Experimental study of the effect of tower shadow on anemometer readings

Meteorological (met) tower measurements are the industry standard for wind resource assessment for wind farm siting. Towers have many potential geometries such as lattice towers but this study concentrated on solid cylindrical towers. Horizontal booms were used to hold cup anemometers at defined distances from the tower. Anemometers are subjected to winds from all directions. At times, the anemometers are located in the wake of the tower or “shadow” and therefore yield measurements which are under-representative of the true wind speed. Relying on a single anemometer at any given elevation can result in significant discrepancies between the measured and actual wind conditions. This experiment investigated the effects of tower “shadowing” on cup anemometer wind speed readings in the wake of common met tower geometries. The objective of this study was to quantify the decrease in measured wind speed with varying wind direction experimentally using full-scale wind facility testing. The results indicate a significant wind speed deficit up to 35% velocity reduction when the anemometer is located in the wake of the tower. The width of the affected area and the magnitude of the effect are functions of wind speed. Additionally, the effect is not greatest when the centreline of the anemometer is directly inline with the centreline of the tower, but rather when the anemometer is located approximately 2∘-5∘ from the centerline due to the geometry of the cup anemometer.