Latent Heat Thermal Storage Unit Research Articles

Numerical analysis of the solidification process of water used as PCM in a rectangular latent heat thermal storage unit

A numerical analysis of the solidification process of water used as phase change material (PCM) has been carried out in a rectangular latent heat thermal storage unit. The major heat transfer phenomena involved in such a process were numerically characterized using the CFD code Star CCM+. During the solidification process, the flow and heat transfer were analysed through vector field, temperature and solid fractions contours. Quantitative global results such as the temporal evolution of the average temperature of the PCM were also provided during the solidification process. The present study shows that the natural convection plays an important role in heat transfer kinetics during solidification process.

Melting behavior of the latent heat thermal energy storage unit with fins and graded metal foam

Metal foam can effectively improve the melting rate of latent heat thermal energy storage units (LHTESU). However, the existing metal foam structure can’t simultaneously solve the problem of non-uniform melting caused by natural convection and slow melting rate in horizontal shell-and-tube LHTESU. To this end, the structure with fins and graded metal foam is proposed to further enhance thermal performance. The porosity of the metal foam increases gradually from the inner tube to the outer shell to reduce thermal resistance and improve thermal conduction globally. The fins (the porosity is 0) are employed to further enhance local heat transfer and balance the difference of melting rate between upper and lower parts. The correctness of the current model is verified by the one-phase Stefan problem with explicit solutions and research results in previous literature. With consideration of natural convection, the enthalpy-porosity method is adopted to compare the melting behavior of LHTESU strengthened by uniform metal foam, graded metal foam, fins and uniform metal foam, and fins and graded metal foam. The results show that the existence of the fins improves the uniformity of the temperature field and significantly shortens the melting time. Based on the same amount of metal materials, the proposed structure obtains the best strengthening effect in the four different structures. Furthermore, the effects of different structural parameters (the fins angle, fins thickness, and porosity gradient) on the melting process are discussed. It can be found that there are optimal values for the three structural parameters. Compared with the graded metal foam structure, the new structure (the fins angle is 25°, the fins thickness is 3 mm and the porosity gradient is 2%) further reduces the total melting time by 27.23% and increases the thermal energy storage rate by 36.52%.

Experimental investigation of a parabolic greenhouse dryer improved with copper oxide nano‐enhanced latent heat thermal energy storage unit

Solar dryers are clean and sustainable systems for preserving agricultural products. In this research, three different active greenhouse dryers, which are grouped in direct solar drying systems, have been designed, fabricated, and experimentally surveyed. Different from other works, this research contains an experimental investigation on nano-embedded thermal storage-assisted greenhouse drying system. Major goal of this study is to analyze the impact of utilizing nano-enhanced thermal energy storage unit on the performance of a greenhouse dryer. In this context, tunnel-type parabolic greenhouse dryers with paraffin wax thermal storage unit (PGD-TES) and nano-enhanced paraffin wax thermal storage unit (PGD-NeTES) have been tested and compared with a conventional parabolic greenhouse dryer (PGD). Copper oxide (CuO) nanoparticles have been employed to upgrade the thermal effectiveness of the thermal storage unit. Tests have been done in two flow rates, which are 0.009 and 0.014 kg/s. Utilizing nano-enhanced modification decreased drying time approximately between 35% and 58% in comparison to the conventional dryer. According to the results, average specific energy consumption values were attained in the ranges of 2.10 to 2.48, 1.93 to 2.09, and 1.63 to 1.89 kWh/kg, respectively, for PGD, PGD-TES, and PGD-NeTES. Also, exergy efficiency was improved by 36% by utilizing the nano-enhanced thermal energy storage system.

Discharging process and thermal evaluation in the thermal energy storage system with fractal tree-like fins

Under microgravity environment for spacecraft operation, the melting rate of phase change material is restricted by its low heat transfer performance and disappearance of natural convection. To accelerate the melting under microgravity, the insertion of fractal tree-like fins into a latent heat thermal energy storage unit is proposed. The dynamic melting behavior and heat transfer performance in cavities with fractal tree-like fins having bifurcation angle of 30°, 60° and 90° are investigated through a transient model based on the enthalpy-porosity method. The relationship between the effects of tree-like fins and thermocapillary convection on melting is disclosed. The results show that during melting driven by combined heat conduction and thermocapillary convection, the presence of tree-like fins significantly accelerates melting, and the increment degree of melting rate increases with the increase of bifurcation angle. Compared with cavity without fin, 30°, 60° and 90° fins reduce melting time by 47.7%, 57.2% and 64.3%, respectively. The fractal tree-like fins also enhance the increment speed of energy storage capacity, while enhancing and slightly deteriorating heat transfer at the beginning and end of melting, respectively. The thermocapillary convection accelerates the melting under microgravity, and the acceleration effect degree increases with the increase of bifurcation angle. With the presence of natural convection, compared with the cavity without fin, 30°, 60° and 90° fins reduce the melting time by 57.8%, 60.6% and 65.3%, respectively; the acceleration effect of fins on melting is enhanced, and the effect of bifurcation angle on melting becomes weaker. The present design innovatively introduces tree-like fins into thermal energy storage under microgravity condition, and can be used for temperature control of spacecraft electronic devices.

Effects of gradient porous metal foam on the melting performance and energy storage of composite phase change materials subjected to an internal heater: A numerical study and PIV experimental validation

This work aims to explore the effects of gradient porous metal foam on the 3D melting heat transfer as well as the heat storage performance of a latent heat thermal energy storage (LHTES) unit inside an internal heated cubic cavity. A modified structure of metal foam with gradient porosity of three stratification layers is proposed to optimize the melting characteristic of the composite phase change materials. The enthalpy-porosity method is adopted to model melting process, and the Darcy-Forchheimer law and local thermal non-equilibrium model are assumed for metal foam. The numerical code of proposed model is in good consistency with the experimental results of particle image velocimetry (PIV) visualization. The effects of porosity gradient with equal and non-equal stratification heights, as well as pore per inch (PPI) on the melting performance, heat transfer and heat storage are investigated. The results indicate that the competitive relation between the conduction and convection induced by the gradient porosity influences the heat transfer and energy storage. The energy storage of negative gradient is greater than that of positive gradient, but the average rate of energy storage of the former is lower than that of the latter. In the middle melting stage, the total average Nusselt number increases with larger porosity gradient for positive cases and decreases for negative cases. With the increase of the stratification height of middle layer, the heat transfer efficiency of positive cases weakens, while it is enhanced for negative cases. Moreover, the increase of PPI can reinforce the conduction effect and improve the average rate of energy storage of the LHTES unit for both positive and negative cases.

Thermal performance evaluation of a latent heat thermal energy storage unit with an embedded multi-tube finned copper heat exchanger

ABSTRACT The experimental thermal characterization during charging and discharging of a prototype compact latent heat thermal energy storage system (LHTESS) with an embedded horizontally oriented finned multi-tube copper heat exchanger is presented. The thermal store was filled with a commercial-grade, CrodaTherm™ 60, phase change material (PCM) with a nominal melting temperature of 60°C. The proposed heat exchanger (HX) configuration mitigates the effect of low PCM thermal conductivity and increases the rates of heat transfer to and from the store during the charging and discharging processes compared to multi-tube copper HX without fins. The experimental testing regime assessed the impact of heat transfer fluid (HTF) volume flow rate and inlet temperature on I) transient PCM temperature distribution, II) the total charging/discharging time and III) instantaneous/average power during the charging and discharging processes. The experimental results showed that the HTF volume flow rate and inlet temperature play a significant role in the thermal performance of the LHTESS during the charging/discharging process. During the charging process, the influence of increasing HTF inlet temperature is greater than that due to increasing HTF volume flow rate. Moreover, it is shown that during the first 30 minutes of the discharging process, the average power output was 4.3, 5.1 and 5.3 kW for HTF volume flow rate of 2.5 and 7.2 and 10.5 l/min, respectively.

Pareto optimal design of a finned latent heat thermal energy storage unit using a novel hybrid technique

A finned latent heat thermal energy storage (LHTES) unit is considered for optimization through a hybrid procedure based on computational fluid dynamics (CFD), grouped method of data handling (GMDH) type of artificial neural network (ANN), non-dominated sorting genetic algorithms (NSGA-II), and multi-criteria decision-making (MCDM). Considering that optimized fins have a tremendous impact on the efficiency of finned LHTES systems, this paper considers the design variables the geometrical parameters of fins (number, length, and volume fraction). Furthermore, two of the most significant thermal energy storage (TES) systems, phase change time and total stored energy are considered objective functions. The primary purpose is to minimize the phase change time and maximize the stored energy. For this purpose, first, the effects of design variables on objective functions are studied by CFD simulations. Afterward, by importing the numerical data into the GMDH-type ANN code, polynomials are derived to predict the values of the objective functions in terms of design variables. Then, Pareto optimal points are presented using the NSGA-II algorithm and GMDH predictive models. Finally, the objective functions' design points per weight are proposed using TOPSIS and VIKOR MCDM methods. The considered finned LHTES unit showed that about 60% of the Pareto optimal points have a dimensionless fin length above 0.98. Also, the volume fraction of fins was observed below 0.1 for about 79% of the optimal cases. Besides, 81% of the total optimal cases had 4 or 5 fins.

