Parametric Study for Savonius Vertical Axis Marine Current Turbine using CFD Simulation

Transcription

1 Parametric Study for Savonius Vertical Axis Marine Current Turbine using CFD Simulation OMAR YAAKOB, M. ARIF ISMAIL, YASSER M. AHMED Department of Marine Technology UniversitiTeknologi Malaysia UTM Skudai MALAYSIA Abstract: - Due to the low current speeds associated with Malaysian tidal currents, Savonius turbine was chosen as a device in extracting this energy. However, this turbine suffers from poor efficiency. The present paper describes some works carried out to improve the turbine. The effect of speeds on performance was found to be minimal while the use of deflectors improved flow into the rotor and contributes significantly to the coefficient of performance improvement. The use of ducts was also studied, indicating improved flow characteristics. Key-Words: Ocean Energy; Marine Renewable Energy; Tidal Current; Computational Fluid Dynamic (CFD) 1 Introduction As one of the renewable energy family, the ocean promises the world for its never ending sources. Malaysia, a country that surrounded by sea can use these nature into benefit by developing an ocean current turbine that able to utilize its known low current speed and shallow water. Water current turbine manipulates the current kinetic energy into electrical energy through generator [1]. Savonius turbine was widely been used in wind energy application. This turbine shows great potential for the use in Malaysia s Seas. However Savonius turbine suffers from a very low efficiency of Betz found the limit which indicates 0.6 for the best possible performance coefficient of a turbine [2]. A study was carried out to find the optimum design of the Savonius turbine in order to increase the rotor performance. Several researches done in the past will be analyzed. Few factors and consideration will be taken into account. Currently, the present study concentrates on the outer structure that can be added to the turbine. The new design will be simulated by Computational Fluid Dynamics (CFD) software. Thus, this parametric study will be shown in this paper with the result of CFD simulation. 2 Literature Review 2.1 Savonius Turbine Performances Performance of Savonius turbine determined by the function of power P, rotor swept area A s, and current speed U as defined in Equation 1. Cp P 1 3 ρasu 2 = (1) Or it can be simplified as the production of Tip Speed Ratio (TSR) and moment of coefficient (Cm). TSR or λ is the ratio between rotational speed of the tip of blade and the actual velocity of the water defined in Equation 2 [3], ωd λ= (2) 2U Cm T 1 2 ρas DU 4 = (3) Equation 3 consists of velocity of incoming water or current speed U to represent mechanical torque T by definition of swept area, A s ; production of Savonius rotor height H and diameter D from Figure Past Improvement on Savonius Rotor Since Savonius turbine suffers from low efficiency, many researches has been carried out to improve the rotor performance. The improvement can be divided into three categories. ISBN:

2 higher torque and smooth running in term of hydrodynamic issue [13]. They found that coefficient of performance encounter at least 50 percent increment Outer Structure Flow Modification Fig. 1: Scheme of a single-step Savonius rotor. (a) Front view; (b) top view (conventional Savonius rotor: e = 0) [4] Rotor Parameters Lot of parametric studies was conducted in order to find the best and optimal Savonius rotor design parameters. An experiment done by Hayashi and Kamoji found the best aspect ratio is ranging from one to two [3] [5]. This ratio produces a low blockage effect lower than 15 percent which is positively affecting the efficiency. Rotor performance seems to increase when 0.1 to 0.15 overlap ratios were used by Blackwell and Akwa [6] [7]. The value decrease when overlap ratio is less than 0.1 and larger than 0.3. Studies by Menet and Yaakob found that the best overlap ratio was in the range 0.2 to 0.25 [4] [8]. The use of end plate, Df was able to increase the rotor performance as described by Ogawa [9]. It is also noted that having the end plate 10% larger than the rotor diameter, D leads to a better power coefficient in terms of its hydrodynamic performance [4]. Number of stage of two with 90 phase different also gives a good improvement on performance as stated by Ushiyama and Khan [10] [11]. The study of the flow by Nakajima robustly concluded with the double-stage rotor with the 90 phase difference within the blades, a lift was generated on the advancing blade s convex side, and the lift suppressed the power coefficient reduction due to the fluid drag on the returning blade s convex side on the other stage [12] Rotor Design Shape Structures Improvements that were made on the rotor structure attract the researchers to study the effect in hydrodynamic performance over certain design. For instant, a study conducted by Kamoji shows that the performance of Savonius rotor can be improve to 0.32 using a zero overlap ratio with 124 blade arc angle, ψ [5]. Twisted Blade was another attempt made by Saha that also improves the rotor efficiency, with the advantage of good self-starting capability, Different Deflector Shape There was a study by Ogawa varying the shape of deflecting plate [9]. The test indicates that in each case, the maximum values of Cp are about at least 14% larger than those in the case without a deflecting plate. Figure 2 shows the flow or velocity flowing to the rotor with several of deflector shape. Fig. 2: Different deflector plate shape [9] Guide vane The research by Hayashi found that the guide vanes increased the torque coefficient on the average in the low tip speed ratio but decreased the torque coefficient in high tip speed ratio [3] Duct Overview Duct can be seen developed constantly in horizontal turbine for wind and water turbine application. The use of duct manipulates an area differences to increase the wind and water speed or incoming flow to the turbine. Horizontal turbine usually used a series of NACA profile for the blade, and known as lift turbine. The higher the speed, the better lift generated to increase the turbine efficiency. Clearly for Savonius rotor that operated mostly from drag generation have no effect by higher velocity. However, there was a research by Yusaku and Gupta where the combination of another vertical turbine helps improve the rotor performance [14] [15]. Thus a study of the flow characteristics along the duct seems practical so that further optimization can consider the effect of combining the Savonius rotor or simply go for lift generated vertical axis current turbine for Malaysia s seas application. Parametric study will be carried out using the above parameters to find the optimum configuration of the turbine. ISBN:

3 3 Purpose of the Present Work By keeping the overlap ratio of d/5, the geometry of the conventional turbine is based on the full scale prototype of the turbine which suits the Malaysia s waters requirement, with a rotor height 15m height. Other main parameters that have been approved by the literature includes the used of end plates, aspect ratio of two for smaller blockage effect will be used [4]. The turbine consists of two stage rotor with 90 phase difference. CFD validation process has been carried out, thoroughly comparing with past research simulation and experimental data [16]. It was shown that 3D analyses is better and is not limited to identical shape only. Thus 3D CFD was chosen for further assessment. In order to understand the effect of increase speed on Savonius rotor, a Reynolds number study to know the comparison was carried out. The incoming flow current speed was varied from 0.5m/s, 1.5m/s and 2.5m/s. These speeds were varied based on Malaysia s lowest current speed and average maximum speed of global ocean current. These speeds are associated with Reynolds number ranging from 4.18x10 6 to 18.69x10 6. The best outer shape from previous studies will be used as initial steps to find the new optimum configuration for two stage Savonius rotor [2] [17]. A deflector will be used as the interference to the flow by outer structure. Various shape of deflector been studied and Ogawa found that the shape of a straight plate shows a better improvement to the rotor performance [8]. Present work will manipulate the optimum deflector geometry to see the effect on two stage Savonius rotor as previously it only applied on single stage and modified zero overlap angle Savonius turbine. The plate used in current work has a thickness of 40mm with the same height of the turbine. The parameters obtained by optimal configuration defined as X 1 /R = , Y 1 /R = and X 2 /R = studied by Mohammed as shown in Figure 3. The 2 deflectors settings were based on the work described in [17]. There was an angle involvement for the second deflector placement. In the present work, the geometry and dimension for the second deflector will be identical with the first deflector by having α = 50 and Z =1.8R. Figure 4 shows the second deflector position. Fig. 4: Schematic of Savonius rotor with space parameters of two deflector plate The duct will be included to see the effect on the flow. The geometry of the duct based on previous work by Azliza[18]. The current work deal with the same geometry under determined formulation and parameters related to the best duct shape. Diameter of cylinder, D cyl equal to D noz / 5 and the nozzle Length, L noz equal to 0.95 x D noz. Figure 5 shows the geometry of the duct. D noz Nozzle L noz Cylinder Diffuser Fig.5: Duct geometry Fig. 3: Schematic description of geometry and parameters X 1, X 2, Y 1 and Y 2 [2] 4 CFD Simulations The flow simulations presented in this work was based on the ANSYS Fluent 6 code. For all three dimensional simulation, the unsteady Reynolds- Averaged Navier-Stokes equations are solved using the SIMPLE (Semi-Implicit Method for Pressure- Linked Equations) algorithm for pressure-velocity coupling) Gauss-Seidel Method is applied to iteratively solved equations used in the simulation ISBN:

4 that in linear algebraic form in obtaining desired parameters [2]. In the present work, second-order upwind scheme were used in the simulation. The unsteady flow is solved by using the Sliding Mesh Model (SMM). Three or more complete revolutions are always computed. Based on Mohamed, three complete revolutions is adequate to measure the parameters [2]. The first complete revolution is to initiates the flow and to be corrected. Average of the rest two revolutions provides the flow properties of torque in time dependent. On a standard PC, one evaluation takes about minutes for three dimensional cases.for each case of three dimensional configurations, a grid-independence study has been first carried out. The grid generator ICEM CFD has been used for generating the unstructured grids required for the CFD code solver. Three dimensional case studies use a grid range from 1,000,000 to 2,500,000 cells. Minimum size of the domain uses 20 times the rotor radius on each side of the turbine for all 3D cases [2]. The out flow boundary was extend to at least another half domain size for reducing backflow effect that might occurred during simulation, and has been approved by other researchers in their simulation work [7]. Figure 6 and 7 shows the boundary domain and unstructured grid used for current work respectively. Flow Computational domain 5 Results and Discussion 5.1 Reynolds Number Effects Speed was varied between 0.5m/s to 2.5m/s resulting in different angular velocity and RPM. The simulations start by manipulating a different TSR, λ in order to obtain the curve of performance. Figure 8 shows the resulting of torque and performance coefficients. Coefficient of Torque, Cm Coefficient of Performance, Cp Tip Speed Ratio, λ (a) 0.5 m/s 1.5 m/s 2.5 m/s m/s m/s m/s Tip Speed Ratio, λ (b) Fig.8: Computational model for different speed (a) torque coefficient; (b) power coefficient Turbine - Rotational domain Fig.6: Domains for numerical Simulation Fig. 7: Unstructured mesh for two stage rotor The results show that water speed and Reynolds number have very little effect on coefficients of torque and performance. The maximum efficiency of the turbine is However, at higher speed and Reynolds number, the RPM of rotor is high; hence more power can be produced. 5.2 Deflector Effect Past research has shown a good improvement when a simple rectangular plate was placed upon the incoming flow of the water to Savonius rotor. The use of deflector in this work was to determine the effect on the efficiency of two stage of Savonius turbine. Figure 9 shows the pressure contour obtained after the simulation. ISBN:

5 Fig. 9: Pressure contour for two stage 90 phase different simulation The interference of deflector to the incoming flow gives a positive effect to the rotor performance. By using one deflector, the rotor performance increase from 0.21 to 0.26 as shown in Figure 10. The deflector helps to cancel out some of the negative torque produced by the rotor. Since two stage Savonius turbine have a 90 phase difference, it tend to improve the flow after passing through the deflector because of the circular movement on the convex advancing blade at second stage. As explained by Nakajima, the flow of two stage Savonius rotor improved because of the lift generated at 90 angle [12]. When two deflectors were in used, the efficiency of the turbine increases to 0.27 which indicates 25 percent of increment. However the maximum performance of two deflectors set up occurred at 0.9 tip speed ratio (TSR) differs with the one deflector which occurred best at 0.8. The second deflector helps improved the flow by directing the flow to the advancing blade of the rotor which later affects the lift generation. In conclusion, the interference of one or two deflectors placing upon the flow improves Savonius rotor performance. Coefficient of Performance, Cp Tip Speed ratio, λ Base Def 1 Def 2 Fig. 10: Comparison of Performance Coefficient between without deflector and with deflector 5.3 Duct Effect on Current Flow in Malaysia Seas For the duct simulation, the rotor was placed inside the duct in order to have more realistic characteristic on the flow rather than an empty duct. Figure 11 shows the velocity vector act along the duct for an average ocean current speed of 0.56m/s or 1knot, typical average speed of current in Malaysian seas. This results in higher velocity of water entering the rotor location. The curvy shape of nozzle helps to accelerate the water speed into the cylinder and vortex seems to form at the outer side of the nozzle. After completing evaluation, the maximum obtained speed is almost twice the velocity inlet; 2knot or 1.1m/s. Fig. 11: Velocity vector along duct (dark grey-low / light grey-high) 6 Conclusions Parametric study for 3D analyses of two stage Savonius turbine has been carried out. Initially, the study of Reynolds number been done to see the effect on the rotor performance. Reynolds number gives a small effect on the rotor turbine efficiency, with a difference that is less than two percent. The interference of deflector to the incoming flow to the turbine gives a good improvement on the rotor performance. Two deflectors achieved higher efficiency than one. Further research can be made by manipulating the number of deflector to see how many deflectors can be put against the flow until the efficiency start to drop. The deflector also can be divided into two separate section so each section have its own angle and dimension for this two stage turbine. Duct will definitely increase the initial incoming velocity. With adequate size and geometrical shape, the duct can be used to increase the poor water flow experienced by Malaysia s seas. Based on ISBN:

6 simulation result, it is possible to prevent the forming vortex by having a cylinder shape cover the whole outer wall of the duct which might improve the duct efficiency hence improve the flow characteristic. Acknowledgement Special thanks are due to Universiti Teknologi Malaysia (UTM) and the Ministry of Science Technology and Innovation Malaysia for funding this project under Science fund Vote No: 4S048. References: [1] Charlier, R.H., and Justus, J.R., Ocean Energies: Environmental, Economic and Technological Aspects of Alternative Power Sources,Elsevier. Sci. Publishers, [2] Mohamed, M.H., Janiga, G., Pap, E., and Th evenin, D., Optimal blade shape of a modified Savonius turbine using an obstacle shielding the returning blade, Energy Conversion Management, Vol. 52, 2010pp [3] Hayashi, T., Li, Y., and Hara, Y., Wind tunnel tests on a different phase three-stage Savonius rotor, JSME International Journal Series B, Vol. 48, No. 1, 2005pp [4] Menet, J.L., A double-step Savonius rotor for local production of electricity: a design study, Renewable Energy, Vol. 29, 2004pp [5] Kamoji, M.A., Kedare, S.B., and Prabhu, S.V., Experimental investigations on single stage, two stage and three stage conventional Savonius rotor, International Journal Energy Res., Vol. 32, 2008pp [6] Blackwell, B.F., Sheldahl, R.E., and Feltz, L.V., Wind tunnel performance data for two and three bucket Savonius rotors, Journal Energy, Vol. 2, 1977,pp [7] Akwa, J.V., Alves, G., and Petry, A.P. (2011). Discussion on the verification of the overlap ratio influence on performance coefficients of a Savonius wind rotor using computational fluid dynamics, Renewable Energy, Vol. 38, 2011pp [8] Yaakob, O.B., Tawi, K.B., and Suprayogi Sunanto, D.T., Computer Simulation Studies on the Effect of Overlap Ratio for Savonius Type Vertical Axis Marine Current Turbine, International Journal of Engineering, IJE Transactions A: Basics, Vol. 23, No. 1, 2010, pp [9] Ogawa, T., and Yoshida, H., The Effects of a Deflecting Plate and Rotor End Plates on Performances of Savonius-Type Wind Turbine, Bulletin of JSME, Vol. 29, No , pp [10] Ushiyama, I., Nagai, H., and Shinoda, J., Experimentally Determining the Optimum Design Configuration for Savonius Rotors, Trans. Japanese Society Mechanical Engineering,Vol. 52, No. 480, 1986,pp [11] Khan, M.N.I., Tariq Iqbal, M., Hinchey, M., and Mase, V., Performance of Savonius rotor as a water current turbine, Journal Ocean Technology, Vol. 4, No. 2, 2009, pp [12] Nakajima, M., Iio, S., and Ikeda, T., Performance of double step Savonius rotor for environmentally friendly hydraulic turbine, Journal Fluid Science Technology, Vol. 3, No. 3, 2008,pp [13] Saha, U.K., and Rajkumar, M.J., On the performance analysis of Savonius rotor with twisted blades, Renewable Energy, Vol. 31, No. 11, 2006, pp [14] Yusaku Kyozuka, An Experimental Study on the Darrieus-Savonius Turbine for Tidal Current Power Generation, Journal Fluid Science Technology, Vol.3, No.3, 2008, pp [15] Gupta. R, R. Das and K.K. Sharma, Experimental Study of Savonius-Darrieus Wind Machine, International Conference on Renewable Energy for Developing Country, [16] Yaakob, O.B., Yasser M. Ahmed, M. Arif Ismail, Validation Study for Savonius Vertical Axis Marine Current Turbine using CFD Simulation, Asia-Pasific Workshop on Marine Hydrodynamics, Vol.6, 2012, pp [17] Golecha K., Eldho T.I., and Prabhu S.V., Investigation on the Performance of a Modified Savonius Water Turbine with Single and Two Deflector Plates, Asian International Conference on Fluid Machinery and Fluid power Technology Exhibition, Vol 11, [18] Azliza A.A, Development of a Ducted Horizontal Axis Marine Current Turbine Rotor, Unpublished Master s Thesis, University of Teknologi Malaysia, Johor Bahru, ISBN: