晴空条件下光照度和辐射照度的关系

利用国际上知名软件ＬＯＷＴＲＡＮ７,分别计算晴空时总日射和散射条件下辐照度与光照度之间的定量关系,得到不同大气能见度和太阳高度对其影响规律。同时发现,光合有效辐射波段的总日射辐照度和散射辐照度与光照度的关系是线性的,而全波段的总日射辐照度和散射辐照度与光照度的关系是非线的。计算结果与Ｂｒｏｗｎ的测量结果相当接近,证明了计算的可靠性。     

分光总日射表校准因子的迭代计算法

详细讨论了分光总日射表的校准方法。论证了以一台直接日射表和一台遮蔽的总日射表为标准的校准方法优于仅用一台直接日射表为标准的校准方法，并提出了分光总日射表校准因子的迭代计算法。该方法可以解决由于遮蔽的总日射表校准因子不准确而产生的误差。最后还讨论了在校准过程中时间准确的重要性，指出至少应准确到分，否则每分的误差通过太阳高度角的正弦会给直接日射值带来０．２％的系统误差。     

PSP总日射表热偏移特征及其测量总辐射误差分析

根据2004年7月～2006年5月期间北京上甸子大气区域本底站Eppley PSP总日射表观测太阳辐射数据,分析了PSP表的热偏移特征及其影响因子,并以Eppley8-48型日射表和NIP直射表分别观测的散射和直射辐射之和作为总辐射的标准值,分析了PSP总辐射的误差特点及其影响因素.研究表明:PSP总日射表的热偏移呈现正态分布特征,平均值为-8.8±2.7W·m-2;PSP表热偏移显著受到净长波辐射、风速、相对湿度和温度的影响,其中与风速和相对湿度的相关性最为明显.与标准的太阳总辐射观测值(水平面上的散射与直射辐射之和)相比,未做热偏移订正的PSP总辐射的平均误差约为-20.0W·m-2,经过当日平均热偏移订正后,平均误差接近-11.0W·m-2.PSP表测量的总辐射绝对误差也呈准正态分布特征,但晴天与云天略有区别.PSP总辐射的相对误差与太阳天顶角的变化以及云量均有相关关系,反映了半球型辐射表的余弦效应对PSP总日射表测量精度的影响.太阳辐射的强弱变化或总辐射中散射与直射所占比率的相对变化也对PSP总日射表的测量精度有一定影响.研究表明:总日射表测量的总辐射应考虑热偏移订正,总日射表标定还应考虑云的影响.     

光电型总日射表性能的初步研究

对光电型总日射表与ＰＳＰ型总日射表进行了对比测试。在带有圆柱形余弦修正器时光电型总日射表的灵敏度尚佳。在太阳高度角小型３５°、没有温度补偿或将传感器与二次仪表做成一体时其误差较大     

TBQ—2型总日射表热滞后效应的理论分析

总日射表自曝光至热稳定经历了从非稳态传热到稳态传热的过程。采用集总热容法建立了热端的换热微分方程，通过求解得到了达到热稳定之前，总日射表输出电压随时间变化的函数关系，估算了热稳定时间的数量级，得到了热稳定时间与总日射表的物性参数，工作条件之间的关系。与实验结果作了对比。     

总日射表石英罩增透效应的研究

对双层球形玻璃罩的增透作用进行理论分析，以ＴＢＱ－２型总日射表为例进行计算，其外罩有效透过率的计算结果与实测结果相近。     

国产紫外总日射表性能测试(Ⅱ):室外测试及与国外同类产品比较

介绍了几种国产紫外总日射表与美国YankeeUVB-1型紫外总日射表和EppleyTUVR型紫外总日射表在室外进行半年对比试验的结果。发现:在几种国产紫外总日射表中,从性能上讲,无论是与TUVR型的相关性,还是余弦响应,均以UVB-2型总日射表为佳,另外,该总日射表还具有自动恒温装置,从而可免受环境温度的干扰;原计划在对比中以YankeeUVB-1型紫外总日射表作为UVB波段总日射表的相对标准,通过试验发现,它与任何一种紫外总日射表之间的关系均呈非线性。通过对其余弦响应实际测量发现,由于该紫外总日射表并无余弦修正装置,余弦响应不良也就不足为奇了。YankeeUVB-1型总日射表虽与EppleyTUVR型总日射表在测量波段上并不一致,但通过理论计算发现,在太阳高度较高时(>40°),二者之间还是存在着较好的线性关系。可以借此判断仪器性能之优劣。     

TBQ－2型总日射表热滞后效应的理论分析

总日射表自曝光至热稳定经历了从非稳态传热到稳态传热的过程。采用集总热容法建立了热端的换热微分方程，通过求解得到了在达到热稳定之前，总日射表输出电压随时间变化的函数关系，估算了热稳定时间的数量级，得到了热稳定时间与总日射表的物性参数、工作条件之间的关系。与实验结果作了对比。     

武汉地区太阳总辐射与气象要素的关系研究

利用武汉市2007年1月1日至2008年12月31日逐时太阳总辐照量和气象要素(气温、相对湿度、云量等)资料,分析武汉地区逐时太阳总辐照量与逐时气象要素的关系,找出对总辐照量有显著贡献的因子,对比各个气象要素对太阳总辐照量的贡献,利用2007年总辐照量和常规气象要素分季节建立逐时多元线性回归方程,并利用回归方程分季节逐时预报2008年总辐照量,最后进行误差分析。结果表明:太阳总辐照量夏季最多,冬季最少;太阳总辐照量与气温、相对湿度、水汽压及云量(低、总)有关,其中相对湿度及云量(低、总)与总辐照量负相关。分季节分时次建立回归方程发现:春季总辐照量与相对湿度、气温和总云量关系较密切;夏季总体上与春季类似,但气温对总辐照量的贡献不大;秋季总辐照量与气温、总云量、相对湿度、水汽压有关;除了和秋季相同的影响因子外,低云量对冬季总辐照量也有一定的贡献。     

总日射表热滞后效应的实验研究—实验装置和结果

介绍了总日射表热滞后效应的测量装置、实验步骤和数据处理方法，并提出总日射表热稳定时间概念。采用微机控制的自动采集系统，测定了常用的六种总日射表的热稳定时间，其值都比铭牌上标明的时间常数长得多。     

Estimation of the diffuse radiation fraction for hourly,daily and monthly-average global radiation

Hourly pyrheliometer and pyranometer data from four U.S. locations are used to establish a relationship between the hourly diffuse fraction and the hourly clearness index k_T. This relationship is compared to the relationship established by Orgill and Hollands and to a set of data from Highett, Australia, and agreement is within a few percent in both cases. The transient simulation program TRNSYS is used to calculate the annual performance of solar energy systems using several correlations. For the systems investigated, the effect of simulating the random distribution of the hourly diffuse fraction is negligible. A seasonally dependent daily diffuse correlation is developed from the data, and this daily relationship is used to derive a correlation for the monthly-average diffuse fraction.     

Analysis of experimental data on diffuse solar radiation in Athens,Greece, for building applications

Proper design and performance prediction of solar energy systems requires accurate information on the availability of solar radiation. The diffuse-to-total radiation correlation, originally developed by Liu and Jordan, has been extensively used as the technique and provided excellent results, although it was latitude dependent and not universally applicable. Thus, diffuse fraction correlations of this type have been developed by few other authors and for different location. This paper presents an analysis of hourly diffuse radiation on a horizontal surface. Hourly pyranometer data from Athens, Greece, are used to establish relationships between the diffuse fraction and the clearness index k _T for hourly and daily values. The results of the proposed equations are then compared with earlier equations. For the urban conditions of Athens the developed correlation fit to the empirical data.     

Study of diffuse and global radiation characteristics in India

Recently, the Indian Meteorological Department has made available, for 13 locations in India, pyranometric data for total and diffuse radiation on an hourly and daily basis. The period of observation is from 1957 to 1975. This data is analysed to reexamine the correlations between monthly-average daily values of diffuse and total insolation and between hourly and daily insolation. The relationship between monthly diffuse/total and total/extraterrestrial ratios is found to be linear. The present correlation implies that the diffuse component is significantly larger than that predicted by other correlations. No noticeable effect of location or seasonal variation was found. Comparison of the present data with earlier studies for India indicates a trend of increasing diffuse radiation with the lapse of time. The ratio of hourly to daily insolation agrees with the Liu and Jordan correlation for total insolation but differs significantly for diffuse radiation. The need for refinement of the Liu and Jordan correlation between hourly and daily diffuse radiation is pointed out.     

