A note on diffuse solar radiation on a tilted surface

We are presenting a note on comparative study of diffuse component of solar radiation on tilted surface with different angle of inclination. The monthly mean daily diffuse radiation have been calculated on inclined surfaces from the data of horizontal surface using the Circumsolar and Isotropic model and the anisotropic model of Klucher and Hay at Lucknow (Latitude 26.75°, Longitude 80.85°), Uttar Pradesh, India. These calculated results are compared with our measured value. The comparison shows that Klucher model who considered the effect of cloudy sky conditions gives comparatively good estimations particularly at low inclination angles.     

Global,direct and diffuse solar radiation on horizontal and tilted surfaces in Jeddah,Saudi Arabia

The measured data of global and diffuse solar radiation on a horizontal surface, the number of bright sunshine hours, mean daily ambient temperature, maximum and minimum ambient temperatures, relative humidity and amount of cloud cover for Jeddah (lat. 21°42′37′′N, long. 39°11′12′′E), Saudi Arabia, during the period (1996–2007) are analyzed. The monthly averages of daily values for these meteorological variables have been calculated. The data are then divided into two sets. The sub-data set I (1996–2004) are employed to develop empirical correlations between the monthly average of daily global solar radiation fraction (H/H₀) and the various weather parameters. The sub-data set II (2005–2007) are then used to evaluate the derived correlations. Furthermore, the total solar radiation on horizontal surfaces is separated into the beam and diffuses components. Empirical correlations for estimating the diffuse solar radiation incident on horizontal surfaces have been proposed. The total solar radiation incident on a tilted surface facing south H_t with different tilt angles is then calculated using both Liu and Jordan isotropic model and Klucher’s anisotropic model. It is inferred that the isotropic model is able to estimate H_t more accurate than the anisotropic one. At the optimum tilt angle, the maximum value of H_t is obtained as ∼36 (MJ/m² day) during January. Comparisons with 22 years average data of NASA SSE Model showed that the proposed correlations are able to predict the total annual energy on horizontal and tilted surfaces in Jeddah with a reasonable accuracy. It is also found that at Jeddah, the solar energy devices have to be tilted to face south with a tilt angle equals the latitude of the place in order to achieve the best performance all year round.     

Statistical comparison of solar radiation correlations Monthly average global and diffuse radiation on horizontal surfaces

For most solar energy applications, it is necessary to predict the amount of global and diffuse radiation arriving on a horizontal surface at any specified location. Scientists have developed methods to achieve this using different input parameters. The purpose of this study is to compare statistically correlations for estimating the monthly average daily global/diffuse radiation incident on a horizontal surface and to recommend one in each catagory that best fits measured data from a number of locations chosen and is the simplest to use. The effects of using the new solar constant of 1367 W⁻² in these correlations are also investigated. As a result, Rietveld's and Page's correlations are recommended for estimating monthly average daily global and diffuse radiation incident on a horizontal surface, respectively. In addition, these correlations seem to be just as accurate with the new solar constant.     

Computing diffuse fraction of global horizontal solar radiation: A model comparison

For simulation-based prediction of buildings’ energy use or expected gains from building-integrated solar energy systems, information on both direct and diffuse component of solar radiation is necessary. Available measured data are, however, typically restricted to global horizontal irradiance. There have been thus many efforts in the past to develop algorithms for the derivation of the diffuse fraction of solar irradiance. In this context, the present paper compares eight models for estimating diffuse fraction of irradiance based on a database of measured irradiance from Vienna, Austria. These models generally involve mathematical formulations with multiple coefficients whose values are typically valid for a specific location. Subsequent to a first comparison of these eight models, three better performing models were selected for a more detailed analysis. Thereby, the coefficients of the models were modified to account for Vienna data. The results suggest that some models can provide relatively reliable estimations of the diffuse fractions of the global irradiance. The calibration procedure could only slightly improve the models’ performance.     

太阳辐射计算讲座第五讲地表斜面上辐射量的计算

在太阳能利用工作中 ,太阳辐射计算十分重要。为了帮助读者掌握太阳辐射计算方法 ,我们请长期从事太阳辐射研究工作的中国气象科学研究院王炳忠研究员编写了《太阳辐射计算讲座》 ,供大家学习、参考。—编者—     

Statistical comparison of correlations for estimation of global horizontal solar radiation

We evaluate the statistical performance of four correlations for estimating the monthly average daily horizontal global radiation. We have used bright sunshine and global radiation data for 140 locations around the world with varied meteorological conditions and geographical locations. The results show that Rietveld's equation generally gives the best estimates. Hay's correlation is next best, whereas the Glover-McCulloch relation gives the maximum error. The computations in this study are based on the most recently adopted solar constant.     

Analysis of solar radiation measurements at Athens Observatory and estimates of solar radiation in Greece

In this paper measurements of Global (G) Solar Radiation on a horizontal surface at Athens during the 16 yr period 1960-75 are analysed. Mean annual, monthly and daily totals, the diurnal variation, frequency distribution of daily totals, frequency of occurrence of daily totals of Global Radiation less than 8.5, 17, and 34 MJ/m² for 2, 3, 4 and 5 successive days are computed and discussed.Direct Solar Radiation at normal incidence (I) is computed and analysed in a similar manner to that of Global Radiation, for the same period. Components of direct radiation of normal incidence have been computed from spot value observations at times when sky conditions permitted.Annual values of global radiation were estimated from sunshine measurements (1960-73) widely distributed throughout Greece. It is considered that this analysis of measurements should closely represent the radiation climatology of Greece.     

Diffuse solar radiation on a horizontal surface for a clear sky

A model of the diffuse or sky radiation as a function of measurable optical properties of the earth's atmosphere is presented. The radiant energy incident upon a horizontal plane at the earth's surface is expressed in an approximate closed-form. The dependence of the sky radiation upon climate type, air mass, and surface reflectivity is given and comparisons with existing empirical correlations show that the model accurately predicts these characteristics.     

太阳能辐射资料库的创建

介绍了所创建的我国太阳辐射资料库,其内容包括适合于太阳能利用的遍布全国的太阳能辐射资料、不同方位和倾角的表面上太阳辐照量的计算方法及相应的计算程序.这些将为系统的优化设计创造有利的条件,对于提高太阳能供热和光伏系统的可靠性和降低投资费用都具有重要的作用.     

Solar global UV (280–380 nm) radiation and its relationship with solar global radiation measured on the island of Cyprus

We report the first observations of solar global UV (280–380 nm) radiation on the island of Cyprus obtained during an ongoing joint research project between the University of Athens and the Meteorological Service of Cyprus. Hourly global UV (G_uv) and global (G_h) solar irradiances are measured and the relationship between the two radiant fluxes is investigated at Athalassa, Cyprus (35°15′N, 33°40′E, 165 m above MSL). These data are used to determine the temporal variability of the percentage ratio of solar global UV to solar global irradiation (G_uv/G_h) and its dependence on various atmospheric conditions. Analyzing the data set, an inverse correlation between ozone column amount and the precise UV/Global ratio was found, which can be rather attributed to the change of various atmospheric parameters than the ozone column. The analysis of hourly percentage ratio values reveals a definite daily pattern with lower values during sunrise/sunset and higher values around noon. The variation of the percentage ratios (G_uv/G_h) ranges from 3.95±0.29% in September to 2.92±0.42% in August for hourly values, while for daily values, the variation is between 2.85±0.32% in August and 3.68±0.1% in September, with annual mean values of 3.33±0.21% for hourly and 3.19±0.17% for daily data. Finally, the present data reveals a seasonal contribution of the aerosol extinction on the precise (G_uv/G_h) ratio values.     

Testing of two models for computing global solar radiation on tilted surfaces

Solar radiation data computerly measured at Amman, Jordan was used to verify the performance accuracy of two models for estimation of global radiation on tilted surfaces. The measurements used in this study were the daily totals of global radiation on a horizontal surface, diffuse radiation on a horizontal surface and global radiation on a 45° tilted surface. The measured radiation using microprocessor based weather station was compared with the computerized results obtained from two widely used models for estimating total radiation on tilted surfaces. These are the isotropic model and Hay’s anisotropic model. The study showed that the models are in good agreement with the isotropic model producing slightly better results during the summer months. Hay’s model resulted in better results for the rest of the year.     

紫外线知识讲座——紫外辐射定义及其分类

臭氧洞和全球气候变化是本世纪人类面临的两大环境危机,关系着全人类文明的延续和毁灭。这两大环境危机,也为人类开启全新的环境史观。从侧重环境污染问题的解决到鉴定国际公约禁止氟氯碳化物使用、限制温室气体排放总量等,现实环境问题全球化的趋势没有任何个人、地区或国家可以置身事外。 就国内来说,紫外线问题长久以来并未受到应有的重视。而国外自上个世纪80年代以来,在世界卫生组织、世界气象组织、联合国开发计划署等国际机构的倡议下,通过电视、广播及报纸等媒体和网络上获知当天或次日的紫外线指数,使得紫外线议题走进普通百姓生活。此举不但可使公众到户外工作或活动之前,能获得正确的紫外线辐射量的信息以助其采取相应的防护措施,更提高了一般民众避免过度曝晒紫外线的危机意识,使公众健康咨询的水准向前迈进一大步,也是近年来环保工作的一大突破。作为宣传和利用太阳能的媒体,在宣传利用太阳能技术的同时,也有义务普及紫外线知识,让公众了解紫外线。紫外线知识讲座从本期开始,由中国气象局研究员王炳忠先生主讲。     

Estimation of global solar irradiation on horizontal and inclined surfaces based on the horizontal measurements

Solar radiation data are essential in the design of solar energy conversion devices. In this regard, empirical models were selected to estimate the global solar radiation on horizontal and inclined surfaces. The hourly solar radiation data measured at the study area during the period of 2004-2007, were used to calculate solar radiations using selected models. The selected models were compared on the basis of statistical methods. Based on the results, a new model, H/H_o = 0.19490 + 0.4771(n/N) + 0.02994 exp(n/N) has been developed, based on Kadir Bakirci linear exponential model. This is highly recommended to estimate monthly mean daily global solar irradiation, on a horizontal surface. Further, a model to convert horizontal solar global radiation to that of radiation on a tilted surface is also presented. It is based upon a relatively simple model proposed by Olmo et al. which requires only measurements of horizontal solar radiation. The developed model appears to give excellent results and has the advantage of being relatively simple for applications. The present work will help to improve the state of knowledge of global solar radiation to the point where it has applications in the estimation of global solar radiation, both on horizontal and inclined surfaces.     

Prediction of hourly diffuse solar radiation from measured hourly global radiation on a horizontal surface

A statistical procedure has been employed to develop correlations between the hourly global horizontal radiation and its diffuse component. Several years', hourly radiation data from three Canadian stations and two French stations have been employed for this purpose. The relationships have been developed in dimensionless form which predict for particular solar altitudes when is given.Under heavily cloudy conditions or when the sky is completely covered ( ), diffuse radiation increases linearly with the global radiation. In this region, solar altitude has no bearing on the fraction of diffuse radiation.As goes beyond 0.35, the effect of solar altitude begins to appear and the region immediately following this may be considered as partly-cloudy-skies conditions. In the beginning of this region, the diffuse component increases briefly with the increase in global radiation and then begins to decrease as the partly cloudy skies become clearer. At particular solar altitudes, a minimum value of the diffuse radiation is reached. The value of where reaches its minimum value varies with solar altitude.The region beyond which a minimum value of is reached may be considered as mainly-clear-sky conditions. In this region, increases again with , lower solar altitudes giving a higher percentage of diffuse radiation.Under partly cloudy skies and under clear skies, solar altitudes lower than 30° had a marked effect on the fraction of diffuse radiation. However, solar altitudes greater than 30° had minimal influence on the fraction of diffuse radiation.     

