Thermal analysis and exergy evaluation of packed bed thermal storage systems Hitesh Bindra * , Pablo Bueno, Jeffrey F. Morris, Reuel Shinnar 1 Department of Chemical Engineering, City College of New York,160 Convent Ave., New York, NY-10031, USA highlights < Packed bed thermal modeling covers intra-particle diffusion, axial dispersion and ambient heat losses. < Exergy recovery is higher for lower axial dispersion and lower ambient heat losses. < Sensible heat materials have higher exergy recovery than phase change materials for higher energy density systems. article info Article history: Received 1 August 2012 Accepted 6 December 2012 Available online 21 December 2012 Keywords: Thermal storage PCM Exergy Packed beds abstract Thermal energy storage in a packed bed is analyzed for dynamic temperature response during cyclic storage and recovery. A robust system-scale heat transfer model for the packed bed is developed and accounts for wall heat transfer and intra-particle diffusion effects. Based upon the temperature field modeling, recovered and lost exergy are calculated. The analysis shows that, for packed beds, sensible heat storage systems can provide much higher exergy recovery as compared to phase change material (PCM) storage systems under similar high temperature storage conditions. The roles of axial dispersion and wall heat losses are considered. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Due to the growing need for extracting energy from renewable sources, which are generally intermittent in nature, much attention has been directed towards energy storage in the last few years. Thermal storage, although inefficiently done in the past, has remained in focus because various sources provide energy in the form of heat. Several methods of storing thermal energy, for different temperature ranges based upon applications, have been discussed and reviewed [1e4]. At present the preferred methods for storing heat in a packed bed are sensible heat (solid particles) and latent heat using encapsulated phase change materials (PCMs). In this work the performance of both methods with different design and operating conditions is evaluated and discussed. In large-scale industrial and high temperature applications, sensible and latent heat storage involving a porous bed have been analyzed and designed extensively. The objective of this study is not to highlight the technical challenges and economic comparisons between different methods but to examine the effect of storage methodology on thermodynamic exergy or availability. Early optimal designs of sensible heat storage systems were based upon first law considerations, which emphasized higher energy density and efficiency. Bejan [5] showed the need of exergy analysis for bath-type systems undergoing thermal storage; this work detailed the optimum thermal storage design on the basis of the second law of thermodynamics for a well- mixed system. Krane [6] extended this approach to show that exergy efficiency for a complete well-mixed storage-recovery cycle is less than 27%. Both analyses assumed the storage element to be a liquid bath and modeled it as a lumped system. As one would expect in the case of a completely mixed flow type of system, the inlet stream temperature is always higher than that of the outlet stream, which is at the bath temperature. Therefore during the recovery process the maximum temperature that can be obtained from a mixed tank is always less than the inlet temperature of the fluid during the storing process. Both studies [5,6] showed that the mean temperature at which energy is recovered from the stored heat is significantly lower than that if * Corresponding author. Tel.: þ1 217 898 6006. E-mail addresses: nuchembins@gmail.com, hbindra@che.ccny.cuny.edu (H. Bindra). 1 Deceased 19 August 2011. Contents lists available at SciVerse ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2012.12.007 Applied Thermal Engineering 52 (2013) 255e263