Measurements of spectral-band solar irradiance in Bangi, Malaysia

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    Solar Energy 89 (2013) 6280Measurements of spectral-band solar irradiance in Bangi, Malaysia

    Yousef A. Eltbaakh a,, M.H. Ruslan b,, M.A. Alghoul b,, M.Y. Othman b, K. Sopian b

    aDepartment of Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, MalaysiabSolar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia

    Received 6 December 2011; received in revised form 10 November 2012; accepted 24 November 2012Available online 19 January 2013

    Communicated by: Associate Editor Christian GueymardAbstract

    In the present study, a series of global spectral-band solar irradiance measurements over a wide range of optical air masses and atmo-spheric conditions in the interval of 4001100 nm is presented. The measurements were obtained continuously using 12 Li-200SA pyr-anometers equipped with different Schott glass, flat, circular, and long-pass filters on a horizontal surface at Universiti KebangsaanMalaysia (2550N, 101460E) between September 1 and November 30, 2011. By combining the measurements obtained using differentfilters, obtaining global solar irradiance in various wavebands is possible. To support the experimental data, the results were comparedwith the simulated results of the Simple Model for the Atmospheric Radiative Transfer of Sunshine (SMARTS2) model. Forecastingperformance parameters such as the normalized root mean square error (NRMSE), the normalized mean bias error (NMBE), and R2

    have been used to test the accuracy of observed data. NRMSE for the whole spectrum varies from 0.7% to 5.3%, whereas NMBE variesfrom 2.1% to 2.3%. The determination coefficient R2 results for all air masses are near 1.0. Simulated and measured data show goodagreement over the whole measured spectrum. The measurement of solar radiation using pyranometers equipped with filters is much lesscomplicated, more compact, and is less costly than using spectroradiometers. 2012 Elsevier Ltd. All rights reserved.

    Keywords: Li-200SA pyranometers; Filters; Experimental measurement; SMARTS2 model; Statistical tests1. Introduction

    Solar radiation energy received at the earths surface isthe basic data in a variety of fields. Knowing the amount(the integral of all electromagnetic radiation, also calledbroadband) of solar radiation is very important formany applications, such as atmospheric energy-balancestudies; analysis of the thermal load on buildings; design-ing, operation, and economic assessment of energy andrenewable energy systems; and for some environmentalimpact analysis (Jacovides et al., 2004; Iqbal, 1983; Muneeret al., 2007). In recent years, due to an increase in terrestrial0038-092X/$ - see front matter 2012 Elsevier Ltd. All rights reserved.

    Corresponding authors.E-mail addresses: (Y.A. Eltbaakh), hafidz- (M.H. Ruslan), (M.A. Al-ghoul).applications of solar energy, the scientific interest hasexpanded from the total amount of solar energy to its spec-tral distribution (Kaskaoutis and Kambezidis, 2009).Many physical and chemical processes are activated morepowerfully at some wavelengths than at others. This condi-tion is especially true and important in the field of solarenergy engineering for the design of certain solar energyapplications such as photovoltaic cells for electric genera-tion and selective absorbers for thermal collectors, andfor practical applications in environmental and agrometeo-rological research (Iqbal, 1983; Jacovides and Kallos, 1993;Gueymard, 2008).

    While global solar irradiance (GSI) measurements arenow routinely made at many locations throughout theworld, global spectral solar irradiance (GSSI) measure-ments are uncommon in many areas throughout the world.This is principally due to the spectral irradiance measuring

  • Nomenclature

    Roman letters

    kt clearness indexk the diffuse ratioV visibilityVr the meteorological rangeVm maximum meteorological range (340.85 km)CA cloud cover amountS sunshine fractionRh relative humidityTLI-200 Li-200SA temperatureTair air temperatureTo reference temperatureE Li-200SA measured irradianceCcorr the factor for correcting the temperature influ-

    ence on Li-200SAf(hz) spectral correction of the global irradiance asso-

    ciated with the solar angle of incidence for Li-200SA

    W(k) smoothing function for broadening the mod-elled spectra to match the bandpass shape andwidth resulting from the combination of differ-ent long-pass filters

    Greek letters

    b Angstrom turbidity coefficienta Angstrom wavelength exponent for the whole

    spectruma1 Angstrom wavelength exponent for k < 500 nma2 Angstrom wavelength exponent for k > 500 nmdA aerosol optical depth

    C1C3 coefficients of Eq. (11)D1D4 coefficients of Eq. (12)hz solar zenith anglek wavelengthac temperature coefficient correction for Li-200SA

    combination with each filterg a constant resulting from the relative definitions

    of V and Vr (g = 1.306)km the wavelength corresponding to 50% of the fil-

    ter transmittance from the main band transmis-sion (also defined as the center of the cutoff)

    kc1 the starting position of the uniform transmit-tance (90%) resulting from the combination ofany two long-pass filters

    kc2 the end position of the uniform transmittance(90%) resulting from the combination of anytwo long-pass filters


    GSI global solar irradianceGSSI global spectral solar irradianceSPPs silicon-photodiode pyranometersUKM Universiti Kebangsaan MalaysiaCC calibration constantAM air massSMARTS Simple Model for the Atmospheric Radiative

    Transfer of SunshinePMs parameterized modelsPRTMs rigorous radiative transfer modelsFWHM full width half maximum

    Y.A. Eltbaakh et al. / Solar Energy 89 (2013) 6280 63systems are more sophisticated and more elaborate calibra-tion process. Furthermore, depending on sensor type, theoperation of spectral radiometers can be labor intensive,often leading to increased costs. Unfortunately, this eco-nomic barrier has contributed to the lack of spectral dat-abases (Morley, 2003). Measurement of GSSI is similarto the measurement of GSI with the important additionalcomplexity that incoming solar radiation must first be sep-arated by wavelength (scanning-type instruments) ordivided based on the interference phenomenon (FourierTransform Instruments) before the detector records the sig-nal (Calisesi et al., 2007; Dyer, 2001).

    Due to the absence of high-resolution spectral measure-ments, which are possible only with very sophisticatedinstruments, a simple and comparatively inexpensiveinstrument, pyrheliometer or pyranometer, in combinationwith glass filters may be used (Jacovides and Kallos, 1993;Iqbal, 1983). Several authors (Kvifte et al., 1983; Iqbal,1983; Jacovides and Kallos, 1993; Utrillas et al., 2000) haveused pyrheliometer equipped with different filters to obtaindirect solar radiation in various wavebands. In this study,an attempt has been made to expand this method toinvestigate global solar radiation, taking into account thediffuse portion of radiation. The present study aims todescribe a simple way of measuring the global spectral-band solar radiation with an inexpensive set-up in the inter-val of 4001100 nm. Experimental and simulation (SimpleModel for the Atmospheric Radiative Transfer of Sunshine[SMARTS2] model) results under cloud-free conditions arecompared in this study.

    2. Experimental site

    The climate of Malaysia is equatorial, being hot andhumid throughout the year. The year can be subdividedinto two distinct seasons. The dry season commences fromthe month of May and continues until September, which isalso the time of the southwest monsoon. The rainy seasonis from the middle of November to March, marked by thearrival of the northeast monsoon (Azhari et al., 2007). Theexperimental work was conducted continuously betweenthe last month of the dry season (September 1) and the firstmonth of the rainy season (November 30, 2011). The atmo-spheric conditions in this period are characterized by heavy

  • 64 Y.A. Eltbaakh et al. / Solar Energy 89 (2013) 6280rainfall. The measurements were taken on the roof of abuilding, at the Physics Department of Universiti Kebang-saan Malaysia (UKM), with geographical coordinates of2550N latitude and 101460E longitude, and with theinstrument altitude of approximately 45 m above sea level.The available area on the roof was about 10 m2 and is sur-rounded by other buildings of the university. The horizonwas unobstructed in all directions from sunrise to sunset,except at the very high solar zenith angles (hz > 85). Themeasurement site was located on the main campus of theuniversity, which spans an area of about 1096.29 hectares,situated in a valley surrounded by hills and green areas inthe district of Bangi, Malaysia. Topographically, the areais moderately flat with several small streams and patchesof swamps and is located at an altitude of 40110 m abovesea level (Bhaskar and Mehta, 2010).

    3. Experimental set-up and methodology

    The system used in the present study has three majorparts:

    3.1. Li-COR pyranometer (Li-200SA)

    Silicon-photodiode pyranometers (SPPs) are nowwidely used for solar irradiance measurements associatedwith solar thermal and photovoltaic power systems, as wellas for agricultural applications (King and Myers, 1997).They are small, light weight, provide the opportunity forlow-cost redundancy, can be calibrated quickly with a solarsimulator and can easily be modified to measure direct nor-mal irradiance (King et al., 1998). SPPs may be handheldor mounted at any required angle, provided that reflectedradiation is not a significant portion of the total. In its mostfrequent application, the pyranometer sensor is set on alevel surface that is free from any obstruction to eitherdirect or diffuse radiation. Among the Silicon-photodiodepyranometers, Li-200SA has been chosen for the presentstudy. Li-200SA is an instrument used to measure globalsolar radiation received from a whole hemisphere in thewavelength range of 4001100 nm. The instrument has arapid response time of about 10 ls, making the sensorappropriate for measuring rapid solar radiation changesFig. 1. Li-200SA spectral response curve.associated with intervals when clouds move in front ofthe sun (Alados-Arboledas et al., 1995). The responseregion of Li-200 SA pyranometer includes about 7075%of the total energy in the terrestrial solar spectral distribu-tion from 300 nm to 4000 nm (Myers, 2011). A typicalresponse curve of Li-200SA pyranometer is presented inFig. 1 (LI-COR, 2005).3.2. Filters

    To determine the irradiance corresponding to differentspectral bands, 12 different Schott glass, flat, circular, andlong-pass filters were used: FGL400, FGL430, FGL495,FGL515, FGL550, FGL590, FGL610, FGL665,FGL715, FGL780, FGL850, and FGL1000. These filtersare specified by their center wavelengths. Angstrom andDrummond (1959) showed that the wavelength corre-sponding to 50% of the filter transmittance from the mainband transmission is defined as the center of the cutoff (km).For longer wavelengths (about 1800 nm), the filter is char-acterized by almost perfected transmission. Shorter wave-lengths (less than cutoff wavelength) are characterized byalmost total opaqueness (Angstrom and Drummond,1961). Long-pass filters are constructed from hard, durablesurface materials covered with dielectric coatings. Thewavelength limits of each filter are nominal values, whichmay vary with temperature. The spectral transmissioncurve of each filter is approximately 90%. Fig. 2 showsthe actual transmissivity curves of two of these filters asobtained from The high cutoffwavelength for all filters is considered as 1100 nm becauseLi-200SA pyranometers are sensitive only up to 1100 nm.Hence, the global solar radiation can be determined inthe following regions: 4001100 nm, 4351100 nm,4951100 nm, 5151100 nm, 5501100 nm, 5901100 nm,6101100 nm, 6651100 nm, 7151100 nm, 7801100 nm,8501100 nm, and 10001100 nm.

    Adding a filter on top of the Li-200SA pyranometer willresult in a complex spectral response because the relativespectral response of Li-200SA does not extend uniformlyover the full solar radiation range of 4001100 nm asshown in Fig. 1. Hence, the only way to correctly calibratethe Li-200SA/filter arrangement would be against a stan-dard lamp in the laboratory or against the sun with theLangley plot method as with sunphotometers or spectrora-diometers. The calibration method used in the presentstudy is as follows: the calibration process is handled foreach Li-200SA-filter independently at the UKM solar sim-ulator laboratory by exposing the Li-200SA-filter to a sta-bilized halogen lamp installed at the end of an opticalbench. The Brillanta halogen lamp is made in Malaysia,with its emission limited to the solar spectrum by a quartzwindow and with output power of 500 W. By reading theoutput current of Li-200SA fixed sensor from the workinglamp, the calibration constant may be found using the fol-lowing equation:

  • Fig. 2. Transmissivity curves of FGL400 and FGL665 long-pass filters. Source:

    Y.A. Eltbaakh et al. / Solar Energy 89 (2013) 6280 65Calibration constant CC

    sensor currentlamp output

    specified unitspecified unit


    The specified unit is an arbitrary amount of measur-able radiation with which a sensor is specified. Each sensortype has a specific value, either 1000 lmol, 100,000 lux, or1000 W m2. For example, if the calibration lamp produces40 W m2 and an Li-200SA sensor puts out 3.6 lA, then

    CC 3:640

    1000 W m2

    1000 W m2 90 lA=1000 W m2

    If the sample sensor produces 12.1 lA, then it is measur-ing radiation of (12.1/90) 1000 = 134.4 W m2.

    Through a series of subtracted reading made with differ-ent filter pyranometers, obtaining incident energy intensi-ties in more wavelength bands is possible. For example,by coupling the filters (FGL400, FGL430, and FGL495),it is possible to evaluate experimentally the GSI in therange of 4001100 nm, 4301100 nm, and 4951100 nm.Z 1100400

    Edk;Z 1100430

    Edk; andZ 1100495

    Edk 2

    where E is the Li-200SA measured irradianceBy combining the three functions, the following bands

    can also be evaluated:Z 430400

    Edk Z 1100400

    Edk Z 1100430

    Edk 3Z 495400

    Edk Z 1100400

    Edk Z 1100495

    Edk 4Z 495430

    Edk Z 1100430

    Edk Z 1100495

    Edk 5

    By combining the 12 filters, it is possible to create a 48-spectral band.

    As previously mentioned, the wavelength limits of eachfilter may vary with temperature and this might introduceuncertainty when evaluating narrow bands, such as withEq. (3). Long-pass filters will shift to longer wavelengthwith increasing temperatures or to shorter wavelength withdecreasing temperature. Thi...


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