Aerodynamically Interacting Vertical-Axis Wind Turbines: Performance Enhancement and Three-Dimensional Flow

Ian D Brownstein,Nathaniel J Wei,John O Dabiri

doi:10.3390/en12142724

Abstract

This study examined three-dimensional, volumetric mean velocity fields and corresponding performance measurements for an isolated vertical-axis wind turbine (VAWT) and for co- and counter-rotating pairs of VAWTs with varying incident wind direction and turbine spacings. The purpose was to identify turbine configurations and flow mechanisms that can improve the power densities of VAWT arrays in wind farms. All experiments were conducted at a Reynolds number of R e D = 7.3 × 10 4 . In the paired arrays, performance enhancement was observed for both the upstream and downstream turbines. Increases in downstream turbine performance correlate with bluff–body accelerations around the upstream turbine, which increase the incident freestream velocity on the downstream turbine in certain positions. Decreases in downstream turbine performance are determined by its position in the upstream turbine’s wake. Changes in upstream turbine performance are related to variations in the surrounding flow field due to the presence of the downstream rotor. For the most robust array configuration studied, an average 14% increase in array performance over approximately a 50° range of wind direction was observed. Additionally, three-dimensional vortex interactions behind pairs of VAWT were observed that can replenish momentum in the wake by advection rather than turbulent diffusion. These effects and their implications for wind-farm design are discussed.

Highlights

There is growing interest in the potential for vertical-axis wind turbine (VAWT) arrays to produce higher footprint energy densities than traditional horizontal-axis wind turbine (HAWT) arrays [1,2,3,4,5,6,7,8].In field demonstrations, small VAWT arrays have achieved 24 W/m2 of output at 10 m/s wind speeds [9,10], compared to 3 W/m2 measured in state-of-the-art horizontal-axis wind farms in similar conditions [11]
The normalized performances of both turbines in co-rotating and counter-rotating arrays for the smallest turbine spacing tested, s = 1.25 D, are plotted in Figure 4 versus φ∗. These results demonstrate that the performance of both turbines exhibits three distinct regimes in φ∗, based on the location of the downstream turbine relative to the wake of the upstream turbine
The results presented in the previous section are further discussed, in order to show that performance enhancements in a pair of VAWTs are due to changes in the mean flow field around both the upstream and downstream turbines

Summary

Introduction

Small VAWT arrays have achieved 24 W/m2 of output at 10 m/s wind speeds [9,10], compared to 3 W/m2 measured in state-of-the-art horizontal-axis wind farms in similar conditions [11]. This VAWT array performance was achieved without optimizing for power generation, suggesting an opportunity for even further improvement with optimization across parameters such as incident wind direction, turbine spacing, and rotational orientation. This rapid recovery has been hypothesized to be due to an induced mean vertical flow in the wake of the turbine

Objectives

Results

Discussion

Conclusion