Direkt Güneş Işınımının Spektral Dağılımının Belirlenmesi

Abstract

Bu çalışmada, İstanbul (41.1°N ve 29.0 E°) için açık bir atmosferde yeryüzeyine ulaşan direkt ışınımın spektral dağılımının belirlenmesi amacıyla, ölçüm çalışmaları ve model hesaplamaları birlikte yürütülmüştür. Bird ve Riordan (1986) tarafından ileri sürülen bu modelde, matematiksel ifadeler ile basınç, sıcaklık, bağıl nem, görüş uzaklığı gibi yer ölçümleri kullanılmaktadır. Görüş uzaklığına bağlı olarak, türbidite katsayısının bulunduğu bağıntılar çıkartılmış, bunların yerine doğrudan doğruya, pirhelyometrik ölçümlerle hesaplanan türbidite katsayıları giriş bilgisi olarak verilmiştir. Atmosfer dışına gelen ışınıma çeşitli atmosfer bilelenlerinin, geçirgenlik fonksiyonlarının etkisi ilave edilmiştir. Rayleigh saçılması, subuhan ve ozon absorbsiyonu, aerosoller ve gazlar tarafından azaltılma ile ilgili geçirgenlik fonksiyonları gözönüne alınmıştır. Hesaplamalar saçılma ve absorbsiyon olaylarında önemli olan 0.3-4.0 um arasındaki 122 dalgaboyunda yapılmıştır. Modelin gerçeklenmesi amacıyla, tüm spektrum boyunca ve belirli spektral bantlardaki pirhelyometrik ölçümler ile modelden elde edilen değerlerin karşılaştırılması yoluna gidilmiştir. Ancak spektral bant değerlerinin bulunmasında, eşit olmayan bu dalgaboyu aralıkları interpole edilerek, sayısal integrasyon yöntemiyle hesaplamalar yapılmıştır. Pirhelyometrik ölçümler san (OG1) ve kırmızı (RG2) filtreleriyle yapılmıştır. Bu filtrelerin ölçüm aralıkları sırasıyla 0.530-2.8 um ve 0.630- 2.8 um dalgaboylandır. Modelden elde edilen değerlerle hesaplanan değerler arasındaki uyum araştırılmış ve ortalama bağıl hataların bu tür çalışmalar için kabul edilebilen şuurlar içerisinde olduğu bulunmuştur. Ayrıca atmosferde değişken olan subuhan miktarı ve aerosollerin güneş ışınımının spektral dağılımı üzerindeki etkileri incelenmiştir. Ultraviyole, görünür ve infrared ışınım bölgeleri için bu etkiler ayn ayn hesaplanarak, absorbsiyon ve saçılma sonucunda spektral ışınımı azaltmaları da detaylı bir şekilde araştırılmıştır.

In recent years, mainly due to its renewable and nonpollutant character, solar energy (utilization) has been a subject of utmost importance. The knowledge of the solar radiation on the earth's surface is essential to many solar conversion systems in terms of their desian, size selection, performance efficiency; heating and cooling the buildings and other energy problems. In the past, researchers were being tought that, knowing the total radiation was enough to solve these problems. But in recent studies, the spectral distribution of solar radiation has been an important subject For industrial, medical and biological applications, not only the total energy but also the spectral distribution of the sunlight is important. For instance, the selection of materials used in the buildings (the degration of colours, paintings or sensitive materials as a fiinction of their uses), the evaluation of electrical energy avahable from the use of solar cells, the study of growth and photosynthetic activity of plants in function of their spectral sensitivity and the evaluation of the UV radiation responsible for skin cancer. Because of having the large application fields, there is a general need for knowing the spectral distribution of radiation and to what extent changes in the meteorological factors effect this energy distribution in addition to the affect on the total energy received With increasing importance of spectral irradiance, the study of the spectral climatological structure of a selected region has become useful in fields of meteorology, architecture, agriculture, hydrology and solar energy. The recent studies are related with the modelling and prediction of the direct spectral radiation at the earth. Different spectral models for the solar radiation have been presented by many researchers which are suitable for technical applications. The spectral distribution of the direct solar radiation reaching the surface of the earth depends on a number of factors; water vapor, ozone, aerosol particle and uniformly mixed gases. The scattering and absorbtion of the solar radiation by these components produce a remarkable attenuation of the direct solar irradiance. In this thesis, a spectral model which has been presented by Bird and Riordan, 1986, has been used in Istanbul (41.1° N ; 29.0°E). The spectral model for cloudless VI days uses simple mathematical expressions and the measurements at the surface which is easily accessiaWe to generate direct horizontal irradiance. The primary significance of this model is its simplicity, which allows its use on small computers. The spectrum produced by this model is limited to 0.3-4.0 urn wavelength. The model gives a description of the physical behavior of the atmosphere such as absorbtion and scattering with related data, as well as the climatological state of the atmosphere. The first chapter of this study is devoted to a state of the art review of the related studies. In doing so, complementary to the models to estimate direct radiation special attention has been given to the studies on solar constants, absorbtion and scattering of solar radiation. Direct solar radiation and spectral distribution of it have been given in the second chapter. Absorbtion of direct component of the spectral solar radiation by ozon, water vapor, uniformly mixed gases and scattering by aerosols have been extensively rewieved. Direct solar radiation has been varied considerably for any given region especially with local atmospheric conditions, time of the day, seasons of the year. The spectral distribution of direct solar radiation is altered as it passes through the atmosphere by absorbtion and scattering. The amount of the attenuated radiation depends on the path length of the solar rays through the atmosphere and the content of the water vapor, ozone, carbondioxide and aerosol particles in the atmosphere. In the third chapter, a spectral model for estimation direct radiation has been represented. In the model, air temperature (degrees Celcius), atmospheric pressure (hPa), percentual relative humidity, horizontal visibility (km) and ground albedo are given for the definition of the climatological state of the atmosphere. Horizontal visibility is used for computing of the Angstrom turbidity cofficient, (3. But the relation between horizontal visibility and turbidity cofficient is valid in the limited range and the observations of the horizontal visibility has not enough sensitivity. So, in this study, turbidity cofficient is used directly instead of horizontal visibility. Pyrheliometric measurements and a computer programme are used for the determination of the turbidity cofficient. In this model, water vapor, ozone, uniformly mixed gases and aerosols were assumed as reducing factors of irradiance. This led to the following expression in which the direct irradiance appears as multiplication of extraterrestrial irradiance and transmittance of atmospheric attenuation components Ij. = hk T,* TaA, ToA. Twx TuX where, Iox. is extraterrestrial spectral irradiance. The spectral atmospheric transmittance functions are Trx ; after Rayleigh scattering T^ ; after the attenuation by aerosol T0}, ; after the absorbtion by ozon layer VII Twx ; after the absorbtion by water vapor Tux ; after the absorbtion of the uniformly mixed gas (02, C02, CH4, v.b.). The selected wavelengths were based on spectra measured at totally 122 wavelenghts between (0.3-4.0 urn). The extraterrestrial irradiance spectrum in the above band, accounting for % 98 of solar constant (1339 of total 1367 W/m2 ) is given according to Neckel and Labs (1981). T,* ; The spectral transmittance after Rayleigh scattering is computed by the function of the relative air mass, (M). The relative air mass (the ratio between the oblique optical path length to the vertical path in the zenit direction), a function of the sun zenith angle, is computed, as indicated by Kasten (1966). The pressure corrected relative air mass, M' = M P/Po where P is the actual atmospheric pressure and Po is the standart atmospheric pressure. Tax is the spectral transmittance after the attenuation by aerosols. It is computed by the function as follows. TaX = exp(-MpX'a) the exponent is the relative air mass times the Angstrom turbidity. Twx. is the spectral transmittance after the absorbtion by the atmospheric water vapor is computed as a function of the relative water vapor mass, the precipitable water vapor height and the spectral water vapor cofficient which is given according to Neckel and Labs (1981). Here the relative water vapor is used instead of air mass, although the difference is very small. The relative water vapor mass is computed as a function of zenith angle. A small departure from relative air mass, increasing with zenith angle, is due to the fact that water vapor is mainly concentrated in the lower troposphere. The spectral transmittances after absorbtion by ozone layer are defined as a function of ozone amount, the spectral ozone absorbtion cofficient and relative ozone mass (ao), according to this equation, To = exp (- ao 03 Mo ) O3 is the surface density of the volume of ozone contained in the vertical column, reduced at normal temperature and pressure, or the NTP ozone amount, computed for each day of the year, at the given site. T"x is the spectral transmittance after the absorbtion of the uniformly mixed gas also defined as a function of the spectral absorbtion cofficient for the uniformly mixed gas (unit: 1/ km) and the pressure corrected relative air mass. VIII With a computer programme using these equations of transmittance, the spectral distributions for the 16 selected clear days which belongs the 1993 and 1994, are defined. The extraterrestrial and direct irradiance at the surface are presented both for the selected day (1 1 Agust 1994). The temperature, atmospheric pressure, relative humidity, turbidity cofficients which define the climatological state of the atmosphere, are represented on the figures. The observed values using this study, are obtained at Istanbul Technical University, Maslak, Meteorological Observation Station. Attenuation of direct solar radiation is maximum in visible region (0.38-0.78 urn). In this region the scattering is important. Water vapor and carbondioxide have absorbtion bands in the ultraviole and infrared region and they caused attenuation of the spectral solar radiation at the earth surface in these regions. For defining the validation of the model a calculated and measured values have been compared in the certain spectral bands (red; 0.630-2.8 urn, yellow; 0.530- 2.8 um ) and total spectrum. Using the spectral distributions of the selected clear days the direct solar irradiances reaching the earth have been calculated with the numerical integration. But for the equation of the wavelength steps interpolation has been made. These results are presented on the respective tables. There is a good agreement between the calculated and measured values. The relative errors belongs to the selected clear days, Red filter : 0.039 Yellow filter : 0.035 The total spectrum: 0.052 The atmospheric components produce a remarkable attenuation of the direct component of solar irradiance. In this study water vapor and aerosols have been considered as the most important components at the lower troposphere. Also the amount of the solar radiation reaching the earth depends on the path length. For determining the attenuation effects caused by these parameters have been calculated for a selected day at the mean earth-sun distance. Atmospheric temperature, relative humidity, atmospheric pressure and turbidity cofficient belonging to this day have been considered as the monthly average values. Effects of these parameters on the spectral distribution of direct radiation have been investigated as considered possible minimum and maximum values in the optical air mass, relative humidity and turbidity cofficient. Direct radiation values have been effected the variations on the optical air mass especially between the (0.3-1. lum). Whenever the optical air mass increases, path length of the solar radiation also increases. Therefore, the scattering caused by ozone, aerosols and air molecule, increases. IX The atmospheric relative vapor varies with time and location. According to the sample results, relative humidity effects the spectral distribution nearly in the infrared region. In order to investigate the effects of atmospheric aerosols the variations in the Angstrom turbidity cofficient have been determined. Angstrom turbidity cofficient has been given as 0.40 for the polluted air. Here the spectral distributions have been calculated for the J3 = 0.10, 0.20, 0.30 and 0.40. Turbity cofficient is directly proportional with the concentration of the aerosols in the atmosphere. Scattering is important especially in the UV and in the visible region. In the infrared region, absorbtion effects are more important than the scattering effects. The difficult calculation of the spectral composition of clear sky direct radiation can be handled with fair accuracy within the framework of the model. This simple model for direct horizontal spectral irradiance for clear sky condition produce results that agree very well with measurement data. The model is simple enough that it should be suitable for anyone desiring spectral data, and it requires a very small computing capability.