Latent Thermal Energy Storage Unit Research Articles

Parameter analysis and rapid design of porosity gradient distribution for open-cell metal foam in the latent thermal energy storage unit

The positive gradient porosity design of metal foam is used to improve the thermal performance of the vertical shell-and-tube latent thermal energy storage (LTES) unit. To optimize the gradient design, quantitative analysis is of vital importance. Therefore, the relative offset number EX is proposed in this study, which indicates the porosity difference between upper/down porosity and the mean porosity. Based on the numerical model of a shell-and-tube LTES unit filled with composite phase change material (CPCM) on the shell side, the effects of parameters, including the inlet HTF temperature and velocity, LTES dimensions, porosity, and thermal conductivity of the metal foam on the optimal gradient design are discussed. The optimal EX exhibits little relation with the inlet temperature but increases with the LTES height and thermal conductivity, while decreases with inlet velocity, porosity, and LTES outer radius. With the correlation between the optimal EX and the relative dimensionless number, the effects of these parameters on the optimal EX are integrated and quantified as experience equations so that the optimal porosity gradient of metal foam and the relative reduction melting time can be rapidly obtained under a wide variety of practical applications.

A New Approach of Solidification Analysis in Modular Latent Thermal Energy Storage Unit Based on Image Processing

The solidification process of RT18HC in a cylindrical shell and tube storage unit has been studied using a new methodology based on image processing. The main idea of the algorithm is to label the region of solidification and use statistical functions to calculate the dimensions of the solidification front over time. Said analysis includes two methods. The first method is to measure the solid fraction changes during solidification. The novelty of this method, as compared to other literature findings, is that pre-processing and calculation process occurs automatically via a calculation algorithm. This method is used to calculate the solid fraction of RT18HC which is reported to be a bit fast at the beginning that 40 % of its volume solidified in 1000 s while the rest of the process is completed in almost 6500 s. The second method is used to measure and calculate the thickness of the solid front by using image processing. This method’s error is calculated to be less than 7% throughout the entire process. The second method also acts as an experimental database of front thickness to use in a novel, simplified, semi-theoretical model proposed to calculate the solid front thickness as a function of time in this paper. It is also worth presenting solution extended by a general definition of thermal resistance for a cylindrical partition. The above study will enable the development of an enhanced and optimized model for complex geometries based on image processing techniques in the future. It will also allow the investigation of both processes i.e. solidification and melting alongside other influencing parameters such as the geometry of the storage unit in future.

Close-contact melting and natural convection in unconstrained melting: A parametric study

Close-contact melting (CCM) is a phenomenon that can be achieved in latent thermal energy storage (LTES) units where the falling of solid PCM is realized. On the other hand, natural convection (NC) plays an important role in LTES units due to circulations based on the buoyant flow of liquid phase change material (PCM). However, the relative contribution of CCM and NC on the melting heat transfer remains unclear when they coexist. This study considers a set of parametric cases that numerically investigate the effects of CCM and NC on an LTES unit with annular fins where CCM is enabled with a shell made of aluminum heated by the heat transfer fluid (HTF). The enthalpy-porosity method is used to model the melting in the LTES unit, and it is established that CCM can be realized using this method with a small value of mushy zone constant. However, with this small value, the mushy zone turns into a highly viscous liquid. The parametric study considers the number of fins and their clearance and concludes that with a decrease in fin clearance and an increase in fins, the effects of CCM are boosted, resulting in faster melting. The behavior of CCM and NC are characterized using the transient behavior of heat transfer rate for different surfaces and defining a factor that calculates the contribution of each surface on the melting. The role of CCM in the melting is up to 50%, while NC plays a less significant role. This study helps with the design of LTES units where CCM is enabled.

Unsteady temperature, heat transfer rate and HTF outlet temperature variation during charging of latent thermal energy storage unit

This work reports a detailed numerical study that is aimed to determine the variation of the three principal parameters, temperature of two phase change materials (PCMs), heat transfer rate and heat transfer fluid (HTF) outlet temperature, during charging of latent thermal energy storage unit (LTES). The unit is consisting of a shell-and-tube type. The shell space is filled with two paraffin wax named PCM1 and PCM2. During charging process, hot fluid heats the PCMs, and when PCMs melts, the heat is stored. A mathematical model based on the conservation energy equations was developed and numerically investigated. Numerical results show that, the variation of the three principal parameters goes through three distinct periods. As time progress, the heat transfer rate increases, reaches its maximum value during the first period, and then decreases, until finally it equals zero. The HTF outlet temperature remains constant and is equal to its initial temperature for approximately 200 seconds. During this time period, the heat storage capacity is greater, PCM1 and PCM2 store energy in the sensible form only. The analysis and discussion of results are employed to evaluate the correspondence between the variations of the three parameters in term of charging time.

Employing perforated copper foam to improve the thermal performance of latent thermal energy storage units

The performance of latent thermal energy storage units (LTESU) is limited by the low thermal conductivity of phase change materials (PCM). Copper foam can enhance PCM's heat conduction, but its structure inhibits the internal natural convection. Based on this, the perforated copper foam was proposed to enhance the natural convection of PCM. The contrast experiment built three LTESUs (Namely, the LTESU only with the copper tube (LTESU-CT), the LTESU with full copper foam (LTESU-FCF) and the LTESU with perforated copper foam (LTESU-PCF). The melting and solidifying state, temperature distribution, and thermal efficiency were compared in three LTESUs during thermal charging and discharging processes. Results showed using copper foam could enhance the thermal performance of LTESU obviously and the perforated copper foam could generate the convective heat exchange, which had a time-average Nusselt number 34.1 % higher in LTESU-PCF than that in LTESU-FCF. Due to the reduced heat transfer area, LTESU-PCF had the lower thermal efficiency than LTESU-FCF, but its thermal storage capacity and release capacity were 6.4 % and 1.4 % higher than that in LTESU-FCF, respectively. It was confirmed that employing perforated copper foam could s enhance the natural convection and heat transfer of PCM significantly, but the perforation parameter still needs to be further explored to achieve higher thermal performance.

Charging performance of structured packed-bed latent thermal energy storage unit with phase change material capsules

A mathematical model of the charging process for a structured packed-bed latent thermal energy storage unit with phase change material capsules is established. The thermal-hydrodynamic characteristics of the unit are investigated. The impacts of the heat transfer fluid inlet velocity, heat transfer fluid inlet temperature, initial temperature of the latent thermal energy storage unit, and diameters of the phase change material capsules are analyzed. The charging performance of a scaled-up latent thermal energy storage unit is studied. The numerical results indicate that the structured packing configuration of the structured packed-bed latent thermal energy storage unit influences the periodic flow characteristics of the latent thermal energy storage unit. The melting behaviors in each phase change material capsule are synchronous due to the excellent heat transfer capacity of the latent thermal energy storage unit. With the increasing initial temperature of the latent thermal energy storage unit, the duration of the charging process varies little, but the total heat storage capacity of the unit decreases. The decreasing diameter of the phase change material capsule significantly shortens the charging process of the latent thermal energy storage unit. The total heat storage capacities of the latent thermal energy storage unit with different phase change material capsule diameters are nearly the same. The heat storage capacity of the phase change material unit can be easily scaled up by adding more phase change material capsules and extending the phase change material capsule zone. The scale-up of the structured packed-bed latent thermal energy storage unit does not affect the charging time of the latent thermal energy storage unit.

Heat transfer analysis during the solidification of RT82 paraffin in big-scale metal foam-based latent thermal storage unit

Metal foam (MF) and nano-sized particles (NSP) are regarded as patent tools for enhancing the thermal performance of phase change materials (PCM)-based latent thermal energy storage unit (LTES), but data on this issue for large-scale installations is very scarce. This study provides a comprehensive computational analysis of the effects of MF and NSP on the solidification process of RT82 paraffin (PCM matrix) in a large-scale shell-and-tube latent thermal energy storage unit (of heat exchanger form). The developed 2D transient model developed on Ansys Fluent 15.0 software was initially verified using available literature experimental data. The process performance was tested for 5% Al2O3 nanoparticles and various MFs [i.e. aluminum (Al), copper (Cu), nickel (Ni) and titanium (Ti)] with varied porosity (96–100%). The computed mean and spatial temperature and solidified degree of the PCM block showed a drastic acceleration of the solidification process with the b MF technique rather than with the nanoparticles system. The solidification performance increased in the direction of MF-thermal conductivity increase, i.e. Cu > Al > Ni > Ti, and material porosity decrease. These conditions allow rapid HTF heat recovery and then stocking considerable thermal energy. However, the MF porosity could not decrease below 95% to avoid a huge loss of material storage (PCM), thereby diminishing the thermal storage capacity of the LTES unit.

Effect of bypassing the heat transfer fluid on charging in a latent thermal energy storage unit

Effect of bypassing the heat transfer fluid on charging in a latent thermal energy storage unit

Effects of mushy-zone parameter on simulating melting and solidification phenomena inside a spherical capsule

The research on latent heat storage technology is beneficial for the large-scale popularization and application of energy storage technology. In order to solve the difficulties in the latent thermal energy storage (LTES) technology, numerical studies of the solid–liquid phase-change process of LTES units using the enthalpy-porosity method have become research hotspots. Among them, the importance of studying the mushy-zone parameter has been neglected. In this paper, a two-dimensional numerical model is created based on the enthalpy-porosity method to investigate the effects of different mushy-zone parameters within a wide range [104–108 kg/(m3·s)] on the melting and solidification processes of the spherical phase change material capsule. A comprehensive examination of the fluid flow and heat transfer characteristics during two phase-change processes is conducted. Meanwhile, the morphology of the solid–liquid phase boundary and the evolution of the mushy zone under different mushy-zone parameters are discussed in detail. It is worth mentioning that a new analytical perspective is innovated by proposing the “eccentricity phenomenon,” and the eccentricity law is further explored. The results show that the influence mechanisms of the mushy-zone parameters on the melting and solidification processes differ greatly. This paper emphasizes the significance of exploring the mushy zone and provides adequate guidance for future simulation studies to determine the mushy-zone parameter.

Experimental and numerical investigation of melting and solidification enhancement using Fibonacci-inspired fins in a latent thermal energy storage unit

The major challenge with phase change materials (PCMs) is their low thermal conductivity, eventuating long melting (charging) and solidification (discharging) times in latent thermal energy storage (LTES) units. Using extended surfaces (fins) is a popular method to resulve this issue, with recent developments in novel designs such as tree-like and fractal fins. The complex fin configurations improve the conduction while decreasing the natural convection effects by introducing additional shear stress to the melted PCM. The current work proposes a new fin design based on the Fibonacci sequence in a vertical LTES unit to improve the charging/discharging characteristic. The finned geometry is manufactured out of a solid block of aluminum using the wire-cutting technique. Based on the volume expansion method, an experimental setup is designed to observe the charging/discharging behavior. The PCM under study is RT42 and its density, volumetric thermal expansion coefficient and solid-state thermal conductivity are measured in house. Water is the chosen heat transfer fluid (HTF) set to a constant mass flow rate of about 99 kg/h and 96 kg/h for the melting and solidification processes, respectively. The water charging temperature is 61.5 °C while the water discharge temperature is 21.5 °C. A numerical model based on the enthalpy-porosity method is implemented to investigate the physics behind melting/solidification. A careful investigation of the mushy zone constant is performed based on comparing the experimental and numerical results for melting and solidification. The average liquid PCM velocity on the melt front is studied to evaluate the natural convection behavior. The effects of secondary flows and mixing on the melting process in the new geometry are studied by investigating the liquid PCM flow behavior during the charging. The proposed fin performed 20% better in total melting time than longitudinal fins by introducing more dispersed natural convection dominated regions. The reduction of 45% in solidification time is also established for the novel fin.

Experimental investigation of latent energy storage systems with the tree-pin-shaped fin

The inefficient heat conduction of the phase change materials is the major bottleneck that hinders the large-scale application of latent thermal energy storage (LTES) systems. In this study, an innovative tree-pin-shaped fin was designed and assembled in a lab-scale shell-and-tube LTES unit, in which adipic acid was used as the phase change material. Moreover, a LTES unit with conventional longitudinal fins was tested under the same conditions for comparison. The temperature evolutions and thermal performance of the two units were studied during the charging and discharging processes. The experimental results suggested that the tree-pin-shaped fins significantly increased the charging and discharging speeds and enhanced the temperature uniformity in the LTES unit. Compared with the LTES unit with the longitudinal fins, the LTES unit with the tree-pin-shaped fins shortens the charging time by 44.6% and the discharge time by 50.6%. Furthermore, the tree-pin-shaped fin exhibited a structural feature that it did not restrain the natural convection when the LTES was placed at any orientation. The results of this study will provide a reference for the development of the LTES system.

