MEASUREMENTS AND THEORETICAL CALCULATIONS OF DIFFUSED RADIATION AND ATMOSPHERE LUCIDITY

Transcription

1 ENGINEERING FOR RURAL DEVELOPMENT Jelgava, MEASUREMENTS AND THEORETICAL CALCULATIONS OF DIFFUSED RADIATION AND ATMOSPHERE LUCIDITY Ilze Pelēce, Uldis Iljins, Imans Ziemelis Lavia Universiy of Agriculure Absrac. Align wih oher environmen friendly renewable energy sources solar energy is widely used in he world. Also in Lavia solar collecors are used. However, in Lavia because of is geographical and climaic condiions here are some specific feaures in comparison wih radiional solar energy using counries. These feaures lead o he necessiy o pay more aenion o diffused irradiance. Anoher facor affecing he received irradiance of any surface is lucidiy of amosphere. This facor has no been no sudied in Lavia ye. This aricle deals wih evaluaion of diffused irradiance, and also of lucidiy of amosphere. The diffused irradiance can be measured direcly or as a difference beween he global irradiance and he beam one. The lucidiy of amosphere can be calculaed from he measuremens of boh global and beam irradiance, if he heigh of he sun is known. Therefore, measuremens of boh global and beam irradiance have been carried ou, and he diffused irradiance calculaed as a difference beween he global irradiance and he beam one. For measuring of he global irradiance he dome solarimeer has been used. For measuring of he direc irradiance racking o sun pirheliomeer has been used. The measuremens were performed in Riga from Ocober 8 ill March 9. The measuremens were execued auomaically afer every 5 minues. The obained resuls have been analyzed aking ino accoun also he daa on nebulosiy from he Sae agency Lavian Environmen, Geology and Meeorology Agency. Also effors o calculae heoreically he diffused irradiance from he heigh of he sun and he daa of he nebulosiy have been done. These calculaed values have been compared wih he measured ones. Good accordance is obained. Keywords: solar energy, diffused radiaion, amosphere lucidiy, nebulosiy Inroducion Solar waer heaing and elecriciy producion sysems are used in many counries all over he world, mosly in souhern ones, bu he use of solar energy is possible also in Lavia [1, ]. However, in Lavia because of is geographical and climaic condiions here are some specific feaures in comparison wih radiional solar energy using counries. One of hem is frequenly grea nebulosiy, which influences boh direc and diffused irradiance. Therefore, usual consrucions of solar energy receivers are no efficien enough in Lavia, and new consrucions are required, which would be able o use he diffused radiaion more efficienly. For beer elaboraion and evaluaion of he new consrucions, also new, more precise, complee and convenien mehods for calculaion and forecasing of he received energy are required. In his aricle an aemp o forecas he diffused solar radiaion has been done. The auhors have deal wih his quesion also in earlier works [3-5]. Now new daa have been measured and herefore new more precise formulae have been developed. Anoher imporan value affecing he received solar radiaion is lucidiy of amosphere. If he value of he lucidiy of amosphere has been esablished, he direc solar radiaion can be forecased. This value can be calculaed from he direc solar radiaion and he heigh of he sun. Such calculaions also have been done in his work. Maherials and mehods Boh he global and he beam solar irradiance were measured from 6 Ocober 8 ill 6 March 9. The measuremens were carried ou Riga. The global irradiance was measured using ISO 1 class piranomeer CMP 6 from Kipp&Zonen shown in Figure 1. Measuremens of boh global and beam radiaion were carried ou a auomaic mode afer every 5 minues. The beam normal solar irradiance (on he surface perpendicular o solar rays) was measured using ISO 1 class pirheliomeer from Kipp&Zonen equipped wih a racking device, shown in Figure. 19

2 ENGINEERING FOR RURAL DEVELOPMENT Jelgava, Fig. 1. Pyranomeer Fig.. Pirheliomeer wih racking device Daa on nebulosiy in he period of measuremens from he Lavia Agency of Environmen, Meeorology and Geology were obained. Nebulosiy was evaluaed in grades afer every hree hours, and hen he mean value of he day was calculaed. Calculaions were done using he mehods saed in [3-8]. The diffused solar irradiance is he difference beween he global solar irradiance and he beam solar irradiance on a horizonal surface. The beam solar irradiance on a horizonal surface can be calculaed from he beam normal irradiance and he heigh of he sun I B = I B sin α where IB beam solar irradiance on horizonal surface, W m-; IB beam normal solar irradiance, W m-; α heigh of he sun, deg..the heigh of he sun can be calculaed using he expression explained in [3-5, 8] sin α = (cos s cos λ + sin s sin λ cos ε ) cos ϕ + sin λ sin ε sin ϕ where s sidereal ime, deg; λ celesial longiude, deg; ε longiude of he place, deg; ϕ - laiude of he place, deg. (1) ()

