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TECHNICAL NOTE

Renewable Energy, Vol. 6, No. 2, pp. 171 174, 1995 Copyright ~ 1995 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0960 I481/95 $9.50+0.00

Study of hourly solar radiation data in Istanbul

S. T O P , U , * S. D i L M A ( ~ a n d Z . A S L A N *

* ITU Faculty of Aeronautics and Astronautics, Depar tment of Meteorology, Maslak 80626, Istanbul, Turkey

t T U B I T A K , Marmara Research Center, P.B. 21, Gebze 41470, Kocaeli, Turkey

(Received 1 March 1994 ; accepted l July 1994)

Abst rac t - -This paper presents a study of the solar radiation data measured in Istanbul (41.1°N, 29.0°E) during 1992 and 1993. The monthly and annual average values of total solar radiation and clearness index are analysed. The month ly averages of daily total radiation are 1.23 k W h m -2 day-~ for January and 6.55 k W h m 2 day l for July. The annual average value of daily total radiation is 3.81 k W h m -2 day -l . The monthly averages of clearness index for January and July are 0.28 and 0.50, respectively. The annual average value of clearness index is 0.38. In the second part of the study, the seasonal relative frequency of hourly total radiation and clearness index is studied. 46% of the annual data corresponds to a value greater than 300 W m 2. The annual average frequency of clear hours is 24%. The analysis points to the conclusion that solar radiation will be efficient and useful between April and September for heating purposes. A polynomial relationship is developed between hourly clearness index and hourly fractional sunshine duration. Some statistical tests are used to check this relationship for four different ranges of optical air mass.

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

With the increasing importance of solar energy, studies of solar climatological structure in selected regions have become useful in the fields of meteorology, architecture, agriculture, hydrology and solar energy. Recent studies of the subject are related to the modelling and prediction of the solar radiation components. More accurate measurements give rise to more reliable model outputs. For regions where solar radiation measurements are sparse, solar energy potential is deter- mined with models based on mean radiation characteristics such as the clearness index, the diffuse fraction, and relative radiation duration.

One of the pioneering studies on the variation of the daily clearness index was carried out by Liu and Jordan [1]. Suehrcke and McCormick have shown the effects of clearness index and air mass on solar radiation distribution [2,3]. The clearness index was also analysed statistically and some relationships among diffuse radiation fraction, direct radi- ation fraction and relative sunshine duration for various locations were suggested [4~6].

Solar data recorded in 1992 and 1993 at the Meteorological Observatory Park o f the Istanbul Technical University are analysed in this study.

2. DATA AND ANALYSIS

The direct, total and ultraviolet radiation, albedo and sunshine durat ion data are continuously loaded on a data- logger at the Istanbul Technical University Meteorological Observatory Park (41. I"N, 29.0°E). Total radiation is mea- sured by using a Kipp Zonen CI 1 solarimeter. Direct radi- ation measurements are carried out with the Eppley Normal

Incidence Pyrheliometer. By employing the World Meteoro- logical Organization (WMO) standards, the sunshine dur- ation is calculated by using pyrheliometric measurements with a threshold of 120 W m 2 [71. The data is recorded every 2 s by using a CR 10 data-logger from Campbell Scientific Instruments.

Annual monthly averages of hourly total radiation observed between 1992 and 1993 are calculated and the sea- sonal relative frequency distribution of hourly total radiation values is obtained.

The clearness index (K0 is defined as the ratio of the total radiation on a horizontal surface to the extraterrestrial radiation for a given period. The clearness index has an extensive application in the field of solar energy potential studies.

Herein, the values of hourly clearness index for Istanbul are analysed. Since haze and smog, especially during sunrise and sunset, increase the clearness index (Kt) and thus lead to values unexpectedly larger than 1, for these conditions, values of clearness index are not considered in the com- putations. The solar constant is considered as 1367 W m -2 to compute the hourly extraterrestrial radiation.

Additionally, the following polynomial relationship is sug- gested between the hourly clearness index (Kt) and hourly fractional sunshine duration (Sh) :

Kt = a+b Sh +cS~. (1)

Hourly values of fractional sunshine duration are considered as the ratio of actual minutes of sunshine to 60 min. The clearness index is a function of sunshine durat ion and optical air mass. The polynomial relationship is analysed for various optical air mass intervals.

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172

3. RESULTS AND C O N C L U S I O N

Technical Note

This study is a preliminary investigation into the solar radiation climatology of Istanbul. A new measuring and recording system, which conformed to W M O recom- mendations, is used.

3.1. Total radiation Annual monthly averages of hourly total radiation and

monthly averages of daily total radiation values are pre- sented in Table 1. Naturally, the hourly radiation values vary with the path length of the solar beam in the atmosphere, The max imum total radiation value under clear atmospheric conditions is observed in July, with the values 978 W m -2 for 1992 and 967 W m-2 for 1993. During partly cloudy conditions, such as 1-2 okta, total radiation values higher than that of clear atmospheric conditions are observed. Since scattering by cloud increases the diffuse radiation compon- ent, the value of hourly total radiation increment is small [2].

At the time of max imum incoming solar radiation, average hourly total radiation values are 586 W m 2 (12 13 h local time), 7 9 6 W m 2(12 1 3 h ) , 4 4 6 W m 2(12_13h) a n d 2 5 0 W m : in spring, summer, au tumn and winter, respectively. The monthly average value of daily total radiation is 1.23 k W h m 2 day in January and 6.55 kW h m 2 day in July. The annual average value is 3.81 k W h m 2 day.

