Diffuse fraction of daily global radiation at Dhahran, Saudi Arabia

Relationship between the diffuse fraction of daily global solar radiation and clearness index (the ratio of global to extraterrestrial radiation) is obtained from the radiation data measured at Dhahran (26°32′N, 50°13′E), Saudi Arabia. Two years of daily radiation data calculated from measured one-minute values are used to develop the relationship. Another year of data is used to validate the relationship. Comparison between the present correlation and previous correlations is presented. The present data confirm the seasonal dependence and the location independence of the correlation reported in literature. The diffuse and global solar radiation are presented for dusty and clear days. The results show that the dust significantly increases the diffuse fraction of the global radiation. The diffuse fraction on a typical clear day in March is 0.11, while on a dusty day, in the same month, may be as high as 0.91.     

Analysis of solar radiation measurements at Al-Arish area, North Sinai, Egypt

This paper presents basic data for a five year period from 1986 to 1990 for global and diffuse solar radiation data at Al-Arish (31°04′N, 33°49′E). These data have been processed, analysed, presented, arranged in tables and graphs and discussed. Mean annual monthly and daily total, the diurnal variation and the frequency of daily totals of global solar radiation are computed and discussed. A correlation between the hourly values of the clearness and diffuse index were obtained and the recommended correlation equations were also given. The isopleths of hourly global radiation were also designed and discussed. The frequency distribution and the frequency of extended periods of low radiation income have been studied which are of particular interest in the field performance of solar energy systems.     

Stochastic modelling of daily global solar radiation measured in Marrakesh, Morocco

Two stochastic models are presented of the daily global solar radiation obtained from three years of data measured on a horizontal surface in Marrakesh, Morocco (latitude 31°37′N, longitude 08°02′W, elevation 463 m). The development of these models is based on the removal of the annual periodicity and seasonal variation of solar radiation using two types of normalisation. The first model is developed using a classical decomposition of the daily radiation as the sum of two components: a trend component and a stochastic component. This model is most useful for long simulated sequences. The second model is developed using a non-dimensional variable, the clearness index, which is modelled as a stochastic process after a preliminary transformation leading to a stationary time series. Both models have satisfactorily passed validation tests for forecasting and simulation of daily global solar radiation data.     

Measurements and predictions of solar radiation incident on horizontal surfaces at Santa Fe, Argentina (31° 39′S, 60° 43′W)

Measurements and predictions of solar radiation during a period of 10 years on horizontal surfaces at Santa Fe (31° 39′ S, 60° 43′ W), Argentina, reported as average daily global radiation for each month, are presented. Data are compared to those obtained with a previously published and verified model for computing solar radiation on horizontal planes at the earth's surface for cloudless sky days. Measurements show an important reduction of global radiation with respect to the cloudless sky model predictions for all months of the year. Conversely, averaged daily diffuse solar radiation calculated with Page's formula shows a small increment with respect to the predicted diffuse solar radiation for cloudless sky conditions. When direct solar radiation data, calculated from global and diffuse solar radiation values, are compared to theoretical prediction, a significant decrease is observed. This trend is similar to that obtained for global solar radiation.     

Total suspended particles and solar radiation over Cairo and Aswan

Measurements were carried out at Cairo (30.05°N, 31.17°E) and Aswan (23.58°N, 32.47°E) in Egypt for three years (1990–1992) by the Egyptian Meteorological Authority. The measurements were done using an Eppley ultraviolet radiometer to measure the global ultraviolet solar radiation (UV), Eppley pyranometers to measure the global solar radiation (G) and the pyrgeometers with silicon dome from Eppley to measure the atmospheric infrared radiation (IR). The clearness index (K_t) and the diffuse fraction (K_d) for both regions have been calculated. Finally the total suspended particles for Cairo and Helwan and their interaction with the solar radiation has been found.     

