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A Novel Small Scale Efficient Wind Turbine for Power Generation

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  A novel small scale ef  󿬁 cient wind turbine for power generation N.A. Ahmed  Aerospace Engineering, School of Mechanical and Manufacturing Engineering, University of New South Wales, NSW 2052, Australia a r t i c l e i n f o  Article history: Received 29 May 2012Accepted 18 January 2013Available online 21 February 2013 Keywords: Wind turbineRenewable energyPower generationCFD a b s t r a c t A  ‘ proof of concept ’  study of a novel wind turbine that overcomes some of the de 󿬁 ciencies and combinesthe advantages of the conventional horizontal and vertical axis wind turbines is presented in this paper.The study conducted using computational  󿬂 uid dynamics and wind tunnel tests clearly demonstrate thatsuch a proposition is feasible and a low cost, low noise, safe and easy to operate but enhanced perfor-mance wind turbine for small scale power generation in low wind speed is viable.   2013 Elsevier Ltd. All rights reserved. 1. Introduction Wind as a source of renewable energy offers an increasinglyviable solution to one of the most pressing issues of our time, thatis, pollution free electricity for sustainable living. The continueddependence on depleting fossil fuel sources or nuclear power hasthe potential to wreck the world ’ s economy and security. If theissue is not addressed with a sense of urgency, then the havoc thatthe recent nuclear power plant meltdown in Japan of 2011 or oilspills of the Gulf of Mexico of 2010 caused, will pale in comparisonthreatening mankind ’ s very existence.In the quest of using wind as a renewable source of energy,increasing attention is given to the production of electricity usingsmall wind turbines for domestic and industrial buildings. In thestate of New South Wales of Australia, for example, laws have beenproposed [1] to allow windmills with a generating capacity of 10 kWor less to be erected in residential areas, and 60 kW in ruraland industrial areas with the addition of solar panels as an optionfordomesticpowergeneration. Aheightlimitof 3mabovetheroof linewill be imposedand turbineswill haveto be at least 25 mfromneighbouringproperties.Aswithsolarpanels,homeownerswillbeable to sell surplus power they generate to the electricity grid,protecting them from skyrocketing power prices. Under the plan,families intending to install a wind turbine would lodge a 10-daycomplying development application with the local council. Strictnoise and location controls would ensure neighbourhoods are notturned into turbine jungles. These steps are intended to make iteasier for property owners to install wind and solar systems turn-ing suburbs and rural areas into renewable energy harvesting areaswith no, or minimal environmental and local amenity impacts.In the USA, small scale residential wind turbines are allowed inparts of New York. A  󿬁 ve storey affordable housing apartmentbuilding in the South Bronx, in New York, for example, hasdeployed 10 kW wind turbines to supplement the facility ’ s con-ventional power usage in the building ’ s hall ways, elevators andother common areas.Despite these positive developments, full exploitation of windturbine technology for domestic use will be possible only whenwind turbines become ef  󿬁 cient at low speed and can be operatedsafely with little or no noise and have the capacity to run withoutshut-down under moderate to extreme variations in windconditions.TheAerodynamicsresearchteamattheUniversityofNewSouthWales has been actively exploring various approaches to harnessrenewable energy for general house hold usage and reduce carbonfoot print to the environment [2 e 19]. To compliment such efforts,the present project was formulated with the objective of develop-ingef  󿬁 cient and effective small scalewind turbines. The authorhasalreadyoutlined qualitativelythe conceptof such awind turbine ina plenary address at the International Conference on RenewableEnergies and Power Quality held in March, 2012 in Spain [20].The present paper provides more qualitative support to thatproposition. E-mail address:  n.ahmed@unsw.edu.au. Contents lists available at SciVerse ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene 0960-1481/$  e  see front matter    