12 Heat Transfer Enhancement In A Latent Heat Storage

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Solar Energy Vol. 65, No. 3, pp. 171–180, 1999  1999 Elsevier Science Ltd S 0 0 3 8 – 0 9 2 X ( 9 8 ) 0 0 1 2 8 – 5 All rights reserved. Printed in Great Britain 0038-092X / 99 / $ - see front matter

HEAT TRANSFER ENHANCEMENT IN A LATENT HEAT STORAGE SYSTEM† R. VELRAJ* , ‡, R. V. SEENIRAJ*, B. HAFNER**, C. FABER** and K. SCHWARZER** , § *Department of Mechanical Engineering, Anna University, Madras 600025, India ¨ ¨ **Solar Institute Julich, Fachhochschule Aachen, Julich, D-52428, Germany Received 18 September 1997; revised version accepted 6 October 1998 Communicated by ERICH HAHNE

Abstract—Commercial acceptance and the economics of solar thermal technologies are tied to the design and development of efficient, cost-effective thermal storage systems. Thermal storage units that utilize latent heat storage materials have received greater attention in the recent years because of their large heat storage capacity and their isothermal behavior during the charging and discharging processes. One major issue that needs to be addressed is that most phase-change materials (PCM) with high energy storage density have an unacceptably low thermal conductivity and hence heat transfer enhancement techniques are required for any latent heat thermal storage (LHTS) applications. In the present paper the various heat transfer enhancement methods for LHTS systems are discussed. Three different experiments to augment heat transfer were conducted and the findings are reported.  1999 Elsevier Science Ltd. All rights reserved.

the LHTS systems. The heat transfer enhancement required for melting or solidification depends on the type of application. Some applications require heat to be charged at a faster rate while others require heat to be discharged at a faster rate. In applications like waste heat recovery, where the process is intermittent and a large amount of waste heat is to be recovered from the process stream during a short time, heat transfer enhancement is required for charging. On the other hand, if the heat is available at a constant rate for a longer time and it is to be removed in a shorter time like solar domestic hot water applications, then the enhancement is required for solidification. As the present work is aimed for solar domestic hot water and space heating applications the study is focused on enhancement methods for the solidification process. This study is performed for a non dynamic latent heat storage system without direct contact between the PCM and the heat transfer fluid (HTF).

1. INTRODUCTION

Efficient and reliable thermal storage systems are an important requirement for solar applications due to the anti cyclic nature of heat demand and availability of solar radiation and also due to the diurnal variation of solar radiation caused by weather variations. Among the thermal energy storage concepts, both sensible heat and latent heat (i.e., phase change) stores are under investigation. The major advantages of phase change stores are their large heat storage capacity and their isothermal behavior during the charging and discharging process. In a latent heat thermal storage (LHTS) system, during phase change the solid–liquid interface moves away from the heat transfer surface. During this process, the surface heat flux decreases due to the increasing thermal resistance of the growing layer of the molten / solidified medium. In the case of solidification, conduction is the sole transport mechanism, and in the case of melting, natural convection occurs in the melt layer and this generally increases the heat transfer rate compared to the solidification process. This decrease of the heat transfer rate calls for the usage of proper heat transfer enhancement techniques in

2. REVIEW OF HEAT TRANSFER ENHANCEMENT METHODS

2.1. Enhancement with fin configurations There are several methods to enhance the heat transfer in a LHTS system. The use of finned tubes with different configurations has been proposed by various researchers as an efficient means to improve the charge / discharge capacity of a

†Paper presented at the ISES Solar World Congress, Taejon, South Korea, 24–29 August 1997. ‡Author to whom correspondence should be addressed. § ISES member. 171

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LHTS system. Experimental studies have been performed by Sparrow et al. (1981) for outward solidification on a longitudinal finned vertical tube, viz. for conduction-controlled or naturalconvection-controlled heat transfer. Conduction is the controlling mode when the liquid is at its fusion temperature, whereas natural convection controls when the liquid is above the fusion temperature. It is concluded that for conduction control, the enhancement of freezing due to finning is less than the area ratio of the finned and unfinned tubes, whereas for natural convection control, the enhancement is very nearly equal to the area ratio. It is also stated that when conduction controls, freezing continues more or less indefinitely, whereas natural convection severely retards the freezing and ultimately terminates it altogether in the vertical tube arrangement. Smith and Koch (1982) have formulated finite difference equations based on conduction heat transfer for phase change occurring adjacent to a cooled flat surface containing fins. They have discussed the effects of fin conduction parameter and fin dimensions on solidification rate and heat transfer. Lacroix (1993) has presented a theoretical model for predicting the transient behavior of a shell-and-tube storage test unit having annular fins externally fixed on the inner tube with the PCM on the shell-side and the HTF flowing inside the tube. The numerical results have also been validated with experimental data for various parameters like shell radius, mass flow rate and inlet temperature of the HTF. Padmanabhan and Krishna Murthy (1986) have also studied the phase change process occurring in a cylindrical annulus in which (i) rectangular, uniformly spaced longitudinal fins, spanning the annulus (ii) annular fins are attached to the outer surface of the inner isothermal tube, while the outer tube is made adiabatic. They have performed parametric analysis and based on the results they have suggested working formulae to obtain the volume fraction solidified at any time for both the cases. All the earlier work relate to the enhancement study for outward solidification on finned tubes. Recently Velraj et al. (1997) have presented theoretical and experimental work for a thermal storage unit consisting of a cylindrical vertical tube with internal longitudinal fins and this tube assembly is, in turn, placed inside another cylindrical vessel containing water. It was concluded that this configuration which forms a V-shaped enclosure for the phase change material gives maximum benefit to the fin arrangement. Sauer (1982) describes a latent heat storage concept that

makes use of inward solidification and outward melting simultaneously. The system consists of two concentric pipes forming an annulus within which the PCM is maintained. Through the inner pipe the warm fluid is circulated and the cold fluid surrounds the outer tube. Fins are uniformly placed in the PCM region spanning the entire annulus. Eftekhar et al. (1984) have experimentally studied a different heat transfer enhancement method for melting of paraffin by constructing a model that consists of vertically arranged fins between two isothermal planes (the bottom one being hotter than the top) which not only provides additional conduction paths but also promotes natural convection within the molten PCM. Their photographs of the molten zone indicate that a buoyant flow induced in the neighborhood of the vertical fin causes rapid melting of the solid wax. The objective of their research was to study the enhancement of heat transfer during heating cycle with paraffin wax as the PCM.

2.2. Other enhancement techniques Several other heat transfer enhancement techniques for LHTS systems have been studied and reported by various researchers. A few important methods among them are, inserting a metal matrix into a PCM, using PCM dispersed with high conductivity particles and micro-encapsulation of PCM. Siegel (1977) has studied the improvement in solidification rate in molten salt dispersed with high conductivity particles. He has presented results of one dimensional analysis for three geometry’s of practical interest, viz., solidification on a flat plate, inside a tube and outside a tube. He has concluded that even though there is improvement in heat transfer rate, there is a compensating effect due to the reduction in volume fraction occupied by the phase change material. For a reasonable fraction of the particles in the PCM, moderate improvement in heat transfer is achieved. He also observed that, compared with a plane layer, the improvement is less for solidification inside a tube and somewhat greater for outside a tube. Hoogendoorn and Bart (1992) have reported that the low value of the thermal conductivity of the PCMs could be greatly improved by embedding a metal matrix structure in them. A numerical simulation model for the transient heat transfer in a PCM heat storage vessel has been formulated by them and is included in TRNSYS. Khan and Rohatgi (1994) have studied the heat transfer characteristics during solidification in the presence of cylindrical

Heat transfer enhancement in a latent heat storage system

reinforcements, including graphite, alumina, iron and copper in an aluminium–silicon alloy base and lead-base composite. They have reported that the rate of movement of interface is strongly dependent on the thermal conductivity ratio of reinforcement to the melt. Tong et al. (1996) have done a theoretical heat transfer study on a vertical annular space filled with water (as PCM) and aluminium matrix. Their numerical results are presented in the form of solid–liquid interface movements, isotherms, streamlines and heat transfer rate for some representative cases. The heat transfer rates for enhanced cases show an orderof-magnitude increase over the base case, where no metal matrix is inserted. Stovall and Arimilli (1988) have evaluated three methods to enhance the thermal conductivity of the thermal storage media using LiH as the PCM. One method is to replace a portion of the PCM with a reticulated metal. The second method uses fins made of SS304 with fin volume fraction ranging from 5 to 50%. The third method uses Li in the form of LiH salt and molten Li slurry. Chow et al. (1996) have evaluated two thermal conductivity enhancement techniques. The first technique focuses on placing PCM in capsules of various shapes in a liquid metal medium. The second technique involves a metal / PCM composite. The work was stimulated by the need of high temperature energy storage for future space power generation systems. 3. EXPERIMENTAL INVESTIGATION

In the present work three different heat transfer enhancement methods are investigated (see Fig. 1). The first enhancement technique uses internal

Fig. 1. Paraffin storage tube cross section and thermocouple locations for the configurations (a) plain tube (b) with fins (c) with lessing rings (d) with bubble agitation.

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longitudinal fins inside a cylindrical vertical storage tube containing paraffin. In the second method, the tube is filled with lessing rings of 1 cm diameter which are commonly used in the chemical reactors to enhance the surface contact, and the molten paraffin is poured into the tube. These lessing rings are made of steel and have a thinwalled hollow cylindrical structure with a partition. Without partition these rings are known as Raschig-rings. A photographic view of the lessing rings is shown in Fig. 2. The molten paraffin occupies 80% of the storage volume. In the third method a very small amount of water is poured into the tube. Molten paraffin is then added and the tube is evacuated by a vacuum pump. The vacuum is maintained such that the saturation temperature of the water inside the tube is nearly equal to the phase change temperature of the PCM. The intention is to create steam bubbles inside the PCM during the phase change that would promote the heat transfer.

3.1. Experimental setup Fig. 3 is a schematic of the storage unit. It consists of the vertical cylindrical storage tube made of aluminium, with an outside diameter of 6 cm, an inside diameter of 5.4 cm and an active length of 60 cm. The tube is, placed inside another cylindrical vessel with 25 cm diameter and having the same height as the tube, containing water and the vessel is well insulated. The temperature of the water bath is controlled by a thermostat, having a variable heating coil capacity of 500, 1000 and 2000 W. A magnetic stirrer is provided at the bottom of the trough to have a uniform temperature in the axial direction of the cylindrical vessel. A small top portion of the tube unit is kept above the water level to provide convenient thermocouple connections and for easy handling of the tube from outside. While conducting experiments with fins, an assembly of four aluminium fins with the dimensions of 0.15 cm thickness, 2.7 cm height and 50 cm length each, and forming a cross shaped cross-section is placed inside the tube and welded at the top and bottom of the tube. The intermediate length of the fin was thermally bonded to the tube by solder. For all the experiments, NiCr–Ni thermocouples are placed in two different horizontal planes, at a distance of 20 cm and 35 cm from the bottom of the tube as shown in Fig. 3. The position of the thermocouples in one plane for all the configurations is shown in Fig. 1. The thermocouples are mounted at suitable locations where the end effects (heat flow from the bottom of the tube

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Fig. 2. A photographic view of the lessing rings.

which could cause heat flow in the axial direction within the paraffin) are not present. Three thermocouples are located in the surrounding water bath at different heights to monitor and ensure that there is minimum temperature variation in the axial direction. The thermocouples are connected to a recorder, which can provide instantaneous analog and digital outputs. The recorder has an accuracy of 60.05% of the reading 1 0.58C, and a resolution of 0.18C for the measurements with K type thermocouple. Molten paraffin is poured into the tube carefully avoiding cavities. For the experiments with the plain tube, paraffin RT 58 was used and for the experiments with heat transfer enhancement methods, paraffin RT 60

was used. Both paraffin have the same thermophysical properties except for the phase change temperature. RT 60 has a phase change temperature range of 58 to 608C and RT 58 has a phase change range of 58 to 598C. Table 1 summarizes the thermophysical properties of paraffin RT 58 and RT 60.

3.2. Experimental procedure Several experimental runs were performed to find the temperature distribution within the paraffin during solidification. Initially the experiments were performed without fins (plain tube configuration) as detailed below. Subsequently, experiments were conducted for the heat transfer enhancement configurations, following the similar procedure. The temperature of the water was maintained at Table 1. Thermophysical properties of paraffin RT 60 and RT 58 (from manufacturer’s a data) Property

Value

Latent heat of fusion Specific heat capacity Thermal conductivity Density Solid Liquid

214.4 kJ kg 21 900 J kg 21 K 21 0.2 W m 21 K 21

a

Fig. 3. Experimental setup.

850 kg m 23 775 kg m 23

Manufacturer address: Hans-Otto Schuemann GmbH & Co KG, Worthdamm 13–27, D-2000, Hamburg, FRG.

Heat transfer enhancement in a latent heat storage system

a constant value, slightly above the solidification temperature through a thermostat control till the entire PCM region attained thermal equilibrium with the surrounding. The temperature of the water was lowered suddenly to a temperature below the solidification temperature and maintained at a constant value. This was achieved by removing part of the water from the surrounding water bath and adding cold water. Due to the presence of the magnetic stirrer at the bottom, a nearly uniform temperature was achieved within a short time. This is confirmed from the readings of the thermocouples located at three different heights in the water bath. The total time lag between the removal of water from the bath and the attainment of nearly uniform temperature was approximately 3 to 4 min. The thermostat is then set to the new surrounding water temperature and the recorder was also switched on. The water bath was maintained at a constant temperature (within a range of 60.28C) throughout the experiment by thermostat control. The transfer of fusion heat from the PCM increases the temperature of the surrounding water. The mass flow rate of the water is hence varied and kept at just sufficient value to maintain a constant temperature during the process. During the initial stage, a slightly higher flow of about 10 to 15 kg h 21 was required to prevent the temperature from rising. This is due to a larger amount of heat transferred from the PCM to the surrounding coolant during the initial stages of the process. Subsequently, a flow of only about 2 kg h 21 was required. As the solidification proceeds, the temperature vs time readings were recorded and monitored. The experimental readings were obtained till all the thermocouples showed temperatures well below the solidification temperature.

3.3. Determination of ‘ h’ using experiments on plain tube configuration Since the height of the paraffin-filled plain tube is high and the rate of circulation of the water is low, convection currents would occur on the outside tube surface. Due to the presence of the magnetic stirrer at the bottom of the vessel, an expression to determine ‘h’ for this type of agitating / circulating flow outside vertical surfaces is not readily available. To obtain more accurate results, the ‘h’ value is determined by conducting experiments on the plain tube configuration, and matching the results with the numerical model for the same. For this and for all further theoretical work, the numerical model developed by Velraj et al. (1997) based on the enthalpy method is used. This model which was originally developed for

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Fig. 4. Experimental and simulated (with h 5 400 W m 22 K 21 ) temperature variation within the paraffin for the plain tube configuration at the thermocouple locations shown in Fig. 1(a).

vertical internally finned tube configuration is reduced for plain tube configuration by assuming the fin thickness equal to zero. The model is initially validated with the exact solution of London and Seban (1943) for inward cylindrical solidification. This is done by obtaining the complete solidification time from the model and comparing it with that obtained from the exact solution for different sets of input parameters. The results are in close agreement and the deviations are within 62% for the selected grid size and time step in the numerical model. To estimate the value of ‘h’, the experimental temperature variation was obtained for three different thermocouple locations shown in Fig. 1(a). Repeated numerical runs were made by varying the ‘h’ value in the model keeping all the thermophysical properties of the paraffin at a constant value, until close matching of the numerically computed temperature profile with the experimental results was obtained. Such numerical results obtained are referred to as simulated results. The value of ‘h’ thus obtained was of the order of 400 Wm 22 K 21 . This value of ‘h’ was used in the computations involved with other heat transfer enhancement methods. Fig. 4 shows the experimental and simulated temperature variation for the above value of ‘h’, at the locations of the thermocouples shown in Fig. 1(a). The numerical and simulated temperature variation are in close agreement with each other. 4. RESULTS AND DISCUSSION

Fig. 5 shows the experimental and predicted temperature variation within the paraffin for the fin configuration at the locations of the thermocouples shown in Fig. 1(b). The predicted temperature variation at the above locations is

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Fig. 5. Experimental and predicted temperature variation within the paraffin for the tube with fin at four thermocouple locations shown in Fig. 1(b).

obtained from the numerical model. The theoretical and the experimental curves are in rather close agreement with each other, except that in the theoretical curves, the rate of temperature drop is slower during phase change and faster after phase change is completed at any location whereas in the experimental curves, there is not much change in the rate of temperature drop even after the lower limit of the phase change temperature range is reached. This could be due to the solidified paraffin, that may still have some amount of latent heat even after the lower limit of the phase change temperature range is reached. This implies that the phase transition temperature range is actually wider than that specified by the manufacturer. This fact has also been observed by Abhat (1983) for most of the paraffin. However, the major phase change takes place within the specified temperature range. This shift of a small amount of latent heat for a short range below the lower limit of the phase change temperature range is also useful from the application point of view. This reduces the sub-cooling of the solidified layer and makes it possible to extract the energy at nearly constant temperature even during the solidification of the paraffin at the center of the

