energies

Article Performance Prediction and Validation of a Small-Capacity Twisted Savonius Turbine

Hyeonmu Jang 1, Insu Paek 1,*, Seungjoo Kim 2 and Deockjin Jeong 3

1 Department of Advanced Mechanical Engineering, Kangwon National University, Chuncheon-si 24341, Korea; [email protected] 2 Korea Testing Certification, 22, Heungan-dearo 27, Gunpo-si 15809, Korea; [email protected] 3 JH Energy, 112, Toegyenonggong-ro, Chuncheon-si 24427, Korea; [email protected] * Correspondence: [email protected]

 Received: 17 April 2019; Accepted: 3 May 2019; Published: 7 May 2019 

Abstract: In this study, an off-grid–type small for street lighting was designed and analyzed. Its performance was predicted using a computational fluid dynamics model. The proposed wind turbine has two blades with a radius of 0.29 m and a height of 1.30 m. Ansys Fluent, a commercial computational fluid dynamics solver, was used to predict the performance, and the k-omega SST model was used as the turbulence model. The simulation result revealed a tip-speed ratio of 0.54 with a maximum power coefficient, or an aerodynamic rotor efficiency of 0.17. A wind turbine was installed at a measurement site to validate the simulation, and a performance test was used to measure the power production. To compare the simulation results obtained from the CFD simulation with the measured electrical power performance, the efficiencies of the generator and the controller were measured using a motor-generator testbed. Also, the control strategy of the controller was found from the field test and applied to the simulation results. Comparing the results of the numerical simulation with the experiment, the maximum power-production error at the same wind speed was found to be 4.32%.

Keywords: ; twisted Savonius wind turbine; computational fluid dynamics

1. Introduction The cumulative installation capacity of small wind turbines achieved 1.7 gigawatts (GW) in 2017 and is expected to reach 2.0 GW by 2020 [1–5]. Small wind turbines are defined slightly differently for different countries and standards; however, in Korea, they are classified as wind turbines with a rotor area smaller than 200 square meters, and a rated voltage lower than 1000 volts alternating current (VAC) or 1500 volts direct current (DC) [6]. Unlike utility-scale wind turbines, which have a rated power of 750 kW or higher, small-capacity wind turbines include various different types, e.g., horizontal axis or vertical axis and lift-type or drag-type [7]. Horizontal-axis wind turbines have higher aerodynamic efficiencies than their vertical-axis counterparts, and lift-type wind turbines have higher aerodynamic efficiencies than their drag-type counterparts. However, because small-capacity wind turbines are often installed in the vicinity of residential areas, other parameters, e.g., rotating speed, durability, noise, and visual impact, are often considered to be more important than efficiency. This is why drag-type wind turbines still survive and are used to generate electricity. Drag-type wind turbines come in two varieties. One is the original Savonius wind turbine. The other is a modified version to increase its aerodynamic efficiency, which is called a twisted (or helical) Savonius wind turbine. Drag-type wind turbines used for electricity generation are mostly twisted Savonius wind turbines, due to their higher aerodynamic efficiency compared to the original Savonius wind turbines. Twisted Savonius wind turbines are commonly used to power street lamps with solar panels and small

Energies 2019, 12, 1721; doi:10.3390/en12091721 www.mdpi.com/journal/energies Energies 2019, 12, 1721 2 of 12 batteries. In most cases, these micro-wind turbines are installed at a street side as a cluster with more than two turbines. Based on recent studies, the energy efficiency of the micro-wind turbine cluster is found to be improved by optimized decision-making for turbine installation locations [8,9]. The blade-element momentum theory is used to analyze the performance of typical horizontal-axis wind turbines with two or three blades. The theory has been implemented in programs such as Bladed [10] and FAST [11], which are officially used by wind-turbine certification companies to verify the overall system performance. The theory has been experimentally validated over a long time and is now considered relatively accurate. Two methods are typically used to analyze the aerodynamic performance of vertical-axis wind turbines. One is the single or multiple streamtube method [12,13]. These methods are implemented in programs, e.g., Qblade [14] and HAWC2 [15], and are used to analyze lift-type wind turbines, e.g., Darrious or H-Darrious wind turbines. The other method includes the RANS (Reynolds-averaged Navier–Stokes equations) or LES (Large Eddy Simulations) methods, which are implemented in general CFD (computational fluid dynamics) programs, e.g., Fluent or Ansys-CFX. Although simulation methods for horizontal-axis wind turbines have been thoroughly verified experimentally because they are the standard technology for utility-scale wind turbines, simulation methods and experimental validations for vertical-axis wind turbines are still limited. Furthermore, performance simulations and experimental validations for drag-type vertical-axis wind turbines are very rare. The drag-type wind turbine, which is the subject of this study, rotates and generates electricity by the drag forces acting on the blade-surface sections. Thus, the shape of the turbine rotor affects the turbine performance. Therefore, selecting an optimized shape is crucial for wind-turbine performance. In the literature, optimum turbine-shape parameters for Savonius rotors have been suggested by researchers. Various studies present optimum values for the wind-turbine blade gap and the aspect ratio, defined as the blade height divided by the blade radius [16–20]. Blackwell [21] and Mahmoud [22] conducted studies on the blade number for optimal performance. Therefore, the wind turbine developed in this study was designed using the optimal parameters presented in the aforementioned literature. As mentioned above, Savonius wind turbines rotate from the drag force on the blade sections. However, most commercial software for analyzing and designing wind turbines is limited to lift-type wind turbines that rotate by lift force. Therefore, many researchers have been analyzing Savonius wind turbines using general CFD software. Fujisawa [23] conducted a wind-turbine flow-field analysis and Modi [24] conducted a Savonius wind-turbine flow-field analysis by adapting the discrete vortex method. The power performance of the wind turbines was not presented in their studies. Shinohara and Ishimatsu analyzed the performance of a Savonius-type wind turbine through CFD simulation [25]. Redchyts and Prykhodko also conducted a wind-turbine analysis using the unstructured finite-volume method [26]. Although most of the studies have carried out wind-turbine performance prediction by simulating the turbine-wake effect, etc., the experimental verification of simulation results using field tests is very rare. Therefore, in this study, an ultra-small wind turbine for urban street lighting was designed, based on the results in the literature, and its aerodynamic performance was predicted using CFD simulations. Based on the simulation results, a torque schedule for maximizing the aerodynamic efficiency was also proposed, and was implemented in the controller of a wind turbine. Finally, the wind-turbine performance predicted by simulation was verified by field tests. In addition, the performance test of the generator and controller assembly was conducted to obtain the efficiency of the power train of the wind turbine with respect to the rotational speed. Through the result of the test, the mechanical power of the rotor could be converted into electrical power and compared with the measured electrical power from the field test for experimental validation.

