Doğal dolaşımlı güneş toplayıcısı ısıl analizi
The Thermal analysis of solar water heaters with natural circulation
- Tez No: 14306
- Danışmanlar: Y.DOÇ.DR. ABDURRAHMAN KILIÇ
- Tez Türü: Yüksek Lisans
- Konular: Enerji, Energy
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1990
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 52
Özet
ÖZET Enerji talebi ve enerji maliyetlerinin artması yanında doğal enerji kaynaklarının azalmaya başlaması güneş enerjisi kullanımına olan ilgiyi son yıllarda arttırmıştır. Bu da araştırmacıları bu konuda çalışmalara yöneltmiştir. Güneş enerjisinden yararlanmada en ekonomik sistem doğal dolaşımlı sıcak su sistemleridir. Bu çalışmada ele alınan termosifon tipi sıcak su sistemi her iklim şar tında yaygın olarak kullanılabilmektedir. Sistem toplayıcı, depo ve bağlantı boruları olarak üç ayrı kısma ayrılarak incelenmiştir, incelemeyi basit leştirmek amacıyla depo ve toplayıcıdaki hız ve sıcaklık dağılımlarının tek boyutlu olduğu, borularda ve toplayıcıda dolaşan suyun ısı depolamadıgı, sistem malzeme sinin ısıl kapasite etkisinin ihmal edildiği kabulleri yapılmıştır. Toplayıcı ve depoya ait enerji denklemleri ve sis tem için momentum denklemi yazılarak sistemin bilgisayar yardımıyla sıcaklık dağılımının belirlenmesine çalışılmıştır. Toplayıcı üst noktası ile depo alt noktası arasındaki mesafenin değişimiyle sistem sıcaklığının nasıl değiştiği incelenmiştir.
Özet (Çeviri)
THE THERMAL ANALYSIS OF SOLAR WATER HEATERS WITH NATURAL CIRCULATION SUMMARY The interest of the use of the solar energy has been developing during the last decades due to the in crease of the costs of energy and the increase in the use of energy. Another reason is that the natural sources of energy seem to decrease. Solar water heaters have been commercially used in Australia, the U.S.A., Israel, and Japon. Water heating with solar systems has been tradi tionally done in two ways: (i) a forced circulation system, in which a pump circulates a fluid from a storage tank through the collectors, and (2) a thermo- syphon system in which circulation of the fluid to and from the storage tank is achieved through natural con vection caused by temperature differences within the collectors and between the collectors and the tank. Many theoritical and experimental works have been carried out by many investigators to study the perfor - mane e of the solar water heaters. Most models are so complex that they have been used only to study perfor mance over a few days or for simplified operating con ditions such as no daytime load. Close [21 first developed a model to describe the performance of the solar thermosyphon collector. With the concept of system mean temperature and assumptions of linear temperature distributions in absorber and tank. Close was able to calculate the performance of collector in a single day. However, a number of defects in Close's model still exist and can be summarized as fol lows. First, the basic performance characteristic para meters of the absorber such as heat loss coefficient and the transmittance of glass cover and the flow resistance of the thermosyphon loop all rely on the theoritical calculations. Since the heat transfer processes in the absorber and the flow network in the thermosyphon loop are extremly complicated, theoritical calculations could VIcause significant errors. Second, the absorber plate ef ficiency was not considered. Third, the effect of ther mal stratification in the tank was not taken into account. Dng [33 later refined Close's model to include the plate efficiency factor which was also based on theoritical calculation and used a finite difference method to compute the collector performance. Another modification made by Ong [41. He con sidered the entire system to be broken up into a finite number of sections, each individual section having a uniform mean temperature, then solved the finite dif ference equations. The solutions were found in very good agreement with experimental results. Huang [5,6] considered the system consisted of by three major parts; absorber, tank and connecting pipes. In this study the thermosyphon system is examined. The solar thermosyphon collector have been widely used in all climates where extended frese protection is not required. The thermosyfhon system is especially prefered for domestic uses, because it is simple in structure, cheap to installate and operate. A thermosyphon collector which is represented in Fig.( 3.2 ) can be divided into three parts s absorber, storage tank and connecting pipes. The solar radiation coming to the absorber causes a rise of water tempera ture inside it. This temperature rise then causes a decrease in density of water and induces a driving force between the absorber and the storage tank. By this driv ing force, the water circulates upward from the ab sorber through the storage tank and returns to the absorber. To simplify the analysis, the following assump tions are made: 1. The velocity and temperature distributions of the fluid in the tank and the absorber are approximately one dimensional. 2. The heat storage effects of the circulating fluid in the absorber and connecting pipes are negligible. 3. The effect of the heat cappacitance due to the constructing materials of the collector can be ignored; The equations obtained with this assumptios can be solved numerically. VI 1The purpose of this thesis is to analyse the tem perature distribution of a natural convection water heater and how the temperature of the system changes with the relative height of tank i.e. with the distance between the bottom of the tank and the top of the ab sorber plate. For this analysis, first, the flat plate solar collectors which collects both diffuse and direct solar radiation and converts into heat, are examined. They have a black solar energy absorbing surface, one or more transparent covers to reduce convection and radia tion losses to atmosphere, back insulation to reduce conduction losses, fluid tubes attached to the absorbing surface and shading which unites all these parts. The temperature of the fluid in the absorber, at any point in the fluid direction, can be calculated by the equa tion derived by Hottel- Whil lier HI. Ta(y ) = Tccv + S/K.+. CTj»a - TCBv~ S/Kj. exp[ - KAtFvy/LmCp,D The fluid outlet temperature is found by sub stituting L by y, if the collector has a lenght L in the flow direction. The second essential part of the system is the tank where the service water is stored. The temperature distribution in the storage tank of a thermosyphon sys tem has a major effect on both the collector inlet tem perature and flow rate. At low collector flow rates, a thermosyphon tank exhibits a large degree of stratification. Most studies have used the finite difference tech niques to simulate the tank temperature stratification. The tank is divided into a series of fixed sized nodes, and the variation' of temperature with time is computed using an energy balance on each tank node. The energy balance on a stationary control volume of a storage tank includes the enthalpies of the fluid entering and leaving, conduction between adjacent sections and heat loss from the outer surface. The degree of mixing be tween incoming fluid and the contents of the tank (and therefore stratification) depends upon the number of sections that are utilised. At low flows. There is very little mixing, and a large number of nodes may be required to predict the degree of stratification. In Section 3.3.2. the the storage tank is examined by dividing iftto N sections. The thermally stratified tank model is first proposed by Close [2]. In this study also, this tank model is used and energy balance is ap plied to each section considering the heat conduction Vlllbetween two adjacent sections. A series of differential equations is obtained. d6i Mi Cp = aA m Cp, (T3-©*) + (3* mi_ CP (Ti_-e*) dt + (UA)A (Tce,v-eA) + t i. Cp (Öi-ı-9i), if £* > o ö i-«-x Cp, ( Q±. 6i+ı), if jji+ı
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