Abstract
The central shaft is an inseparable part of a vertical axis wind turbine (VAWT). For small turbines such as those typically used in urban environments, the shaft could operate in the subcritical regime, resulting in large drag and considerable aerodynamic power loss. The current study aims to (i) quantify the turbine power loss due to the presence of the shaft for different shaft-to-turbine diameter ratios δ from 0 to 16%, (ii) investigate the impact of different operational and geometrical parameters on the quantified power loss and (iii) evaluate the impact of the addition of surface roughness on turbine performance improvement. Unsteady Reynolds-averaged Navier-Stokes (URANS) calculations are performed on a high-resolution computational grid. The evaluation is based on validation with wind-tunnel measurements. The results show that the power loss increases asymptotically with increasing δ due to the higher width and length of the shaft wake as the blades pass through a larger region with lower velocity in the downwind area. A maximum power loss of 5.5% compared to the hypothetical case without shaft is observed for δ=16%. The addition of surface roughness is shown to be an effective approach to shift the flow over the shaft into the critical regime, reducing the shaft drag and wake width as a result of a delay in separation. For an optimal dimensionless equivalent sand-grain roughness height of 0.08, the turbine power coefficient at δ=4% improves by 1.7%, which is equivalent to a 69% recovery of the corresponding turbine power loss. The results are found to be virtually independent of the shaft-to-turbine rotational speed ratio.
Highlights
Vertical axis wind turbines (VAWTs) have regained interest during the last decade for application as large-scale multi-MW turbines in offshore areas [1,2,3] and as small-scale turbines in urban environments [4,5]
For small turbines such as those typically used in urban environments, the shaft could operate in the subcritical regime, resulting in large drag and considerable aerodynamic power loss
For an upwind horizontal axis wind turbine (HAWT) the effect is due to the blades passing through the stagnation region in front of the tower, while for a VAWT or a downwind HAWT this is due to the blades passing through the wake of the tower
Summary
Vertical axis wind turbines (VAWTs) have regained interest during the last decade for application as large-scale multi-MW turbines in offshore areas [1,2,3] and as small-scale turbines in urban environments [4,5]. This results in a large relative distance between the tower (which can be referred to as the shaft) and the blades, in which the effect of the shaft wake on the turbine aerodynamic performance was assumed to be negligible and was not quantified in existing research [4] This is not the case for smaller urban-scale VAWTs with comparatively large shaft-to-turbine diameter ratios, as the turbine power loss due to the blades passing through the wake of the shaft could be substantial. Note that for a smooth cylinder the critical diameter-based Reynolds number corresponds to Res < 2.5 × 105 [38] This leads to a relatively large drag and massive flow separation [39], which can subsequently magnify the corresponding turbine power loss.
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