Global solar radiation estimation with sunshine duration in Tibet,China

Eight models using only the sunshine duration for estimating the global solar radiation on a horizontal surface are evaluated at Changdu, Geer, Lasa and Naqu stations in Tibet, China against the measured meteorological data during 1994–2008. Based on statistical error tests, results show that the simple linear Ångström–Prescott model is reasonably accurate in practice, and the modifications with complex expression are not necessary in Tibet. Then, two general Ångström–Prescott models for estimating the daily and monthly average daily global solar radiation respectively in Tibet are proposed.     

Estimation and prediction of global solar radiation over Greece

A regression analysis of the relative monthly values of global solar radiation ( ) the corresponding values of sunshine (n/N), for the period 1961-75, was performed to determine the constants a and b of the Ångström formula, for Athens. The constants a and b were also determined by a graphical relationship between the average annual relative sunshine (n/N) and these constants, for the same station. The latter method was then used to determine the constants a and b for 33 other stations widely distributed throughout Greece, after making a relative correction to these constants. In this way, monthly and annual values of global solar radiation were estimated for 34 stations over Greece from sunshine measurements. The geographical distribution of the annual totals of global solar radiation over Greece was mapped and some types and sub-types were identified.Further, a stepwise multiple regression analysis of the annual total amounts of global solar radiation and the three factors (latitude, longitude and altitude) was carried out; the validity of the assumption of the linear relationship between the annual totals of global solar radiation and the three factors was examined.     

Estimation of solar radiation over Jordan—predicted tables

As measured solar radiation data for all parts of Jordan are not available, they have to be estimated using correlation relations and models. This paper presents, for the first time, values of solar radiation over Jordan as estimated from these relations. Measurements of global solar irradiance on a horizontal surface and sunshine duration at nine meteorological stations in Jordan are correlated and used for prediction of the regression coefficients of an Angstrom type correlation relation at these stations and others which only have records of sunshine duration. Regional regression coefficients are obtained and used for prediction of global solar irradiance. The agreement with measurements is better than 5% and 1% on monthly and yearly basis respectively. Estimation of diffuse solar irradiance by Page's and also Liu and Jordan's correlations, as well as the direct beam component are also performed and the results are examined and presented. The abundance of solar energy in Jordan is evident from the daily average global solar irradiance which ranges between 5 and 7 kWh/m². A correlation of Angstrom type of the form: H/Ho = 0.448+0.203 S/So is found suitable for Jordan with correlation coefficient r = 0.93.     

Daily global solar radiation modeling for Gran Canaria Island

The main aim of this paper is the daily global solar radiation modeling with different estimation models for the island of Gran Canaria (Spain). The errors of those ones will be studied and, moreover, the results will be compared with a solar radiation numerical model previously developed and validated. We have tested five different models starting from the Ångström linear model. Logarithmic, exponential, cubic models, and an exponential-linear composed model have been developed. The best results were obtained with the composed model. The development of these models will allow getting accurate results far from the measurements stations.     

Uses of sunshine duration to estimate the global solar radiation over eight meteorological stations in Egypt

The climatological Ångstrom regression coefficients have been determined by three methods and used to predict the global solar radiation over eight meteorological stations. Each of the three methods depends on the correlation between two ratios. The first ratio is between the long period of monthly average sunshine duration and the corresponding maximum of daily sunshine duration (day length) N, and the second ratio is between the measured monthly average daily global solar radiation and the corresponding monthly mean daily extraterrestrial solar radiation on the horizontal surface ₀. A comparison between the measured data and the estimated values has been done. The t-statistics is used as a statistical indicator to choose the coefficients of the best method that gives a percentage of error less than 10%.     

Solar radiation model

Solar radiation models for predicting the average daily and hourly global radiation, beam radiation and diffuse radiation are reviewed in this paper. Seven models using the Ångström–Prescott equation to predict the average daily global radiation with hours of sunshine are considered. The average daily global radiation for Hong Kong (22.3°N latitude, 114.3°E longitude) is predicted. Estimations of monthly average hourly global radiation are discussed. Two parametric models are reviewed and used to predict the hourly irradiance of Hong Kong. Comparisons among model predictions with measured data are made.     

