Solar & Wind Technology Vol. 4, No. 4, pp. 483-490, 1987 0741-983X/87 $3.00+.00 Printed in Great Britain. Pergamon Journals Ltd.

DIRECT AND SPECTRAL DIRECT SOLAR BEAM MEASUREMENTS FOR BAGHDAD, IRAQ

A. H. MUSA*, M. AL-RIAHI t and S. HIKMATJ" * Physics Department, College of Science; Baghdad University, Jadiriyah (Iraq) ; t Solar Energy Research

Centre, Scientific Research Council, Jadiriyah, P.O. Box 13026, Baghdad, Iraq

(Received 30 September, 1986 ; accepted l0 January 1987)

Abstract--The direct normal incidence solar radiation and its spectral are measured using Schott filters : OG1, RG2 and RG8 over a period of one year in Baghdad, Iraq. The fractions of the normal incidence solar radiation are 70 and 52% at 80 ° solar altitude and increase uniformly to 75 and 58% at 40 ° solar altitude for OGI and RG2 filters, respectively. But for RG8 filter, the fraction remains almost constant at 50% for solar altitudes between 50 and 80 °. Then these fractions increase rapidly to 83, 72 and 65% as the altitude decreases to 10 ° for OG1, RG2 and RG8, respectively. The fitted relationships between the measured fractions and the air mass and the solar altitude are second order equations. The range of monthly average of atmospheric extinction coefficients is 0.2754).544 for direct normal radiation, the ranges are 0.256-0.599, 0.2904).683 and 0.311~.622 for the energy measured by the OG1, RG2 and RG8 filters respectively.

1. INTRODUCTION

Measurement of direct normal incidence solar radia- tion (DNR) and its spectral components (SDNR) are useful, not only for climatic studies, but also for the practical purposes of selective surfaces behaviour and in photovoltaic and biomass technologies.

Much work has been done in analysing the relative importance of the various atmospheric constituents [1, 2] in extinguishing the direct radiation and to con- struct models of the spectral solar direct irradiance to simulate the overall behaviour of the atmosphere [3-8].

Several studies indicate that the urban atmosphere affects the DNR by decreasing its value by 20-30% [9, 10]. Few workers attempt to analyze how aerosols affect the visible spectrum [11, 12].

The characteristics of clear sky DNR measured with the RG2 filter have been reported in [13]. The annual variation in energy received at solar noon under clear sky conditions using Schott filters (OG1, RG2 and RG8) for Edmonton (Canada) and Dhahran (Saudi Arabia) have been reported in Refs [14] and [15], respectively.

This paper presents for the first time the annual variation in the DNR and SDNR received at different solar hour angles under clear sky conditions using the three filters (OG1, RG2 and RG8) for Futhailia in

Baghdad, Iraq. The record period is from 1 November 1984 to 31 October 1985.

2. MEASUREMENT SITE AND INSTRUMENTATIONS

The measurement site, Futhailia, is located 35 km north-east of Baghdad city centre, Iraq (Latitude 33 ° 14'N, Longitude 44 ° 14'E, Elevation 34 m above M.S.L.). The heavy brick factories implemented in the surroundings qualify the site to be "urban".

The instruments comprised four Epply normal inci- dence Pyrheliometers, with Schott filters mounted on two tracking units to monitor DNR and SDNR pass- ing through the appropriate filters. The International Radiation Commission recommends the use of the sharp cut-off Schott filters to divide the solar spectrum into spectral regions [16]. The filters' designation and their lower sharp cut-off values are superimposed in Fig. 1, upon a typical solar spectrum as observed at sea level.

The measurements were carried out each hour of the day, under clear sky conditions. The pyr- heliometers generated a millivolt signal through a shielded cable to a Lab where the signal was fed into an Epply Record/Integrator so that hourly integrated

483

484 A. H, MusA et al.

IOO

E 80

._~ 60

_,~ 4o

2O jl OG I 525 m #

RG2 6 3 0 mp. RG8 710 m/z

Sc hot'c Lower Cutoff

200 400 600800 1200 1600 2000 2400 28(30

WaveLength (m/~)

Fig. 1. The shorter wavelength cut-off values of the filters superimposed upon a typical solar spectrum as observed at

sea level.

digitized values of DNR and SDNR were available, the printed output was fed into a computer (Model 9845 HP) to format and display the experimental data and to calculate the mean hourly values of DNR and SDNR for each month.

3. DATA COLLECTION

To eliminate the effects of clouds, all the measure- ments were carried out on clear days. Thus, this imposed a restrictive factor on the number of measurements. On average, each month had 8-10 days of measurements taken from sunrise to sunset.

4. RESULTS AND ANALYSIS

Whenever the sky over Baghdad was clear, the measured full day values of the DNR and SDNR were used in the analysis. For each forenoon and afternoon, the DNR and SDNR at different hour angles from solar noon for each month are tabulated in Tables 1 4. From these tables it can be seen that the highest energy occurs in July when the mean monthly relative humidity (RH) is low (18%). While the lowest energy level occurs during March, August and November. In March and November, the solar altitude range is 11.61-54.19 ° and the RH is high (50-67%). In August the atmosphere of the measurement site is at its highest level of pollution due to the increased activities of the brick factories and rising dust storms.

Figure 2, shows the shape of the mean hourly DNR and SDNR curves in July 1985 as a typical example. The extreme values of DNR and SDNR from Fig. 2 (at solar noon and at hour angle 6) are listed in Table 5 for the three filters. The DNR and SDNR decrease as the hour angle from solar noon increases and accordingly the fraction of DNR decreases. This trend was observed for all the measured days of each month of the year, forming a characteristic pattern that occurs for all days.

Figure 3a, shows the monthly variations of the mean hourly values of DNR and SDNR at solar noon. The curves in this figure show a slight annual variation

Month

Table 1. Monthly average DNR (W h m 2) ~r a clear day

Hour angle from solar noon

O 1 2 3 4 5 6 ,,, ,,

Jan. 870 825 729 539 205

Feb. 803 793 736 583 317

Max'. 817 794 755 643 430

Apr. 764 764 711 658 507

May 759 749 711 635 495

June 848 830 796 710 613

July 890 861 816 755 659

Aug. 744 718 680 625 480

Sept. 791 774 736 659 493

Oct. 773 734 669 574 444

Nov. 706 666 569 442 250

Dec. 791 758 682 557 145

116

252

294

440

544

206

177

175

29

117

255

Direct and spectral direct solar beam measurements for Baghdad, Iraq 485

Table 2. Monthly average solar radiation measured by the OG1 filter (W h m-2) for a clear day

Hour angle from solar noon M~ath

0 1 2 3 4 5 6

Jan. 652 624 560 436 175

Peb. 604 598 557 467 264

Mar. 601 591 554 485 338

Apr. 563 567 527 482 410

May 518 518 494 459 372

June 580 575 557 523 461

July 626 608 582 544 486

2mg. 510 495 474 444 351

Sept. 551 540 519 473 368

Oct. 547 529 485 427 331

Nov. 529 511 447 347 191

Dee. 619 593 551 468 128

, , , ,

98

197

230

342

411

154

138

107

26

95

208

Table 3. Monthly average solar radiation measured by the RG2filter (W hm - : ) fo r a dear day

Hour angle from solar noom Month

0 1 2 3 4 5 6

Jan. 510 480 445 353 154

Feb. 448 436 427 369 233

Mar. 434 434 419 372 251

Apr. 422 423 384 361 298

May 400 393 383 354 295

June 437 432 420 399 357

July 474 460 439 413 371

Aug. 392 382 368 346 276

Sept. 423 416 401 366 291

Oct. 424 407 375 330 264

Nov. 423 407 359 292 172

Dee. 494 473 450 394 119

83

163

187

266

318

127

113

107

19

83

161

486 A.H. MUSA et al.

Table 4. Monthly average solar radiation measured by the RG8 filter (W h m 2) for a

Month

clearday

Hour angle from solar noon

0 1 2 3 4 5 6

Jan. 440 417 371 297 136

Feb. 401 392 375 325 186

Mar. 419 418 403 363 248

Apr. 389 387 351 328 266

~ay 378 373 364 336 277

~ne 416 412 401 379 342

July 443 430 412 386 345

Aug. 372 361 346 324 256

Sept. 396 390 375 342 271

Oct. 383 364 336 304 244

Nov. 378 345 306 251 152

Dec. 431 405 385 348 104

68

148

175

261

3O8

119

106

98

18

83

147

for both DNR and SDNR. Comparison between these curves reveals a remarkable fact that seems worth emphasizing. It is the close parallelism between the values of mean hourly DNR and that of SDNR. This parallelism means a constancy of these fluxes passing through the OG1, RG2 and RG8 filters, as a proportion of the DRN values for all wavelengths. In this figure, it is clear that the DNR and SDNR curves are characterized by three minima in May¢ August and November, and two maxima, the sharper appears

in July and the other between September and October. To study the annual variation of the DNR and of

SDNR received at the earth's surface, the monthly average of atmospheric extinction coefficients (B) for DNR and SDNR measured at solar noon were cal- culated and tabulated in Table 6. The relationship used in the calculation is given by Lambert-Bouger's law [17]:

l = Io e x p ( - B / s i n h )

where

1200 /

I lOOp <~ I O 0 0 p JuLy

9oo~-~.._~ p_ 8ooF ~,

~°°F..._.+ oG1 ~-~ 600 -- " " - . . -+ ~ + ~ e

4oo- 300 - ~ - ~ , , ~ °

20C

I00 I L I 1 1 ]

0 I 2 3 4 5 6 Hour ongb~s from soLor noon

Fig. 2. Mean hourly variation in DNR and SDNR.

I Direct normal incidence solar radiation (watt m ~)

I,, The irradiance at air mass zero (watt m ~) B Atmospheric extinction coefficients (dimen-

sionless) h Altitude angle of the sun (degree)

The equation is often applied to specific wavelength bands and to the complete solar spectrum. The extinc- tion coefficients were calculated using the air mass, and the extraterrestrial band energy term, the latter term was determined using the solar spectrum [18] and the lower centre of cut-off for the filters.

The plot of B values is presented in Fig. 3b. These curves show the difference in magnitude of the DNR and SDNR extinction coefficients, any annual vari-

Direct and spectral direct solar beam measurements for Baghdad, Iraq

Table 5. The extreme values of the mean hourly DNR, SDNR and the fraction of DNR at solar noon and at hour angle 6

Fllteri

A~c solar noon

DNR = 890 w.hr/m 2

8D1TR

w.hr/m 2

OG1 626

RG2 474

RC.-8 443

~ r a c t i o n o f

D~R

7O

53

50

At hour angle six

DNR = 255 w.hr/m 2

8IIRR

w.hr/m 2

2O8

161

147

~ s c t i o n o f

82

63

58

487

12OO~_ • DNR l loo (o) + OGI iO00p * RG2 [ 900L o RG8 I

- 6 0 0 ~ . . . . + ~ _ _ _ _ + I + \ / ~ =^,~ , ~ + / \+.....+---+..,,,~ - ~

8 rr

2O0 I00

I I I [ I I I I I I J F M A M d d A S O N D

Months

Fig. 3a. Annual variation of the D N R and SDNR at solar noon as measured at Futhailia.

I

0.9 - ( b ) • DNR + OGI

o.~ w RG2 o RG8

~ O.E

8_ ~ 0.4

o.~

't [ ().2

0.1 o I I I I I I J I I I d F M A M J d A S O N D

Months

Fig. 3b. Annual variation of the extinction coefficients for DNR and SDNR at solar noon.

ation of these extinction coefficients is due to changing atmospheric conditions throughout the year. The maxima of the B values coincide with the minima of DNR and SDNR values and vice versa. It can be deduced that there is a correlation between B and RH, which can be attributed to the fact that more humidity can enhance the growth and development of water droplets and then increases turbidity, this in turn causes the annual change of the DNR and SDNR received at the earth surface.

The average fraction of the DNR measured by the three filters was plotted against solar altitude ranges from 10 to 80 °, Fig. 4. The fraction increases uniformly from 70 and 52% at 80 ° to 75 and 58 at 40 ° solar altitudes for OGI and RG2 filters, respectively. For the RG8 filter, the fraction remains almost constant at 50% for solar altitudes between 50 and 80 ° . This figure also shows that the fraction increases rapidly to 83, 72 and 65% at 10 ° solar altitude for OGI, RG2 and RG8 respectively. The better fitting is observed at greater altitudes which is due to data obtained during the summer months where RH remains almost at the same level. The fitted second order equations of the fractions are given in Table 7.

Figure 5, shows the plots of the fractions of DNR measured by the three filters as a function of the air mass. The fitted second order equations of the fractions in this case are given in Table 8.

5. CONCLUSIONS

The fractions of the DNR measured by the OGI, RG2 and RG8 filters increase rapidly to 83, 72 and 65% respectively as the solar altitude decreases from about 40 to 10 °. The annual variation in DNR and

488 A . H . MUSA et al.

E

o t~

Fig. 4.

0 8 - -

0 . 7 - -

0 6 - -

0"5 t

O4 0

O G I o R G 2

+ R G 8

0 -~ .~¢ ~ ' ~ " ~

~ t t ~ * .

o ~ %0 + 0 0 0 0

++ + + + + o o o

+ , , ; ~ ÷ _ . 4 _ ~ _ ~ _ ~ . _ _ + + _ ~ _ ~ . o + ~ + - ~ + - . ~ . ~

t I t 1 I 1 I I I0 20 30 40 50 60 70 80 90

SoLor a l t i t u d e ( d e g r e e )

Fractions of DNR measured by the filters as a function of solar altitude.

I

0 . 9 - -

0 . e - -

0 7 - -

0 . 6 -

0 5 - -

0.4 o f f

~ O G I

o R G 2

+ R G 8

j o

oaooO o o o

~ - ~ ' ~ . ,

I I I [ I I 1 I I L I 15 2 Z.5 3 35 4 4.5 5 55

A i r m o s s

Fig. 5. Fractions of DNR measured by the filters as a function of air mass.

S D N R for the three filters have similar characteristic changes. The band energy and extinction coefficient show the usual form of annual variation. The differ- ence in band energy between measurements with the three filters is almost constant throughout the year.

Figure 6, presents a comparison between our results of D N R and S D N R at solar noon and that from other parts of the world, namely Edmonton [14], (Canada, Lat. 53 ° 34'N, Long. 113 ° 31'W), and Dhahran [15], (Saudi Arabia, Lat. 26 ° 23'N, Long. 50 ° 00'). The selection of these sites is due to the fact that our site

falls between them. On comparing these curves, it appears that the maximum values of D N R and S D N R for Futhailia in Baghdad occurs in July, while the maximum D N R and S D N R of Dhahran appears in August and September. The minimum values occur in August at Futhailia while those of Dhahran occur in July, this was attributed to high turbidity in this month. Dhahran data show the largest variation of D N R and S D N R from 1 month to another, while data for Edmonton show the lowest variation in these values. However, the monthly variation of D N R and

Direct and spectral direct solar beam measurements for Baghdad, Iraq

Table 6. Extinction coefficient for band energy

Month DNR 001 RG2 RG8

Jan. 0.275 0.256 0.298 0.315

Feb. 0.374 0.351 0.437 0.430

Mar. 0.418 0.406 0.523 0.462

Apr. 0.518 0,504 0.611 0.567

May 0.539 0.599 0.683 0.622

June 0.430 0.488 0.596 0.528

Ju ly 0.380 0.411 0.513 0.450

Aug. 0.544 0.597 O.683 O.619

Sept. 0.453 0.490 0.570 0.524

Oct. 0.421 0.439 0.504 0.487

Nov. 0.416 0.397 0.430 0.423

Dec. 0.328 0.276 0.290 0.311

489

Table 7. The fitted second order equations of the fractions of DNR as a function of solar altitude for the three filters

l~llter

OGI

RG2

R~

F i t t e d e q u a t l u n s o f t h e f r a c t i o n o f DNR

0.867 - 4.027x10 -3 h + 2.301xi0 -5 h 2

0.799 - 8.24x10 -3 h + 6.268xi0 -5 h 2

0.704 - 6.828xi0 -3 h + 5.608xi0 -5 h 2

h = solar altitude

Table 8. The fitted second order equations of the fractions of DNR as a function of air mass for the three filters

Filter

OG1

RG2

Re,8

Fitted equations of the fraction of DNR

0.636 + 0.077 M - 0.007 M 2

0.456 + 0.079 M - 0.005 M 2

0.451 + 0.048 M - 0.001 M 2

M = a l r ~ , 8 s

490 A. H. MUSA et al.

MonthLy average DNR and SDNR futhaiLia (

Dhahran (------ 900 Eclmonton ( ....