Sky conditions at Ilorin as characterized by clearness index and relative sunshine

Clearness index (K_T) and relative sunshine (S) are used to characterize the sky conditions at Ilorin, Nigeria (latitude: 8°32′N, longitude: 4°34′E, elevation: 375 m), a tropical location. Daily analysis shows that the majority of the days are relatively cloudy with hardly any very clear or overcast days. Based on monthly averaged K_T, the monthly variations of cumulative frequency of K_T, f, and prevalent climatic conditions, the annual sky conditions have been classified into six patterns: three patterns each for the two broad seasons, dry and rainy. The trend in the seasonal variation of S lends further credence to this six seasonal pattern. Determined cumulative frequency curves further confirm the assertion that Liu and Jordan’s generalized cumulative distribution curves (CDC) are not applicable to tropical locations hence available design procedures based on these curves should be reconsidered [Liu, B. Y. U and Jordan, R. C. (1960). The Inter-relationaship and characteristic distribution of direct, diffuse and total solar radiation. Solar Energy 4, 1–19]. An empirical expression relating monthly maximum clearness index (K_max) and the monthly average clearness index (K_T) is presented and recommended for tropical locations. Comparative analysis shows that a similar expression by Saunier et al. is also appropriate to this location while that of Hollands and Huget is quite inadequate. [Saunier G. Y., Reddy T. A. and Kumar S. (1987). On monthly probability distribution function of daily global irradiation values appropriate for both tropical and temperate locations. Solar Energy 38, 169–177; Hollands K. G. T. and Huget R. G. (1983) A probability density function for clearness index, with applications. Solar Energy 30, 195]. Furthermore, the values of the minimum clearness index (0.12–0.24) for Ilorin are noticed to be much higher than the generally accepted value of 0.05 for overcast sky conditions. The results are further compared, especially with those of Ideriah and Suleman [Ideriah, F. J. K. and Suleman, S. O. (1989). Sky conditions at Ibadan during 1975–1980. Solar Energy 43, 325–330.] and Akuffo and Brew-Hammond [Akuffo, F. O. and Brew-Hammond, A. (1993). The frequency distribution of global solar radiation at Kumasi. Solar Energy 50, 145–154.] for Ibadan and Kumasi, all in the West African sub-region. Although the locations are slightly different in agro-climatic conditions, results are found to be very comparable.     

First solar radiation atlas for the Arab world

In the year 1998, the Arab League Educational, Cultural and Scientific Organization (ALESCO), Directorate of Science and Scientific Research, Tunis, had launched the “Solar Radiation Atlas for the Arab World”. This atlas contains three sets of maps (using Mercator projection) for monthly means, where each stands for one month. These are sunshine duration, global solar radiation and diffuse solar radiation. The atlas contains data for nearly 280 stations from 19 Arab states which cover latitudes from 0° (tropic) to 37°N and longitudes 19°E to nearly 60°E with different elevations from the sea level. It also contains useful tables of the monthly recorded means of the direct, diffuse and global solar radiation as well as the sunshine duration for 16 Arab states including 207 cities.The maximum recorded annual mean (10 years) of the global solar radiation in the Arab world was 6.7 kW h/m²/day in Nouakchott (latitude 20°56′N, longitude 17°02′E), Mauritania, and 6.6 kW h/m²/day in Tamenraset (latitude 36°11′N and longitude 5°31′E), Algeria, while the lowest recorded annual mean global solar radiation was 4.1 kW h/m²/day in Mosul (latitude 43°N and longitude 36°E), Iraq. Furthermore, the maximum recorded annual mean sunshine duration in the Arab world was 10.7 h in Aswan (latitude 23°58′N, longitude 32°47′E), Egypt, and the lowest was 7.5 h in Tunis (latitude 36°50′N, longitude 10°14′E), Tunisia.     

Study of hourly solar radiation data in Istanbul

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 monthly averages of daily total radiation are 1.23 kW h m⁻² day⁻¹ for January and 6.55 kW h m⁻² day⁻¹ for July. The annual average value of daily total radiation is 3.81 kW h m⁻² day⁻¹. 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.     