2. Simulation Modeling of a Wind Turbine Figure1 shows the simulation procedure to estimate the turbine performance. As the first step, a modeling was performed by smoothing sharp edges, eliminating unnecessary components including Energies 2019, 12, x FOR PEER REVIEW 3 of 12 Energies 2019, 12, x FOR PEER REVIEW 3 of 12 Energies 2019, 12, 1721 3 of 12 a modeling was performed by smoothing sharp edges, eliminating unnecessary components a modeling was performed by smoothing sharp edges, eliminating unnecessary components including the generator, and generating meshes with suitable partitions. In this study, tetrahedral includingthe generator, the generator, and generating and generating meshes with meshes suitable with partitions. suitable partitions. In this study, In this tetrahedral study, tetrahedral mesh was mesh was used for simulation. As the second step, the solution of the flow field was obtained. A meshused forwas simulation. used for simulation. As the second As step,the second the solution step, ofthe the solution flow field of wasthe flow obtained. field Awas pressure obtained. based A pressure based transient method was used for the simulation, and the pressure and velocity pressuretransient methodbased transient was used method for the was simulation, used for and the the simulation, pressure and and velocity the pressure distributions and velocity around distributions around the rotor were obtained. Finally as the third step, the rotor torque was obtained distributionsthe rotor were around obtained. the rotor Finally were as obtained. the third step,Finally the as rotor the third torque step, was the obtained rotor torque from was the obtained pressure from the pressure distribution, and finally the mechanical power of the rotor was obtained based on fromdistribution, the pressure and finally distribution, the mechanical and finally power the ofmech theanical rotor waspower obtained of the basedrotor was on the obtained rotational based speed on the rotational speed of the rotor. The entire procedure from step 2 to step 3 was repeated with theof the rotational rotor. The speed entire of procedure the rotor. from The step entire 2 to proc stepedure 3 was from repeated step with 2 to di stepfferent 3 was rotational repeated speeds with to different rotational speeds to obtain simulations with different tip speed ratios. differentobtain simulations rotational withspeeds di fftoerent obtain tip si speedmulations ratios. with different tip speed ratios.

Figure 1. Procedure of a simulation to analyze the Savonius wind turbine. FigureFigure 1. ProcedureProcedure of of a a simulation simulation to to anal analyzeyze the Savonius wind turbine.

2.1. Wind-Turbine Numerical Model and Parameters 2.1. Wind-Turbine Wind-Turbine Numerical Model and Parameters The target wind turbine in this study is a drag-type twisted Savonius wind turbine with 2 blades. The target wind turbine in this this study study is is a a drag-type drag-type twisted twisted Savonius Savonius wind wind turbine turbine with with 2 2 blades. blades. As shown in Figure 2 and Table 1, the rotor of the target Savonius wind turbine is composed of two As shown in Figure Figure 22 andand TableTable1 ,1, the the rotor rotor of of the the target target Savonius Savonius wind wind turbine turbine is is composed composed of of two two semi-circular blades with a diameter of 0.58 m and a height of 1.3 m. It has a gap of 0.15 m between semi-circular bladesblades withwith aa diameter diameter of of 0.58 0.58 m m and and a heighta height of of 1.3 1.3 m. m. It hasIt has a gap a gap of 0.15of 0.15 m betweenm between the the two blades so that air can pass through the gap. This type of air gap is shown in the literature to thetwo two blades blades so that so that air can air passcan pass through through the gap. the Thisgap. typeThis of type air gapof air is gap shown is shown in the literaturein the literature to slightly to slightly increase the aerodynamic efficiency of the Savonius rotor. The mechanical power extracted slightlyincrease increase the aerodynamic the aerodynamic efficiency efficiency of the Savonius of the rotor.Savonius The mechanicalrotor. The mechanical power extracted power by extracted the rotor by the rotor is converted into electrical power via a permanent-magnet synchronous generator, and byis convertedthe rotor is into converted electrical into power electrical via a permanent-magnetpower via a permanent-magnet synchronous generator,synchronous and generator, the generated and the generated electrical power is stored in a 24 VDC battery. As generated power is stored in a battery, theelectrical generated power electrical is stored power in a 24 is VDC stored battery. in a 24 As VD generatedC battery. power As generated is stored power in a battery, is stored a type in ofa battery, turbine a type of turbine grid-off or independent grid system, then the main purpose of the turbine is lighting agrid-off type of or turbine independent grid-off grid or independent system, then grid the main system purpose, then the of the main turbine purpose is lighting of the turbine LED street is lighting lamps. LED street lamps. LED street lamps.

