Small-scale horizontal axis wind-turbines (SHAWTs) are acquiring
relevance within the regulatory policies of the wind sector aiming at
net-zero-emissions, while reducing visual and environmental impact by
means of distributed grids. SHAWTs operate transitionally, at Reynolds
numbers that fall between 10^5 < Re < 5·10^5.
Furthermore, environmental turbulence and roughness affect the energetic
outcome of the turbines. In this study, the combined effect of
turbulence and roughness is analysed via wind tunnel experiments upon a
transitionally-operating NACA0021 airfoil. The combined effects cause a
negative synergy, inducing higher drops in lift and efficiency values
than when considering the perturbing agents individually. Besides, such
losses are Reynolds-dependent, with higher numbers increasing the
difference between clean and real configurations, reaching efficiency
decrements above 60% in the worst-case scenario. Thus, these
experimental measurements are employed for obtaining the power curves
and estimating the annual energy production (AEP) of a 7.8 kW-rated
SHAWT design by means of a BEM code. The simulations show a worst-case
scenario in which the AEP reduces above 70% when compared to the
baseline configuration, with such a loss getting attenuated when a
pitch-regulated control is assumed. These results highlight the
relevance of performing tests that consider the joint effect of
turbulence and roughness.
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