Pergamon Energy Convers. Mgmt Vol. 35, No. 10, pp. 843-856, 1994 Copyright © 1994 ElsevierScienceLtd 0196-8904(94)E0010-V Printed in Great Britain. All rights reserved 0196-8904/94 $7.00+ 0.00 THERMAL ENERGY STORAGE SYSTEM WITH STEARIC ACID AS PHASE CHANGE MATERIAL AFIF HASAN Mechanical Engineering Department, Birzeit University, P.O. Box 14, Birzeit, West Bank via Israel (Received 7 June 1993; received for publication 18 April 1994) Abstraet--A simple tube-in-tube heat exchanger system for thermal energy storage employing stearic acid as PCM has been investigated in this work. The performance and heat transfer characteristics of such a system were studied. Phase transition temperature range and times were measured and the speed of the phase transition front was computed. The melting front was found to move in the radial direction inward as well as in the axial direction from the top toward the bottom of the phase change material PCM tube. The speed of the melting front is enhanced by a convection heat transfer mechanism in the melted PCM. The heat transfer rate and, consequently, phase transition time can be altered by changing the water inlet temperature to the heat exchanger. In addition, a faster phase transition is realized by placing the heat exchanger in a horizontal position rather than a vertical one. Fatty acids Latent heat Phase change material Phase transition Stearic acid Thermal energy storage NOMENCLATURE C = Specific heat of PCM (kJ/kg °C) H = Latent heat of PCM (kJ/kg) R = Inside radius of PCM tube (m) St = Stefan number defined as C(T m - Tw)/H (dimensionless) t = Time (s) t*= Dimensionless phase transition time defined as c~t/R 2 Tm= Melting temperature of PCM (°C) Tp~ m= PCM temperature (°C) T~ = Temperature of PCM tube wall (°C) T,,~ = Inlet water temperature (°C) Tw2= Outlet water temperature (°C) = Thermal diffusivity (m2/s) INTRODUCTION The storage of thermal energy as the latent heat of a phase change material PCM represents a good attractive option to thermal energy storage. A wide range of PCMs have been investigated, including salt hydrates, paraffin waxes, and non-paraffin organic compounds [1]. Matching of the transition temperature range of the PCM to the delivered energy temperature for a given application is an important aspect of PCM energy storage. Eliminating the problems of subcooling, phase separation and stability over a long period of application are important criteria for successful application of the PCM [2]. Some fatty acids were investigated as PCM for thermal energy storage [3, 4]. Stearic acid can be a suitable material for energy storage in domestic water heating systems, since it has a relatively high latent heat and an appropriate transition temperature of 65-69°C and shows a good stability over a large number of heating and cooling cycles [4]. Utilization of PCM for thermal energy storage requires a proper heat exchanger system for charging and discharging the thermal energy. A tube-in-tube heat exchanger system employing stearic acid for thermal energy storage is investigated in this work. The heat transfer aspects and performance of the system, as well as the rate of the phase transition, are examined here. 843