Dominant roles of eccentricity, fin design, and nanoparticles in performance enhancement of latent thermal energy storage unit

In this paper, numerical and experimental investigations of thermal performance enhancement of horizontal longitudinal shell and tube latent heat thermal energy storage unit (LHTESU) via eccentricity (e), fin design optimization, and nanoparticles in the phase change material (PCM) of stearic acid are presented. An enthalpy-porosity-based two-dimensional, transient numerical methodology is used after validating against the experimental results and the literature. Five different eccentric positions, e = 0.14, 0.28, 0.42, 0.56, and 0.63 are investigated by modifying the design of Y-finned tube. The eccentricity and fin design modifications improved natural convection effects as compared to concentric Y-finned tube. Thermal performances of LHTESU are analyzed on the basis of melting time, heat storage capacity, heat storage rate, and performance enhancement ratio. Based on the performance analysis, an optimum eccentric configuration of LHTESU is proposed. For optimized eccentric unit (e=0.42), a reduction of 34.14% in melting time along with 30.7% improvement in thermal energy storage rate is achieved as compared to concentric (e=0) LHTESU. The effect of temperature of the tube on PCM's melting time and heat transfer is also analyzed and two important correlations are proposed. The thermal performance of the optimized eccentric unit is further enhanced by adding nanoparticles of Al2O3 and CuO ranging from 0.5% to 10% by volume in the pure PCM. The nano-enhanced PCM with 1% of Al2O3 further improves melting and energy storage rates of optimum LHTESU by 10%.

Performance investigation on an air source heat pump system with latent heat thermal energy storage

This paper proposes an air source heat pump (ASHP) system integrated with a latent heat thermal energy storage (LTES) unit based on a specially-designed heat exchanger, condensing heat storage unit, to further improve system performance in cold region. The innovation of this device is that heat is transferred directly between refrigerant and phase change material (PCM), which significantly reduces heat loss. Experimental investigation on thermal performance of the system is carried out. Besides, a mathematical model of condensing heat storage unit is developed to investigate charging/discharging characteristics and validated by experimental data. Experimental results show that heat storage capacity reaches 31.83 kWh and average heat release power is 4.73 kW. Simulation results indicate that both smaller fin pitch and larger fin thickness accelerate the melting rate of PCM, but variations of tube diameter show little impact on it. In order to maintain heat release power, circulating water flow rate should be increased with discharging time. Three-stage PCM improves both energy and exergy performance compared to single-stage PCM. COP improvement could reach 4.01% and overall exergy efficiency is increased by 4.65%. Therefore, ASHP system combined with LTES unit is a potential approach to expand its application under extremely low temperature.

Enhanced heat transfer in a PCM shell-and-tube thermal energy storage system

The dominant technology among latent heat thermal energy storage methods relies on solid–liquid phase change. Since the primary disadvantage of phase change materials is low thermal conductivity, heat transfer enhancement techniques are required for these types of systems. In this paper, we propose a new double-tube latent heat thermal energy storage units. The melting time and exergy efficiency are compared for eight different design solutions. 3D simulations of the phase change material melting process were performed using the enthalpy-porosity model, Boussinesq approximation, and select temperature-dependent phase change material properties. This study shows that the proposed latent heat thermal energy storage unit (M06) significantly reduces PCM melting time compared with vertical (76%), horizontal (66%), and helical-coiled (53%) systems. The helical-coiled unit with spiral fins (M05) has the highest exergy efficiency (0.77) at the end of melting time. The M05, M06 and M08 units have the highest exergy efficiency at times t = 1200 s and t = 3307 s, which is essential for climates with time constraints on latent heat thermal energy storage. • Proposition of a new helical-coiled tube enhanced with spiral fins and conical shell. • 60% PCM melting time reduction, compared with horizontal LHTES unit. • The helical-coiled unit with spiral fins has the highest exergy efficiency (0.77). • Analysis of the effect of fins and conical shell design on melting process.

Experimental investigation of a latent heat thermal energy storage unit encapsulated with molten salt/metal foam composite seeded with nanoparticles

Molten salt has been widely used in latent heat thermal energy storage (LHTES) system, which can be incorporated into hybrid photovoltaic/thermal solar system to accommodate the built environment. Solar salt (60 wt.% NaNO3 and 40 wt.% KNO3) was employed as the phase change materials (PCMs) in this study, and both aluminum oxide (Al2O3) nanopowder and metal foam were used to improve the properties of pure solar salt. The synthesis of the salt/metal foam composites seeded with Al2O3 nanopowder were performed with the two-step and impregnation methods, and the composite PCMs were characterized morphologically and thermally. Then pure solar salt, the salt/2 wt.% Al2O3 nanopowder and salt/copper foam composite seeded with 2 wt.% Al2O3 nanopowder were encapsulated in a pilot test rig, respectively, where a heater of 380.0 W was located in the center of the LHTES unit. The charging and discharging processes of the LHTES unit were conducted extensively, whereas the heating temperatures were controlled at 240 °C, 260 °C and 280 °C respectively. Temperature evolutions at radial, angular and axial positions were recorded, and the time-durations and volumetric mean powers during the charging and discharging processes were obtained and calculated subsequently. The results show that physical bonding between Al2O3 nanopowder and nitrate molecule has been formed from the morphological pictures together with XRD and FTIR curves. Slight changes are found between the melting/freezing phase change temperatures of the salt/metal foam composites seeded with Al2O3 nanopowder and those of pure solar salt, and the specific heats of the salt/Al2O3 nanopowder composite slightly increase with the addition of Al2O3 nanopowder. The time-duration of the charging process for the salt/copper foam composite seeded with Al2O3 nanopowder at the heating temperature of 240 °C can be reduced by about 74.0%, compared to that of pure solar salt, indicating that the heat transfer characteristics of the LHTES unit encapsulated with the salt/copper foam composite seeded with Al2O3 nanopowder can be enhanced significantly. Consequently, the mean volumetric powers of the charging process were distinctly enhanced, e.g., the volumetric mean power of heat storage can reach 110.76 kW/m3, compared to 31.94 kW/m3 of pure solar salt. However, the additive has little effect on the volumetric mean power of heat retrieval because of the domination of natural air cooling.

Phase change material heat transfer enhancement in latent heat thermal energy storage unit with single fin: Comprehensive effect of position and length

In this study, the comprehensive effect of position and length of the fin in a latent heat thermal energy storage (LHTES) unit with a single fin on the melting and solidification of the phase-change materials (PCMs) was explored by transient numerical simulations. By analyzing the liquid-solid interfaces, temperature distributions, velocity vector and phase change rate, the melting and solidification characteristics of PCM and heat transfer enhancement mechanism of the fin with various length ratios and height ratios were understood in detail. The results illustrated that the total melting time was significantly reduced by increasing the fin length and lowing the fin position simultaneously. On the other hand, lowering the fin position would result in non-uniform temperature distribution during the solidification process. More importantly, considering the contradiction in fin position between melting and solidification, an optimal position of the fin corresponding to different fin lengths was suggested. Besides, a disparity between heat storage/release and phase transition was quantitatively discussed.

Effect of the circumferential and radial graded metal foam on horizontal shell-and-tube latent heat thermal energy storage unit

The latent heat thermal energy storage unit (LHTESU) strengthened by metal foam can effectively store solar energy and realize the sustainable utilization of solar energy. In this paper, the metal foam with a two-dimensional (radial and circumferential direction) porosity gradient is proposed for the problem of slow melting rate and non-uniform melting in the horizontal shell-and-tube LHTESU. The non-uniform index and thermal energy storage rate are introduced to evaluate different metal foam structures. Under the same amount of metal foam, the influence of one-dimensional and two-dimensional graded structures is numerically studied. The results show that circumferential graded structure can alleviate the non-uniform melting problem. Compared with the uniform structure, the non-uniformity index decreases by 36.63% and the thermal energy storage rate is increased by 38.60%. In addition, the optimum circumferential porosity gradient also increases with the increase of Ra*. The strengthening effect of the circumferential and radial graded structures is also compared. It can be found that with the increase of porosity gradient, the strengthening effect of the circumferential graded structure increases gradually and that of radial graded structure first increases and then decreases. When the circumferential graded structure is combined with the radial graded structure, the two-dimensional porosity gradient structure can further improve the thermal performance of LHTESU. Compared with the uniform structure, the thermal energy storage rate can be increased by 47.40%.

