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[MAG32-03] The effects of distance from the reference met mast on the verification of floating LiDAR systems
Keywords:Offshore Wind Energy, Wind Condition, Floating LiDAR System, Mutsu-Ogawara Offshore Wind Observation Test Site
IEA Wind recommends that the accuracy of floating LiDAR systems (FLSs) be verified within a radius of 500 m from the reference met mast. As part of the NEDO project “Establishment of Offshore Wind Resource Assessment Methods”, the accuracy verification campaign of FLS was carried out for one year from November 2020 to November 2021 at Mutsu Ogawara, Aomori Prefecture (currently the Mutsu-Ogawara Offshore Wind Observation Test Site; MTS), and FLSs were verified within a radius of 500 m of the offshore met mast. The four FLSs used were Fugro SEAWATCH (vertical LiDAR (VL): ZX300M), AXYS WindSentinel (VL: ZX300M and Windcube v2.0), and Nagasaki Prefecture 5 Companies (now MIA) MIA (VL: DIABREZZA).
As a result of the one-year accuracy verification, it was found that the difference in wind speed between the cup anemometer on the met mast and the FLSs becomes larger when the wind blows from the land (called “land wind”) than when the wind blows from the sea (called “sea wind”). In general, the uniformity of the wind in space is higher for the sea wind than for the land wind, so the above fact suggests that the distance from the reference met mast to the FLS installation position affects the accuracy verification results. Thus, in this study, we examined the possibility of applying the analysis of spatial wind speed distribution based on dual scanning LiDAR (DSL) observation (wind vector measurements) to the accuracy verification of FLS, with the aim of improving the quality of the FLS accuracy verification at MTS.
The DSL observation was conducted by placing two Windcube 100S and Windcube 200S units at two land sites, and was conducted during the last three and a half months of the one-year period in which the FLSs were verified. In order to compare wind speeds at 63 m above the four locations (i.e., met mast, Fugro, AXYS and MIA), DSL observations were carried out at a cycle of 1 sec per point and 9.3 sec per cycle. As a result, it was found that at the MIA location, which is about 480 m offshore from the mast, land winds on average have wind speeds that are around 3 % higher than at the met mast. In addition, it was found that this increase in wind speed was also seen for sea winds, by around 1 %, which suggests a convergence effect of winds approaching the land near the coastline.
In order to take the difference in wind speed between the met mast and the FLS locations into account, an empirical correction formula based on the DSL observation was created and applied to accuracy verification of FLS. As a result, it was found that the degree of agreement between the wind speed of FLSs and the met mast was clearly improved, compared to that before correction. In addition, the results of comparing with the fixed VL (Windcube v2.1), installed next to the met mast, at multiple altitudes (63m, 120m, 180m) showed that the pulse-wave LiDAR (AXYS-Windcube and MIA-DIABREZZA), had higher observation accuracy at higher altitudes. This means that the effect of the higher uniformity of wind speed at higher altitudes overcomes the effect of the lower observation accuracy caused by the larger inspection volume of VL at higher altitudes.
From the above, it was shown that the quality of the accuracy verification of FLS at MTS can be improved by using DSL observations in combination, and that the degree of agreement between the wind speeds of FLS and the met mast is better at higher altitudes (e.g., 120-150m, which is equivalent to the hub height of a wind turbine) than at the met mast observation altitude (63m). From now on, we plan to continue investigating methods for correcting the differences in standard deviation of wind speed (turbulence intensity) at the positions of the met mast and FLS, as well as wind speed.
Acknowledgements:
This paper is based on results obtained from project JPNP07015 commissioned by the New Energy and Industrial Technology Development Organization (NEDO).
As a result of the one-year accuracy verification, it was found that the difference in wind speed between the cup anemometer on the met mast and the FLSs becomes larger when the wind blows from the land (called “land wind”) than when the wind blows from the sea (called “sea wind”). In general, the uniformity of the wind in space is higher for the sea wind than for the land wind, so the above fact suggests that the distance from the reference met mast to the FLS installation position affects the accuracy verification results. Thus, in this study, we examined the possibility of applying the analysis of spatial wind speed distribution based on dual scanning LiDAR (DSL) observation (wind vector measurements) to the accuracy verification of FLS, with the aim of improving the quality of the FLS accuracy verification at MTS.
The DSL observation was conducted by placing two Windcube 100S and Windcube 200S units at two land sites, and was conducted during the last three and a half months of the one-year period in which the FLSs were verified. In order to compare wind speeds at 63 m above the four locations (i.e., met mast, Fugro, AXYS and MIA), DSL observations were carried out at a cycle of 1 sec per point and 9.3 sec per cycle. As a result, it was found that at the MIA location, which is about 480 m offshore from the mast, land winds on average have wind speeds that are around 3 % higher than at the met mast. In addition, it was found that this increase in wind speed was also seen for sea winds, by around 1 %, which suggests a convergence effect of winds approaching the land near the coastline.
In order to take the difference in wind speed between the met mast and the FLS locations into account, an empirical correction formula based on the DSL observation was created and applied to accuracy verification of FLS. As a result, it was found that the degree of agreement between the wind speed of FLSs and the met mast was clearly improved, compared to that before correction. In addition, the results of comparing with the fixed VL (Windcube v2.1), installed next to the met mast, at multiple altitudes (63m, 120m, 180m) showed that the pulse-wave LiDAR (AXYS-Windcube and MIA-DIABREZZA), had higher observation accuracy at higher altitudes. This means that the effect of the higher uniformity of wind speed at higher altitudes overcomes the effect of the lower observation accuracy caused by the larger inspection volume of VL at higher altitudes.
From the above, it was shown that the quality of the accuracy verification of FLS at MTS can be improved by using DSL observations in combination, and that the degree of agreement between the wind speeds of FLS and the met mast is better at higher altitudes (e.g., 120-150m, which is equivalent to the hub height of a wind turbine) than at the met mast observation altitude (63m). From now on, we plan to continue investigating methods for correcting the differences in standard deviation of wind speed (turbulence intensity) at the positions of the met mast and FLS, as well as wind speed.
Acknowledgements:
This paper is based on results obtained from project JPNP07015 commissioned by the New Energy and Industrial Technology Development Organization (NEDO).