Dual-Rotor Wind Turbine System And Optimization Of Its Performance
Introduction
The demand for electric energy has only increased exponentially to a great extent in the society which indicates a rise in living quality of humans. Leap in electricity consumption has become such an important factor such that a country’s progress itself is determined by the same. Conventional power sources like the coal, petroleum are fast depleting and the only reliable method to secure the future of the world’s increasing power demand keeping in mind the escalating pollution levels are renewable resources. Although many types of renewable sources exist, this paper deals with one very reliable energy source, wind energy. Wind energy can be harnessed by various types of wind turbine systems, namely, horizontal axis, vertical axis turbine system integrated with generators to supply power to the power grid. Numerous methods have become available to tap the kinetic energy of the wind turbine system to electrical energy.
One of them considers increasing the wind turbine power coefficient. But we know by Betz law, a law governing wind aerodynamic theory that the power coefficient also has certain limitations. Thereby, increasing this coefficient requires special techniques. One of these techniques consists in connecting two wind rotors spinning in opposite directions, each of them driving a separate electric armature. Such a solution with two wind rotors produces mechanical power equivalent to that of a single wind turbine system with a larger diameter. Another important advantage that the double rotor solution consists is the possibility of orienting the wind turbine blades more easily according to the wind blowing direction. As the two wind turbines rotate in opposite directions, the relative speed between the two electric armatures is larger, the electric generator being able to produce a higher power at the same frame size, which implies a reduced consumption of active materials. The proposed model promises to be among the efficient and cleaner energy solutions in the wind energy sector. It may be easily and locally manufactured from blades to grid connection.
Modelling Of Dual-Rotor Wind Turbine System
The proposed wind turbine system consists of dual rotor system positioned horizontally at upwind and downwind locations. It also contains a drive train installed horizontally inside the tower as shown in the below figure 1. Two separate rotors of different dimensions are found in this rotor system. The appropriate distance between the rotors and their relative sizes are to be found to ensure peak performance. Aerodynamic efficiency, Control and safety and drive train with appropriate lubrication are the aspects to be discussed in this paper.
Safety and Control of the system
Safe working of the system is determined by various components in the Control and Safety system. Operational information is passed to the operator by the control system. The most difficult component to be designed in the entire system is the blades, and they also possess a lot of scope for development. In addition to this, blades are prone to damage and when they are damaged they tend to cause failure of other parts of the system as well. The loading and damages caused to the blades can be identified by embedding sensors into the blades thus making them smart rotor blades. A smart embedded sensor system (SESS) is used to have a check on the conditions of the wind turbine. Real-time changes are monitored by sensors incorporated in bearings, gear box, generator, relay so as to ensure maximum efficiency and to find future problems. The operator of the wind turbine makes use of Programmable logic controller (PLC) which manages the SESS to prevent any malfunction, thus ensuring efficient and safe operation. The setup below was used after discarding the previous setup for poor gear performance. This consists of a sensor which is plugged into a microphone jack to record the results and this setup has been used to measure varying degrees of vibration. A noise reduction was seen due to the reduced vibrations due to the presence of lubrication oil.
Efficiency of Energy Conversion
Through the years many enhancements have been made to improve the energy conversion efficiency of single rotor systems. Increased transmission efficiency, reduced noise, enhanced aerodynamic characteristics are some of the examples of improvements in this field in the current years. But the total efficiency of single rotor systems has not changed much despite the technological advancements. The Dual rotor system proves to solve the proposed problem. The dual rotor system consists of a similar structure to that of a single rotor system except for the fact that it has two rotors separated by a specific distance and the rotors rotate in opposite directions to each other. The aerodynamic aspect of this system proves to possess a better efficiency then the conventional system. Many studies prove that this concept can be made as a reliable efficient source of energy for the long term. Results prove that dual rotor system has the capacity to produce 43. 5% more annual energy than a single rotor turbine of same type. In case of the same output, a smaller gear ratio is needed due to higher tip speeds achieved by smaller blade length when compared with conventional systems. The main aspects of a two rotor wind turbine is the rotational speed and the distance between the two rotors.