A model for calculating hourly global solar radiation from satellite data in the tropics

A model for calculating global solar radiation from geostationary satellite data is presented. The model is designed to calculate the monthly average hourly global radiation in the tropics with high aerosol load. This model represents a physical relation between the earth-atmospheric albedo derived from GMS5 satellite data and the absorption and scattering coefficients of various atmospheric constituents. The absorption of solar radiation by water vapour which is important for the tropics, was calculated from ambient temperature and relative humidity. The relationship between the visibility and solar radiation depletion due to aerosols was developed for a high aerosol load environment. This relationship was used to calculate solar radiation depletion by aerosols in the model. The total column ozone from TOMS/EP satellite was employed for the determination of solar radiation absorbed by ozone. Solar radiation from four pyranometer stations was used to formulate the relationship between the satellite band earth-atmospheric albedo and broadband earth-atmospheric albedo required by the model. To test its performance, the model was used to compute the monthly average hourly global radiation at 25 solar radiation monitoring stations in tropical areas in Thailand. It was found that the values of monthly average of hourly global radiations calculated from the model were in good agreement with those obtained from the measurements, with the root mean square difference of 10%. After the validation the model was employed to generate hourly solar radiation maps of Thailand. These maps reveal the diurnal and season variation of solar radiation over the country.     

Solar radiation models—A review

Solar radiation models for predicting the average daily and hourly global radiation, beam radiation and diffuse radiation on horizontal surface are reviewed in this article. Estimations of monthly average hourly global radiation from daily summations are discussed. It was observed that Collares‐Pereira and Rabl model as modified by Gueymard (CPRG) yielded the best performance for estimating mean hourly global radiation incident on a horizontal surface for Indian regions. Estimations of monthly average hourly beam and diffuse radiation are discussed. It was observed that Singh‐Tiwari and Jamil‐Tiwari both models generally give better results for climatic conditions of Indian regions. Therefore, their use is recommended for composite climate of Indian regions. Empirical correlations developed to establish a relationship between the hourly diffuse fraction and the hourly clearness index using hourly global and diffuse irradiation measurements on a horizontal surface are discussed. Fifty models using the Angstrom–Prescott equation to predict the average daily global radiation with hours of sunshine are considered. It was reported that Ertekin and Yaldiz model showed the best performance against measured data of Konya, Turkey. Copyright © 2010 John Wiley & Sons, Ltd.     

Assessment of diffuse solar energy under general sky condition using artificial neural network

In this paper, artificial neural network (ANN) models are developed for estimating monthly mean hourly and daily diffuse solar radiation. Solar radiation data from 10 Indian stations, having different climatic conditions, all over India have been used for training and testing the ANN model. The coefficient of determination (R²) for all the stations are higher than 0.85, indicating strong correlation between diffuse solar radiation and selected input parameters. The feedforward back-propagation algorithm is used in this analysis. Results of ANN models have been compared with the measured data on the basis of percentage root-mean-square error (RMSE) and mean bias error (MBE). It is found that maximum value of RMSE in ANN model is 8.8% (Vishakhapatnam, September) in the prediction of hourly diffuse solar radiation. However, for other stations same error is less than 5.1%. The computation of monthly mean daily diffuse solar radiation is also carried out and the results so obtained have been compared with those of other empirical models. The ANN model shows the maximum RMSE of 4.5% for daily diffuse radiation, while for other empirical models the same error is 37.4%. This shows that ANN model is more accurate and versatile as compared to other models to predict hourly and daily diffuse solar radiation.     

