Experimental and simulated temperature distribution of an oil-pebble bed thermal energy storage system with a variable heat source A. Mawire * , M. McPherson Department of Physics and Electronics, North West University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa article info Article history: Received 1 June 2007 Accepted 26 May 2008 Available online 7 June 2008 Keywords: Oil-pebble bed Thermal energy storage Temperature distribution Modelling Simulation Charging and discharging abstract Axial temperature distributions of a thermal energy storage (TES) system under variable electrical heat- ing have been investigated. An electrical hot plate in thermal contact with a hollow copper spiral coil through which the oil flows simulates a solar collector/concentrator system. The hot plate heats up the oil which flows through the storage thus charging the TES system at a constant controlled temperature. The Schumann model and the modified Schumann model for the dynamic temperature distributions in the TES system are implemented in Simulink. The simulated results are compared with experimental results during the charging and discharging of the TES system. The Schumann model is in close agree- ment with experiment at lower TES temperatures during the early stages of the charging process. How- ever, larger deviations between experiment and simulation are seen at later stages of the charging process and this is due to heat losses that are unaccounted for. The modified Schumann model is in closer agreement with experiment at later stages of the charging process. The discrepancies between experi- ment and simulation are also discussed. Discharging simulation results using both models are compara- ble to the experimental results. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction There has been considerable research carried out on the charac- terization of a packed bed as a means of thermal energy storage (TES) for various applications [1–3]. Most applications, however, are limited to generally low temperature cases [4–6] in which air is the heat transfer medium to the various solid particulate matter. Air-rock pebble bed TES systems have generally been applied to space heating and cooling applications. Air has the disadvantage of effective containment at high temperatures since it has a high diffusion rate. For higher temperature TES systems, liquid systems become a viable option since the liquids are easier to contain at higher temperatures. Water has an excellent heat capacity but suf- fers from the disadvantage that it vapourises at 100 °C. This means that a method of pressurizing it is necessary if it should be used for temperatures above its boiling point. Heat transfer oils that have low heat capacities than water can be used for TES systems as is applied at the solar thermal power station described in Ref. [7]. The use of heat transfer oils rather than water reduces the need for expensive pressurizing equipment. These heat transfer oils are usually expensive but can be used in contact with solid media to reduce the cost of the TES system. Most applications of solid–liquid phase TES systems have been limited to solar thermal power plants as is reported by Pacheco et al. [8] and by Bhavsar and Balakrishnan [9]. Most solar cookers [10–12] reported recently tend to use phase change material (PCM) which has a very high TES density as opposed to sensible heat material. PCM is rather expensive when temperatures of high- er than 150 °C are concerned. High TES temperatures and high rates of TES extraction result in faster cooking speeds. Sandy pebbles in thermal contact with a heat transfer oil can be used for a medium to high temperature sensible heat TES system for a solar cooker. To characterize the thermal response of a TES system, it is nec- essary to develop a dynamic model for the system and to verify it experimentally. Different models of varying complexity [13–15] for a packed bed porous media have been reported over recent years. Most models assume that the specific heat capacity and the density of the heat transfer fluid are constant during the charg- ing process. This assumption is only valid when the temperature range of use of the system is small for example in space heating. For high temperature ranges, the density and the specific heat capacity change significantly with an increase in temperature. Also as noted by Al-Nimr et al. [16], during the charging process the in- let temperature does not remain constant but varies according to the power value of the heating source, the charging flow rate and the outlet charging temperature. For solar applications, the heat source is not constant but varies sinusoidally [17]. It is thus necessary to simulate the variation of 1359-4311/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2008.05.028 * Corresponding author. Tel.: +27 18 3892174; fax: +27 18 3892052. E-mail addresses: ashmawire02@yahoo.co.uk, 18027938@student.nwu.ac.za (A. Mawire). Applied Thermal Engineering 29 (2009) 1086–1095 Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng