EXPERIMENTAL ANALYSIS OF PARTIAL AND FULLY CHARGED THERMAL STRATIFIED HOT WATER STORAGE TANKS

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1 INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING AND TECHNOLOGY (IJMET) International Journal of Mechanical Engineering and Technology (IJMET), ISSN (Print), ISSN (Print) ISSN (Online) Volume 6, Issue 2, February (2015), pp IAEME: Journal Impact Factor (2015): (Calculated by GISI) IJMET I A E M E EXPERIMENTAL ANALYSIS OF PARTIAL AND FULLY CHARGED THERMAL STRATIFIED HOT WATER STORAGE TANKS Dr V.Krishna Reddy a, Mr.M.Mastanaiah b, Dr.S.Rama Krishna Reddy c a Professor, Department of Mechanical Engineering, KCIT, Markapur, B department of Mechanical Engineering, SGIT, Markapur, c department of Chemistry, SGIT, Markapur ABSTRACT Developing competent and economical energy storage devices such as thermal energy storage have great importance as it reduces the gap between demand and supply of energy Presently, the most common storage devices utilize phase change materials (commonly known as eutectic salts), rock beds and hot water storage. In most cases stratified storage tanks with various capacities are employed for storing energy which was most economical. Some analytical simulations and experimental investigations of the effect of thermal stratification in storage systems were carried out by a number of investigators who showed that stratification improves the performance of solar systems. By considering the phenomenon of mixing and other degradation mechanisms will provide an higher limit to the performance of a stratified thermocline hot water storage tank as they influenced a lot and provides closure picture to the practical situation. An attempt was made to study Thermal stratification in energy storage tanks systems using hot water experimentally as the works available so far are very few. The experiments were carried out in static mode with hot water inlet at the centre of the tank. This paper presents temperature stratification i.e. degradation of heat in stratified thermocline storage was studied experimentally. Emphasis was given in this study for the effects of aspect ratio, mix number, exergy efficiency and initial temperature difference. Data were taken on fully, partially charged storage tanks and the temperature changes in axial and radial directions were studied. The results showed that heat conduction to atmosphere through the insulation contribute to greater loss than conduction across the thermocline in small diameter tubes and is not happened in large diameter tanks with thick insulation. Key words: Thermal storage, Thermocline, Stratification, Conduction Heat loss, Convection; Mixing parameter. 62

2 INTRODUCTION Thermal energy storage in the form of sensible heat of water in stratified hot-water tanks is a widely accepted practice. Energy storage schemes are necessary in solar energy systems because of the intermittent nature of solar radiation. Presently, the most common storage devices utilize phase change materials (commonly known as eutectic salts), rock beds and hot water storage. Water is nontoxic, has a very high specific heat, is inexpensive and its vapor-liquid phase equilibrium is suitable for the temperature range required for space and water heating. Both for thermosyphon and forced-circulation types of solar domestic hot-water (SDHW) systems, stratified storage tanks with capacities varying from tens to thousands of liters are employed. These storage systems can act as buffer medium between the collection system and the point of application. The hot water from the storage tank may then be used on demand at various flow rates. In stratified storage systems, we can store both the hot and cold fluids in a single tank. The principle is based on the gravity (dense) separation of fluids of different densities. This intermediate zone between the hot and cold water in the storage tank, where a considerable temperature gradient is obtained, is called the thermo cline, and its thickness should be as little as possible. The increase in thermo cline thickness with time is the degree of stratification decay in the Hot-water. These kinds of systems are being suggested in the heating of buildings, as they can readily be retrofitted into existing Hot-water systems [8]. Number of works on stratification has done through experiments and numerical studies for storage tanks having different geometries, inlet and outlet positions, different tank wall materials, and with various flow rates. The experimental observations of the degradation in a static thermocline on static stratification, charging and discharging cycles are made by the different researchers. LITERATURE SURVEY Lavan and Thomson (1) done through investigation with storage tanks having different L/D ratios and observed that 3.0 L/D ratio is optimum for better performance. These studies indicate that the extraction efficiency decreases with flow rate, and the degree of stratification depends upon the flow distributors used in the tank. Increasing the length/diameter ratio also increases the degree of stratification, but increases heat losses through the walls. A drawback in their study is that the L/D ratio was varied by changing the level of the exit pipe in the same tank because the storage volume also varies with L/D. A one-dimensional model of a thermally-stratified tank was presented by Cole and Bellinger (2), who used their experimental results to estimate the empirical constants in a correlation for a mixing parameter expressed in terms of Fourier and Richardson numbers. They identified a critical Richardson number of about 0.25, below which mixing occurs, and concluded that the best stratification occurs with tall tanks, low inlet velocities and large top-to-bottom temperature differences. Shyu et al. (3) studied experimentally the influence of interior lining on the stratification. They confirmed that the outside insulation can improve the tank wall axial conduction, which leads to decrease the degree of stratification. All these works are mainly focused on static stratification, tank charging and discharging cycles. Dobbin (4) has studied stratification experimentally in water tanks for closed-loop thermosyphon solar collectors. To achieve improved stratification, he recommended the use of side-arm thermosyphon heat exchangers in preference to immersed coil types. He also observed that the degree of stratification is highly dependent on the pressure drop in the collector which, in turn, determines the flow rate. Fanney and Klein (5) have observed that 63

3 stratification exists at all times within the storage tank for the lower collector flow rates (= kg/set m2 collector area); they also presented temperature profiles at different flow rates. Ghaddar and Al-Marafie [6] published their numerical and experimental works on the effect of finite wall thickness on stratification while charging of solar thermal storage tanks. In their work they used a 2-dimensional spectral element model. The temperatures observed from their model were compared with experimental data and with another one dimensional plug flow model. They confirmed that thermocline degradation is because of the conducting wall. They also noticed that the radial temperature distribution is almost uniform at low flow rates, but varies considerably at high flow rates. Wildin and Truman [7] conducted experiments to identify the factors influencing the performance of stratified storage tanks, using natural stratified, diaphragm type and multi-tank systems. Performance of the naturally stratified storage systems are observed to be the best out of all above systems. While these studies provide a qualitative description of the phenomena of stratified thermocline storage, it was felt that a systematic study of thermocline properties under controlled conditions in static and dynamic modes to get the possibility of stratification closure to the practical prevailing conditions. OBJECTIVES OF THE WORK Even though much work has been published on stratified heat storages but systematic parametric study has not been done in the case of hot water storages. The degradation of thermoclines and loss of available heating or cooling energy in the static mode of operation are mainly because of mixing taking place in the tank between the incoming water and the water in the tank. The level of mixing is a function of inlet stream velocity, temperature difference between the water available in the tank and incoming water, and the thermo physical properties of the water. Quantification of mixing is done in several cases by using some empirical relationships to compare the experimental temperature profiles. The objective of this study is to experimentally determine the mixing parameter which is measured as a function Reynolds number and Richardson number, and the effect of several geometric and dynamic parameters such as aspect ratio on thermal stratification. RESULTS AND DISCUSSIONS Static Temperature Profiles Temperature profile for an uninsulated fully charged hot water storage tank in static case with varying time instant is shown in Fig.4.1. Initial temperature profile could not be maintained uniform due to heat loss to ambient because of the large temperature differences between the stored hot water and ambient. The top and bottom portions of the storage tank consist of radial diffusers made of low thermal conductive material, so the heat transfer from the top and bottom surfaces can be considered to be negligible. In addition, the top and bottom plates are insulated using 15mm thick thermo resin material. It is therefore stated in the figures that convection takes place only through sidewalls. The temperature profile in Fig.4.1 shows the top surface being high and bottom being lower. As time increases this temperature decreases due to convective heat losses to the ambient from the bulk fluid resulting in the formation of temperature gradients out the bulk fluid. Mix Number (a) Effect of aspect Ratio The variation of Mix number with aspect ratio for increasing time in the case of fully charged hot water storage rank is seen in Fig

4 Fig 4.1 Fig 4.2 It is evident that more mixing (less stratification) takes place for small aspect ratio tanks. As the aspect ratio increases, the Mix Number decreases due to increased thermal stratification inside the storage tank. (b) Effect of initial temperature Figure 4.3 shows the variation of Mix Number with aspect ratio for different initial temperatures. It is noted in this case that with increase in aspect ratio mixing decreases. The result also shows mixing decreases with increase in the initial temperature differences. 65

5 Exergy efficiency The exergy efficiency for a typical case of hot water storage tank at larger aspect ratio is seen to be more compared to lower aspect ratio as seen in Fig.4.4 with initial temperature difference. The reason is convective heat loss from the tank wall to the ambient by natural Fig 4.3 Convection leads to development of temperature gradient, as a result the stored energy inside the tank is decreased, leading to decrease in the value of exergy efficiency. The development of temperature gradients is attributed due to mixing inside the storage tank. At higher initial temperature differences heat leak is more resulting convective mixing hence showing lesser value in the exergy efficiency. Fig

