Abstract
Abstract. The aerodynamic design of a ducted wind turbine for maximum total power coefficient was studied numerically using the axisymmetric Reynolds-averaged Navier–Stokes equations and an actuator disc model. The total power coefficient characterizes the rotor power per total device area rather than the rotor area. This is a useful metric to compare the performance of a ducted wind turbine with an open rotor and can be an important design objective in certain applications. The design variables included the duct length, the rotor thrust coefficient, the angle of attack of the duct cross section, the rotor gap, and the axial location of the rotor. The results indicated that there exists an upper limit for the total power coefficient of ducted wind turbines. Using an Eppler E423 airfoil as the duct cross section, an optimal total power coefficient of 0.70 was achieved at a duct length of about 15 % of the rotor diameter. The optimal thrust coefficient was approximately 0.9, independent of the duct length and in agreement with the axial momentum analysis. Similarly independent of duct length, the optimal normal rotor gap was found to be approximately the duct boundary layer thickness at the rotor. The optimal axial position of the rotor was near the rear of the duct but moved upstream with increasing duct length, while the optimal angle of attack of the duct cross section decreased.
Highlights
The power output of a wind turbine can be augmented by surrounding it with a duct, typically referred to as a ducted wind turbine (DWT), a diffuser augmented wind turbine, or a shrouded wind turbine
This study investigates the effect of the duct length on the optimal design for maximizing the total power coefficient (CP,total) of a DWT having the Eppler E423 airfoil as the cross section
The results show that there is an upper limit to CP,total for a DWT, which is similar to the Betz–Joukowsky limit for open rotors
Summary
The power output of a wind turbine can be augmented by surrounding it with a duct, typically referred to as a ducted wind turbine (DWT), a diffuser augmented wind turbine, or a shrouded wind turbine. For a given rotor area, significantly more power can be obtained for a DWT compared to an open wind turbine. By adding a duct, the total area of the device facing the wind direction is increased. If the power produced per total projected frontal area of the device is calculated for DWTs, often values closer to that of open wind turbines are found (van Bussel, 2007). Achieving values of CP,total greater than the Betz– Joukowsky limit (Okulov and van Kuik, 2012) of 0.593 for a DWT is significant as it means a DWT can capture more power per unit area of the device than an open rotor turbine. CP,total is a useful metric to compare DWTs with open rotor wind turbines but could be an important design objective in certain problems like fully integrated DWTs for sustainable buildings (Ishugah et al, 2014; Agha and Chaudhry, 2017) or other applications where a designer seeks to maximize the power output from the limited space allocated to a wind turbine
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