|Table of Contents
Watts Up With Torque!
Multiple Rotors: A High Efficiency Windmill Design
The overall efficiency of a windmill is the amount of electricity that can be generated over time on a cost basis. Two important factors that determine overall windmill efficiency are the ability to use low velocity wind and the ability of the windmill to convert the kinetic energy of the wind into electrical energy (conversion efficiency).
Currently, the most popular windmill design utilizes a large, single, three-blade rotor. It is obvious that "unused" wind passes between the blades of these three-blade systems. Rotors with more blades, such as those used on farms for irrigation, will turn at lower wind speeds, and have high conversion efficiency. However, these multiple blade rotor systems are subject to higher loads and unable to withstand extreme winds. My design is to utilize multiple three-bladed rotors. Multiple small rotors weigh less than a single large rotor, are easier to produce and transport, and less subject to fatigue.
The purpose of this project was to determine if multiple rotors would increase the electrical output of a horizontal axis windmill. Torque, the force created by the rotating windmill axis, was used to turn the axle of various sized DC motors and generate electricity. Three sizes of DC motors were used to assess whether the rotor combinations produced enough torque to start, and continuously turn larger motors. Electrical current in mAmps and electrical force in mVolts were measured and electrical energy in mWatts calculated.
My engineering objectives were to design and build a multiple rotor, horizontal axis, laboratory scale windmill, complete with couplers to connect the axis to the axle of various DC motors. The scale model was used to determine the effect of different rotor attributes (size, number, distance apart and orientation of the rotors) on the resulting torque and electricity generated with the various sizes of DC motors. The rotor arrangement and motor that produced the most electricity was identified.
My hypothesis was that multiple rotors, blades offset, closely spaced would produce more torque and generate the most electrical energy. The experiment was designed to measure the effect of the independent variables (rotor size, number, placement, fan speed and motor size) on the dependent variables (wind speed, RPM, mAmps and mVolts). Wind speed was measured with an anemometer, RPM with a digital tachometer and mAmps and mVolts with a digital multimeter. In all, 12 rotor variations were tested at medium and high fan speeds, using three sizes of DC motors. Each measurement of wind speed, RPM, mAmps and mVolts was repeated ten times. The RPM and wind speed measurements were used to calculate tip speed. Statistical analysis was performed to assess the quality of the data collected.
My engineering objectives were met and my hypothesis was correct. Multiple rotors produced more electricity than a single rotor. There were substantial differences in the amount of electrical energy produced depending on rotor size, number, orientation, spacing, wind speed and size of the DC motor. Adding a second rotor produced the largest increase (over 2000 %) in electricity generated. Over all, three 28-centimeter (cm) rotors, placed side by side with the blades offset and coupled to a 12 V DC motor produced the most electrical energy. This rotor combination consistently turned at the highest RPM and produced the most electrical energy output at both fan speeds and with each of the three motors. However, the addition this third rotor only increased the amount of energy produced by two rotors by an average of 29 %.
My results suggest that the overall efficiency of windmills could be substantially increased through the addition of a second rotor.