|M.Sc Student||Shkolnik Rami|
|Subject||Shrouded Turbine Control in "Energy Towers"|
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Raphael Linker|
|Dr. Rami Guetta|
“Energy Tower” is a technology for producing clean electricity by means of an "artificial wind" generated by spraying water droplets at the top of a very large tower in arid and warm environment. Shrouded turbines located at the bottom of the tower transform the airflow into electricity. Previous studies have shown that the performance of the whole system depends strongly on the control of those turbines. In order to study the operation of such a shrouded turbine, a 1:50 scale model (580mm diameter) was built and placed in a 6.7 meter long shroud (600mm diameter) equipped with a strong variable-speed bellows that produced the airflow. The turbine was connected to a DC generator by means of a planetary transmission. The excitation current If in the generator was created by a 12[V] battery connected to a pulse width modulation (PWM) card, and could be varied continuously. The electricity generated was dissipated in a 1[W] resistor. The system was connected to a personal computer that recorded the current produced by the generator, together with the wind velocity pressure, the turbine rotation speed and the excitation current, and determined the desired excitation current (PWM duty cycle).
Experiments consisting of step changes in both the excitation current and bellows velocity were conducted. The data was used to determine the optimal turbine speed and excitation current for each airflow velocity (bellows setting), and to calibrate simplified (black-box) dynamic models of the system. Two first-order transfer functions with uncertain parameters were calibrated and validated, the first one from the bellows control signal to the turbine rotation speed, and the second one from the excitation current to the turbine rotation speed. Thereafter, a robust two-degree-of-freedom feedback control loop was designed for controlling the turbine velocity via the generator excitation current. The design was conducted according to the quantitative feedback theory (QFT) approach, and resulted in a proportional-integral (PI) and lead feedback controller, and a second-order pre-filter. After validation of the design by computer simulation, the control loop was implemented in the experimental system.
The controlled system was tested for various bellows settings and turbine rotation speeds. The controlled system was found to comply with the design specifications. Simpler control schemes (e.g. open-loop) were also tested and the average electric power generated by the velocity closed-loop scheme was found to be about 2% to 8% higher than for all the other schemes.