Doğal dolaşımlı güneş toplayıcısı ısıl analizi

Abstract

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.

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 <£ O + Nf[eNi(9i<-ı-ei) + eii(ei-x-8i) The solution of these differential equations gives the temperature distribution of the storage tank. The first tern» of the left hand side of the dif ferential equation presents the energy of the fluid com ing from the absorber, while the second term the energy of the make- up fluid. The heat loss from the tank to the atmosphere is defined in the third term. The fourth term gives the heat transfer with the net mass flow rate. And the final term is the conduction term between adjacent sections. This term can be neglected when the net mass flow rate through the tank is not small. The thermosyphon head induced in the thermosyphon loop can be evaluated by integrating the specific gravity of the water around the loop. This is balanced by the pressure head opposing flow due to the friction and other losses in the pipes, i. e. bends, tees, restriction losses. These local losses are calculated in Section 3.4.İ. To determine the rate of flow, due to thermosyphon action, generated by heating of water in the collector and the consequent movement by natural convection, it is necessary to consider the density at various points of flow circuit at any instant. The thermosyhon flow rate will be such that at every instant, the thermal pressure head is balanced by the frictional head loss in the flow circuit. In Section 4. the dimensions of the system are given. The overall heat loss coefficient of the absorb- erand the tank are calculated and also simulation proce dures sire given in this section. The energy equations of the absorber and the storage tank can be solved numerically with the initial IXcondition that the water temperature is uniform and as sumed to be the same as the ambient temperature for simplicity. In the numerical solution the finite dif ference method is used for the tank energy equations. The numerical solution can be found when proper values of the system Are given. The first step of the solution is to evaluate the temperature distribution around the thermosyphon loop for the flow rate of the previous time step. The inlet temperature to the collector is computed from the temperature of the section in the bottom of the tank. After allowance for heat loss from the inlet pipe, the temperature of each of the fixed nodes used to present the collector temperature profile is evaluated from the above equation. The tank inlet temperature is computed from the collector outlet temperature and the temperature drop across the return pipe to the tank. A new tank temperature profile is then evaluated. The thermosyphon pressure head due to the density dif ferences around the loop is determined from the system temperature profile. The equality of the later and the pressure losses in the circuit gives a new mass flow rate value. This procedure continue until the mass flow rates be equal. The numerical techniques used in the present study to solve for the mass flow rate and the tank temperature distribution are the repeated substitu tion and Euler methods, respectively. The temperature distribution of the system is ob tained by the computer program. The results shows that the ambient temperature does not vary much throughout the day while the insident solar radiation varies. The temperature distribution of the water in the storage tank represents different features during the daytime. Three phases of heating- up periods can be observed. At the beginning the water in the tank hardly warms up- redistribution of the temperature within the tank is tanking place. After this there is a very rapid incrsase in the rate of heat collection; the water in the tank heats up very fast. Then when there is a very little heat collection redistribution of tank temperature again take place. The relation between tank temperature and the relative height of tank is also examined. The results shows that the mean tank temperature does not vary much with increasing relative height of tank.