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Development of multi-module arranged in series using U-type longitudinal fin tube in thermal energy storage system
Shortening the melting time of phase change materials is a fundamental research problem in thermal energy storage systems (TES). The present research numerically explores improving the thermal performance of a Paraffin wax shell and tube TES unit through various anchor-type fins. The main studied factors are the fin shape and curvature. Three different anchor-type fins are investigated: the inside curved (Configuration 1), outside curved (Configuration 2), and typically straight (Configuration 3). Galerkin finite element method (GFEM) is employed to model the storage unit and melting process. Results show that Configuration 2 reports the optimum anchor-type fin configuration by reducing the thermal charging time by 47 % and increasing the averaged Nusselt number by > 20 % compared to the other configurations. In addition, the lowest recorded Bejan number (the thermal irreversibility) and the fastest stored energy were also achieved by Configuration 2.
In the current study, free convection heat transfer of a suspension of Nano–Encapsulated Phase Change Materials (NEPCMs) is simulated and discussed in an inclined porous cavity. The phase change materials are capsulated in nano-shells layers, while the core stores/releases large amounts of energy during melting/solidification in the vicinity of the hot/cold walls. The governing equations are introduced and transformed into non–dimensional form before being solved by using the finite element method. Simulation results are validated thoroughly. Thereafter, the consequences of the fusion temperature and the Stefan number on the distributions of streamlines, isotherms, and the heat capacity ratio, as well as the heat transfer characteristics, are analyzed for different inclination angles of the cavity. Inspection of the results demonstrates that the best heat transfer performance occurs for the non–dimensional fusion temperature of 0.5 and the inclination angle of 42°. It is found that a decrease in the Stefan number improves heat transfer. The results also show that the presence of the NEPCM particles generally leads to heat transfer improvement.
For conventional power plants, the integration of thermal energy storage opens up a promising opportunity to meet future technical requirements in terms of flexibility while at the same time improving cost-effectiveness. In the FLEXI- TES joint project, the flexibilization of coal-fired steam power plants by integrating thermal energy storage (TES) into the power plant process is being investigated. In the concept phase at the beginning of the research project, various storage integration concepts were developed and evaluated. Finally, three lead concepts with different storage technologies and integration points in the power plant were identified. By means of stationary system simulations, the changes of net power output during charging and discharging as well as different storage efficiencies were calculated. Depending on the concept and the operating strategy, a reduction of the minimum load by up to 4% of the net capacity during charging and a load increase by up to 5% of the net capacity during discharging are possible. Storage efficiencies of up to 80% can be achieved.
The paper presents the recent research in study of the strategies for the power plant flexible operation to serve the requirement of grid frequency control and load balance. The study aims to investigate whether it is feasible to bring the High Temperature Thermal Storage (HTTS) to the thermal power plant steam-water cycle, to identify the suitable thermal charge and discharge locations in the cycle and to test how the HTTS integration can help support grid operation via power plant dynamic mathematical modelling and simulation. The simulation software named SimuEngine is adopted and a 600 MW supercritical coal-fired power plant model is implemented onto the software platform. Three HTTS charging strategies and two HTTS discharging strategies are proposed and tested via the simulation platform. The simulation results show that it is feasible to extract steam from the steam turbine to charge the HTTS, and to discharge the stored thermal energy back to the power generation processes. With the integration of the HTTS charge and discharge processes, the power plant simulation model is also connected to a simplified GB (Great Britain) grid model. Then the study is extended to test the improved capability of the plant flexible operation in supporting the responses to the grid load demand changes. The simulation results demonstrate that the power plant with HTTS integration has faster dynamic responses to the load demand changes and, in turn, faster response to grid frequency services.
In this article, latent heat thermal energy storage (LHTES) is analyzed for nano phase change material enclosed in a hexagonal triplex-tube with solidification acceleration. Optimized curve shape fins are adjusted inside the tube. The shape of the structure is a hexagonal annular. Therefore, both inner hexagonal walls are cold. Corresponding to six vertices of the hexagonal shaped tube, parallel but symmetrically curved fins are adjusted to every corner. Nanoparticles Enhanced Phase Change Material (NEPCM) is filled inside the annulus, and the mathematical model is constructed based upon the system of nonlinear partial differential equations through the heat equation. Single wall carbon nanotubes (SWCNT) and Copper driven through liquid phase and solid phase within the entire duct. Galerkin Finite Element Method (GFEM) is employed to model the solidification procedure. The smaller symmetric fins achieving an angle of 10o≤β1≤45o and the larger symmetric fins making an angle of 10o≤β2≤45o with the vertical rod. However, the length of rod is 10mm≤L≤20mm. It is found that the optimized curve shape fin penetrate the intensity of thermal storage into the Phase change materials (PCM) and the shape of the fin prevents the phase trade manner from final unsolidified PCM near the outer shell, in which the phase alternate front passes the top of the fins. It is further analyzed that at the mean position of the triplex tube, the solidification process is minimum due to the high thermal performance of nanoparticle volume fraction and time perfection.
District heating has been widely implemented in residences because of its environmental and economic advantages. The hot water load of residences generally peaks in the morning and at night because of the lifestyle of the residents. An imbalance in the heat load presents high capital costs and low operating efficiency of the district heating facilities. In this study, a latent heat storage system using a solid-liquid phase change material was developed for the peak load shifting control of hot water supplied from district heating in an apartment complex. The latent heat storage system was designed using a shell-tube structure and can store heat for more than 100 h using an insulation of 50 mm. A field demonstration was performed by installing the developed latent heat storage system in a machinery room in the basement of the apartment complex. The demonstration showed that the total heat flow of hot water during a day and its standard deviation with the latent heat storage are lower than those without latent heat storage by 2.69% and 59.9%, respectively. Therefore, the latent heat storage system is crucial in the peak load shifting of hot water in the apartment complex.
The objective of this research work is to evaluate thermal performance capabilities of a heat exchanger having multiple circular tubes incorporated with three and six fins and a phase change material (PCM) charged in the shell region of the heat exchanger. Present study has been performed using two-dimensional computational fluid dynamics (CFD) model. To study the phase transition of the solid PCM and optimize the design of the cylindrical tubes based on the operating circumstances, a two-dimensional CFD simulation with a physical enthalpy-porosity approach is utilized. The effects of incorporating a number of fins on the tubes on heat transmission have been investigated in terms of liquid percentage and PCM mean temperature at two applied temperatures of 50°C and 60°C. While using a fixed temperature of 50°C, it was discovered that cylindrical tubes with three and six fins improved heat transmission in the PCM and reduced melting time from 780 s for a design having no fin to 650 s for a design having three fins and 580 s for a design having six fins. Results shows, when PCM melting begins, conduction is predominant and around the tubes, the melt zones are similar. However, convection heat transmission in PCM begins after 100 s in a configuration without fins, 80 s in a configuration with three fins, and 60 s in a configuration with six fins at 50oC. After 60 s, 24.6% of PCM is transformed into liquid in a setup without fins, 35% in a configuration with three fins, and 43.3% in a configuration with six fins. After 180 s, liquid percentage is 57.4% in the design without fins, 71.3% with 3 fins, and 79.6% with 6 fins. Results shows that adding 6 fins on tubes perimeter increases heat transmission. Results have been validated with literature results.
In view of the high energy consumption of heating and air conditioning in buildings, the study takes the unit radiation plate filled with Phase Change Material (PCM) as the research object, and proposes an energy storage scheme combining double-layer energy storage floor with ceiling-mounted energy storage radiant panel air conditioning to improve the utilization rate of valley electricity. Numerical simulation method is used to simulate the phase change heat transfer process, and the heat transfer characteristics of the phase change energy storage radiant air conditioning system under different ambient temperatures and indoor heat fluxes are studied. The results show that the floor surface can maintain a warm surface of 24–18 °C in the heat storage environment of 19 °C and the heat release environment of 12 °C in winter. And the floor surface can maintain a cool surface of 19–24 °C, and the radiant panel surface can maintain a cool surface of 18–23 °C in the cold storage environment of 26 °C and the cold release environment of 32 °C in summer. The system can not only meet the requirements of indoor heating and ventilation, but also alleviate the uncomfortable problem of rapid temperature change of radiant panel, and effectively solve the dewing problem of radiant panel.
This study aims to analyze the effect of fin geometry on the thermal performance of longitudinally finned-tube horizontal latent heat thermal energy storage (LHTES) systems. The longitudinal fins with different fin heights, thickness, and numbers were applied in the horizontal shell-and-tube LHTES system and the paraffin was packed in the annulus. The melting fronts, average temperature, and velocity profiles of Phase Change Material (PCM) were graphically illustrated and compared for charging processes. It was found that the incorporation of longitudinal fins in a small quantity (2.85% of total volume) could reduce the complete melting time by 34% in comparison to the bare-tube configuration. Melting characteristics were compared using average liquid fraction, average PCM temperature, and stored energy. It was recommended that the fin height should not exceed half of the annulus gap. The optimum fin volume was identified by comparing the enhancement ratio for examined configurations. Furthermore, for the same fin volume configurations, this study evaluated (i) whether a large number of short fins are beneficial or a smaller number of long fins and (ii) whether thick short fins are beneficial or thin long fins? Comparison results showed that a small number of long fins were more beneficial than that of more short fins and the melting characteristics of thin long fins were superior to that of the thick short fins. Finally, the optimum fin configuration was justified, and the priority sequence was suggested for the design of longitudinally finned-tube energy storage systems.
Tubular heat exchangers for natural convection can be found in many industrial applications. In order to develop the high-efficiency natural convection radiator, the heat dissipation characteristics of vertically oriented 3-dimensional (3-D) finned tubes under the condition of non-forced convection were studied experimentally. The airflow and temperature distribution outside the tubes were analyzed by computational fluid dynamics. Results showed that the 3-D finned tube with fin height of 7 mm achieved the highest Nusselt number, which was up to 207% higher than that of a smooth tube. Nusselt numbers increased with the increasing of the fin height and the decreasing of the axial fin pitch, but were minimally affected by fin width and circular fin pitch. For comparison, the most important fin geometric parameter that affects the heat dissipation is the fin height. Finally, an empirical correlation for predicting Nusselt number was obtained based on the experimental data, which can be used in practical design.
To improve the thermal performance of horizontal latent heat storage (LHS) units, this study investigates two innovative LHS units using uniform and gradient tree-shaped fins. By coupling visualization experiments and three-dimensional numerical simulations, the melting/solidification behavior and thermal properties of innovative LHS units are comprehensively analyzed and compared with traditional LHS units. The results show that tree-shaped fins facilitate heat diffusion from point to area, breaking the heat transport hysteresis in traditional LHS units, thus accelerating the melting/solidification rate. During the melting process, the gradient tree-shaped fins strengthen the heat transfer in lower parts of the LHS unit and extend the convective heat transfer duration in upper regions, promoting the synergistic strengthening of natural convection and heat conduction in the late melting stage. Compared to the uniform fin layout, the gradient tree-shaped fins effectively increase the melting rate, shortens the melting duration by 9%, and further improve the overall temperature uniformity of LHS units. However, the non-uniform transport path of gradient tree-shaped fins is not conducive to solidification heat transfer with conduction as the primary thermal regime, which increases the temperature gradient of LHS units and prolongs the solidification duration by 57.4% compared to the uniform tree-shaped fins.
In the current study, the melting process of phase change material (PCM) embedded with nanoparticles and metal foams (MF) in a large scale shell-and-tube based latent thermal energy storage unit (LTES) has been numerically studied. Initially, the developed model was validated with experimental data. Then, the effect of Al2O3 (5%) nanoparticles addition and several MFs, i.e. Aluminum (Al), copper (Cu), Nickel (Ni) and Titanium (Ti), of different porosity (96–99%) on the performance of the melting process has been compared based on temperature and liquid fraction evaluations during the melting operation. The MF-PCM systems showed the best performance over the nano- and pure- PCM, with a huge reduction in the melting time. The beneficial effect of MFs for the melting process followed the order Cu > Al > Ni > Ti, which is the same order of the thermal conductivity. Increasing the metal foam porosity results in shorter melting time; however, since the surface temperature of the porous-PCM unit is almost constant for different metal foam porosities, a system with higher porosity (99%) is desirable. This study optimizes the design to enhance practical application performance and to reduce waste energy.