FigureFigure 2. 2. GeometryGeometry of of a a Savonius Savonius wind wind turbine turbine.. Figure 2. Geometry of a Savonius wind turbine. Table 1. Wind-turbine specifications. Table 1. Wind-turbine specifications. Table 1. Wind-turbine specifications. Subject Unit Specification Subject Unit Specification Subject Unit Specification RotorRotor Diameter Diameter m m 0.58 0.58with with 0.15 0.15 gap gap RotorRotor Height Diameter m m 0.58 with 0.15 1.30 gap Rotor Height m 1.30 TurbineRotor Type Height -m Twisted 1.30 Savonius Turbine Type - Twisted Savonius Turbine Type - Twisted Savonius

ForFor a a wind turbine, thethe mostmost importantimportant parameter parameter representing representing its its power-production power-production ability ability is the is For a wind turbine, the most important parameter representing its power-production ability is power coefficient, C [26]. It can be defined as the non-dimensional ratio of the rotor’s mechanical the power coefficient,P C [26]. It can be defined as the non-dimensional ratio of the rotor’s the power coefficient, C [26]. It can be defined as the non-dimensional ratio of the rotor’s mechanicalpower to the power power to carried the power by the carried wind by passing the wind through passing the through rotor-swept the rotor-swept area. The power area. coe Theffi powercient is mechanical power to the power carried by the wind passing through the rotor-swept area. The power coefficientalso known is asalso the known aerodynamic as the aerodynamic efficiency of theeffic rotor,iency andof the can rotor, be calculated and can be by calculated by coefficient is also known as the aerodynamic efficiency of the rotor, and can be calculated by 𝑃Pmech C = 𝑃 (1) 𝐶P = 3 (1) 𝐶 =0.5𝜌𝐴𝑉0.5ρAV (1) 0.5𝜌𝐴𝑉 3 2 wherewhere P𝑃mech [W] [W] is is the the mechanical mechanical power power extracted extracted by by the the rotor, rotor,ρ [kg 𝜌/ m[kg/m] is the] is air the density, air density,A [m ]𝐴 is where 𝑃 [W] is the mechanical power extracted by the rotor, 𝜌 [kg/m ] is the air density, 𝐴 [them] rotor-swept is the rotor-swept area, and area,V [mand/s] 𝑉 is [m/s]the wind is the speed. wind speed. [m] is the rotor-swept area, and 𝑉 [m/s] is the wind speed.

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Another variable that can explain the turbine’s characteristics is the tip-speed ratio. It is the ratio Another variable that can explain the turbine’s characteristics is the tip-speed ratio. It is the ratio of the linear velocity at the blade tip to the wind speed [27], and is mathematically defined as of the linear velocity at the blade tip to the wind speed [27], and is mathematically defined as 𝑅Ω λ = RΩ (2) λ = 𝑉 (2) V wherewhere 𝑅R [m] [m] isis the radius of of the the wind wind turbine turbine rotor, rotor, and and 𝛺Ω [rad [rad/s]/s] is the rotational rotational speed speed of of the the wind wind turbineturbine rotor. rotor. ForFor all all wind turbines,turbines, the the power power coe coefficientfficient is ais nonlinear a nonlinear function function of the of tip-speed the tip-speed ratio. Itratio. initially It initiallyincreases increases with the with tip-speed the tip-speed ratio, reaches ratio, reaches its maximum, its maximum, and then and decreases. then decreases. Therefore, Therefore, after the after rotor theis designed,rotor is designed, it is essential it is essential to find out to itsfind maximum out its maximum power coe powerfficient coefficient and the tip-speed and the tip-speed ratio where ratio the wheremaximum the maximum power coe powerfficient coefficient is achieved. is Then,achieved. this Then, information this information is applied tois theapplied controller to the tocontroller maintain tothe maintain optimum the tip-speed optimum ratio, tip-speed regardless ratio, ofregardless the wind of speed. the wind speed.

2.2.2.2. Computational Computational Fluid Fluid Dynamics Dynamics Model Model ToTo simulate simulate the the performance performance of of the the designed designed wind-turbine wind-turbine rotor, rotor, Ansys Ansys Fluent, Fluent, a a commercial commercial computationalcomputational fluid fluid dynamics dynamics solver, solver, was was used, used, with with a a𝑘𝜔k ω shear stress stress transport transport (SST) (SST) turbulent turbulent − model.model. All All the the walls walls except except the the inlet, inlet, outlet, outlet, and androtor rotor surface surface were wereset as set non-slip as non-slip walls. walls.For the Forinitial the andinitial boundary and boundary conditions, conditions, the wind the windspeed speed was wasgiven given at the at theinlet inlet area, area, and and the the pressure-release pressure-release conditioncondition at at the the outlet outlet was was used. used. ToTo conduct andand calculate calculate the the rotor rotor mechanical mechanical power power produced produced by the bladeby the rotation, blade arotation, simulation a simulationwas conducted was conducted with initial with conditions, initial conditions, including in thecluding inlet speed, the inlet outlet speed, pressure, outlet pressure, and rotor and rotational rotor rotationalspeed, in rpm.speed, in rpm. ForFor the the simulation, thethe analysis analysis domain domain is dividedis divided into into four four sections sections that arethat the are upstream the upstream section, section,the internal the internal section, section, the rotor, the and rotor, the and downstream the downstream section section as shown as shown in Figure in3 Figurea. The 3(a). rotor The is a rotor solid isgeometry a solid geometry section. section. After the After simulation, the simulation, the rotor the torquerotor torque is calculated is calculated from from this section’sthis section’s area area that thatcontacts contacts the internalthe internal section. section. This This internal internal section section rotates rotates at the at same the same speed speed as the as rotor the rotor section section and is andthe is shape the shape of a cylinder, of a cylinder, including including an entire an entire rotor dimensionrotor dimension of φ 0.6 of m𝜙 0.6h 1.30 mm .h This1.30 internal. m This internal section × sectionis where is where the air the particles air particles flow around flow around the rotor the and roto causesr and causes rotor torque;rotor torque; hence, hence, the rotor the torquerotor torque can be canobtained be obtained in this in section this section from from the relation the relation between between the forces the forces generated generated by the by airthe drag air drag and and lift, lift, and andthe the distance distance from from the the rotational rotational axis axis of theof the rotor. rotor. The The inter-sectional inter-sectional area, area, with with the the rotor rotor part part and and the theboundary boundary area area similar similar to a to wind a wind tunnel, tunnel, contains cont layeredains layered meshes meshes that can that help can transfer help transfer the flowing the flowingparticles’ particles’ momentum. momentum. The internal The internal section section is sandwiched is sandwiched by upstream by upstream and downstream and downstream sections. sections.These sections These sections have a 6-mhave width, a 6-m 3-mwidth, height, 3-m height, and 5-m and length. 5-m length. The length The length of the of sections the sections is about is about10 rotor 10 diametersrotor diameters (D). (D).

