Simulated performance of storage materials for pebble bed thermal energy storage (TES) systems A. Mawire * , M. McPherson, R.R.J. van den Heetkamp, S.J.P. Mlatho Department of Physics and Electronics, North West University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa article info Article history: Received 10 December 2007 Received in revised form 22 August 2008 Accepted 5 September 2008 Available online 26 October 2008 Keywords: Thermal performance Thermal energy storage Sensible heat materials Oil/pebble-bed abstract A simplified one dimensional single phase model for an oil pebble thermal energy storage system is used to examine the thermal performance of three solid sensible heat pebble materials. These are fused silica glass, alumina and stainless steel. The model is validated with experimental results and reasonable agree- ment is achieved between experiment and simulation. The thermal performance of these materials is evaluated in terms of the axial temperature distribution, the total energy stored, the total exergy stored and the transient charging efficiency. The results indicate that not only is the value of the total amount of energy stored important for the thermal performance of oil-pebble-bed systems but also that the amount of exergy stored and the degree of thermal stratification should be considered. A high ratio of the total exergy to the total energy stored is suggested as a good measure of the thermal performance of the peb- ble material. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Sensible heat storage materials are cheaper and usually have lar- ger thermal conductivities when compared to phase change mate- rial (PCM) for solar applications. Although having a larger energy storage density than sensible heat material, PCM degrades with thermal cycling especially at high temperatures and thus it cannot be used effectively for long-term TES applications. A single phase sensible heat TES system is usually expensive to operate and hence dual phase systems using air as the heat transfer fluid and pebbles as the solid phase have been applied in recent years for different solar thermal applications [1–4]. A main disadvantage of air as a heat transfer fluid is that for operation, a large-sized heat exchanger is re- quired. As a result, a high pump power is required to store a large amount of heat due to the low density of the air. Air can, however, be replaced with a liquid and this offers the advantage of a lower va- pour pressure. This then allows for the use of a low cost storage tank. Furthermore, since the solid bed materials allow for both hot and cold fluid to be stored within a single tank, an additional storage tank is not required and this reduces heat losses. Water is the most widely available liquid and it can be used in liquid–solid bed TES systems as reported in Refs. [5–6]. Water can not easily, however, be used at temperatures higher than 100 °C since expensive pressure equipment would be required in order to raise its operating temperature range. In place of water, thermal heat transfer oils can be used with solid pebbles and this allows for an increase in the operating temperature without requiring expen- sive pressure equipment. Thermal oils are particularly attractive for relatively medium temperature domestic solar thermal applica- tions like solar cooking in the range of 100 to 300 °C. This operating range is below the thermal degradation temperatures of most of these oils. For TES systems for solar applications, a high degree of thermal stratification along the height of the storage increases the overall efficiency of the system by ensuring that the top of the storage is hotter than the bottom. A larger temperature difference between the top and the bottom implies that the TES system can store more energy. A high value of the transient charging efficiency implies that a good degree of thermal stratification in a TES system is maintained during charging. The total amount of energy stored should be high for a sensible heat TES system to be economically viable. The total energy is related to the specific heat capacity of the storage media. Although a high specific heat capacity material is required for a greater amount of the energy stored, a material with a high specific heat capacity does not necessary give the best thermal performance in terms of the quality of the energy stored. The quality of energy stored based on the second law of thermody- namics is given by the exergetic content in the process of charging a TES system. The exergetic content accounts for the irreversibili- ties leading to a loss in the quality of the energy stored during the charging process of the TES system. The exergetic loss in a TES system is caused by an external parameter such as heat loss to the surroundings which can not be accounted for by an energetic evaluation of the system. It is thus essential to evaluate a sensible 0306-2619/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2008.09.009 * Corresponding author. Tel.: +27 18 3892449; fax: +27 18 3892052. E-mail addresses: ashmawire02@yahoo.co.uk, 18027938@student.nwu.ac.za (A. Mawire). Applied Energy 86 (2009) 1246–1252 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy