Doğal dolaşımlı, kendinden depolu güneş toplayıcısının ısıl analizi
The Thermal analysis of a novel built-in-stroge solar water heater
- Tez No: 39291
- Danışmanlar: DOÇ.DR. ABDURAHMAN KILIÇ
- Tez Türü: Doktora
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1994
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 100
Özet
ÖZET Sıcak su ısıtıcıları, güneş enerjisinden yararlanılan sistemler içerisinde en yaygın kullanılan ve en ekonomik olan sistemlerden biridir. Sıcak su sistemlerinde toplayıcı ve depo ayrı veya birarada bulunabilmektedir. Toplayıcı ile deponun birarada bulunduğu kendinden depolu su ısıtıcıları, ayrı bir depo bulunmadığından maliyetinin azalması, ayrıca şebeke basıncında çalıştırılabilmesi gibi nedenlerden dolayı tercih edilmek tedirler. Bu çalışmada yeni tip kendinden depolu su ısıtıcısının teorik ve deneysel incelemesi yapılmıştır. Tamamen alüminyumdan imal edilen ve siyah yüzeyli su ısıtıcısı, yutucu yüzey ve depo vazifesi gören beş adet kanatlı borudan oluşmaktadır. Borular, doğal dolaşım halinde sirkülasyonun daha iyi olabilmesi için bir ayırıcı levha ile bölünmüştür. Toplayıcı yüzey alam yaklaşık 2 m, depo hacmi ise 87 litredir. Teorik çalışmada, bu toplayıcıya ait verim faktörleri çıkarılmış ve bir takım kabuller yapılarak ifade edilen enerji denklemleri bilgisayarda sayısal olarak çözülmüştür. Deneysel çalışmada ise; toplayıcı, laboratuvar şartlarında lambalar kullanılarak elde edilen değişik sabit ışınım akılarında denenmiş ve borular içerisinde değişik noktalarda su sıcaklığının zamanla değişimi incelenmiştir. Elde edilen deney sonuçlan, sayısal çözümle karşılaştınlmışür. Deneyde ölçülen su sıcaklıklarının ortalama değerleri ile sayısal çözümdeki değerleri arasında iyi bir uyum olduğu görülmüştür. Benzer şey boru orta noktalarında ölçülen su sıcaklıklarında da görülmüştür. Sayısal çözümden bulunan su sıcaklıklarının değerleri, deneyde ölçülen değerlere göre boru giriş kısımlarında daha büyük; orta kısımlarında çok yakın; çıkış kısımlarında ise daha küçük olmaktadır. Geliştirilen ve analizi yapılan yeni tip kendinden depolu toplayıcıda su sirkülasyonun iyi olması, istenilen yere konulabilmesi, ayrı deposu olmaması ve basınç altında kullanılabilmesi gibi güneş enerjisinden yararlanılan diğer sıcak su sistemlerine göre üstünlükleri olup, verim eğrisinden de uygun olduğu görülmüştür. XV
Özet (Çeviri)
THE THERMAL ANALYSIS OF A NOVEL BUILT-IN-STORAGE SOLAR WATER HEATER SUMMARY Today, solar energy İs used in many areas such as water heating, refrigeration, air conditioning, heating and cooling of houses, water desalination, production of hydrogen and conversion of solar energy into electricity. Solar Water Heating for domestic use is at present the most attractive way of utilizing solar energy. Solar water heaters can be divided into the following two categories: (a) collection and storage in separate units, (b) collection and storage in a single unit. In domestic solar water heaters of which absorber and storage tank are separate, water can be circulated by natural or forced circulation. Such systems can be designed to operate either at full line pressure or at a reduced pressure, the former is usually termed as a pressurized system and the latter a non-pressurized system. Numerous studies have been performed on thermosyphon (natural circulation) solar water heating systems since they have the advantage of avoiding a water pump for circulating water in the collector. Close [14] has presented a method of estimating the performance of solar water heaters circulating to a storage tank by thermosyphon. By assuming a sinusiodal variation of solar insolation and ambient air temperature, he obtained an analytical expression for average tank temperature as a function time, when hot water is not withdrawn from the storage tank. He tested two absorber and tank systems and compared the results with those estimated from the theoretical model. The transient analysis of Close has been improved by Gupta and Garg [15] by incorporating a collector plate efficiency factor and expanding solar radiation and ambient air temperature as a Fourier series in time. The analysis of Close has also been extended by Ong [16, 17] by incorporating a different formulation of plate efficiency factor and retaining the assumption of equality of the mean collector and storage tank temperatures. Shitzer, Kalmanoviz, Zvirin and Grossman [19] have tested a typical Israeli water heating system in thermosyphon flow. Morrison and Ranatunga [20, 21] have used a laser Doppler anemometer to measure the thermosyphon flow rate; their studies included investigation of system responses to step changes in the solar radiation. Huang [22] has derived ten dimensionless groups or system characteristic parameters which uniquely determine the performance of the collector under thermosyphon mode. Tiwari, Shukla and Sodha [35] have studied two large solar water heating systems (non-pressurized type), each having 1000-1. capacity, under thermosyphon mode. Solar water heater of built-in-storage type incorporating the storage volume and collector in a single unit is one attractive alternative to the system in which collection and storage are in separate units. The elimination of vertical storage tank in the xvicompact unit could markedly reduce the cost of solar water heater and may well remove aesthetic objections to roof top installations. However, such systems suffer from heavy energy losses during nights or periods of insufficient radiation. Several methods have been tried to reduce these losses, such as using a movable insulation cover at night, using an insulated baffle plate or phase changing material inside the tank. Another way to reduce heat losses from the top is to use higly transparent insulation at the top. Numerous studies have also been performed on the built-in-storage type solar water heaters [45-71]. Garg [45] has studied a built-in-storage type solar water heater with a capacity of 90 liters and made up of 112x80x10 cm rectangular tank. The performance tests carried out at the Central Arid Zone Research Institute, Jodhpur, indicated an efficiency factor reaching as high as 70 per cent. Chauhan and Kadambi [46] have tested an inexpensive solar water heater of about 70 liter capacity, combining collection and storage. They have tested the water heater under four different modes of operation: (a) water circulation with a small pump (b) natural convection conditions (c) water draw-offs taking place when the water is around 50-60°C (d) water flowing continuously past the absorber plate with flow rates of 38, 60, and 75.9 kg/hr. Garg and Rani [48] carried out extensive theoretical and experimental studies on a built-in-storage solar water heater which was developed earlier by Garg [45] in India. Sokolov and Vaxman [49] have analysed the thermal performance of an integral compact solar water heater numerically and compared with experimental data. They have investigated two geometries of the proposed solar water heater. Sodha, Shukla and Tiwari [50] have analysed the thermal performance of n-built-in storage water heaters (or shallow solar ponds), connected in series. Vaxman and Sokolov [51] have presented experimental results for an Integral Compact Solar Water Heater which was described and numerically simulated in [49]. Dutt, Rai and Tiwari [53] have presented a simple, straight forward, transient analysis of a Shallow Solar Pond water heater by incorporating the effect of a baffle plate. Schmidt, Goetzberger and Schmid [55] have investigated two integrated collector storage prototypes with about 1 m2 absorber surface each, installed at the Institute für Solare Energiesysteme (ISE). Each prototype consists of a water tank with a highly transparent insulating material and is very well insulated on the sides and at the back. Ecevit, Al-Shariah and Apaydın [56] have studied triangular built-in- storage solar water heaters of different volumes. Kumar and Tiwari [58] have presented transient analysis of collection-cum-storage water heater integrated with a heat exchanger. Prakash, Garg and Datta [59] have investigated the performance a novel built-in-storage water heater containing a layer of phase changing material (PCM)-filled capsules at the bottom. Zollner, Klein and Beckman [62] have developed a performance prediction methodology which is applicable to most commercially available integral collection-storage passive solar domestic hot water systems. Chinnappa and Gnanalingam [69] have tested a pressurised solar water heater of the combined collector and storage type which consists of a square coil of 3 in. diameter pipe, 44.3 ft in length, in a wooden box with heat insulation at the bottom and two glass covers, in Ceylon. xvuIn this work, a novel built-in-storage type solar water heater has been investigated theoreticaly and experimentally under thermosyphon mode with no drawoff. The improved solar water heater detailed in Figure 1 performs the dual function of absorbing and storing hot water. The tank is made of 5 pipes, each of length 1.8 m and diameter 12 cm. Pipes are connected to each other by tubes. A baffle plate is placed inside each pipe to enchance circulation. The material of pipe and plate is aluminium. The collection area and tank volume are approximately 2 m2 (0.99x2 m) and 87 liters respectively. The glazing is 3 mm ordinary glass and there is a thermal insulation of 5 cm-thick glass wool at the bottom. Transparent cover Baffle plate ft Absorber channel Absorber Connecting tube B *< Figure 1 Schematic diagram of a novel built-in-storage water heater The thermal analysis of such a system is very complicated, therefore some simplifying assumptions need to be made. The analysis is based on a time dependent, one dimensional finite difference method. The entire system is divided into sections in the direction of flow as shown Figure 2. The following assumptions are made: (a) the temperature is uniform in each control volume, (b) thermal capacity of each section includes the thermal capacities of the fluid and of the construction material, (c) thermal capacity of the transparent cover is neglected, xvin(d) the maximum temperature in the plate occurs at midpoint between the pipes, (e) the water temperature inside the absorber channel or the tank changes only in the flow direction, depending on time. (f) the thermosyphon flow in every pipe occurs in the same way, so there is no fluid flow from one pipe to another. (g) There are no losses in the connecting tubes. In the theoretical study, transient performance of the system is predicted by solving the mathematical model consisting of energy balance equations written for each control volume comprising one length of pipe. These equations are converted to finite difference form and then solved by a digital computer. Baffle plate Absorbing surface of the pipe ? Bottom surface of the pipe Figure 2 Control volumes inside