A comparison of experimental thermal stratiﬁcation parameters for an oil/pebble-bed thermal energy storage (TES) system during charging Ashmore Mawire ⇑ , Simeon H. Taole Department of Physics and Electronics, North West University (Maﬁkeng Campus), Private Bag X2046, Mmabatho 2735, South Africa article info Article history: Received 2 March 2011 Received in revised form 3 June 2011 Accepted 11 June 2011 Available online 7 July 2011 Keywords: Thermal stratiﬁcation Oil/pebble-bed Thermal energy storage Charging abstract Six different experimental thermal stratiﬁcation evaluation parameters during charging for an oil/pebble- bed TES system are presented. The six parameters are the temperature distribution along the height of the storage tank at different time intervals, the charging energy efﬁciency, the charging exergy efﬁciency, the stratiﬁcation number, the Reynolds number and the Richardson number. These parameters are eval- uated under six different experimental charging conditions. Temperature distribution along the height of the storage tank at different time intervals and the stratiﬁcation number are parameters found to describe thermal stratiﬁcation quantitatively adequately. On the other-hand, the charging exergy efﬁ- ciency and the Reynolds number give important information about describing thermal stratiﬁcation qual- itatively and should be used with care. The charging energy efﬁciency and the Richardson number have no clear relationship with thermal stratiﬁcation. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Thermal stratiﬁcation in solar thermal energy storage (TES) sys- tems is important in that it increases the efﬁciency of the system. The increase in the operational efﬁciency due to stratiﬁcation comes from the fact that a hot ﬂuid is delivered at the top of the storage and a cold ﬂuid exits at the bottom of the storage resulting in a larger thermal gradient. The larger thermal gradient results in a greater potential of thermal energy to be stored. Stratiﬁcation distributes the stored heat making it easier to use. Another advan- tage of stratiﬁcation is that the efﬁciency of a solar collector is in- creased as a cooler ﬂuid from the bottom of the storage is heated in the collector. Concerted efforts have been done by earlier works to evaluate thermal stratiﬁcation within the hot water storage tank [1–10]. Brumleve [1] demonstrated the possibility of separating hot and cold water inside a water tank using a natural thermocline. A numerical study with a two-dimensional model of solar storage tanks was done by Cabelli [2]. The effects of the entrance Reynolds number and the contribution of buoyancy in promoting stratiﬁca- tion were studied. Stratiﬁcation in thermal storage during charging was studied in [3]. A set of dimensionless parameters for predicting the occurrence and the degree of thermal stratiﬁcation in thermal energy storage tanks was identiﬁed and it was found out that there was a critical value of the Richardson number below which strati- ﬁcation did not occur. Thermally stratiﬁed hot water storage tanks were studied experimentally by Wood et al. in 1981 [4]. Design correlations suitable for the use with cubical storage tanks were derived and analytical predictions of the thermocline showed good agreement with experimental results. Thermally stratiﬁed tanks were studied by Cole and Bellinger [5] and they concluded that an aspect ratio of 4 maximised thermal stratiﬁcation. Modelling and operation characterization of thermosyphon solar water heat- ers was carried out in [6]. The results indicated that thermosyphon systems had optimum performance when the daily collector ﬂow was approximately equal to the daily load volume. Zurigat et al. [7] performed a study on the inﬂuence of inlet geometry on the degree of stratiﬁcation attainable in thermocline thermal energy storages. Based on the obtained correlations it was concluded that the inlet geometry started to inﬂuence thermal stratiﬁcation in a thermocline thermal energy storage tank for Richardson numbers below 3.6. A numerical study of the effect of the inlet geometry on stratiﬁcation in a thermal energy storage system was done by [8]. Results showed that the effect of the inlet geometry is negligi- ble for Richardson numbers above 10. Lin et al. [9] investigated on a solar water heating system with natural circulation assisted by an auxiliary electric heater with the use of a model. The model agreed well with experimental results. An analytical and experi- mental investigation of thermal stratiﬁcation in storage tanks was carried out in [10]. Parametric studies showed that the turbu- lent mixing factor due to hydrodynamic disturbances at the inlet ports was the most signiﬁcant element in the performance of ther- mal stratiﬁcation in storage tanks. There has been considerable research carried out on evaluating different thermal stratiﬁcation parameters for solar hot water 0306-2619/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apenergy.2011.06.019 ⇑ Corresponding author. Tel.: +27 18 3892127; fax: +27 18 3892052/2377. E-mail addresses: ashmawire02@yahoo.co.uk, 18027938@nwu.ac.za (A. Mawire). Applied Energy 88 (2011) 4766–4778 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy