Opak yüzeylerde güneş ışınımı etkisinin yüzey pürüzlülüğü ile değişimi
The Variation of solarenergy gain on opaque surfaces with surface roughness
- Tez No: 46490
- Danışmanlar: PROF.DR. İMER SUNGUROĞLU
- Tez Türü: Doktora
- Konular: Mimarlık, Architecture
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1995
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 95
Özet
ÖZET Bu çalışmada, opak dış yüzey pürüzlülük özelliğinin yüzeyin güneş ışınım emiciliğine ve sıcaklığına etkisi deneysel olarak araştırılmıştır. Soğuk veya sıcak dönem etkisindeki opak dış kabuk eleman yüzeyinin güneş ışınım emiciliği, binanın ısı kazanç ve kayıplarını değiştirmektedir. Bu durum iç ortam konfor koşulları ile ısıtma, soğutma amaçlı enerji tüketimi üzerinde etkili olmaktadır. Çalışmanın ikinci bölümünde, güneş ışınımı ile opak yüzeylerin etkileşimi çerçevesinde opak yüzeyleri etkileyen güneş enerjisinin özellikleri, opak yüzeylerin güneş ışınımı karşısında gösterdikleri davranışlar (emme, yansıma, yayınlama) ve renk, iç yapı, nemlilik, pürüzlülük gibi opak yüzey özelliklerinin bu davranışlara olan etkileri açıklanmıştır. Üçüncü bölümde, opak yüzeylerin pürüzlülük özelliği standartlara göre geometrik ve sayısal olarak tanımlanmış ve değerlendirilmesine yönelik yaklaşımlar açıklanmıştır. Çalışmada yüzeyin pürüzlülüğü nedeniyle yüzeye gelen güneş ışınının, pürüzler arasında birkaç defa yansımaya uğrayacağı ve yansıyan ışınların yüzeye tekrar çarpmasının emme oranını arttırdığı vurgulanmıştır. Pürüzlülük özelliğinin güneş ışınım emme oranına ve yüzey sıcaklığına etkisinin belirlenmesi amacıyla yapılan deneyler üç ana grupta toplanmıştır. Birinci grup deneylerde, opak yüzeylerin pürüzlülüklerinin standartlara uygun olarak belirlenmesine yönelik ölçüm metodlan denenmiş ve deneysel çalışmalarda kullanılan malzemelerin pürüzlülüklerinin belirlenebilmesi için bir pürüzlülük ölçüm metodu geliştirilmiştir. İkinci grup deneylerde, malzemenin kısa dalga ışınıma karşı emiciliklerinin belirlenmesi için monokromatik yönlenmiş emme oranlan ölçülmüştür. Üçüncü grup deneylerde ise, güneş ışınımı, kısa dalga ışınım özelliklerine sahip bir lamba ile simüle edilerek lamba ışınımı etkisinde bırakılan malzemelerin yüzey sıcaklıkları ölçülmüştür. Malzemelerin pürüzlülüklerinin güneş ışınım emme oranına ve yüzey sıcaklıklarına etkisinin araştırılmasında, renk ve malzeme iç yapısı gibi yüzey özelliklerinin sabit tutulabilmesi için deneyler, beyaz renkli ve aynı iç yapı özelliklerine sahip farklı pürüzlülükte hazırlanmış dış cephe sıva örnekleri üzerinde gerçekleştirilmiştir. Deneysel çalışmalar sonunda yapılan ölçümler değerlendirilerek, malzemelerin yüzey pürüzlülüklerine göre güneş ışınım emme oranlarının ve yüzey sıcaklıklarının değişimi karşılaştırmalı olarak irdelenmiştir. Opak yüzeylerde pürüzlülük ile güneş ışınım emme oranı etkileşimi ortaya konduktan sonra, emme oranına bağlı dış yüzey sıcaklığı değişiminin opak eleman bünyesindeki ısı akımına etkisi araştırılmıştır. Çalışmanın sonunda, bölgenin iklimsel özelliklerine bağlı olarak opak yüzey özellikleri seçiminin güneş ışınımından kazanılan enerji miktarını etkilediği ve böylece ısıtma, soğutma amaçlı enerji tüketiminin azaltılmasında katkı sağlanabileceği vurgulanmıştır. XII
Özet (Çeviri)
THE VARIATION OF SOLAR ENERGY GAIN ON OPAQUE SURFACES WITH SURFACE ROUGHNESS SUMMARY The subject of employing solar energy for heating and cooling purposes along with providing energy conservation in buildings, is gaining importance in recent years because of the increasing cost of energy. Worldwide research continues aimed at using solar energy as a substitude for conventional sources of energy for heating and cooling of especially buildings. Solar radiation is an important climatic parameter affecting the heating and cooling requirements of buildings. The intensity of solar radiation affecting the earth' s surface, displays a variation with the latitute of a region. Therefore, the requirement for the use of or the protection from solar radiation depends on the location of that region. The capacity of opaque external envelope elements to absorb effectively the incident solar energy in the cold region and to transfer the former to the internal space, increases the solar energy gain of buildings. Their capacity to reflect this energy as much as possible in warm period reduces the gain. From the viewpoint of the problem under consideration, the solar radiation absorptance ratio as a basic opaque surface characteristic is regarded as an important parameter. It represents the amount of solar radiation absorbed by the surface. The increase of external surface temperature of opaque components depends on their ability to absorb more of the solar energy incident on them. Surface characteristics of the opaque component determine the increase of solar absorptance. These are the inner structure, colour, moisture content and surface roughness of the material. In this study the roughness characteristic of material surface is taken into consideration and the effect of surface roughness on solar absorptance and surface temperature are investigated experiments. The effect of surface roughness on surface temperatures appears by changing the solar absorptance of the surface. For this reason, first the effect of surface roughness on solar absorptance is examined, and then by considering surface roughness and solar absorptance together, the change in surface temperature is investigated according to the experimental results. Towards this objective, the characteristics of solar radiation reaching the building envelope are studied theoretically in the second part of this thesis together with the xmradiation characteristics of opaque surfaces and the variables affecting these characteristics. The following points, emerging from the study of previous theoretical and experimental works, are important in reaching the objective:. The amount of solar energy affecting the earth and hence the building surface and the distribution of this energy according to wavelength, acquires different values depending on lattitute, height above sea level and meteorological character of the region as well as on the type of settlement. The size, shape, height, orientation and surface properties of the building envelope affect the amount of solar radiation received by the building at different times during the year. Different regions have different properties and amount of energy gain.. The amount of energy absorbed by the opaque surfaces is affected by the spectral distribution of solar energy.. The solar absorptance of the opaque surface varies with the incidence angle of solar radiation on the surface.. The solar energy absorbed by surface increases the external surface temperature which is an important property as it determines the heat transfer through the material.. The amount of energy absorbed by the opaque surface varies according to the inner structure property of the material. The arrangement of atoms in the material or the arrangement of electrons in the molecules, type of band, crystal or amorphic properties affect the energy absorbed by the opaque surface.. The surface colour is effective in the visible part of the spectrum. The hue, value and chroma variables of colour affect the energy absorbed by the surface in different ways. In general, as the colour gets darker, the amount of energy absorbed by the surface increases.. Increasing the roughness of an opaque surface affected by solar radiation, increases the probability of a solar ray reflected from any point on the surface, hitting the surface again. As a result of the solar ray undergoing multiple reflections between the irregularities, the surface is expected to absorb more energy. In order to verify this investigation, surface roughness of materials have to be defined in a realistic way. That is why, in the third part of the thesis, the surface roughness characteristic is defined in optical, geometrical and numerical terms for an analytical determination of the roughness characteristic of building external surface materials. Furthermore, approaches are explained, which are related to determining the variables defining surface roughness. The complete texture of any surface is a combination of irregularities of various kinds and magnitudes arising from different production processes. The variables definining surface roughness are determined as a result of investigating the irregularities of surfaces within a framework of a comparison system. The complex problem of assesment of surface roughness which basically is three dimensional xivin character, is reduced to two dimensional metrology. The determination of the surface roughness characteristics comprises the following steps: 1. Measurement of the surface with a particular method. 2. Graphical display of the numerical data obtained through measurement. 3. Determination of“Ra”(the distance of irregularities from neutral axis),“Ry”(the distance between the minimum and maximum point of irregularities),“Re”(height of irregularities) parameters defining surface roughness from graphs or directly depending on the method of measurement. The methods used for determining surface roughness are classified as below:. Assessment of the surface by visual and optical methods.. Assessment of the surface by mechanical and electronic contact methods.. Assessment of the surface by photogrammetric projection method. Theoretical assessments reveal that the roughness of opaque surfaces affects the amount of solar energy gained from the surface. Accordingly, differences are expected between the absorbtances and surface temperatures of materials with different roughnesses but the same colour and inner structure properties for a particular range of wavelength and intensity of radiation. The experimental work clarifying this expectation is described in the fourth part of the thesis. This work consists of three series: 1. Tests for determining surface roughness 2. Tests for determining solar absorbtances of materials. 3. Tests for determining surface temperatures of materials under the influence of radiation of certain characteristics. In the first series of tests, applicability of mechanical and electronical contact methods to the measurement of roughness of opaque external surface materials, is investigated. However, since not all samples could be measured with these methods, a new method is devized. In the development of this method an anology is drawn between the roughness of material surfaces and the surface of the earth. The photometric method (photographical projection method) used for surveying the earth surface is applied here. In the second series of tests, the monochomatic directional absorbtances (a^ (n,)) of samples are measured in the 350 nm - 2500 nm wavelength range and 50 nm intervals with a spectrophotometer, in order to express the solar absorbtance of materials with different roughnesses as hemispherical absorbtance (a). The monochomatic directional absorbtance can be defined as the fraction of the incident energy from the direction \i, $ at the wavelength X that is absorbed. The hemispherical solar absorbtance (a) includes all directions as well as all wavelengths and can be considered as a property as the spectral distribution is essentially fixed. xvIn the third series of tests, solar radiation is simulated in laboratory conditions using a lamp emitting radiation with the same spectrum as that of solar radiation. Surface temperatures of the samples subjected to radiation emitted by this lamp, are measured at points with equal distances and the mean surface temperatures are determined. The following conditions are present in the experimental work:. All samples used in tests are prepared in same white colour in order to eliminate the effects of colour on the absoptance.. The samples used in tests are made of the same polymer based commercial external render. Since they all share the same inner structural characteristic, the effect of this characteristic on absorbtance is taken as a constant.. All tests are performed in laboratory conditions and protected from wind effects. Therefore the effect of air movement is also eliminated and the external surface convection coefficient is assumed to be constant.. In the measurement of monochromatic directional absorbtance and surface temperatures, the test is set up so that rays hit the surface perpendicularly. Hence the variation of the absorbtance with the angle of incidence can be neglected.. The effect of direct radiation is taken into consideration on the surface in the tests, but the effects of diffuse and reflected radiations are neglected. In the fifth part of the thesis, measurements of roughness, absorptance ratios and surface temperatures have been evaluated comparatively by means of quantitative results obtained from experiments conducted in line with the objectives of this work. The effect of the roughness characteristic of building external opaque surface material on the energy absorbed by the surface is analized within the framework of following relationships:. The effect of material surface roughness on solar absorbtance.. The effect of surface roughness of materials subjected to solar radiation on surface temperatures. At the end of this part, the effect of the external surface temperatures varying with surface roughness of opaque external envelope element on the heat flow occuring in the opaque element, is theoretically investigated. In the sixth and final part of the thesis, general conclusions reached as a result of the experimental research accomplished within the context of this work:. The solar absorptance of the material increases as the Ra, Ry, Re roughness parameters acquire greater values.. The photogrammetric method is the most accurate measurement method for determining roughness. xvi“Ra”(average roughness value) is the variable which best defines the surface among all roughness variables. As the wavelength of radiation increases, the values of monochromatic directional absorbtances of materials increase as well. Monochromatic directional absorptance ratio curves present similar characteristics. The shape of irregularities on the material surface affects the form and magnitude of the reflection, giving rise to variation in surface temperatures. According to the experimental results, as a result of increasing the average roughness value (Ra) to 1 mm by roughening a white smooth surface, an increase of %23 occurs in the solar radiation absorbtance of this material. Even though the effect of roughness on surface temperature is not very significant for materials with small roughness values, this effect increases with greater values of roughness. According to the experimental results, an increase of 4.8 °C in the surface temperature is obtained by increasing the average roughness value (Ra) to 1 mm when a white smooth surface is subjected to solar energy. A method for evaluating rough surfaces has been developed within the scope of this work. However a further method for achieving more accurate measurements on rough surfaces is also required to be developed. Determination of external surface roughness characteristic with respect to cold and warm period conditions and contribution to the reduction of energy consumption for heating and cooling becomes possible. The combined effect of colour and fabric characteristics on the solar radiation absorption of opaque external envelope elements is important. This effect may be researched in a seperate study. XVII
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