Direct beam and hemispherical terrestrial solar spectral distributions derived from broadband hourly solar radiation data

Multiple junction and thin film photovoltaic (PV) technologies respond differently to varying terrestrial spectral distributions of solar energy. PV device and system designers are concerned with the impact of spectral variation on PV specific technologies. Spectral distribution data are generally very rare, expensive, and difficult to obtain. We modified an existing empirical spectral conversion model to convert hourly broadband global (total hemispherical) horizontal and direct normal solar radiation to representative spectral distributions. Hourly average total hemispherical and direct normal beam solar radiation, such as provided in Typical Meteorological Year (TMY) data, are spectral model input data. Default or prescribed atmospheric aerosols and water vapor are possible inputs. Individual hourly and monthly and annual average spectral distributions are computed for a specified tilted surface. The spectral range is from 300 nm to 1800 nm. The model is a modified version of the Nann and Riordan SEDES2 model. Measured hemispherical spectral distributions for a wide variety of conditions at the Solar Radiation Research Laboratory at the National Renewable Energy Laboratory, Golden, Co. and Florida Solar Energy Center (Cocoa, FL) show that reasonable spectral accuracy of about ±10% is obtainable with exceptions for weather events such as snow. Differing cloud climatology and variable albedo and aerosol optical depth atmospheric conditions can lead to spectral model differences of 30–40%.     

Investigating the effects of solar modelling using different solar irradiation data sets and sources within South Africa

When designing a solar photovoltaic (PV) system, the main aim of the designer is to optimise the system through the correct choice in sizing of system components (i.e.: size of PV array versus number of batteries for storage), whilst at the same time keeping the overall systems costs as low as possible. The choice of data provider and method for measuring the solar irradiance data in W/m² for a specific geographical location is therefore a critical determinant to ensure maintenance of the desired level of accuracy for the system design. The use of a data set which either over predicts or under predicts the amount of available solar irradiation at a specific location will therefore affect the electrical performance of the system, as the real world conditions may differ considerably to the data set used in the modelling of the system design. This paper specifically deals with the modelling of an off-grid photovoltaic power supply system using three different sources of solar irradiance data for two specific geographical locations within South Africa. The principal aim of the paper is to show that satellite data over-estimates solar irradiance and hence underestimates the initial cost of a solar PV system needed to meet the load in question.     

Estimates of uncertainty for measured spectra in the SERI spectral solar radiation data base

The Solar Energy Research Institute's (SERI) Solar Radiation Resource Assessment Project collected spectral solar irradiance data and coincident insolation and meterological data from October 1986 to May 1988. This article describes the methods used to derive the wavelength-dependent measurement uncertainty for each spectral scan (i.e., spectra). We list the sources of uncertainty in calibration and field measurements, and present summaries of the magnitudes for each. Finally, we show derived uncertainty envelopes, representative of the 21 combinations of instruments, measurement modes, and measurement periods used during the project. Our results indicate uncertainly of greater than ±20% for the ultraviolet (300 to 400 nm), +5% to − 13% in the visible (400 to 900 nm), and + 8% to − 15% in the near infrared (900 to 1100 nm). Complete details of the analysis summarized here are available in separate SERI technical reports detailing the data base, its structure; and the uncertainty analysis.     

Using reduced data sets ISCCP-B2 from the Meteosat satellites to assess surface solar irradiance

This paper explores the capabilities of a combination of the reduced data set ISCCP-B2 from the Meteosat satellites and the recently developed method Heliosat-2 to assess the daily mean of the surface solar irradiance at any geographical site in Europe and Africa. Firstly, we discuss the implementation of the method Heliosat-2. Secondly, B2-derived irradiances are compared to coincident measurements made in meteorological networks for 90 stations from 1994 to 1997. Bias is less than 1 W m⁻² for the whole set. Larger bias may be observed at individual sites, ranging from −15 to +32 W m⁻². For the whole set, the root mean square difference is 35 W m⁻² (17%) for daily mean irradiance and 25 W m⁻² (12%) for monthly mean irradiance. These accuracies are close to those of similar data sets of irradiance, such as Medias and NASA Surface Radiation Budget. It is concluded that B2 data can be used in a reliable way to produce long-term time-series of irradiance for Europe, Africa and the Atlantic Ocean.     

Enhancing solar cell efficiency by using spectral converters

Planar converters containing quantum dots as wavelength-shifting moieties on top of a multi-crystalline silicon and an amorphous silicon solar cell were studied. The highly efficient quantum dots are to shift the wavelengths where the spectral response of the solar cell is low to wavelengths where the spectral response is high, in order to improve the conversion efficiency of the solar cell. It was calculated that quantum dots with an emission at 603 nm increase the multi-crystalline solar cell short-circuit current by nearly 10%. Simulation results for planar converters on hydrogenated amorphous silicon solar cells show no beneficial effects, due to the high spectral response at low wavelength.     

A method for spectral analysis of concentrated solar radiation

In the year 1994 the DLR in Cologne inaugurated a 20 kW solar furnace for technological and scientific experiments. One of the main applications of the solar furnace is the research on photochemical reactions using concentrated sunlight. For this purpose exact knowledge of the spectral composition and its local distribution is essential. The evaluation of experiments and the validation of computer codes rely on these informations.The solar radiation that is concentrated to high flux densities (up to 4 MW/m²) shows short-time fluctuations and therefore a special measurement technique is required.It is shown how to use the imaging properties of a Czerney-Turner monochromator in combination with video camera detection to achieve a fast spectral analysis. This measurement is actually done along an intersection line through the focus. Moreover the calibration is described and first test pictures are analysed.     

Diffuse and global solar spectral irradiance under cloudless skies

A simple empirical model to calculate solar spectral diffuse and global irradiance under cloudless skies is presented here. This formulation takes into account absorption of radiation by molecules such as O₃, H₂O and the uniformly-mixed gases. Attenuation by Rayleigh scattering and aerosol extinction are included. Aerosol attenuation is calculated through Ångström's turbidity parameters α and β. The diffuse radiation is assumed to be composed of three parts: (1) Rayleigh-scattered diffuse irradiance; (2) aerosol-scattered diffuse irradiance; and (3) irradiance arising out of multiple reflections between the atmosphere and the ground. The global irradiance is the sum of these three components of diffuse irradiance plus the direct irradiance. The input parameters include an extraterrestrial spectrum, zenith angle, turbidity coefficient β, wavelength exponent α, ground albedo _g, water vapor content and ozone content. The model is shown to yield very good results up to air mass two when compared to accurate theoretical calculations. No comparisons with measured spectra are presented because of a lack of accurate specifications of input parameters. Results are presented to show the effect of variation of certain of the input parameters.     

硅光电池的光谱响应测试技术

以精密单色仪、斩波器SR540、卤钨灯光源、样品室和自主研发的交流放大滤波电路等硬件搭建成一套光谱响应测试系统。该系统软件以VC++6.0为开发平台,主要实现了计算机对单色仪中的扫描仪参数设置以及波长连续扫描控制,精确测量波长400~1 100 nm的光谱响应值;利用光谱响应理论公式,采用最小二乘法可对单晶硅的性能参数(SP、LP和SN、LN)实现自动拟合。     

Experimental validation of a spectral direct solar radiation model

A model of spectral direct radiation of the sun at the ground compared with the spectral measurements performed by a monochromator has evidenced a satisfactory agreement. However, some discrepancies, mainly in the shorter wavelength range, have been evaluated. In order to point out the causes of these discrepancies, long series of data of direct integral radiation, water vapour content, aerosol turbidity together with standard meteorological data have been performed. These causes are defined by means of a statistical analysis.     

Quality control for solar radiation data

Solar radiation values are difficult to predict or to estimate, due to the multitude of factors affecting radiative transfer in the atmosphere. Whether considering a network of stations or several radiation sensors at a single station, proper calibration procedures of both sensors and data acquisition systems are mandatory.This paper presents a method whereby the interrelationships of signals received from different radiometers at a single site are systematically checked as a means of data control. The various measurements discussed include direct solar, global, diffuse solar and net radiation, albedo, measurements on inclined surfaces and in various bandpasses. The data control method is accomplished by examining ratios, combinations, and comparisons of various measured parameters in addition to calculating some of these parameters by the use of empirical formulae.     

Analysis of short-term solar radiation data

Solar radiation data are available for many locations on an hourly basis. Simulation studies of solar energy systems have generally used these hourly values to estimate long-term annual performance, although solar radiation can exhibit wide variations during an hour. Variations in solar radiation during an hour, such as on a minute basis, could result in inaccurate performance estimates for systems that respond quickly and non-linearly to solar radiation. In addition, diffuse fraction regressions and cumulative frequency distribution curves have been developed using hourly data and the accuracy of these regressions when applied to short-term radiation has not been established. The purpose of this research is to investigate the inaccuracies caused by using hourly rather than short-term (i.e., minute and 3 min) radiation data on the estimated performance of solar energy systems. The inaccuracies are determined by examination of the frequency distribution and diffuse fraction relationships for short-term solar radiation data as compared to existing regressions and by comparing calculated radiation on tilted surfaces and utilizability based on hourly and short-term radiation data.     

Bidirectional spectral reflectance and directional-hemispherical spectral reflectance of six materials used as absorbers of solar energy

Measurements are presented of the bidirectional spectral reflectance and directional-hemispherical spectral reflectance of Nextel-2010 black velvet, Solarcoat-50, Cusorb, Skysorb, Maxorb and Solarcoat-100 at wavelengths of 633 nm and 1152 nm. The data are consistent with surface form as revealed by scanning electron micrographs and surface roughness measurements.     

Short wavelength spectral response of the tandem junction solar cell

The effect of surface recombination velocity on the short wavelength response of the tandem junction solar cell is investigated theoretically. The analysis predicts approximately the same dependence of the short-circuit current on surface recombination velocity as for a conventional p-n junction solar cell. It is concluded that the experimentally observed improvement in the blue response of the tandem junction solar cell is probably due to a small surface recombination velocity as a result of the absence of front-surface metallization.     

Estimation of continuous solar spectral distributions from discrete filter measurements

Techniques exist for estimating a well-behaved continuous function given a relatively small number of carefully chosen samples of that function. Two of these methods are applied to the problem of obtaining a low-resolution spectral distribution of direct solar radiation from a few filter samples. The minimum variance estimation and cubic spline fitting techniques are different approaches which yield integrated irradiances within one percent of each other and the test spectrum. They both produce reasonable averaged spectra over most of the spectral interval tested.     

A presentation of solar irradiation data suitable for solar energy application

Temporal global solar radiation patterns are discussed for two climatically different U.S. stations (Phoenix, Arizona and Geneseo, New York). A concept of the solar utilization season is developed along with methodology to assess the frequency of periods of consecutive days when solar irradiation values do not attain specified threshold values. This analysis is considered more appropriate in applications of solar energy than the common use of means and assumed Gaussian data properties that can be obtained from several compilations and summaries of national solar irradiation data.     

Reliability analysis of solar photovoltaic system using hourly mean solar radiation data

This paper presents the hourly mean solar radiation and standard deviation as inputs to simulate the solar radiation over a year. Monte Carlo simulation (MCS) technique is applied and MATLAB program is developed for reliability analysis of small isolated power system using solar photovoltaic (SPV). This paper is distributed in two parts. Firstly various solar radiation prediction methods along with hourly mean solar radiation (HMSR) method are compared. The comparison is carried on the basis of predicted electrical power generation with actual power generated by SPV system. Estimation of solar photovoltaic power using HMSR method is close to the actual power generated by SPV system. The deviation in monsoon months is due to the cloud cover. In later part of the paper various reliability indices are obtained by HMSR method using MCS technique. Load model used is IEEE-RTS. Reliability indices, additional load hours (ALH) and additional power (AP) reduces exponentially with increase in load indicates that a SPV source will offset maximum fuel when all of its generated energy is utilized. Fuel saving calculation is also investigated. Case studies are presented for Sagardeep Island in West Bengal state of India.     

Quality control of solar radiation data within the RMIB solar measurements network

Assessment of the solar resource is based upon measured data, where available. However, with any measurement there exist errors. Consequently, solar radiation data do not exhibit necessarily the same reliability and it often happens that users face time series of measurements containing questionable values though preliminary technical control has been done before the data release. To overcome such a situation, a major effort has been undertaken at the Royal Meteorological Institute of Belgium (RMIB) to develop procedures and software for performing post-measurement quality control of solar data from the radiometric stations of our in situ solar monitoring network. Moreover, because solar energy applications usually need continuous time series of solar radiation data, additional procedures have also been established to fill missing values (data initially lacking or removed via quality checks).     

The sun’s total and spectral irradiance for solar energy applications and solar radiation models

Using the most recent composite time series of total solar irradiance spaceborne measurements, a solar constant value of 1366.1 W m⁻² is confirmed, and simple quadratic expressions are proposed to predict its daily value from the Zurich sunspot number, the MgII index, or the 10.7 cm radio flux index. Whenever these three indices are available on a daily basis (since 1978), it is possible to predict the sun’s irradiance within 0.1% on average, as accurately as current measurements.Based on this value of the solar constant, an improved approximation of the extraterrestrial solar spectrum from 0 to 1000 μm is proposed. It is obtained by dividing the spectrum into nine bands and selecting representative (and recent) spectra, as well as appropriate scaling coefficients for each band. Comparisons with frequently used spectra are discussed, confirming previous findings of the literature.This synthetic and composite spectrum is proposed at 0.5-nm intervals in the UV (280–400 nm), 1-nm intervals between 0–280 and 400–1705 nm, 5-nm intervals between 1705 and 4000 nm, and progressively larger intervals beyond 4 μm, for a total of 2460 wavelengths.