JJMIE Jordan Journal of Mechanical and Industrial Engineering

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1 JJMIE Jordan Journal of Mechancal and Industral Engneerng Volume 11 Number 4, December ISSN Pages Modelng and Smulaton of Shrouded Horzontal Axs Wnd Turbne Usng RANS Method C. Ghena a *, T. Salameh b, I. Janareh b a Sustanable and Renewable Energy Engneerng Department, College of Engneerng- Unversty of Sharah, Sharah 27272, Unted Arab Emrates b Mecancal Engneerng Program, Khalfa Unversty of Scence and Technology, Masdar Insttute- Abu Dhab, 54224, Unted Arab Emrates Receved... Accepted... Abstract The vast deployment of wnd energy leads to ts techncal maturty and cost reducton that compete wth natural gas. Low wnd regons can be also targeted when shrouded or optmally desgned turbne s utlzed. The obectve of the present study s to assess the performance of the shrouded Horzontal Axs Wnd Turbne (HAWT) numercally subected to low wnd speed. The goal s to develop wnd energy technologes that can maxmze the power extracted from the turbne under low wnd condtons. Three blades horzontal axs wnd turbne surrounded wth a shroud was tested n the present study. The Reynolds Averaged Naves-Stoes (RANS) modelng approach and fnte volume numercal method was used to solve the governng equatons. These equatons are based on the equatons of conservaton of mass, momentum and turbulence scalars (from the computed velocty profles. The turbne power coeffcent (Cp) was determned usng the nducton factor. A comparson between the Cp wth and wthout shroud at dfferent tp speed ratos s presented n the present paper. The results show that the shrouded turbnes can generate quadruple of the power of the non-shrouded baselne HAWT. Keywords: Jordan Journal of Mechancal and Industral Engneerng. All rghts reserved 1. Introducton Nearly eghty two percent of the world energy demand (electrcty generaton, transportaton, buldngs, and ndustral applcatons) s provded through the nonrenewable and unsustanable fossl fuels,.e., coal, ol and natural gas. Renewable and clean energy systems contnue to evolve and deploy to mtgate the problems of clmate change (hgh GHS emssons from combuston of fossl fuels), energy securty, the rasng future energy demand, and the depleton of fossl fuel resources (ol, natural gas and coal) [1]. A comparson between the resource ntenstes (materal, area, energy, CO 2 and captal ntenstes related to the constructon of renewable energy systems) and operatonal parameters (system effcency, capacty factor, and lfetme) of fossl fuel, renewable and nuclear power systems has been presented by Ghena and Janareh [1]. Among these renewable energy technologes, wnd energy wll contnue to play an mportant role n the future. Accordng to the World Wnd Energy Assocaton, the world has seen a new record (63.69 GW) n ts nstallatons n 2015 and nearly coverng 3% of the global electrcty supply n 2012 [2]. It also observed global growth rate of 17.2 % n The world total wnd capacty reached 435 GW n 2015, wth 46.2%, 33%, 29%, and ncreases n Brazl, Poland, Chna, and Turey, respectvely (most dynamc countres wth the strongest growth rates n 2015). Ths growth can be accelerated n the future should renewable energy polces contnue to emerge from countres around the world [3]. Whle wnd ntermttency and low speed reduce turbne capacty factor, shrouded wnd turbne can mtgate ths loss and help n ncreasng wnd energy deployment. Ths technology can be adopted to generate hgher power n regons characterzed wth low speed condtons. The turbne s enclosed n shroud or duct that wll allow the turbne to operate at hgher effcences and thereby capacty compared to non-shrouded turbne. The shroud s used to ncrease the flow velocty or the volume of arflow around the turbne. More power s then generated wth greater return n nvestment. Hort and Larsen [4] nvestgated the desgn requrement for Dffuser Augmented Wnd Turbne (DAWT) rotor to effcently convert the avalable energy shaft energy. Three dfferent confguratons were studed: bare propeller (HAWT), classcal dffuser augmenter wnd turbne (DAWT) and hgh performance mult element DAWT. The results showed that the DAWT rotors can successfully convert the avalable energy to shaft energy f the swrl and axal loadng dstrbuton were selected correctly. Bala [5] performed expermental and numercal study of dffuser augmented wnd turbne. The wnd turbne power performance was evaluated usng the power coeffcent values. The expermental date showed * Correspondng author e-mal: cghena@sharah.ac.ae.

2 Jordan Journal of Mechancal and Industral Engneerng. All rghts reserved - Volume 11 Number 4 (ISSN ) mprovements for the electrcal power values, resultng n an average of 90% n the power coeffcent and the more pronounced enhancement was obtaned at low wnd speed values [5]. A shrouded wnd turbne generatng hgh output power wth wnd lens technology was nvestgated by Yu Ohya and Taah Karasudan [6]. They showed that the shrouded wnd turbne wth brmmed dffuser augmented the power by 2-5 compared to bare wnd turbne for gven wnd speed and rotor dameter. Ths s due to low-pressure regon, due to the strong vortex formaton behnd the broad brm leadng to draw more mass flow to the wnd turbne nsde the dffuser shroud. Kosash and Tondell [7] focused, n ther study, on the effects of dffuser shape and geometres, and blade arfols on the wnd turbne performance enhancement. Three dfferent geometrcal features: straght dffuser, nozzledffuser combnaton, and brmmed dffuser combnaton have been nvestgated. The tests confrmed that placng the mcro turbne model nsde a shroud can substantally mprove ts performance. The results showed that the dffuser shroud mproves the performance by 60% compared to the bare turbne. The nozzle-dffuser enhancement was 63% slghtly better than dffuser only. The fndngs from ther wor suggested that shroudng mcro wnd turbne not only mproves ts performance, but also provde gudelnes on the desgn of the dffuser geometrcal parameters (L/D) and/or (H/D) wth a customzed performance curve. Hu and Wang [8] used a self-adaptve flange for the wnd turbne shroud. The self-adaptve flange can mantan the advantages of the flanged dffuser at wnd veloctes lower than the rated velocty and reduce the wnd loads actng on the dffuser and blades at hgher wnd veloctes. Numercal results show that the wnd load actng on the total flanged dffuser can be reduced by about 35% at 60 m/s due to the reconfguraton of the self-adaptve flange. Anet et al. [9] nvestgated the flow and performance of shrouded turbnes usng Reynolds Averaged Naver- Stoes (RANS) equatons. Dfferent shroud geometres were evaluated for ther augmentaton of mass flow through the turbne. The results showed an augmentaton ratos of up to 1.9 can be acheved wth shrouded turbnes. El-Zahaby et al. [9-10] used 2D axsymmetrc CFD model of flanged dffuser as a casng for small wnd turbnes to ncrease the generated power. The numercal smulaton shows the creaton of vortces behnd flange that cause pressure drop whch ncreases mass flow rate through the dffuser leadng to an ncrease n the generated power. The man obectve of the present study s to perform numercal smulatons to test the performance of shrouded horzontal axs wnd turbne under the low speed wnd condtons of the Araban Pennsula eastern coastlne as ndcated elsewhere wth an average annual wnd at the borderlne cut-n wnd turbne speed of 4.5 m/s [11]. The goal s to develop wnd energy technologes that can maxmze the power extracted from the turbne under low wnd condtons and to accelerate the penetraton of wnd energy technologes n the Araban Pennsula. 2. Governng Equatons A steady state soluton s sought for the rotatng turbne. Ths can be acheved through a "rotatng reference frame" formulaton n whch the turbne rotor rotaton s assgned a specfc rotatng speed wthout need to temporally trace ts tme advancement. Ths can drastcally reduce the computatonal tme and remove the burden n carryng senstvty studes. Although the detals of the flow due to the rotatng blade aganst the outer doman and there nteracton n generated vortcty trals wll be undermned, nevertheless an average of these phenomena s captured at lower cost than the lengthy real tme transent calculaton. These latter real tme calculatons follow transent arbtrary Lagrangan Formulaton, whch nvolves doman movement and mesh sldng as appears n a prevous wor by the present authors [12], are lmted to smaller flow doman to brng more flow nsght or ts physcs valdaton. The equatons of conservaton of mass, momentum, and the two scale turbulence equatons (turbulent netc energy and the dsspaton of the netc energy ) are solved to predct the flow feld around the wnd turbne [13-14]. The tme averaged gas phase equatons for steady turbulent flow are: (1) u S x x x where s the dependent varable that can represent the velocty u, the turbulent netc energy, and the dsspaton rate of the turbulent netc energy The governng equatons n steady state form are gven such that, the conservaton of energy s wrtten as: u 0 x where s the densty, u are the three velocty components. The momentum equaton s wrtten as: u u t P (3) x x x where s the p s the statc pressure and vscous stress tensor defned as: t u u 2 u x x 3 x f and 0 f (2) t s the s the average Reynolds stress tensor defned and modeled usng the eddy vscosty turbulence model as: ' u ' u u u 2 u 2 t x x 3 x 3 where s the average turbulent netc energy defned as: 1 2 u ' u ' and t s the turbulent eddy vscosty expressed n terms of and the dsspaton rate ( as:

3 2017 Jordan Journal of Mechancal and Industral Engneerng. All rghts reserved - Volume 11 Number 4 (ISSN ) 237 c t 2 where C s constant (C = 0.09) and s the average dsspaton rate of the turbulent netc energy and defned as: ' ' u u x x where s the nematc vscosty. The two scalar transport equatons for each of the Turbulence Knetc Energy () and ts dsspaton are wrtten as ( ): t u x (4) G x x where = 1 and G s the producton of the turbulent netc energy defned as: u u u 2 u u G K t t x x x 3 x x C u x C x 2 t 2 x 1 G where C 1 =1.44, C =1.92, and =1.3 are the tunng model constants. (5) 3. Geometry, Bounadar Condtons, Mesh and Numercal Method Fg. 1 shows a schematc of the three dmensonal wnd turbne. The wnd turbne blade characterstcs are summarzed n Table 1. For the boundary condtons, the ar enters from the left boundary at a velocty U wu (Inlet flow velocty nlet). The computatonal doman has a cylndrcal shape as shown n Fg. 1 (symmetry). The ar exts the computatonal doman from the rght sde (Outflow). The rotor s modeled as rotatng mmersed frame wth and angular velocty [rad/s] whch remove the hassle of seeng transent analyses. Thus, an averaged steady state soluton s sought. The geometry of the shroud s shown n Fg. 2. The shroud characterstcs are summarzed n Table 1. A Qualty mesh was generated for the shrouded and unshrouded turbnes. The number of nodes selected for the wnd turbne smulaton study was 188,000. A grd ndependent study was performed wth three dfferent meshes (coarse mesh wth 94,000 nodes, medum mesh wth 188,000 nodes and fne mesh wth 365,000 nodes) and the results of the smulaton were compared. The dfference between the smulatons results obtaned wth medum and fne mesh was less than 1.5%. Based on ths grd ndependent study, the medum mesh was selected for ths numercal study. A control volume based fnte dfference method s used n order to solve a system of partal dfferental equatons governng the conservaton of mass, momentum, and turbulent flow. In ths CFD analyss, the SIMPLER algorthm s used to solve explctly for the velocty and pressure felds. SIMPLER (revsed verson of SIMPLE algorthm) s a sem-mplct method used as a numercal procedure for solvng the Naver-Stoes equatons [15-17]. The SIMPLE algorthm uses a relatonshp between velocty and pressure correctons (pressure-velocty couplng algorthm) to enforce the contnuty equaton (mass conservaton) and to obtan the pressure feld. Fgure 1: Unshrouded wnd turbne geometry, mesh and computatonal doman

4 Jordan Journal of Mechancal and Industral Engneerng. All rghts reserved - Volume 11 Number 4 (ISSN ) Fgure 2: Shrouded Wnd Turbne Geometry Table 1: Wnd turbne and shroud characterstcs Blade - arfol Blade length (m) 0.5 SG6043 Throat dameter D (m) 1.17 Rotor dameter 1.16 Dffuser Length L (m) 1.46 Dffuser angle 12 o 4. Performance Characterstcs of the Wnd Turbne 4.1. Unshrouded Baselne Wnd Turbne The effcency of the wnd turbne represented by the power coeffcent, C p, and the power extracted from the turbne, P t, are used as the performance characterstcs of the wnd turbne. The power coeffcent s gven by: C p = P t = T.ω (6) P wnd P wnd where T s the torque (N.M), s the angular velocty of the turbne (rad/s), and P wnd s the maxmum avalable power n the wnd (W). The wnd power s gven by: P wnd = 0.5 ρ A U w 3 (7) where s the ar densty (g/m 3 ), A s the rotor dameter (m 2 ) and U w s the wnd speed. The C p s also gven [12] by: C p = 4 a (1 a) 2 (8) where the nducton factor a s gven by: a = 1 U wrot U wu (9) U wrot = 1 2 (U wu + U wd ) (10) where U wrot, U wu, and U wd are the ar veloctes at the rotor, up-steam and down-stream of the turbne blades, respectvely Shrouded Wnd Turbne The power coeffcent of the shrouded wnd turbne [6, 18, 19] s gven by: C p = ε L μ 4 a (1 a ) 2 (11) ε L = A out A rot (12) μ = U out U wu (13) a = 1 U wd U wu (14) C p)dffuser = μ 4 a (1 a ) 2 (15) where A out and A rot are the surface areas at the ext from the shroud and at the rotor (see Fg. 3), L and are the area rato and the bac pressure coeffcent, and Cp) Dffuser s the power coeffcent due to the dffuser. Fgure 3: Control volume for shrouded turbne [17-18] 5. Results The smualton results of the unshrouded and shrouded turbne are evaluated and presneted. The effect of turbne tp speed rato (TSR = 4 10) on the power extracted and the Cp s nvestgated n the present study. The wnd speed at the nlet of the computaonal doman was set to as low as U wu = 4 m/s and the corrospondng rotatonal speed (rad/s) of the turbne was determned from the tp speed rato equaton (TSR = R/U wu, where R s the blade length). Fg. 4 shows the velocty magntude of the ar flow at the poston of the unshrouded wnd turbne (y =0) and behnde the wnd turbne (y = 0.1, 0.2, 0.3, 0.4 and 0.5

5 2017 Jordan Journal of Mechancal and Industral Engneerng. All rghts reserved - Volume 11 Number 4 (ISSN ) 239 m). The wnd approched the the turbne wth a wnd speed of U wu. The wnd speed decreases as t approaches the turbne before t passes thorugh the rotor plane. The axal velocty defct s the amount of decrease n the frestream velocty. The nducton factor a gven by the equaton 9 represnets the fracton by whch the axal component of the velocty s reduced. The axal component of the velocty reduces further after passng the rotor plane (y = 0.1, 0.2, 0.3, 0.4 and 0.5 m) as shown n Fg. 4. There s a pressure drop across the rotor plane when the ar passes the rotor plane. Downtream the rotor plane, the pressure s recovered and bac to the atomospherc pressure where the average ar velocty s U wd as shown n Fg. 5. Ths average ar velocty downtream the wnd turbne and where the y = 0 m pressure s recovered bac to the atmospherc pressure s used to calculate the nducton factor usng the equatons 9 and 10. The Cp s then caluclated usng the equaton 8 for the unshrouded turbne. The varaton of the Cp and a wth the tp speed rato are show n Fg. 6. The results show a maxmum value for Cp of 0.38 at a tp speed rato bewteen 7 and 8. The value of Cp dereases to 0.2 at low and hgh tp speed rato (TSR = 4 and TSR = 10). The nducton factor a for the range of TSR tested n the present study was between 0.06 and The maxum value for the nducton factor a = 0.13 was obatned at TSR = 7 and s less than the maxmum theoretcal value of nducton fcator of a = 1/3 (Betz lmt). y = 0.1 m y = 0.2 m y = 0.3 m y = 0.4 m y = 0.5 m Fgure 4: Velocty magntude at the rotor plane and at dfferent statons behnd the wnd turbne: Unshrouded Turbne, U wu = 4 m/s and TSR = 7

6 Power coeffcent Cp & Inducton factor a Jordan Journal of Mechancal and Industral Engneerng. All rghts reserved - Volume 11 Number 4 (ISSN ) Inlet Outlet HAWT Turbne Fgure 5: Axal varaton of the pressure and velocty magntde for the Unshouded Wnd Turbne (U wu = 4 m/s, and TSR = 7) 0.5 Power Coeffcent Cp Inducton factor a Tp Speed Rato TSR Fgure 6: Power coeffcent Cp and nducton factor a for the unshrouded wnd turbne

7 2017 Jordan Journal of Mechancal and Industral Engneerng. All rghts reserved - Volume 11 Number 4 (ISSN ) 241 For the shrouded turbne, the equatons are used to calculate the C p. The area rato, L, was calculated usng the selected geometry of the shroud (A out /A rot ). The bac pressure coeffcent,, was calculated usng equaton 13 based on the average wnd speed at the ext of the shroud U out. Fgs. 7 and 8 show the velocty magntude n the X-Y and X-Z planes respectvelly; for the shrouded wnd turbne. The velocty contour results show clearly an ncrease (1.8 tmes the freestream velocty U wu = 4 m/s) of the wnd speed nsde the shrouded turbne. Ths wll draw more mass flow nsde the shrouded wnd turbne leadng to an ncrease n the shaft power,.e., the power extracted from the turbne. Fgure 7: Velocty magntude contours n the X-Y plane for the shrouded turbne (U wu = 4 m/s and TSR = 7) y = m y = 0 m y = 0.05 m y = 0.1 m y = 0.2 m y = 0.3 m Fgure 8: Velocty magntude contours n the X-Z plane for the shrouded turbne (U wu = 4 m/s and TSR = 7)

8 Inducton factor, Bac pressure coeffcent,and Cp- Dffuser Power Coeffcent Cp Jordan Journal of Mechancal and Industral Engneerng. All rghts reserved - Volume 11 Number 4 (ISSN ) Le the unshrouded turbne, the wnd approched the shrouded wnd turbne wth a wnd speed of U wu = 4 m/s. The wnd speed decreases as t approaches the turbne before t passes thorugh the rotor plane. The axal velocty defct s the amount of decrease n the frestream velocty, as shown n Fg. 9. The nducton factor a gven by the equaton 16 represents the fracton by whch the axal component of the velocty s reduced. The average ar velocty downtream the wnd turbne and where the pressure s recovered bac (See Fg. 9) to the atmospherc pressure s used to calculate the nducton factor. The bac pressure coeffcent s calculated from the average wnd speed at the shroud ext usng equaton 13. The power coeffcent Cp s then caluclated usng the equaton 11 for the shrouded turbne. The power coeffcents versus the TSR were then determned for the shrouded turbnes as shown n Fgure 10. The results n Fg. 10 show an ncrease of the Cp by a factor rangng from 2.46 to 4.33 for the shrouded turbne compared to unshrouded turbne. The numercal results show a net mprovement and more pronounced enhancement of the electrcal power extracted from the shrouded turbne. These results are n good agreement wth the expermental data obtaned by Yu and Taash [6] for the shrouded wnd turbne wth a brmmed dffuser. Hs expermental results show a power augmentaton by a factor of about 2 5 compared wth a bare wnd turbne, for a gven turbne dameter and wnd speed. The expermental results also show that for long dffuser (Dffuser length Lt = 1.47 D), a remarable ncrease n the output power of approxmately 4-5 tmes that of a conventonal wnd turbne s acheved [6]. Fgure 9: Axal varaton of the pressure and velocty magntde for the shouded Wnd Turbne (U wu 4 m/s, and TSR = 7) 0.8 Inducton factor a Bac pressure coeffcent n Cp - Dffuser Power coeffcent Cp Tp Speed Rato TSR 0.8 Fgure 10: Varaton of the power coeffcent Cp and nducton factor a wth the tp speed rato - Shrouded wnd turbne (Uwu = 4 m/s)

9 2017 Jordan Journal of Mechancal and Industral Engneerng. All rghts reserved - Volume 11 Number 4 (ISSN ) Conclusons Computatonal flud dynamcs analyss was performed n the present study to assess the performance of shrouded HAWT. The governng equatons of mass, momentum and turbulence equatons were solved usng fnte volume method to obtan the nformaton about the flow feld around the turbne. The nducton factor and the power coeffcent (Cp) were calculated from the computed velocty profles. The Computatonal Flud Dynamcs results and the varaton of the Cp versus the tp speed rato of the unshrouded and shrouded turbnes show that the shrouded turbnes can generate greater power than the non-shrouded baselne wnd turbne; and the varaton of the Cp versus the tp speed rato shows an ncrease by a four folds compared to the unshrouded turbne. Ths ncrease s due to the ncrease of the ar speed nsde the shrouded turbnes. The present study suggests that shrouded HAWT can be a good soluton for energy producton n regons wth low speed condtons offsettng more fossl fuel usage and reducng ther subsequent greenhouse gas emssons. References [1] Chaou Ghena, Isam Janareh, Comparson of resource ntenstes and operatonal parameters of renewable, fossl fuel, and nuclear power systems, Internatonal Journal of Thermal and Envronmental Engneerng, Volume 5, No2, , [2] Kanyao F., Isam Janareh, Implementaton and economcal study of HAWT under dfferent wnd, Sustanable Ctes and Socety, 15, 2015, [3] Lew, D. (2000). Alternatve to coal and candles: wnd power n Chna. Energy Polcy, 28, [4] Hort S., and Larsen, H., Rotor desgn for dffuser augmented wnd turbnes, Energes, 8, , [5] Maa, L.A.B, Expermental and Numercal study of dffuser augmented wnd turbne, DAWT, Master thess, December [6] Yu Ohya and Taah Krasudan, A Shrouded wnd turbne generatng hgh output power wth wnd lens technology, Energes, 3, , [7] Buyung Kosash, and Andrea Tondell, Expermental study of shrouded mcro-wnd turbne, Evolvng Energy- IEFInternatonal Energy Congress (IEF-IEC2012), Proceda Engneerng 49 ( 2012 ), [8] Jun-Feng Hu, and Wen-Xue Wang, Upgradng a Shrouded Wnd Turbne wth a Self-Adaptve Flanged Dffuser, Energes 8, 2015, [9] Anet C. Aranae, Vnod K. Lashmnarayan, and Karth Durasamy, Computatonal analyss of shrouded wnd turbne confguratons usng a 3-dmensonal RANS solver, Renewable Energy 75 (2015) [10] Aly M. El-Zahaby, A.E. Kabeel,, S.S. Elsayed, M.F. Obaa, CFD analyss of flow felds for shrouded wnd turbne's dffuser model wth dfferent flange angles, Alexandra Engneerng Journal, 56, Issue 1, 2017, [11] Isam Janareh, Ilham Talab, Wnd Data Collecton and Analyses at Masdar Cty for Wnd TurbneAssessment, Int. J. of Thermal & Envronmental Engneerng, Vol. 1, No. 1 (2010) [12] Isam Janareh, Ilham Talab, and Jll Macpherson, Numercal Smulaton of Tower Rotor Interacton for Downwnd Wnd Turbne, Hndaw: Modellng and Smulaton n Engneerng, Volume 2010, Artcle ID , 11 pages, do: /2010/ [13] Ghena, C., Sargsyan, A., and Janareh, I., Three dmensonal modelng of flow feld around a horzontal axs wnd turbne (HAWT), Computatonal Flud Dynamcs n the Development of Renewable Energy Applcatons Boo, Internatonal Energy and Envronment Foundaton, [14] I Janareh, R Qudah, I Talab, C Ghena, Aerodynamc flow smulaton of wnd turbne: downwnd versus upwnd confguraton, Energy Converson and Management 51 (8), [15] Patanar, S. V. (1980). Numercal Heat Transfer and Flud Flow. Taylor & Francs. ISBN [16] Ferzger, J. H.; Perc, M. (2001). Computatonal Methods for Flud Dynamcs. Sprnger-Verlag. ISBN [17] Tannehll, J. C.; Anderson, D. A.; Pletcher, R. H. (1997). Computatonal Flud Mechancs and Heat Transfer. Taylor & Francs. [18] Bussel, G.J.W., An assessment of the performance of dffuser augmented wnd turbnes, Delft Unversty of Technology, Proceedngs of the 3th ASME Jont Fluds Engneerng Conference, San Francsco, Calforna, July 18-23, [19] Bussel, G.J.W., The Scence of mang more torque from wnd: dffuser experment and theory revsted, Delft Unversty of Technology, Journal of Physcs confer.