Melting Rate Of Phase Change Material Research Articles

Melting of PCM-graphite foam composites with contact thermal resistance: Pore-scale simulation

Latent heat energy storage systems have emerged as a solution to intermittency and instability in renewable energy sources, the phase transition of phase change materials (PCMs) within these systems is hindered by their poor thermal conductivity. Graphite foam, renowned for its superior thermal properties, stands as an ideal choice to improve the melting efficiency of PCMs. In the current study, the melting behavior of PCM-graphite foam composites is simulated by the pore-scale numerical simulation (PNS) by employing cubic idealized cells with spherical pores and cylindrical channels. The computed results demonstrate that the porosity and contact thermal resistance of graphite foam are key factors affecting the melting efficiency of PCMs. The full melting time (FMT) of PCMs increases with porosity, achieving a 53.1 % improvement as porosity decreases from 0.90 to 0.75. In addition, the absence of natural convection markedly diminishes the maximum melting rate of PCM, reducing it by 33.5 % to 50.3 % for porosity from 0.75 to 0.90. Notably, it needs to be emphasized that the melting rate of PCM is significantly reduced when the contact thermal resistance surpasses 1.0 × 10–4 m2·K·W−1. Compared to the idealized condition without contact thermal resistance, the dimensionless FMTs for porosities of 0.75, 0.80, 0.85, and 0.90 extend by 27.3 %, 27.1 %, 22.2 % and 23.9 % for a contact thermal resistance of 5.0 × 10–4 m2·K·W−1, respectively.

Experimental assessment of the heat shielding performance by the integration of phase change material and liquid cooling plate

This research focuses on innovative stealth equipment by integrating a liquid cooling plate and phase change material (PCM), and the temperature rise on the cold surface and PCM melting rate is reported to illustrate the PCM thermal behavior in lifting the heat shielding performance. Results show that the integrated PCM system is effective in delaying the temperature rise within a certain time. A slower temperature rise on the cold surface is found at the first 1200 s, and a significant acceleration in the temperature rise is given within 1200 s ∼ 2000 s as the heat flood breaks through the PCM layer. The PCM melting rate increases rapidly and then rises slowly, and the complete melting time is shortened as increasing heat power. The heat shielding performance is significantly reduced with increasing the liquid flow rate, and the influence of the coolant temperature on the temperature rise becomes more and more prominent as the completion of the PCM melting process. A slower temperature rise is observed at the lower PCM phase transition temperature, and a further increase in the PCM covering layer has little influence on the temperature rise at the initial stage.

Heat transfer performance of copper foam/paraffin composite phase change material under different centrifugal forces-A visual experimental study

Copper foam is used to enhance the heat transfer of phase change material (PCM) due to its high thermal conductivity. However, its porous structure can restrict the convection of PCM. The natural convection of PCM embedded in copper foam under external force remains unclear. A visual experimental system was established to investigate the effect of copper foam on the heat transfer process of PCM under external force. The phase interface, temperature distribution, and heat storage rate of composite PCM (CPCM) were conducted under centrifugal forces from -5 g to 5 g. Results indicate that there is a significant difference in the melting rate and temperature distribution of PCM under different forces. Under forward force, the convection of PCM is inhibited, and the isotherm is parallel to the end-state isobars line, increasing the total melting time of CPCM and creating a large temperature gradient near the heat source. Conversely, under the backward force, the melting rate of CPCM is increased due to the enhanced natural convection of liquid PCM. The development of isotherms is influenced by buoyancy, which results in a more uniform temperature distribution of CPCM. However, the enhancement effect of the backward force is limited when it exceeds -3 g. In general, CPCM exhibits more apparent improvement in heat transfer performance under the backward force. This article provides references for the optimization design of thermal management systems.

Thermal performance enhancement of multiple tubes latent heat thermal energy storage system using sinusoidal wavy fins and tubes geometry modification

In this paper, performance enhancement of a shell and multiple tube latent heat thermal energy storage system (LHTESS) is numerically investigated by implementing sinusoidal wavy fins and different geometrical designs of heat transfer tubes. The annular space of the shell is filled with the paraffin wax RT35 as phase change material (PCM) and absorbs heat from hot water through the multiple tubes. With the constant total cross-section area of all tubes and fins as the constraint condition, the effect of tube number and tube shape incorporated with the proposed fins on the heat transfer properties of PCM during the melting process is evaluated. The computational results demonstrate that the sinusoidal wavy fins can improve the phase change efficiency compared to the conventional straight fins. The double-pitch sinusoidal wavy fins decrease the total melting time by 9.5% compared to the straight fins for the single-tube case. As the number of HTF tubes increased from one to four, the complete PCM melting time decreased by 38.3%. Geometrical design alterations of heat transfer tubes from circular to petal-shaped are proposed to further improve the thermal performance of multiple tubes LHTESS. The melting rate of PCM is accelerated with an increase in the number of petals. The petal tubes with nine petals can save the charging time up to 24 % in comparison with the circular tubes in a four-tube case with double-pitch sinusoidal wavy fins, and up to 57% compared to the base case with a single circular HTF tube and straight fins.

A Pore Scale Study on Heat Transfer Characteristics of Integrated Thermal Protection Structures with Phase Change Material

Phase change material (PCM) are characterized by their high latent heat and low density. Combining PCM with building walls, aircraft fuselages, and other structures can significantly enhance the thermal sink capability of these structures. In order to address the issue of low heat storage efficiency resulting from the low thermal conductivity of PCM, a novel integrated thermal protection structure (ITPS) architecture with a supportive structure based on a porous lattice has been designed. Experimental and numerical methods were employed to investigate the thermal response characteristics of the ITPS with and without PCM, the melting behavior of PCM within the porous lattice, and the effects of lattice configuration and pore size on the PCM melting rate. The current ITPS study includes evaluation of two types of lattice configurations and three different pore sizes. The results indicate that the inclusion of PCM reduces the internal panel temperature of the ITPS by approximately 15%. The melting of PCM occurs primarily at the central region of the porous lattice and gradually spreads towards the periphery until complete melting is achieved. Specifically, the Gibson–Ashby lattice configuration enhances the PCM melting rate by 43.5%, while the tetradecahedron lattice configuration yields a 53.1% improvement. Furthermore, for PCM with different pore sizes, smaller pores exhibit faster melting rates during the early and intermediate stages, whereas larger pores exhibit faster melting rates in the later stages as the proportion of liquid PCM increases. The conclusions of this study provide valuable insights for the application of PCM in the field of thermal management.

Investigation of phase change heat transfer in a rectangular case as function of fin placement for solar applications

This paper attempts to maximize solar energy utilization by employing a latent heat thermal energy storage (LHTES) unit integrated with a solar receiver of concentrating collectors or thermal management unit for photovoltaic panels. The study investigated an LHTES unit with a rectangular shape and equipped with a T-shaped fin subjected to heat flux. The paper examined the effect of eight T-fin positions under the applied heat flux on the thermal performance of phase change material (PCM). The PCM melting process was modeled using the enthalpy-porosity method using the finite element approach. The obtained numerical results revealed that PCM melting rate strongly depends on fin position, with superiority for fins located at the upper part of the chamber when heat flux is used as the primary heating source. Under applied heat flux, locating the fin at the upper section of the vertical LHTES unit enhanced the melting rate of the PCM, reaching 10 % better than the fin's middle and bottom positions, which is against the cases reported for the fixed temperature heating process that prefer bottom fins. Thus, these results recommend using T-shaped fins in the upper parts of PCM-based units used for solar collector receivers and thermal management of photovoltaic panels.

Experimental and numerical investigation on the cold harvest of composite phase change materials for building energy conservation

This paper investigates the cold energy storage of a horizontal shell-tube latent heat thermal energy storage (LHTES) unit for the cold harvest during night time. An experimental apparatus containing a horizontal circular tube with circumferential heating/cooling was designed and developed. Paraffin based phase change material (PCM) with a melting temperature of 27 °C was adopted as the cold harvest media. To improve the heat transfer rate, multi-walled carbon nanotubes (MWCNTs) were mixed with the paraffin to form a composite PCM with high thermal conductivity. A corresponding model of the composite PCM was developed and verified against the experimental data. Numerical simulations were performed for different boundary temperatures and concentrations of MWCNTs. The results show that when the heat transfer temperature difference is 20 °C, 0.05 wt% nanoparticles increase the melting rate of PCMs. When the heat transfer temperature difference was 40 °C, 0.10 wt% nanoparticles significantly affect the PCM melting rate. For low or high temperature (heat transfer temperature difference of 10 °C and 80 °C), the addition of nanoparticles is not conducive to improving the melting rate.

