Type of Wind Turbine Horizontal Axis Wind Turbine

7

2.2. Basic Theory

2.2.1. Type of Wind Turbine

Wind energy basically is available because of the kinetic energy of large masses of air moving over earth. Blades of the wind turbine receive this kinetic energy, which is transformed into mechanical or electrical forms. The efficiency of wind convert to other energy forms greatly depends on the efficiency with which the rotor interacts with the wind stream Mathew 2006. There are various machines and designs that were developed since the wind energy conversion introduced. Wind turbines are categorized in several ways. According to the aerodynamic force characteristic, wind turbines are classified into drag- based wind turbine concept and lift-based wind turbine concept fig. 2.1. And based on the 6. Damak et al Damak, Driss et al. 2013 Proposing twisted- helical Savonius wind turbine to reach better performance. Helical one met better performance than conventional one. 7. Nasef et al Nasef, El- Askary et al. 2013 Determining the influence of air flow around stationery and rotating Savonius rotor toward its performance. The best performance reached at β = 0.15. 8. Kumbernuss et al Kumbernus s, Chen et al. 2012 Determining the effect of overlap ratio and shift angles on Savonius rotor performance. Higher overlap ratios give higher impact than shift angle changes. 9. Jeon et al Jeon, Jeong et al. 2014 The influence of end plates design to improve a 180° twist angle Savonius rotor. Cp increased up to 36 compared without end plates. commit to user 8 axis rotation, wind turbine is classified into horizontal axis wind turbine and vertical axis wind turbine. Figure 2.1 Aaerodynamic forces of Wind Turbine Hau 2006

2.2.2. Horizontal Axis Wind Turbine

Horizontal axis wind turbines HAWT have rotation axis horizontal to the ground and parallel to the wind stream Fig. 2.2. Most of the commercial wind turbines are based on this category. Horizontal axis machines have some advantages such as low cut-in wind speed and easy furling. In general, they show relatively high power coefficient. The generator and gearbox of turbines are placed over the tower. Which it makes the design is more complex and expensive. Another disadvantage is the need for the tail or yaw drive to orient the turbine towards wind. Figure 2.2 Horizontal axis wind turbine perpustakaan.uns.ac.id commit to user 9 Based on the number of blades, horizontal axis wind turbines are classified as single bladed, two bladed, three bladed and multi bladed. The ratio between the actual blade areas to the swept area of a rotor is termed as the solidity. Multi bladed rotors are also called high solidity rotors. These rotors can start easily as more rotor area interacts with the wind initially. Some low solidity designs may require external starting. Based on the direction of receiving the wind, HAWT can be classified as upwind and down wind turbines. Upwind turbine‟s rotors face the wind directly. On the other hand, downwind machines are more flexible and may not require a yaw mechanism.

2.2.3. Vertical Axis Wind Turbine