Advanced heat transfer fluids for direct molten salt line-focusing CSP plants TaggedPAlexander Bonk a, *, Salvatore Sau b , Nerea Uranga c , Marta Hernaiz c , Thomas Bauer d TaggedP a German Aerospace Center (DLR), Institute of Engineering Thermodynamics, Stuttgart 70569, Germany b ENEA DTE-STT-ITES, Casaccia Research Center, Rome 00123, Italy c IK4-TEKNIKER Research Center, Eibar 20600, Spain d German Aerospace Center (DLR), Institute of Engineering Thermodynamics, Cologne 51147, Germany TAGGEDPARTICLE INFO Article History: Received 17 July 2017 Accepted 9 February 2018 Available online xxx TAGGEDPABSTRACT Concentrating solar power coupled to thermal energy storage (TES) is a vastly growing industrial process allowing for the generation of dispatchable and green electricity. This paper focuses on direct molten salt line-focusing technology using linear Fresnel and parabolic trough collector systems. Direct molten salt technology utilizes molten salt as heat transfer fluid in solar field and TES medium. Nitrate salts can be applied since they cover a wide temperature range. As storage medium Solar Salt, a binary NaNO 3  KNO 3 (60-40 wt%) mixture, is most commonly used but variations of this system have promising thermal proper- ties in terms of a lower melting temperature to minimize the risk of undesired salt freezing events. These modified salts are typically ternary, ternary reciprocal or higher order systems formed by adding additional cations, anions or both. In this study five molten salt systems Solar Salt, HitecXL (CaKNa//NO3), LiNaK- Nitrate, Hitec (NaK//NO23) and CaLiNaK//NO23 are both investigated and critically reviewed. Their thermo- physical properties including phase diagrams, composition, melting ranges, melting temperature, minimum operation temperature, thermal stability, maximum operation temperature, density, heat capacity, thermal conductivity, viscosity and handling are evaluated and the most recommended values are discussed and highlighted. This review contributes to a better understanding of how the listed properties can be deter- mined in terms of measurement conditions and provides temperature dependent data useful for future sim- ulations of direct molten salt LF CSP plants. © 2018 Elsevier Ltd. All rights reserved. TaggedPKeywords: Thermal energy storage (TES) Concentrating solar power (CPS) Molten nitrate salt Heat transfer fluids (HTF) Review of thermo-physical properties Contents 1. Introduction ......................................................................................................................................................... 70 1.1. Concentrating solar power plants ....................................................................................................................... 70 1.2. State-of-the-art in thermal energy storage for CSP .................................................................................................. 71 1.3. Nitrate salts as working fluids............................................................................................................................ 72 2. Material aspects .................................................................................................................................................... 73 2.1. Definition of salt mixtures ................................................................................................................................ 73 2.2. Correlation of salt properties with molten salt technology ........................................................................................ 73 2.2.1. Liquidus temperature............................................................................................................................ 73 2.2.2. Thermal stability and maximum operating temperature ................................................................................ 74 2.2.3. Specific heat ....................................................................................................................................... 74 2.2.4. Density ............................................................................................................................................. 75 2.2.5. Viscosity ............................................................................................................................................ 75 2.2.6. Thermal conductivity and thermal diffusivity .............................................................................................. 75 2.3. Decomposition mechanism of nitrate salts ............................................................................................................ 75 3. Thermal properties of molten salts ............................................................................................................................. 76 3.1. Liquidus temperature ...................................................................................................................................... 76 3.2. Thermal stability ............................................................................................................................................ 77 * Corresponding author. E-mail address: alexander.bonk@dlr.de (A. Bonk). http://dx.doi.org/10.1016/j.pecs.2018.02.002 0360-1285/© 2018 Elsevier Ltd. All rights reserved. Progress in Energy and Combustion Science 67 (2018) 6987 Contents lists available at ScienceDirect Progress in Energy and Combustion Science journal homepage: www.elsevier.com/locate/pecs