Abstract
This study presents the impact of seasonal variation in air density on the operating tip-speed ratio of small wind turbines. The air density, which varies depending on the temperature, atmospheric pressure, and relative humidity, has an annual amplitude of about 5% in Tokyo, Japan. This study quantified this impact using the rotational speed equation of motion in a small wind turbine informed by previous work. This governing equation has been simplified by expanding the aerodynamic torque coefficient profile for a wind turbine rotor to the tip-speed ratio. Furthermore, this governing equation is simplified by using nondimensional forms of the air density, inflow wind velocity, and rotational speed with their characteristic values. In this study, the generator’s load is set to be constant based on a previous analysis of a small wind turbine. By considering the equilibrium between the aerodynamic torque and the load torque of the governing equation at the optimum tip-speed ratio, the impact of the variation in the air density on the operating tip-speed ratio was expressed using a simple mathematical form. As shown in this derived form, the operating tip-speed ratio was found to be less sensitive to a variation in air density than that in inflow wind velocity.
Highlights
Wind energy is one of the promising renewable energy sources; it is essential to obtain knowledge critical to enhancing the operation and production of wind energy
The approximate expression of the aerodynamic torque coefficient is used in this study based on the non-dimensional inflow wind velocity and rotational speed
The purpose of this study was to investigate the effect of seasonal air density changes on the driving tip-speed ratio of small wind turbines quantitatively
Summary
Wind energy is one of the promising renewable energy sources; it is essential to obtain knowledge critical to enhancing the operation and production of wind energy. There are two main types of wind turbines used for wind energy production: a large wind turbine and a small wind turbine [1]. The characteristics of wind energy using small wind turbines are different from those of large wind turbines. Pitch angle control, often used in large wind turbines, is not often used in small wind turbines [2,3,4]. There are various types of small wind turbines, such as a horizontal axis wind turbine and a vertical axis wind turbine [1]. This study considers that obtaining a universal result is challenging in wind energy using small wind turbines. The density of the air flowing into the actuator disc of a small wind turbine may not be constant throughout the year
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