Numerical investigation and optimal design of partially filled sectorial metal foam configuration in horizontal latent heat storage unit

A numerical model of a horizontal shell-and-tube latent heat thermal energy storage (LHTES) unit partially filled with sectorial metal foam (MF) was established and validated. Different filling angles (θ = 60°–210°) and thicknesses (l = 7/12–11/12 lo) of the metal foam configuration were simulated to investigate their effects on the melting performance. The results illustrate that the enhancement of the melting rate caused through increasing the filling angle will rapidly slow down when the filling angle increases from 150° to 210°. Increasing the thickness of the configuration deteriorates cost performance, but the excessive reduction on the thickness conversely restricts the melting rate enhancement when increasing the filling angle. The optimal case with a ladder shape design can reduce the full melting time by 45.9 % compared with the case without metal foam. This indicates that the amount of metal foam using in the ladder shape design can be reduced by 16.8 % compared with that of the benchmark design. Therefore, the ladder shape design is a more economical strategy for the partially filled metal foam configuration in the latent heat storage unit.

Shell-and-Tube Latent Heat Thermal Energy Storage Design Methodology with Material Selection, Storage Performance Evaluation, and Cost Minimization

Shell-and-tube latent heat thermal energy storage units employ phase change materials to store and release heat at a nearly constant temperature, deliver high effectiveness of heat transfer, as well as high charging/discharging power. Even though many studies have investigated the material formulation, heat transfer through simulation, and experimental studies, there is limited research dedicated to the storage unit design methodology. This study proposes a comprehensive methodology that includes the material assessment with multi-attribute decision-making and multi-objective decision-making tools, epsilon-NTU method, and cost minimization using Genetic Algorithm. The methodology is validated by a series of experimental results, and implemented in the optimization of a storage unit for solar absorption chiller application. A unit cost of as low as USD 8396 per unit is reported with a power of 1.42 kW. The methodology proves to be an efficient, reliable, and systematic tool to fulfill the preliminary design of shell-and-tube LHTES before the computational fluid dynamics or detailed experimental studies are engaged.

Latent heat storage integration into heat pump based heating systems for energy-efficient load shifting

Integrating latent heat thermal energy storage (LHTES) units into building heating systems has been increasingly investigated as a heat load management technology. A conventional LHTES integration method for heat pump based heating systems is to connect the heat pump’s condenser for charging the LHTES unit. This integrating layout however usually leads to increased electricity input to the heating system. To underline this issue and provide solutions, this paper presents three new LHTES integrating layouts where the LHTES unit is connected with the de-superheater of the main heat pump (Case 2), the condenser of a cascaded booster heat pump cycle (Case 3), or a combination of using both the de-superheater and the booster cycle (Case 4). In the context of a multi-family house in Stockholm, a quasi-steady state heating system model was developed to evaluate the new integrating layouts, which were benchmarked against the baseline heating system without storage (Case 0) and the conventional integrating layout using the main heat pump condenser (Case 1). Hourly electric power input to the heating system was modelled for calculating the performance indicators including the heating performance factor, the operational expense and justifiable capital expense, and the indirect CO2 emissions. Two load shifting strategies were simulated for an evaluation period of Week 1, 2019: 1) charge during off-peak hours (8 pm to 6 am) and 2) charge during daytime hours (10 am to 7 pm). The simulation results of the off-peak charging strategy show that, in Cases 2–4, the heating performance factor is 22%-26% higher than Case 1 and the operational expense can be reduced by 2%-5% as compared with Case 0. The savings in the operational expense can justify the capital expense of 11 k-25 k Swedish Krona (SEK) for the LHTES systems in Cases 2–4 assuming a 15-year operation. Furthermore, the advantage of using the daytime charging strategy is principally the mitigation of CO2 emissions, which is up to 14% lower than the off-peak charging strategy. In summary, higher energy efficiency for heating is validated in the three new proposed integration layouts (Cases 2–4) against the condenser charging layout.

