INVESTIGATION OF POWER LAW WIND EXPONENT WITHIN THE LOWER BOUNDARY LAYER AT ILE-IFE, NIGERIA. Akinlade, G.O. and Ayoola, M.A.

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1 Ife Jurnal f Science vl. 14, n. 2 (212) 325 INVESTIGATION OF POWER LAW WIND EXPONENT WITHIN THE LOWER BOUNDARY LAYER AT ILE-IFE, NIGERIA. Akinlade, G.O. and Ayla, M.A. Department f Physics, Obafemi Awlw University, Ile-Ife th th (Received 26 July, 212; Accepted 8 Octber, 212) ABSTRACT A sund detectin and ranging (SODAR) equipment installed n the rf f a tw-strey building at the Department f Physics, Obafemi Awlw University Ile-Ife, Nigeria (7.52 N and 4.52 E), 294 m height abve mean sea level and abut 2 m abve the grund level has been used t study n cntinuus basis the planetary bundary layer (PBL) wind. Eighteen thusand (18) sets f hurly averaged data fr the wind speed frm 3 m abve the surface t 5 m height at 2 m intervals had been used t study the pwer law wind expnent ver Ile-Ife Nigeria. The data cvered abut tw and half years (frm June 29 t December 211). Expnent was fund t increase with height and decrease with surface wind speed. It was als discvered that had bth diurnal and seasnal variatins. The values at daytime were lwer than at nighttime and the wet mnths had higher values than the dry mnths. Its average fr the dry mnths was.33 while the average fr the wet mnths was.45. INTRODUCTION Bundary layer winds had been studied in time past fr varius reasns ranging frm the study f weather system dynamics t wind energy applicatins. In the design f tall buildings, civil engineers need infrmatin abut the vertical wind structure arund the building and the maximum wind speed the building needs t withstand. The nature f the vertical wind prfile als dictates the structure f buildings, bridges, pllutant dispersin and wind turbines amng thers (Stull, 1988). In mst weather statins in Nigeria, wind speed are bserved at 1 m height r belw, whereas in studying the suitability f wind energy generatin in the cuntry, infrmatin abut wind speed is needed at the hub height which is usually sme height greater than 1 m. Hence there must be a way f extraplating the wind speed at the hub height frm the available 1 m wind. Wind speed prfiles within the bundary layer are generally represented by tw relatinships; the lgarithmic and pwer law prfiles. While the lgarithmic wind prfile finds its applicatin within the lwest 1 m abve the surface, the pwer law wind prfile is valid within the lwest 6 m abve the grund. Hence fr mst meterlgical purpses and engineering applicatins, the pwer law wind prfile is used (Linsley et al., 1988). The wind field in the bundary layer is largely cntrlled by frictinal drag impsed n the flw by the underlying rigid surface. The drag retards air mtin that is clse t the grund and gives rise t a sharp decrease f mean hrizntal wind speed as the surface is apprached. In the absence f strng thermal effects, the depth f this frictinal influence depends n the rughness f the surface. The wind therefre increases with height in the bundary layer. Ile-Ife, Nigeria, is lcated within the trpics. Here, there are tw prminent seasns, namely; dry and wet seasns. The meridinal mvement f the Inter-trpical discntinuity (ITD) dictates the tw seasns. ITD is the meeting pint ver land f mist suthwesterly stream frm the cean and the dust-laden dry nrtheasterly trade wind frm the Saharan desert, while the meeting pint ver the cean is called the Inter-Trpical Cnvergence Zne (ITCZ) (Balgun, 198). The psitin f the ITD n surface chart usually cincides with the psitin f a lw-pressure system als knwn as a heat lw. The characteristic behaviur f the ITD is such that any statin nrth f it experiences cld dry dust-laden winds lcally knwn as 'Harmattan' in West Africa, while any statin suth f it is brught under the influence f warm suth westerlies, which brings abut cnvective clud frmatin leading t thunderstrm and rain, depending n the psitin f the statin relative t the ITD (Adedkun, 1978). Previus studies revealed that the ITD