3 ENGINEERING FOR RURAL DEVELOPMENT Jelgava, An aemp o find he dependence of he daily sum of he diffused irradiance on nebulosiy and he maximal heigh of he sun has been made using a graphical mehod [6], used also in previous works by he auhors [3-5]. This mehod conains a graphical plo of calculaed values via measured ones. The validiy of he model (formula) under consideraion can be evaluaed from he slope of linear graph (mus be near o one), inercep (mus be near o zero), and larges coefficien of deerminaion R. On he oher hand, he direc normal irradiance can be expressed wih he well known formula where I D direc normal solar irradiance, W m - ; S solar consan, W m - ; S = 1367 W m - P amosphere lucidiy, relaive unis; m amosphere mass, relaive unis. I m D = S P (3) The amosphere mass can be calculaed using he formula given in [7] where z rue zenih angle, deg. 1.43cos z cos z m = (4) 3 cos z cos z cos z If he direc normal solar irradiance is measured, he amosphere lucidiy can be calculaed from he formula (3). Resuls and discussion The daily course of solar irradiance is various and depends no only on nebulosiy, bu also on he kind of clouds. There is he daily course of global (a), beam normal (b) and diffused irradiaion on 18 November shown in Fig. 3. The mean nebulosiy on his day was 5 grades. I is ineresing ha approximaely a 1:3 when he beam irradiaion rapidly decreases obviously due o he clouds he diffused irradiaion iniially increases and decreases laer. I is no a paricular case, such course is usual for he diffused irradiance. However, regardless of he various course of he diffused irradiance is daily sum shows a raher srong dependence on he maximal heigh of he sun (or lengh of he day hey are correlaed values) and on nebulosiy. From analyzing his dependence he following formula for calculaion of he daily sum of he diffused radiaion can be obained D = ( Asin α + B sinα + C) ( E M + F M + G) (5) where D daily sum of he diffused irradiance, MJ m - day -1 ; α heigh of he sun, deg; M nebulosiy, grades; A, B, C, E, F, G coefficiens. From he measuremens he values of he coefficiens obained are hose shown in Table 1. Table 1 Values of coefficiens in formula (5) Coefficien A B C E F G Value The calculaed from his formula values of he daily sum of diffused irradiance have been ploed via he measured ones. This plo is shown in Fig. 4. Approximaing his dependence wih linear graph he slope 1.4±.1 and inercep.53±.36 have been obained, and he coefficien of deerminaion R =.87. 1

4 ENGINEERING FOR RURAL DEVELOPMENT Jelgava, Radiaion, W m - Radiaion, W m - Radiaion, W m : 8: 1: 1: 14: 16: 18: : 8: 1: 1: 14: 16: 18: 3 1 6: 8: 1: 1: 14: 16: 18: Fig. 3. Solar irradiance: a global, b beam and c diffused, on 1 January 9 Calculaed energy, MJ m - day -1 a b c Measured energy, MJ m - day -1 Fig. 4. Measured and calculaed daily sums of diffused energy, MJ m - day -1 The amosphere lucidiy has been calculaed from he measuremens of beam normal irradiance using he formula (3). For example, in Figure 5 he daily course of he amosphere lucidiy on 17 November is shown. I is raher difficul o separae he influence of he amosphere lucidiy from ha of he clouds. Larges values can be assumed as amosphere lucidiy, while sharp decreases are

5 ENGINEERING FOR RURAL DEVELOPMENT Jelgava, relaed wih clouds. If so, he amosphere lucidiy in mos cases is beween.65 and.77. However, a larger number of more precise measuremens is required. Amosphere : 8: 1: 1: 14: 16: 18: Fig. 5. Amosphere lucidiy on 17 November 9 Conclusions 1. The diffused solar irradiance can be prediced from he maximal heigh of he sun and nebulosiy.. Such model gives good accordance (R =.87) wih he measuremens. 3. The amosphere lucidiy is beween.65 and.77, bu addiional measuremens are required. References 1. Kancevica L., Navickas J., Ziemelis E., Ziemelis I. 6. Increase of he Efficiency of Solar Collecors. Second Inernaional Scienific Conference: Biomerics and Informaion Technologies in Agriculure: Research and Developmen.- Lihuania, Kaunas, 89-9 pp.. Ziemelis I., Iljins U., Navickas J. 4. Economical Comparison of Some Parameers of Fla-Plae Solar Collecors. Inernaional Research Conference: The Role of Chemisry and Physics in he Developmen of Agriculural Technologies.- Lihuania, Kaunas, 3 5 pp. 3. Pelēce Ilze. Influence of nebulosiy on use of solar energy in Lavia // Engineering for Rural Developmen. - Jelgava : LLU, 8. - lpp ISSN Pelēce Ilze, Iljins Uldis, Ziemelis Imans. Forecas of Energy Received by Solar Collecors [Konference] // Applied Informaion and Communicaion Technologies. - Jelgava : LLU, 8. - lpp ISBN Pelēce Ilze, Iljins Uldis, Ziemelis Imans. Theoreical calculaion of energy received by semispherical solar collecor // Engineering of Agriculural Technonolies / red. Lillak Rein. Kaunas, 8. - Vol pp ISSN x. 6. Younes S. un Muneer T. Improvemens in solar radiaion models based on cloud daa. - Building Servicies Engineering Research Technology : Vol. 7(1). - pp Young A. T. Air mass and refracion - Applied Opics : Vol pp Žagars J. un Vilks I. Asronomija augsskolām. - Rīga : LU, 5. 3