Experimental evaluation of vitrified waste as solid fillers used in thermocline thermal energy storage with parametric analysis

Abstract

This work presents the first use of Cofalit® (vitrified asbestos-containing waste) as a solid filler in pilot-scale thermocline thermal energy storage (TES). The thermocline size is 4 m³ connected to the MicroSol-R installation at the PROMES research facility in Odeillo, France. The study compares the thermal performance of the thermocline filled with Cofalit® to the reference case of alumina spheres for typical charge and discharge operations. It evaluates the actual thermal behavior of thermocline considering three leading performance indicators: process duration, thermocline thickness, and process efficiency. The investigation shows a 22% shorter charge duration in Cofalit® compared to alumina and 16% shorter discharge duration. Cofalit® exhibits about 16% lower thermocline thickness during both charge and discharge compared to alumina. The charge efficiency is slightly better in Cofalit® than alumina with an efficiency of 82% and 78%, respectively. Also, Cofalit® has a better discharge efficiency, 90% with respect to 84% for alumina. These results confim a good thermal performance of Cofalit® as filler material in thermocline TES. Considering the cost-saving and positive environmental impact of using Cofalit® as well as the good thermal performance of the thermocline filled with it, Cofalit® appears a very good filler material in TES. The obtained temperatures from radial positions indicate no significant variation during charge and discharge, and this confirms the one-dimensional thermal behavior of this setup. A parametric analysis is performed using a 1D continuous solid (C-S) model to investigate the influence of particle diameter, porosity, thermal conductivity, and volumetric heat capacity on the thermal performance of the thermocline. The analysis confirms the experimental findings, and it indicates that about a 10.9% longer process duration is associated with a 10% larger volumetric heat capacity and less affected with other parameters. Thermocline thickness is mainly affected by the diameter as well as the volumetric heat capacity of the solid filler; it grows 2.2% for each 10% diameter increase and around 3.23% for doubling the volumetric heat capacity. Charge efficiency demonstrates independency from evaluated properties. While discharge efficiency increases sharply at a tiny particle diameter before an optimum diameter value is reached, then it starts to decrease by 1.4% to each 10% bigger diameter.

Single-tank thermal energy storage systems for concentrated solar power: Flow distribution optimization for thermocline evolution management

“…Compared to conventional two-tank TES systems, the single-tank thermocline storage is a more cost competitive option (about 35% cheaper) [Grirate et al, 2016;Motte et al, 2015], i.e., the reduced amount of high-priced HTF by about 70% because of using cheap solid or industrial waste as energy storage material [Esence et al, 2017;Galione et al, 2015]. As a result, the interest for this technology is rapidly growing worldwide with a variety of new numerical and experimental developments being reported in the recent literature [Ahmed et al, 2019;Keilany et al, 2020;Pizzolato et al, 2017;Vigneshwaran et al, 2019].…”

Experimental and numerical study of combining encapsulated phase change material to sensible heat storage material in one-tank pilot scale thermal energy storage

“…The sensible heat storage material is 2 cm in diameter alumina spheres that fills the lower part of the tank and has a mass of 4.66 tons. The same materials was used in our previous work inside MICROSOL-R [5].…”

“…For the heat convection coefficient h f-p , Nusselt number (Nu) is calculated from the correlation suggested by Wakao et al [41] Eq. (5).…”

“…Eqs. ( 10) and ( 11) express the effective thermal conductivities of the fluid and the solid particle [5]:…”

“…However, during the charge and discharge process of the TES, a thermal gradient layer, typically called the thermocline thickness (or region), develops between the hot and cold zones of the tank. The quality of stored and released energy degrades inside this region, because the fluid will be at a temperature lower than the useful temperature [5]. While this layer is expanding during the charge and discharge operations, it could account for up to 33% of total tank height [6], which reduces the storage efficiency of the system and influences the outlet temperature of the HTF flowing out the TES.…”