Comparison of different approaches for thermal performance improvement of a phase change energy storage system
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Comparison of different approaches for thermal performance improvement of a phase change energy storage system Omid Ahmadi1 · Sahand Majidi1 · Pooyan Hashemi Tari1 Received: 28 February 2020 / Accepted: 7 May 2020 © Akadémiai Kiadó, Budapest, Hungary 2020
Abstract Performance improvement of a phase change material (PCM) thermal storage system is numerically investigated. A finite volume solver is employed to simulate the melting process of the PCM in different geometrical and boundary conditions. The numerical predictions are initially validated against available experimental data. Afterward, four different scenarios are investigated to improve the thermal characteristics of the phase change process. These scenarios include insertion of radial fins with different radial lengths, insertion of a porous layer on the PCM side of the heat transfer fluid (HTF) tube, doubling the HTF mass flow rate, and also increasing the HTF inlet temperature. The results indicate that all of these scenarios expedite the melting process, but at different rates. The insertion of the porous medium is shown to be more effective than using of radial fins. Moreover, according to the second-law analysis of the thermal storage system, using the porous layer provides a superior exergy efficiency compared to other enhancement scenarios. Overall, the addition of a metallic porous layer around the HTF tube is proven to be the most effective as well as the most efficient approach to improve the thermal characteristics of the energy storage system. Keywords Thermal storage · Phase change material · Porous medium · Radial fins · Second-law analysis List of symbols Notations t Time (s) V Velocity (m s−1) P Pressure (Pa) h Convection heat transfer coefficient (W m−2 K−1) T Temperature (K) q Heat flux (W m−2) Cp Specific heat (J kg−1 K−1) g Gravitational acceleration (m s−2) k Conduction heat transfer coefficient (W m−1 K−1) H Enthalpy (kJ kg−1) λ Melted fraction μ Viscosity (kg m−1 s−1) ε Porosity α Thermal diffusivity ν Kinematic viscosity (m2 s) S Source term * Sahand Majidi [email protected] 1
Faculty of Mechanical and Energy Engineering, Shahid Beheshti University, Tehran, Iran
ex Exergy (J) ρ Density (kg m−3) Subscripts s Steady state PCM Phase change material sur Surface B Bulk M Melted liquid Liquid state Solid Solid state ref Reference
Introduction An increase in using solar energy as the heat source in home industry has led to the design of efficient storage systems. The most important role of these systems is to overcome the difference between the supply and the demand for energy at different times of a day [1]. Phase change materials as storage units can store 5–14 times more energy in comparison with other materials like water, stone, etc. [2]. One of the great advantages of these materials is that they can release stored energy in nearly constant temperature. Therefore, they
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are great candidates to be utilized as storage unit buildings in order to have a more efficient energy
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