Role of applying PCMs on thermal behavior of innovative unit roof enclosure

As a technical approach, using Phase Change Materials (PCM) can be an effective method to prevent energy losses through building enclosures. In the present paper, different roof structures containing PCM have been examined to analyze their thermal behavior. The governing equations were developed and solved for a sample unit extracted from a typical roof. The control volume based finite element method has been applied for solving coupled equations governing heat transfer through the roof and flow of melted PCM. The effect of key parameters including thermal diffusivity of the PCM and the wall, Prandtl number (Pr), Rayleigh number (Ra) and solid wall thickness to total thickness has been examined. The main results indicate an enhancement of heat transfer through the increase of the thermal diffusivity of the PCM with respect to the building material. Increasing the thermal diffusivity ratio from 1 to 25 doubles the heat transfer rate and decreases the full melting time by 80% for high Rayleigh. Pr affects the mode of heat transfer but has little impact on PCM melting rate. Raising Ra improves heat transfer under the condition of high thermal diffusivity in the PCM. Increasing Rayleigh number from 104 to 106 doubles the heat transfer rate and reduces the melting by 67% for high thermal diffusivity ratio. Finally, increasing the size of the wall thickness does not affect the fraction of melted PCM but reduces the heat transfer rate. The heat transfer rate can be increased by up to 75% when the non-dimensional thickness of the solid layer is reduced from 0.5 to 0.005.

Numerical simulation and analysis of heat transfer and melting rate of nano-enhanced PCM composite embedded in a concentrator photovoltaic system

The primary objective of this research is to explore the impact of nanoparticles-infused PCM (phase change material) in the context of nano-PCM-PV technology (PV: photovoltaic panel). Computational investigations were conducted to evaluate the effectiveness of RT25HC, a paraffin wax PCM, in conjunction with various nanoparticles (MgO, TiO2, ZnO and CuO) in solar panel cooling. The study also considers the influence of PV module inclination angles (β: 0 to 90°). High-resolution numerical simulations using the ANSYS-Fluent CFD platform were employed. The findings highlight the intricate relationship between nanoparticle addition, melting kinetics, electrical efficiency and PCM enthalpy. Specifically, the melting rate of RT25HC PCM exhibits notable differences between horizontal (β = 0°) and inclined panels (β ≠ 0°), with a melting fraction of 12 % at t = 2 h for β = 0° compared to 40–44 % for β ≠ 0°. Nanoparticles maintain a higher electrical efficiency (11.6 %) for horizontal panels, whereas inclined panels (β ≠ 0°) experience declining electrical efficiency (7.5 % at t = 8 h). For β = 0°, the PCM system effectively maintains the surface panel's temperature constant at 44 °C for a long time (up to 8 h). However, when nano-PCM systems are introduced, this temperature stabilizes at a slightly lower value of 40 °C, representing a 4 °C reduction attributed to the presence of nanoparticles. Conversely, for β ≠ 0, the surface panel's temperature stabilizes at 44 °C for up to t = 2 h, then experiences a sharp increase, ultimately reaching 120 °C at t = 8 h, regardless of the type or absence of nanomaterials. Overall, the most effective application of nano-PCM for cooling the PV panel was predicted at a horizontal orientation of the panel, regardless of the nanomaterial type.

Simulation and optimization of square fin concentric tube heat storage exchanger

The square fin was used to improve the heat transfer rate of the concentric tube phase change heat exchanger. The effects of fin spacing, fin arrangement, and fin length on the melting process of phase change material (PCM) were studied. The influence and mechanism of non-uniform fin arrangement were discussed in detail. The liquid phase distribution and temperature distribution of PCM with time were analyzed. The results indicated that the heat exchanger with square fins can significantly improve the heat storage rate and shortened the complete melting time of PCM. Compared with heat exchangers without fins, the melting time of PCM in a 40 mm equidistant fin heat exchanger was shortened by 15.85%, and that in a 70 mm, non-uniformly spaced fin heat exchanger was shortened by 36.79%. Due to the different distances between the square fins and the outer boundary of the concentric tube, the temperature distribution near the square fin showed anisotropy, which enhanced the melting rate of PCM. The sensitivity order was fin arrangement > fin length > fin spacing. The research results can provide a reference for the design of phase change heat exchangers.