6 Fig 4.5 Partially charged storage tank A case with convective heat loss at top region and convective heat gain at the bottom region is taken for presenting the results Temperature profile for partially charged storage tank with different levels of initial charge Temperature K /4 CHARGE 0 MINUTES 1/4 CHARGE 30 MINUTES 1/4 CHARGE 60 MINUTES 1/4 CHARGE 120 MINUTES 1/2 CHARGE 0 MINUTES 1/2 CHARGE 30 MINUTES 1/2 CHARGE 60 MINUTES 1/2 CHARGE 120 MINUTES 3/4 CHARGE 0 MINUTES 3/4 CHARGE 30 MINUTES 3/4 CHARGE 60MINUTES 3/4 CHARGE 120 MINUTES Dimensionless height Fig 4.6 Static Temperature Profiles A clear variation of temperature for different levels of initial charge (½) is shown in Figs. 4.5 & figure 4.6 shows the temperature profile considering three levels of initial charge for an aspect ratio of 3 with varying time. Figure 4.6 also shows temperature variation for partially charged tank wherein the top region is at a temperature higher than the ambient(heat loss condition).in partially charged storage tank for different levels of initial charge, degradation of themocline is to be more in case of ¼ and ¾ charged tank as compared to ½ charged tank. Thermocline is a zone between the warm fluid layers and cold fluid layers in which there is a large temperature and density gradients. The results show less dense warm water tries to induce convective mixing at the top of the storage tank. Similar mixing effect can be explained for ½ and ¾ tank because of more mixing due to buoyancy effects. 67

7 Mix Number The Mix Number variations with time for varying aspect ratios at different levels of initial charges are seen in Figs. 4.7when the initial charge is 348K. The results shows Fig 4.7 Degradation of thermocline in all the cases. Figure shows the quantitative degradation in terms of Mix Number. The Mix Number values increases with time for a given condition. The degradation is seen to be more ¼ and ¾ charged tank compared to ½ charged storage tank. Providing insulation reduces mixing as indicated by the dotted lines in Figure. It is also seen that varying trend of mixing for the three different cases in which top and bottom region acts as either convective heat loss or heat gain. The effect of insulating the storage tank considerably reduces mixing which is shown clearly in all the cases of partial charging. Fig 4.8 Exergy efficiency Figure 4.8 shows the variation of exergy efficiency for different levels of initial charge in case of partially charged storage tank. At ¼ charged storage tank, thermal degradation and convective mixing leads to greater loss of energy whereas in case of ¾ charged tank, significant loss is attributed due to axial conductive effects and thermal degradation. The storage tank with ½ charged accounts for lesser mixing and hence more exergy efficiency compared to ¾ and ¼ charged 68

8 storage tank. It is to be noted that exergy efficiency calculation is not done for the case of dynamic operating conditions as mixed convective effects leads to lesser accurate prediction of exergy efficiency. CONCLUSIONS Numerical and experimental heat transfer analysis which can be described well by all major factors relating to thermal degradation and the convective mixing is required in thermal stratification. Majority of the present works focused on static or dynamic mode in a single mode of operation. Very limited works are available in dynamic mode with inlet mixing and partially charged containers. In the present work various studies are carried out for convection and stratification in both partially and fully charged in both static and dynamic situations which resemble the practical situation. From this work it is observed that the parameters like initial charge level, temperature difference, insulation and aspect ratio between hot and cold water have substantial effect on the decay of stratification in tanks. Mix number increases with time for all aspect ratios and more mixing at lower aspect ratios in fully charged tanks. In partially charged tanks also Mix number increases with time up to particular values of aspect ratio and initial charge. With time mix number increases with aspect ratio and elevated aspect ratios guide to more thermal degradation because of axial conduction and heat gain through convection. One fourth charged tank gives higher mix number than in ½ and ¾ charged tanks and more stratification can be established with the raise in temperature gradients. In case of dynamic charging and discharging storage efficiency was much influenced by non dimensional numbers like peclet, Fourier number, tank wall material, insulation and inlet mixing. Three dimensional geometry study can give better results and visualization of contour diagrams can be done with numerical studies. REFERENCES 1. Lavan Z, Thompson J., Experimental study of thermally stratified hot water storage tanks, Solar Energy 1977, 19, R.L. Cole and F.0. Bellinger, ASHARE, Trans. 88, 1005 (1982). 3. Shyu, R.J., Hsieh, C.K., Unsteady natural convection in enclosure with stratified medium. Journal of Solar Energy Engineering , W. Dobbin, Proc. ENERGEX &4, p. 501, F.A.Curtised. Saskatchewan, Canada (May 1984). 5. A. H. Fanney and S. A. Klein, Sol. Energy 40, 1 (1988). 6. Ghaddar N.K,Al-Marafie A.M. Study of charging of stratified storage tanks with finite wall thickness. Int J Energy Res 1997; 21: Wildin M.W, Truman C.R. Performance of stratified vertical cylindrical thermal storage tanks part I ASHRAE, Transcactions, 1989, 95(Part1): (1989) 9. Yogesh Dhote and S.B. Thombre, A Review on Natural Convection Heat Transfer through Inclined Parallel Plates International Journal of Advanced Research in Engineering & Technology (IJARET), Volume 4, Issue , pp , ISSN Print: , ISSN Online: Prof.Alpesh V Mehta, Nimit M Patel, Dinesh K Tantia and Nilsh M Jha, Mini Heat Exchanger Using Al2o3-Water Based Nano Fluid International Journal of Mechanical Engineering & Technology (IJMET), Volume 4, Issue 2, 2013, pp , ISSN Print: , ISSN Online: