Solar Energy Vol. 37, No. 3, pp. 205-213, 1986 0038-092X/86 $3.00 + 00 Printed in the U.S.A. © 1986 Pergamon Journals Ltd.

MONTHLY AND YEARLY AVERAGE MAPS OF TOTAL AND 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 eco- nomic viabili ty 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 spe- cific application.

One practical way of presenting spatial varia- tions of solar radiation is by means of maps pre- pared on a temporal basis. A number of local stud- ies 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 docu- mentation of the monthly mean solar radiation has more utility rather than presenting the seasonal av- erages.

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 world- wide[4].

2. PREPARATION OF THE MAPS

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

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 radia- tions 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 MJ .m-2 .d - 1.

Even though the contour mapping techniques are carefully applied, some incongruities may be encountered. It should be recognized that the lo- cations of the plotting points are not purposely se- lected by solar mapping considerations, as they are the old established meteorological stations. Be- cause 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 for- ests, deep valleys or other local topographical con- ditions 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 meth- ods applied. We will, therefore, compare these re- sults instead of commenting on the distribution of

205

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212

solar radiation over the country. Such comments have also been made with respect to the geograph- ical 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 nor th-south gradient ranges ap- proximately from a low of about 14 MJ.m-2.d - t at latitude 41 ° to a high of 16-17 MJ.m-2 .d -1 at lat- itude 36 ° . On the other hand, the contour lines for the same latitudes are found to be about 15 MJ.m-Z.d - ~ and 18 MJ.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 per- formed by Notaridou and Lalas[16], using the Bu- dyko ' 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 ac- cordance 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 differ- ent 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 sun- shine hours values. Region two, which extends from about 35 ° to 38 ° latitudes, seems to have sim- ilar meteorological and topographical conditions to the southeastern part of our country, throughout the border with Iran. There the annual global solar radiation is est imated to be between about 15.5 and 17.7 MJ .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 ra- diation maps. In their work, yearly average global radiation throughout the frontier between these two countries is about 15 MJ.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 November to April, the monthly mean results are well in accordance. But some incon- sistencies 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

E. TA~DEMIRO(3LU and R. SEVER

throughout the border of these countries are about 14.6, 29.3, 25 and 7.3 MJ.m-2 .d - j for the months of March, June, September and December, respec- tively. 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' equip- ment 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 ra- diation is required. For such applications it is rec- ommended 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 esti- mated 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 uni- versally accepted and gives acceptable results for midlatitude regions, as shown by several au- thors[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 coun- tries. These would probably be caused by the mea- surement techniques and the diversity of the meth- ods 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 accumulat- ing become available and the applied techniques are improved.

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Total and direct solar radiation in Turkey 213

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