An estimation of global solar radiation at ground level using clear-sky radiation in Hebron city, Palestine

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1 Internatinal Jurnal f Renewable Energy, Vl. 8, N. 2, July - December 2013 An estimatin f glbal slar radiatin at grund level using clear-sky radiatin in Hebrn city, Palestine ABSTRACT Husain Alsamamra Al-Quds University, Department f Pysics, Jerusalem, P O Bx 20002, Abu-Dies, Palestine samamra@science.alquds.edu Due t its gegrapical psitin in te slar belt, Palestine is blessed wit an abundance f slar energy and as te pprtunity t utilize tis bunty f natural energy effectively, prmting a clean envirnment and develping renewable energy tecnlgies in te regin. Te present apprac prpses ne equatin specific fr every mnt f te year t simulate te glbal slar radiatin under clear-sky in Hebrn city. Fur-years measured daily glbal slar radiatin data n rizntal surfaces were used fr te mdel assessment. In rder t indicate te perfrmance f te mdel, te statistical test metds f mean bias errr (MBE) and rt mean square errr (RMSE) are used. Te results btained indicate tat te mdel predict te glbal slar radiatin reasnably well. Keywrds: Glbal slar radiatin, Clearness index, Hebrn, MBE, RMSE. 1. Intrductin Knwledge f te lcal glbal slar radiatin is required by mst mdels tat simulate crp grwt, and is als essential fr many applicatins, including evaptranspiratin estimates, arcitectural design, and slar energy systems. Design f a slar energy cnversin system requires precise knwledge regarding te availability f glbal slar radiatin at te lcatin f interest [1]. A prerequisite t te design f slar cllectr systems is te availability f slar irradiance data at te required lcatin. It wuld be cst effective fr slar cllectr system wen te utility lad and slar resurce prfiles are well matced. Due t ig and reliable slar irradiance f abut 5.4 kw/m 2 day a dmestic usage fr slar energy in Palestine ver te life time as te ptential t prduce a dmestic t water fr abut 330 sunny days per year using slar cllectrs [2]. Tere is n dubt tat systematic lng-term data measurements are regarded as te mst effective and accurate metd f setting up te slar irradiance database. In many parts f te wrld, wever, te basic slar irradiance data fr te surfaces f interest are nt always readily btainable. Cnventinally, different matematical mdels ave been develped in te literature t predict te slar irradiance n varius inclined-surfaces using rizntal data [3-4]. Fr places were measured data are nt btainable, teretical and empirical mdels ave been pstulated t cmpute te slar radiatin wuld als be an apprpriate slutin [5-6]. Te tecnical literature prpses a series f empirical mdels t estimate te glbal, direct and diffuse slar radiatin variatin [7]. Altug tese mdels are simple frm a matematical pint f view, and tese are easy t use in practice, tey ave te disadvantage f a limited area f applicability, tese being specific t a particular area. Te existent empirical mdels ave been develped based n te specific measured data specific t a gegrapic area (slar radiatin, temperature, umidity, etc.). Te present wrk prpses a new empirical mdel t estimate te glbal slar radiatin n rizntal surfaces at Hebrn city, Palestine. Specific equatins fr every mnt f te year are als prpsed. 2. Glbal slar radiatin data Palestine as a ig slar energy ptential, were average slar energy is between 2.63 kw/m 2 day in December t 8.4 kw/m 2 day in June, and te daily average f slar radiatin intensity n rizntal surface is 5.4 kw/m 2 day [8]. Hebrn City is lcated in te sutern part f west bank at latitude f 31 57' N, and its climate is t in summer and cld in winter. Als, it receives a large quantity f

