Session: 12-02: Process Heat for Industrial Decarbonization

Paper Number: 156354

156354 - Numerical Design Optimization of a Compact Heat Exchanger Gas to Molten Salts for High Temperature Heat Pump Integration 

Abstract: 

Integrating thermal energy storage (TES) systems with high-temperature heat pumps (HTHPs) combines two highly efficient technologies to develop a robust energy storage and flexible heat supply system. This integration package is predominantly valuable for industrial processes, renewable energy storage and supporting the grid where a consistent and intermittent demand for high-temperature heat (above 100 °C) exists. HTHPs are regarded as an emerging technology in the industrial heating sector that allow for efficient heat recovery, are financially competitive and significantly aid in decarbonization to meet the global energy demands. To achieve greater productivity in thermal storage processes, higher magnitudes of heat transfer temperature are preferred, wherein the heat transfer fluids such as molten salt are favoured. Molten salts TES is a mature and effective technology for storing large amounts of thermal energy for the emerging high temperature applications in concentrating solar power plants (CSP). In order to facilitate a sustainable operation of a seamlessly integrated TES and HTHP system, development of robust heat exchangers is key to withstand the high temperatures and maintain an effective heat transfer performance of the system. Previous studies have focussed on compact heat exchanger design optimization for either the heat pump or thermal energy storage systems by primarily experimental investigations. As per the authors’ knowledge this forms the first case numerical analysis of a compact heat exchanger design for an integrated high-temperature heat pump and thermal energy storage system. This study aims at optimizing the design of a HTHP integrated compact shell and tube heat exchanger that operates between pressurised Helium gas from the HTHP side and a ternary molten salt from the TES side. The considered HTHP is based on a Stirling cycle, thus ideally characterized by isothermal transformations. This study aims to evaluate the thermal-hydraulic performance of the heat exchanger capable of operating at working temperatures of up to 400 °C. A three-dimensional computational model is developed using ANSYS Fluent. The pressurised Helium gas at 50 bar flows via dense, closely packed tubes with a triangular tube pattern arrangement. While, the molten salt flows around the shell side. A symmetry plane is applied along the duct axis to reduce the computational effort and a k-epsilon turbulence model with enhanced wall treatment is adopted for both tube side and shell side flows. Mesh is finely resolved near the boundary regions to capture the near-wall features of the flow with a satisfactory value of y+. Molten salts feature high specific heat capacity and offer wide temperature range of operation, and is modelled with temperature dependent thermo-physical properties. Simulations are performed with cases involving baffles and no baffles to compare their effectiveness, and accordingly the molten salt flow ducts are positioned to maximise the heat transfer and mitigate the salt flow resistance. Numerical results are validated against a well-established analytical model for shell and tube heat exchangers with a heating capacity target of up to 50 kW. Overall heat transfer coefficient (U) and pressure drop (dP) are the key parameters of interest to analyse the heat exchanger characteristics. Simulations are performed for a varying tube pattern (N_t) arrangement in the range 550 < N_t < 800, overall heat exchanger length of 203 mm, shell diameter of 263 mm, with an outer tube diameter of 6 mm, wall thickness is 2 mm and the molten salt ducts are placed colinear to the flow. For a case of N_t = 625, molten salt Re = 3500, an U = 704 W/m².K and a molten salt relative dP of 0.45% is obtained. Further work will explore a comprehensive design optimization to maximize the thermal-hydraulic performance of the compact heat exchanger over a range of operational flowrates and temperature parameters. The expected outcomes will potentially contribute towards a wider development of integrated heat and power solutions to achieve the industrial decarbonization targets.

Presenting Author: Parth Kumavat KTH Royal Institute of Technology

Presenting Author Biography: He received a Ph.D. degree in mechanical engineering from Trinity College, University of Dublin,Dublin, in the year 2023 and further worked as a postdoc at Trinity College. His Ph.D. research was focused on active heat transfer enhancement in liquid cooling systems using combined numerical and experimental methods. He is currently working as a postdoctoral researcher in the heat and power division of the Energy department at KTH. His current area of work is involved with the design and optimisation of heat exchangers for an integrated system of high-temperature thermal energy storage and heat pumps.