Melting performance enhancement of a phase change material using branched fins and nanoparticles for energy storage applications

This study presents the application of branched fins and nanoparticles to enhance the melting of phase change material (PCM) placed in a horizontally configured latent heat thermal energy storage (LHTES) unit. The mathematical model governing the transient melting of PCM is numerically solved for 2D planar geometries. The numerical solution procedure is validated against literature as well as our experimental results. First, the effects of different fin designs on melting performance are analyzed using pure PCM. All-straight, single-branched and double-branched fin designs are seen to enhance melting through efficient heat transfer resulting in time savings of 22.9%, 35.4% and 45.9%, respectively, as compared to a Y-oriented triple-fin Base Case. Additionally, the energy storage capacity is observed to increase in the same order with the double-branched design storing the highest at ∼3600 kJ. Next, the combined effects of the most efficient double-branched fin design and Al2O3 nanoparticles on melting performance are assessed. An addition of 1%, 5% and 10% nanoparticles by volume in the PCM results in an additional time savings of 11.5%, 19.2% and 26.8%, respectively, taking pure PCM case as the reference. The corresponding energy storage, however, is seen to decrease with an increase in the nanoparticles concentration.

Effect of metal foam with two-dimensional porosity gradient on melting behavior in a rectangular cavity

The thermal performance of the latent heat thermal energy storage unit (LHTESU) can be effectively promoted by metal foam. In the present work, a new structure of metal foam with a two-dimensional porosity gradient (MFTDPG) is proposed to further accelerate the melting process inside the rectangular cavity. The MFTDPG is obtained by placing the metal foam with small porosity near the left wall and the bottom wall. With consideration of natural convection, the melting behavior under different parameters is numerically analyzed, including the direction of the porosity gradient, the temperature of the left wall, and the aspect ratio of the rectangular cavity. The results show that the structure of metal foam with vertical porosity gradient (MFVPG) and horizontal porosity gradient (MFHPG) can reduce the total melting time by 7.65% and 3.37% respectively. The MFTDPG can shorten the total melting time by 12.07% compared with the structure with uniform porosity. Furthermore, the MFTDPG can achieve the shortest melting time and the maximum thermal energy storage rate (TESR) with different wall temperatures and aspect ratios. For example, when the aspect ratio is 2.5, the total melting time of MFTDPG is reduced by 24.35%, and TESR is increased by 25.88%.

The Thermal Charging Performance of Finned Conical Thermal Storage System Filled with Nano-Enhanced Phase Change Material.

A latent heat thermal energy storage (LHTES) unit can store a notable amount of heat in a compact volume. However, the charging time could be tediously long due to weak heat transfer. Thus, an improvement of heat transfer and a reduction in charging time is an essential task. The present research aims to improve the thermal charging of a conical shell-tube LHTES unit by optimizing the shell-shape and fin-inclination angle in the presence of nanoadditives. The governing equations for the natural convection heat transfer and phase change heat transfer are written as partial differential equations. The finite element method is applied to solve the equations numerically. The Taguchi optimization approach is then invoked to optimize the fin-inclination angle, shell aspect ratio, and the type and volume fraction of nanoparticles. The results showed that the shell-aspect ratio and fin inclination angle are the most important design parameters influencing the charging time. The charging time could be changed by 40% by variation of design parameters. Interestingly a conical shell with a small radius at the bottom and a large radius at the top (small aspect ratio) is the best shell design. However, a too-small aspect ratio could entrap the liquid-PCM between fins and increase the charging time. An optimum volume fraction of 4% is found for nanoparticle concentration.

Thermal Charging Optimization of a Wavy-Shaped Nano-Enhanced Thermal Storage Unit.

A wavy shape was used to enhance the thermal heat transfer in a shell-tube latent heat thermal energy storage (LHTES) unit. The thermal storage unit was filled with CuO–coconut oil nano-enhanced phase change material (NePCM). The enthalpy-porosity approach was employed to model the phase change heat transfer in the presence of natural convection effects in the molten NePCM. The finite element method was applied to integrate the governing equations for fluid motion and phase change heat transfer. The impact of wave amplitude and wave number of the heated tube, as well as the volume concertation of nanoparticles on the full-charging time of the LHTES unit, was addressed. The Taguchi optimization method was used to find an optimum design of the LHTES unit. The results showed that an increase in the volume fraction of nanoparticles reduces the charging time. Moreover, the waviness of the tube resists the natural convection flow circulation in the phase change domain and could increase the charging time.