Melting and solidification heat transfer characteristics of different paraffin/ethylene-vinyl acetate/graphene aerogel (Pa/EVA/GA) composites were predicted by the volume-averaged method (VAM). In parameter settings, Pa/EVA was treated as fluid, and GA was set to solid. The two-temperature model based on the assumption of local thermal nonequilibrium was adopted to solve the large difference in thermal conductivity between GA and Pa/EVA. The simulation results were validated by experimental data. The numerical results denoted that GA and EVA significantly improved heat transfer performance of Pa and reduced the melting/solidification time. The heat transfer of the composites was dominated by heat conduction, as the three-dimensional skeleton of GA and the high viscosity of EVA inhibited the natural convection of paraffin. Compared with Pa/EVA0/GA, Pa/EVA5/GA and Pa/EVA10/GA had a higher surface heat flux, decreased the velocity of paraffin, and had a more even temperature distribution.
In the present study, a numerical model of a vertical shell and tube latent heat storage system validated with the experimental data is presented. The developed model comprises of three blocks of phase change materials (PCMs) having melting point temperatures (Tm) 360 °C, 335.8 °C and 305.4 °C, respectively. A non-uniform distribution of fins in three PCM blocks is initially employed to study the performance of the single PCM system (Tm=335.8 °C). The effect of inlet heat transfer fluid temperature on the charging and discharging performances of the single PCM and multiple PCM (m-PCM) systems is analysed by varying a Stefan number (Steref) parameter, calculated based on the single PCM system. The charging and discharging times for the m-PCM system are either similar or lesser than the single PCM system for Steref≥1, however, there is an improvement of 21-25% in the specific power charged and discharged by the m-PCM system for all the Sterefvalues (0.5, 1, 1.5 and 2) considered. By employing a compound enhancement technique, which is a combination of non-uniform fin-distribution and PCM blocks length ratio optimization for the m-PCM system, 30% and 9% reduction in the charging and discharging time, respectively, over the single PCM system is achieved.
This paper presents a numerical investigation and experimental validation of the melting process of phase change material (PCM) in a double-pipe latent heat storage unit (LHSU) at different inclinations. The aim of this paper is determining the optimal inclination angle at different PCM thicknesses for the melting process in the double-pipe LHSU. The PCM is placed in the annulus while hot water flows through the inner tube. ANSYS-FLUENT 17 is used to create a transient multi-phase numerical model to simulate the thermal characteristic of the storage unit. Three masses of PCM have been numerically studied; namely 0.154, 0.308, and 0.46 kg which presents three PCM annulus thicknesses of 7, 11.28, and 14.5 mm. The length of the double-pipe LHSU is 500mm and the inclination angles are 0 ̊, 15 ̊, 30 ̊, 45 ̊, 60 ̊, 75 ̊, and 90 ̊. The numerical model has been verified through the comparison between its predicted results and those obtained from previous experimental published work. In addition, the comparison between the present experimental and numerical results shows good agreement. It has been found that the inclination of the storage unit has a significant effect on the melting behavior when the thickness of PCM is enough such that natural convection contributes to the melting process. For this condition, 45 degrees was the optimum inclination angle. Conversely, for thin PCM thickness, the horizontal orientation produced the least total melting time.
Adding longitudinal fins is an efficient method to enhance the melting behavior of a horizontal heat-storage tube. Nevertheless, the inhomogeneous heat transfer intensity, which is brought by the natural convection of liquid phase change material, causes a hard-melting region in the bottom half of the tube. In this study, two-dimensional models considering natural convection have been established to investigate the melting of phase change material in the latent heat storage unit with longitudinal fins. Optimization of the structural parameters, including fin-thickness and fin-length, are conducted and numerically calculated to accelerate the melting process. To enhance the charging of the hard-melting region, four strategies are employed and compared with the conventional model 1, whose fin length and fin thickness are constant. It is observed that the intensity of heat transfer is higher in the top-half domain, and it is lower in the bottom half. Moreover, thickening the bottom fins and thinning the top fins can efficiently enhance the charging process. An 8.7% reduction in the complete melting time can be realized by model 6 with the thickest bottom fins as compared to model 1 with equal length and equal thickness fins. Meanwhile, model 9, who has the longest bottom fins, can reduce the complete melting time by 47.1% as compared to model 1, so that the method of lengthening the bottom fins and shortening the top fins is likewise useful in accelerating the melting process. Based on the two strategies, we proposed an optimal model 10, who has the thickest and longest bottom fins, is conducted, and the complete melting time can be reduced by 54.1% compared with the original model 1. Consequently, the key limitation of heat storage in the horizontal tube is located in the bottom half, which is defined as hard-melting region, and thickening and lengthening the bottom fins is conducive to enhance the heat storage in the horizontal tube.
In this paper, the flow channel layout in latent heat storage unit is optimized to balance the effect of heat transfer and flow resistance based on topology optimization. A simplified two-dimensional latent heat storage unit is established and a binary objective function is adopted. The influence of different weighting coefficient ratio on the channel structure is discussed. Results show that as the weighting coefficient of heat transfer increases, the flow channel topology becomes more and more bent, and some small bifurcated channels gradually appear which means the enhancement of one factor usually comes along with the cost of the other factor. Subsequently, the optimized channel structures are reconstructed and then numerical simulation is carried out on these reconstructed models to validate the results obtained by topology optimization. Heat transfer performance and flow characteristics of these channel structures during the entire heat release process are explored. Results of the simulation confirm a better heat transfer performance of the optimized channel structure and clearly reflect that the heat transfer enhancement is accompanied by the increase of pressure drop between the inlet and outlet. A numerical comparison made between channel and fin shows that both of them enhance the heat transfer effect by increasing the thermal contact surface, while the structural continuity as well as the irregular shape of the optimized channel may increase the manufacturing difficulty. Prototype manufacturing and experimental test have been carried out to validate the channel structure.
Solar storage tanks are key to ensuring the high efficiency of concentrated solar power plants, and phase change materials are the most important storage energy media influencing system efficiency. Therefore, as energy storage or release mechanisms are a focus of related research. In this study, numerically analysed the thermal performance of a small capsule of three different phase change materials for a packed bed solar energy storage system. Air and molten salt are used as the heat transfer fluid (HTF) and the phase change material (PCM), respectively. A model based on a concentric-dispersion model and the enthalpy method was used to analyse the phase transition of the PCM. The equation was solved using the finite-difference method, and the results were verified using previous experimental data. The influence of particle diameter, porosity, and height-to-diameter ratio of the storage tank on the total storage energy, storage capacity ratio, axial temperature curve, and utilization ratio of the PCM were studied. It was found that he storage capacity and utilization rate of 3-PCM energy storage tanks are relatively high. And that increase from 86.07% to 86.65% and 86.07%–86.67%, respectively, when the porosity is reduced from 0.6 to 0.1. This results in an increase in the total storage energy of 5.2 × 10¹² Wh to 1.3 × 10¹³ Wh. Similarly, when the particle diameter decreases from 0.6 to 0.1, the storage capacity ratio and utilization rate increase from 85.8% to 87.3% and 85.6%–87.4%, respectively. However, although these increases are larger, the increase in total energy storage is small. Finally, it was found that the shape of the tank has no effect on the storage capacity at a fixed tank volume. The proposed model provides a reference value for energy storage in a concentrating solar thermal power (CSP) system.
In this study, melting (charge) and solidification (discharge) characteristics of a phase change material (PCM) in a vertical annular gap between a heat transfer tube and a shell are investigated experimentally. Different heat transfer tube surface configurations as finless tube and finned tube are considered in order to optimize the thermal performance of the system. Fin geometries with various edge lengths’ ratios (upper edge length / lower edge length) are examined for different inlet fluid temperatures. For each configuration tested, transient temperature variations at some radial and axial points inside PCM are obtained. For the finless tube case, natural convection recirculation is obtained to be weak in the lower half region of the annulus when compared to that in the upper half. Results show that adding a conducting fin to the inner tube intensifies the recirculation in the lower half region of the annular space. Accordingly, it is disclosed that decreasing the fin edge lengths’ ratio (w1/w2) significantly shortens the melting time while it has an insignificant effect on the solidification time.
A comparative thermal performance assessment is reported for vertical and horizontal orientation of shell and
tube type Latent Heat Storage Unit (LHSU) using stearic acid (melting point of 55.7–56.6 °C) as Phase Change
Material (PCM) and water as Heat Transfer Fluid (HTF). Total 45 thermocouples are used in radial, axial and
angular directions to measure the temperature distribution in the PCM. In horizontal LHSU, temperature in the
upper half of the horizontal diametric plane reaches the melting point faster due to natural convection. As a
result, the upper half melts earlier than the lower half. In order to demonstrate the motion of solid-liquid
interface, liquid fraction contours are presented. For vertical LHSU, melting front moves in a conical fashion
around the vertical plane with faster melting at upper axial locations. Experimental analysis suggests that
horizontal LHSU is better for part load operation as compared to vertical LHSU, as it requires lesser time to melt
half of the PCM mass. Energy storage/retrieval rate during melting and solidification is also compared for both
the LHSU configurations.
This paper is aimed at analyzing the behavior of a packed bed latent heat thermal energy storage system in concentrating solar power (CSP). One way of improving the performance of a latent thermal energy storage system is by implementing the multiple phase change materials (PCMs) design. The behavior of a packed bed latent heat thermal energy storage system at different cases is numerically analyzed. The molten salt is considered for the heat transfer fluid (HTF) with phase change material (PCM) capsules as the filler. In this design, spherical capsules filled with PCMs of different thermo-physical properties are used. The capsules are packed in the bed at different sections based on the PCM melting temperature. The model developed using the Concentric-Dispersion (C-D) equations. The governing equations are solved in MATLAB, and the results obtained are validated against experimental data from the literature. The performance of the systems is calculated. The results show that the three-stage PCMs system with different melting point exhibited the highest energy and exergy efficiency during a charging discharging cycle. Moreover, results show that the three-stage PCMs unit can improve the heat transfer rate greatly and shorten the heat storage time effectively.
The thermal conductivity of commonly used phase change materials (PCM) for thermal energy storage (TES), such as, fatty acids, paraffin etc., is relatively poor, which is one of the main drawbacks for limiting their utility. In the recent past, few attempts have been made to enhance the thermal conductivity of PCM by mixing different additives in the appropriate amount. Graphene nanoparticles, having higher thermal conductivity may be a potential candidate for the same, when mixed appropriately with different PCM. In present study authors have carried out the numerical investigation for the melting of graphene nano-particles dispersed PCM filled in an aluminum square cavity heated from one side. In this work, the graphene nanoparticles are mixed in three different volumetric ratios (1%, 3%, and 5%), with three different commonly used categories of organic, inorganic and paraffin PCM (namely, Capric Acid, CaCl2.6H2O, and n-octadecane) to see the effect on melting of composite PCM developed. The resulting transient isotherms, velocity fields, and melting front and melt fractions thus have been deliberated in detail. These results clearly indicate that the addition of graphene nanoparticles increases melting rate but can also hamper the convection heat transfer within large cavities.The study also shows that such enhanced PCM can be effectively used for different TES applications in different fields. The prediction of temperature variation and rate of melting or solidification may be found useful especially for designing such TES devices.
Latent Heat Storage Unit (LHSU) employing Phase Change Materials (PCMs) is an effective means of thermal energy storage for solar applications. The practical use of such energy storage unit is however limited by the low thermal conductivity of the available PCMs. Significant augmentation in the heat transfer rate of PCMs is possible by installation of longitudinal fins. The augmentation in heat transfer for a shell and tube type LHSU is estimated by carrying out experimental analysis with three longitudinal fins installed on the heat transfer fluid (HTF) tube. The heat transfer augmentation is established in terms of melting and solidification time for varying fluid inlet temperatures and flow rates of heat transfer fluid (HTF). Experimental results show that the heat transfer augmentation is more sensitive to increase in HTF inlet temperature as compared to increase in mass flow rate of HTF. Solidification time has been observed to reduce up to 43.6% by installation of three fins.
Storing sun’s energy in the form of latent thermal energy of a phase change material (PCM) offers high volumetric energy storage density resulting in low capital cost. The objective of this paper is to analyze the dynamic behavior of a packed bed thermal energy storage system with encapsulated PCMs, subjected to partial charging and discharging cycles, and constraints on charge and discharge temperatures as encountered in a concentrating solar power (CSP) plant operation. A transient, numerical analysis of a molten salt, single tank latent thermocline energy storage system (LTES) is performed for repeated charging and discharging cycles to investigate its dynamic response. The influence of the design configuration and operating parameters on the dynamic storage and delivery performance of the system is studied to identify configurations that maximize utilization of the storage system. Based on the parametric studies, guidelines are derived for designing a packed bed PCM based storage system for CSP plant operating conditions.
An experimental analysis is presented to establish the thermal performance of a latent heat thermal storage (LHTS) unit. Paraffin is used as the phase change material (PCM) on the shell side of the shell and tube-type LHTS unit while water is used as the heat transfer fluid (HTF) flowing through the inner tube. The fluid inlet temperature and the mass flow rate of HTF are varied and the temperature distribution of paraffin in the shell side is measured along the radial and axial direction during melting and solidification process. The total melting time is established for different mass flow rates and fluid inlet temperature of HTF. The motion of the solid–liquid interface of the PCM with time along axial and radial direction of the test unit is critically evaluated. The experimental results indicate that the melting front moves from top to bottom along the axial direction while the solidification front moves only in the radial direction. The total melting time of PCM increases as the mass flow rate and inlet temperature of HTF decreases. A correlation is proposed for the dimensionless melting time in terms of Reynolds number and Stefan number of HTF. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21120
An experimental energy storage system has been designed using a horizontal concentric tube heat exchanger incorporating a medium temperature phase change material (PCM) Erythritol, with a melting point of 117.7 °C. Three experimental configurations, a control system with no heat transfer enhancement and systems augmented with circular and longitudinal fins have been studied. The results presented compare the system heat transfer characteristics using isotherm plots and temperature–time curves. The system with longitudinal fins gave the best performance with increased thermal response during charging and reduced subcooling in the melt during discharging. The experimentally measured data for the control, circular finned and longitudinal finned systems have been shown to vindicate the assumption of axissymmetry (direction parallel to the heat transfer fluid flow) using temperature gradients in the axial, radial and angular directions in the double pipe PCM system.
To determine the heat transfer coefficient by natural convection for specific geometries, experimental correlations are used. No correlations were found in the literature for the geometries studied in this work. These geometries consisted of a cylindrical module of 88 mm of diameter and 315 mm height with external vertical fins of 310 mm height and 20 and 40 mm length. To determine the heat transfer coefficient by natural convection, experimental work was done. This module, containing PCM (sodium acetate trihydrate), was situated in the middle upper part of a cylindrical water tank of 440 mm of diameter and 450 mm height. The calculated heat transfer coefficient changed by using external fins, as the heat transfer surface was increased. The temperature variation of the PCM and the water are presented as a function of time, and the heat transfer coefficient for different fins is presented as a function of the temperature difference. Experimental correlations were obtained, presenting the Nusselt number as a function of different dimensionless numbers. Different correlations were analysed to find which one fit better to the experimental data.
The influence of expanded graphite (EG) and carbon fiber (CF) as heat diffusion promoters on thermal conductivity improvement of stearic acid (SA), as a phase change material (PCM), was evaluated. EG and CF in different mass fractions (2%, 4%, 7%, and 10%) were added to SA, and thermal conductivities of SA/EG and SA/CF composites were measured by using hot-wire method. An almost linear relationship between mass fractions of EG and CF additives, and thermal conductivity of SA was found. Thermal conductivity of SA (0.30 W/mK) increased by 266.6% (206.6%) by adding 10% mass fraction EG (CF). The improvement in thermal conductivity of SA was also experimentally tested by comparing melting time of the pure SA with that of SA/EG and SA/CF composites. The results indicated that the melting times of composite PCMs were reduced significantly with respect to that of pure SA. Furthermore, the latent heat capacities of the SA/EG and SA/CF (90/10 wt%) composite PCMs were determined by differential scanning calorimetry (DSC) technique and compared with that of pure SA. On the basis of all results, it was concluded that the use of EG and CF can be considered an effective method to improve thermal conductivity of SA without reducing much its latent heat storage capacity.