Diffuser augmented wind turbine for low-speed wind

Abstract

Wind energy is the cheapest way to create additional renewable electric generating capacity. Nonetheless, it is proportional to the cubic of wind speed, which means that regions with low wind speeds, such as countries near the equator, have limited wind energy potential. Such limitation can be addressed with Diffuser Augmented Wind Turbine (DAWT), which can accelerate the flow and increase the efficiency of the wind turbine to exceed the Betz limit. However, the flow separation that occurs on the inner walls of DAWT is the main problem in degrading the DAWT performance. Furthermore, the flow is vulnerable to laminar separation at low wind speeds. Therefore, this study aimed to develop a DAWT that can operate at wind speeds lower than 5 m/s and investigate the possibility of using the number of rotor blades as a passive boundary layer control. The design of DAWT was developed by designing a bare wind turbine and a diffuser suitable for low wind speeds. The thin airfoil SD2030 was used as the blade profile of the bare turbine, while the suction surface of the same airfoil was also used as the profile of the diffuser to overcome laminar separation. In addition, the diffuser design was completed following a parameter study that evaluated flange configuration and height, diffuser cross-section profile, and diffuser opening angle. The final diffuser configuration has a flat flange with a height to rotor diameter ratio of 0.05 to minimize the total reference area of DAWT. Velocity vector analysis revealed that employing the suction surface of the airfoil SD2030 as the diffuser cross-section profile improved the flow structure by delaying stall at a wider opening angle, allowing the opening angle to be extended to 15°. After finalizing the DAWT design and its parameters, the effects of the rotor's blade numbers and solidity were evaluated by studying the 2-blade, 3-blade, and 4-blade DAWT. This research was conducted using computational fluid dynamics (CFD) simulations with Ansys CFX. Eventually, the final design was tested in a wind tunnel to validate the CFD results. The test findings show that the developed 3-blade DAWT has a low starting speed of 1m/s and effectively operates with a power coefficient of 0.599 at a low wind speed of 5m/s, which agrees with CFD results. By analyzing the velocity contours behind the rotor's plane, it was found that increasing the number of the rotor's blades increased the kinetic energy at the rotor's tips which helped energize the flow close to the diffuser’s inner walls. Consequently, this helped the boundary layer stay attached to the diffuser's walls and avoid separation. Velocity vectors from CFD results showed that the 4-blade DAWT has the flow fully attached to the diffuser at an opening angle of 20° compared to the 3-blade and 2-blade DAWT, which had the flow completely separated at the same opening angle. These results proved that the rotor's blade number can be used as a passive boundary layer control. In conclusion, a DAWT which is suitable for low wind speeds has been successfully developed, benefiting nations with low average wind speeds like Malaysia. The potential of rotor's blade number as a passive boundary layer control has also been successfully investigated, which contributed to broadening the DAWT's understanding.