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Advances in Mechanical Engineering
Volume 2012 (2012), Article ID 380986, 7 pages
Experimental Investigation on Power Output in Aged Wind Turbines
1Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli 620015, India
2Department of Electrical Engineering, National Institute of Technology, Tiruchirappalli 620015, India
3Department of Electrical Engineering, Thiagarajar College of Engineering, Madurai 625015, India
Received 4 June 2012; Accepted 24 June 2012
Academic Editor: Oronzio Manca
Copyright © 2012 N. Murugan et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
An investigation on the power output on effect of tower height with same diameter of rotor was conducted in a wind turbine site. As the wind acceleration is varying with height, 3 levels were selected according to the availability of tower. The responses of power output with respect to variation of wind speed are changing for the tower heights of 30, 40, and 50 m. The study showed that the actual ideal power output and measured real power output follow the same trend within range of operating wind speed. The empirical model used for calculation of actual ideal power output was compared with real power output and the overall concepts in power output also had been analysed.
The increase in negative effects of fossil fuels on the environment has forced many countries, especially the developed ones, to use renewable energy sources. From various viewpoints including environment, energy security, social and rehabilitation issues and economics, the World is seriously forced on the options for meeting energy demand in future. The only manner, in which the scenario of electricity shortages and serious environmental risks can be avoided, is through a major shift to renewable energies. Among the various renewable energies, in terms of historical development, commercial viability as well as wide spread availability, wind turns out to be the most viable resource.
Wind energy is one of the most promising and potential sources of energy. One of the greatest advantages of wind energy is that it is abundant, renewable, and has advantage over traditional methods of creating energy. Some other benefits of wind energy are that it is relatively cheap and also reducing toxic gas emissions. Wind energy may soon be the cheapest way to produce energy on a large scale. By adopting the proper location, the micro climate change also can be avoided. Along with economy, wind energy is also said to diminish the greenhouse effect. The cost of producing wind energy has come down by at least 30 percent in three decades. Wind Energy is also a more permanent type of energy. The wind will exist till the time the sun exists, which is roughly another four billion years.
One other advantage of wind energy that it is readily available around the globe, and therefore there would be no need of dependence of other energy in any country. Even though its availability varies from place to place, it is environmentally friendly. Numerous works on wind energy availability and the effects were reported [1–9], and many technical studies were also reported [10–16]. The wind zone above ground can be divided into two parts namely upper wind and surface wind. The wind above 500 metres  height is upper wind, and towers are used for measurement. The wind turbines are usually installed within the surface wind to harness the wind energy. The wind speed is the main component for the power output of the wind turbine, and it varies with the terrain type and height. Generally, wind turbines are designed by taking into accountof wind speed at a particular location including all terrain factors, and so forth. In actual situation, original wind flow condition is getting changed gradually due to the development of new buildings and other infrastructure in many sites. This topography changes affect the wind speed which in turn affectsthe performance namely, power output. As the wind turbines are getting aged, thepower output decreases due to increase in various mechanical losses. In theliterature cited above, no work is reported on the performance oldwind turbines. Therefore, the basic objective of thispaper is topresent experimental investigation of power output of fifteen-year-oldwind turbines by validating the experimental data obtained from the conventionalmodels used for calculation of power output.
2. Brief Background of Wind Machines
The term low and high wind stands with respect to speed of wind which, normally flows in any location related to harnessing the power. Any modern wind electric generator (WEG) is basically made up of rotor which has 2 or 3 blades with hub, shaft connected to gearbox and alternator usually. The fixed speed and fixed pitch (FS-FP) are normally common in all applications for lower capacities up to 250 kW. For low wind and higher capacity, gearless variable speed and pitch control will be optimum called direct drive. Due to high cost in earlier days, permanent magnets (PMs) were used rarely in few machines but due to updated technology of manufacturing, many manufacturers are now towards the application of PM. This is enclosed in the housing called as Nacelle, and the entire system is mounted on yaw plate through vibration isolation system (VIS) or antivibration mount (AVM). The nacelle assembly on the yaw plate along with rotating blades can be slewed about vertical axis of tower called as axis of yaw by electrical yaw motors which are connected under the yaw plate. Hence, no latest WEG are provided with rudders at the backend. The yaw plate is loaded on yaw adopter which is bolted to the top of the tower.
