Contribution to the study of the solar radiation climate of the Baghdad environment

Solar Energy Vol. 44, No. 1, pp. 7-12, 1990 Printed in the U.S.A.

0038-092X/90 $3.00 + .00 Copyright ~ 1990 Pergamon Press plc

CONTRIBUTION TO THE STUDY OF THE SOLAR RADIATION CLIMATE OF THE BAGHDAD ENVIRONMENT M. AL-RLAHI, N. AL-HAMDANI, and K. TAHIR Solar Energy Research Center, Scientific Research Council, Jadiriyah, Baghdad, Iraq AbstractmMeasurements of global and diffuse solar radiation on a horizontal surface at Fudhaliyah Experimental Station, Baghdad-Iraq during the period August 1984 to August 1987 are analysed and presented in this paper. The annual mean of the daily global and diffuse solar radiation is about 18.28 MJ/m 2 and 5.86 MJ/m 2 respectively. On a yearly average basis, the percentage frequency of cloudy days (Kr < 0.35) is quite low, namely 6 percent, whereas that for clear days is quite high (Kr > 0.64), 26 percent. The average monthly clearness index varies from 0.663 in September to 0.529 in December, While the average monthly diffuse fraction of global radiation ranges between 0.364 in November and 0.256 in September. The highest fraction of the day's radiation was observed in the moderate radiation period (9-11 + 13-15) and confined between 51 percent in winter and 37 percent in summer.

1. I N T R O D U C T I O N

Knowledge of the incoming solar radiation is of fundamental importance for all of the solar energy research and development programs which are presently being undertaken in various countries. Actually daily radiation on a horizontal surface is measured at most recording stations. However, the mean daily radiation is not always the most appropriate figure to characterize the potential utility of solar energy utilization systems. While designing solar energy systems, one also needs to know radiation values at hourly intervals. Hourly values of solar radiation allow us to derive very precise information about the performance of solar energy systems. Among the climatological parameters, the global and diffuse solar radiation are of vital importance, being the forcing functions for climate and a supplement to the more conventional and non-renewable sources of energy. Hence, detailed studies of daily and hourly solar radiation under local climate conditions have been carried out at various places[ 1-4]. In this paper, the monthly average of daily totals of extraterrestrial radiation, global radiation and diffuse radiation in Fudhaliyah-Baghdad were analysed and the seasonal variations were discussed. The monthly average global and diffuse radiation for each hour of the day between 5 a.m. and 7 p.m. are presented and studied. A frequency histogram of the clearness index and diffuse fraction of global radiation are also presented and discussed. 2. M E A S U R E M E N T S

SITE AND INSTRUMENTATION

The need for a detailed and accurate measurement of meteorological parameters in the design and evaluation of solar energy systems stimulated the installation of an Automatic Weather Observing Station (FA 511 Wilh. Lambrecht GmbH) at Fudhaliyah, 35 km N.E. of Baghdad city center (Lat. = 33014 ' N, Long. = 44014 ' E, Elev. = 34 m above MSL.). The observation site affords optimum exposure to the sen-

sors without any appreciable obstacle for the incoming radiation. A complete weather station consists of the measuring sensors (6 radiation parameters and 8 collateral weather parameters), signal convertor, scanner, data acquisition system and matrix printer. More details about sensors characteristics, data acquisition, and processing are described elsewhere[5]. Measurements of global solar radiation (wavelength range from 0 . 3 0 - 3 . 0 ttm) over a horizontal surface in W h / m 2 are made with the star black and white pyranometer, manufactured by Schenk, and, described by Dirmhim[6]. A similar pyranometer screened by a shadowring was used to measure the diffuse component. Since the shade-ring blocks not only the solar disk but also a sizeable portion of the diffuse radiation, there is a need to apply a geometric multiplying factor to the recorded diffuse readings. Drummond[7] has suggested the use of an additive value of 3 - 7 % , which depend on insolation and incident angle, to correct for the sky obscured. However, in this investigation no correction was made to the measured diffuse radiation reading. Before commencement of the experiment both pyranometers were factory-calibrated, in accordance to the International Pyrheliometric Scale (IPS) of 1956. To insure accuracy of records, the sensors were calibrated periodically against a standard Eppley PSP kept inside the laboratory. No change in the calibration factors was found. In this paper, the data for a 30-month period between August 1984 and August 1987, for which we obtained continuous measurements, were included. Seven months of data are excluded due to a number of interruptions which occurred during the measurements as a result of malfunctioning of instruments and lack of spare equipment. The data for a 30-month period was valid for this study since the systems are of high quality data output and the station is maintained on a dally basis to provide reliable measurements.

M. AL-RIArHet al. 3.

ANALYSIS

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3.1 Monthly variation of global and diffuse radiation

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The monthly average of daily global radiation, diffuse radiation, and extraterrestrial radiation[8] on a horizontal surface are shown in Fig. 1. The annual mean of daily global and diffuse radiation at Fudhaliyah-Baghdad is about 18.28 MJ/m z and 5.86 MJ/ m z respectively, with a seasonal variation from 9.57 M J / m z for global radiation and 3.29 MJ/m 2 for diffuse radiation in mid-winter to 25.71 MJ/m 2 for global radiation and 8.5 M J / m 2 for diffuse radiation in midsummer. The summer period (April-September) contributes about 63 percent of the annual mean of daily global and diffuse radiation. December contributes least, being responsible for only about 4.5 percent of the annual global and diffuse radiation. A plot of the monthly averages of, daily clearness index Kr, Kr = H/Ho (H = daily global radiation, Ho = daily extraterrestrial radiation), and the diffuse fraction of global radiation/Ca, Kd = Ha/H(Hd = daily diffuse radiation), are shown in Fig. 2. It appears that the monthly variations are consistent since the maxima of/
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Fig. 2. The monthly variations of H,/H and H//~o for Fudhaliya6-Baghdad. the lowest values occur in December (/
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M. AL-RIAHIet al.

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timating the average hourly global radiation, appears adequate and quite accurate as far as the predictions of diffuse radiation is concerned. Their equation for rd is represented by expression (1) which holds very well under verification with actual experimental data:

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dn is the day number of the year (starting 1 January). Figure 5 demonstrates the verification of eqn (1) where the solid lines are obtained from the fight-hand side of this equation. The measured data in this diagram represent mean values of the hour pairs (1/2, 1-1/2, 2-1/2, etc.) around solar noon. In a further investigation by Collares-Pereira and Rabl[12] new data were combined with the earlier results to produce the plots of f,. The least-squares fits to the results are of the form (4) which holds very well under verification with measured data. This is clearly demonstrated by the plots of Fig. 6.

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f, = (~r/24)(a + b cos W~) cos W~ - cos W, sin 14,',- (~/180)W, cos W, Hourly global radiation Daily global radiation a = 0.4090 + 0.5016 sin(W, - 60 °) b = 0.6609 - 0.4767 sin(Ws - 60°)

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It should be borne in mind that eqns (1) and (4) are, in general, inadequate for high latitudes (higher than 50°).

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Because of the variability of the weather the distribution of solar radiation is mostly irregular, both in time and in space. Therefore, the clearness index Kr and the diffuse fraction of global radiation/Ca, are useful under average conditions as a measure of cloudiness and other atmospheric constituents which attenuate the solar radiation. A study of the percentage frequency distribution of Kr and Ka is also important since it gives us information about the percentage frequency to have clear or cloudy days. For this purpose we have calculated, on a daily basis, the ratio of Kr and Ka from measured data of global radiation, diffuse radiation, and the estimate value of extraterrestrial radiation at Fudhaliyah-Baghdad, then the percentage frequency of Kr and Ka have been determined using interval of 0.05. A frequency histogram over the year is shown in Figs. 7 and 8. It appears that the distribution of Kr skews toward high values and is sharply peaked in the range 0.6 ~< Kr ~< 0.64. Th~

The solar radiation climate of the Baghdad environment SO

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data in this range of Kr represents 29 percent of the total data, more of these percentages are observed in the summer than in winter. The percentage frequency of cloudy days (Kr < 0.35) is quite low, viz 6 percent, whereas that for clear clays is quite high (Kr > 0.64), viz 26 percent. The intermediate range of 0.35 <~Kr <~0.64 represent partly cloudy skies for Baghdad area and contributes 68 percent of the data points. Thus, it appears that the skies over Baghdad are fairly clear during most of the year. A reverse behaviour was observed in the distribution of Kd where the histogram skews toward low values of Kd and the peak falls to near Kd = 0.24, with 17 percent of the total days lying in this range. It is obvious to note that, the days which fall within that range were associated with the days which have Kr values around 0.64. This was expected since both Kr and Kd were dependently affected by the state of the sky (dust content, cloud coverage, amount of precipitable water vapour, and so on) which varies with time of the day and year. Hence, increasing Kr value accompanied the decrease in Kd value.

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3.5 Distribution of hourly radiation as a fraction of day's radiation

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At any time of the year and at a specified location (at latitudes lower than 50°), approximately 90 percent of day's radiation is received during the middle two-thirds of the day[ 11 ]. This means that for a given fixed-position solar energy devices, practically all the useful heat gain, comes from the middle two-thirds of the day. This observation indicates that, at both early morning and late afternoon the solar radiation data are of little practical consequences in such applications. The day's radiation can be classified into 3 periods[8]: (a) Peak radiation period, 11-13. (b) Moderate radiation period, 9-11 + 13-15. (c) Low radiation period, 7 - 9 + 15-17. On the basis of monthly average of hourly global and diffuse radiation for Fudhaliyah-Baghdad, we calculated the percentage of radiation in the abovementioned time interval as a fraction of day's radiation which presented graphically in Fig. 9.

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Fig. 9. Percentage distribution of hourly global and diffuse radiation as a fraction of day's radiation.

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M. AL-RtArll et al. 4. CONCLUSIONS

REFERENCES

In summarizing the results obtained with regard to the global and diffuse solar radiation measurements in Fudhaliyah-Baghdad, it may be concluded that:

1. B. D. Katsoulis and C. E. Papachristopoulos, Analysis of solar radiation measurements at Athens observatory and estimates of solar radiation in Greece, Solar Energy 21, 217-236 (1978). 2. C. T. Leung, The fluctuation of solar irradiance in Hong Kong, Solar Energy 25, 485-494 (1980). 3. R. H. B. Exetl, The solar radiation climate of Thailand, Solar Energy 18, 349-354 (1976). 4. M. Iqbal, A study of Canadian diffuse and total solar radiation data-II; monthly average hourly horizontal radiation, Solar Energy 22, 87-90 (1979). 5. M. AI-Riahi and A. Akrawi, Automation of meteorological observation at Fudhaliyah field station. First symposium on solar energy application in agriculture, Baghdad, Dec. 15-18 (1985). 6. I. Dirmhim, Utersuchungen an Stempyranometern. Archiv Met., Geoph. u. Biokl. Ser. B.9, 124-148 (1958). 7. A. J. Drummond, On the measurement of sky radiation, Arch. Met. Geoph. 8iokl. Serie B, 7, 413-436 (1956). 8. M. lqbal, Introduction to solar radiation, Academic Press, Toronto, Canada (1983). 9. B. Y. H. Liu and R. C. Jordan, The interrelationship and characteristic distribution of direct, diffuse and total solar radiation, Solar Energy 4, 1-19 (1960). 10. A. WhiUier, The determination of hourly values of total solar radiation from daily summation. Arch. Meteorol. Geophys. Bioklimatol. Ser. B7, 197-204 (1956). II. A. Whillier, Solar radiation graphs, Solar Energy 9, 164-165 (1965). 12. M. Collares-Pereira and A. Rabl, The average distribution of solar radiation-correlations between diffuse and hemispherical and between daily and hourly insolation values, Solar Energy 22, 155-164 (1979).

1. The annual mean of daily global and diffuse solar radiation is about 18,28 M J / m 2 and 5.86 M J / m 2 respectively. 2. The maximal monthly mean of hourly radiation occurs in June at noon, being 3.163 M J / m Z . h r for global radiation and 0.924 M J / m "~. hr for diffuse radiation. The minimum occurs in December, being 1.589 M J / m 2. hi" for global radiation and 0.483 M J / m 2. hr for diffuse radiation. 3. The h i g h e s t / ( r values occur in September (/~r = 0.663) and the lowest value in December (/~r = 0.529), while the highest/~a value occur in November (/~d = 0.364) and the lowest value in September (/~d = 0.256). 4. The peak of the frequency histogram of KT falls in the range 0.6 ~< Kr ~< 0.64, the data in this range of Kr contributes 29% of the total data. The peak of frequency histogram of K,~ lie around Ka = 0.24, with 17% of the total days observed in this range. 5. The highest seasonal change in the fraction of the 3 radiation periods was observed in the moderate radiation period, ranging between 51% in winter and 37% in summer.