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Phase diagram, thermodynamic properties and long-term isothermal stability of quaternary molten nitrate salts for thermal energy storage

Bonk, Alexander and Braun, Markus and Bauer, Thomas (2021) Phase diagram, thermodynamic properties and long-term isothermal stability of quaternary molten nitrate salts for thermal energy storage. Solar Energy (231), pp. 1061-1071. Elsevier. doi: 10.1016/j.solener.2021.12.020. ISSN 0038-092X.

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Official URL: https://www.sciencedirect.com/science/article/abs/pii/S0038092X21010690

Abstract

Thermal energy storage (TES) is a vastly growing technique that allows for the generation of dispatchable electricity in modern concentrating solar power (CSP) plants. In solar tower systems the key to success is the use of a heat transfer fluid (HTF) and storage medium known as Solar Salt. This nitrate salt mixture of NaNO3 and KNO3 is considered thermally stable, non-toxic and environmentally friendly. In line-focusing CSP plants Solar Salt is still not being used as heat transfer fluid due to the inherent risk of freezing and related systematic failure. Accordingly, there is a need to develop nitrate salts with lower melting temperatures but yet acceptably high thermal stability. The information on molten nitrate mixtures with low melting points (=<100 °C) is limited, especially in terms of isothermal stability tests. This work presents thermo-physical data of the complex quaternary Ca,Li,Na,K//NO3 system. Melting temperatures of more than 100 mixtures were assessed and compositions with melting points below 100 °C were identified. The heat capacity of selected mixtures was in the range of 1.5-1.6 kJ/kg K and generally increased with increasing Li-content. Thermal stability, with Solar Salt as reference salt, indicated that the stability of all mixtures did not exceed 500 °C but that the achievable dT was 360 °C, about 90 °C higher than that of Solar Salt. Some compositions are therefore are potential HTF and storage media but the overall Ca-content plays a crucial role in the decomposition of the quaternary mixtures during operation at high temperatures.

Item URL in elib:https://elib.dlr.de/148154/
Document Type:Article
Title:Phase diagram, thermodynamic properties and long-term isothermal stability of quaternary molten nitrate salts for thermal energy storage
Authors:
AuthorsInstitution or Email of AuthorsAuthor's ORCID iD
Bonk, AlexanderAlexander.Bonk (at) dlr.dehttps://orcid.org/0000-0002-0676-7267
Braun, Markusmar.braun (at) dlr.deUNSPECIFIED
Bauer, Thomasthomas.bauer (at) dlr.dehttps://orcid.org/0000-0003-4080-7944
Date:28 December 2021
Journal or Publication Title:Solar Energy
Refereed publication:Yes
Open Access:No
Gold Open Access:No
In SCOPUS:Yes
In ISI Web of Science:Yes
DOI :10.1016/j.solener.2021.12.020
Page Range:pp. 1061-1071
Publisher:Elsevier
ISSN:0038-092X
Status:Published
Keywords:thermal stability, molten salt, concentrating solar power (CSP), heat transfer fluids (HTF)
HGF - Research field:Energy
HGF - Program:Materials and Technologies for the Energy Transition
HGF - Program Themes:High-Temperature Thermal Technologies
DLR - Research area:Energy
DLR - Program:E SP - Energy Storage
DLR - Research theme (Project):E - Thermochemical Processes
Location: Stuttgart
Institutes and Institutions:Institute of Engineering Thermodynamics > Thermal Process Technology
Deposited By: Bonk, Alexander
Deposited On:11 Jan 2022 09:48
Last Modified:24 May 2022 23:48

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