the pine Consider the plate-pipe configuration as shown in Figure 2. The distance between the pipes is w, the outside pipe diameter is Dd, the inside pipe diameter is Dj, the plate is thin with a thickness 5, pipe or base temperature is Tb, pipe length is L, the water temperature in the absorber channel is Tûoo, the water temperature in the tank is Taoo, and ambient temperature is T.ev. The region between the centerline separating the pipes and pipe base can be considered as a classical fin problem. The fin, shown in Figure 3 is of length w/2. w/2 b w/2. o>- >x S=(tü hiıü L (Tb - Tü>00) + * di>a hi>a L (Tb - Ta>00) (4) where hiu is the heat transfer coefficient between the fluid in absorber channel and the pipe wall, hj a is the heat transfer coefficient between the fluid in water tank and the pipe wall, diü is hydraulic diameter of the absorber channel and di,a is the hydraulic diameter of the water tank. Solving Equation (4) for Tb, substituting into (3), and then solving for the useful gain, one obtains: 0f = (b + w) L fa [S - K (Tüoo - Tçev)] + Fvd [S - K (Ta>00 - T^ )] } (5) where F^ and Fvd are the collector and tank efficiency factors respectively: Fvt = f -?,...1 (6)“ 7C d;. h; a F*%T^F« (7) Useful energy for the absorber channel and the tank are given in Eqs. (4.22) and (4.24) respectively. Energy balance equation for the absorber channel is dTü)00 _ -. dTû,oo where mü is the water-equivalent mass for the absorber channel, Cpjü is the specific heat of water, rh is the thermosyphon mass flow rate, z is the flow direction and the useful gain per unit area for absorber channel is as follows: xxqm Qfii (b+w)L. = F, vt S-K(TÜ00-Tçey) * di,a hi,a b+WT]k A^tLoo ^a.oo) (9) Energy balance equation for the water tank is ax m”c a,oo _ a ~p,a at = qfa(b + w)Az-rhcP)a ax a,oo dz Az (10) where ma is the water-equivalent mass for the water tank and the useful gain per unit area for water tank is as follows: q& = (b+w)L vd S - K (Xoq - Tçev ) + ~r~~ ~ vmi,oo - Ta «, ) y b+wrik (11) If an assumption is made that the fluid flow is laminar one obtains the thermosyphon mass flow rate as: m = - - APS' g(Pa-Po)LiSma 128 % Vo(Li+Le) | vaLt d 1,0 di,a (12) where g is the accelaration due to gravity, p is the density, Lj is the lengt of the baffle plate, a is the angle of inclination, v is kinematic viscosity and Le is equivalent pipe lengtjL Writing the energy equations in finite-difference form, after a small finite increment of time At, the temperatures of control volumes i and j can be found as given in Equations \S.\0) and (5.11) respectively. These equations and the thermosyphon mass flow rate were programmed for solution on a personel computer. The solution procedure is outlined by the flow-diagram of the computer programme shown in Figure 5.3. The experiments have been performed inside the laboratory using an artificial sun consisting of twenty seven sun lamps (Philips, IR) of 250 W each. A total of 24 Chromel-Alumel thermocouples were placed at various locations in the system. Eleven thermocouples were placed in the tank section and eleven in the collector channel. One thermocouple was placed on the plate and another was used for measuring the ambient temperature. The distrubition of thermocouples is shown Fig. 6.8. Solar radiation was measured by a Kipp&Zonen Solar Integrator, placed parallel XXIto the collector plane. In this work, the test results for no-draw operation of the system are presented. Measurements taken from 6 experiments, performed at various constant insolations, are shown in Figures 6.10-6.15. The experimental results for pipe inlet, midpoint and outlet temperatures as. well as the model predictions are shown in Figures 6.16-6.27. The measured pipe inlet temperatures are always less than the predicted ones, while the measured pipe outlet temperatures are generally greater than the predicted ones. The measured water temperatures at the midpoint of the pipe is in very good agreement with the predicted ones. Figures 6.28-6.33 depict the temperature distribution along the length of the collector channel at given times. The solid lines represent theoretical results. The temperature seems to rise almost linearly. Figures 6.34-6.39 depict the temperature distribution along the length of the collector channel and the tank after '4 hours. The experimental temperature difference between the pipe inlet and the pipe outlet is always greater than the predicted one. Agreement between the experimental and predicted average water temperatures are very good as shown in Figures 6.40-6.45. In this work, an improved solar water heater has been tested in the laboratory. Although agreement between the experimental and predicted mean water temperatures are very good, the difference between experimental temperatures of pipe inlet and outlet are always greater than predicted ones. This may be related to the assumptions made in numerical analysis, the predicted thermosyphon flow rate and heat transfer coefficients, and to that the mean value of insolation is constant along the test time but not uniform in every point on collector surface. In this work, thermosyphon mass flow rate was not measured because of difficulties in accurate measurement of low flow rates and the collector geometry, it requires further experimental work. This will be the subject of future studies together with the testing of the collector under outside conditions. xxn
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