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 solarsi


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