Research paper Numerical and experimental studies on heat transfer characteristics of thermal energy storage system packed with molten salt PCM capsules Selvan Bellan a , Tanvir E. Alam b , Jos  e Gonz  alez-Aguilar a, * , Manuel Romero a , Muhammad M. Rahman b, c , D.Yogi Goswami b, d , Elias K. Stefanakos b, e a IMDEA Energy Institute, Ramon de la Sagra 3, 28935 M ostoles, Spain b Clean Energy Research Center, University of South Florida, Tampa, FL, USA c Department of Mechanical Engineering, University of South Florida, Tampa, FL, USA d Department of Chemical & Biomedical Engineering, University of South Florida, Tampa, FL, USA e Department of Electrical Engineering, University of South Florida, Tampa, FL, USA highlights  Presents numerical analysis of heat transfer characteristics of TES system packed with molten-salt PCM capsules.  Presents a lab scale experimental setup of high temperature latent thermal energy storage system.  Presents continues solid phase and effective packed bed modeling approach of the system.  Presents the inﬂuence of thermal conductivity and thickness of the shell on the thermal performance of the system. article info Article history: Received 20 April 2015 Accepted 18 July 2015 Available online 30 July 2015 Keywords: Molten salt capsules Latent thermal energy storage Thermocline system Encapsulated phase change material Concentrating solar power abstract In order to avoid intermittent energy supply problems, thermal energy storage system plays an impor- tant role in concentrated solar power plants. Thus, a signiﬁcant focus has been given on the improvement of thermal energy storage systems from the past few decades. In this paper, the dynamic thermal per- formance of high temperature latent thermal energy storage system packed with spherical capsules is analyzed experimentally and numerically. The spherical capsules are encapsulated by sodium nitrate and air is used as heat transfer ﬂuid. Transient two-dimensional continuous solid phase and effective packed bed models are developed and validated by comparing to the experimental results. Using these models, detailed characteristics of the heat transfer between the capsules and heat transfer ﬂuid are analyzed. Parametric analyses are conducted to study the inﬂuence of mass ﬂow rate, Stefan number, thickness and the thermal conductivity of the shell. The results indicate that the Stefan number plays a vital role on the total heat storage capacity due to sensible heat, and the shell properties of the capsule signiﬁcantly inﬂuence the thermal performance of the system; the inﬂuence of the shell thickness increases (de- creases) when the thermal conductivity of the shell is low (high). © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Thermal energy storage (TES) system plays a vital role in concentrating solar power (CSP) plants since it stores the solar thermal energy for later use. Consequently, the CSP plant increases the duration of electricity generation and provides high annual capacity factors [1,2]. Although the two tank molten salt TES system has been commercially used in several power plants (e.g Gemasolar tower plant, Spain (19.9 MW)), there has been lack of space for cost reduction [2]. Thus, in order to reduce the cost, one tank TES system packed with solid ﬁllers has been used, e.g., silica sand and quartzite rock have been used as solid ﬁllers [3], which reduces the capital cost compared to the two-tank system, about 35% [4]. Latent heat storage technique can be used to reduce the size of the storage tank and the capital costs as it provides higher storage density than the sensible heat storage technique. Thus, several investigations have been conducted to store the thermal energy in the PCM based TES systems by latent heat of fusion e.g. [5e15]. TES systems based * Corresponding author. E-mail address: jose.gonzalez@imdea.org (J. Gonz alez-Aguilar). Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng http://dx.doi.org/10.1016/j.applthermaleng.2015.07.056 1359-4311/© 2015 Elsevier Ltd. All rights reserved. Applied Thermal Engineering 90 (2015) 970e979