Session: 06-02: Heat Transfer in CSP Applications 1

Paper Number: 131260

131260 - Numerical Analysis of Solidification in Molten Salt-Air Shell-and-Tube Heat Exchangers 

Abstract: 

In high-temperature applications, such as concentrated solar power, pumped thermal energy storage, and waste heat recovery systems alike, molten salt-gas-cooled heat exchangers employing air, helium, argon, or nitrogen are often more suitable than molten salt-oil heat exchangers, because the gases are inert, and they are cheaper, more environmentally friendly and can sustain higher temperatures than oils. However, a significant concern during the cyclic operation of molten salt heat exchangers is the proneness of molten salts to freeze due to their relatively higher freezing point (~495 K for solar salt). During the operation of the plant, freezing can occur due to one or more unavoidable factors, for example, failure in heat tracing, maldistribution of flow, unprecedented weather patterns, and pump blockage. Hence, it is essential to implement freeze protection strategies. However, such strategies may amplify the parasitic costs, such as operation and maintenance costs. In this context, there is a need to identify the onset of salt freezing and predict the phase change behavior of molten salts in high-temperature heat exchange processes, as it governs the design and operation of the heat exchanger and the freeze protection system.

A major share of the reported work on molten salt-gas-cooled heat exchangers addresses latent heat thermal energy storage concentric tube/shell and tube heat exchangers often used as energy storage units where the molten salt is static, allocated in the annular space. Apart from the storage perspective, experimental studies on molten-salt gas-cooled heat exchangers focused on testing and understanding different heat transfer enhancement techniques were reported. All studies reported in the open literature on molten salt-gas-cooled heat exchangers used atmospheric air as the heat transfer fluid, though it has low heat capacity and thermal conductivity.

This paper presents a 3-dimensional, transient computational fluid dynamics analysis using ANSYS Fluent^® of the latent heat transfer of a binary salt (solar salt) mixture in a molten salt-air heat exchanger of shell-and-tube type. The air is placed on the shell side, while the molten salt is placed on the tube side. The primary objective is to determine the salt freezing rate under different abnormal conditions, such as failure in heat tracing, maldistribution of flow, and unprecedented weather patterns. The influence of variations in mass flow rates and inlet temperature of both fluids on the freezing rate of the molten salt under different abnormal conditions is examined. The thermophysical properties of both fluids were modeled as a function of temperature. A realizable k-epsilon turbulence model and pressure implicit with splitting of operators algorithm were used to solve the turbulence viscosity and the pressure-velocity coupling. The developed numerical solver was validated using experimental results of a eutectic salt mixture latent heat thermal energy storage unit. Based on the results, recommendations for implementing freeze protection strategies are provided. The main novel contribution to state-of-the-art of the work is that it provides an improved understanding of the dynamics of the salt freezing phenomena under abnormal operating conditions in shell-and-tube heat exchangers with air on the shell side and molten salt on the tube side.

The preliminary results suggest that the results of the numerical model are in good agreement with the experimental results, achieving a maximum deviation between numerical and experimental results below ±7 %. The result will support the industry in developing control strategies for freeze protection in molten salt-air heat exchangers, while the results will benefit academia by providing a benchmark for modelling freezing in heat exchanger tubes using computational fluid dynamics software for high-temperature applications.

Presenting Author: Arun K Raj Technical University of Denmark

Presenting Author Biography: I am a Marie Skłodowska-Curie Postdoctoral Fellow at DTU Construct, Department of Civil and Mechanical Engineering, Thermal Energy Section (TES), Danmarks Tekniske Universitet (DTU), Denmark. I am mainly interested in Thermal and Energy Engineering, which involves design and optimization, integration, and pilot scale/real-scale testing and analysis (experiments/CFD).

I have 7+ years of research experience (including Ph.D., nearly two years as a Postdoc & 1 year as a Research Track Assistant Professor) in the field of Water-Energy Nexus (i.e., solar thermal systems, heat transfer & fluid flow, thermal energy storage, and energy conversion & management) covering fundamental and applied research. I have designed flexible integration strategies for thermal energy storage to be incorporated into several solar thermal systems. I have been involved in multidisciplinary collaborative projects in different countries (India, Hungary, the Kingdom of Saudi Arabia, South Korea, Denmark, Netherlands, and the United States), including a few with attractive research funding.

Moving forward, I will continue to focus on research in developing sustainable devices and solutions for meeting the futuristic decarbonization goals for applications in solar energy and thermal energy storage & conversion.

Authors:

Arun K Raj Technical University of Denmark
Nishith B. Desai Technical University of Denmark
Fredrik Haglind Technical University of Denmark