Performance enhancement of a horizontal latent thermal energy storage unit with elliptical fins

The practical use of latent thermal energy storage (LTES) system is limited by the low thermal conductivity of the available Phase Change Materials (PCMs). In this study, the elliptical fin is proposed and applied to Horizontal Double Tube LTES Unit (HDT-LTESU) for improving the performance of thermal storage. Paraffin wax is used as PCM that is placed in the annulus between tube and shell. Three-dimensional numerical simulation based on enthalpy porosity method is performed to study the characteristics of the PCM melting/solidification and the performance of charging/discharging thermal energy of HDT-LTESU with different aspect ratios elliptical fins. The mechanism of enhanced thermal energy storage is also verified and analyzed by the entransy theory. It was found that the addition of elliptical fins can effectively improve convection heat transfer of PCM at the top and bottom of the annulus. Further, the high aspect ratio fins are more favorable for improving the comprehensive performance enhancement, and enhancement ratio can reach up to maximum 16.9% and 13.8% for melting and solidification, respectively. The entransy dissipation is reduced, and the phase change process of heat transfer is improved.

Experimental study on thermal performance of the novel two-path thermal energy storage for emergency cooling in data center

Emergency cooling system is necessary to guarantee high security for data center. Thermal energy storage is one of the feasible methods for emergency cooling with quick response ability to supply higher thermal safety. To simplify the traditional thermal energy storage system and improve its heat transfer efficiency, the novel two-path latent thermal energy storage unit developed for emergency cooling is experimentally studied. The effects of inlet temperature, flow rate on thermal performance of LTES, including heat transfer rate, accumulated energy, effectiveness and overall heat transfer coefficient, are investigated during both charging and discharging processes. Results from the experimental analysis show that the released energy of system could reach up to 2.9 MJ with the average heat transfer rate 1575W-2286 W. The overall heat transfer coefficient ranges from about 297–660 W/K when the flow rate of HTF varied from 12 to 16 L/min. The experimental LTES is then studied integrated with CRAC for emergency cooling. For air flow rate higher than 300 m3/h, and thermal load lower than 1800 W, the LTES meets 15 min of emergency cooling. The inlet and outlet air temperature of CRAC are successfully controlled under 40.5 °C and 23.9 °C. The newly designed LTES shows good thermal performance, and provides more flexibility and adaptability for real application in data center.

Investigating the effect of distribution form of copper foam fins on the thermal performance improvement of latent thermal energy storage units

Investigating the effect of distribution form of copper foam fins on the thermal performance improvement of latent thermal energy storage units

Heat Transfer Analysis of Melting Process over A Horizontal Three Fins Tube With Different Orientations

In this paper, an experimental study was achieved for the melting process of a phase change material around a copper tube with three longitudinal copper fins that placed inside a rectangular plastic container as a horizontally latent thermal energy storage unit (LTESU). A paraffin wax was used as a phase change material (PCM). Some parameters were considerable in this work like the directions of fins at angles of (0°, 30°, 60° and 90°), temperature of the water entering to the finned tube at (55, 65 and 75) °C and the effect of water flow rate inside the finned tube at (7.5, 15 and 22.5) liters / min. The law of energy conservation was used to find the amount of potential energy stored inside paraffin wax by calculating the energy liberated from the hot water entering the system. The results showed that the effect of the temperature of water entering to the system has given increases in the melting with rate of (64.3)% at a temperature of (75) °C compared to the water temperature at (55) ° C. Also the increases in melting rate has reached to (17.5)% at the flow rate (22.5) liters / min compared to the flow rate (7.5) liters / min . The melting has reached to (25.6)% at the angle of (30) degrees compared to the angle of (90) degrees. The results also showed that when using temperature of (55) ° C, water flow rate (7.5) l/min and rotation angle of (30°) that the maximum enhancement ratio in the melting fraction reached to (5.74)% and when using the water temperature (65) degrees and flow rate (15) l/min and the angle of rotation (30°) the maximum enhancement ratio was (11.9)%. When using the incoming water temperature (75) °C and flow rate (22.5) l/min and the angle of rotation at (30°) the maximum enhancement ratio was (12.03)%. The highest percentage of time savings was also reached (12.03%) at the angle of (30°) compared with (90°). Finally, the highest percentage of energy liberated was reached at a water temperature (75) ° C and fin rotation angle (30°) and the flow rate (22.5) l / min was reached (5355) watts.

A New Approach of Solidification Analysis of in Modular Latent Thermal Energy Storage Unit Based on Image Processing.

A New Approach of Solidification Analysis of in Modular Latent Thermal Energy Storage Unit Based on Image Processing.