(a) (b)

FigureFigure 3. Analysis 3. Analysis domain domain and and generation generation of mesh of mesh for simulation:for simulation: (a) analysis(a) analysis domain; domain; (b) ( generationb) generation of mesh. of mesh.

With a divided fluid field and target bodies, a mesh was generated for a turbine analysis in the shape of a triangular pyramid, called a “Tetra” in Fluent and other solver tools. To prevent the

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Energies 2019, 12, x FOR PEER REVIEW 5 of 12 With a divided fluid field and target bodies, a mesh was generated for a turbine analysis in the shapeanalysis of time a triangular delay and pyramid, the convergence called a “Tetra” error incaus Fluented by and the other mesh solver densification tools. To around prevent complex the analysis and timesharp delay elements, and thee.g., convergence nuts and bolts, error the caused frames by are the simplified mesh densification or suppressed around within complex a range and that sharp does elements,not affect the e.g., aerodynamic nuts and bolts, performance the frames and are analys simplifiedis. The or element suppressed distribution within aand range shape that are does shown not ainff ectFigure the 3(b). aerodynamic The meshed performance model consists and analysis. of 5,539,770 The elements elementdistribution and 1,032,475 and nodes. shape are shown in FigureTo3 b.perform The meshed the flow model analysis, consists the initial of 5,539,770 conditions elements for the and velocity 1,032,475 inlet nodes. and pressure outlet were definedTo performas the wind the flowspeed analysis, and zero the Pascals initial in conditions gauge pressure, for the velocityrespectively. inlet For and the pressure velocity outlet inlet, were the definedinput wind as the speed wind was speed selected and zero in Pascalsthe range in gaugeof 3 m/ pressure,s to 15 respectively.m/s with intervals For the of velocity 2 m/s, inlet,and the inputrotational wind speed speed of was the selectedrotor was in selected the range in of the 3 m range/s to 15of m10/ srpm with to intervals 250 rpm. of Considering 2 m/s, and the the rotational analysis speedtime, the of theinterval rotor defining was selected the rota in thetional range speed of 10 was rpm selected to 250 rpm.as 30 Consideringrpm. The final the output analysis is time,the force the intervaland pressure defining acting the rotationalon the rotor speed surface was selectedgenerated as 30by rpm. the airflow The final and output the torque is the force calculated and pressure in the actingrotational on theaxis rotor to calculate surface the generated mechanical by the power. airflow The and analysis the torque was performed calculated inat 0.01-s the rotational intervals, axis with to calculate10-s for case. the mechanical power. The analysis was performed at 0.01-s intervals, with 10-s for case.

3. Simulation Results

3.1. Pressure and Velocity Distribution Figure 44 shows the the CFD CFD simulation simulation results. results. Because Because of the of thetetrahedral tetrahedral mesh, mesh, the wind the wind flow could flow couldnot be notaligned be aligned to the grid. to the In grid. this case, In this the case, 2nd order the 2nd accuracy order accuracy method is method known is to known be more to accurate be more accuratethan the 1 thanst order the method. 1st order So, method. the level So, of the the level solution’s of the convergence solution’s convergence and the accuracy and the of accuracy simulation of simulationwas carried was out carriedusing the out 2 usingnd order the accuracy 2nd order method. accuracy The method. rotor shown The rotor in the shown figure in theis in figure a state is inof arotation state of with rotation a speed with of a speed200 RPM of 200 under RPM the under mean the wind mean speed wind of speed 13 m/s. of 13 Figure m/s. Figure 4(a) shows4a shows the thepressure pressure distribution distribution on the on therotor rotor surface surface and andthe thesurrounding surrounding area area around around the therotor. rotor. Based Based on the on thefigure, figure, the maximum the maximum pressure pressure can canbe found be found to occur to occur on the on inner the inner (concave) (concave) surface surface of the of rotor the rotor that thatdirectly directly faces facesthe incoming the incoming wind. wind. At the Atsame the time, same it time, can be it seen can bethat seen the thatdirection the direction of rotor rotation of rotor rotationis counter-clockwise is counter-clockwise as the lowest as the pressure lowest pressureis distributed is distributed on the opposite on the oppositeouter (convex) outer (convex)surface. surface.Figure 4(b) Figure shows4b shows the velocity the velocity of air of predicted air predicted under under the thesame same condition condition as asFigure Figure 4(a).4a. AsAs thethe pressure ofof thethe fluid fluid increases, increases, the the flow flow velocity velocity decreases, decreases, resulting resulting in the in lowestthe lowest value value inside inside the rotor the androtor the and maximum the maximum value value on the on outer the outer surface. surface.

(a) (b)

FigureFigure 4. Simulation 4. Simulation results results with inletwith windinlet speedwind speed of 13 m of/s: 13 (a )m/s: pressure (a) pressure distribution; distribution; (b) velocity (b) velocity distribution. distribution. 3.2. Power Coefficient vs. Tip-Speed Ratio 3.2. PowerBased Coefficient on the simulation vs. Tip-Speed model Ratio described in 2.2, a wind turbine performance prediction simulation was performedBased on andthe thesimulation power coe modelfficients described were obtained in 2.2, with a variouswind turbine operating performance conditions asprediction shown in Figuresimulation5. The was horizontal performed axis ofand the figurethe power is the tipcoeffi speedcients ratio were mentioned obtained in Equation with various (2). As operating expected, theconditions power coeas shownfficient in increased Figure 5. nonlinearly The horizontal as the axis tip of speed the figure increased is the and tip reached speed ratio its maximum mentioned and in decreased.Equation (2). According As expected, to the the analysis,power coefficient the maximum increased efficiency nonlinearly of the as Savonius the tip speed rotor increased was found and to bereached 0.17(17%) its maximum and the optimumand decreased. tip speed According ratio of theto the turbine analysis, was predictedthe maximum to be 0.54.efficiency The powerof the coeSavoniusfficients rotor of other was Savoniusfound to windbe 0.17 turbines (17%) availableand the optimum in the literature tip speed are foundratio of to the be betweenturbine 0.12was predicted to be 0.54. The power coefficients of other Savonius wind turbines available in the literature are found to be between 0.12 and 0.22 with the tip speed ratio between 0.4 and 0.6 [28,29]. Therefore