A transient numerical approach for phase change material effect on mixed convection in an open cavity

The effects of heated wall locations on mixed convection flow in a 2-D rectangular open cavity are investigated numerically using ANSYS-Fluent software. The flow is supposed to be a two-dimensional laminar and is regulated by parameters such as Reynolds, Re (100 and 200), and Richardson numbers, Ri (1, 5, 10, 15 and 20). One of the walls is heated uniformly, while the others are adiabatic. The heated wall might be vertical on the inflow and outflow sides or horizontal on the bottom of the cavity. The flow is stable at all Re and Ri values except at Ri = 20 and Re = 100 for bottom heated wall. It is found that using phase change material (PCM) reduces flow variations in the cavity. Further, the mixed convection effects are developed with increased Richardson number. This pushes the recirculating zone upstream and creates an unstable flow behavior. According to the results, the condition that minimized the fluctuations in the flow direction was the application of PCM to the bottom heated wall case. Therefore, thermal control of the flow is most efficient in this condition. The PCM melting rate is quite high for the Re = 200 condition as there is higher convective heat transfer in all cases than the Re = 100. Dimensionless wall temperature values with PCM case are lower than without PCM case for all conditions. The decrease of this parameter in bottom heated case is about 18.8% while 43.1% and 56.3% for left and right heated case, respectively. Additionally, the highest average Nusselt number is found for the bottom heated case for the same Re, with and without PCM cases.

Effect of phase change heat storage tank with gradient fin structure on solar energy storage: A numerical study

In this paper, the heat storage process of a latent heat thermal energy storage (LHTES) tank is studied numerically. A new type of gradient fin is added to the heat storage process in a latent heat storage tank to improve the heat transfer performance of the internal phase change material (PCM). The numerical model is verified by the experimental data. The influences of different structures on the melting rate, temperature distribution, velocity distribution, and maximum melting rate of PCM in LHTES tank are investigated, and the influence of natural convection on the energy storage system is further quantified. The results show that the melting time can be significantly reduced by 16.9% by using the four straight fins with gradient shape, and the melting time can be further reduced by 41.1% by reconstruction of the gradient fin position. The deformation in the opposite direction will slow down the melting speed and extend the melting time by more than 4.66 times. The convection criterion for judging the buoyancy convection of the melt in this paper shows that natural convection is dominant in both the upper and lower part of the melt. The closer the convection coefficient is to the overall heat transfer coefficient, the better the melting efficiency and energy efficiency will be.

A hybrid battery thermal management system coupling with PCM and optimized thermoelectric cooling for high-rate discharge condition

The lithium-ion battery is now abundantly available in the market due to its excellent performance. However, battery overheating is increasingly prominent when the battery discharges at high speed. In this study, an active-passive BTMS (Battery Thermal Management System) combining PCM (Phase Change Material) and TEC (Thermoelectric Cooler) was proposed. The effects of different TEC currents and delayed cooling at different PCM melting rates were analyzed under the 4C discharge condition to enhance the thermal performance. The results demonstrate that battery temperature gets effectively controlled as the TEC current increases, but the temperature difference and PCM utilization are poor. However, applying a delayed current after the PCM melting rate reaches 80% reduces energy consumption and provides better temperature uniformity. Considering the large battery heat under 4C discharge, combined with the transient supercooling effect of TEC, a continuous pulse current was used to provide additional cooling power. Results show that the battery with TEC pulsed current operates 57.8% longer at 40 °C with a temperature difference maintained at 2.5 K under 4C discharge conditions compared with the PCM model.

Melting of phase change materials inside metal foams with uniform/graded porosity: Pore-scale simulation

Latent heat thermal storage system has the potential for industrial applications to reduce excessive consumption of fossil fuels. However, the commonly employed phase change materials (PCMs) have the drawback of poor heat transfer capability, causing slow charging/discharging rates for thermal storage systems. To enhance the melting rate of PCM, metal foams (MFs) with good thermal conductivity were exploited, focusing on the contribution of porosity gradients in foam structures to the melting characteristics. Based on tetradecahedron cells, geometric models of MFs with uniform porosity or porosity gradients were created, including negative gradient foam (NGF), homogeneous foam (HF), and positive gradient foam (PGF). Pore-scale numerical simulations (PNS) based on the tetradecahedron cells with porosity gradients were carried out. Results of PNS demonstrated that the complete melting time for the NGF, the HF and the PGF were 92 s, 92 s and 132 s, respectively. However, the NGF had the fastest integrated average temperature response rate (IATRR). Specifically, compared with the HF, the IATRR of NGF increased by 15.4%, while that of the PGF decreased by 36.1%. In addition, natural convection enhanced the heat transfer efficiency between the PCM and the metallic skeletons of the MF. The impact of natural convection on the phase transition process was analyzed from the microscopic pores. Under the influence of natural convection, the top PCM melted faster than the bottom PCM in the z-direction.

Evaluation and analysis of the improvement in charging time for the high-power fast charging module using phase change material

The advancement of charging time for the fast charging piles facilitates the full adoption of EVs. The benefits of adding the suitable phase change material (PCM) to the thermal control system of the high-power fast charging power module are demonstrated in this experimental study. The effects of different PCM's melting temperature, thermal conductivity, and filling thickness on the improvement of charging time are explored, and the PCM melting rate is visualized to reveal the mechanism of PCM in improving the charging time. The results show that the extension in charging time of 2370 s is found for the charging power module coupled with PCM at the heat generating power of 15 W, and it is extended by 54% and 39% at heat generating power of 25 W and 35 W, respectively. The benefit of applying the PCM in the improvement of charging time for the charging power module is lessened with increasing the air velocity. The optimal PCM melting temperature is 47.5 °C, and it achieves the maximum charging time of 1540 s. The maximum available charging time is 1620 s at the filling PCM thickness of 5.5 mm. Also, the charging time is extended by 510 s when the thermal conductivity of PCM increases from 0.2 W/(m K) to 5 W/(m K). Increasing the heat generating power or decreasing the air velocity will shorten the completed melting time of the used PCM, and the initial melting time is also ahead. A faster PCM melting rate is observed at the lower PCM melting temperature and filling thickness, and this shortens the PCM latent heat role in the thermal control for the charging power module.

Effect of Asymmetric Fins on Thermal Performance of Phase Change Material-Based Thermal Energy Storage Unit.

Phase change material (PCM)-based thermal energy storage units (TESU) have very low thermal conductivity that compromise their charging and discharging rate. The present study focuses on an enhancement in charging rate as well as an increase in the uniformity of the melting rate. A rectangular cavity consisting of two horizontal partial fins is studied. The horizontal partial fins are placed symmetrically in a PCM-based TESU. In the current work, the melting rate of PCM was enhanced using asymmetric arrangement while keeping all other parameters the same, thus showing the positive effect of asymmetric configuration in such storage systems. The position and the pitch of each fin is optimized to improve heat transfer characteristics of the TESU. The numerical investigation of the problem is performed. TESU with asymmetrically placed fins show better performance in terms of higher charging rate as well as uniformity of the charging rate. The asymmetric placement of the fins suggested by present study increased the charging rate by 74.3% on average as compared to the symmetrically placed fins in the storage system. The charging rate uniformity is improved by 43.7%. The asymmetric fin's placement conserved the convection strength for a longer melting duration and so increased the Nusselt number by 80.2% as compared to the symmetrically placed fins. Thus, it can be concluded that the performance of asymmetric fins is better in the charging of PCMs than the symmetrically placed fins in a PCM-based TESU.

A comprehensive approach combining gradient porous metal foam and the magnetic field to regulate latent heat storage performance

This study proposes a comprehensive regulation method of gradient porous metal structure combined with the magnetic field to improve the controllability of heat utilization of latent heat thermal energy storage (LHTES) unit. Melting and solidification are considered. The enthalpy porosity method and magnetic force model describe the phase change of phase change material (PCM) and the movement of Fe3O4 nanoparticles under the magnetic field and assume the local thermal non-equilibrium model of metal foam. The numerical model is consistent with the visual experimental results of melting front evolution. The effects of magnetic nanoparticle concentration and gradient porosity on heat transfer and thermal energy change during melting and solidification are numerically studied. The results show that the phase change rate increases with the concentration of nanoparticles. The positive magnetic field further accelerates the phase change and heat energy change. The addition of copper foam considerably shortened the phase change time. The positive magnetic field accelerated the melting and heat energy change rate of PCM in copper foam by 18.2% and 23.1%, but the improvement of solidification was weak. Based on magnetic field promotion, the setting of porosity spacing with different gradients can regulate the phase interface uniformity, total heat energy, and heat energy change rate. Finally, according to different heat utilization scenarios of the LHTES system, this study gives recommended cases to optimize the performance of melting, solidification, and the phase change cycle.

Numerical simulation study of the effect of mechanical vibration on heat transfer in a six-fin latent heat thermal energy storage unit

The optimization of heat transfer in latent heat thermal energy storage units (LHTES) has received a lot of research during the past decade, and the use of fins to enhance heat transfer in phase change materials (PCM) is one of the efficient ways. However, the existing studies do not consider the influence of mechanical vibration which widely exists in LHTES. To fill the research gap in this important topic, a horizontal shell-and-tube six-fin latent heat thermal energy storage unit is investigated in this study, and the effects of mechanical vibration are investigated by numerically simulating the PCM charging process in LHTES using the Enthalpy-porosity technique. In this paper, the charging rate of PCM, variation of key parameters such as Nusselt number, temperature and liquid fraction are analyzed in detail under vibration conditions. And the following conclusions can be obtained. (1) Mechanical vibration can significantly increase the charging rate of PCM, under the condition of vibration amplitude of 10mm and frequency of 1Hz (the melting time of PCM is reduced by 46.6%). (2) The enhancement effect of mechanical vibration on the melting rate of PCM increases with the increase of amplitude and frequency, but the enhancement efficiency keeps decreasing. These observations result from that the mechanical vibration changes the velocity of the liquid PCM, so the forced convective heat transfer of the PCM is significantly enhanced. Furthermore, the melting process of PCM is sensitively affected by increasing the amplitude than by increasing the frequency. Increasing the amplitude or frequency alone (amplitude from 10mm to 40mm; frequency from 1Hz to 4Hz) can reduce the total melting time of PCM by 75% and 68.7%, respectively.

Thermal and geometrical investigation of an original double-pipe helical coil heat storage system with kock snowflake cross-section containing phase-change material

In our study, a double-pipe helical coil heat storage system for water heaters containing Phase-change Material (PCM) with a Koch snowflake cross-section of the inner pipe is numerically investigated. Instead of using a simple geometry as a cross-section and fins in order to improve the heat transfer efficiency, as a novelty, the Koch snowflake cross-section with angled sides and sharp corners increases the effective surface area and also acts like fins. Water in the inner pipe is Heat Transfer Fluid (HTF), and RT-35 is located in the annulus as PCM. Hypotheses such as the insulation of the outer pipe surface and the incompressibility of the Newtonian fluid have been applied to solve the equations in the simulation. Validation has been done by comparing the results with a previous study in the literature. Important geometrical and fluid parameters, such as the first three iterations of Koch snowflake for the inner pipe cross-section, coil diameter, helical pitch, Reynolds number, and operating fluid inlet temperature for this heat storage system, were discussed in detail. The outcomes showed that the second iteration is the optimum iteration in the first three iterations of the Koch snowflake for heat exchanging. As a matter of fact, the second iteration enhances the PCM's melting rate by 5% at the maximum state (liquid fraction changes from 0.79 to 0.83), while the third iteration, compared to the second iteration, maximum decreases it by 1.2% (liquid fraction changes from 0.83 to 0.82). Furthermore, the coil diameter parameter has an inverse relationship with melting; thus, with a 33% increase in the size of the coil diameter (90 to 120 mm), the melting decreases by 1.1%. Also, increasing the helical pitch by 100% (40 to 80 mm) raises the melting by 3.2%. The Reynolds number change does not influence the melting in the range of laminar flow; however, rising the inlet temperature of heat transfer fluid by 2.9% (340 to 350 K) increases the liquid fraction by about 23%.

Improving the melting performance of phase change material (PCM) in a latent heat thermal energy storage unit via a non-uniform arrangement of longitudinal fin

The application of latent heat thermal energy storage (LHTES) technology in solar energy utilization is greatly restricted by the low thermal conductivity of phase change material (PCM). This paper proposed a non-uniform arrangement of a longitudinal fin in a typical LHTES unit to improve the melting performance of PCM. Water is used as the heat transfer fluid (HTF) and paraffin (RT35) is selected as the PCM. A three-dimensional computational model is built to simulate the melting process of PCM, and its result is verified by experimental data. Firstly, the melting performance of PCM in the LHTES unit with four kinds of fin arrangements is comparatively studied, and its heat transfer mechanism is characterized via the maximum velocity of liquid PCM. In addition, the influences of HTF inlet temperature and flow rate, as well as heat transfer tube material on the complete melting time of three typical fin arrangements are also investigated. The results implied that the complete melting time of the non-uniform arrangement of the longitudinal fin reduces by 30.91% relative to the conventional longitudinal fin (Case 2). The results also revealed that the melting rate of PCM is related not only to the intensity of natural convection but also to the distribution of natural convection in the whole melting process.