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Acta Physica Universitatis Comenianae Volume (2016) 5769 The Design, Construction and Analysis of Horizontal Wind Turbine for Small Scale Electric Energy Production* ¼. Staòo, V. Chudoba, M. Morvová Department of Astronomy, Physics of the Earth and Meteorology, Faculty of Mathematics, Physics and Informatics, Comenius University Bratislava, [email protected] Abstract: This paper is handling with the design, construction and study of a low-cost horizontal wind turbine as a small scale power source without the use of mechanical gears. It is focused on custom design of individual components of the system including the turbine rotor, alternator, regulation, measuring equipment and electric energy storage system. Manufacture and technological process is also described. Turbine output has been compared with the record of wind speed obtained from the anemometer that is installed on the same site, triggering the establishment of performance characteristics. Based on data analysis, the potential for energy production for various wind speed has been evaluated in order to assess the potential of using the turbine in different wind conditions. 1. Introduction The wind manifests a transformation of solar radiation into kinetic energy of an air mass. Thermal power transformed into airflow represents only about 12 % of the Sun radiation reaching the Earth's surface [8]. Nevertheless, that represents in absolute numbers 1015 W [1]. In fact, it is about 100 times the worldwide human energy consumption in all its forms. Wind flow is generally unstable, mainly due to the general circulation, climatic zone, landscape relief etc. In the lowest layer of the atmosphere (up to 50 m), the wind velocity increases roughly logarithmically over an open landscape. That is why it is important to locate WT as high above the terrain as possible. In urban areas, the situation is quite different due to the potential to form funnel effect between buildings [8]. Unstable wind velocity generates operational problems in practice. To manage this problem, it is necessary to develop sophisticated regulation and control systems that eliminate power fluctuations by means of energy storage or transformation. However, the advantage of wind is that it occurs in different intensity almost everywhere, making it one of the most affordable and sustainable source of renewable energy worldwide [1]. The conversion of this form of energy into electricity is handled for decades by means of wind turbines (hereinafter referred to as WT) of various types. Horizontal axis wind turbine (HAWT) is a type of WT with the axis of rotation directed horizontally. The advantage of HAWT compared to vertical wind turbines VAWT is that it achieves higher performance, according to comparable cross-section area. The base is very similar - the wind flows through blades and rotates it together with an axis. *) Dedicated to Prof. V. Martiovit 75-th anniversary 58 ¼. STAÒO, V. CHUDOBA, M. MORVOVÁ The axis is connected to an electric generator, either directly or by means of a gear system. However, physical principle of the wind interaction with the blade is significantly different. Considering vertical wind turbines VAWT, pressure effects (except of Darrieus WT) are utilized predominantly. In contrast, the principle of wing lift effect occurs on HAWT and that force component, which is perpendicular to the axis of rotation, generates the torque. Principles of aerodynamics, relevant to the wing lift effect generation, tend to be disrupted by turbulent airflow, resulting in disadvantage of HAWT. Our intention was to prove, that even at urban areas, this type of turbine could be harvested throughout this deficiency. 2. Theoretical background 2.1 Wind energy and efficiency of turbines The kinetic energy of air mass that flows at velocity u through an active area A and per unit of time is given as follows PW = 1 rAu 3 2 (1) To prevent any distortion of airflow, the kinetic energy cannot be converted to the torque completely, based on the theory of inertia conservation. As it can be obtained, optimal performance is achieved, when the wind speed leaving the turbine u out reaches certain value, defined as Betz condition. u out = 1 / 3 u a u1 = 2 / 3 u (2a) The efficiency of energy transformation in turbines is defined by the relationship Ptot = cp PW (2b) where c p is the power coefficient and relates the power that can be extracted from turbine Ptot to the theoretical wind power PW . One can further derive the Betz limit upon (2a), c p( Betz) = 0.59, that is the maximum of the power coefficient. 