Monthly and yearly average maps of total and direct solar radiation in Turkey

SolarEnergyVol. 37, No. 3, pp. 205-213, 1986

0038-092X/86 $3.00 + 00 © 1986Pergamon Journals Ltd.

Printed in the U.S.A.

MONTHLY A N D YEARLY AVERAGE MAPS OF TOTAL A N D DIRECT SOLAR RADIATION IN TURKEY E. TA~DEMIROt3LU and R. SEVER Physics Department, Middle East Technical University, Ankara, Turkey (Received for publication

19

March

1986)

Abstract--Total and direct solar radiation are calculated using the bright sunshine hours measurements from 83 stations over Turkey. The results are presented in the form of monthly and yearly mean maps. The values associated with the isoradiation lines, which pass throughout the frontiers of the country, are compared with the results obtained in some neighbourhood countries.

1. INTRODUCTION

In estimating the thermal performance and economic viability of solar energy systems, knowledge of the solar radiation data in a place in question is essential. These data may be available in several forms in terms of the type of the irradiance (total, direct and diffuse), surface placement (horizontal, slanted or deviated from true south) and the time basis (hourly, daily, monthly, seasonal or yearly). Each of these data has its particular usage in a specific application. One practical way of presenting spatial variations of solar radiation is by means of maps prepared on a temporal basis. A number of local studies were carried out to prepare solar radiation maps similar to those given in [1-18]. These maps reflect the levels of solar energy potential and give at a glance a clear idea of the degree of applicability of various solar energy conversion systems in these specific locations. Seasonal solar radiations maps for Turkey were previously published[19], in which sunshine hours values were used in estimating radiation from 50 stations. However, for general interest, the documentation of the monthly mean solar radiation has more utility rather than presenting the seasonal averages. In this present study the data basis is improved compared to the previous one[19], including the measurements of 33 new stations. The isoradiation lines of the monthly and yearly averages of both total and direct radiation values are thus drawn on the maps. The results are also discussed with the ones obtained in some neighbouring countries, Greece[15-17], Iran[12] and Iraq[18] and worldwide[4].

are found by splitting the total radiation into its components[22-24] with the aid of Liu and Jordan's method[25]. Spatial variations of both total and direct radiations throughout the country are thus compiled in the form of 26 maps. Figures l(a) and l(b) present the 12 monthly maps for total radiation, and Figs 2(a) and 2(b) for the direct radiations, respectively. In Figs. 3 and 4 annual averages of total and direct radiations as well as the location of the 83 stations of which sunshine recordings used are presented. In elaboration of the maps, contour lines are either drawn through stations with identical values or are interpolated proportionally between them. Values represented by the contour lines are rounded to the nearest integer, being the selected interval 1 M J . m - 2 . d - 1. Even though the contour mapping techniques are carefully applied, some incongruities may be encountered. It should be recognized that the locations of the plotting points are not purposely selected by solar mapping considerations, as they are the old established meteorological stations. Because of this uneven distribution of the plotting points over the country, solar radiation could not be well predicted in some small areas relatively far from the measurement stations. Moreover, there are some regions where high mountains, dense forests, deep valleys or other local topographical conditions cause the sharp variations in diffusivity of solar radiation between points a few kilometers apart. However, it is believed that the maximum error of a specific point on a map will be less than half the difference between two adjacent contour lines. 3. CONSISTENCY WITH SOME LOCAL STUDIES

Similar local studies of solar radiation were made in the neighboring countries. A global review of the results throughout the frontiers with some of these studies may show the accuracy of the methods applied. We will, therefore, compare these results instead of commenting on the distribution of

2. PREPARATION OF THE MAPS

Total solar radiation used in the zone mapping are estimated from the bright sunshine hours data[20], using a quadratic equation of modified Angstrom type[21]. Direct and diffuse components 205

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E. TA~DEMIRO(3LUand R. SEVER

solar radiation over the country. Such comments have also been made with respect to the geographical regions of the country in our previous paper[ 19]. In the papers of Flocas[17] and Katsoulis and Papachristopoulos[15], distribution of the yearly total solar radiation through the Aegean sea and the Thrace, where actually Turkey has a large frontier with Greece, are well depicted by the isoradiation lines. Dominant n o r t h - s o u t h gradient ranges approximately from a low of about 14 M J . m - 2 . d - t at latitude 41 ° to a high of 16-17 M J . m - 2 . d -1 at latitude 36 ° . On the other hand, the contour lines for the same latitudes are found to be about 15 MJ.m-Z.d - ~ and 18 M J . m - 2 . d - 1, respectively, in this present study, compare Fig. 3. Our figures have a fairly good agreement with these Greece studies which also used the sunshine recordings to estimate the solar radiation. Another interesting study for Greece was performed by Notaridou and Lalas[16], using the Bud y k o ' s equation[l]. They have drawn the seasonal average solar radiation maps. With respect to the latitude level, the averages of winter and spring throughout the frontier are relatively good in accordance with the results found in this present work. But differences up to 22% are seen between these values during summer and fall seasons. These incongruities can be attributed to the selections of the seasonal periods and the applications of different methods in estimating the radiation. In the work of Daneshyar[12], monthly mean total and direct radiations were computed for the four different climatic regions in Iran, using the sunshine hours values. Region two, which extends from about 35 ° to 38 ° latitudes, seems to have similar meteorological and topographical conditions to the southeastern part of our country, throughout the border with Iran. There the annual global solar radiation is estimated to be between about 15.5 and 17.7 M J . m - 2 . d -1, which are well in accordance with our results, compare Fig. 3. The results for direct radiation vary between 10.2 and 13 M J . - 2 . d - 1 at the same region in Iran. These figures also show very good agreement with the ones found in our study, compare Fig. 3. Ahmad et al. estimated the monthly mean total radiation from the sunshine recordings in Iraq[18]. They also drew monthly mean and yearly total radiation maps. In their work, yearly average global radiation throughout the frontier between these two countries is about 15 M J . m - 2 . d -~, which is nearly 17% less than our results, compare Fig. 3. When month by month comparison is performed between these studies, it is seen that during the period of six months from N o v e m b e r to April, the monthly mean results are well in accordance. But some inconsistencies begin to appear for the hotter months. Both results are then checked with reference to a worldwide study accomplished by L r f et al. [4]. The values represented by the contour lines passing

throughout the border of these countries are about 14.6, 29.3, 25 and 7.3 M J . m - 2 . d - j for the months of March, June, September and December, respectively. These values are slightly higher than those computed in our work, compare Fig. 3, even much more so when comparing with the results of Iraqi study where the figures for the same months are shown to be 12, 23, 18 and 7 MJ.m-Z.d t. Such discrepancies may probably be caused from the measurement period, as the nature of the methods applied in both works is the same. 4. REMARKS

In this study, global and direct solar radiation distributions throughout the country are presented in the form of maps as the contour lines make the general patterns of radiation visible. This helps to an easy representation of the solar information to the users. These maps can be used to estimate the amount of energy which can be recovered by a square meter of horizontal solar collector in a month or year at any location in the country. Useful energy obtained from a solar system can also be found a priori by the application of these maps if the systems' equipment efficiency is known. In designing a solar system in a particular area and in appraising the economics of the proposed application, more precise information of solar radiation is required. F o r such applications it is recommended that the necessary data of the point of interest be obtained from [22-24]. This study is performed with the latest available data in the country. Total solar radiation is estimated from the sunshine duration measurements within an acceptable error margin, as verified in [20]. In the estimation of the direct radiation, Liu and Jordan's approach was used, since it is universally accepted and gives acceptable results for midlatitude regions, as shown by several authors[25-33]. However, as the long measurements of direct radiation are not yet available, we can not tell about the percentage of accuracy of this method for the country. Some discrepancies were observed when we compared the results of three neighbouring countries. These would probably be caused by the measurement techniques and the diversity of the methods applied in estimating the solar radiation. Compared with the previous world solar radiation maps[l, 4-8], they show in much greater detail the intensity of solar radiations. However, they are by no means a definitive set. More precise maps can be certainly developed as the data now accumulating become available and the applied techniques are improved. REFERENCES 1. M. I. Budyko, Atlas o f the Heat Balance o f the Earth.

Leningrad (1963).

Total and direct solar radiation in Turkey 2. F. Bennet, Monthly maps of mean daily insolation for the United States. Solar Energy 8, 145 (1965). 3. G. Stanhill, Diffuse sky and cloud radiation in Israel. Solar Energy 10, 96 (1966). 4. G. O. G. L f f et al., World distribution of solar radiation. Solar Energy 10, 27 (1966). 5. G. T. Trewartha et al., Elements of Geography. McGraw Hill New York (1967). 6. G. T. Trewartha et al., An Introduction to Climate. Fourth edition, McGraw Hill, New York (1968). 7. W. H. Terjung, A global classification of solar radiation. Solar Energy 13, 67 (1970). 8. B. deLong, Net Radiation Received by a Horizontal Surface at the Earth. Delft University Press, Netherlands (1973). 9. A. Mani and O. Chacko, Solar radiation climate of India. Solar Energy 14, 155 (1976). 10. R. H. B. Excell, The solar radiation climate of Thailand. Solar Energy 18, 349 (1976). 11. G. W. Pattridge and D. Proctor, Monthly mean solar radiation statistics for Australia. Solar Energy 18, 235 (1976). 12. M. Daneshyar, Solar radiation statistics in Iran. Solar Energy 21, 345 (1978). 13. S. Barbaro et al., Global solar radiation in Italy. Solar Energy 20, 431 (1978). 14. R. Almanza and S. Lopez, total solar radiation in Mexico using sunshine hours and meteorological data. Solar Energy 21,441 (1978). 15. Bas D. Katsoulis and Const. E. Papachristopoulos, Analysis of solar radiation measurements at Athen's observatory and estimates of solar radiation in Greece. Solar Energy 21,217 (1978). 16. V. A. Notaridou and D. P. Lalas, Distribution of global radiation over Greece. Solar Energy 22, 505 (1978). 17. A. A. Flocas, Estimation and predition of global solar radiation over Greece. Solar Energy 24, 63 (1980). 18. I. Ahmad et al., Global solar radiation maps for lraq. Solar Energy 31, 29 (1983). 19. E. Ta~demiro~lu, Regional and seasonal distributions

213

of total and direct irradiations over Turkey. Solar and Wind Technology 2, 85 (1985). 20. H. tggelman et al., A new method for estimating solar radiation from bright sunshine. Solar Energy 35, 619 (1984). 21. A. Angstrom, Georg. Am. 11, 156 (1929). 22. E. Ta~demiroglu and A. Ecevit, Solar Radiation Handbook of Turkey. TBAG-653, Ankara, Turkey (1984). 23. E. Ta~demiroglu et al., Solar radiation potential in Turkey. Alternative Energy Sources VI, 1, 41, Editorial:Veziro~lu, Hemisphere Publ. Co. (1985). 24. E. Ta~demiro~lu and A. Ecevit, Comparisons of the hourly and daily global irradiances of Turkey on nonhorizontal surfaces. Energy Conversation and Management 25, 119 (1985). 25. B. Y. H. Liu and R. C. Jordan, The interrelationship and characteristic distribution of direct, total and diffuse solar radiation. Solar Energy 4, 1 (1960). 26. J. E. Hay, A revised method for determining the direct and diffuse components of the total short-wave radiation. Atmosphere 14, 4 (1976). 27. B. Goldberg et al., A comparison of some simple methods used to predict solar irradiances on a horizontal surface. Solar Energy 23, 81 (1979). 28. J. E. Hay, Calculation ofmonthly mean solar radiation for horizontal and inclined surfaces. Solar Energy 23, 301 (1979). 28. T. H. Klucher, Evaluation of models to predict insolation on tilted surfaces. Solar Energy 23, 111 (1979). 30. V. Modi and S. P. Sukhatme, Estimation of daily total and diffuse insolation in India from weather data. Solar Energy 22, 407 (1980). 31. M. Villarrubia et al., Solar radiation incident on tilted fiat surfaces in Barcelona, Spain. Solar Energy 25, 259 (1980). 32. F. Newwirth, The estimation of global and sky radiation in Austria. Solar Energy 24, 421 (1980). 33. U. V. Desnica et al., Climatic predispositions in Yugoslavia for solar energy application. Solar Energy 30, 401 (1983).