Investigation of the sky optical conditions over selected stations in the major vegetation zones of Nigeria

Data-set of monthly global solar irradiance covering periods of at least 20 years over selected stations in Nigeria have been utilised to investigate the sky optical conditions over the five major vegetation zones of the country. The monthly variation of the clearness index for the selected stations has been investigated. Results show that the clearness index has its minimum in August which indicates reduced insolation during the monsoon month of August and maximum around November, December and February which shows high insolation. Statistical analysis of the monthly clearness index which gives the percentage depletion by the sky of the incoming insolation and the subsequent characterisation of the sky conditions based on this were also done. The results show that cloudy sky conditions prevail in the rainforest and mangrove swamp zones (e.g. Lagos, K_T=0.40) while clear sky conditions prevail in the Sudan and Sahel vegetation (e.g. Nguru K_T=0.66) and Kano (K_T=0.65).     

An assessment of the solar resource for Durban,South Africa

Renewable energy’s role as an alternative to fossil-based power is growing in the developing world. The city of Durban, South Africa, is an example of a rapidly expanding urban center which can benefit from the implementation of solar energy technologies. This paper presents a year-long data record of the solar flux intensity for the city of Durban (29°58′N 30°55′E). Global horizontal irradiance (G_t), direct normal irradiance (G_DN), diffuse horizontal irradiance (G_d) and daily average clearness index (K_T) are used. The data were recorded at the Solar Thermal Applications Research Laboratory (STARlab) at Mangosuthu University of Technology. Ground-based measurements obtained from STARlab are compared with data from a variety of sources including NASA’s SSE database and the literature. The aim of this study is to build a reliable record of the solar resource for urban planning, engineering design and effective operation of solar energy systems and applications. Results show that Durban has a considerable solar energy resource, which remains to be exploited.     

Evaluation of models for predicting insolation on slopes within the Colorado alpine tundra

This empirical study evaluates insolation predictions for the Colorado tundra from models based upon isotropic and anisotropic distribution approximations for diffuse sky and reflected solar radiation. The data set of hourly insolation values was obtained from 40 locations on an alpine ridge by simultaneous measurement of direct beam irradiance and total insolation to the horizontal ridge crest and two nearby sloping surfaces. Six models are used to predict insolation, two based on the isotropic distribution of diffuse solar radiation and four on anisotropic diffuse distribution fields. Three models (one isotropic and two anisotropic) employ measurements of normal direct beam irradiance while the other 3 models incorporate the correlation between the “clearness index” and the ratio of diffuse sky to total insolation. The precision of insolation estimates from models using the correlation method is only slightly less than from other models. Accounting for the increase of diffuse radiation in the circumsolar sky improves insolation predictions. However, inclusion of additional regions of diffuse radiation anisotropy decreases model accuracy. Errors of insolation estimates for the alpine tundra from all models vary in a systematic manner as a function of relative azimuth and ground slope angles.     

Global solar radiation: Multiple on-site assessments in Abu Dhabi,UAE

Renewable energy technology and in particular solar energy is being considered worldwide due to the fluctuations in oil prices, global warming and the growing demand for energy supply. This paper investigates the climate conditions available in the United Arab Emirates (UAE) in particular Abu Dhabi to implement Photovoltaic (PV) technology. Measured solar radiation was analyzed for five different geographical locations to ensure the suitability of this region. Hourly, daily and monthly global horizontal irradiation (GHI) were collected and processed. Statistical methods were used to evaluate the computed GHI and showed high values especially during the summer period. Moreover, clearness index was calculated to investigate the frequency of cloudy sky days and results have shown a high percentage of clear days during the year. This paper highlights a promising future for Abu Dhabi in the solar energy sector and in particular Photovoltaic (PV) technology.     

基于光伏组件的太阳能资源观测

鉴于太阳能资源的测量和评价是太阳能开发利用的重要基础,按照光伏电池两种主要安装方式(倾斜固定和太阳跟踪),利用单晶、多晶和非晶三种典型的光伏组件设计进行了太阳能光伏资源观测试验,获得了各季节典型晴天条件下各类型光伏组件辐照度的日变化特征和倾斜面光伏组件一年中月均每日可发电量的极大值、极小值及其月份。通过对比各类型光伏组件在太阳跟踪器上和纬度倾斜面上光伏辐照度变化,得出跟踪光伏组件日均光伏曝辐量与倾斜光伏组件日均光伏曝辐量的相比较优势。根据光伏组件的观测结果推算出各类型光伏组件的光伏反演辐照度,与气象辐射观测用总辐射表的总辐射辐照度趋势非常一致,在太阳能光伏主要利用时段相对误差基本在10%以内。     

Estimation of daily Linke turbidity factor by using global irradiance measurements at solar noon

This work presents a methodology for estimating daily Linke turbidity factor for clear sky days from global horizontal irradiance information at solar noon and monthly mean values of the Linke turbidity factor. The analysis of the method proposed here have been made using the ESRA clear sky model to recalculate the direct normal irradiance using as input the new Linke turbidities. Ground data of three BSRN and six AEMet radiometric meteorological stations have been used for assessing the method. Linke turbidity factor estimated here exhibits higher fluctuations than the monthly means and the comparison of the solar irradiance recalculated with the ground data have shown a noticeable reduction of the root mean square deviation. On the other hand the new Linke turbidity estimations have been compared to those values obtained from normal beam irradiance measures. The discrepancies found point out the high dependence of the reliability of this method with the quality of the ground measurements used.     

Solar irradiance on non-horizontal surfaces at the East-Mediterranean coastal plain of Israel

A large volume of global, diffuse and direct solar radiation observed at the National Radiation Center in Bet-Dagan (the East-Mediterranean coastal plain of Israel) has been analysed to evaluate the solar irradiance climatology on non-horizontal surfaces of various slope aspects and tilt angles. The isotropic approximation was used with respect to the diffuse sky radiation, as well as with regard to the reflected radiation from the ground. The feasibility of applying this approximation for the assessment of the insolation climate on non-horizontal surfaces is discussed. The monthly curves of daily total insolation on inclined surfaces were drawn, and their characteristic patterns for the various slope aspects and seasons are examined.     

An analytical comparison of four approaches to modelling the daily variability of solar irradiance using meteorological records

Temporal solar variability significantly affects the integration of solar power systems into the grid. It is thus essential to predict temporal solar variability, particularly given the increasing popularity of solar power generation globally. In this paper, the daily variability of solar irradiance at four sites across Australia is quantified using observed time series of global horizontal irradiance for 2003–2012. It is shown that the daily variability strongly depends on sky clearness with generally low values under a clear or overcast condition and high values under an intermittent cloudiness condition. Various statistical techniques are adopted to model the daily variability using meteorological variables selected from the ERA-Interim reanalysis as predictors. The nonlinear regression technique (i.e. random forest) is demonstrated to perform the best while the performance of the simple analog method is only slightly worse. Among the four sites, Alice Springs has the lowest daily variability index on average and Rockhampton has the highest daily variability index on average. The modelling results of the four sites produced by random forest have a correlation coefficient of above 0.7 and a median relative error around 40%. While the approach of statistical downscaling from a large spatial domain has been applied for other problems, it is shown in this study that it generally suffices to use only the predictors at a single near point for the problem of solar variability. The relative importance of the involved meteorological variables and the effects of clearness on the modelling of the daily variability are also explored.     

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%.     

Estimating surface solar radiation from upper-air humidity

A numerical model is developed to estimate global solar irradiance from upper-air humidity. In this model, solar radiation under clear skies is calculated through a simple model with radiation-damping processes under consideration. A sky clearness indicator is parameterized from relative humidity profiles within three atmospheric sublayers, and the indicator is used to connect global solar radiation under clear skies and that under cloudy skies. Model inter-comparisons at 18 sites in Japan suggest (1) global solar radiation strongly depends on the sky clearness indicator, (2) the new model generally gives better estimation to hourly-mean solar irradiance than the other three methods used in numerical weather predictions, and (3) the new model may be applied to estimate long-term solar radiation. In addition, a study at one site in the Tibetan Plateau shows vigorous convective activities in the region may cause some uncertainties to radiation estimations due to the small-scale and short life of convective systems.     

An assessment of a revised Olmo et al. model to predict solar global radiation on a tilted surface at Beer Sheva,Israel

A model to convert horizontal solar global radiation to that on a tilted surface is presented. It is based upon a relatively simple model proposed by [Olmo FJ, Vida J, Foyo I, Castro-Diez Y, Alados-Arboledas L. Prediction of global irradiance on inclined surfaces from horizontal global irradiance. Energy 24 (1999) 689–704]., which requires only measurements of horizontal solar radiation but was found to produce significant errors when tested with data from another site. The present model assumes the availability of databases for at least two of the three solar radiation types, viz., global, beam and diffuse. The horizontal global radiation is converted to that on a tilted surface by applying the Olmo model to the diffuse component, whereas the beam component is converted by using the geometrical relationship between the two surfaces. The original Olmo anisotropic radiation correction factor is now assumed to be a function of sky conditions. The solar radiation databases were converted to subsets corresponding to clear, partially cloudy and cloudy sky based upon clearness index values. The three anisotropic correction factors were determined by fitting to a 12-months database. The present model was then tested by applying it to a second database consisting of 24-months not involved in the model development. It was found to give better results than three highly regarded more complex models.     

Computer simulation of solar pond thermal behavior

A computer model of salt gradient solar pond thermal behavior has been developed and used to verify the validity of assuming constant salt solution physical parameters and long term averaging schemes for ambient temperature and insolation in previous solar pond analytical models. A theoretical limit for pond transparency is calculated which is significantly higher than that previously assumed. It is suggested that a transparent membrane be placed just below the air/water interface of solar ponds to maintain pond solution purity and approach the theoretical limit for transparency. A means of estimating the diffuse insolation input into a solar pond is given which utilizes sky color temperatures for different values of the clearness index (K_T). A single sky color temperature is calculated for each average clearness index value ( ).     

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.     

Solar radiation modelling for the simulation of photovoltaic systems

The effect of 12 different combinations of diffuse–global correlations and tilted surface radiation models on the accuracy of PV output simulation of a grid-connected photovoltaic (PV) system was studied using statistical methods. A site specific diffuse–global correlation was developed using local insolation data and the performance of this model was compared with those of two other diffuse–global correlations. The impact of diffuse–global correlations on the calculated inclined insolation for four different tilted surface radiation models was investigated on annual, seasonal and monthly basis. The accuracies of predicted 45° inclined plane insolation and PV output were improved when the site specific diffuse–global correlation was used in the main simulation model. The error between measured and predicted inclined insolation was higher in winter than summer. The prediction of PV output was improved by using an isotropic sky tilted surface radiation model instead of the anisotropic models. The accuracy of PV output was also improved when the proposed diffuse–global correlation was used.     

Estimated monthly average global radiation for Turkey and its comparison with observations

Global solar radiation data obtained from actinographs of the Turkish State Meterological Service were compared with data obtained from pyrheliometers that were established recently to determine if the actinograph data were usable in practice. It has been found that the observed actinograph data have a rather high error rate with 14.7% annual and 42.1% monthly averages. It has, however, not been possible to smooth these errors as they are absolutely random. Thus, a well known quadratic model was used to produce available radiation data. A quadratic relationship between solar insolation and duration of solar radiation data has been investigated in order to estimate monthly average global irradiance for Ankara, Antalya, Samsun, Konya, Urfa and zmir. The data from August 1993 to July 1995 have been used in the quadratic model. But, observations of zmir differ from other stations covered for about four years. Solar insolation data used in the quadratic model were obtained from the pyrheliometer with model CR10. Duration of bright sunshine data were obtained from a Siap, Frans or Müller sunshine recorder with 60° global lens. A general quadratic formula was found that represents the whole of Turkey. The estimated monthly average global solar radiation data, then, were produced from this quadratic formula. Comparison of the estimated and measured values showed that the quadratic model was able to estimate global radiation with about a 4% annual relative error and the estimated data seemed to be more reliable than the data measured by actinograph.     

Solar radiation for Shiraz, Iran: a comparative study for two periods

Instantaneous solar radiation data read from the chart of global solar radiation on a horizontal surface as well as sunshine hours in Shiraz during two periods (1980–1983 and 1988–1990) are analysed and compared with solar conditions in the early 1970s. It is apparent from the recorded data and subsequent analysis that while this area still possesses a relatively high abundance of sunshine, there has been a gradual decay in clearness index. Frequency of clear sky days, the average daily irradiation and the mean cumulative annual irradiation for the two periods under study are presented and a decrease in solar radiation received on a horizontal surface due to down grading of Shiraz from the clear sky conditions enjoyed in the 1960s and 1970s is demonstrated.     

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.     

A MULTIPLE LINEAR CORRELATION FOR DIFFUSE RADIATION FROM GLOBAL AND SUNSHINE DATA

Several years of measured data of global and diffuse radiation together with sunshine duration, for five locations in Spain are used to establish empirical relationships to connect monthly mean daily diffuse irradiation with clearness index and relative sunshine duration. A correlation connecting sky radiation with both clearness index and percent possible sunshine together is found to be most accurate for locations in Spain and Portugal. When clearness index and relative sunshine duration are combined together, it is observed that the accuracy of the estimated diffuse radiation data is better than when they are used separately.     

Making full use of the clearness index for parameterizing hourly insolation conditions

An enhanced parameterization of insolation conditions based only on the knowledge of global irradiance is presented. Two limitations associated with the current approach using the clearness index are pointed out: its dependence on solar elevation and its inability to differentiate between different conditions that produce the same global irradiance. Suggestions are provided which could overcome part of these limitations. Arguments are substantiated with solid experimental evidence. It is further shown that noticeable gains in accuracy for the decomposition of global into direct and diffuse irradiance are possible if one makes optimum use of the information available within a global irradiance time series.     

Correlations for estimating solar radiation using sunshine hours and temperature measurement in Osogbo,Osun State,Nigeria

In this study, the global solar radiation on horizontal surface in Osogbo, Osun state, Nigeria was analyzed using 11-year data (1997–2007). Correlations using linear and quadratic expressions were developed to relate the global solar radiation on horizontal surface based on relative sunshine hours and temperature measurements for evaluating the monthly average daily global solar radiation. The calculated monthly clearness index values indicate that the prevailing weather condition in Osogbo is heavily overcast. All the developed quadratic correlations gave better correlation coefficients (0.834, 0.872 and 0.823 respectively) than the linear models. However, the Hargreaves and Samani related based quadratic model gave the best among the three developed quadratic expressions and is therefore suggested for estimating the monthly global radiation in this site and its surroundings.