2 Internatinal Jurnal f Renewable Energy, Vl. 8, N. 2, July - December slar radiatin, especially in summer [9]. Te glbal slar radiatin data emplyed in tis wrk were supplied by te Palestinian meterlgical ffice. A radimetric statin was establised at te Hebrn city (1000 m abve sea level). Te measurements are carried ut in a cntinuus manner since January 2009 t December 2012, being recrded at a 10 minute interval, data fr ter perids were cecked but unfrtunately tere were big gaps encuntered. Data cllectin started befre sunrise and finised after sunset and all measurements were referred t true slar-time. 3. Results and discussin Slar radiatin data is required t cnduct feasibility studies fr slar energy systems, but grund irradiance measurement sites are nt always available, requiring te use f mdels t estimate irradiance in lieu f measurements [10]. Clear sky mdels are essential t estimating irradiance levels. Clearness index, k t is a parameter tat prvides infrmatin cncerning te real slar radiatin cmpared wit te available slar radiatin. Te clearness index is defined as te rati f te glbal radiatin at grund level n a rizntal surface G and te extraterrestrial glbal irradiatin [11]. In rder t btain mre accurate values f te clearness index te extraterrestrial glbal radiatin is calculated accrding t its dependence n te eccentricity f te eart rbit and te altitude angle. Te clearness index is given by: G k t = ε sinα (1) I were I 0 is te extraterrestrial radiatin; ε is te eccentricity crrectin factr f te Eart s rbit; α is te altitude angle. Te clearness index functins was acieved fr every mnt by mdeling te urly values; ence, a mntly expnential frmula f te clearness index was btained depending n te slar time and ten te mntly mean values was calculated alng te study perid. Figure 1 presents te diagrams fr mntly mean values f te clearness index; te diagrams presented are pltted fr every year cnsidered in te study. Figure 1 Mntly mean values f te clearness index.

3 Internatinal Jurnal f Renewable Energy, Vl. 8, N. 2, July - December Figure 1 presents seasnal variatins f te clearness index wit lw values caracterized in winter seasn and ig values in summer seasn, wever, spring and autumn are transitin seasns. Te iger mntly mean value f te clearness index fr Hebrn city was bserved t be 0.55 in July 2012, wereas, te lwest value (0.21) was btained in December Te prpsed glbal radiatin mdel estimates te slar radiatin fr clear sky cnditins. Tus, te real data underlying te new mdel determinatin are data tat cmply wit te WMO clear sky cnditin. Based n te selected clear sky days te estimatin equatins fr te glbal slar radiatin were determined specific fr eac mnt f te year. Te estimatin equatins fr glbal slar radiatin are riginal and specific t every mnt f te year; te expressins f tese equatins are presented in Table I. Te estimatin equatins f te glbal radiatin are expressed as functins depending n slar time, altitude angle and extraterrestrial radiatin. Te glbal radiatin depends n te gegrapical caracteristics f te area trug te altitudinal angle; lcal particularizatin and seasnal are realized trug te slar time (a spring and autumn day ave abut 9 urs f sun and a maximum altitudinal angle f 45 ; a summer day as 13 urs f sun and te maximum value fr te altitudinal angle is 68 ; fr winter te maximum altitudinal angle is 22 ). Table I. Specific equatins fr glbal radiatin estimatin fr eac mnt. Mnt Equatin January G I * sin( α ) *(0.0452* time ) = slar sin( ) *(0.0369* timeslar sin( ) *(0.0296* timeslar sin( ) *(0.0162* timeslar sin( ) *(0.0262* timeslar sin( ) *( * timeslar sin( ) *(0.0362* timeslar sin( ) *(0.0316* timeslar sin( )*(0.0331* timeslar sin( ) *(0.0268* timeslar sin( ) *(0.0263* timeslar sin( ) *(0.0582* timeslar February G * α ) Marc G * α ) April G * α ) May G * α ) June G * α ) July G * α ) August G * α ) September G * α ) Octber G * α ) Nvember G * α ) December G * α ) Te estimated glbal radiatin data frm te equatins in Table I and te real data are presented in Fig.2; te June and July mnts ave been pltted separately in Fig.3. In Fig.2 and Fig.3 it is clear tat te prpsed mntly equatins estimate in a crrect manner te glbal slar radiatin fr values greater tan 100 W/m 2 ; te estimated values are nt widely spread frm te 1:1 line, especially fr te interval W/m 2. A larger scattering f te simulated values twards te real values are bserved fr ig values f te glbal slar radiatin.

