Contents lists available at ScienceDirect The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu Thermal energy storage with supercritical carbon dioxide in a packed bed: Modeling charge-discharge cycles Erick Johnson a,c , Liana Bates b , April Dower b , Pablo C. Bueno d , Ryan Anderson b,c, ⁎ a Department of Mechanical Engineering, Montana State University, Bozeman, MT 59717, United States b Department of Chemical and Biological Engineering, Montana State University, Bozeman, MT 59717, United States c Energy Research Institute, Montana State University, Bozeman, MT 59717, United States d Southwest Research Institute, Mechanical Engineering Division, San Antonio, TX 78238, United States GRAPHICAL ABSTRACT ARTICLE INFO Keywords: Supercritical carbon dioxide Thermal energy storage Sensible heat Packed beds Brayton cycle ABSTRACT Thermal energy storage in concentrated solar power systems extends the duration of power production. Packed bed thermal energy storage is studied in this work with supercritical carbon dioxide as the working fluid and α- alumina as the storage material. The operating conditions are appropriate for use in a supercritical Brayton cycle. An axisymmetric model produces temperature profiles in the bed, insulation, and pressure vessel in the axial and radial directions over time. The packed bed system has a mass flow rate of 8.17 kg s −1 at 275 bar. The inlet temperature is 750 °C for storage. In discharge, gas at 500 °C enters the bed to recover the stored energy. Discharge continues until the outlet temperature drops below 700 °C, the minimum temperature required for the turbine inlet. Ten charge-discharge cycles are considered and thermal exergetic efficiency is calculated. Due to thermal dispersion and heat losses, the exergetic efficiency varies from 0.795 to 0.844. 1. Introduction Concentrated solar power (CSP) is an appealing renewable source of energy. Distinct from solar photovoltaics, CSP systems concentrate solar energy by reflecting sunlight to a receiver with an array of specialized mirrors. At the receiver, a thermal energy carrier is heated, and the heat is used to generate power [1]. In 2016, global CSP systems accounted for 4805 MW with the U.S. and Spain at 1745 MW and 2304 MW, re- spectively. Current state-of-the-art systems utilized molten salts at temperatures of 565 °C with a steam-Rankine power cycle with esti- mated costs in the 10–14 ¢/kWh range [2]. However, this cost remains higher than the goal of 6¢/kWh e by 2020 under the SunShot Initiative of the US Department of Energy (DOE) [3,4]. One limiting factor in CSP applications that increases cost is that solar energy is variable and in- termittent, such as from summer to winter or day to night [1], ne- cessitating some type of thermal energy storage (TES) system [4,5]. Various pathways are being considered including improvements to molten salt technology, falling particle systems, and gas phase systems https://doi.org/10.1016/j.supflu.2018.03.009 Received 23 December 2017; Received in revised form 13 March 2018; Accepted 14 March 2018 ⁎ Corresponding author at: Montana State University, 306 Cobleigh Hal, Bozeman, MT 59717, United States. E-mail address: ryan.anderson@montana.edu (R. Anderson). The Journal of Supercritical Fluids 137 (2018) 57–65 Available online 17 March 2018 0896-8446/ © 2018 Elsevier B.V. All rights reserved. T