Abstract—The present study investigated combined enhancement of heat conduction and thermal radiation in a finned cylinder during the solidification (energy recovery) of a non-gray, non-opaque phase change material. Transient heat transfer in a symmetric, two-dimensional design is considered. The radiative transport equation is solved by using the Discrete Ordinate Method (DOM) while Finite Volume Method is used to discretize and solve equations for the conservation of mass, momentum, and energy. It was found that energy recovery time can be reduced by 74% by controlling the optical thickness property of the PCM with embedded radiation absorbing particles. Index Terms—Phase change material, radiation, solidification, thermal energy storage module. I. INTRODUCTION Designing an effective energy storage system is essential for base load power generation using solar energy, which is gaining considerable attention in the renewable energy community. Currently, most thermal energy storage systems that have been commercially deployed use sensible energy storage materials. Alternatively, it has been already proven by researchers that latent heat energy storage systems using phase change materials (PCMs) can provide significant advantage compared to that of the sensible energy storage technology. PCM can keep up the temperature of the heat transfer fluid (HTF) required to maintain its potential to produce work in the power generation system. Using PCMs as Thermal Energy Storage (TES) medium can provide relatively low cost due to their high heat of fusion which results in smaller storage volume. One major drawback of PCMs, especially in large-scale systems, is their low thermal conductivity. This would cause a negative effect by delaying the phase change; hence force slow down the charge/discharge time. Fortunately, for high temperature energy storage applications geared towards electric power generation using solar energy, one can take advantage of enhancing thermal radiation alongside enhancing thermal conduction. Several studies aimed at overcoming this issue of low energy storage and recovery rates by increasing the heat transfer area between the storage material and the heat Manuscript received April 8, 2020; revised June 9, 2020. Marwan Belaed is with the Department of Mechanical Engineering, University of South Florida, Tampa, Florida 33620, USA (e-mail: marwan1@ mail.usf.edu). Muhammad Mustafizur Rahman is with the Department of Mechanical Engineering, Wichita State University, Wichita, Kansas 67260, USA (e-mail: muhammad.rahman@wichita.edu). transfer fluid. This was achieved by adding fins, heat pipes or microencapsulation of the PCM. Improving the thermal properties of the PCM such as density and thermal conductivity by including highly conductive nanoparticles and foams were also considered in some investigations. Yang et al. [1] numerically investigated the annular fins with different number of fins, height and thickness. They found that using annular fins in the PCM can reduce the melting time by 65%. Nithyanandam and Pitchumani [2] designed a thermal resistance network to study the response of PCM with four embedded heat pipes in terms of their effectiveness. It was found that the effectiveness of the heat pipes reduced with increase in the HTF mass flow rate, radius of the tube, and length of the system. On the other hand, the effectiveness could be increased by increasing the length of the evaporator and condenser sections. Nithyanandam and Pitchumani [3] presented a study of the latent heat energy storage system with embedded gravity-assisted heat pipes. The analysis found that a larger spacing between the pipes yields a lower heat transfer rate between the PCM and the HTF. Zeng et al. [4] experimentally developed a phase change material with enhanced thermal conductivity via in-situ polymerization. Elgafy and Lafdi [5] experimentally and analytically studied the thermal behavior of carbon nanofiber filled paraffin wax PCM. It was found that including the carbon nanofiber into the system enhanced the thermal conductivity. Khodadadi and Hosseinizadeh [6] improved the solidification time by using the nanoparticle enhanced PCM and nanofluid as HTF resulting an increase in the heat transfer rate between the PCM and the HTF. Mahdi and Nsofor [7] studied the effect of fins, nanoparticles, and a combination of both on the thermal behavior of PCM. The investigation found that using the fins alone gave more optimum results compared to the other two methods. Using fins as means to improve heat transfer is an attractive method due to their compactness and simple structure. Popular fin configurations include annular, longitudinal, pin, and plate. Longitudinal fins are quite desirable as they allow for easier design and fabrication. The convenience of heat exchange with the storage medium is another advantage of these fins. Experimental and numerical studies by Velraj et al. [8] found that the thermal resistance between the liquid PCM and the module surface increased as solid phase started to form next to the inner wall during energy recovery. However, the presence of fins in the storage system led to an overall decrease in the solidification time. Castell et al. [9] experimentally investigated the natural convection heat transfer coefficient in a cylindrical phase change energy storage module with external vertical fins. The Enhancement of Energy Recovery in a Phase Change Energy Storage Module with Embedded Radiation Absorbing Particles Marwan Belaed and Muhammad Mustafizur Rahman Journal of Clean Energy Technologies, Vol. 8, No. 3, July 2020 24 doi: 10.18178/jocet.2020.8.3.521