Assessment of solar and wind energy resources in Ethiopia. I. Solar energy

This paper describes how data from a variety of sources are merged to present new countrywide maps of the solar energy distribution over Ethiopia. The spatial coverage of stations with radiation data was found to be unsatisfactory for the purpose of a countrywide solar energy assessment exercise. Therefore, radiation had to be predicted from sunshine hours by employing empirical models. Using data from seven stations in Ethiopia, linear and quadratic correlation relationships between monthly mean daily solar radiation and sunshine hours per day have been developed. These regional models show a distinct improvement over previously employed countrywide models. To produce a national solar-energy distribution profile, a spatial extension of the radiation/sunshine relationships had to be carried out. To do this, the intercepts (a) and slopes (b) of each of the seven linear regression equations and another six from previous studies, completed in neighbouring Sudan, Kenya and Yemen, were used to interpolate the corresponding values to areas between them. Subsequent to these procedures, 142 stations providing only sunshine data were assigned their “appropriate” a and b values to estimate the amount of solar radiation received, which was then used to produce annual and monthly solar radiation distribution maps for Ethiopia. The results show that in all regions solar energy is an abundant resource.     

Evaluation of a cloud cover based model for estimation of hourly global solar radiation in Western Canada

Cloud cover based solar radiation models are relatively simple and convenient as the models require the input of cloud cover data which are mostly available from the meteorological stations. In this study, the performance of a cloud cover based solar radiation model (Kasten–Czeplak model) with original or locally fitted coefficients was evaluated for estimating the hourly global solar radiation for four different locations in Western Canada. The average value of R², mean bias error, and root mean square error are 0.69, ?61.6, and 157.9?W?m^?2, respectively, for the model with original coefficients, whereas 0.82, 4.4, 107.1?W?m^?2 with locally fitted coefficients. Results show that the Kasten–Czeplak model with locally fitted coefficients satisfactorily estimated the hourly solar radiation of four different locations in Western Canada. Also, the results indicate that the model with original coefficients has very limited accuracy under intermediate cloud cover conditions.     

A correlation for estimation of global solar radiation

We present a simple Ångstrom-type correlation for estimating the monthly average daily horizontal global radiation. We have developed this correlation using the bright sunshine and global radiation data of 48 locations around the world, with varied meteorological conditions and a wide distribution of geographical locations. Our study shows that this correlation provides good estimates of solar radiation. We have also statistically compared the performance of our correlation with Rietveld's equation, which is believed to be applicable anywhere in the world and has been found by Ma and Iqbal (1984) to give better estimates than other published correlations. Our correlation yields better estimates than Rietveld's equation for a majority of the stations considered.     

Solar radiation and sunshine hour maps in Castilla and León region (Spain)

From global solar radiation and sunshine hour data of Castilla and León region (Spain), six different Angstrom-type correlations have been performed using the least square technique and in each of them the parameters a and b have been calculated. In two of these correlations we have taken into account the optic effects of atmosphere and not burning the sunshine recorder chart. The results have been compared using statistical tests based on MBE, RMSE and CC and from this calculation we have obtained the best correlation for Castilla and León locations. Finally from experimental and calculated data the monthly mean solar global radiation and sunshine isoline maps have been plotted for each month of the year and for the year as a whole.     

A simplified model for generating sequences of global solar radiation data for isolated sites: Using artificial neural network and a library of Markov transition matrices approach

The purpose of this work is to develop a hybrid model which will be used to predict the daily global solar radiation data by combining between an artificial neural network (ANN) and a library of Markov transition matrices (MTM) approach. Developed model can generate a sequence of global solar radiation data using a minimum of input data (latitude, longitude and altitude), especially in isolated sites. A data base of daily global solar radiation data has been collected from 60 meteorological stations in Algeria during 1991–2000. Also a typical meteorological year (TMY) has been built from this database. Firstly, a neural network block has been trained based on 60 known monthly solar radiation data from the TMY. In this way, the network was trained to accept and even handle a number of unusual cases. The neural network can generate the monthly solar radiation data. Secondly, these data have been divided by corresponding extraterrestrial value in order to obtain the monthly clearness index values. Based on these monthly clearness indexes and using a library of MTM block we can generate the sequences of daily clearness indexes. Known data were subsequently used to investigate the accuracy of the prediction. Furthermore, the unknown validation data set produced very accurate prediction; with an RMSE error not exceeding 8% between the measured and predicted data. A correlation coefficient ranging from 90% and 92% have been obtained; also this model has been compared to the traditional models AR, ARMA, Markov chain, MTM and measured data. Results obtained indicate that the proposed model can successfully be used for the estimation of the daily solar radiation data for any locations in Algeria by using as input the altitude, the longitude, and the latitude. Also, the model can be generalized for any location in the world. An application of sizing PV systems in isolated sites has been applied in order to confirm the validity of this model.     

Correlation of global solar radiation with meteorological parameters over Egypt

Global solar radiation measurements on a horizontal surface (G_m), mean daily maximum temperature (T), mean daily relative humidity (RH), mean daily sea level pressure (MSL), mean daily vapor pressure (V) and hours of bright sunshine (S) are presented, analyzed, arranged in tables and graphs and discussed for five selected locations over Egypt. The locations chosen represent the different weather conditions of Egypt. Matrough and Al Arish are in the north, Cairo in the middle and Kharga and Aswan in the south of Egypt. A correlation between the measurements of global solar radiation and the meteorological parameters were given for the considered locations. The common relationship for all Egypt was also estimated. The correlation and the regression coefficients and the standard errors of estimate are listed in Table 1. The values of correlation coefficients vary from 89% to 99% and the errors of estimation are between 0.01 and 0.04.

Table 1. Geographic location of the selected stations and correlation, regression coefficients and standard errors of estimate for the model ((5), (6), (7), (8) and (9))

Full-size table (<1K)

     

Empirical model development and comparison with existing correlations

This study utilized monthly mean daily values of global solar-radiation and sunshine duration at 41 locations in Saudi Arabia and developed an empirical correlation for the estimation of global solar radiation at locations where it is not measured. The paper also presents the comparison between the present correlation and other models developed under different geographical and varied meteorological conditions. The comparisons are made using standard statistical tests, namely mean bias error (MBE), root mean square error (RMSE), mean percentage error (MPE), and mean absolute bias error (MABE) tests. The errors are calculated using monthly mean daily measured and estimated values of global solar radiation at all 41 locations. The study found that the present correlation produced the best estimates of global solar radiation.     

An atmospheric model applied to some actinometric stations of Sicily

In this paper the application of an atmospheric model proposed by S. Barbaro et al. in a previous work for computing the direct and diffuse solar radiation is extended to some actinometric stations in Sicily, namely Gela, Messina, Pantelleria, Trapani and Ustica. The comparison of the monthly average values of the global radiation computed by this method with the experimental data shows a good agreement, that confirms the encouraging results obtained for Palermo in the previous work. This further test of the validity of the proposed model allows this one may be used with good reliability to predict the radiation on locations lacking in actinometric instruments. Finally, the influence of the diffuse component on the global radiation is computed and the results are plotted with the relative regression curves.     

基于GA-BP神经网络的逐时总辐射分组模型研究

通过分析影响太阳辐射的主要因素,提出以太阳高度角、季节和天气（晴空指数）作为数据划分依据的分组模型建立方法。以拉萨和西安地区的逐时气象数据和辐射数据为例,基于遗传算法（genetic algorithm,GA）优化的BP神经网络,建立太阳高度角、季节和天气类型的逐时总辐射分组模型。该研究揭示分组模型误差变化的规律,并将其估算误差与AllData模型比较。结果显示,相较于AllData模型,分组模型的估算误差均有降低。其中,天气分组模型误差最小,且西安的天气分组模型结果优于拉萨。西安天气分组模型平均绝对百分比误差（MAPE）和相对均方根误差（rRMSE）相较AllData模型结果分别下降3.96%和4.18%。研究结果表明分组模型能够降低逐时总辐射估算误差,可为估算逐时总辐射提供方法借鉴。     

Solar attenuation by aerosols: An overview

One of the most important factors affecting the attenuation of solar radiation reaching the earth's surface under cloudless sky conditions is the presence of aerosol particles. A direct measurement of the aerosol transmittance is not possible due to the strong influence of the other atmospheric components. Thus the extinction caused by the atmospheric aerosol can be calculated only indirectly using fundamental physical techniques. This work provides an overview of the effect of aerosols on solar radiation budget by considering two common turbidity parameters including the Linke turbidity factor T_L and Ångström turbidity coefficients β and α. Total extinction of solar radiation due to the absorption and the scattering caused by the atmospheric aerosol accounts for 10–20% for zero zenith angle. The influence of aerosol on radiation passing through the atmosphere cannot be neglected, especially in urban or industrialized areas. The attenuation of solar radiation through a real atmosphere versus that through a clean dry atmosphere gives an indication of the atmospheric turbidity.     

Estimation of global solar radiation in the Indonesian climatic region

In this paper the monthly average daily global solar radiation correlation applicable to the Indonesian climatic region is presented. The correlation developed is based on the meteorological data collected from seven meteorological stations, namely: Banjarbaru, Denpasar, Jakarta, Kupang, Manado, Palembang and Semarang. The correlation developed evolves from the Sayigh's “Universal Formula” with a slight modification in the form of a correction factor (I_CF) to suit the Indonesian climatic conditions.     

Global solar radiation in Northeastern Saudi Arabia

This paper presents the actual global solar radiation on a horizontal surface along with the prevailing meteorological conditions encountered during the measurement period from 1 January–31 December, for one complete year, in the Arabian Gulf Coast near the city of Dhahran. High resolution, real time solar radiation and meteorological data were collected, and processed. Hourly, daily, and monthly statistics of solar radiation was made from the one-minute averaged recorded values. The highest measured daily, and monthly mean solar radiation were found to be 351 and 328 W/m⁻², respectively. The highest one-minute averaged solar radiation values up to 1183 W/m⁻² were observed in the summer season, from May–September. The highest hourly solar radiation value was recorded as 1053 W/m⁻² in the middle of June. Beside the global solar radiation measurements, the main observed meteorological parameters were temperature, pressure, wind speed, precipitation, and relative humidity. On the other hand, the estimation of daily and monthly mean global solar radiation was performed based upon two empirical formulas which relate the solar radiation to the sunshine duration, relative humidity, maximum temperature, the latitude of the monitoring location, sunset hour and declination angles. The agreement between the measured and estimated solar radiation values was found to be satisfactory. Nevertheless, the empirical formula under-estimates the solar radiation values during summer, and over-estimates during winter.     

Effect of sunshine and solar declination on the computation of monthly mean daily diffuse solar radiation

Several years of measured data for 17 European locations have been used to develop models for estimating monthly mean daily values of diffuse radiation (Hd) from combinations of the following: clearness index, sunshine fraction, and solar declination. Two models giving the highest correlation coefficients and the lowest standard errors of estimation are tested with data for 10 European locations not used in their development. From consideration of the MBE and RMSE values, a model which estimates Hd values from clearness index, relative sunshine duration and solar declination is found to be the most accurate. Comparison with Hd values predicted with the European Community solar radiation model (ECM) confirms this conclusion.     

Comprehensive study of solar conditions in Mozambique: the effect of trade winds on solar components

A new algorithm to simulate all solar components and optimum slopes, β_opt, based on new models for direct normal beam and diffuse radiation and an analytical model to predict β_opt, developed at the University of Botswana is applied for complete study of solar conditions in Mozambique. The components of solar radiation depend to a large extent on the number of h of sunshine. However, it is obvious that cloud-cover is determined mainly by the prevailing trade winds, which carry moisture and rain clouds. This is of especial concern in coastal areas. In the current work, hourly, I, daily, H and monthly mean, components of solar radiation and the optimum slopes of a north–south aligned collector are simulated and analyzed for 21 synoptic stations in Mozambique. Monthly mean daily direct normal, solar radiation maps are plotted for December and June and discussed. It is found that, to a great extent, isoinsolation curves are determined by the prevailing trade winds, mountain chains and coastal conditions. Plotted maps of annual mean daily direct normal and global solar radiation also show tremendous dependence on the prevailing winds. Several special locations in Mozambique with quite high or very low solar radiation components are pointed out and the reasons explained.