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 Navier- Stokes 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.
Highlights
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
The results presented here focus on a counter-rotating design
Parametric sweeps using Reynolds Averaged Navier Stokes (RANS) simulations show that secondary rotor turbine size should be 25% of the main rotor and it should be axially separated from the main rotor by a distance of 0.2 times the main rotor radius
Summary
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 referred to as the Betz limit.
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