· Chapter 3 details the procedure undertaken to design a permanent magnet synchronous generator for Ga-Rampuru village wind turbine.
· Chapter 4, the generator geometry discussed in chapter 3 is modelled in FEMN using recyclable and commercial magnets to analyse and estimate both machine designs.
· Chapter 5 discusses the results found in chapter 4.
· Chapter 6 details all the steps that were taken in an attempt to assemble a prototype of the wind generator.
· Chapter 7 & 8 concludes the discussion based on the analyses and finally presents recommendations.
Chapter 2. Design of the wind turbine prototype
2.1 Background on wind energy
Wind powered systems have been widely used since the tenth century for water pumping, grinding grain and other low power applications [9]. Since then, this has lead to an investigation and attempt to build large wind energy systems to generate electricity.
Wind energy has proven to be cost effective and reliable in the past years. The main development of this technology has been with large wind turbines in the industrialized world, but there is scope to deliver decentralized energy service in the rural areas of developing countries [6].
Furthermore, wind energy is an attractive option to generate electricity since it does not consume fossil fuels nor emit greenhouse gases. The land on which the wind generators are build may also be used for agricultural purposes such as ploughing the land or domestic animal gazing.
During its transition from the earlier day’s wind ‘mills’ to the modern electric generators, the wind energy conversion systems (WECS) have transformed to various sizes, shapes and designs, to suit the applications for which they are intended for [5]. In this chapter, the main components of a simple small wind generator will be investigated and a wind generator suitable for Ga-Rampuru village will be designed using recyclable materials found in the area.
The available wind resource is governed by the climatology of the region concerned and has a large variability from one location to the other and also from season to season at any fixed location [9]. Hence, it is important that the wind generator is designed for a specific area; this will ensure that the wind energy in that specific area is exploited to generate maximum power from the wind.
2.2 Wind turbine basic principles
The wind generators are specially designed and build to extract power from turning blades with the maximum efficiency and minimum complexity [6]. The magnet rotor disk rotates as a result of the force of the wind on the turbine’s blades.
A typical small wind generator consisting of blades, tower, PM generator and the cabling is illustrated in figure 2.1. The main components, which are common to most wind generators, will be discussed below.
Fig 2.1 Basic features of a typical small wind generator [6]
2.2.1 The blades
Modern wind turbine rotors usually have two or three wooden blades. A larger number of blades would create more turning force (torque), but would not be capable of driving the PM generator fast enough to generate the required voltage, because the rotor would turn more slowly [6]. The rotor blades are designed in such a way that they extract the maximum power from the wind.
Power supplied by the blades to the generator is [7]:
(Eq 2.1)
whereis the air density (Kg/m3), C is the dimensionless power coefficient and A the area swept by the blades in m3.
In equation 2.1 above, the power drawn from the wind is proportional to the cube of the wind speed. This means that if the wind speed doubles, there is 8 times as much power available from it [7].
A further important parameter is the tip speed ratio. The tip speed ratio is defined as the ratio of the tip of the blade to that of the undisturbed wind velocity entering the blades [11]. The ratio is given by [7]:
(Eq 2.2)
where R is the radius of the blades, ωr is the rotor speed in rad/s and W the wind speed (m/s).
Multi bladed rotors operate at low tip speed ratios of 1 or 2, where else, one, two or three bladed rotors operate at higher tip speeds of 6 to 10. The power coefficient in equation 2.1 depends on tip speed ratio as shown in figure 2.2. For a particular wind rotor design there exists a tip speed ratio which will produce the maximum value of power coefficient [11].
Fig. 2.2 Power coefficient Cp versus tip speed ratio [11]
2.2.2 Permanent magnet generator
Using permanent-magnet generators for small wind turbines is very commonly used world wide. Usually an AC generator with many poles operates between 10-100 Hz. Many configurations use surface mounted three phase permanent magnet synchronous generators with a rectifier connected to the generator terminals. [16]
A simple PM generator consists of the stator, magnet rotor disk and a shaft. The magnet rotor disk is mounted on a bearing hub so that it can rotate on the shaft due to the rotating blades of the wind generator.
The stator has coils of copper wire wound around them, which are accommodated in the slots. Electricity is then generated when the magnets on the rotor disks rotate past the coils embedded in the stator. The magnetic field that is created induces a voltage in the coils [6].
2.2.3 Rotor design
There are two types of rotor configurations commonly used world wide, these are the disk and the cup as shown in figure 2.3 below [20].
Fig. 2.3 Disk and cup rotor designs
The radius of the rotor primarily depends on the power expected from the turbine and the strength of the wind regime in which it operates [5].
2.2.4 Tower
The main function of the tower is to raise the blades and the generator to a height where the wind is stronger and smoother than the ground level. The wind speed increases with height because of the earth surface [9]. The tower should be high enough to avoid any obstacles such as trees, building, etc. Practical considerations such as expense, safety and maintenance limit the tower to between 10m to 20m [6] above ground level.
2.3 Design of a wind turbine for Ga-Rampuru village
In this section a wind generator that is designed specifically for Ga-Rampuru village will be discussed. The generator will be designed using recyclable materials such as car brake plates, cables and drums found in the village [See appendix A]; this will clearly ensure a cost effective design. The wind turbine will be designed in such a way that the local people can easily assemble and manufacture it themselves.
All the recyclable materials that will be used in this design will be discussed below and an artist impression of the wind generator will be sketched.
2.3.1 The drum
The output of the wind generator depend on the amount of wind swept by the blades, therefore the wind extracting materials in a wind generator are very significant. A plastic drum will be used in this design to extract the wind since it can be easily shaped and carefully balanced to run smoothly. Also, it is resistant to fatigue braking and has a very light weight.
The drum will be assembled as follows:
1. The top and the bottom part of the drum will be cut carefully by using a knife or pair of scissors to make a cylinder with open ends.
2. The cylindrical drum is then cut length-wise into two equal halves.
3. The two halves are then glued together similar to the drum shown in figure 2.4.
Figure 2.4 An S-shaped drum
To prevent the over speeding of the drum, the permanent magnet generator should always be connected to a battery or other electrical load. If this is not done the wind turbine will become noisy and may vibrate so much that some parts come loose and fall to the ground [6].
2.3.2 Magnet rotor disk
After a tour around the village neighbourhood dumpsites it was discovered that there are many discarded loud-speakers that are no longer in use in the village. These loud-speakers have permanents mounted to their back. Since the PM generator requires magnets, these loud-speakers will be recycled and the magnets on them will be used in this design. Figure 2.5 shows one such magnet that was found in the village.
There are many factors such as heat, radiation and strong electrical currents that can affect the strength of a magnet [8], especially in such discarded state. These factors will be discussed later to investigate exactly how much surface magnetic flux density these magnets loose in the dumpsites.
And later on in this thesis the performance of a PM wind generator designed using standard commercial magnets will be compared to a generator using the recycled loudspeaker magnets as substitutes.
Designing a generator using the speaker magnets will pose the following challenges due to their shape and strength:
· How does one design a machine with these magnets?
· Do they have to be smashed and aligned to work?
· Or should they be used the way they are?
· How much flux density do these magnets have, in other word, can they give out any power when used in the generator design?
· Can different magnet types be used on one machine? As this magnets are picked randomly in the rural area.
2.3.3 Rotor Disk
A cylindrically shaped rotor is preferred as it allows the proper distribution of flux over the armature surface as the field coils are spread over the periphery of the cylindrical rotor. Hence, a brake plate from an old car like the one in figure 2.6 will be used as the rotor in this design to hold and house the magnets.
2.3.4 Distribution cables
All the cabling that will be required in the construction of the wind generator was found in an old car in the village [See figure 2.7].
2.3.5 Artist impression of the wind turbine
Figure 2.8 below shows the artist impression of the wind generator designed exclusively for Ga-Rampuru village.
Figure 2.8 Ga-Rampuru wind generator
The following chapters describe the steps taken by the author to investigate the performance of a synchronous permanent magnet machine constructed using recyclable loudspeaker magnets.
Chapter 3. Generator Design
3.1 A brief background
This chapter will detail a simple procedure undertaken to design the wind generator from recyclable materials. Permanent magnet machines are preferred for this application as they reduce the excitation losses significantly and hence a substantial increase in the efficiency of the machine. In addition, permanent magnet machines are simple to construct and maintain [10].