region between the adjacent fins. Further, in the theoretical case, the initial temperature was fixed at 60.58C for the entire domain, whereas it can be observed from Fig. 5 that in the experiments the initial temperatures could only be maintained at 60.560.38C. However, as soon as the temperature of the water bath is lowered, the whole region attains the temperature of saturated liquid. It is also seen from the above figure that initially the temperature drop is faster at thermocouple location 2 than in location 1 and vice versa after a certain interval of time. This could be because location 2 is more closer to the fin and this fin conduction is more pronounced initially. Later, the corner effect (cooling effect from the fin and the boundary wall) predominates at location 1 and this results in a faster drop in temperature. This kind of temperature variation is observed both in the theoretical and experimental curves. A similar corner effect is discussed by Shamsundar and Sparrow (1975) for the case of a square container. The unique feature in the numerical model (Velraj et al., 1997) is that it takes into account the heat flow in the circumferential direction along the tube wall. Initially, when the numerical

Heat transfer enhancement in a latent heat storage system

trials were done without taking into account the circumferential heat flow from the fin tip to the wall, more time was taken to lower the fin temperature. Consequently, the time for complete solidification was much higher than the experimental value. When the above circumferential heat flow is taken into account, it is observed that approximately 80% of the total heat flow from the fin is through this path. This is because the convective surface area at the fin tip is not sufficient to pass away all the heat from the fin directly into the surrounding fluid. Fig. 6 shows both the experimental and simulated temperature distribution within the paraffin for the enhancement technique employing paraffin-lessing ring mixture at the thermocouple locations shown in Fig. 1(c). The numerical model for the plain tube configuration was used to find the effective thermal conductivity, k e , for this heat transfer enhancement method. For this method, the experimental temperature–time variation obtained from three different thermocouple locations was used as a base (standard). The value of k e was varied by trial and error in the numerical model until close matching of the predicted temperature profile with the experimental results was obtained. The value of c p was obtained by mixture rule and was used in the energy equation. Since the paraffin occupies only 80% of the total volume, the corresponding reduction in amount of latent heat per unit of the total volume is taken into account in the energy equation by assuming the value of the density of the paraffin as 80% of its actual value. The value of k e thus obtained was 2 Wm 21 K 21 which is ten times greater than the ‘k’ of paraffin. Hence, the thermal conductivity enhancement factor which is defined as the ratio

Fig. 6. Experimental and simulated (with k e 5 2 W m 21 K 21 ) temperature variation within the paraffin for the tube with lessing rings at three thermocouple locations shown in Fig. 1(c).

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of k e obtained with lessing rings to that of the paraffin, is ten for this configuration. This enhancement is achieved with a loss of 20% volume of paraffin for the particular tube diameter of 5.4 cm and lessing ring diameter of 1 cm. The increase in effective thermal conductivity depends on the ratio of the diameter of the tube and the dimensions of the ring and it is well known that for larger tube diameter this enhancement is much higher. Fig. 7 shows the experimental temperature distribution within the paraffin for the case employing bubble agitation at the thermocouple locations shown in Fig. 1(d). It is found that there is not much increase in heat transfer during solidification. Though the movement of bubbles induces convection during melting, the vapor pocket or cavity entrapped within the solid paraffin during solidification reduces the conduction heat transfer as the vapor is a poor conductor of heat. Initially when the temperature of the PCM is above saturation temperature, the water is present in vapor form. During solidification the water condenses and since it is denser than paraffin, it slides down over the solidified paraffin. When this water comes in contact with the interior paraffin which is at a higher temperature, again it evaporates and forms steam bubbles which tend to raise up. This water condensing and bubble formation takes place alternately throughout the solidification process. During this process some steam bubbles may get entrapped within the solidified paraffin and will affect the conduction heat transfer. However from melting experiments conducted (not reported in the present work), it was observed that the increase in heat transfer during melting is appreciable, which may be suitable for some waste heat recovery applications where the pro-

Fig. 7. Experimental temperature variation within the paraffin for the tube with enhancement through bubble agitation at three thermocouple locations shown in Fig. 1(d).

