Abstract
The current study systematically analyzes the impact of solidity (σ) and number of blades (n) on the aerodynamic performance of 2-, 3- and 4-bladed Darrieus H-type vertical axis wind turbines (VAWTs). Solidity varies within the wide range of 0.09–0.36. A large number of operational parameters, i.e., tip speed ratio (λ), Reynolds number (Re), turbulence intensity and reduced frequency (K) are investigated to provide a deeper insight into the impact of σ and n on the dynamic loads on blades, the turbine performance and the wake. High-fidelity unsteady Reynolds-averaged Navier-Stokes (URANS) simulations, extensively validated with experiments, are employed. The results show that the turbine optimal tip speed ratio (λopt) is invariant to a newly-introduced parameter ‘σλ3’, regardless of the turbine geometrical and operational characteristics. In addition, a new correlation is derived to estimate λopt as a function of σ, which can also be employed to predict the optimal σ for a turbine with a given λ. It is also found that: (i) for constant-speed urban VAWTs, which due to the low mean wind speed in the urban environment, frequently operate at moderate to high λ, a relatively-low σ is optimal; (ii) an optimal VAWT is a moderately-high-solidity variable-speed rotor maintaining a relatively-low λ, where due to the large blade chord length the resulting Re and K are favorably high; (iii) within the turbine optimal operational range, turbine power coefficient (CP) is almost independent of n. The present findings support the optimal aerodynamic design of small-to large-scale VAWTs.
Highlights
Vertical axis wind turbines (VAWTs) have recently received renewed interest for wind energy harvesting in two new potential locations, i.e., far offshore and in urban and rural environments [1e6]
VAWTs have several advantages compared to horizontal axis wind turbines (HAWTs): omni-directionality, low noise, simple design, low manufacturing, installation and maintenance costs, scalability, compactness, small shadow flickering, less visual aesthetic disturbance and higher bird and bat safety [19e24]
Their aerodynamic performance is currently lower than HAWTs due to the comparatively small
Summary
Vertical axis wind turbines (VAWTs) have recently received renewed interest for wind energy harvesting in two new potential locations, i.e., far offshore and in urban and rural environments [1e6]. The complex aerodynamic performance of VAWTs is influenced by flow phenomena such as unsteady separation and dynamic stall [32e40], blade-wake interactions [41], flow curvature effects [42,43] and rotational and Coriolis effects on boundary layer and shed vortices [44]. (v) A new correlation is derived to estimate lopt only as a function of s, which can be helpful during the initial design phase of VAWTs; (vi) Detailed analyses are performed at constant Rec to better clarify the impact of solidity and number of blades, unaffected by any potential Reynolds number effects; (vii) In addition to the turbine averaged performance, the turbine wake and dynamic loads on the blades are investigated for various solidities and number of blades; (viii) Reduced frequency is shown to be an influential parameter on the aerodynamic performance of VAWTs. The evaluation is based on high-fidelity CFD simulations, extensively validated with experiments.
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