Global Blockage Effects in Wind Farms

Abstract

An experimental study of wind farm blockage has been performed to quantify the velocity decrease that the first row of a wind farm experiences due to the presence of the other turbines downstream. The general perception has been that turbines downstream of the first row are only influenced by the wakes from upstream turbines without any upstream effect. In the present study, an attempt is made to demonstrate the existence of a two‐way coupling between individual turbines and turbines in the wind farm. Several staggered layouts were tested in the wind tunnel experiments by changing the spacing between rows, spacing between turbines in the rows, and the amount of wind turbines involved. The experiments focused on turbines located in the center of the first row as well as the two turbines located in the row edges, usually believed to experience a speedup. The present results show that no speedup is present and that all the turbines in the first row are subjected to a reduced wind speed. This phenomenon has been considered to be due to “global blockage.” An empirical correlation formula between spacing, number of rows, and velocity decrease is proposed to quantify such effect for the center turbine as well as for the turbines at the edges.

A Study of Blockage Effects at the Wind Turbine and Wind Farm Scales

“…These effects induce a power decrease and a stronger induction zone for the first turbine. This result is consistent with the experimental observations done by Segalini and Dahlberg [12], which indicates that the velocity in the first row of a wind farm depends on rows behind.…”

“…The impact of this effect increases from edge to center, in concordance with the induction zone of the second row-see Figure 19. Effects of the second row on the first row have been observed experimentally by Segalini and Dahlberg [12]. They reported a velocity decrease in the first row of the wind farm due to the presence of rows behind.…”

“…The work done by Ebenhoch et al [11] and Segalini and Dahlberg [12] indicate that blockage is a complex phenomenon that can affect wind farm energy production in positive and negative ways. Segalini and Dahlberg [12] showed experimentally that the velocity in the front row depends on the layout: the number of rows and spacing between the first row and the rest of the wind farm. They proved that the first line is not affected only by the flow perpendicular to it, but also by the presence of the downstream turbine.…”

Boundary-layer evolution over long wind farms

“…Up to 152 turbines were involved in the present measurement campaign. These were freely rotating two-blade plastic rotors with diameter D = 45 mm, similar to what was already used by Ebenhoch et al (2017) and Segalini & Dahlberg (2020). The hub (located at z hub = 60 mm = 1.33D from the ground) consisted of a tube of 3 mm diameter with friction bearings holding the rotor, while a tower with the same diameter was used to support nacelle and rotor.…”

Modelling wakes and blockage i large-scale wind farms

“…Bleeg [104] suggested employing RANS models in combination with a graph neural network approach to model turbine interaction losses and blockage because traditional wake models tend to ignore blockage effects. Segalini et al [90,105] studied wind farm scale blockage in wind tunnel experiments and showed that the front row of a wind farm is negatively impacted by the presence of downstream turbines such that…”