A review on packed bed solar energy storage systems Harmeet Singh a, *, R.P. Saini a , J.S. Saini b a Alternate Hydro Energy Centre, Indian Institute of Technology Roorkee, Roorkee 247667, India b Mechanical and Industrial Engineering Department, Indian Institute of Technology Roorkee, Roorkee 247667, India Contents 1. Introduction .................................................................................................... 1059 2. Packed bed energy storage system .................................................................................. 1060 3. Analytical studies................................................................................................ 1061 4. Experimental investigations ....................................................................................... 1064 5. Conclusions .................................................................................................... 1068 References ..................................................................................................... 1068 1. Introduction Conversion of solar energy into thermal energy is the easiest and the most widely accepted method. However, solar energy is a time dependent energy resource. Due to intermittent nature of solar energy, an energy storage unit is required to be attached with solar collectors to store energy for use when sunshine is not available. A storage system therefore constitutes an important component of the solar energy utilization system. A line diagram of a typical solar energy utilization system is shown in Fig. 1. The different forms of energy that can be stored include mechanical, electrical and thermal energy [1]. Thermal energy storage is of particular interest and significance in using this technique for solar thermal applications. Thermal energy can be stored as sensible heat, latent heat or chemical energy. An overview of major technique of storage of solar thermal energy is shown in Fig. 2. In sensible heat storage, heat is stored by increasing the storage medium temperature. In case of latent heat storage systems, the energy is stored in phase change materials. The heat is stored when the material changes phase from solid to a liquid. Thermo chemical storage is a technique, which involves chemical reactions. Sensible heat storage is the most simple and inexpensive way of energy storage system although there are few advantages of phase change energy storage over sensible heat storage, but the technological and economical aspects make sensible heat storage superior. Packed beds represent the most suitable storage units for air-based solar system. A packed bed storage system consists of loosely packed solid material through which the heat transport fluid is circulated. Heated fluid (usually air) flows from solar collectors into a bed of graded particles from top to bottom in which thermal energy is transferred during the charging phase as shown in Fig. 3. Coutier and Farber [2] mentioned that packed bed generally represents the most suitable energy storage unit for air based solar Renewable and Sustainable Energy Reviews 14 (2010) 1059–1069 ARTICLE INFO Article history: Received 31 August 2009 Accepted 20 October 2009 Keywords: Sensible heat storage Packed beds Sphericity Void fraction ABSTRACT Because of intermittent nature of solar energy, storage is required for uninterrupted supply in order to match the needs. Packed beds are generally used for storage of thermal energy from solar air heaters. A packed bed is a volume of porus media obtained by packing particles of selected material into a container. A number of studies carried out on packed beds for their performance analysis were reported in the literature. These studies included the design of packed beds, materials used for storage, heat transfer enhancement, flow phenomenon and pressure drop through packed beds. This paper presents an extensive review on the research carried out on packed beds. Based on the literature review, it is concluded that most of the studies carried out are on rocks and pebbles as packing material. A very few studies were conducted on large sized packing materials. Further no study has been reported so far on medium sized storage elements in packed beds. ß 2009 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +91 9411111692. E-mail address: harmeetpawar@gmail.com (H. Singh). Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser 1364-0321/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.rser.2009.10.022