A Novel Dual-Rotor Turbine for Increased Wind Energy Capture

Abstract. Horizontal axis wind turbines suffer from aerodynamic inefficiencies in the blade root region (near the hub) due to several non-aerodynamic constraints. Aerodynamic interactions between turbines in a wind farm also lead to significant loss of wind farm efficiency. A new dual-rotor wind turbine (DRWT) concept is proposed that aims at mitigating these two losses. A DRWT is designed that uses an existing turbine rotor for the main rotor, while the secondary rotor is designed using a high lift-to-drag ratio airfoil. Reynolds Averaged NavierStokes computational fluid dynamics simulations are used to optimize the design. Large eddy simulations confirm the increase energy capture potential of the DRWT. Wake comparisons however do not show enhanced entrainment of axial momentum. Introduction A single-rotor horizontal axis wind turbine (HAWT) can capture a maximum of 59.3% of the flow energy passing through the turbine rotor disk. This remarkable result can be derived by applying mass, momentum, and energy conservations laws across a rotor disk assuming the flow to be one-dimensional, steady, and incompressible. This limit was found at around the same time by Albert Betz, Frederick Lanchester, and Nikolay Zhukovsky, but is now referred to simply as the Betz limit. It should be emphasized that this limit assumes the turbine to have a single, un-ducted rotor. On one hand, this limit can be easily surpassed if multiple rotors, or ducted rotors are employed. On the other hand, the Betz limit of achieving 59.3% aerodynamic efficiency is not realizable by any real single-rotor, finite-bladed, un-ducted HAWT. Physical laws and other constraints limit the efficiency to be no greater than around 53% (see Maximum C P of modern utility-scale HAWT's is only a few percentage points short of this practical limit. A good portion of this shortfall is due to aerodynamic losses in blade root region. Rotor blades in the root region (approximately bottom 25% of a rotor) is designed to withstand blade root bending moments. For structural integrity, relatively thick (high thickness-to-chord ratio) airfoils are chosen in the root region, which are aerodynamically poor. Flow separation in the root region causes span-wise (cross) flow, which deteriorates the aerodynamic performance of the outboard blade sections as well. Blade thickness, twist, and chord in the root region ar