The assessment of different models to predict the global solar radiation on a surface tilted to the south

Global and diffuse solar radiation intensities are, in general, measured on horizontal surfaces, whereas stationary solar conversion systems (both flat plate solar collector and PV) are tilted towards the sun in order to maximize the amount of solar radiation incident on the collector surface. Consequently, the solar radiation incident on a tilted surface must be determined by converting the solar radiation intensities measured on a horizontal surface to that incident on the tilted surface of interest. There exist a large number of models designed to perform such a conversion. 11 such models have been tested utilizing data measured in Beer Sheva, Israel. The data consist of hourly global and diffuse solar radiation on a horizontal surface, normal incidence beam and global radiation on a south-oriented surface tilted at 40°. The horizontal diffuse radiation measured using a shadow ring was corrected using four different correction models. This resulted in 44 model permutations. The individual model performance is assessed by an inter-comparison between the calculated and measured solar global radiation on the south-oriented surface tilted at 40° using both graphical and statistical methods. The relative performance of the different models under different sky conditions has been studied. Different grading systems have been applied in an attempt to score the relative performance of the models.     

Parameterization of aerosols from burning biomass in the Brazil-SR radiative transfer model

A new aerosol parameterization was developed and implemented for operational use with the BRASIL-SR model. The goal is to improve the assessment of solar energy resources in Brazil. Optical properties of the aerosols from burning biomass were obtained using software package OPAC and are in good agreement with previous field measurements made in Brazil. Three different mixture ratios of black carbon were used to cover the full range of typical measured values. The atmospheric transport model SMOKE provided the aerosol profile. An evaluating period of 11 days in August/1995 and ground measurements from six sites situated in Amazon region was used to validate the results. The global solar irradiation estimates obtained with new aerosol parameterization, presented smaller mean bias error in all ground sites. The correlation among estimates and measured values for surface global solar irradiation improved about 2.5 times by adopting an aerosol composition with 5% of black carbon.     

A simple hourly all-sky solar radiation model based on meteorological parameters

An hourly solar radiation model for cloudy skies, based on meteorological data, was developed andtested. As a means of comparison, the SOLMET regression and Watt models were also tested. The present model was examined for individual cloud types using measured solar radiation to judge the effectiveness of the model in the presence of particular clouds.     

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.     

Experimental verification of some clear-sky insolation models

Clear-sky solar irradiance can be predicted when a number of essential atmospheric parameters are known. A number of parameterization methods to predict solar irradiance with various degrees of difficulty are available in the literature. In this study, three models called model A, model B and model C, with medium degree of difficulty, have been examined. In these models, the solar transmittance due to each atmospheric parameter is available in simple algebraic form. Based on these algebraic equations, the direct normal, diffuse, and global horizontal irradiance can be predicted. These models have been compared with measured data from Carpentras, a French radiometric station. At this station, several daily observations of the clear-sky irradiance are carried out. Corresponding instantaneous values of the Ångström turbidity coefficient β and several other necessary surface meteorological observations are also made. For diffuse irradiance, a value of 0.95 is assumed for the single-scattering albedo of the aerosols. Based on the calculation of the mean bias error and root mean square error, model C has the best correspondance with the measurements as for as direct irradiance is concerned. Model B appears to be more accurate for prediction of diffuse and global irradiance.Regression equations are provided to help the user of any one of the three models for better prediction of solar irradiance.     

PREDICTING THE BROADBAND AEROSOL TRANSMITTANCE FOR SOLAR RADIATION MODELS

Based on a detailed spectral aerosol model (1), the total scattering and absorption broadband aerosol transmission functions are estimated. Using parameterization techniques, analytical expressions for the broadband aerosol transmission functions are obtained. The accuracy of the proposed expressions are verified with various tests. The proposed broadband aerosol transmission functions can be combined directly with the solar radiation models to predict accurately the direct beam, the diffuse and the total solar radiation at a given place.     

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.     

Performance assessment of global solar irradiation models under Romanian climate

Six global solar irradiation models were tested against data measured at three stations from Romania in the year 2000. One of the tested models, called EIM, is an empirical global solar irradiation model fitted by us, which requires as input a single meteorological parameter associated with the mean daily cloud amount. The accuracy of the EIM is acceptable and comparable to that of parametric models, which need more than one meteorological datum as input. The main conclusion is that such simple empirical “local-models” are a useful alternative for the more complicated one. In addition, the details of obtaining EIM are presented as a driven-tool, which may serve as a guide to elaborate similar simple solar irradiation models in any other location as well.     

Predicting the spectral and broadband aerosol transmittance in the atmosphere for solar radiation modelling

Calculations of the spectral transmittance of the atmospheric aerosol, using Mie theory, for wavelengths between 0 and 40 μm is presented. The chemical composition of the aerosol particles has been modelled in order to correspond to the atmospheric conditions of medium and large coastal or near coastal cities with important industrial and other anthropogenic emission sources. Individual size distributions and optical properties for each aerosol constituent have been considered.Based on the detailed aerosol model, and using parameterization techniques, analytical broadband aerosol transmission functions for the absorption and total attenuation are obtained. The accuracy of the proposed expressions are verified with various tests, using data from the National Observatory of Athens (NOA). The proposed broadband aerosol transmission functions can be incorporated directly into solar radiation models to predict accurately the beam, diffuse and global solar radiation at a given place.     

总辐射表性能的测试研究

根据WMO和ISO关于总辐射表的性能指标，对我国气象观测台站使用的总辐射表（型号为DFY4和TBQ－2）进行了全面的性能测试，主要研究了灵敏度和长期稳定性，以及非线性，温度特性，余弦响应，方位响应，倾斜响应，时间响应，零位漂移等对灵敏度的影响，通过大量的实验数据和分析研究，得出了总辐射表测量误差的主要来源，这对于控制仪器质量，提高太阳总辐射测量的准确度以及对太阳辐射的研究是非常有益的。     

硅太阳电池工程用数学模型

为实现光伏电站、光伏户用系统、光伏水泵系统、"风 -光-柴-蓄"等各种涉及太阳能光伏利用复合能源系统的设计、数字仿真和动态模拟实验 ,以电子学理论为基础,讨论了满足工程应用精度且便于运算的太阳电池数学模型.该模型的特点是仅采用生产厂家为用户提供的太阳电池组件在标准测试条件(STC)下测出的I sc、Voc、Im、Vm作为参数,通过引入相应系数来考虑环境影响,并给出系数的典型值.实验结果表明模型的误差一般都在6%以下,可以满足对绝大多数工程项目进行物理模拟的精度要求.     

A new neural network model for evaluating the performance of various hourly slope irradiation models: Implementation for the region of Athens

The present study is divided into two parts. The first part deals with the comparison of various hourly slope irradiation models, found in the literature, and the selection of the most accurate for the region of Athens. In the second part the prediction of global solar irradiance on inclined surfaces is performed, based on neural network techniques.The models tested are classified as isotropic (Liu and Jordan, Koronakis, Jimenez and Castro, Badescu, Tian) and anisotropic (Bugler, Temps and Coulson, Klucher, Ma and Iqbal, Reindl) based on the treatment of diffuse irradiance. For the aforementioned models, a qualitative comparison, based on diagrams, was carried out, and several statistical indices were calculated (coefficient of determination R², mean bias error MBE, relative mean bias error MBE/A(%), root mean square error RMSE, relative root mean square error RMSE/A(%),statistical index t-stat), in order to select the optimal.The isotropic models of “Tian” and “Badescu” show the best accordance to the recorded values. The anisotropic model of “Ma&Iqbal” and the pseudo-isotropic model of “Jimenez&Castro”, show poor performance compared to other models. Finally, a neural network model is developed, which predicts the global solar irradiance on a tilted surface, using as input data the total solar irradiance on a horizontal surface, the extraterrestrial radiation, the solar zenith angle and the solar incidence angle on a tilted plane. The comparison with the aforementioned models has shown that the neural network model, predicts more realistically the total solar irradiance on a tilted surface, as it performs better in regions where the other models show underestimation or overestimation in their calculations.     

Estimating the global solar radiation for solar energy projects – Egypt case study

The main objective of this paper is to determine the best models for estimating the global solar radiation (GSR) on a horizontal surface over Egypt. A model selection criterion is used to determine the best model for each site. A general two-stage method is developed. In the first stage, 40 popular empirical models are evaluated for a specific set of sites in Egypt. In the second stage, Egypt is divided into a set of regions each of which is described by the best found GSR model. The determined regions and regional models are based on the results of the first stage and determined based on the determined best model for the sites contained in a specific region. The results show that the solar radiation over the whole lands of Egypt can be characterised by two models: the Iranna and Bapat 10P GOB 1 and the K?l?c and Ozturk.     

A new approach for predicting vertical global solar irradiance

Solar irradiance, particularly on vertical surfaces, plays a major role in determining the thermal and energy performance of a building. It is important to the design and analysis of both active solar systems and passive solar buildings. Many mathematical models are mainly developed to predict the sky-diffuse irradiance on inclined surfaces from the measured horizontal diffuse component. This paper presents an approach to estimate the vertical global irradiance based on direct beam and ground-reflected components which can be accurately determined. Hourly data recorded from January 1996 to December 1998 in Hong Kong were used for the model development. The performance of the proposed model and two well-known anisotropic inclined surface models (Muneer and Perez) was evaluated against data measured in 1999. Statistical analysis indicated that the proposed model gives reasonably good agreement with measured data for all vertical planes. Although the new model has been found less effective than the Perez model, its simplicity nature provides buildings designers a convenient and reliable alternative in the estimation of vertical solar irradiance.     

Solar thermal performance of a nickel-pigmented aluminium oxide selective absorber

The solar thermal performance of a nickel-pigmented aluminium oxide selective absorber was determined by using an experimental model (prototype model). The constructed models were tested outdoors during daylight under a clear sky. Different volumes of prototypes were used. The flat-plate collector used was of the glazed type. The thermodynamic characteristics of each flat-plate collector were determined. The maximum calculated collective flux of the selective absorbers was in the range 590–699 W/m². The maximum conversion efficiency for our systems was in the range 0.73–0.78. However, the maximum Carnot efficiency was 0.04–0.16. The absorption of merit of the samples lay in the range 0.91–0.97. The selective absorbers used have absorptivity in the range 92.0–97.0% while they have emissivity in the range 11.0–22.5%. The effects of absorptivity, emissivity, and prototype volume on the collective flux, conversion efficiency, and absorption of merit are discussed.     

Prediction of global solar irradiance based on time series analysis: Application to solar thermal power plants energy production planning

Due to strong increase of solar power generation, the predictions of incoming solar energy are acquiring more importance. Photovoltaic and solar thermal are the main sources of electricity generation from solar energy. In the case of solar thermal energy plants with storage energy system, its management and operation need reliable predictions of solar irradiance with the same temporal resolution as the temporal capacity of the back-up system. These plants can work like a conventional power plant and compete in the energy stock market avoiding intermittence in electricity production.This work presents a comparisons of statistical models based on time series applied to predict half daily values of global solar irradiance with a temporal horizon of 3 days. Half daily values consist of accumulated hourly global solar irradiance from solar raise to solar noon and from noon until dawn for each day. The dataset of ground solar radiation used belongs to stations of Spanish National Weather Service (AEMet). The models tested are autoregressive, neural networks and fuzzy logic models. Due to the fact that half daily solar irradiance time series is non-stationary, it has been necessary to transform it to two new stationary variables (clearness index and lost component) which are used as input of the predictive models. Improvement in terms of RMSD of the models essayed is compared against the model based on persistence. The validation process shows that all models essayed improve persistence. The best approach to forecast half daily values of solar irradiance is neural network models with lost component as input, except Lerida station where models based on clearness index have less uncertainty because this magnitude has a linear behaviour and it is easier to simulate by models.     

Modeling and experimental studies on a domestic solar dryer

A domestic solar dryer with transparent external surfaces was designed, built and tested. Thin-layer drying models that describe the drying phenomena in a unified way, regardless of the controlling mechanism; have been used to estimate the drying period for several products. Temperature of the drying medium is one of the factors that affect the drying rate constant of the exponential model. This fact introduces a problem when used to predict the drying rate under solar drying conditions since the temperature of the drying medium is rarely constant. This paper aims to propose a solar dryer with a uniform temperature profile that meets the requirements of the exponential model over a wide range of cases, thus, providing a simple and accurate design tool. The dryer is characterized by collecting the maximum possible solar energy by having a longer drying period, and allows the fixed dryer to approach with its performance the tracked one with all technical and economical advantages of the tracking system. The performance was tested under different operational conditions and the drying characteristics were experimentally investigated by conducting the experiments on two local herbs, Jew's mallow and mint leaves. The dryer was able to reduce moisture of the tested products to the recommended level (6% wb) in about a 12 h period. The reliability of the exponential model was evaluated by comparing the experimental with the predicted curves. A reasonable agreement was found for the different tests carried out for the entire drying period.     

Performance analysis of solar water heater combined with heat pump using refrigerant mixture

In the research presented in this paper the thermal performance of a solar water heater combined with a heat pump is studied. A solar collector was modified from corrugated metal roofing with a copper tube attached beneath. The performance of the solar water heater was tested, and models for the collector efficiency and storage tank were developed and used for the evaluation of their performance when combined with a heat pump system.