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Performance response of packed-bed thermal storage to cycle duration perturbations

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Title: Performance response of packed-bed thermal storage to cycle duration perturbations
Authors: McTigue, JD
Markides, C
White, AJ
Item Type: Journal Article
Abstract: Packed-bed thermal stores are integral components in numerous bulk electricity storage systems and may also be integrated into renewable generation and process heat systems. In such applications, the store may undergo charging and discharging periods of irregular durations. Previous work has typically concentrated on the initial charging cycles, or on steady-state cyclic operation. Understanding the impact of unpredictable charging periods on the storage behavior is necessary to improve design and operation. In this article, the influence of the cycle duration (or ‘partial-charge’ cycles) on the performance of such thermal stores is investigated. The response to perturbations is explained and provides a framework for understanding the response to realistic load cycles. The packed beds considered here have a rock filler material and air as the heat transfer fluid. The thermodynamic model is based on a modified form of the Schumann equations. Major sources of exergy loss are described, and the various irreversibility generating mechanisms are quantified. It is known that repeated charge-discharge cycles lead to steady-state behavior, which exhibits a trade-off between round-trip efficiency and stored exergy, and the underlying reasons for this are described. The steady state is then perturbed by cycles with a different duration. Short duration perturbations lead to a transient decrease in exergy losses, while longer perturbations increase it. The magnitude of the change in losses is related to the perturbation size and initial cycle period, but changes of 1–10 % are typical. The perturbations also affect the time to return to a steady-state, which may take up to 50 cycles. Segmenting the packed bed into layers reduces the effect of the perturbations, particularly short durations. Operational guidelines are developed, and it is found that packed beds are more resilient to changes in available energy if the store is not suddenly over-charged (i.e. longer perturbations), and if the steady-state cycle duration is relatively long. Furthermore, using the gas exit temperature to control cycle duration reduces the impact of perturbations on the performance, and reduces the time to return to steady-state operation.
Issue Date: 1-Oct-2018
Date of Acceptance: 20-Aug-2018
URI: http://hdl.handle.net/10044/1/63743
DOI: https://dx.doi.org/10.1016/j.est.2018.08.016
ISSN: 2352-152X
Publisher: Elsevier
Start Page: 379
End Page: 392
Journal / Book Title: Journal of Energy Storage
Volume: 19
Copyright Statement: © 2018 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/)
Sponsor/Funder: Engineering & Physical Science Research Council (EPSRC)
Engineering & Physical Science Research Council (EPSRC)
Funder's Grant Number: EP/J006041/1
EP/P004709/1
Keywords: Science & Technology
Technology
Energy & Fuels
Thermal energy storage
Thermocline
Packed bed
Exergy analysis
Heat transfer
Partial-charge
CONCENTRATING SOLAR POWER
PILOT-SCALE DEMONSTRATION
AIR ENERGY-STORAGE
ELECTRICITY STORAGE
SYSTEM
OPTIMIZATION
PLANT
HEAT
Publication Status: Published
Online Publication Date: 2018-09-01
Appears in Collections:Faculty of Engineering
Chemical Engineering



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