This paper deals with dual rotor wind turbine generator system consisting of two rotors, an auxiliary rotor at the upwind location and the other relatively larger main rotor at the downwind location. The power that can be obtained from the wind depends on the area of the turbine that is in contact with the air and the height at which the rotor can be placed. But the swept area of the wind turbine is directly proportional to mass of the rotor which in turn leads to higher maintenance costs during high wind occasions. Hence an optimum size of the rotor set to ensure a balance between being economical and efficient. When the torques produced by the two rotors counterbalance each other, there is increased stress on the tower that leads to bending. But the system discussed here makes sure bending stress is reduced and efficiency is enhanced. The following equation represents the dynamics of a single system, JΩ = Ta – Tl – Td (1) Where J is the equivalent moment of inertia, Ω is the rotational speed of either rotor or shaft, or generator depending on the approach of the modelling and, Ta, Tl and Td represent the input torque from the wind, the load torque for power production and the disturbance torque due to the internal friction etc. , respectively. With the turbine type, Tl and Td will vary. Ta is given by: Where Cp represents the power coefficient, and is a function of the tip speed ratio and the blade pitch angle, R and S represent the rotor blade radius and the swept area by the rotor blade, respectively; vw is the wind speed; and is the sir density at the hub height. Also, Cp is computed before-hand based on the theoretically expected performance of the turbine system.
Power extracted from the air stream can be given as: Where is the air density, A is the disk area and V, Vo and V1 are the flow velocity components along the axis of the stream tube. Power coefficient can be found by: Where a represents the axial induction factor. Generator and its importanceTwo main types of generator exist, variable and fixed. In fixed speed generators, their speed dictated by the frequency of operation and the number of magnetic poles. Hence, they are not ideal for wind systems because wind speed is variable. Generators like DC generators and induction generators (squirrel-cage or doubly-fed wound rotor) are variable speed generators. The induction generator is most widely used in applications that supply power directly to the grid. Most stand-alone systems make use of Brushless DC machines because it can be operated without any external power supply. Since these are permanent magnet synchronous generators they can’t be extended to large-scale power because they involve the use of big and heavy permanent magnets. Mostly wind turbine make use of six-pole induction generators, while others use directly driven synchronous generators. Induction generators are better than synchronous generators for wind turbine systems.
The factors that make them better are brushless and rugged construction, low cost, maintenance and operational simplicity, self-protection against faults, good dynamic response, and capability to generate power at varying speed. An AC source is required by the induction generator to produce the stator magnetic field that is required for generation of power. Though power factors are connected by exterior capacitor banks, the harmful effects of voltage harmonics still persist, hence utility companies don’t allow induction generators to interconnect them to their systems. An upgraded form of induction generator called TRIAS generator is used in this project. This generator has a unique symmetry of winding technique and it is placed in the dual-rotor turbine system. Very poor power factor and high total harmonic distortion (THD) are the main drawbacks of a typical induction generator. The TRIAS generator possesses characteristics of both, the induction and the synchronous generators as it has the capacity to produce its own excitation [11]. Since the TRIAS generator has two stator windings per phase, it can ensure maximum energy transfer and efficiency. Based on testing (Fig. 5) we can say that the generator provides higher operating performance in terms of signal distortion and harmonics, resistive losses, overheating, failure of capacitor banks, and power factor. Fig. 6 compares the power factor for both generators. At full load, the generator provides a power factor of almost 0. 99. Fig. 7 compares the effect of voltage imbalance on losses in both the types of machines. Losses have dropped by almost 27% using the TRIAS generator.
Integration
A prototype is used to scale the performance of the proposed dual-rotor wind turbine system. A data acquisition system has been built based on LabView (Fig. 9 and Fig. 10) in order to determine the optimal settings of the turbine design parameters such as diameter, rotational speed, and the clearance between the two rotors. The results shown (Fig. 11), indicate that the built model produces 60% more electrical energy than the conventional single rotor wind turbine system of the same size.
Assuming that: the incoming air approaching the wind turbine is almost uniform so that the wind tunnel test data obtained for the wake characteristic near the rotor can be used and there is no aerodynamic interference effect between the main rotor and the auxiliary rotor, the test has been performed. Parameters such as distance between the two rotors and rotational speed should be studied to determine the most efficient setting of the turbine. The information about the relative size and the optimum placement of the auxiliary rotor and the main rotor in the system will be provided by the ultimate test results of the wind turbine.
Conclusion And Future Direction
In this paper, a scaled down version of the dual-rotor wind turbine system was built. Though this miniature version proves to produce 60% more power than a single rotor wind turbine system, our literature denotes that the dual rotor wind turbine system is capable of producing 20-30% more power than the former when scaled to its actual size. The new “TRIAS generator” has also been implemented with higher efficiency, power quality and performance. In the future, the experiments will be extended to conduct field test measurements to correlate the analysis results and to study the overall performance of the proposed system to the single rotor system. A 50 and 100 kW system will be built if the tests prove to be positive. Following this the successful model will be integrated with the power grid and studied on how the generator and the entire system reacts to the same. In due course of time, the model will be converted to a product with a recognized license and given to the public.