The towers can be either lattice tower or steel pylons or tubular towers with applicable foundation at the base which depends on the location and type of soil or off-shore. The tower is designed for tower frontal loads due to wind, gyroscopic, and other dynamic loads including contingency and emergency loads. The towers are made up of low carbon steel and fully field-bolted type with square-based lattice tower design. The profile of towers in vertical plane is designed based on the rotor blade type, nacelle type, and deflection criteria. Some of the lattice towers of lower capacities are with uniform slanting towards top, tower cap flange which is called as yaw adopter mounting flange and welded at the top most part of the corner angles of tower. In larger type, the lattice towers are with waist or neck, with shallow and steep slanting below and above the waist, respectively. This type of single waist design is common in all fixed speed and fixed pitch design. In case of variable speed and variable pitch control design, there will be second waist or neck near the top of the tower. This is designed to have mechanical clearance for free pitching of turbine blades about pitch axis of blade and at any yawing position.
All the steel, welds and bolts are to be designed by considering allowable stress loss due to fatigue also. The entire tower has vertical bracings and horizontal bracings. The provision of bracing pattern in a lattice design is an art of controlling the forces and also taking care of aesthetics. The style of vertical bracings is the best if connected with nodes on columns and transfers frontal loads to corner columns by acting as skew columns. Hence, a well- and symmetrically braced lattice tower is good due to higher mass participation ratio. In early-stage designs, these frames were analysed in plane frame methods, which were conservative. The latest designs are analysed as space frame which is optimum and reduces the cost. In case of high-power lattice, hexagonal towers also being used so that the response for resistance would be better against sudden and higher actual gust forces.
The higher power machines are provided generally with tubular towers for these reasons. The base width is not limited in case of lattice, but it is dictated by transport limitations in case of tubular tower which calls for higher thickness at base or warrants high-tensile steel where the welding becomes highly complicated and increases the cost. The tubular towers are fully shop welded, and circumferential joints are bolted at site. Depending on the height and loading factors, the tubular towers are either fully embedded into the foundation or bolted to the foundation studs. The tubular tower encloses the control panel room inside the bottom of tower and can be approached through an elliptical manhole in bottom of shell. For same applications of load and height, the tubular tower is more costly due to higher steel grade and weight. As fabricated and shipped as assemblies, the logistic costs also higher due to (ODC) overdimensional consignments.
The special varieties of self assembled mast type crawler hydraulic cranes are also available to reach any height of erection. The technology of wind turbine design within this decade has crossed with phenomenal jump because of availability of design software, high end machine and welding tools, enhanced transport facilities, improved roads, and skilled man power.
In case of other turbines, the design can be done in a stipulated pattern as the prime mover can be stopped by closing the flow of propulsion fluids like steam, water, and gas in to the turbines. This is being done by HP and LP bypass systems in a steam-based thermal power plant in between boiler and TG circuit, straight bypass penstocks to tailrace to bypass the scroll casing of hydraulic turbines, annular bypass duct in gas turbines, and so on. In case of wind turbines, wind does not need to stop while the turbine is tripping or while turbine is necessitated to stop. Hence WEG’s rotating components must weigh light enough to start at low wind and should be strong enough to withstand high wind at the same time including survival wind speed. This important design factor is to be considered in WEG engineering which is totally different from other turbine designs.
As per standards of wind turbines, machine starts generation at minimum wind speed of ~4 m/s by rotating the turbine generator after attaining predicted revolutions in generator and it is called as cut-in speed. The operation mode of turbine till the WEG is electrically in to cut-in or as generation mode, it is called freewheeling. The turbines are designed for delivering the rated output as (MCR) maximum continuous rating at approximately 16 m/s  generally. The machines can run continuously up to higher wind speed of 18, 19, 20 m/s and 25 m/s till it crosses the set value of over load current or power for a predicted time. The machine is applied with emergency brake and tripped beyond any one of these values. This is called as cutoff speed. While the wind speed is reducing continuously below 4 m/s, it draws power from grid and shows the power output as kW in dashboard. This is called motoring mode, running the turbine in motoring mode is back levied by EB as penalty. Hence the generator shall be cutout beyond set time provided the wind speed not gusted up within time frame which can be set time to time. Therefore, the machine will be in free-wheeling. Similarly, manual intervention is required to start back the machine once the machine is tripped due to over load, high current, or wind speed depending on necessity. Hence, adjustment can be done in machine control panel depending on various factors like season, condition of load, and machine.
In mechanical design, the generator is coupled with high speed shaft, turbine is coupled with low-speed shaft and hence whenever the turbine rotates, the generator also physically rotates. In cutoff speed both are stopped simultaneously as applied with emergency brake. The various factors like low frequency, high frequency, low voltage, high voltage, voltage unbalance, high temperature, high ambient temperature, high generator temperature, high coolant temperature, VIS abnormalities and higher rotor speed, high wind speed are considered for tripping which depends on the machine design and software. In case of out-of-control, which sometimes damages the installation due to over speeding and it is named as runaway. Apart from this, blade hitting, lightning damage, uprooting, Nacelle falling, generator fire, and bolt failures are the events faced in few installations.
These WEG installations are banned near bird’s sanctuary and along the path way of bird’s migration. The downstream of wake may have micro climate change also .
3. Models for Power Prediction
The induced power generated in WEG is proportional to density of wind, area of rotor, and cube of wind speed and this is called as ideal power.
The following formula  governs the ideal power output in kW for estimation:
This will be increasing due to increase in wind speed but turbine power output which is called as real power output will be different. This depends on system design, weight, and sensitivity to variation of wind speed. The real power produced in each turbine also varies due to type of machine, power train system, location of machine, crowding or cluster factor in wind farm which is called as array efficiency, effective hub height, surface roughness factor due to seashore, paddy field, forest, desert, suburban or urban and concealment against the vicinity of wind due to mountain or hills.
The mechanical condition and aging of machine also plays role in power output. As this formula is quantifying the ideal power as induced power, this can be further validated with consideration up to the extent of accuracy of losses due to farm factors and design of mechanical and electrical systems in series. This can be considered as actual ideal power output. where is electrical power in kW (actual ideal power output), is density of wind in kg/m3, is swept area of rotor in m2, is speed of wind in m/s, is efficiency of power trains from rotor to generator in Nacelle system in series, and is array efficiency , and so forth.
The ideal power generated in wind rotor is reduced due to transmission of torque from rotor to gear box. This is due to various stages of speed step up in gear box to meet the high generator speed requirement. Usually, the machines are stepping up the speed in 3 stages inside the gear box either with gear trains or with planetary gear box arrangement. Due to this, there will be a difference in input and output, this efficiency is gear box efficiency and taken as , which is equal to 0.89 for the gear boxes of wind turbines used for the present study as all are installed by the same manufacturer and same years of service.
There are few designs with hydraulic couplings and clutch arranged in series at upstream of generator shaft and after the gear box. The hydraulic coupling and clutch efficiency are taken as and , respectively. As wind turbines used for the present study are directly coupled with gear box and generator without any hydraulic coupling and clutch, the values and were taken as 1. This is because of no loss of power due to nonexistence of such clutch and hydraulic coupling machine parts.
The wind machines generators are supposed to be highly efficient both mechanically and electrically as fitted on high antifriction bearings. The generator efficiency is taken as e4, whose value is 0.90 due to aging and the maximum efficiency had been proven up to 0.958 as given by manufacturer.
As the values are in series, the ideal power output in kW should be multiplied with the actual values of according to the downstream system to calculate the actual ideal power output. As test machines used for the present study only have gear box and generator, only and are to be considered.
There are situation where wind turbine erected in plain land may become dense due to construction of building and development of other infrastructure known as degradation of terrain, which will result in change in terrain roughness length. This will reduce the wind flow and hence a degradation of power. The wind near the ground is retarded by surface roughness. The lower layers of air then retard those above them. When the winds are strong, the mechanical mixing of wind destroys the thermal gradients giving rise to convection process, and the layer is neutral. Under these conditions, it is found that the mean wind speed  over relatively uniform surfaces is given by the equation where is mean wind speed at height of above ground level; is the friction velocity; is Von Karman’s constant and taken approximately as 0.4; is the roughness length of terrain.
It is favourable to use empirical power law to approximate the boundary layer profiles as follows: where is power law exponent; is wind speed at reference height above the ground level; is mean wind speed at height above the ground level.
Table 1 shows the values of the roughness length of terrain and the applicable power law index.
Therefore, the above wind speed in wind farm is changing due to plain land conversion into suburb or urban, and these effects are to be considered only if wind speed is measured in ground level to calculate the wind speed at hub levels of 30 to 50 m. To avoid this cumbersome working, wind masts are fitted with anemometers at the levels of tip of blades and hub levels during estimation work in a site. As the hub level wind speed is directly measured in all the machines by hub level anemometers, there is no necessary for the usage of the power law exponent.
Another aspect to be considered is the best of land usage. The loss of induced power is higher with closely arrayed WEG in wind farms. This type of defect will affect highly in low wind zone. As the ideal power is the function of wind speed, it is known that the change in wind speed will vary the ideal power. But the factors, due to which the wind speed is getting degraded before wind reaches the machine, can be considered as separate factor and not as reduced wind speed. This wind flow degradation is due to wake called as array efficiency which can be taken as “.” The “” as a direct reduction factor of power output and can be multiplied with ideal power output.
This array efficiency is calculated as follows: where is actual output from the machine in the wind farm in kW; is machine output in kW without disturbance of neighbourhood machines elsewhere while as an ideal standalone machine.
As per present norms of micro site guidelines, WEG are located 5 times the diametric spacing in transverse axis and row pitch by 7 times the diametric basis in zig-zag array. As the wind turbines used for present study are widely well placed, the array efficiency is taken as 1.
By taking into account the above factors, the formula for actual ideal power output is as follows: where and .
4. Experimental Setup and Procedure
The photograph of wind turbines used for the present study is shown in Figure 1. These three WEG each of 250 kW rating are located among approximately 3370 machines in the Muppandal wind park in south India. The machines are located in high wind area spread over 40 km length with total installed capacity of 1670 MW. The machines where the readings were taken almost have logged in 15 years of operation in the past. All the 3 are located in same region, one nearby other. Due to this, the terrain constants are almost same.
The brief description of experimental setup test machines used for the present study is as follows. The 30 m and 40 m high towers are of tubular type and 50 m tower is of lattice type. All the turbines have the same rotor diameter of 25 m. The machines are fitted with same blades. The above towers are embedded into steel-reinforced cement concrete raft. The type of tower does not have any effect on the power output. The generated power is passed to the control room at the bottom of tower. The parameters like wind speed, net power output produced in kW and other variables namely, temperature, rotor rpm and turbine rpm are displayed in a LCD panel.
Figure 2 shows the digital LCD display showing wind speed in m/s and power output in kW. As this display is too dynamic to note down instantaneously due to fluctuations in wind speed and kW rating, it was video graphed during each trial. The videos were replayed; the wind speed and the corresponding power output were tabulated for all trials.
5. Results and Discussion
The plot of actual ideal power output and real power output versus wind speed is shown in Figure 3.
The actual ideal power is almost matching with real power output as seen from Figure 3. It is also observed that the actual ideal power increases with increase in wind speed and the real power output transforms to right side of actual ideal power curve in to shallow characteristic. Actually, the decline of FS-FP (fixed speed-fixed pitch) wind turbines is mainly due to the bad power quality, rather than to reduced power capture . This is due to control strategy in fixed speed and fixed pitch (FS-FP) machines beyond 15 m/s. The similar behaviour was reported by Olsina et al. .
Although the experiments were carried out within the speed range from cut-in speed 4 m/s to the maximum available wind speed 18 m/s, the calculation for actual ideal power has been done up to the wind speed of 12 m/s. This is because the actual ideal power output crosses the maximum rated value of machine at 12 m/s.
Figure 4 presents the excursion of turbine power at 30 m, 40 m and 50 m heights. The power excursion is the harmonic of power output due to fluctuations and degree of change of wind speed. It is seen from Figure 4 that the wind speed was higher in 50 m height tower than the other two towers of 40 and 30 m. In the same day, it can be seen that the power output has touched the maximum point in 50 m hub height. It is also seen that the 50 m tower height power curve is not conjoined with the power curve at 30 and 40 m heights. The power curves of higher height are oriented towards vertical axis. This is because of consistency in high wind speed at higher elevation with less fluctuation in power output. When the wind speed is getting raised suddenly, the response to go with high wind will take some time as the rotor has inertia. This is applicable while the wind turbine is facing reduction in wind speed also. When the wind speed droops, there is no necessary that the wind speed should be descending continuously. The end value of wind speed could be lesser than the starting wind speed but with in this spell, the wind would have hiked and dropped in a saw teeth manner also. This type of fluctuation and variation of wind speed can be felt in lower elevations, and there will be fairly steady wind at higher and higher elevations within limits.
The degree of excursion can be seen by the flow of power curve from higher power to lower power with the extent of lateral meandering. If more lateral shift is found in power curve, it indicates that the turbine is under variation of power output. If the power curve is without lateral shift, it is maintaining a constant turbine speed to the extent possible indicating the smooth real power output. This type of smoother performance is observed only in 50 m tower. Hence it can be concluded that 50 m tower height is preferred option for this 250 kW machine. While installing any machine in wind farm, the interest is shown in higher diameter and height for the same rated power. The tower in which the studies made shows that the performance is better in 50 m height tower only.
(i)Experimental investigation on performance assessment study of existing fifteen-year-old wind turbine has been presented;(ii)experimental investigation on the power output on effect of tower height with same diameter of rotor is presented and found to be higher at 50 m height;(iii)empirical models were proposed to calculate actual ideal power output and are almost matching with real power output on dashboard;(iv)the power fluctuation is observed to be minimal in 50 m height tower;(v)for 250 kW machine, 50 m tower height is ideal for the wind turbine site chosen for present study.
|:||Swept area of rotor in|
|:||Power law exponent|
|:||Coefficient of power|
|:||Actual output from the machine in the wind farm in kW|
|:||Machine output without disturbance due to neighbour machines elsewhere while as a standalone machine in kW|
|:||Efficiency of power trains within rotor to generator in Nacelle system in series|
|:||Constants (0.89 and 0.90, resp.)|
|:||Wind farm factors like terrain and array efficiency|
|:||Von Karman’s constant and taken approximately as 0.4|
|:||Shaft power in kW|
|:||Density of wind in kg/|
|:||Friction velocity in m/s|
|:||Speed of wind in m/s|
|:||Mean wind speed at height above ground level in m/s|
|:||Wind speed at reference height above ground level in m/s|
|:||Height in m|
|:||Roughness length of terrain|
|:||Reference height in m.|
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