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the performance of the twisted Savonius wind turbine in this study is not much different from the Energiesperformances2019, 12, 1721 of other twisted Savonius wind turbines with different capacities in the literature.6 of 12

Energies 2019, 12, x FOR PEER REVIEW 6 of 12 and 0.22 with the tip speed ratio between 0.4 and 0.6 [28,29]. Therefore the performance of the twisted Savoniusthe performance wind turbine of the in twisted this study Savonius is not muchwind turbine different in from this study the performances is not much ofdifferent other twisted from the Savoniusperformances wind turbines of other withtwisted diff erentSavonius capacities wind turbines in the literature. with different capacities in the literature.

Figure 5. Estimated power coefficient.

3.3. Power Curve Based on Maximum Power Coefficient

Figure 6 shows the aerodynamic power and the rotor speed with respect to the wind speed calculated with the assumptionFigure thatFigure the 5. Estimated5.rotor Estimated speed power power is controlled coe coefficient.fficient. to maintain the optimal tip speed ratio to achieve the maximum power coefficient as shown in Figure 5. The aerodynamic power in 3.3.Figure3.3. Power Power 6(a) Curve Curvewas Based calculated Based on on Maximum Maximum using Equation Power Power Coe Coefficient(1).fficient The power coefficient used in the equation was 0.17, and the optimum tip speed ratio was 0.54. However, actual wind turbines need to consider external FigureFigure6 shows 6 shows the the aerodynamic aerodynamic power power and and the the rotor rotor speed speed with with respect respect to theto the wind wind speed speed environmental conditions, such as electrical losses and overspeed protection, which may alter the calculatedcalculated with with the the assumption assumption that that the the rotor rotor speed speed is controlledis controlled to to maintain maintain the the optimal optimal tip tip speed speed torque-speed schedule of the generator and finally the turbine power performance. The power in ratioratio to to achieve achieve the the maximum maximum power powe coer coefficientfficient as as shown shown in in Figure Figure5. The5. The aerodynamic aerodynamic power power in in Figure 5 does not reflect these external constraints and is just an ideal power production. FigureFigure6a 6(a) was was calculated calculated using using Equation Equation (1). (1). The Th powere power coe coefficientfficient used used in in the the equation equation was was 0.17, 0.17, Figure 6(b) shows the rotational speed schedule calculated with the assumption that the andand the the optimum optimum tip tip speed speed ratio ratio was was 0.54. 0.54. However, However, actual actual wind wind turbines turbines need need to to consider consider external external maximum power point tracking (MPPT) control is performed with the optimum tip speed ratio of environmentalenvironmental conditions, conditions, such such as as electrical electrical losses losses and and overspeed overspeed protection, protection, which which may may alter alter the the 0.54. Through the aforementioned Equation (2), the rotational speed can be calculated through the torque-speedtorque-speed schedule schedule of of the the generator generator and and finally finally the the turbine turbine power power performance. performance. The The power power in in relationship of tip speed ratio and the wind speed. The result is a linear line shown in Figure 6(b). FigureFigure5 does 5 does not not reflect reflect these these external external constraints constraints and and is just is just an idealan ideal power power production. production. Figure 6(b) shows the rotational speed schedule calculated with the assumption that the maximum power point tracking (MPPT) control is performed with the optimum tip speed ratio of 0.54. Through the aforementioned Equation (2), the rotational speed can be calculated through the relationship of tip speed ratio and the wind speed. The result is a linear line shown in Figure 6(b).

(a) (b)

FigureFigure 6. 6.Simulation Simulation results results on on power power and and rotor rotor speed: speed: (a ()a mechanical) mechanical power; power; (b ()b rotor) rotor speed. speed.

4. Power-PerformanceFigure6b shows the rotationalTest for Validation. speed schedule calculated with the assumption that the maximum power point tracking (MPPT) control is performed with the optimum tip speed ratio of 0.54. Through For the verification(a )and validation of the wind-turbine performa(bnce) estimated by CFD the aforementioned Equation (2), the rotational speed can be calculated through the relationship of tip simulation, the target twisted Savonius wind turbine was manufactured. Before the power speed ratioFigure and the 6. Simulation wind speed. results The on result power is and a linear rotor linespeed: shown (a) mechanical in Figure power;6b. (b) rotor speed. performance testing of the wind turbine, a performance test of the generator was performed. Also, the4. Power-Performance power regulation scheme Test for was Validation. determined and applied to the controller of the wind turbine. For the verification and validation of the wind-turbine performance estimated by CFD simulation, the target twisted Savonius wind turbine was manufactured. Before the power performance testing of the wind turbine, a performance test of the generator was performed. Also, the power regulation scheme was determined and applied to the controller of the wind turbine.