4 Internatinal Jurnal f Renewable Energy, Vl. 8, N. 2, July - December Figure 2. Real and estimated glbal slar radiatin data fr all days wit clear sky cnditins. Figure 3. Real and estimated glbal slar radiatin data fr June and July mnts, days wit clear sky cnditins. Figure 3 present te estimated and real glbal slar radiatin fr June and July mnts, pints crrespnding t iger values tan 800 W/m 2 f glbal slar radiatin values are situated belw te 1:1 line tat means te estimated values are smaller tan te real values. Te underestimatin tendency is predminant fr te summer days, wen te altitudinal angle as ig values. In Figure 4, te estimated and real curves f te glbal slar radiatin fr clear sky days fr January, Marc, June, July and Octber mnts are als presented.

5 Internatinal Jurnal f Renewable Energy, Vl. 8, N. 2, July - December Figure 4. Real and simulated glbal radiatin data f selected mnts. Frm Figure 4 It can be bserved tat te glbal slar radiatin curves simulated wit te prpsed equatins in Table I prvide an accurate estimatin in cmparisn wit te real radiatin curves. Te maximum value f glbal slar radiatin is bserved t be between 12 and 13 urs in June mnt. Als it is clear tat te mdel predicts well te glbal slar radiatin in te secnd alf f te day and te maximum values wit sligtly underestimatin. Als by cmparing te results f June and July mnts in Fig.3 and Fig.4, it is clear tat glbal slar radiatin wit values iger tan 800 W/m 2 is sligtly underestimated. Overall, it can be cncluded tat te estimatin mdel simulates satisfactry te real variatin curves in te afternn and sunset perid, wit a sligtly verestimatin in te sunrise perid alng te year.

Internatinal Jurnal f Renewable Energy, Vl. 8, N. 2, July - December 2013 18 4.

6 Internatinal Jurnal f Renewable Energy, Vl. 8, N. 2, July - December Statistical perfrmance In rder t evaluate te perfrmance f te equatins prpsed t estimate te glbal slar radiatin in Hebrn city fr te perid ( ) a statistical cmparisn is perfrmed using te indicatrs Rt Mean Square Errr (RMSE) and Mean Bias Errr (MBE) [12]. Te test n RMSE prvides infrmatin n te srt-term perfrmance f te mdel as it fllws a term-by term cmparisn f te actual deviatin between calculated and measured value [13]. Te test f MBE prvides infrmatin n te lng term perfrmance f te mdel studied. A psitive MBE value gives te average amunt f verestimatin in calculated values and vice versa. Te mntly values f MBE and RMSE are calculated and presented in figures 5 and 6 respectively. Te Mean Abslute Errr (MAE) values prvide an underestimatin f te glbal slar radiatin during te study perid wit iger values in summer mnts and lwer values in winter mnts. As seen in Fig. 5 in te year 2009 MAE values in summer mnts are iger tan ter years f te study perid wereas, in winter mnts te underestimatin f te glbal slar radiatin is apprximately equal during te study perid. Als, MAE calculated fr September 2010 was iger tan te same mnt f te ter years. As a cmparisn between te calculated and measured values f te glbal slar radiatin, te RMSE presented in figure 6 prvides iger values in mnts frm June t August f 2009 tan te ter mnts f te study perid. Te lwest values f RMSE were btained fr January. Figure 5. Mntly MBE values f glbal slar radiatin.

Internatinal Jurnal f Renewable Energy, Vl. 8, N. 2, July - December 2013 19 Figure 6. Mntly RMSE values f glbal slar radiatin.