L DNR ~ . ~ ~ 0 " ~ ' ~

8 0 0 - " ' ~

<

E 7 0 0 / - N ~ • /

/OGL . . . . . . . . . . . . . . _ ,

600~ / ° / "-.% DNR / [ \ /RGZ = , _ . ~ _ ~ OGl~

• i ~ O 0 O~. . ' ~ TM ~ • ~ ~ ~ ~ • ~ .

- ~ q P " = ' ~ - " . . RG2 o.~,,'-- / ~o ~ . a t "

RG8 300

2oo I I I I y. Aug Sep t Oc t . Nov

Months

Fig. 6. Monthly average DNR and SDNR (

Futhailia -), Dhahran ( 3, Edmonton ( • • ).

S D N R for Futhail ia in Baghdad falls between those of the two sites. This could be at tr ibuted to Baghdad 's latitude which is between that o f the two sites. However, f rom the foregoing measurements of D N R and SDNR, we can determine the a tmospheric tur-

bidity for Futhailia in Baghdad, Iraq. This will be our

goal in the near future.

REFERENCES

1. R. M. Goody, Atmosphere Radiation, 1 Theoretical Basis. Clarendon Press, Oxford (1964).

2. K. ya. Kondratyev, Radiation in the Atmosphere, Int. Geoph. Series, Vol. 12. Academic Press, New York (1969).

3. K. W. B6er, The solar spectrum for a typical clear weather day. Solar Eneryy 19, 525 537 (1977).

4. B. Leckner, The spectral distribution of solar radiation at the earth surface : elements of a model. Solar Energy 20, 143 150 (1978).

5. R. Guzzi, R. Rizzi and S. Vindignl, Reconstruction of the solar spectrum at the ground using measured tur- bidity and pricipitable water as input parameters. 2nd Int. Solar Forum, Hamburg (1978).

6. H. A. McCartney and M. A. Unsworth, Spectral dis- tribution of solar radiation I : direct radiation. Q. J. R. Met. Soc. 104, 699-718 (1978).

7. R. Guzzi, G. Lo Vecchio, R. Rizzi and G. Scalabrn, Experimental validation of a spectral direct solar radi- ation model. Solar Energy 31,359 363 (1983).

8. R. E. Bird, A simple solar spectral model for direct normal and diffuse horizontal irradiance. Solar Energy 32, 461~471 (1984).

9. D. Randerson, Acomparisonofthespectraldistribution of solar radiation in a polluted and a clear air mass. J. Air Pollut. Contrd. Ass. 20, 54~548 (1970).

10. W. A. Sprigg and W. E. Reifsnyder, Solar radiation through the lowest 100 metres of an urban atmosphere. J. geophys. Res. 77, 6499 6507 (1972).

11. O. Steinvall and C. H. A, gren, Measurements of atmo- spheric extinction in spectral region 36(~800 nm by spec- troradiometry. J. appl. Meteor. 14, 603-608 (1975).

12. H. A. Bridgman, Direct visible spectra and aerosol optical depths at urban and rural locations during the summer of 1975 at Milwaukee. Solar Energy 21, 139 148 (1978).

13. G. W. Sadler, Characteristics of clear sky normal inci- dence solar radiation measured with RG2 filter. Solar Energy 20, 139 142 (1978).

14. G. W. Sadler, Turbidity of the atmosphere at solar noon for Edmonton, Alberta, Canada. Solar Energy 21,339 - 342 (1978).

15. M. A. Abdelrahman and B. G. Nimo, Measurements of atmospheric turbidity in Dhahran, Saudi Arabia. Proc. ISES Congress, Brighton, England, Vol. 3. Pergamon Press, Oxford. August 1981.

16. Annals o/ the International Geophysical Year, Vol. 5, Parts IV VI. Pergamon Press, Oxford (1958).

17. G. F. Lothian, Beer's law and its use in Analysis, Analyst 88, 678 685 (1963).

18. M.P. Thekaekara, Proposed standard values of the solar constant and the solar spectrum. J. environ. Sci. 13, 6 9 (1970).