2 326 Akinlade and Ayla: Investigatin f Pwer Law Wind Expnent migrates between 4 N and 25 N during the curse f a year (Hamiltn and Archbld 1945). The intersectin n the surface migrates with the slar cycle attaining its mst nrtherly psitin between latitudes in August. As a result, the mnsn winds penetrate far int the West/Nrth Africa hinterland cvering the whle f Nigeria during this perid, bringing misture and rain int the Sahel and Suth Sahara. Between late August and early September, the ITD line starts t recede suthwards till it reaches the mst sutherly psitin abut latitudes 4-6 N (in January). During this time, the nrth easterly winds, (lcally knwn as Harmattan) dminates ver the regin, bringing dust as far as suth f West African casts (Adedkun, 1978). Balgun (198) further affirmed that as suthwest mist air becmes thicker ver West Africa. The thicker layer cluds cut ff inslatin and cnsequently reduce cnvectin develpment and thunderstrm activity (the august break phenmena). Generally, the regin under study is characterized by lw winds. It is therefre imprtant t investigate the suitability f wind energy generatin vis-à-vis the pwer law wind expnent. THEORY AND METHODOLOGY Thery Accrding t Jhnsn (1959) and Geiger (1965), the pwer-law is represented as; =( ) U Z U Z Where U is the wind speed at height Z, U is the wind speed at a reference height Z usually 1 m. is an expnent which varies with surface rughness, surface stability and surface wind speed. Accrding t Alvi and Abdalla (199), the value f varies between.1 and.6, while ther researchers maintained that the value varies between.1 and.4 (e.g. Jarass, 1981). Frm existing recrds, it was estimated that fr castal areas, expnent takes the value f.15, while in inland lcatins, the parameter is higher and takes the value f.3 (Pashardes and Christfides, 1995). This pwer law wind expnent is investigated in this wrk t ascertain whether the value at Ile-Ife agrees with what btains in the mid-latitudes as fund in the literature. Experimental Set-up The instrument used fr this study is a METEK PCS.2/24 acustic sunder (sdar) system which uses a phased array antenna cnsisting f 24 ludspeakers f 1W maximum utput pwer, serving as transmitters as well as receivers. In rder t reduce an interference with envirnmental nise and t suppress the influence f eches frm fixed targets, the acustic antenna is lined with an absrbing material. The speakers are switched t 4 different phases (, 9, 18 and 27 ) t direct the beam t ne f the 5 directins; the fifth directin is vertically upward (METEK, 26). In the present peratin, the PCS.2/24 sdar system was cnfigured t give return values f the wind speed and directin frm 3 m t abut 83 m (tpmst measurement height was 83 m). A vertical spacing f 2 m was selected between successive measurement levels. In 21, the sdar system was cnfigured t include surface data. Fr this reasn the 21 data had wind speed values at height 1 m. While fr ther perids, the lwest measurement height was 3 m. In rder t be free f bstructin, the sdar antenna (Figure 1) was installed n the rf f a tw-strey building at the Department f Physics, Obafemi Awlw University Ile-Ife, Nigeria with crdinates 7.52 N and 4.52 E and 294 m height abve mean sea level and abut 2 m abve the grund level. The system was cnnected t an indr desktp cmputer (Figure 2; a UPS was prvided t slve the prblem f pwer utage that was nt mre than tw hurs) which was cntrlled by prprietary sftware prvided by the instrument manufacturer. Subsequently, the wind prfiles were then stred as 1 minutes averages. The system was installed in June 29 and had been in peratin since then, fr the mnitring f wind prfiles in the bundary layer.

3 Akinlade and Ayla: Investigatin f Pwer Law Wind Expnent 327 Fig. 1: Metek Sdar Antenna Installed at the Rf Tp f Physics Building, Obafemi Awlw University, Ile-Ife Fig.2: The Indr Desktp Cmputer that Cntrls the Sdar Antenna with the UPS; the Mnitr Displays the 1 Minutes Averages f Wind Prfile Measured by the System, the Gaps Indicates Perids f Pwer Outage.

4 328 Akinlade and Ayla: Investigatin f Pwer Law Wind Expnent Data Prcessing and Analysis As a way f data quality cntrl prvided within the METEK sftware (METEK graphics), wind speed values abve 3 m/s were screened ut withut replacement frm the data set (3 m/s being the upper limit f valid wind speed value measurable by this mdel f the METEK sdar). The btained data were then subjected t data reductin by means f lcally develped sftware using the AutIt Vs.3 language. This reduced the 1 minutes averages t daily hurly averages. The data used fr this investigatin cmprised f abut 18, data pints f daily hurly averages starting frm June 29 t Nvember 211 with the exemptin f Nvember and December 29, January 21 and March 211 when the equipment was nt functinal r that there was a prlnged electric pwer utage. The wind shear expnent was investigated in this study t see hw it varies with height, mnth f the year, surface wind speed and time f day. Wind speed prfile was als studied under different stability cnditin fr the area under study. RESULTS Variatin f Pwer Law Wind Expnent With Height Figure 3 presents the variatin f pwer law wind expnent with height when the wind speeds are averaged ver all the perid and all the hurs. The values f crrespnding t Z values f 3 and 5 m were cmputed frm the whle data sets while values crrespnding t Z value f 1 m are based n nly 21 data (abut 8, data pints). It is generally bserved that values increase with height, this is in cntradictin t what was reprted by Alvi and Abdalla (199) when they studied the wind speed prfile ver Bahrain, a desert regin in the Middle East, they reprted a decrease f with height. This cntradictin may be attributed t the lcatin and the canpy level where the experiment was cnducted. Hwever, that the values tend twards a cnstant at abut 5 m height is in cnsrt with what the authrs reprted. This implied that extraplating the wind at a particular height frm the surface is dependent n the height at which extraplatin is dne, the expnent is applicable t the height belw 5 m. Hwever, abve 5 m, a cnstant value can be used such that an increase in value with Z is als bserved frm Figure 3. A significant increase is bservable abve 15 m height. Fig 3: Variatin f Pwer Law Wind Expnent with Height fr Different Values f Z