Analysis of the solar radiation data for Beer Sheva, Israel, and its environs

The solar radiation climate of Beer Sheva, Israel, is reported upon in detail. The database utilized in this analysis consisted of global radiation on a horizontal surface, normal incidence beam radiation, and global radiation on a south-facing surface tilted at 40°. Monthly-average hourly and daily values are reported for each of these three types of measured radiations, together with the calculated monthly-average daily values for the components of the global radiation, viz. the horizontal beam and diffuse radiations. The monthly-average hourly and daily clearness index values have also been calculated and analyzed. Monthly-average daily frequency distributions of the clearness index values are reported for each month. The solar radiation climate of Beer Sheva has also been compared to those reported for a number of countries in this region. The annual-average daily global radiation incident on a horizontal surface is 18.91 MJ/m² and that for normal incidence beam radiation is 21.17 MJ/m². The annual-average daily fraction of the horizontal global radiation that is beam is 0.72. The annual-average daily value for the clearness index is 0.587 and the average frequency of clear days annually is 58.6%. We conclude, based upon the above analysis, that Beer Sheva and its environs are characterized by relatively high, average-daily irradiation rates, both global and beam, and a relatively high frequency of clear days.     

Measurement of solar-energy (direct beam radiation) in Abu Dhabi, UAE

This paper presents actual measurements of direct solar radiation in Abu Dhabi (24.43°N, 54.45°E) with the existing meteorological conditions encountered during the measurement throughout the year. High resolution, real-time solar radiation and other meteorological data were collected and processed. Daily and monthly statistics of direct solar radiation were calculated from the one-minute average recorded by a Middleton Solar DN5-E Pyroheliometer. The highest daily and monthly mean solar radiation values were recorded as 730 and 493.5 W/m², respectively. The highest one-minute average daily solar radiation was recorded as 937 W/m². In addition to direct beam radiation, the daily average clearness indexes, surface temperature variations, wind speeds and relative humidity variations are discussed. When possible, direct beam radiation and some meteorological data are compared with corresponding data of the 22-year average of NASA's surface meteorology and solar-energy model. The measured data (direct beam radiation and meteorological) are in close agreement with the NASA SSE model with some discrepancy.     

Ultraviolet solar radiation at Dhahran, Saudi Arabia

Measurements of ultraviolet solar radiation for the period January 1985 to December 1987 at Dhahran (26°32′N, 50°13′E), Saudi Arabia, are analyzed. Dependence of ultraviolet radiation on the atmospheric clearness index is investigated. For a clearness index above 0.65, the ratio of ultraviolet radiation to total horizontal radiation is a constant value of 3.45%. For a clearness index below 0.65, the ratio was found to vary significantly with clearness index but it was also a strong function of the meteorological events (i.e., rainfall, clouds and suspended dust) bringing about the clearness index change. A ratio of ultraviolet to total horizontal radiation as high as 4.6% was reached on a rainy day, while a ratio as low as 2.1% was obtained for a dusty day.     

Equations for estimating global solar radiation in data sparse regions

The knowledge of the amount of solar radiation in an area/region is very essential in the field of Solar Energy Physics. In this work two equations are put forward for estimating global solar radiation from common climate variables in data sparse regions. The first is the Hargreaves equation, R_s=0.16R_aT_d^0.5 where R_a is the extraterrestrial solar radiation and T_d is the temperature difference (maximum minus minimum), while the second is the Angstrom equation, R_s=R_a(0.28+0.39n/N) where n and N are the measured sunshine hours and the maximum daylight duration respectively. The global solar radiation estimated by the two equations for three sites, Owerri (5°28′N, 7°2′E), Umudike (5°29′N, 7°33′E) and Ilorin (8°32′N, 4°46′E), located in different climate zones of in Nigeria, West Africa, are in agreement with those of earlier workers and that from Photovoltaic Geographic Information System (PVGIS) project. The implication of this in solar photovoltaic applications has been stressed.     

Estimation of global solar radiation on horizontal surfaces in Jeddah, Saudi Arabia

The measured data of global solar radiation on a horizontal surface, as well as the number of sunshine hours, mean daily ambient temperature, maximum and minimum ambient temperatures, relative humidity and amount of cloud cover, for Jeddah (latitude 21° 42′37″N, longitude 39° 11′12″E), Saudi Arabia for the period 1996–2006 are analyzed. The data are divided into two sets. The sub-data set 1 (1996–2004) are employed to develop empirical correlations between the monthly average of daily global solar radiation fraction (H/H₀) and various meteorological parameters. The nonlinear Angström type model developed by Sen and the trigonometric function model proposed by Bulut and Büyükalaca are also evaluated. New empirical constants for these two models have been obtained for Jeddah. The sub-data set 2 (2005, 2006) are then used to evaluate the derived correlations. Comparisons between measured and calculated values of H have been performed. It is indicated that, the Sen and Bulut and Büyükalaca models satisfactorily describe the horizontal global solar radiation for Jeddah. All the proposed correlations are found to be able to predict the annual average of daily global solar radiation with excellent accuracy. Therefore, the long term performance of solar energy devices can be estimated.     

Determination of atmospheric turbidity in Bangladesh

Atmospheric turbidity parameters have been determined following Angstrom's method at three locations in Bangladesh namely, Dhaka city (23.7°N) and two rural locations, Haripur (26.03°N) and Sripur (24.11°N). The parameters were obtained from direct solar radiation data for specific spectral regions and for the whole spectrum as measured using a normal incidence pyrheliometer provided with cut-off glass filters. A considerable variation of Angstrom's turbidity parameters for Dhaka over the year was observed with a maximum value in March. The value for Sripur for the month of March was somewhat lower and the value for Haripur for the month of April was not much different from that of Dhaka. Linke turbidity factor T_L was also determined using the new value of solar constant 1367 W/m² for all three locations.     

Global solar radiation measurements in Maceió, Brazil

This work focuses on the variability of the global solar radiation over the area of Maceió (9°40′S, 35°42′W, 127 m), located in Northeastern State of Alagoas, Brazil, during the1997–1999 period. Solar radiation variability was evaluated on 5 min, hourly, daily, monthly and seasonal scales. The results showed that the maximum values of the hourly global solar irradiation, , in the dry (September–February) and rainy (March–August) seasons were 3.18 and 2.50 MJ m⁻², respectively. The peaks of the hourly average, , for both periods were 2.79 MJ m⁻² and the daily average of the global solar irradiation, , was 19.89 MJ m⁻². The daily clearness index, , was found to be 0.53 (rainy period) and 0.59 (dry period). In clear, partially cloudy (the most frequent) and overcast days, the daily averages of global solar irradiation were 25.20, 19.00 and 8.00 MJ m⁻², respectively. On an annual scale the global solar irradiation changed from 15.00 MJ m⁻² by August to 24.04 MJ m⁻² by November.     

Relationship between “clearness index” and “cloudiness index” at a tropical station (Ilorin, Nigeria)

Total (global) solar radiation, H, and diffuse solar radiation, H_d were measured at Ilorin (8° 30′N 4° 42′E) Nigeria. From these, the daily values of H/H₀ and H_d/H were computed, where H₀ is the extraterrestrial radiation. The relationship between the two ratios and their variation with the prevailing atmospheric conditions were examined.The ratios were found to be opposite in characteristics. H/H₀ has high values in clear atmospheres and low values in cloudy or turbid atmospheres; and vice-versa for H_d/H. The two ratios are negatively linearly related and with this fact, two simple mathematical models were obtained for estimating H_d/H in terms of H/H₀.     

Solar radiation climate in Malawi

Recently, Diabaté et al. [Diabaté, L., Blanc, Ph., Wald, L., 2004. Solar climate in Africa. Solar Energy 76, 733–744] created a map of solar radiation climate in Africa using clearness index for 62 sites. They established a solar climate class II located in Malawi and Madagascar. However, their analysis did not include any irradiation data from a site in Malawi. In this study, the solar radiation climate of Malawi has been studied using long-term global radiation data from a local site. The mean monthly (K_tm) and seasonal (K_ts) daily clearness indices for the site were computed. It is observed that K_tm has two maxima in a year (K_tm = 0.58 in May and K_tm = 0.64 in September), in close conformity with findings of Diabaté et al. (2004). Other results are presented and discussed.     

An application of the multilayer perceptron: Solar radiation maps in Spain

In this work an application of a methodology to obtain solar radiation maps is presented. This methodology is based on a neural network system [Lippmann, R.P., 1987. An introduction to computing with neural nets. IEEE ASSP Magazine, 4–22] called Multi-Layer Perceptron (MLP) [Haykin, S., 1994. Neural Networks. A Comprehensive Foundation. Macmillan Publishing Company; Hornik, K., Stinchcombe, M., White, H., 1989. Multilayer feedforward networks are universal approximators. Neural Networks, 2(5), 359–366]. To obtain a solar radiation map it is necessary to know the solar radiation of many points spread wide across the zone of the map where it is going to be drawn. For most of the locations all over the world the records of these data (solar radiation in whatever scale, daily or hourly values) are non-existent. Only very few locations have the privilege of having good meteorological stations where records of solar radiation have being registered. But even in those locations with historical records of solar data, the quality of these solar series is not as good as it should be for most purposes. In addition, to draw solar radiation maps the number of points on the maps (real sites) that it is necessary to work with makes this problem difficult to solve. Nevertheless, with the application of the methodology proposed in this paper, this problem has been solved and solar radiation maps have been obtained for a small region of Spain: Jaén province, a southern province of Spain between parallels 38°25′ N and 37°25′ N, and meridians 4°10′ W and 2°10′ W, and for a larger region: Andalucía, the most southern region of Spain situated between parallels 38°40′ N and 36°00′ N, and meridians 7°30′ W and 1°40′ W.     

Experimental and theoretical investigation of global and diffuse solar radiation in the United Arab Emirates

In the present study, the global, direct and diffuse components of solar radiation as well as temperature, relative humidity and wind speed have been continuously monitored and analysed on an hourly, daily and monthly basis. The monthly average daily total solar radiation varies from 2700 W h/m² in December to 8000 W h/m² in June with an average clearness index of 0.65. Experimental data are compared to the predictions of different theoretical models as functions of declination and hour angles. Correlations are obtained describing the variation of hourly, daily and monthly averages of total and diffuse solar radiation using polynomial expressions. Empirical correlations describing the dependence of the daily average diffuse to total radiation ratio on the clearness index are also obtained. Data for the daily diffuse to total radiation ratio are compared to correlations obtained by other investigators. The comparison shows a reasonable agreement with some scatter due to the seasonal dependence of the correlation. Comparison of calculations with experimental measurements under clear sky conditions show excellent agreement with a maximum error of 8%.     

Evaluation of clear-sky conditions in high altitude sites

The correct assessment of cloud conditions in any site is important for determining the characteristics of solar resource availability. The criterion proposed by Iqbal has been widely used for this purpose, through the classification of the daily clearness index K_t. However, evidence was found to suggest that this criterion may not be applicable to data measured in high altitude sites, because partially cloudy days are incorrectly classified as clear-sky days. This paper analyzes the cloudiness state of the sky by observing the values of the clearness index K_t and the clear-sky index K_c. These indices were obtained from reliable global solar irradiance data measured over a year in three places of Argentina located at different altitudes (25, 1200 and 3700 m.a.s.l). ESRA and ARG-P models were used to estimate daily clear-sky global solar irradiation values H_c. They also reveal that the most probable value of K_c which corresponds to an actual clear-sky day of a particular location and time depends on the model used to estimate the clear-sky global solar radiation values. Thus, it was confirmed that, unlike the index K_c, the Iqbal's criterion does not correctly classify the cloud cover status from data measured in high altitude sites (>1000 m.a.s.l.).