DESCRIPTION OF WIND TECHNOLOGY
In the EU that type of the RES is actually the most common. It is caused by potent the EU policy, historical reasons, relative low cost of investment and operation. In Europe, people used to use wind to drive their machines for many years. Netherlands is considered as a European capitol of windmills and it is from there comes to us the oldest wind turbine technology. Nowadays people are familiar with a few technology type of wind turbine. We can split them on a two groups: with vertical and horizontal axle position. A main factor deciding about usefulness of turbine is rotor efficiency (depended from high-speed factor). The highest value of this factor is related to propeller with horizontal position of turbine axis.
Velocity and power relations
Energy of wind is definition as kinetic energy: movement of air mass m, with speed v. Formula below shows that dependence:
Related to above, formula of wind stream power looks like below (only for constant wind speed):
where: m’ – mass flow per second
If treated m as a mass flow rate of the air per second, it given by the following expression:
where: ρ – air density A – area swept by the rotating blades v – wind speed
It easy to see that mechanical power have a cubic relation with wind speed. It is considerable advantage in situation when the wind is blowing with constant speed but this condition is happens rarely in real. Therefore that cubic relation is significant disadvantages of wind turbine making them very floating resource.
Mechanical power extracted from wind
Humanity are familiar with the theorem that the perpetum – mobile does not exist, thus it is impossible to convert kinetic energy of wind to mechanical energy of turbine, with 100% efficiency. It is related with many types of wastages like e.g. theoretical wastages or mechanical efficiency. Therefore the actual power extracted by the turbine from the wind is given the following equation (gauge area on an entrance and on an exit can be difference Ain ≠ Aout) [1]
where: vin – upstream wind velocity at the entrance of the propeller vout – downstream wind velocity at the exit of the propeller
Below graph depict the rotor efficiency (power coefficient) for different types of propellers. (Fig. 4)
Humanity are familiar with the theorem that the perpetum – mobile does not exist, thus it is impossible to convert kinetic energy of wind to mechanical energy of turbine, with 100% efficiency. It is related with many types of wastages like e.g. theoretical wastages or mechanical efficiency. Therefore the actual power extracted by the turbine from the wind is given the following equation (gauge area on an entrance and on an exit can be difference Ain ≠ Aout) [1], [3]. One of the main technological categories of wind farm is localization. There are two methods of location of wind power farms: on shore and off shore. First method (on shore) depends on locate farm landlocked and second one (off shore) on locate farm on seas and oceans shallows. Not to long ago the most common method was on shore due to low cost of investment, easiness of construction and low cost of exploitation. But recently off shore method are more and more popular. The reason are people, who don’t want to allow for the localization of wind power farms on their own land. [1], [3]
Wind power system
Even if the wind turbine is operating as a single turbine or as a farm, each turbine is made of the following components: tower structure, propeller (rotor with blades attached to the hub), shaft with mechanical gear, electrical generator, yaw mechanism, sensors and control (with stall controller), anemometers, numerous sensors, power electronic to convert and condition power, control electronics, battery and transmission link. Below is a short description each one of the most important elements including information about main function: Tower This is the biggest part of wind turbine. Tower structure is assembled from tower and nacelle where the yaw mechanism are installed as well as an electrical generator, stall control and all of the mechanical gears. The picture bellow depicts the components of nacelle.
Large tower are build as tubular steal towers, lattice towers, concrete towers or in small turbine case, guyed tubular towers. The height of the tower is in few rotor diameter but not lower than from 25 to 30 meters in order to avoid turbulence caused e.g. by trees, high rocks or buildings. The main problem in the tower design is vibration caused by rotor rotation and wind speed fluctuation. Therefore designers have to design the tower avoiding the resonance frequencies. The cost of the tower is nearly 20 per cent of a total cost of wind turbine. [1], [2] Rotor The most common type of a rotor is a three-blade rotor. The main factor of the rotor is the swept rotor area. The turbine power is in linear relation, correspondingly to equation (2.3), therefore turbines with short blades have lower power that rotor with long blades. In modern turbines with large swept area, blades can also swirl for regulation. A large rotor cost is relatively lower than of a smaller, it has better parameters and covers less area but on the other hand farm assembled from many small rotors has lower the cost of infrastructure, the energy in farm out put has less fluctuation, several machines decrease the risk of turn off of a farm in case when one of them temporarily fails and smaller machines can be installed in areas when the grid is to weak to handle too much energy from large turbines. [1], [2] Shaft with mechanical gear The construction of this element is not very complicated. The main part is a gear box. It is a mechanism which converts slow speed rotation of a wind rotor to high speed rotation of a generator.[1] Electrical generator In modern wind turbines there are two types of three-phase generators: synchronous and asynchronous generator. But nowadays, the most common is that second one due to the method of rotor feeds. A synchronous machine required DC current to feed rotor, therefore each generator needs their own DC current source or permanent magnet inside a stator. This solution can operate with low speed rotation, thus can operate also without a gearbox and allow full control of active and reactive power.[1], [3], [10]
An asynchronous machine (induction machine) has squirrel cage rotor which does not need to be feed. Other advantages of the induction machine are e.g.: low capital cost, low maintenance and better transient performance.[1], [3], [10]
Relatively new technology is double fed induction generator. This solution allowed to transmission energy in two directions, into and from rotor. This feature has a few significant advantages: output power optimization (allows to change rotation speed what increase turbine efficiency related to wind speed changes), to use rotor rotation as energy inventory (overflow is warehouse in kinetic form when wind speed is fall off), fast adjust to wind speed changes (in this case is allowed to fast changes of wind rotor speed and in the same time to use blasts energy), passive energy regulation without using capacitor battery, lower level of loud emission, less exhaustion of gear box, blades and tower, energy quality improvement, active curtail of power and voltage fluctuation also higher current and voltage harmonics.[1], [3], [10]
Yaw mechanism This mechanism allows to conform wind turbine position to wind flow direction. This mechanism cooperates with anemometers and sensors.[1] Sensors and control This system is responsible for measure important values and control prime parameters of a turbine. Speed control methods fall into few categories: no speed control whatsoever (to control extreme wind speed), yaw and tilt control (to orients the rotor in the direction of the wind), pitch control (to change the pitch of the blade with wind speed changes), stall control (to position rotor and blades in case when turbine does not works). In older versions of wind farms, have central control but in modern turbines, each turbine must have their own control system what is related with lower the moment of inertia than in a farms with central control system.[1], [3]
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Description of technology|
Economic aspects|
Environment and public awareness|
Legislation|
Final comparison|
References