7 Internatinal Jurnal f Renewable Energy, Vl. 8, N. 2, July - December Figure 6. Mntly RMSE values f glbal slar radiatin. Overall, it can be cncluded tat te deviatin values between te real and estimated glbal slar radiatin are satisfactry results. Te igest deviatins ave been btained fr te summer mnts, wever, during te summer; te termal inversin penmenn manifests te greatest influence ver te slar radiatin due t increased temperature wit altitude wic causes mre scattering f slar radiatin, and meanwile fr te cld mnts te lwest values fr te errrs were btained. Te verestimatin f te slar radiatin in te sunrise perid is due t te sadw caused by tpgrapy because te sun is at lwer altitude angles. Hwever, te underestimatin f glbal slar radiatin may caused by tpgrapic (slpe, aspect, sky-view factr, air mass) and atmsperic parameters (temperature and umidity) tat did nt tk int accunt by te mdel. 5. Cnclusin Te present wrk as te aim t elp engineers and designers in evaluating te Hebrn area ptential fr slar energy applicatins. An evaluatin f te predicted slar-irradiance fr Hebrn rizntal surface using measured data fr fur years as been cnducted in te present wrk. Te prpsed mdel was prduced and implemented fr Hebrn city. Te perfrmances f te prpsed mdel (caracterized by 12 equatins specific t every mnt) fr estimating te glbal slar radiatin fr Hebrn area ave been cnfirmed based n te applicatin f statistical indicatrs MBE and RMSE. Acknwledgements We wuld like t acknwledge te Palestinian meterlgical statins netwrk ffice fr teir kindly elp in prviding te data. References [1] Im-Seuti T, Can-Ting V, Taufa V, (2010). Feasibility study n wind energy in Sama. Internatinal Jurnal f Renewable Energy, vl5: [2] Alsamamra H, (2013). Statistical apprac fr mdeling f daily glbal slar radiatin n rizntal surfaces ver Hebrn city, Palestine. Jurnal f Tecnlgy Innvatins in Renewable Energy, vl2: 1-7. [3] Katiwra N, (2009). Energy efficiency plicy instruments and measures. Internatinal Jurnal f Renewable Energy, vl4:

8 Internatinal Jurnal f Renewable Energy, Vl. 8, N. 2, July - December [4] Emad Amed, El-Nuby Adam, (2013). Estimate f glbal slar radiatin by using artificial neural netwrk in Qena, Egypt. Jurnal f Clean Energy Tecnlgies, vl1: [5] Gadiwala M, Usman A, Aktar M, Jamil K, (2013). Empirical mdels fr te estimatin f glbal slar radiatin wit sunsine urs n rizntal surface in varius cities f Pakistan. Pakistan Jurnal f Meterlgy, vl9: [6] Musa B, Zangina U, Aminu M, (2012). Estimatin f glbal slar radiatin in Maidguri, Nigeria using Angstrm mdel. ARPN Jurnal f Engineering and Applied Sciences, vl7: [7] Kalil S, Faty A, (2012). An empirical metd fr estimating glbal slar radiatin ver Egypt. Acta Plytecnica, vl7: [8] Mammed T, Sadi N, (2011). Re-evaluatin and re-design stand-alne PV slar ligting prjects in Gaza strip, Palestine. Te Islamic University Jurnal, vl4: [9] Mamud M, Ibrik I, (2003). Field experience n slar electric pwer systems and teir ptential in Palestine. Renewable and Sustainable Energy Review, vl7: [10] Badescu V, (1997). Verificatin f sme very simple clear and cludy sky mdels t evaluate glbal slar irradiance. Slar Energy, vl61: [11] Al-Salii A, Kadum M, Mammed A, (2010). Estimatin f glbal slar radiatin n rizntal surface using rutine meterlgical measurements fr different cities in Iraq. Asian Jurnal f Scientific Researc, 1-9. [12] Memet K, (2012). Estimatin f glbal slar radiatin n rizntal surface in Erzican, Turkey. Internatinal Jurnal f Pysical Sciences, vl7: [13] Alsamamra H, Ruiz-Arias J, Pz-Vazquez D, Tvar-Pescadr J. A cmparative study f rdinary and residual kriging fr mapping slar radiatin ver sutern Spain, Agriculture and Frest Meterlgy 3 (2009)

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