2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.renene.2013.01.023 Renewable Energy 57 (2013) 79 e 85  2. The novel wind turbine  2.1. The rationale behind the concept  Harnessing wind energy has always been accomplished by twomain types of wind turbine, namely the horizontal axis wind tur-bine and the vertical axis wind turbine. The two have been com-peting with each other in terms of ef  󿬁 ciency, ease of maintenance,environmental safety and aesthetic looks.The horizontal axis wind turbine is known for its high ef  󿬁 ciencywhen facing the direction of the wind. However, under high windconditions, the blades fail in  󿬂 ap wise bending of the rotor blade asthey approach the speeds higher than it is designed for, a commonoccurrence observed in wind farms. This problem is prevented byhaving to stall, wasting untapped power. The gearboxof the HAWTis also located at the top of tower, making it dif  󿬁 cult to access forinstallation and maintenance. To the environmentalists, the toweris not desirable, as the blades pose a threat to fowls.The vertical axis wind turbine promises the versatility of beingable to capture the wind from all directions without having to stallunder high wind conditions. The location of its generator andgearbox are housed at ground level for easy access during main-tenance due to its design. Given a wider range of designs, it is alsoaesthetically pleasing to the eye. However, due to the circular cycleof motion, the vertical axis wind turbine often have a portion of itsblades constantly backtracking against the wind, making ef  󿬁 ciencyits major disadvantage, and lowering it by as much as 30 timescompared to horizontal axis wind turbine.The proposed novel wind turbine is an attempt to overcomesome of the de 󿬁 ciencies but retain the advantages of both thehorizontal and vertical axis wind turbines. The con 󿬁 guration issimple and requires mainly an additional cowling with air guidevanes enclosing a rotor used in a horizontal axis wind turbine.However, the whole con 󿬁 guration would be installed parallel oraligned to the wind direction in a manner that a vertical axis windturbine would be oriented and guide the wind to the rotor in anef  󿬁 cient manner.  2.2. Con  󿬁  guration Thenovelwindturbineconceivedcanbebrie 󿬂 ydescribedusingthe schematic presented in Fig. 1. The wind turbine, 1, is mounted ontheturbinesupportstructure,2,withdirectionvanes,3,todirectthe wind towards the turbine blades, 4. This arrangement makesthe wind turbine operate like a horizontal axis wind turbine. Theturbine blades can be of   󿬁 xed or variable pitch. A second row of counter rotating blades can also be used to improve ef  󿬁 ciency, if needed. The surrounding structure or the cowling, 5, directs the air 󿬂 ow to provide improved conversion of the wind energy throughthe turbine blades into either mechanical or electrical energy.Mechanical or electrical devices can be mounted at the base of thestructure, 6, which allow ease of maintenance and lighter supportstructures for the turbine. The speed of the turbine can be con-trolled byrotatingthesurroundingstructure,5,varyingtheturbineblade pitch, 4, or varying the directional vanes, 3. Flow modifyingstructures attached to, 5, can also be varied to control turbinespeed. The shape of the surrounding structure, 5, of the turbineconditions the  󿬂 ow and minimises the impact of sudden gusts onthe turbine blades and facilitate the increase of the air velocity atthe turbine blade. This would result in improved low wind speedstart up and overall ef  󿬁 ciency. A protective screen, 7, can beincluded to protect the turbine from bird strikes. Part or all of the Fig. 1.  Schematic of a novel wind turbine. Fig. 2.  Schematic of the numerical model. CD  ¼  constriction diameter; CH  ¼  cowlingheight; COA  ¼  Cowling outlet angle; VG  ¼  Vane gap. Fig. 3.  Mesh topology of the proposed wind turbine con 󿬁 guration. N.A. Ahmed / Renewable Energy 57 (2013) 79 e 85 80  surrounding structure, 5, could be rotated to face the direction of the wind by any external motor and direction system such as anymechanical wind vane. The vertical wind turbine,1, combined withthe surrounding structure, 5, also will help decrease noise levels byacting as a noise absorbing chamber. 3.  ‘ Proof of concept ’  investigation The success of the concept proposed depends on ensuring thatwind from the surrounding atmosphere can be guided to the tur-bine rotors with minimum losses. This involves determining theoptimum geometry of the structure with the goal of achieving thehighest velocity at the constriction where the rotor will be located.The proof of concept nature of the work, therefore did not entailmodelling of the  󿬂 ow  󿬁 eld in the presence of rotor or the roof  Fig. 4.  Flow  󿬁 eld with cowling outlet at angles of 5  and 50  . 0102030400 10 20 30 40 50 60    I  n  c  r  e  a  s  e   i  n   W   i  n   d   S  p  e  e   d   (   %   ) Cowling outlet angle (degrees) Fig. 5.  Increase in wind speed with changes in cowling angle. Fig. 6.  Flow  󿬁 elds for different turning vanes gaps (a) 5 mm (b) 20 mm. N.A. Ahmed / Renewable Energy 57 (2013) 79 e 85  81  boundary layer. The present work was, therefore, undertaken toexplore such geometrical con 󿬁 guration and identify the key con- 󿬁 guration parameters of the cowling using computational  󿬂 uiddynamics (CFD) and then validating the results through windtunnel experiments.  3.1. Identifying the key con  󿬁  guration parameters using CFD Since CFD as a research tool is especially cost effective and 󿬂 exible,itwasdecidedtousethismodeofinvestigationtoestablishthe interrelations between the various important geometrical pa-rameters of the novel concept, such as cowlingoutlet angle turningvanegap,andconstrictiondiameter wherethe rotor willbelocated(Fig. 2).As the  󿬂 ow features and variables of interests of this study areon the global scale, the Reynolds-averaged Navier e Stokes (RANS)equations, which model the turbulence rather than solve it, wereconsidered of suf  󿬁 cient accuracy to describe the air motions. TheRNG k-epsilon turbulence model was selected to close the RANSequations because of its superior overall performance among theeddy viscosity models in modelling internal turbulent  󿬂 ows [21].The discretization and computation of the RANS equations and theturbulence closure model was conducted using ANSYS FLUENT. Atime resolution of 0.05 s was found suf  󿬁 ciently small enough tocapture the  󿬂 ow features of interest since halving the time stepfurther did not have any signi 󿬁 cant in 󿬂 uence on the numericalresults.Fig. 3 shows the mesh topology of the proposed wind turbine.The cowling was maintained at a constant height of 180 mm.Meshes were re 󿬁 ned and were denser at the critical areas. Thesewereattheturningvanesatthetopandconstrictionatthecowling.For mesh re 󿬁 nements, an  ‘ area of in 󿬂 uence ’  was created using CFX-Mesh, and area with an  ‘ in 󿬂 ation layer ’  was set along the surfacewith a default body spacing of 15 mm. The default face spacing hadangular resolution of 30  with minimum and maximum lengths of 1 mm and 15 mm respectively.For residential areas, the wind speed is expected to be low. Thisconsideration led to conducting numerical simulations for windspeed of 5 m/s.  3.1.1. Cowling outlet angle Numerical simulations were conducted at six outlet angle of 5  ,15  , 20  , 30  , 40  and 50  respectively. During these simulations,the dimensions of the turning vane gap and constriction diameterwere kept at 15 mm and 75 mm respectively.The main aim of these simulations was to determine for each of the six cowling outlet angles the highest velocity occurring withinthe symmetry plane at the constriction diameter. For demonstra-tive purposes, the  󿬂 ow  󿬁 eld with the smallest and the largestcowling outlet angles are shown in Fig. 4.Fig. 5 was plotted with the collated values to show the per-centageincreaseofwindvelocityattheconstrictionwithrespecttothe inlet wind speed of 5 m/s. From this  󿬁 gure, a sharp percentageincrease in velocity at low cowling outlet angles is observed. Thesteepness of the curve then drops and the increase in velocity Fig. 8.  Flow  󿬁 elds with constriction diameters of 55 mm and 105 mm (a) 55 mm (b) 105 mm. 30354045500 5 10 15 20 25    I  n  c  r  e  a  s  e   i  n   W   i  n   d   S  p  e  e   d  a   t   (   %   ) Turning Gap Fig. 7.  Increase in wind speed with changes in turning vane gap. 01020304050607050 60 70 80 90 100 110    I  n  c  r  e  a  s  e   i  n   W   i  n   d   S  p  e  e   d   (   %   ) Constriction diameter (mm) Fig. 9.  Increase in wind speed with changes in constriction diameter. N.A. Ahmed / Renewable Energy 57 (2013) 79 e 85 82
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