Prediction and validation of cloudless shortwave solar spectra incident on horizontal, tilted, or tracking surfaces

The SMARTS spectral model can advantageously be used to predict clear-sky irradiance spectra on surfaces of any tilt and orientation, e.g., for the simulation of spectrally selective technologies like photovoltaic devices or coated fenestration systems. To evaluate the intrinsic accuracy of the model, its current version undergoes here a three-step validation exercise, involving reference radiative transfer codes, and two series of sophisticated spectral and ancillary measurements performed at different locations. Provided that the most important inputs are known with sufficient accuracy, it is concluded that the model performance is very high, with typical differences of 1–2% when compared to reference models, and uncertainties largely within the overall experimental error (≈5%) when compared to spectroradiometric measurements. The effect of imprecise spectral reflectance data of the foreground on the diffuse irradiance incident on shaded vertical surfaces is discussed.     

The estimation of daily, clear sky, solar radiation intercepted by a tilted surface

The amount of solar energy that is intercepted by surfaces of any orientation is estimated from a new model of the clear sky, spatial distribution of solar radiation. The model was developed from measurements made during clear sky conditions and uses direct, isotropic reflected, and anisotropic diffuse radiation. The effects of azimuth, tilt, season, latitude, atmospheric turbidity, and reflectivity of the surroundings were computed using hourly measurements of normal beam and horizontal total radiation at four stations in the United States. A transformation of the co-ordinates of orientation produced a general relationship between orientation and intercepted energy. The general relationship was tested against measurements from six locations in the Northern Hemisphere and was found to be valid. The model is also a better estimator of energy intercepted by a tilted surface than are the more commonly used models.     

第四讲地外斜面辐射量的计算

在太阳能利用工作中,太阳辐射计算十分重要.为了帮助读者掌握太阳辐射计算方法,我们请长期从事太阳辐射研究工作的中国气象科学研究院王炳忠研究员编写了<太阳辐射计算讲座>,供大家学习、参考.     

竖直面上太阳辐射的模拟计算研究

以内蒙古呼和浩特、锡林浩特和阿拉善盟3个地区太阳辐照气象参数为基础,选用Klein模型作为竖直面的辐射计算模型,利用Matlab仿真软件对这3个地区-90~90°太阳方位角内竖直面单位面积所接收到太阳辐照量进行模拟计算。研究结果表明:对于内蒙古各地区,太阳能建筑最佳朝向的太阳方位角为-15~15°,在此太阳方位角范围内竖直面上的太阳辐照量较大;内蒙古各地区太阳能利用的最佳时段为冬季和春季,秋季次之,夏季竖直面上的太阳辐照量最小;内蒙古各地区年总太阳辐照量均为4 604 MJ/m2以上,部分地区年总太阳辐照量最高可达5 515 MJ/m2,整体上属于太阳能资源较丰富区,太阳能资源可开发利用的潜能较大。     

A study of Canadian diffuse and total solar radiation data—II Monthly average hourly horizontal radiation

Optimum collector slope for a liquid base active solar heating system employing flat-plate collectors was investigated. The optimum collector slope was studied as a function of (a) collector area, (b) yearly total heating load and (c) the ratio of space heating load to service hot water load. Collectors facing equator only were considered. Such a system was studied in four different Canadian locations having widely different climates. Under the above conditions, optimum collector slope varied with the amount of collector area employed. The optimum collector slope was invariant with the yearly total load itself, or the space heating to hot water load ratio. Contrary to the widely held belief, for the four locations investigated, the optimum collector slope varied from lat. − 10° to lat. + 15°; depending upon f_y, the fraction of load supplied by the solar system. When f_y is in 10–20 per cent range, optimum collector slope is lat. − 10° and increases almost linearly to lat.+ 15° at f_y in 80 per cent range. Consequently, when the fraction of load by the solar system is low, a flat roof may be profitably employed. On the other hand, when the fraction by the solar system is high, a south facing (for northern hemisphere) vertical wall may be profitably employed.