Investigation on dynamics, performance and pitch angle optimization of vertical-axis turbines
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Date
2019-06
Authors
. Lim Yew Hao
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Abstract
To meet increasing global demand for energy, renewable energy sources are endorsed as a solution for the energy crisis. Hydrokinetic energy, the eco-friendly alternative for
traditional hydropower that extract kinetic energy from water flow through hydrokinetic
turbines has potential to be the replacement for the depleting fossil fuels. Vertical-axis
hydrokinetic turbine, with its scalability and omnidirectional characteristic, is a fitting
renewable energy supply for off-grid rural areas. However, the existing issue of low
performance efficiency hinders the large-scale application of vertical-axis turbine. It is
suggested that applying pitch angle to turbine blade is a simple modification that can
improve the low performance efficiency of the vertical-axis turbine. This research aims
to investigate the effects of pitch angle to vertical-axis turbine dynamics and power
performance through numerical simulation. The problems associated with this research
are the effects of pitch angle on turbine performance and the optimum pitch design for
different operating conditions. Pitch angle effects would be investigated through analyses
of simulation results in terms of instantaneous aerodynamic loadings and average power
coefficient. Pitch angle optimizations are then performed on the same turbine model to
determine optimum pitch angles that maximize turbine performance at different tip speed
ratio. To conduct the investigation, a numerical simulation model of a Darrieus type
straight-bladed vertical-axis turbine with 3 blades is completed using MATLAB with
incorporation of NACA 0021 airfoil data. Blade Element Theory and various
assumptions that represent physical turbine operating conditions are applied to create
turbine simulation that can adequately predict turbine performance. Validation studies
are then conducted by comparing the simulated results with computational fluid
dynamics simulation and experimental data, to ensure that the simulation results are
accurate and reliable. It is found that, the simulation model is able to simulate results
with less than 10% of error at low tip speed ratio. Fixed-pitch angle and dynamic-pitch
angle optimizations are then carried out by modifying the numerical model to determine
optimum pitch angles. With the simulation results, the effects of pitch angle on the
instantaneous angle of attack, tangential force coefficient, normal force coefficient, and
power coefficient are analyzed. It is found that fixed-pitch optimization improves on
turbine performance efficiency by reducing drag-induced tangential force component in
the downstream region. Fixed pitch optimization is able to increase turbine power
coefficient by 5.24% at the tip speed ratio of 1. Dynamic-pitch optimization, however,
increases turbine power coefficients by maximizing the lift-induced tangential force
component while minimizing drag-induced component. Dynamic-pitch optimized
turbine model produces power coefficient that improves for 626.92 % compared to zero
pitch turbine model. Due to the limitations and assumptions made, the simulation model
is suitable to be applied in fundamental understanding and preliminary design of vertical
axis turbine.