Elsevier

Solar Energy

Volume 65, Issue 3, 1 February 1999, Pages 171-180
Solar Energy

HEAT TRANSFER ENHANCEMENT IN A LATENT HEAT STORAGE SYSTEM1

https://doi.org/10.1016/S0038-092X(98)00128-5Get rights and content

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.

Section snippets

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

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 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 natural-convection-controlled heat transfer. Conduction is the controlling mode when

EXPERIMENTAL INVESTIGATION

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

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 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

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

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1

Paper presented at the ISES Solar World Congress, Taejon, South Korea, 24–29 August 1997.

2

ISES member.

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