2.2 The rotor blade profile The efficiency of airflow energy transformation in the turbine of cross sectional area A (swept area) is strongly dependent on the rotational speed. The dependence is more significant by decreasing the number of blades. With the speed of rotation, the blade velocity v proportionally changes. At given distance from the axis of rotation r, relative velocity of air ua at a given speed of the wind changes, that is crucial for the lift effect (Fig. 1). Since the lifting force L (that results from airflow around the blade profile) is perpendicular to u a , the value of angle F between v and u a is important. As far as the component L sin F is parallel to the velocity v, only this component is involved to the torque generation. In Fig. 1 it can be seen that for angle F applies: THE DESIGN, CONSTRUCTION AND ANALYSIS ... 59 Fig. 1. The balance of forces on the HAWT blade aerodynamic profile (the profile represents a blade section). tgF = u1 v (3) At low speed or start-up, the ratio of inertia transferred to the turbine, compared to the inertia of air is small. It is because conditions are not suitable for lift generation and more over, pressure effect on the profile is negligible. In fact, the only parameter for a specific blade profile, which affects the lift force at given wind speed, is the angle of attack a. Because a (angle between the blade chord and u a ) is large at start-up situation, airflow around the profile forms turbulences on its end, which prevents from formation of lift. In such circumstances, the angle F is large, but the useful component of lift is also increased. Therefore, there is the speed v of blade section, at which the angle is optimal to reach maximum of useful lift component. Low torque at start-up is the main disadvantage of lift-effect turbine types. When the rotational speed approaches the speed for which blades are designed, the angle of attack became optimized and the torque rises to the maximum. Practically, the angle of attack is continuously optimized by tilting blades according to actual wind and rotor speeds. However, it applies only on large scale power plants over 10 kW for economical reasons [9]. 2.3 The wind speed and rotation TSR (Tip-Speed Ratio l), namely the ratio of the blade tip-speed v t to the wind speed u is one of the parameters of WT design. TSR refers to the maximal rotational speed of the turbine of radius R, which is kept constant after the balance of forces is achieved. l= v t 2 pRf = u u (4) Angle F may then be expressed using (3) and (4) together with Betz condition u 1 = 2u / 3 as: tgF = 2R 3rl (5) Eqn. (5) shows that for fixed point in the distance r, F is only a function of l. Therefore, TSR is critical in the design of lift type WT. To maintain the value of angle a optimized for the whole blade (tgF »1/r) it can be seen that along the blade, the angle between blade chord and a plane of rotation called tilt angle is given as (Fig. 1): Q=F -a (6) 60 ¼. STAÒO, V. CHUDOBA, M. MORVOVÁ where the optimal value of a ranges from 6 to 10 [9]. The maximal lift during start-up (F = p / 2) is therefore situated near to the rotor base, where the tilt angle is maximal. 2.4 The dependence of power coefficient on the tip-speed ratio Subtracting air friction losses D (Drag), the power spinning the turbine will be: v (R) p= ò L sin F (1 - gl)dv (7) v ( 0) where g = D / L (Fig. 1) is the draft to lift ratio. Of course, analytical expression of g does not exist and numerical methods (computer simulations) should be used to determine it [1]. We mentioned that the maximum of theoretical performance PBetz resulting from the Betz limit is proportional to c p( Betz) . That is, under real conditions, decreased due to friction looses to reduce the maximal efficiency of turbine, expressed by the Eqn. 8 and visualized in Fig. 2 (for various WT designs). c p (max) = (1 - gl )c p ( Betz) (8) Fig. 2. The power coefficient and TSR dependence for various WT types. 3. The design and construction 3.1 Rotor design We decided to use a three-bladed rotor. It was designed to cooperate with electrical generator, which converts generated torque into electric energy, so that optimization is needed to extract the power efficiently. The wind speed at which the electrical energy generation starts upon this system is called cut-on wind speed u c . For fixed number of turbine blades with radius R and resulting typical value of TSR, there is cut-on speed of rotation THE DESIGN, CONSTRUCTION AND ANALYSIS ... 61 fc obtained, at which electromagnetic braking is stimulated and the generator starts to slow down the turbine. fc = lu c 2 pR (9) In our system as discussed later, this rotational speed changes a little as a result of variable actual accumulator battery voltage. We consider average wind speed to be u c as principal parameter, which plays an important role in our calculations and is based on long term wind speed measurements and observations on Faculty of Mathematics, Physics and Informatics meteorological station. The average wind speed during period of years 20102013 shows to be between 2.7 and 3.1 m/s. If u c is considered to be too low according to the average wind speed, the torque of turbine is actually not sufficient and it stalls rapidly. In other case, too high u c means that the probability of exceeding it in local conditions becomes smaller. We chase 3 m/s as optimal cut-on wind speed. According to Eqn. 9, the value of cut-on frequency fc = 3.14 Hz was calculated. Three-bladed rotor usually has a typical TSR ranging 610, the higher the value, the more precisely fabricated rotor in general. Perfect blade design was not expected, so that resulting aerodynamics should not be overestimated, because drag to lift ratio affects the efficiency of turbine. The design of aerodynamic profile was based on Eqn. 5 and Eqn. 6 (tilt along the blade). 3.2 Generator In our project we designed a three-phase alternator to work within 12 V system. It is a type of axial-direction magnetic flux alternator with permanent neodymium magnets. Each of three phases represents three series-connected coils mutual angle of 120°. This layout determines the total induced voltage. Dimensions of the alternator components are summarized in Fig. 4. Fig. 3. The layout of magnets on the rotor disc. The source of magnetic flux is a set of 12 magnets made of neodymium iron boron (NdFeB) alloy with a magnetization of N45 and dimensions (l/w/h) 30/12/5 mm. Magnetic poles are located on 30´12 mm surfaces. Their contact force is 8.5 kg in touch. Adja- 62 ¼. STAÒO, V. CHUDOBA, M. MORVOVÁ cent magnets according to each other are attached to a rotor disc with opposite polarity orientation as shown in Fig. 3 in order to obtain required alternating magnetic flux. Based on the strength of magnetic flux, we obtained the value of cut-in rotor speed and thus the design of stator winding. Considering the speed of circular motion of magnet v m = 2pfc rm , the total induced voltage Ui on phase can be calculated (Eqn. 10) if the rotor magnets are located at a distancerm = ( R i + R o ) / 2 = 8 cm from the axis of rotation. Shape of the coil is designed so that it corresponds with magnet dimensions (especially length l = 3 cm) to maximize magnetic induction flux through the coil. U i = 3U ic = 3 - Dj = -3NBl 2 prm fc Dt (10) One can find that the number of turns in one coil should be N = 200 if Ui =14 V has to be achieved at fc =3.14 Hz. The number of turns gives the final thickness of the coil 8 mm and 15 mm in width around the perimeter. The diameter of wire is a compromise between the number of turns and minimizing the total winding resistance (to achieve minimal losses in winding that converts electric power into heat. The maximal net power that can be obtained depends on the system voltage. When the current I produced at voltage U, the net power is given as P = UI, where maximal current is determined by the nominal current load of the wiring and is roughly directly proportional to its cross section. The total electrical resistance of the copper wire R cu causes ohm losses in the winding Pcu , relating to the current consumption as Pcu=I2Rcu. Thus, electrical generator effectiveness relates to the current consumption as 1/I. The final layout of the coils was connected and embedded all together Fig. 4. The assembled alternator scheme (stator red, rotor disc blue, dimensions in mm). in a bed of epoxy resin and glass fiber composite. Phase terminals were led out for later contact connections. GND contacts are led separately for each phase to allow any changes between star and delta phase connection. In the star connection, phases are connected as parallel, so there is a sum of currents in the main terminal. 