Thermal performance improvement of thermal energy storage systems by employing a contrastive experiment

The thermal storage system is widely used to reduce the mismatch between thermal energy supply and demand in time and space, and its thermal performance is a critical factor in the system's thermal efficiency. The thermal performance of the Sensible Thermal Energy Storage Unit (STESU), the Latent Thermal Energy Storage Unit (LTESU), and the LTESU integrated with Copper Foam Fin (CFF) were compared through comparative experiments. The experimental results showed that the thermal changing rate of STESU was higher than that of other units, and there was no apparent thermal stratification. However, LTESU had the lowest thermal changing rate with apparent thermal stratification. During the thermal charging process, the solid-liquid interface on the LTESU cross-section formed an inverted-triangle shape. The lower part of the PCM could not change its phase as a dead zone and failed to participate in the thermal charging and discharging. Even so, the thermal efficiency of LTESU was 18.7% higher than that of STESU. Integrating with CFF could increase the thermal charging capacity, the thermal discharging capacity and the thermal efficiency by 51.1%, 55.9% and 10.8%, respectively.

FEM simulation of the entropy due to a buoyancy-induced flow confined novel prismatic enclosures using nano-encapsulated phase change materials

The flow, heat transfer, and melting-solidification behaviors within novel prismatic enclosures using the nano-encapsulated phase change materials (NEPCMs) are examined in this study. The flow area has nonfacing isothermal boundaries and it is filled by a homogeneous porous medium (glass balls). The Brinkman-extended non-Darcy formulations are applied while the latent heat of the change phase is computed using sine profiles. The host fluid is water, and the core and shell of the NEPCMs are nonadecane and polyurethane, respectively. The numerical simulations are carried out using the finite element method based on the characteristics-based split algorithm. The irreversibility of the system, flow, and thermal fields together with melting-solidification area is examined for wide ranges of the governing parameters. The major findings disclosed that the flow, heat transfer, and irreversibility of the system are affected by the fusion temperature and permeability of the medium. Also, an augmentation in values of up to 75% is obtained as is altered from 1% to 5%. Additionally, the heat transfer rate gets higher values at the lower values of the inclination angle. Furthermore, this numerical investigation is germane to the latent thermal energy storage units.

Experimental comparison of latent thermal energy storage units with different metal macro-additives

The inherent low thermal conductivity of phase change materials hinders the industry's deployment of practical latent heat storage systems. Adding macro-additives into PCMs is the most economical way to overcome this problem. This study experimentally compares three mid-temperature shell-and-tube LTES units with macro-additives of fins, copper foam, and shavings under the same conditions. The heat transfer mechanism of three macro-additives is investigated by analyzing the temperature evolution curves and comparing their structural characteristics. Experimental results show that the charging time of the fin unit is 68.4% shorter than the copper foam unit and 59.5% shorter than the copper shaving unit. Although the three heat storage units have the same compactness factor, the fin unit has the best thermal conduction performance for two reasons. One is that the local porosity of the fin unit decreases as the radius increases, achieving a higher effective thermal conductivity in the regions with a larger temperature gradient. The other is that the fins have a concentrated thermal energy flow pathway in the radial direction, which aligns the primary effective thermal conductivity direction with the heat flux direction. In addition, the larger pore size in the shaving unit is the main reason for its better performance than the foam unit.

Performance study on forced convection mode V-trough solar air heater with thermal storage: An experimental approach

This paper presents the performance evaluation of Forced Convection V-Trough Solar Air Heater (FCVTSAH) with latent thermal energy storage unit. The FCVTSAH contains three pass solar air heater, an absorber unit reserved with paraffin wax, an air blower and the cylindrical drying unit. The thermodynamic performance is evaluated using the drying of agricultural commodities in the dry air temperature having the temperature in the range of 35 °C to 68 °C. The experimental test is carried out from 20th April 2020 to 22nd April 2020 at (12.96° N and 77.63°E). The energy and exergy performance analysis of each component is done using standard mathematical expression available from litrature. It is evaluated that the average energy efficiency is fluctuates from 2 % to 50.5 % and exergy efficiency fluctuates from 2.4 % to 25 %. The latent heat storage unit energy and exergy efficiency fluctuates from 20.3 % to 24.5 % and 8.9 % to 12.8 %. The drying unit exergy efficiency is in the range of 20.5 % to 74.3 % respectively.