The seasonal relative frequency distribution of hourly total radiation is shown in Fig. 1. The number of data used in this study is 2344 for spring, 2631 for summer, 2046 for au tumn and 1539 for winter. The annual average value of hourly total radiation is found to be approx. 300 W m 2.46.6% of spring data, 60.0% of summer data, 47.7% of au tumn data and 20.6% of winter data are in excess of 300 W m : of average radiation.

3.2. Clearness index The monthly average clearness index values in Istanbul

are presented in Table 2. Values of hourly clearness index at noon are relatively high in June, July, August and September. According to the monthly mean value, the max imum clear- ness index is observed in July (0.50) and the min imum in

December (0.25). The annual average clearness index is found to be 0.38.

The seasonal relative frequency distribution of calculated hourly clearness index values is presented in Fig. 2. When the frequency distribution is analysed, the average clearness index values are 0.28 in winter, 0.49 in summer, and 0.37 in spring and autumn. The number of data used in this analysis is 1588 in summer, 941 in winter, 1291 in spring and 1009 in autumn. It is found that 21.1% of the clearness index data are in the range of 0.264).30 in winter, 25.2% of data are in the range of 0.60-0.70 and 17.7% of data are in the range of 0.50-0.60 in summer. The relative frequency distribution shows a bimodal distribution in spring and summer. 16.7% of spring data are in the range of 0.30-0.40 and 16.4% are in the range of 0.60 0.70. In the au tumn 17.1% of data are in the range of 0.20~).30 and 18.1% are in the range of 0.60- 0.70 of clearness index. In general, values of Kt less than 0.60 show clear atmospheric conditions [4]. The clear atmospheric conditions in Istanbul are more frequent in September and rarer in May. This causes greater values of clearness index in au tumn than in the spring months. Indeed, rainy atmospheric conditions are more often observed in Istanbul in April and May. It is concluded that the seasonal average frequency of clear hours (K, >/0.6) is 35% in summer, 22% in spring and 23% in autumn.

3.3. The relationship between hourly clearness index and hourly fractional sunshine value

Results of the polynomial regression analysis and errors for various optical air masses (m) are presented in Table 3. N is the number of data. The correlation coefficient (r) varied from 0.841 to 0.903. The standard error (SE) and the relative error (RE) in the estimation of clearness index are calculated. For the relative error the following equation is used :

r/K,m--~;,o )]/ RE= [ y ~ / ~ x 100 /N, (2)

where K,m is the ith measured value and K,~ is the value estimated from eqn (1).

7 0 ' ' ' - - ' . . . . ' ' ' 4 0 ; ] ' ' ' ' ' ' ' ' I

[ ] Spring [ ] Summer i [ I [ ] Spring [ ] Summer I

6 0 ~ I I I , Autumn I~J] Winter ] H " A u t u m n [ ] Winter J

5 0 3 0

4 0 g o 2 0

30

20 ~ l0

10

0 75 225 375 525 625 825 975 0 0.05 0.15 0.25 0.35 0.45 0.55 0.65 0,75

Total radiation (W/m 2) Clearness index

Fig. 1. Seasonal relative frequency distribution of hourly Fig. 2. Seasonal relative frequency distribution of hourly total radiation, clearness index.

Technical Note 173

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174 Technical Note

Table 3. Regression coefficients and errors of polynomial relationship between Kt and Sh

Air mass N a b c r Kt SE RE (%)

1 ~< m < 2 790 0.143 0.835 - 0 . 2 5 0 0.841 0.649 0.092 4.10 2 <~ m < 3 174 0.134 0.871 --0.309 0.899 0.554 0.081 5.96 3 ~< m < 4 72 0.110 0.893 - 0 . 3 2 9 0.903 0.513 0.105 5.07 4 ~< m < 5 94 0.109 0.810 -0 .533 0.891 0.355 0.095 6.33

The values of mean measured clearness index (glrn) for various optical air mass intervals are 0.649, 0.554, 0.513 and 0.355, as given in Table 3. The mean clearness index of all data is 0.518. The means of SE and RE are 0.093 and 5.37% respectively. These errors are well within reasonable limits [6]. It can be concluded that the empirical equation given for estimating the clearness index has good accuracy.

As a conclusion, the potential of average total radiation and the clearness index are not too high in the near vicinity of Istanbul. However, for assessment of the solar energy, the period between April and September is more suitable. For more conclusive results, the observation period should be in climatological time limits and more data belonging to differ- ent geographical regions should be analysed.

Acknowledgements--This study is a part of an ongoing pro- ject supported by the Turkish State Planning Organization (DPT). The authors thank the DPT for its support.

REFERENCES

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characteristic distribution of direct, diffuse and total solar radiation. Solar Energy 4, 1-19 (1960).

2. H. Suehrcke and P. G. McCormick, The frequency dis- tribution of instantaneous values. Solar Energy 40, 413- 422 (1988).

3. H. Suehrcke and P. G. McCormick, The distribution of average instantaneous terrestrial solar radiation over the day. Solar Energy 42, 303-309 (1989).

4. A. I. Kudish and A. Ianetz, Analysis of the solar radiation data for Beer Shave, Israel, and its environs. Solar Energy 48, 97-106 (1992).

5. A. Louche, G, Notton, P. Poggi and G. Simonnot, Cor- relations for direct, normal and global horizontal irradiation on a French Mediterranean site. Solar Energy 46, 261-266 (1991).

6. P. C. Jain, A model for diffuse and global irradiation on horizontal surfaces. Solar Energy 45, 301-308 (1990).

7. J .J . Michalsky, Compar ison of a national weather service foster sunshine recorder and the World Meteorological Organization s tandard for sunshine duration. Solar Energy 48, 133-141 (1992).