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cess is not continuous and large quantity of heat should be stored in a short period. This bubble formation was observed by conducting the initial experiments with glass tube instead of aluminium tube. Since there was no remarkable augmentation effect on solidification and also due to the complexities of the process, further analysis was not attempted. Fig. 8 shows the time for complete solidification (obtained from the numerical model) of the paraffin with the surrounding water bath temperature at 508C for the plain tube and for the two heat transfer enhancement configurations shown in Fig. 1(b) and (c). These results are obtained from the numerical model, since experimentally it is difficult to determine the portion of the PCM which solidifies at the end, and place the thermocouple in the appropriate location, especially for the fin configuration. The fins occupy 7% of the tube volume and it is seen from the figure, that the time for complete solidification is approximately one-fourth that of the plain tube. In the other method the lessing rings occupy 20% of the tube volume and the time for complete solidification is nearly one-ninth that of the plain tube. Comparing the volume occupied by the fins and the lessing rings, the latter occupy more volume

without proportionate reduction in time for complete solidification. However, the time factor which is defined as the ratio of time for complete solidification with heat transfer enhancement to that of plain tube configuration, depends on the diameter of the tube for the lessing ring configuration. The time factor in this case decreases with increase in diameter of the tube. With the fin configuration, it was observed from the numerical runs that the time factor remains more or less unchanged for a fixed number of fins irrespective of the diameter of the tube. The figure also shows the heat stored which represents the amount of latent heat that can be stored per unit meter length in a tube with 5.4-cm inner diameter with paraffin RT 60 as PCM. In the plain tube configuration 490 kJ of energy can be stored per meter tube length and with fins and lessing ring configurations the reduction in energy stored is 7 and 20% respectively. In a LHTS system the total resistance to the heat flow is the sum of convective resistance on the tube surface and the conductive resistance of solidified PCM. During the extraction of heat from the system, as the solidification proceeds, the conductive resistance offered by the PCM increases whereas the convective resistance is

Fig. 8. Comparison of total solidification time and total quantity of heat stored for different configuration.

Heat transfer enhancement in a latent heat storage system

constant throughout the process for a given flow condition. Hence the variation of surface heat flux depends on the predominance of the convective resistance (fixed resistance) and the conductive resistance (variable resistance). When the convective resistance is dominant nearly uniform surface heat flux with time can be achieved from the LHTS system and if the conductive resistance is dominant the surface heat flux will have a decreasing trend with respect to time. In a LHTS system when water is the heat transfer fluid, the convective resistance is less and the variable conductive resistance offered by the PCM becomes dominant during the process and hence it is very difficult to achieve a uniform surface heat flux. The surface heat flux variations with water as heat transfer fluid has been discussed by Velraj et al. (1997). However, when air is the heat transfer fluid, nearly uniform surface heat flux can be achieved as the surface convective resistance is high compared to the variable conductive resistance. Based on the above observation the following configurations are suggested for water heating and air heating applications. For domestic solar hot water applications, when used as single LHTS unit, the surface heat flux will not be uniform. Therefore a combined sensible and latent heat storage system with a innerfinned tube configuration such as that shown in Fig. 9 is recommended. This type of storage system utilizes the advantages of both sensible and latent heat storage systems. For domestic water heating, the hot water may be required at a higher rate for a short duration and this requirement can be met by the sensible heat of the water in the storage tank. Before the next usage, the water in the tank is allowed to heat up again by slow extraction of latent heat from the PCM. Thereby the problem of nonuniform heat flux during the withdrawal of heat from the LHTS system is minimized

Fig. 9. Latent heat storage system for solar thermal applications.

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For air heating applications, since more or less uniform heat flux can be achieved as discussed above, the LHTS system can be used as a single large module and the performance of the unit can be improved by increasing the effective thermal conductivity of the PCM by the addition of high conductivity material like lessing rings into the PCM. The addition of high conductivity material increases the surface heat flux and also more uniform surface heat flux can be achieved as the maximum conductive resistance (variable resistance) gets reduced.

5. CONCLUSION

A detailed investigation of the different heat transfer enhancement methods for the latent heat thermal storage system has been carried out. The heat transfer enhancement with fin configuration for storage tubes and by using lessing rings in storage tanks is appreciable, and these two methods are highly suitable for solidification enhancement. The storage configuration employing bubble agitation may be suitable for applications where heat transfer enhancement for melting is required. Though the two LHTS systems suggested are technically feasible, their commercialization requires an economical analysis. Ongoing research and close cooperation with industry will bring the LHTS system more simple and costeffective in the near future for many applications.

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