3.3 The regulation and storage system Because of the fact, that weather conditions are generally unstable, renewable energy sources such as wind cannot be simply used directly in small scale system. In our system, THE DESIGN, CONSTRUCTION AND ANALYSIS ... 63 Fig. 5. The scheme of alternator and regulator electrical connection circuit. electric energy is stored in two parallel connected 12 V lead-acid battery bank, each capacity of 90 Ah, thus together providing capacity of 180 Ah, 2.16 kWh. To ensure optimal charging cycle of batteries to prolong its lifetime and to gain maximum of the stored energy during charge-discharge cycle, electronic regulation circuit is used (the scheme is shown in Fig. 5). The main element of the circuit is an electromagnetic switch controller, which leads rectified current from the alternator either into the battery bank or, when fully charged, to feed a dummy load, which is basically a variable resistor and has a function of an electro-dynamic brake. The battery voltage is controlled to hold an optimal charging cycle. For lead-acid batteries, it is appropriate to maintain the voltage between 12 and 14 V though, at peak maximum up to 14.7 V. At discharged state (11 V), any extra discharging should not be longer allowed. This is when relay is switched to "charging" state and the generator terminal is connected to the battery positive. The state "dummy load" (putting the switch into active state) switches if the battery voltage exceeds the maximum of 14 V. It is possible to change the states, when voltage is in defined range using momentary buttons, while designed to perform a hysteresis effect automatically. To avoid discharging the battery below 11 V, internal protection of an inverter 300 W (DC/AC 12V/230V) electronics has been used. The inverter is used for generation AC of 50 Hz from DC battery voltage, which is then usable for most appliances. When the stored energy used directly from the battery (12 V appliances), an additional undercharge protection is needed. 3.4 Measurement setup The whole construction consisting of the rotor, alternator, rectifiers and a hall-effect sensor are placed on a bearing that provides vertical rotation according to wind direction by means of a tail. Rectified voltage is transferred over two-pole rotary contacts. The net power delivered to the battery was quantified as a product of battery voltage and charging current. The analog voltage signals are processed through an AD converter with the microcontroller interface, which is connected through a COM port to a computer. 64 ¼. STAÒO, V. CHUDOBA, M. MORVOVÁ To correlate the net power extracted from the turbine with the wind speed, measurement by wind speed sensor connected to a wireless Vantage Pro2 weather station from DAVIS also has been digitalized. Values of the speed and direction are averaged over minute intervals before storing in memory by associated software. 4. Experimental results 4.1 Alternator testing and calibration An important parameter that characterizes the alternator is the dependence of voltage on the rotational speed. After the alternator has been assembled, we measured voltage signals of respecting phases by means of an oscilloscope in order to verify the values calculated at specific rotor frequency. Fig. 6a. The voltage signal on one phase. Fig. 6b. The net power (blue), the loss power (red) and the efficiency (green) as a function of gained current. THE DESIGN, CONSTRUCTION AND ANALYSIS ... 65 In one phase winding, there are 6 periods of AC voltage signal generated per one revolution of the rotor with 12 magnets. The frequency of the voltage is thus firmly determined and corresponds to six times the rotor frequency in Hz or, 1/10 of revolutions per minute (rpm). This is typical for synchronous machines. As a result, the time-voltage diagram (shown in Fig. 6a) contains the information about the speed of rotation. Table 1. The technical parameters of constructed alternator. Dimensions of magnets Number of poles Rotor disc diameter 30´12´5 mm 12 100 mm Air gap 12 mm Magnetic induction 0.5 T Number of phases 3 Winding turns number 500 Wire diameter 0.8 mm Winding resistance Rated current Voltage frequency by 200 rpm 3.6 11.4 A 20 Hz Rated power 140 W Weight of winding 1.08 Kg Total weight 7 Kg Since the storage system is designed to use DC voltage, the phase outputs are rectified (the wiring diagram is shown in Fig. 5). All phases have the same waveform voltage and the same amplitude at given speed. Measurement of DC signal involving all phases shows a linear dependence on the speed with coefficient of 3.78 V/Hz, or 0.063 V/lrpm. Charging voltage 14 V is generated at the frequency of 3.7 Hz. Compared to the value in computations considered ( fc = 3.18 Hz) it represents a deviation of 16 %. 12 V is achieved at 3.3 Hz (200 rpm). This is because in calculations amplitudes are considered, while after rectification it is the effective value of AC. To specify the effective value of such non-harmonic signal would be more complicated problem and would require more complex method to determine it. In addition, rectified voltage signals of three phases overlaps, so that the effective value is higher. Additional voltage drop is caused by semiconducting rectifier. 4.2 Wind speed measurements In local wind conditions, where the turbine is situated, there is turbulent wind, because of buildings and others objects higher than the turbine. The turbine is situated 20 m from south-east corner of a building, so east and north-east winds are not utilized. South wind provides optimal conditions, with low wind speed fluctuation. However, there is west and north-west preference of airflow on this site. Measuring of the wind speed was carried out from 27.3.2015 00:00 to 2.4.2015 17:00. From anemometric data we calculated the average wind speed of 2.61 m/s and a histogram 66 ¼. STAÒO, V. CHUDOBA, M. MORVOVÁ that shows the wind speed duration (Fig. 7). Produced energy in kWh has been evaluated for this period using calibration curve (Fig. 8). Fig. 7. The histogram of wind duration (wide) and extracted power (narrow) according to the wind speed. Fig. 8. The turbine calibration curves (net power, total power, wind power and power coefficient as a function of wind speed). 4.3. Turbine performance characteristics Precise correlation of the power with wind speed is very complicated because of the presence of wind speed fluctuation. The turbine has its own momentum of inertia, like anemometer has, so measurement is actually just an average through certain time intervals and there is not the same response for the whole speed range. The reason why this happens is discussed in the section of the rotor blade profile. To minimize inaccuracies, the calibration was performed at roughly constant speed. It is difficult to quantify the turbine reaction on gusty wind, but there are studies dealing with the problem of time reaction THE DESIGN, CONSTRUCTION AND ANALYSIS ... 67 characteristics of HAWT [9]. To obtain relevant wind turbine calibration it is necessary to use a wind tunnel with well-defined conditions. This wind turbine is not designed for wind speed lower than u c , thus the reaction time is long. If fluctuations of the wind speed are around u c = 3.4 m/s (determined as the cut-in speed according to measurements) the turbine reaches necessary rotational speed for optimal operation, so reaction is dramatically different. At this speeds, fluctuations are captured efficiently. This is observed on rapid electric current increase. Data of all variables were collected every 2 seconds. The result is the calibration curve shown in Fig. 8. The power loss (heat dissipation in windings) Pcu and total electrical power Ptot = P + Pcu gathered from the turbine were also computed. The power coefficient cp has been specified according to the Eqn. (2b). It is clearly visible that cp increases up to wind speed of about 5.5 m/s. Above 5.5 m/s it remains almost constant. That means the turbine reaches optimal operating speed for maximum performance. The ratio of total electrical power to Betz limit Ptot/PBetz reaches a value »44 %. Energy efficiency h is the ratio of P/Pw. Ideally this is the product of the turbine power coefficient cp and the alternator efficiency h a . The value cp is characteristic for the turbine itself and refers to the conversion of kinetic energy of the wind into mechanical energy. Total efficiency h characterizes a whole system including more energy losses as were considered. Nevertheless, the value h is more accurate because the value was set from ratio of precisely-measured net power and wind energy (calculated from theory). In fact, there are additional losses on contacts, losses by the transfer, friction in bearings etc. that indicate inaccuracy of the power coefficient value. The efficiency of alternator rapidly decreases at high electric current. Maximum h = 17 % has been recorded at wind speed 5.5 m/s and power P = 48 W. Higher wind speed, around 8 m/s were also recorded, but just like gusty wind, therefore these data were excluded from calculations. Rated power has been observed at gusty wind 12 m/s. Fig. 9. The wind turbine power system constructed. 68 ¼. STAÒO, V. CHUDOBA, M. MORVOVÁ Table 2: Parameters of the wind turbine. Rotor radius 0.9 m Swept area 2.5 m2 Height of the axis of rotation above ground 8.5 m Tip speed ratio 5.8 Power coefficient 0.26 Cut-in wind speed 3.4 m/s Cut-in revolutions (12V) 200 rpm Cut-out wind speed Rated power Maximum efficiency factor of the system 12 m/s 140 W 17 % 5. Conclusions In this project, we used long term measurements of the wind speed in order to design the turbine rotor and adapted alternator (system for generating electrical energy) for a specific urban site. On the constructed alternator, we performed measurements intended to diagnose voltage signals. Three-bladed rotor using lifting principle, own design, has been tested in cooperation with three phase generator and calibrated on output performance according to wind speeds. Within the evaluation, the power coefficient of the turbine relative to consumed output cp = 0.26 and overall energy efficiency of the system h = 17 % has been determined. We consider important to mention that at constant wind speed, the turbine is able to spin-up already at 2 m/s. This would be impossible by using an alternator with cored winding. Even high speeds of the wind provide very low torque to run up steady HAWT. The start-up is possible only at sufficiently low static resistance, in our case there are coreless coils meaning no additional inductive losses when alternator not loaded and the resistance is generated only in bearings. Acknowledgement Effort sponsored by the Slovak Research and Development Agency APVV-0134-12, and Slovak grant agency VEGA 1/0918/15. References [1] [2] [3] [4] Andrews J., Jelley N.: Energy Science - Principles, Technologies and Impacts, Oxford Univ. Press, 2007. ISBN: 978-0-19-928112-1. Bumby J. R., Martin R.: Axial-Flux Permanent-Magnet Air-Cored Generator For Small-Scale Wind Turbines, IEE Proc.-Electr. Power Appl., Vol. 152, No. 5 (2005). Chudoba V.: túdium moností návrhu, kontrukcie a vyuitia vertikálnych veterných ruíc v podmienkach Bratislavy, Bakalárska práca, 2013. Hosseini S. M., Mirsalim M. A., Mirzaei M.: Design, Prototyping, and Analysis of a Low Cost Axial-Flux Coreless Permanent-Magnet Generator, IEEE Trans. on Magnetics, vol. 44, 1 (2008) 7580. THE DESIGN, CONSTRUCTION AND ANALYSIS ... [5] 69 Chalmers B. J., Wu W.: An Axial-Flux, Premanent-Magnet Generator for a Gearless Wind Energy System, IEEE Trans. on Energy Conv., vol. 14, 2 (1999) 251257. [6] Chan T. F., Lai L. L.: An Axial-Flux Permanent-Magnet Synchronous Generator for a Direct-Coupled Wind-Turbine System, IEEE Trans. on energy conv., vol. 22, No. 1 (2007). [7] Morady H., Afjei E.: Analysis of Brushless DC Generator Incorporating an Axial Field Coil, Energy Conversion and Management, vol. 52 (2011) 27122723. [8] Morvová M.: Princípy metód a vyuitie obnovite¾ných zdrojov energie, Kninièné a editaèné centrum FMFI UK, 2008. ISBN: 978-80-89186-28-0. [9] Wright A.K. , Wood D.H.: The Starting and Low Wind Speed Behaviour of a small Horizontal Axis Wind Turbine, Journal of Wind Engineering and Industrial Aerodynamics, vol. 92 (2004) 12651279 [10] Wang R. J., Kamper M. J., Calculation of Eddy Current Loss in Axial Field Permanent-Magnet Machine With Coreless Stator, IEEE Trans. on Energy Conv., vol. 19, 3 (2004) 532538.