Pergamon

Renewable Energy, Vol. 14. Nos. I-4, pp. 479-484, 1998 0 1998 Published by Elsevier Science Ltd. All rights reserved

Printed in Great Britain PII: SO960-1481 (98) 00107-4 0960-1481/98 $19.00+0.00

SOME COMMENTS ON TIME VARIATION IN SOLAR

RADIATION OVER BAGHDAD, IRAQ

Rf. AL-Riahi and A. AGKayssi

ABSTRICT

In the present \\ork an attempt xvas made to study and examine some aspects ofradiation climatology which

are important in solar energy utilization. The yearly cumulative global radiation for Bashdad is 216 MJ/mz

per bear. while the annual total of daily dike radiation at Baghdad is about 70.52 \lJ/mz. The mean

monthly \.alues oiclearness index lir present a maximum of 65.9% in August and a minimum value of

-IS.??0 in Januav The annual mean ofdaily global radiation and its spectral OCI. RGZ and RG8 at Baghdad

is about ,18.03.13 53.1 O.S6 and 9.36 A4J/m2,respectively. Over the year, the highest LX radiarjon \\ere

recei\,ed during June and July (243 WWm2) and the lowest in December (79 \Vh;mz). Furthermore, UV

radiation constituted on a\.erage 3.25% of global radiation.

0 1998 Published by Elsevier Science Ltd. All rights reserved.

KEYWORDS

Global and difise radiation, Solar radiation transmission. UV radiation, Spectral radiation

INTRODUCTION

The need for meteorological parameters is essential in the design and study of solar energy conversion

devices. Other uses of such information include agricultural studies, meteorological forecasting, environment

and energy con\.ersion (Lougeay and Braze], 1984; Lund, 1985; Oliver e/ nl. 1987; Fohr and Figueiredb,

1957). An important base for all solar applications is a weather station. It is necessary to establish an accurate

data base for iolar radiatioi measurements and the collection of relevant meteorological data. An accurate

data base will enable a proper assessement to be made for solar energy’s contribution to the naticn‘s t&al

enel-gy supply (Atwater, 1978).

The Solar-Meteorological Station at Solar Energy Research Center in Jadiriyah, Baghdad (Lat. 33* 14‘ N,

Long 440 14‘ E, Elek,. 34m above MSL) has Automatic Weather Observation Station measuring the global

solar radiation on a horizontal and inclined surfaces, diffised radiation, global and direct normal spectral

radiation on an hourly basis throughout the year. Readings for other radiation and meteorological parameters are also monitored. The \Veather Station was designed for automatic installation, but minor maintenance,

adjustment, cleaning of the instruments. etc., were performed daily. Description of the complete weather

479

480 M. AL-RIAHI and A. AL-KAYSSI

station (sensors characteristics, data acquisition and processing) were mentioned in a previous report (AI-Riahi et al., 1992).

The objective of the present investigation is to study and examines from the available records, some aspects of radiation climatology which are important in solar energy utilizations. The mean annual, monthly and daily global and diffuse radiation are studied and presented in and graphical forms. The number of days in which the radiation has exceeded certain given limits and the period of successive days having radiation less than specified values have been examined and presented graphically in forms suitable for direct applications. Another important parameters which reflect the general atmospheric transparancy of Baghdad environment are also studied and discussed.. The annual variation in the global radiation and their spectral components are also presented and discussed.

To acheive this purpose, the data for a 12-month period between September 1995 and August 1996, for which we obtained continuous measurements, were used.

ANALYSIS OF THE MEASUREMENTS RESULTS

Hourly Distribution of Global and Diffuse Radiation

Figures l&2 sho~s the mean diurnal cycle of global and diffuse radiation for each month. As in Fig.l, the highest values of global radiation occur during the months of June, July and August,and the lowest values in the months of December, January and Febreuary. During the daily cycle, the highest values &global radiation occure in early part of the afternoon, approximately,12:00 and 13:00 hours. These values, which vary from more than 400 W.hr/m z during the Winter months to more than 800 W.hr/m 2 during the Summer months, decrease ~oward the morning and evening hours. The seasonal and diurnal distribution of diffuse radiation shows basically the same pattern as the distribution of global radiation. The highest values occure during the early afternoon of the day, approximately, between 12:00 and 13:00 hours. These values, which vary between more than 160 W.hdm 2 during the Winter months to approximately more than 220 W.hr/m z during the Summer. also decrease toward the evening and morning period of the day.

: :_ ,-=- •

Fig.1 : Isopteths of monthly average of houdy global radiation

iL. o! Fig 2: Isopleths of monthly avelage of I"oarly

diiffuse radiation

Time variation in solar radiation over Baghdad 481

Monthly Variation o f f;Iobal and Diffuse Radiation

The monthly average of daily global radiation H, maximum global radiation (assuming the atmosphere is perfectly clear He), diffuse radiation Ha, and extraterrestrial radiation on a horizontal surface H o, are shown in

Fig.3. Notable points of this figure are:

1.The yearly cumulative global radiation for Baghdad is 216.3 ik~/m 2 per),ear. The Summer period (June- August) contributes about 35.9% of the annual total. The worst month December contributes least, being responsible for only about 4.14%. Over the year, the monthly average o f daily incoming global solar radiation has its nlaximum value in July (26.993 MJ/m2), while the minimum values occur during December with about 8.95 M J/m:.

2.Comparing the monthly average of daily global radiation Hand the monthly average values of clear days global radiation H c, the two cun'es have same trend with a marked increase in the values of clear days global

radiation especially during the months January-May. This could be mainly due to the characteristic weather (cloud coverage, pollutants content, turbidity, etc.) which varies v,'ith time of day and year, and produces different attenuation of the solar radiation.

3.The annual total of daily diffuse radiation at Baghdad is about 70.82 MJ/m 2, with a seasonal variation from 3.81 M J/m: in mid Winter (December) to 8.54 .MJ/m 2 in May. The summer period contributes about 31,13%. December contributes least, being responsible for only about 5.78%.

Some Consideration Over the Attenuation of Solar Radiation

To give an opinion about the transmission phenomena at Baghdad, we calculated the average daily values of the e~traterrestrial radiation H o for each month using equations given by lqbal (1983) [7], and the potential

~adiation as that v.hich corresponds to completely cloudless days H c. With these values and those of the

monthly average of daily global radiation on a horizontal surface H, we calculated the attenuation for an atmosphere of normal composition, KT=H, Hr, and for a clear atmosphere,KTc=H c Hr,, these ratios could be considered as a good estimate of the irradiance absorbed by the aerosols, cloud etc., plus the irradiance scattered upv, ard bv clouds and gaseous and solid aerosols. The lower part of Fig3 shows the mean monthly values of global radiation attenuation for Baghdad. It is clear that the Krc and K T curves have a similar trend.

The smaller differences between K~c and K r values occur during Summer and never exceed 9.5%. Values of

](7C a r e relatively high in \Vinter,this is due to atmosphere is more clear in Winter (in absence of clouds ) than

in Summer. The mean monthly values o fK r present a maximum of 65.9% in August and a minimum values v f

484% in January. The KTc curves show a maximum of 70.9% in September and a minimum value of 61.8% in December.

Seasonal Variqtion o f The Clearness hldex

The study of tile frequency distribution of daily clearness index were determined by using llistograms of fiequency. The K 7 values are classified into eight intervals from 0.0 to 0.79 with a step 0.1 (e.g. 0.40

<Kz<0.49). The results are shown in Fig .4 for each season of the year. It can be seen that, the daily clearness

index distribution shows a similar behaviour and assimlabte to normal distribution with almost negative asymmetry. The interval of frequencies is relatively high in Winter and Spring. In Summer, the negative asymmetry and the absolute interval decrease and the distribution skewe towards high value of daily clearness index. It can also be seen that the peaks of the histograms remain around a narrow interval (0.60</~r<0.69). The data in this range ofK r represents 29%, 42%,67% and 50% for Winter, Spring, Summer, and Autumn, respectively. For all seasons, the percentage frequency of cloudy days (Kr<029) is quite low In the

482 M. AL-RIAHI and A. AL-KAYSSI

intermediate range of (0 39<Kr<0.59) which represents partly cloudy skies, the percentage frequency of

occurence is equally distributed in the Spring and Autumn months being 38%, while this percentage are 48 and 25 lbr Winter and Summer months. Generally speaking, it appears that the skies over Baghdad are fairly clear during a large time of the year.

Global Radiation on Vertical and hzclined Surfaces

Measurement and analysis of the solar radiation of vertical surfaces have received much attention in connection with the design of building air conditioning system. Fig.5, for example, shows radiation data for horizontal surface, vertical and 30 ~ tilt surfaces facing south. This figure shows that the radiation of a south vertical surface in Winter is much greater than that of a horizontal surface, while the reverse is correct for the rest months of the year. Such exposure is, therefore, particularly advantageous in winter. The 30 o tilt surface appears to offer the best collection possibility around the year compared with the horizontal surface, since the annual total of global radiation are 216 .MJ/m 2 and 237 MJ/m 2 for horizontal and 30 ° tilt surfaces,

respectively.

- - .

MONTHS Fig3: hlontnly awrage of s.ocar

radiation of clearness index.

F~g4: Percentage frequency dlstnbution of dairy clearness index.

L ~ i i ~ i d i J j i t - - ~ -

r , p ~ i i i i i i i r _ t _

~ o , n . s

FigS: Monthly average of global radiation on different surfaces f a c i n g south,

Solar Spectral Distributions from Discrete Filter Measurements

In order to know how the global radiation and its spectral components (OGI, RG2, and RG8) is distributed over the year, the monthly averages of these values are plotted in the upper part of Fig.6. From this figure, its clear that the curves are similar in pattern and indicates different attenuation of the solar radiation. The annual mean of daily global radiation and its spectral OG1, RG2 and RG8 at Baghdad is about 18.03, 13.53, 10.86 and 9.36 MJ/m 2. respectively. The Summer period (June-August) contributes about 36% of the annual for global radiation and its spectral components. The worst month (December) contributes least, being responsible for only about 4%.

The lower pat~ of Fig.6 shows the plots of the fraction of global radiation measured by the three filters OG1, RG2 and RG8. The fractions in the band energy with the three filters show a slight annualvariatibn. Comparision between these curves reveals a remarkable fact that seems worth emphasizing. It is the close parallelism between the values of the three filters, OGI, RG2 and RG8. This parallelism means aconstancy of the flux passing through the OGI, RG2 and RG8 filters, as a proportion to the global radiation values for all wavelengths. Over the year, the average fraction of the global radiation measured by the three filters was 75%, 60% and 52% for OG1, RG2 and RG8 filters, respectively.

Time variation in solar radiation over Baghdad 483

lntensit.r Variations o f Global Solar UV Radiation in the Range (0.29-0.385 /z m)

The received amount of UV radiation at any location depends on solar zenith angle, atmospheric ozone content overhead, and transparency of the atmosphere. Fig.7 depicts the annual variations of the intensity of global UV radiation and horizontal global radiation. Over the year, the highest UV radiation were received during June and July 243 Wh/m 2 and the lowest in December 79 Wh/m:. Furthermore, UV ladiation constituted on average 3.25% of global radiation with a maximum amount of 3.3% recorded during April and the lowest amount of 3.13% in September. In outer space, UV radiation between (0.290-0.385 /an) represents 6.12% of solar radiation. Most of the 2.87% loss in received incoming global solar UV radiation is probably due to scattering and absorption through the atmosphere.

CONCLUSIONS

Most of fl~ese conclusions have already been mentioned in previous sections of this work; there are however a few more general conclusions, which, we believe should be stressed also here.

1. The annual mean of daily global and diffuse solar radiation is about 18.03MJmLday q and 5.90 MJnr- 'dayL respectively.

2 The maximal monthly mean of hourly radiation occurs in June at noon, being 940 W.hr/m 2 for global radiation and 236 W.hr/m 2 for diffuse radiation. The minimum occurs in December for global radiation being 437 W.hr/m-' and in November for diffuse radiation being 155 W,hr/m:.

3. The monthly average ofK r values does not exceeds 0.66 in Summer and not lower than 0.49 in Winter.

4. For all seasons, the percentage frequency of cloudy days (Kr < 0,29) is quite low, which indicate that the

skies over Baghdad are fairly clear during a large time of the 5'ear.

5. The 30- tilt surface appears to offer the best collection possibility around tile 3'ear compared with the horizontal surface

6. Over fl~e year, the average fraction of the global radialion measured by the three filters was 75%, 60% and 52% for OG1, RG2 and RGS, respectively.

7. UV radiation constituted on average 3.25% of global radiation.

REFERENCES

AI-Riahi, M., A~ AI-Douri, A..Katchoo and A. AI-Kayssi (1992), Modem automated Solar meteorological. station, h~ternal l?eport.

Atwater M. A and J. T. Ball (1978). A numerical solar radiation model based on standard meteorological observation, 5blar Energy, ~ 163-170.

Fohr J. P. and A. R. Figueiredo (1987). Agricultural solar air collectors: Design and performance, Sohtr Energo." 5~311-321.

Iqbal, M. (1983). An Introduction to solar radiation, Academic Press.

484 M. AL-RIAHI and A. AL-KAYSSI

Lougeay R.L and A.J. Brazel (1984). A presentation of solar irradialion dala suitable for solar energy applicalion. S~htr Ener~k~,, 5_., 409-415+

Lund P. D.(1985). On the effects of solar radiation variations on solar heating systems performance. Eiwrgj, Res, ~ 53-64.

Oliver, S.A., HR.Oliver ,J.S Wallace and A.M. Roberts (1987). Soil heat flux and temperature variation wifla vegetation, Soil type and climate. AgTq¢ & Fore. Meteorol., ~ 257-269.

i,0 =

s

¢+

o=01

c= 7+

I t I l - - + - ~ - - t I I l - - - ~ - - ~ -

c~+i

i I i ? L I I I 1 ! r r

FIg6: Monthly average of global radiation and its spectral components with fraction in band measured by filters.

+ 3'

,~g 7: Monthly average of global radiation and gioba UV radiator1.