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  • Featured in Physics
  • Open Access

Unsteady Inherent Convective Mixing in Thermal-Energy-Storage Systems during Standby Periods

Henning Otto1, Clemens Naumann1, Christian Odenthal2, and Christian Cierpka1,*

  • 1Institute of Thermodynamics and Fluid Mechanics, Technische Universität Ilmenau, Am Helmholtzring 1 Ilmenau 98693, Germany
  • 2Institute of Engineering Thermodynamics, German Aerospace Center (DLR), Linder Höhe, Köln 51147, Germany

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PRX Energy 2, 043001 – Published 2 October, 2023

DOI: https://doi.org/10.1103/PRXEnergy.2.043001

Abstract

Recent studies on the flow phenomena in stratified thermal-energy-storage (TES) systems have shown that heat conduction from the hot upper fluid layer through the vertical tank sidewall into the lower cold fluid layer leads to counterdirected wall jets adjacent to the vertical sidewalls. It was shown that these phenomena destroyed half of the total exergy content in less than a tenth of the storage time constant of a 2-m3 stratified TES system. This paper investigates short-term fluctuations of the wall jets since these fluctuations can potentially mix the hot and cold zones of the thermal stratification that are separated by the thermocline region. Using particle-image velocimetry measurements in two regions of a TES model experiment (near-wall region and far-field region) and analyzing the frequency content of the velocity fields revealed characteristic oscillations for different regions. In the near-wall region, observed fluctuations agreed well with an adjusted boundary layer frequency from the literature, showing that the wall jet is transitioning from laminar to turbulent flow. In the far-field region, the oscillations are related to the Brunt-Väisälä frequency. It is shown that the fluctuations from the boundaries of the thermocline region are most dominant and propagate into deeper regions of the thermocline. A comparison to data from the large-scale test facility for thermal energy storage in molten salt at the German Aerospace Center in Cologne showed good agreement. The consensus between the two experiments proves firstly that a small-scale model experiment with water as a storage liquid can be used to analyze the physical phenomena of large-scale molten salt storage facilities and secondly that these fluctuations are relevant for exergy destruction in real-scale TES.

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synopsis

Turbulent Jets Doubly Detrimental to Fluid-Based Batteries

Published 2 October, 2023

Jets that develop along the walls of fluid-based thermal-energy-storage systems induce multiple flows that limit the devices’ ability to store energy.

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Popular Summary

The efficiency of a sensible-heat thermal-energy-storage system relies on minimizing the mixing between the hot and cold fluid regions. Here, the authors show that in a stratified thermal-energy-storage tank an unsteady inherent flow develops and promotes mixing of the hot and cold regions. They studied this mixing behavior using laser-based measurements in a model system, and frequency analysis showed that flow near the walls of the thermal storage tank fluctuates periodically. This wall-jet phenomenon was assigned to a transition from laminar to turbulent flow, and it triggers additional fluctuations in the tank that further mix the hot and cold layers. The authors complemented their study by additional measurements on an industrially-relevant, large-scale thermal-energy-storage system using molten salt as the thermal-storage fluid and found similar fluctuations. They suggest that this detrimental mixing behavior could be limited by modifying the tank walls with additional insulation or structures to reduce flow. These findings will contribute to improving the efficiency of future thermal-energy-storage systems.

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Outline

Information

PRX Energy 2, 043001– Published 2 October, 2023
  • Received 3 April 2023
  • Revised 25 July 2023
  • Accepted 4 August 2023
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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