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4.4.1. Power-Performance Generator – Controller-related Test for ValidationTest ForThe theperformance verification test and of validation the generator of the was wind-turbine conducted performanceat Korea Test estimated and Certification by CFD simulation,(KTC). As theshown target in twistedFigure 7, Savonius the test wind facility turbine consists was of manufactured. a motor that Before can simulate the power the performance mechanical testingpower ofgenerated the wind by turbine, a rotor, a and performance a generator test with of the a contro generatorller of was the performed.actual turbine Also, system. the power The mechanical regulation schemepower generated was determined by the andmotor applied is transferred to the controller to the generator of the wind through turbine. coupling, and the torque is measured through a torque sensor attached to the rotary shaft. The mechanical power, which is the 4.1.input Generator—Controller-related to the generator, the electrical Test power output from the generator (AC), and the power output fromThe the performancecontroller (DC) test are of the all generatorobtained wasduring conducted the test. at The Korea test Test was and carried Certification out with (KTC). 10 rpm As shownintervals in from Figure 607, to the 230 test rpm, facility and consists the detailed of a motor test procedures that can simulate with a the schematic mechanical are poweras follows. generated by aFirstly, rotor, and the arotational generator speed with aof controller the motor of theis adjusted, actual turbine and the system. mechanical The mechanical power output power is generatedcalculated bythrough the motor the rotational is transferred speed to theof the generator motor throughand the torque coupling, measured and the through torque is the measured torque throughsensor. Also, a torque the generator sensor attached output is to measured the rotary using shaft. a Thepower mechanical analyzer. power,At this whichpoint, it is is the possible input to thecalculate generator, the generator the electrical efficiency power by output comparing from theit with generator the mechanical (AC), and power. the power Finally, output the output from the of controllerthe rectified (DC) power are allis analyzed obtained through during the the test. controller The test and was the carried efficiency out withof the 10 controller rpm intervals is calculated from 60 toby 230comparing rpm, and it thewith detailed the measured test procedures generator with power. a schematic are as follows.

Figure 7. Generator and controller test set-up.

Firstly,Figure the8 shows rotational the speedelectrical of the efficiency motor is of adjusted, the generator and the mechanicaland controller power measured output isfor calculated various throughrotational the speeds. rotational The dots speed and of thecircles motor are andactual the measured torque measured points and through the dotted the torque and dashed sensor. curves Also, theare their generator regression output curves. is measured The reason using why a power the data analyzer. have some At this scatters point, is itthat is possiblethey include to calculate transients. the generatorBased on the effi figure,ciency the by comparinggenerator efficiency it with the (dashe mechanicald line) increases power. Finally, slowly the with output the rotational of the rectified speed powerof the generator. is analyzed It through is fluctuating the controller but mostly and thelower effi ciencythan 0.7 of when the controller the rotational is calculated speed byis lower comparing than itabout with 150 the rpm, measured and becomes generator higher power. than 0.7 when the rotational speed is higher than 150 rpm. While the generatorFigure8 shows efficiency the electricalshows a relatively e fficiency slow of the increasing generator trend, and the controller controller measured efficiency for (dashed various rotationalline) significantly speeds. Thechanges dots with and circlesthe rotational are actual speed. measured It starts points with and very the lo dottedw efficiency and dashed around curves 0.2 arewhen their the regression rotational curves. speed is The about reason 60 whyrpm and the dataincreases have somenonlinearly scatters with is that the they rotational include speed. transients. The Basedefficiency on thereaches figure, about the generator 0.6 when e ffitheciency rotational (dashed speed line) is increases about 140 slowly rpm, with and the after rotational that an speed abrupt of theincrease generator. in efficiency It is fluctuating is observed. but mostly Between lower 150 than rpm 0.7 whenand 190 the rpm rotational the efficiency speed is lower of about than about0.8 is 150maintained rpm, and and becomes after that higher it decreases than 0.7 to when 0.74. theBased rotational on Figure speed 7, at isleast higher 130 rpm than is 150 needed rpm. to While achieve the generatorthe controller effi ciencyefficiency shows of 0.6. a relatively slow increasing trend, the controller efficiency (dashed line) significantly changes with the rotational speed. It starts with very low efficiency around 0.2 when the rotational speed is about 60 rpm and increases nonlinearly with the rotational speed. The efficiency reaches about 0.6 when the rotational speed is about 140 rpm, and after that an abrupt increase in efficiency is observed. Between 150 rpm and 190 rpm the efficiency of about 0.8 is maintained and after that it decreases to 0.74. Based on Figure7, at least 130 rpm is needed to achieve the controller efficiency of 0.6.

Energies 2019, 12, 1721 8 of 12 Energies 2019, 12, x FOR PEER REVIEW 8 of 12

FigureFigure 8. Generator 8. Generator and and controller controller e ffiefficiencyciency correspondingcorresponding to to the the ro rotationaltational speed. speed.

4.2. Test4.2. Environments Test Environments The power-performanceThe power-performance test test of of the the wind wind turbine turbine waswas carried out out at at the the small small wind-turbine wind-turbine test test site ofsite Kangwon of Kangwon National National University, University, which which is is located located at Pyeongchang, Gangwon Gangwon Province, Province, Korea. Korea. The averageThe average wind wind speed speed at the at the 10-m 10-m hub hub height height was was 4.6 4.6 mm/s/s during the the test. test. The powerThe power performance performance test test of of a winda wind turbine turbine consistsconsists of of the the power power measurement measurement of the of thewind wind turbineturbine and theand measurementthe measurement of of wind wind data. data. Wind Wind datadata including wind wind speed speed and and direction direction were were measured through sensors installed to a mast at the hub height of the wind turbine. The mast was measured through sensors installed to a mast at the hub height of the wind turbine. The mast was located about 4 rotor diameter (RD) away from the wind turbine. The wind speed was measured locatedusing about an 4anemometer rotor diameter and (RD)an F–V away Converter from the was wind used turbine. to convert The the wind frequency speed obtained was measured from the using an anemometersensor to voltage. and an To F–V measure Converter the wind was turbine used to power, convert a power the frequency analyzer obtainedwas used. fromThe generated the sensor to voltage.power To measureand the wind the winddata were turbine stored power, in a personal a power computer analyzer through was used. a data The acquisition generated system. power All and the windthe data data were were saved stored with in aa personal1 Hz sampling computer rate. through a data acquisition system. All the data were saved with a 1 Hz sampling rate. 4.3. Measured Test Results 4.3. MeasuredFigure Test 9 Resultsshows the measurement results of the wind turbine. Figure 9(a) shows the measured Figureelectrical9 shows power thewith measurement respect to wind results speed. ofThe the dots wind in the turbine. figure represent Figure9a one-minute shows the averaged measured electricalpower power which with is used respect to show to windthe power speed. performance The dots of in a thesmall figure capacity represent wind turbine one-minute based on averaged the standard. The solid line with asterisks in the figure represents the averaged value in a bin, which has power which is used to show the power performance of a small capacity wind turbine based on the an interval of 0.5 m/s. Figure 9(b) shows the rotational speed of the generator with respect to the wind standard. The solid line with asterisks in the figure represents the averaged value in a bin, which has speed. The rotational speed of the generator was calculated using, an interval of 0.5 m/s. Figure9b shows the rotational speed of the generator with respect to the wind 120𝑓 speed. The rotational speed of the generator wasN = calculated using, (3) 𝑃 120 f In Equation (3), f is the frequency ofN as single-phase= voltage signal, p is the number of the (3) Energies 2019, 12, x FOR PEER REVIEW P 9 of 12 generator poles, and Ns is the rotational speed in revolutions per minute (RPM). The number of poles of the generator used for the Savonius wind turbine was 20. The dots in Figure 9(b) represent one- minute averaged values like the dots in Figure 9(a).

(a) (b)

Figure 9. FigureTest results9. Test results with with respect respect to to wind speed: speed: (a) electrical (a) electrical power output power from output the controller; from ( theb) controller; generator speed. (b) generator speed. 4.4. Control The result shown in Figure 10(a) is the change in the tip speed ratio as the wind speed increases. The tip speed ratio values of the one minute averaged data were obtained using Equation (2) with the measured rotational speed of the generator in Figure 9(b). Based on the figure, the tip speed ratio was high when the wind speed was small and decreased nonlinearly. When the wind speed was between 7.5 m/s and 9 m/s, the tip speed ratio was about 0.57, which is the optimal tip speed ratio to achieve the maximum power coefficient, 0.54. Also, the tip speed ratio decreased as the wind speed increased after 9 m/s. Based on Figure 10(a), the control strategy of the controller can be obtained. When the wind speed is small, the generator torque values are lower than the optimum values to achieve the optimum tip speed ratio and as a result, the generator speed becomes higher and finally the tip speed ratio becomes higher. When the wind speed is high, the generator torque values are higher than the optimum values to achieve the optimum tip speed ratio and as the result, the generator speed becomes lower and finally the tip speed ratio becomes smaller. This control strategy is suitable to avoid the low efficiency region based on the efficiency characteristic of the controller shown in Figure 8 when the wind speed is low, and also to protect the system by preventing overspeed of the generator. Figure 10(b) shows the power coefficient calculated from the simulation data in Figure 4 with the tip speed ratios obtained from the test data in Figure 10(a). It is clearly shown in the figure that the power coefficient decreases as the tip speed ratio deviates from the optimal value.

(a) (b)

Energies 2019, 12, x FOR PEER REVIEW 9 of 12

(a) (b) Energies 2019, 12, 1721 9 of 12 Figure 9. Test results with respect to wind speed: (a) electrical power output from the controller; (b) generator speed. In Equation (3), f is the frequency of a single-phase voltage signal, p is the number of the generator poles,4.4. Control and Ns is the rotational speed in revolutions per minute (RPM). The number of poles of the generatorThe result used forshown the Savoniusin Figure 10(a) wind is turbine the change was in 20. the The tip dots speed in ratio Figure as9 theb represent wind speed one-minute increases. averagedThe tip speed values ratio like values the dots of in the Figure one 9minutea. averaged data were obtained using Equation (2) with the measured rotational speed of the generator in Figure 9(b). Based on the figure, the tip speed ratio 4.4. Control was high when the wind speed was small and decreased nonlinearly. When the wind speed was betweenThe result7.5 m/s shown and 9 inm/s, Figure the tip10 aspeed is the ratio change was in about the tip 0.57, speed which ratio is asthe the optimal wind tip speed speed increases. ratio to Theachieve tip speed the maximum ratio values power of the coefficient, one minute 0.54. averaged Also, the data tip were speed obtained ratio decreased using Equation as the (2)wind with speed the measuredincreased rotationalafter 9 m/s. speed Based of theon Figure generator 10(a), in Figurethe control9b. Based strategy on the of figure,the controller the tip speedcan be ratio obtained. was highWhen when the thewind wind speed speed is small, was small the generator and decreased torque nonlinearly. values are When lower the than wind the speedoptimum was betweenvalues to 7.5achieve m/s and the 9 optimum m/s, the tip tip speed speed ratio ratio was and about as a result 0.57, which, the generator is the optimal speed tip becomes speed ratio higher to achieve and finally the maximumthe tip speed power ratio coe becomesfficient, higher. 0.54. Also, When the the tip wind speed speed ratio decreasedis high, the as generator the wind torque speed increasedvalues are afterhigher 9 mthan/s. Based the optimum on Figure values 10a, the to controlachieve strategythe optimum of the controllertip speed canratio be and obtained. as the Whenresult, thethe windgenerator speed speed is small, becomes the generator lower and torque finally values the tip are sp lowereed ratio than becomes the optimum smaller. values This control to achieve strategy the optimumis suitable tip to speed avoid ratio the andlow asefficiency a result, region the generator based on speed the becomesefficiency higher characteristic and finally of the tipcontroller speed ratioshown becomes in Figure higher. 8 when When the the wind wind speed isis high,low, theand generator also to protect torque valuesthe system are higher by preventing than the optimumoverspeed values of the to generator. achieve the optimum tip speed ratio and as the result, the generator speed becomes lowerFigure and finally 10(b) the shows tip speed the power ratio becomes coefficient smaller. calculated This controlfrom the strategy simulation is suitable data toin avoidFigure the 4 with low ethefficiency tip speed region ratios based obtained on the from efficiency the test characteristic data in Figure of the 10(a). controller It is clearly shown shown in Figure in the8 whenfigure thethat windthe power speed coefficient is low, and decreases also to protect as the the tip systemspeed ratio by preventing deviates from overspeed the optimal of the value. generator.

(a) (b)

Figure 10. Calculated tip speed ratio and power coefficient based on measured rotational speed. (a) Tip speed ratio calculated with measure generator speed and wind speed. (b) Power coefficient

calculated with the tip speed ratio and the power coefficient in Figure4.

Figure 10b shows the power coefficient calculated from the simulation data in Figure4 with the tip speed ratios obtained from the test data in Figure 10a. It is clearly shown in the figure that the power coefficient decreases as the tip speed ratio deviates from the optimal value. Figure 11 shows the aerodynamic power with the maximum power point tracking strategy obtained from the simulation (dashed line), and the aerodynamic power calculated with the tip speed ratios obtained from the test results in Figure 10a (solid line). Due to the algorithm in the controller, it can be found that the aerodynamic power of the rotor slightly deviates when the wind speed is lower than about 7 m/s or higher than about 9 m/s. Energies 2019, 12, x FOR PEER REVIEW 10 of 12 Energies 2019, 12, x FOR PEER REVIEW 10 of 12 Figure 10. Calculated tip speed ratio and power coefficient based on measured rotational speed. (a) TipFigure speed 10. Calculatedratio calculated tip speed with ratio measure and powergenerator coeffici speedent basedand wind on measured speed. (b rotational) Power coefficientspeed. (a) calculatedTip speed withratio thecalculated tip speed with ratio measure and the powergenerator coefficient speed andin Figure wind 4. speed. (b) Power coefficient calculated with the tip speed ratio and the power coefficient in Figure 4. Figure 11 shows the aerodynamic power with the maximum power point tracking strategy obtainedFigure from 11 theshows simulation the aerodynamic (dashed line), power and thwithe aerodynamic the maximum power power calculated point withtracking the tip strategy speed ratiosobtained obtained from the from simulation the test results (dashed in line),Figure and 10(a th)e (solid aerodynamic line). Due power to the calculated algorithm with in the the controller, tip speed itratios can obtainedbe found from that the testaerodynamic results in Figurepower 10(aof the) (solid rotor line). slightly Due deviates to the algorithm when the in windthe controller, speed is it can be found that the aerodynamic power of the rotor slightly deviates when the wind speed is Energieslower 2019than, 12 about, 1721 7 m/s or higher than about 9 m/s. 10 of 12 lower than about 7 m/s or higher than about 9 m/s.

Figure 11. Wind turbine power output comparison by different control method. FigureFigure 11.11. WindWind turbineturbine powerpower outputoutput comparisoncomparison byby didifferentfferent control control method. method. 5.5. DiscussionDiscussion 5. Discussion FigureFigure 12 12 shows shows aa comparisoncomparison ofof thethe measuredmeasured windwind turbineturbine outputoutput withwith thethe simulationsimulation results.results. Figure 12 shows a comparison of the measured wind turbine output with the simulation results. TheThe solid solid line line with with squares squares shown shown in the in figure the representfigure represent the measured the measured wind turbine turbine converted power The solid line with squares shown in the figure represent the measured wind turbine power toconverted Bin data. to The Bin curve data. with The circles curve shows with thecircles predicted shows power the predicted performance power of wind performance turbines throughof wind converted to Bin data. The curve with circles shows the predicted power performance of wind simulation.turbines through To compare simulation. errors withTo compar simulatione errors results, with a simulation10% error results, range is a described ± 10% error by error range bars, is turbines through simulation. To compare errors with ±simulation results, a ± 10% error range is anddescribed the data by interval error bars, for simulation and the data was interval chosen for as 0.5 simulation m/s. The was simulation chosen results as 0.5 inm/s. Figure The 12simulation include described by error bars, and the data interval for simulation was chosen as 0.5 m/s. The simulation bothresults the in effi Figureciency 12 of include the generator both the and efficiency the controller of the generator with respect and to the the controller rotor speed with in Figurerespect8 toand the results in Figure 12 include both the efficiency of the generator and the controller with respect to the therotor power speed coe inffi Figurecient based 8 and on the the power torque coefficient schedule ofbased the windon the turbine torque in schedule Figure9 b.of Thethe wind comparison turbine rotor speed in Figure 8 and the power coefficient based on the torque schedule of the wind turbine showsin Figure that 9(b). the two The data comparison have the sameshows increasing that the trend,two data and thehave maximum the same prediction increasing error trend, is found and the to in Figure 9(b). The comparison shows that the two data have the same increasing trend, and the bemaximum about 4.32% prediction (@12m /errors) within is found the measured to be about and 4. predicted32% (@12m/s) ranges. within the measured and predicted ranges.maximum prediction error is found to be about 4.32% (@12m/s) within the measured and predicted ranges.

FigureFigure 12.12. ComparisonComparison ofof estimatedestimated powerpower applyingapplying extractedextracted rotorrotor scheduleschedule andand measuredmeasured power. power. Figure 12. Comparison of estimated power applying extracted rotor schedule and measured power. 6. Conclusions

In this study, a drag-type Savonious wind turbine rotor with a diameter of 0.58 m and a height of 1.32 m was designed and tested. The rotor was modelled in a commercial CFD program and its aerodynamic performance was predicted through simulations. The simulation confirmed that the maximum power coefficient of wind turbines is 0.17 at a tip speed ratio of 0.54. Based on this, the power curve with the maximum power point tracking was obtained. The simulation results were compared with test results of the designed wind turbine with a generator and a controller. For comparisons, the aerodynamic power was obtained with tip speed ratio data from the field test based on the control strategy imbedded into the controller of the wind Energies 2019, 12, 1721 11 of 12 turbine. Also the electrical conversion efficiencies of the generator and the controller were found from a motor-generator test and considered for the comparison. From the comparison, the simulation results were validated with a maximum error of 4.32% in power estimation at 12 m/s.

Author Contributions: H.J. performed the design and simulation of the wind turbine rotor, and the field test. I.P. supervised the research and revised the paper. S.K. performed the generator-controller test and analyzed the test result. D.J. manufactured the proto-type wind turbine and analyzed the field test results. Funding: This work was supported by the Human Resources Program in Energy Technology and the New and -Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) with a granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (Grants No. 20153010024470 and 20184030201940). Acknowledgments: The authors acknowledge Deockjin Jeong from JH Energy for support concerning the manufacturing proto-type wind turbine. Conflicts of Interest: The authors declare no conflicts of interest.

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