A Comprehensive Comparative Investigation of the Lifting Line Theory and Blade Element Momentum Theory Applied to Small Wind Turbines

Abstract

Small wind turbines (WTs) are adequate for electricity generation in isolated areas to promote local expansion of commercial activities and social inclusion. Blade element momentum (BEM) method is usually used for performance prediction, but it generally produces overestimated predictions since the wake effects are not precisely accounted for. Lifting line theory (LLT) can represent the blade and wake effects more precisely. In the present investigation, the two methods are analyzed and their predictions of the aerodynamic performance of small WTs are compared. Conducted simulations showed a computational time of about 149.32 s for the Gottingen GO 398 based rotor simulated by the BEM and 1007.7 s for simulation by the LLT. The analysis of the power coefficient showed a maximum difference between the predictions of the two methods of about 4.4% in the case of Gottingen GO 398 airfoil-based rotor and 6.3% for simulations of the Joukowski J 0021 airfoil. In the case of the annual energy production, a difference of 2.35% is found between the predictions of the two methods. The effects of the blade geometrical variants such as twist angle and chord distributions increase the numerical deviations between the two methods due to the big number of iterations in the case of LLT. The cases analyzed showed deviations between 3.4% and 4.1%. As a whole, the results showed good performance of both methods

however, the LLT provides more precise results and more information on the local flow over the rotor blades.