On shadowband correction methods for diffuse irradiance measurements

Diffuse irradiance, G_d, is an important variable in solar resource assessment. The diffuse irradiance can be worked out from global, G, and direct, G_b, irradiance measurements, but this method involves the use of relatively expensive tracking mechanisms. Alternatively, a widely accepted technique uses a pyranometer with a shadowband. Because the shadowband screens the sensor from part of the diffuse radiation coming in from the sky, a correction must be made to the measurements. However, because of the anisotropy of diffuse radiation it is difficult to compute an exact theoretical correction. In this study we use two data sets registered in two locations in Spain. The first one consists in coincident hourly values of global, direct, and diffuse irradiance; the latter by means of shadowband. The other data set includes the same variables but as 5-minute values. Our goal is to study the necessary correction factor for diffuse irradiance measurements obtained by means of shadowband. After testing several well-known correction methods, we have developed two different correction models, using two-thirds of the hourly data set, while the remaining one-third and the whole 5-minute data set have been used for validation purposes. The last validation test suggests that our anisotropic models provide reliable corrections for conditions different than the ones where they have been developed. The results obtained by the developed models show a negligible mean bias deviation. Approximately 55% of cases present deviations lower than 5% over the mean value of diffuse irradiance.     

An operational system for mapping insolation from goes satellite data

A system for mapping the mesoscale distribution of insolation from GOES satellite data has become operational at the University of Wisconsin Space Science and Engineering Center. An energy balance model of the earth-atmosphere system is used in conjunction with radiometrically calibrated GOES measurements to estimate surface insolation from satellite images taken at hourly intervals. Integration of these hourly estimates into daily insolation totals has produced results within 9 per cent (standard error of the mean) of pyranometer measurements on a much finer space scale than is available from the surface pyranometer network.The model has been applied, using satellite data display facilities and software designed for large satellite data sets, to produce monthly maps of insolation over large geographical areas at a resolution of approx. 12 × 12 km. Although the density of the surface pyranometer network is inadequate to directly verify the mapping results, the quality of the daily insolation estimates compared with the measurements of single pyranometers suggests that the insolation maps will have a similar accuracy.     

Analysis of diffuse radiation data for Beer Sheva: Measured (shadow ring) versus calculated (global-horizontal beam) values

We have utilized concurrently measured global, normal incidence beam, and diffuse radiation data, the latter measured by means of a shadow ring pyranometer to study the relative magnitude of the anisotropic contribution (circumsolar region and nonuniform sky conditions) to the diffuse radiation. In the case of Beer Sheva, the monthly average hourly anisotropic correction factor varies from 2.9 to 20.9%, whereas the “standard” geometric correction factor varies from 5.6 to 14.0%. The monthly average hourly overall correction factor (combined anisotropic and geometric factors) varies from 8.9 to 37.7%. The data have also been analyzed using a simple model of sky radiance developed by Steven in 1984. His anisotropic correction factor is a function of the relative strength and angular width of the circumsolar radiation region. The results of this analysis are in agreement with those previously reported for Quidron on the Dead Sea, viz, the anisotropy and relative strength of the circumsolar radiation are significantly greater than at any of the sites analyzed by Steven. In addition, the data have been utilized to validate a model developed by LeBaron et al. in 1990 for correcting shadow ring diffuse radiation data. The monthly average deviation between the corrected and true diffuse radiation values varies from 4.55 to 7.92%.     

The assessment of four different correction models applied to the diffuse radiation measured with a shadow ring using global and normal beam radiation measurements for Beer Sheva, Israel

The measurement of the diffuse radiation incident on a horizontal surface, a priori a straightforward task, is fraught with difficulties. It is possible to measure the diffuse radiation by three different techniques: two of which measure it directly and the third indirectly. The most accurate is the indirect one, which is based upon the concurrent measurements of the horizontal global and the normal incidence beam radiation. The disadvantage of this being the relatively expensive tracking system required for measuring the latter. The diffuse radiation can be measured directly with a pyranometer outfitted with either an occulting disk or shadow ring, which prevent the beam radiation from impinging on the pyranometer sensor. The occulting disk can provide accurate measurements of the diffuse radiation but it requires a relatively expensive sun tracking system in the east–west axis. The shadow ring is a stationary device with regard to the east–west axis and blocks the beam radiation component by creating a permanent shadow on the pyranometer sensor. The major disadvantage of the shadow ring is that it also blocks that portion of the diffuse radiation obscured by the shadow ring. This introduces a measurement error that must be corrected to account for that portion of the sky obscured by the shadow band. In addition to this geometric correction factor there is a need to correct for anisotropic sky conditions. Four correction models have been applied to the data for Beer Sheva, Israel and the results have been evaluated both graphically and statistically. An attempt has been made to score the relative performance of the models under different sky conditions.