Wind Turbine

Design and Construction of on-campus stall-regulated small Horizontal Axis Wind Turbine (HAWT) for low wind speeds.

With the ready availability of 22-month wind data, the target is to capture about 250-350 W of electrical power. The rotor swept area (and therefore, the blade length) is decided to maximize power capture. The available wind energy is partially converted into the rotational kinetic energy of the blades. The aerodynamic torque generated through the aerofoil-shaped blade sections gives the mechanical power to the rotor, which is to be converted into electrical power. The rotor is connected through the main shaft to the machinery, which consists of a combination of a step-up gearbox (which steps up the rotor RPM) and a 3-phase induction motor (asynchronous machine), which converts mechanical energy into electrical energy. As long as good wind speeds are available, the turbine rotates and generates electricity. The system is grid-connected (as opposed to a stand-alone system with an expensive storage battery). The turbine’s initial rotation from rest is started by the torque provided by the induction machine, with the aerodynamic torque taking over with increasing RPM and reversing the direction of energy flow, and generating electrical energy. This is possible because the induction motor, when operated above its synchronous RPM, acts as a generator. The low RPM of the turbine/ main shaft has to be stepped up to the higher RPM of the generator shaft with the planetary gearing system in the gearbox. The main shaft is supported by bearings fixed on the bed plate, and also helps in resisting the axial rotor thrust. The bed plate is supported on the tower through a slewing bearing, operated by a worm gear powered by a 2-phase stepper motor. The nacelle is free to yaw and align itself to the wind direction (which changes during the day and the year). The yawing is controlled actively through a microcontroller after receiving inputs from the nacelle-mounted anemometer. The pitch control of the blades is avoided to reduce costs and sophistication. Thus, this is a stall-regulated rotor (as opposed to pitch-regulated rotor) operating at a fixed speed (and hence various Tip-Speed-Ratios for changing wind speeds) as opposed to a variable-speed turbine (which has a fixed TSR). The supporting structure of the wind turbine is a hollow cylindrical tower, through which the electrical cables are safely fitted without exposing them to the weather. The tower is designed and reinforced at its base to withstand the horizontal wind-thrust loads on the rotor (and therefore withstand the base bending moments and stresses). The concrete foundation is designed to withstand the overturning moments for wind speeds as high as 50 m/s, and provide more than sufficient restoring moments over and above the self-weight-restoring-moment of the wind turbine. The blades are hollow and unstiffenned, and are fabricated in-house using glass-fibre reinforced plastic (GFRP), which provides a high strength-to-weight ratio. The pre-tested composite (chopped strand mats and woven roving mats bound with epoxy resins and hardeners) provide sufficient rigidity against axial/centrifugal loads, bending loads, and twisting moments due to a combination of rotational forces and wind loads. The rotor is protected against over-speeding with an automatic brake. The induction generator is also protected against over-current by circuit breakers. Because there is no decoupling between the gearbox and the main shaft, the generator shaft RPM is monitored through an incremental rotary encoder (mated to the main shaft), which signals the microcontroller to actuate the electromagnetic brake upon over-speeding. Also, when the wind speeds are very high, the active yaw system is actuated to un-align the nacelle with respect to the wind direction.

BACKGROUND

Theoretically, a maximum of 59.3% of the available wind power can be captured (Betz limit, determined from Bernoulli's principle). The best wind turbines in the world capture 40%-45% of the available wind power, using the sophisticated (and expensive) pitch-regulation system of the blades. Smaller wind turbines are stall-regulated (to be cost-effective) and have a little lower efficiency. The IIT-KGP on-campus wind turbine works at 30%-35% efficiency. Kharagpur has erratic and turbulent winds : it is not a commercially viable wind energy location. This work was done to begin wind technology research at IIT KGP. We need infrastructure first to do any research : and wind turbines suitable for academic research is not available directly in the market. We did not want to go for portable simplified miniaturized unrealistic wind turbines, to be tested inside wind tunnels, which do not replicate actual wind patterns and can give only unidirectional wind flow (hence the control systems cannot be studied at all). All big wind turbine companies have their own prototype as a test facility, where they continuously study the performance to improve their designs.