5 Akinlade and Ayla: Investigatin f Pwer Law Wind Expnent Variatin f With Surface Winds Figure 4a presents the variatin f the pwer law wind expnent with surface winds i.e. wind at 1 m when Z is taken t be 1 m. The data used fr this plt were that f the daily hurly averages fr the year 21 alne crrespnding t abut 8, data pints but further reduced int mnthly hurly averages. It can be bserved that has an 329 inverse relatinship with the surface wind speed, that is, it decreases with increasing surface wind speed in cnsistence with the bservatin f Alvi and Abdalla (199). Figure 4b is the same as Figure 4a except that Z is 3 m. It als established the fact that decreases with increasing 3 m height wind speed. Fig. 4a: Variatin f with Surface Wind Speed (Wind at 1 m) Fig. 4b: Variatin f with Wind Speed at 3 m

6 33 Akinlade and Ayla: Investigatin f Pwer Law Wind Expnent Effects f Different Stability Cnditins n Expnent The values f expnent, calculated fr different stability cnditins are presented in Table 1. Figure 5 presents values fr different times f the day crrespnding t different stability cnditins. Tw segments are bvius frm the Figure; the first segment crrespnds t when Z is 3 m and the secnd segment crrespnds t Z = 1 m. When the atmsphere is transiting frm unstable cnditin t stable cnditin, the surface layer first becmes stable and decuples frm the upper layer. Fr this reasn, the atmsphere may be stable at height 1 m and may nt be stable at 3 m height and abve. This is the majr reasn why there is difference between the shapes f the tw segments in Figure 5. Fr the first segment, values crrespnding t Z = 3 m shw a unilateral increase frm predawn t evening. This is a bit different frm what was bserved frm the secnd segment where Z = 1 m. The newly established unstable cnditins (Mrning and afternn) are assciated with lw values f, while stable cnditins (evening and predawn) are assciated with relatively higher values f. Thus, values are expected t be higher during nighttime than during the day. This is because turbulent mixing is expected t weaken the wind shear during the day due t high inslatin, this effect is absent during the nighttime giving rise t strng wind shear and higher value. Because the shape f the secnd segment agrees with established facts frm literature mre than the first segment, therefre, the wind at 1 m is mre realistic t use as initial wind value fr extraplating upper level winds using the pwer law relatinship. Table 1: Values During Different Stability Cnditins Stable Neutral Unstable Neutral Stable Predawn Dawn Mrning Afternn Dusk Evening (12-6 am) (6-7 am) (7-12 am) (12-5 pm) (5-6 pm) (5-12 midnight) 3, , , , , , , , Fig. 5: Values fr Different Stability Cnditins

7 Akinlade and Ayla: Investigatin f Pwer Law Wind Expnent Diurnal Variatin f The diurnal variatin f is presented in Figure 6. It is bserved that the values f are relatively higher during the night time perids than during the day. Tw minima are nticeable; ne is between 4 5 hrs and the ther between 18 and 19 hr. (1hr refers t average f 1 minutes measurements taken between 1 and 15) with the ne arund 4 hr being mre prnunced. The lw values f during the day are expected as cnvectin tends t weaken vertical wind shear during the day. When Figure 6 is cmpared with Figure 7, the reasn fr the deep at abut 4 hr is bvius. It has been established earlier in Figure 4 that wind shear parameter decreases with increasing surface wind speed. Figure 7 shws an increased surface wind speed at 4 hr. A careful lk at the 331 data set prir t averaging reveals a wind speed maximum between the hurs f 4hr and 5hr, this is what Figure 7 reveals at 4hr. It had als been established by Akinlade and Ayla (212) that the diurnal wind pattern ver the statin reveal tw maxima; ne between 4 and 5 hrs and the ther between 2 and 21 hrs. It is this increase in wind speed values at predawn that is respnsible fr the characteristic different wind prfile bserved at 4 hr in Figure 7 and als resulted in the deep nticed at 4 hr in Figure 6. Apart frm this unique behaviur f the wind prfile at 4hr belw 12 m, the vertical wind prfiles are in agreement with what is btained in the literature except that there seems t be little r n disparity between night time and day time situatins when the data are averaged ver the entire perid. Fig.6a: Diurnal Variatin f fr Different Step Changes when Z = 3 m (whle data)

8 332 Akinlade and Ayla: Investigatin f Pwer Law Wind Expnent Fig.6b: Diurnal Variatin f fr Different Step Changes when Z = 1 m (21 data alne). Fig.7: Mean Wind Speed Prfile Over the Statin fr Different Times f the Day (whle data).

9 Akinlade and Ayla: Investigatin f Pwer Law Wind Expnent Mnthly Variatin f The mean variatin f as a functin f the mnth f the year separated int the tw seasns is presented in Table 2 and the averaged value acrss the heights up t height 5 m is as shwn in Fig 8. The estimated number f hurly data pints is enclsed in brackets. Pwer utages and mechanical faults f the sdar are respnsible fr the sparse data in the dry mnths. It is shwn that the wet mnths have higher values than the dry mnths; unlike what is bserved by Pashardes and 333 Christfides (1995) that winter mnths recrds higher mean wind speeds than the summer mnths althugh they nted that the difference is nt significant. This implies that strnger upper level wind speeds are t be expected during wet mnths than during the dry mnths. While this infrmatin serves as precautin fr civil engineers in building skyscrapers, it is indeed an advantage in wind energy generatin meaning greater energy during the wet mnths Values (868) (54) (936) (682) (1,731) (1,994) (1,244) (1,578) Dec Jan Feb March June July Aug Sept Mnth Fig 8: Variatin f With Mnth Averaged Over the Entire Perid and Height up t 5 m

10 334 Akinlade and Ayla: Investigatin f Pwer Law Wind Expnent Table 2: Mean Variatin f With Height (Z = 3 m) fr Tw Seasns f the Year Averaged Over the Entire Perid and Height up t 5 m Dry Mnths Wet Mnths Heights (m) Dec Jan Feb March Mean June July Aug Sep Mean Mean CONCLUSION The pwer law wind prfile has been studied ver Ile-Ife (7.52 N and 4.52 E) Nigeria using an acustic sunder technique (METEK PCS 2 mini sdar). The pwer law wind expnent being the parameter needed in extraplating the wind speed abve the surface frm the available surface wind speed have been fund t vary with a number f parameters. Pwer law wind expnent increases with increasing height but tends twards a cnstant value at heights abve 5 m, it decreases with increasing surface wind speed, and its values are higher at nights than at day-time. Hwever, it has its minimum value between 4 and 5hrs. Relatively, its values are higher during the wet mnths than the dry mnths and its average value fr the dry mnths being.33 while its average value fr the wet mnths is.45. REFERENCES Adedkun, J.A West African precipitatin and dminant Atmspheric mechanisms. Archives fr Meterlgy, Gephysics Biclimatlgy Series A 27: Akinlade, G.O. and Ayla, M.A Assessment f wind energy prspect at the Obafemi Awlw University (OAU), Ile-Ife campus using a sdar system. Advances in Meterlgy (Submitted) Alvi, S. H. and Abdalla Yusef, A.G The vertical lw-level wind speed prfiles ver Bahrain. Wind Engineering 14: Balgun E.E Seasnal and spatial variatins in thunderstrm activities ver Nigeria. Weather 36 pp Geiger, R., The climate near the grund.harvard University Press, Cambridge.

11 Akinlade and Ayla: Investigatin f Pwer Law Wind Expnent Hamiltn, R.A. and Archbld, J.W Meterlgy f Nigeria and adjacent territry. Quarterly Jurnal f the Ryal Meterlgical Sciety 72, Jhnsn, O An examinatin f the vertical wind prfile in the lwest layers f the Atmsphere. Jurnal f Meterlgy, Vl.16, N 2 Jarass, L Wind Energy. Springer, Berlin 335 Linsley, R.K., Khler, M.A., and Paulhus, J.L.H Hydrlgy fr Engineers, McGraw- Hill Bk C., SI Metric Editin Pashardes, S. and Christfides, C Statistical analysis f wind speed and directin in Cyprus. Slar Energy 55: Stull, R.B., Intrductin t bundary layer meterlgy. Kluwer Academic Publishers, the Netherlands.