Heat transfer enhancement in medium temperature thermal energy storage system using a multitube heat transfer array
Introduction
A major drawback in the development of latent heat thermal energy storage systems has been the low thermal conductivities possessed by most phase change materials (PCMs). Due to their low thermal conductivities, most phase change materials require the use of heat transfer enhancement techniques to improve the rates of charging and discharging of energy. Some common techniques employed include finned tubes [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], rings and bubble agitation [12], insertion of a metal matrix into the PCM [13], [14], [15], using PCM dispersed with high conductivity materials [16], microencapsulation of PCM [17], [18] and multitubes [1], [19], [20], [21], [22]. A latent heat storage system that promises high energy density and efficient charging for a minimum volume is the shell and tube heat exchanger, with the PCM filling the shell side and the heat transfer fluid (HTF) flowing through the tube(s) conveying the stored energy to and from the storage unit [20]. Several authors have studied shell and tube systems, analysing the thermal characteristics of the heat exchange [22], [23], [24], [25], [26], [27], [28]. One approach to the analysis of a shell and tube system which utilises many tubes (multitubes) to improve the rate of heat transfer into the PCM has been to represent the physical system by a simpler single tube at the centre of the PCM [21], [29]. Ghoneim [21] employed theoretical models, representing multitubes with a single tube, to study the variation of solar fraction with storage volume for air based and water based systems using the thermophysical properties of P116Wax, medicinal paraffin, and rock. The author investigated storage mass per unit collector area, applying an energy balance to a control volume element of the representative single tube based on a low Biot number in order to ignore temperature variations normal to the flow direction. The results were compared to ref. [30]. Both studies concluded that PCMs showed higher heat capacities for storage compared to sensible heat storage systems (rock). Another approach to analysing multitube system has been to choose a numerical domain within a cross-section of the system. Hamada et al. [15] investigated experimentally and numerically the effect of carbon-fibre chips and carbon brushes as additives on the thermal conductivity enhancement of PCM, n-octadecane using four steel tubes arranged vertically in a cylindrical container made of acrylic resin. In the experimental system, the cylindrical container was packed with carbon-fibre chips and carbon brushes. The computational domain in the multitube system was chosen such that the tube pitch, used as a characteristic length, extended between two heat transfer tubes and the PCM at the periphery. The mathematical model was numerically solved using the control volume method by Patankar [31]. Results showed that the carbon brushes were superior to the carbon-fibre chips in enhancing heat transfer in the PCM. The objective of this paper is two fold, study the performance of multitube system compared to a single tube system and test the validity of axisymmetric assumption using a medium temperature thermal energy storage material, Erythritol whose suitability to power a LiBr/H2O absorption cooling system has been reported [32].
Section snippets
Experimental setup and procedure
A schematic diagram of the experimental setup is shown in Fig. 1. The PCM store consisted of a 3 mm thick horizontally mounted cylindrical storage shell made of aluminium with inner diameter of 146.4 mm. A 54 mm diameter copper tube embedded in the PCM served as the heat transfer tube in the control (single tube) system. The second system (multitube system) consisted of the same shell size with four 28 mm cylindrical heat transfer tubes spaced at 120 mm.
The experimental procedure required two fluid
Temperature profile comparison
Fig. 3 shows a comparison of the variation of average PCM and output temperatures with time for the control and multitube systems. Average PCM temperatures were calculated from the thermocouple readings listed in Fig. 2. Sensible heat energy was absorbed by the solid PCM at the beginning of the charging process and the temperature initially increased linearly from 21 °C and tended to asymptote. Average temperatures in the multitube system were higher than those of the control system during the
Conclusion
The experimental study compared the performance of multitube to a single tube shell and tube system. The multitube system improved heat transfer rate during charging and produced output temperature suitable to power a LiBr/H2O absorption cooling system but showed large subcooling. The validity of ignoring the thermal conductivity in the direction parallel to the heat transfer fluid (as reported by most previous authors) has been experimentally verified using temperature gradients in the axial,
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