidar leads to a reduced measurement uncertainty compared to conventional lidar systems. Furthermore, the comparison demonstrates that the deviation between the bistatic lidar and the wind met mast lies well within the measurement uncertainty of the cup anemometers of the wind met mast for both homogeneous and inhomogeneous wind fields. At PTB, the aim is to use the bistatic wind lidar as a traceable reference standard to calibrate other remote sensing devices, necessitating an in-depth validation of the bistatic lidar system and its measurement uncertainty. To this end, a new, specially designed wind tunnel with a laser Doppler anemometer (LDA) as flow velocity reference has been erected on a platform at a height of 8 m; this allows the new wind lidar to be positioned below the wind tunnel test section to be validated for wind vector measurements that are traceable to the SI units. A first validation measurement within the wind tunnel test section is presented, showing a deviation between the bistatic lidar system and the LDA clearly below 0.1 %.
Highlights
The accurate determination of the wind velocity vector is a key factor in the field of wind energy
A new, specially designed wind tunnel with a laser Doppler anemometer (LDA) as flow velocity reference has been erected on a platform at a height of 8 m; this allows the new wind lidar to be positioned below the wind tunnel test section to be validated for wind vector measurements that are traceable to the SI units
As the power derived from the wind is proportional to the cube of the wind velocity, the measurement uncertainty of the wind speed measuring device used for the potential analysis strongly influences the reliability of the power output forecast of projected wind farms
Summary
The accurate determination of the wind velocity vector is a key factor in the field of wind energy. These methods are very costly and time consuming (Bradley et al, 2015) Another approach to determine the wind velocity in complex terrain with high precision using optical remote sensing is to combine multiple monostatic lidar systems that are focused to one point (Pauscher et al, 2016; Sørensen et al, 2018). With this multi-lidar technique higher accuracy can potentially be achieved, but the measurement setup is spacious, and the superposition of multiple lidar beams in one point is intricate. To ensure a mobile operation with stable working conditions in the field, especially with respect to requirements on the mechanical setup and the optoelectronics, the bistatic lidar system has been enclosed in a temperature-controlled housing unit mounted on a trailer (see Figs. 4 and 9)
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