1.3 Wind Energy Electric. same as the one before

A brief overview of geothermal energy

1. Wind Energy Electricity Generation 1

2. 2 Wind Turbine Power Wind energy can be used for electrical energy generation a certain wind speed can be maintained. The favorable wind speed is 5- 12 m/s The number of blades reduces as the wind speed is reaching 15 -20 m/s The shaft power output is not normally used directly, usually coupled to a load through a transmission or gear box. The load may be a pump, compressor, grinder, electrical generator, and so on. Lets consider the load to be an electrical generator.

3. Wind Turbine Power Power in the wind, Pw, after the power pass through the turbine, we have mechanical power Pm, at the turbine angular velocity wm Which is then supplied to the transmission The transmission output power Pt is given by the product the turbine out power Pm and the transmission efficiency. Similarly it happens to the generator out put power. 3

4. Typical System 4

5. Wind Turbine Power Transmission losses are primarily due to viscous friction of the gears and bearings turning in oil. At fixed rotational speed, the losses do not vary strongly with transmitted torque. It is therefore reasonable to assume that the transmission loss is a fixed percentage of the low speed shaft rated power. The actual percentage will vary with the quality of the transmission, but a reasonable value seems to be 2 percent of rated power per stage of gears. The maximum practical gear ratio per stage is approximately 6:1, so two or three stages of gears are typically required. Two stages would have a maximum allowable gear ratio of (6) 2 :1 = 36:1 so any design requiring a larger gear ratio than this would use three stages. 5

6. 6 Wind Turbine Power The generator losses may be considered in three categories: hysteresis and eddy current losses, which are functions of the operating voltage and frequency, windage and bearing friction losses, which vary with rotational speed, and copper losses, which vary as the square of the load or output current. Normal operation with the generator connected to the utility grid will be with fixed voltage and frequency, and either fixed or almost fixed angular velocity depending on whether the generator is of the synchronous or induction type

7. 7 Wind Turbine Power The losses into two categories: fixed and variable, with hysteresis, eddy currents, windage, and bearing friction considered fixed, and copper losses being variable. The relative magnitudes of these losses will vary with the design of the generator. It is considered good design to have the two categories approximately equal to each other when the generator is delivering rated power Larger generators are inherently more efficient than smaller generators. Some losses are proportional to the surface area of the rotor while the rated electrical power is proportional to the volume.

8. 8 Wind Turbine Power The ratio of volume to area increases with increased physical size, hence the efficiency goes up. Good quality generators may have full load efficiencies of 0.85 for a 2- kW rating, 0.9 for a 20- kW rating, 0.93 for a 200- kW rating, and 0.96 for a 2- MW rating. The efficiency continues to climb with size, exceeding 0.98 for the very large generators . This variation in efficiency with rating is different from the efficiencies of the turbine and transmission, which were assumed to not vary with size.

9. Toque at constant speed Most wind turbines extract power from the wind in mechanical form and transmit it to the load by rotating shafts. These shafts must be properly designed to transmit this power. When power is being transmitted through a shaft, a torque T will be present. This torque is given by N.m/rad where P is mechanical power in watts and in rad/sec. The torque in the low speed shaft of is while the torque in the high speed shaft is is angular velocity 9

10. Toque at constant speed The application of torque T to a shaft causes internal forces or pressures on the shaft material. Such a pressure is called the stress fs with units Pa or N/m2. J is the polar moment of inertia r o is the radius of the shaft. Since this pressure is trying to shear the shaft is referred to as shearing stress . It varies with the distance from the axis of the shaft. The design of shaft is to carry maximum shearing stress which is allowed for a given shaft materials. 10

11. 11 AC Generators Almost all electrical power is generated by three- phase ac generators which are synchronized with the utility grid. Single- phase generators would be used for wind turbines only when power requirements are small (less than perhaps 20 kW) and when utility service is only single- phase. A three- phase machine would normally be used whenever the wind turbine is adjacent to a three- phase transmission or distribution line. Three- phase machines tend to be smaller, less expensive, and more efficient than single- phase machines of the same power rating, which explains their use whenever possible.

12. 12 THE SYNCHRONOUS GENERATOR Generators are often rated in terms of apparent power rather than real power. The reason for this is the fact that generator losses and the need for generator cooling are not directly proportional to the real power. The generator will have hysteresis and eddy current losses which are determined by the voltage, and ohmic losses which are determined by the current. The generator can be operated at rated voltage and rated current, and therefore with rated losses. A generator may be operated at power factors, between 1.0 and 0.7 or even lower depending on the requirements of the grid, so the product of rated voltage and rated current (the rated apparent power) is a better measure of generator capability than real power.

13. THE SYNCHRONOUS GENERATOR A construction diagram of a three- phase ac generator is shown as shown below. There is a rotor which is supplied a direct current If through slip rings. The current If produces a flux Φ. This flux couples into three identical coils, This flux couples into three identical coils, marked aa’, bb’, and cc’, produces three voltage waveforms of the same magnitude but 120 electrical degrees apart. 13

14. The Phase angles 120 14

15. THE SYNCHRONOUS GENERATOR The equivalent circuit for one phase of this ac generator is the magnitude of the generated rms electromotive force (emf) E is given by : Where , Flux per pole Φ, and k1 is a constant which includes the number of poles and the number of turns in each winding. 15

16. THE SYNCHRONOUS GENERATOR The reactance Xs is the synchronous reactance of the generator in ohms/phase. The generator reactance changes from steady- state to transient operation, and Xs is the steady- state value. The resistance Rs represents the resistance of the conductors in the generator windings. The synchronous impedance of the winding is given the symbol Zs. The frequency is given by Where p is the number of poles and n is the number of revolutions rpm. 16

17. 17 THE SYNCHRONOUS GENERATOR If a turbine is operating near rated power, and a sharp gust of wind causes the input power to exceed the pullout power from the generator, the rotor will accelerate above rated speed. Large generator currents will flow and the generator will have to be switched off the power line. Then the rotor will have to be slowed down and the generator resynchronized with the grid. Rapid pitch control of the rotor can prevent this, but the control system will have to be well designed.

18. 18 INDUCTION GENERATORS A large fraction of all electrical power is consumed by induction motors. For power inputs of less than 5 kW, these may be either single- phase or three- phase, while the larger machines are almost invariably designed for three- phase operation. Three- phase machines produce a constant torque, as opposed to the pulsating torque of a single- phase machine. They also produce more power per unit mass of materials than the single- phase machine. The three- phase motor is a very rugged piece of equipment, often lasting for 50 years with only an occasional change of bearings.

19. Induction Machine 19

20. 20 Induction Motor/Generators It is simple to construct, and with mass production is relatively inexpensive. The same machine will operate as either a motor or a generator with no modifications, which allows us to have a rugged, inexpensive generator on a wind turbine with rather simple control systems. The basic wiring diagram for a three-phase induction motor is shown in Fig. 16. The motor consists of two main parts, the stator or stationary part and the rotor.

21. Induction Motor/Generator • The most common type of rotor is the squirrel cage, where aluminum or copper bars are formed in longitudinal slots in the iron rotor and are short circuited by a conducting ring at each end of the rotor 21

22. 22 Induction Generators The circuit of the induction generator is identical to that of the induction motor, except that we sometimes draw it reversed, with reversed conventions . The induction generator requires reactive power for excitation. It cannot operate without this reactive power. This makes it somewhat less versatile than the synchronous generator which is able to supply both real and reactive power to the grid. The induction generator requirement for reactive power can also be met by capacitors connected across the generator terminals.

23. 23 Induction Generators A wind turbine application presents a much better cooling environment to an induction machine than most applications, and this needs to be included in the system design. The second effect is that the wind is not constant. Short periods of overload would normally be followed by operation at less than rated power, which would allow the machine to cool. These cooling effects should allow the generator rating to be equal to the motor rating for a given induction machine.

24. 24 Induction motors as generators The operation of an induction machine as both a motor and generator connected to the utility grid. The induction generator is generally simpler, cheaper, more reliable, and perhaps more efficient than either the ac generator or the dc generator. The induction generator and the PM generator are similar in construction, except for the rotor, so complexity, reliability, and efficiency should be quite similar for these two types of machines. The induction generator is likely to be cheaper than the PM generator by perhaps a factor of two, however, because of the differences in the numbers produced. Induction motors are used very widely, and it may be expected that many will be used as induction generators because of such factors as good availability, reliability, and reasonable cost

25. 25 Induction motors as generators Most induction motors in sizes up to 100 kW or more are built with 208-, 230-, or 460-V ratings, so the available capacitors can readily handle the line to line voltages. We have seen that a three- phase induction generator will supply power to a balanced three phase resistive load without significant problems. There will be times, however, when single phase or unbalanced three- phase loads will need to be supplied Single- phase loads may be supplied either from line- to- line or from line- to- neutral voltages.

26. 26 Control System The electrical grid was assumed to be able to accept all the power that could be generated from the wind turbines. The grid was also able to maintain voltage and frequency, and was able to supply any reactive power that was needed. When the system is isolated from the grid, these advantages disappear . There is a need of adding additional equipment. The wind system design will be different from the synchronous system and will contain additional features.

27. Block diagram of asynchronous electrical system 27

28. 28 Control Systems In this system, the microcomputer accepts inputs such as wind speed and direction, turbine speed, load requirements, amount of energy in storage, and the voltage and frequency being delivered to the load. The microcomputer sends signals to the turbine to establish proper yaw (direction control) and blade pitch, and to set the brakes in high winds. It sends signals to the generator to change the output voltage, if the generator has a separate field. It may turn off non critical loads in times of light winds and it may turn on optional loads in strong winds. It may adjust the power conditioner to change the load voltage and frequency. It may also adjust the storage system to optimize its performance.

29. 29 Control System/Small Units many wind electric systems have been built which have worked well without a microcomputer. Yaw was controlled by a tail, the blade pitch was fixed, and the brake was set by hand. (Battery storage) Such systems have the advantages of simplicity, reliability, and minimum cost, with the disadvantages of regularly requiring human attention and the elimination of more nearly optimum controls which demand a microcomputer to function. The microcomputer and the necessary sensors tend to have a fixed cost regardless of the size of turbine. The microcomputer easier to justify for the larger wind turbines.

30. 30 Control/ Asynchronous System The asynchronous system has rather interesting mode of operation different from electric utilities The turbine speed can be controlled by the load rather than by adjusting the turbine. Electric utilities do have some load management capability, but most of their load is not controllable by the utilities. The utilities therefore adjust the prime mover input to follow the variation in load. That is how supply follows demand. In the case of wind turbines, the turbine input power is just the power in the wind and is not subject to control. Turbine speed still needs to be controlled for optimum performance.

31. 31 Control/Economics Economics must be carefully considered in any asynchronous system. First, a given task must be performed at an acceptable price. Second, as many combinations as possible should be examined to make sure the least expensive combination. And third, the alternatives should be examined. That is, a wind turbine delivers either rotational mechanical power or electrical power to a load, both of which are high forms of energy, and inherently expensive. If it is desired to heat domestic hot water to 40oC, a flat plate solar collector would normally be the preferred choice since only low grade heat is required

32. Pitch Control 32

33. 33 Pitch Regulator The control system is able to adjust the pitch of the blade by a fraction of a degree at a time, corresponding to a change in the wind speed, in order to maintain a constant power output. This requires a rather complicated active regulation system, which can be sensitive to turbulence in high winds. Therefore pitch regulation in practice requires a special generator or variable speed, allowing for a slight acceleration in speed. Otherwise the active regulation is unable to follow the variations of the wind excessive peak loads will occur.

34. 34 Pitch Controller The thrust of the rotor on the tower and foundation is substantially lower for pitch-controlled turbines than for stall- regulated turbines. The pitch regulated turbine can adjust the pitch blade angle to reduce the aerodynamics loads, hence reducing g the thrust forces. This allows for a reduction of material and weight. On a pitch controlled wind turbine the turbine's electronic controller checks the power output of the turbine several times per second. When the power output becomes too high, it sends an order to the blade pitch mechanism which immediately pitches (turns) the rotor blades slightly out of the wind. Conversely, the blades are turned back into the wind whenever the wind drops again.

35. 35 Stall regulation Stall is a condition in which an airfoil experience an interruption of airflow resulting in a lost of lift. Passive stall controlled wind turbines have the rotor blades bolted onto the hub at a fixed angle. Stalling works by increasing the angle at which the relative wind strikes the blades and reduce the induced drag (drag associated with lift). Stalling is simple because it increases automatically when the winds speed up, but it increases the cross- section of the blade face- on to the wind, and thus the ordinary drag. A fully stalled turbine blade, when stopped, has the flat side of the blade facing directly into the wind This principle requires a constant rotational speed (i.e. independent of the wind speed). A constant rotational speed can be achieved with a grid-connected induction generator. •

36. 36 Stall regulator • • • • This movement will increase the angle of attack of the rotor blades in order to make the blades go into a deeper stall in order to get a reasonably large torque at low wind speeds. . If the wind speed reaches cut- out wind speed (usually between 20 and 30 m/s), the wind turbine shuts off and the entire rotor is turned out of the wind to protect the overall turbine structure. One of the advantages of active stall is that one can control the power output more accurately than with passive stall, so as to avoid overshooting the rated power of the machine at the beginning of a gust of wind. Another advantage is that the machine can be run almost exactly at rated power at all high wind speeds. A normal passive stall controlled wind turbine will usually have a drop in the electrical power output for higher wind speeds, as the rotor blades go into deeper stall.

37. Yaw Control System 37

38. 38 Performance The nacelle weighs about 56 tonnes and the rotor weighs about 31 tonnes. Total weight varies, depending on tower height, from about 205 to 318 tonnes at heights ranging from 60 to 100 m. The unit is designed to maintain a constant 2 MW. output over a wide range of operating conditions. Its microprocessor- based control system allows the rotational speed of both the rotor and the generator to vary by about 60%. For example, at a distance of 340 m , its sound emissions can be reduced from 44.9 to 40.4 dba by reducing its blade tip speed.

39. Lighting Protection 39

40. Catastrophic Failures 40

41. Thrust on Turbine 41

42. 42 Small Power Systems Diesel engine-generators are mostly commonly used in large mixed (hybrid) systems either to share load with the WECS or as back- up, other technologies such as battery storage may be used. Batteries serve to stabilize power fluctuations from the turbine and store excess electricity produced for later use. Battery banks most frequently use lead- acid batteries and range in capacity from 1 to 3 days’ supply. Small capacity WECS will often rely solely on batteries for storage/back- up, while larger systems more often rely on Diesel engine back- up, or a combination of engines and

43. 43 Application/Economics If the wind turbine were driving a heat pump or charging batteries as a primary function, then heating domestic hot water with surplus wind power might make economic sense. The basic rule is to not go to any higher form of work than is necessary to do the job. Fixed frequency and fixed voltage systems represent a higher form of work than variable frequency, variable voltage systems, so the actual needs of the load need to be examined to determine just how sophisticated the system really needs to be. If a simpler system will accomplish the task at less cost, it should be used.

44. 44 Application/Economics The energy needs of small rural communities fall into three categories: energy to improve living conditions, energy to improve agricultural productivity, and energy for small-scale industries. The other is a decentralized system of solar and wind equipment installed at the village level. It is difficult to set priorities among these needs, but living conditions certainly have to be improved if the people are to have any hope in the future

45. 45 Application/Economics Comparatively small amounts of energy could meet the basic needs for cooking of food, pumping and purifying drinking water, and lighting of dwellings. Once these needs are met, work can begin on increasing agricultural and industrial productivity. A rough estimate of the energy needs of a typical village of 200 families is as follows: 88,000 kWh per year for cooking food, 1,000 kWh per year for pumping water, and 26,000 kWh per year for lighting. This averages 315 kWh per day, most of which must be supplied during a three hour period in the evening.

46. 46 Applications /economics A preliminary indication of an area’s wind energy potential can be based on its rated wind power class. In addition to wind resource, the economic viability of wind power will vary from region to region, depending on numerous factors, including production/demand match (seasonal and daily), market electricity costs, transmission and access constraints, public acceptance, and public policy support.

47. 47 Maintenance Several specialized terms are usually used to describe the reference indicators of machine reliability and maintenance team- work efficiency: Mean Time Till Failure – basic reliability factor; Mean Time Between Failures – operation time for the machine divided to number of failures per year; Mean Time To Repair – amount of time the turbine is out of operation divided to number of failures; Corrective Maintenance Rate – time spent for corrective maintenance activities divided to total maintenance time.

48. 48 Most Common Faults Most vulnerable components in a typical wind turbine are usually the: Gearbox (preventive maintenance by frequent oil change); Yawing brake system (preventive checking and tuning); Damping elements and vibration sensors; Blades need to be constantly monitored and may require to be cleaned or exchanged after 5- 10 years of operation; Electronic control system; All types of bearings, whose failure, if not identified on time, can lead to very expensive repairs and component changes.