Heat Transfer Fluid Inlet Temperature Research Articles

Thermal analysis of packed bed thermal energy storage system with dimpled spherical capsules

The thermal storage potential of a packed bed filled with paraffin wax capsules was examined. Heat transfer fluid (HTF) at 70 °C inlet temperature for dimpled and plain stainless-steel capsules was compared for three different flow rates, 1 L/min, 3 L/min and 5 L/min. In general, dimpled spherical capsules sustain more heat than plain spherical capsules at the same flow rate, according to instantaneous and cumulative heat stored. Spherical capsules with dimples exhibited a 30 % improvement in charging rate compared to smooth spheres, particularly at elevated flow rates. Specifically, at a flow rate of 5 L/min, dimpled capsules achieved faster charging and sustained higher temperature compared to plain capsules. Higher flow rates (3 and 5 L/min) result in steeper temperature increases and an earlier peak temperature phase compared to the lower flow rate (1 L/min). Key dimensionless heat transfer numbers were examined. The Stefan number was found to be 0.1966 for an HTF inlet temperature of 70 °C and a latent heat of fusion (Lm) of 213 kJ/kg. Heat transfer increases when Reynolds numbers increased from laminar (123.8 at 1 L/min) to turbulent (619.1 at 5 L/min). Rayleigh number decreased over time but increased for dimpled capsules, indicating improved convection. Dimpled capsules had higher Nusselt numbers and increases with flow rate, indicating better convective heat transfer. Dimpled capsules absorbed heat faster and stored more energy, giving them a viable and efficient Packed Bed Latent Thermal Energy Storage system design.

Tracking of the melting front in tube-in-tube latent thermal energy storage heat exchangers

In this study, the shrinking solid regime is investigated in a high aspect ratio tube-in-tube latent thermal energy storage (LTES) heat exchanger by tracking the phase change front evolution. In the setup, water flows through the inner tube as heat transfer fluid (HTF) and the shell contains a paraffin RT35HC as phase change material (PCM). A series of melting experiments have been performed in which the HTF mass flow rate, HTF inlet temperature and initial PCM temperature were varied. During each experiment, the movement of the phase change front was tracked using a camera placed next to the PCM tube. The front position as a function of time follows an S-shape curve. The effect of the operating conditions on the evolution of the phase change front is analysed. It is found that the temperature difference between the HTF and PCM exerts a more pronounced influence on the front position compared to the HTF Reynolds number. Finally, a correlation was developed to determine the temporal evolution of the front position. This research contributes to a deeper understanding of phase change phenomena in LTES systems and the found correlation can be a valuable tool for predicting and optimizing the performance of tube-in-tube LTES heat exchangers.

Experimental investigation on thermodynamic performance of a copper foam-based cascaded latent heat storage tubes

Cascaded latent heat storage (CLHS) systems offer an effective means for energy storage from moderate to medium and low temperature heat sources (100-200℃). In this study, a three-stage shell-and-tube cascaded latent heat storage system was designed and fabricated. Three types of paraffin wax with distinct melting points were employed as phase change materials (PCMs), and foam copper was utilized to enhance heat transfer. By adjusting the inlet temperature and flow rate of heat transfer fluid (HTF), the thermodynamic performance of each LHS stage and the entire system under different operating conditions were investigated. Exergy analysis, entransy analysis, and sensitivity analysis were employed to study the primary influential factors on system performance and the impact weights of diverse operating conditions. Experimental results highlighted the crucial significance of maintaining consistency in the physical parameters of PCMs across different stages in the CLHS system for its thermodynamic performance. The high thermal conductivity and low latent heat of PCM#2 in the second stage result in substantial energy losses during the heat storage process, reducing the efficiency of energy cascaded utilization within the system. With an increase in the HTF inlet temperature and flow rate, the energy storage capacity and rate increase, while the energy storage efficiency exhibits a decreasing trend. Moreover, the effect of varying inlet temperature on the energy storage efficiency is much smaller compared to that of inlet flow rate. This study emphasizes that heat dissipation is a critical factor affecting the thermodynamic performance of the cascaded latent heat storage system. For CLHS systems targeting high-temperature and high-flow rate heat sources, enhancing insulation is crucial for ensuring the sustained high-efficiency operation.

Effect of steam flow parameters on the partial load behaviour of a PCM storage

To estimate the achievable thermal power in passive latent heat thermal energy storage (LH-TES) systems and the corresponding charging/discharging times for customised storage design and operation, relevant influencing parameters and their impact must be sufficiently known and understood. These parameters include not only the well-researched parameters on the phase change material (PCM) side, but also the parameters on the heat transfer fluid (HTF) side, such as the inlet temperature, the mass flow rate and the resulting heat transfer coefficient (HTC) between the HTF flow and the tube wall. For heat exchangers, these influences have been studied well, however for highly transient LH-TES studies are scarce. This article presents the influence of relevant parameters focusing on the ones related to the HTF side, using analytical and numerical parameter studies in Matlab®. The effect of different HTCs for varying flow conditions of the HTF in different zones of the tube is utilised to introduce the dominant heat transfer surface between the storage and the HTF (DHTS) as an indirect operating parameter. It corresponds to the area where the HTF phase change occurs and where most of the total energy is transferred. The numerical study indicates that the size of the dominant heat transfer area and thus the DHTS can be adapted by the HTF mass flow rate and the temperature difference between the HTF inlet temperature and the PCM phase change temperature. Based on this knowledge, new conjugate design and operating strategies for a test rig with a nominal thermal power of 1 kW using the commercial PCM PLUSICE A133 as storage material are proposed. They lead to a peak shaving behaviour of the storage with defined charging and discharging times.

Experimental study on the thermal response of a metal foam dual phase change unit thermal storage tank

Adding fins or metal foam to phase change units can effectively enhance the heat storage efficiency of thermal energy storage (TES) tanks, thereby improving the reliability of TES tanks in coordinating renewable energy supply and demand. In this study, an experimental TES tank platform was designed and constructed to investigate how the addition of straight fins, metal foam, and straight fin/metal foam composite materials to paraffin affects the temperature response during the heat storage process. High-temperature water was selected as the heat transfer fluid (HTF) and injected from the top into the TES tank at the same paraffin fill level, and T-type thermocouples were arranged along the central axis of the phase change unit. TES tanks filled with either pure paraffin or composite phase change materials were tested by varying the HTF inlet temperature and velocity, and temperature response data were captured and recorded using a KEYSIGHT DAQ970A data logger. The results indicated that increasing the HTF inlet temperature significantly enhanced the heat storage rate while increasing the HTF inlet velocity had varying effects on different phase change units. A comprehensive analysis suggested that an inlet velocity of 1.3 m3/h (from a range of 0.8–1.8 m3/h) was reasonable. The addition of enhancement measures improved the temperature drift phenomena and effectively increased the temperature response rates and distribution uniformity. Moreover, the straight fin/metal foam composite material exhibited the best performance.

A novel flat coil tube heat exchanger for metal hydride hydrogen storage reactors

Metal hydride (MH) based hydrogen storage is one of the potential methods for achieving higher volumetric storage densities. However, its poor thermal conductivity limits heat transfer during exothermic hydriding and endothermic dehydriding processes. Hence, heat transfer augmentation is essential in MH reactors to speed up the charging and discharging of hydrogen. The present study proposes a novel flat coil tube heat exchanger with a spiral fin for heat transfer augmentation. It offered superior performance than conventional single and double helical tube heat exchangers, taking 35.3 and 16.7% less time for 90% storage time. Further, optimization of design parameters (pitch and fin thickness) is carried out considering several key parameters such as weight ratio (WR), gravimetric exergy output rate (GEOR), and energy efficiency (EE). A pitch of 30 mm with a fin thickness of 0.25 mm is optimum according to a novel performance index. The influence of the operating parameters (H2 supply pressure and heat transfer fluid inlet temperature) on the absorption performance is investigated for the optimized design. Finally, a single desorption case is presented at 1 bar and 333 K to gain insights about desorption behavior.

CFD Analysis of a Latent Thermal Storage System (PCM) for Integration with an Air-Water Heat Pump

Heat pumps driven by sustainable electricity sources have been identified as a technology that can contribute to reduce carbon dioxide emissions. Furthermore, a heat pump can also provide energy savings when combined with a thermal energy storage system. Heat pumps can optimize their efficiency by accumulating thermal energy during periods of lower electricity demand, resulting in shorter operational durations and decreased overall energy consumption. In this work, the combination of a latent heat storage system with an air-water heat pump has been numerically analysed and experimentally tested. A phase change material (PCM) heat exchanger with an immersed plate was designed using a 3D CFD model (COMSOL Multiphysics®). The heat exchanger configuration with six steel plates immersed in the phase change material tank was proposed to enhance heat transfer in the storage system. The developed model is validated against experimental data from a real case study, demonstrating a maximum error of approximately 3% during the discharging phase. Additionally, the study explores the effects of different inlet heat transfer fluid temperatures and flow rates on the PCM solidification time.

An experimental investigation on the heat transfer characteristics of cascaded thermal storage units based on multichannel flat tubes

A latent heat storage unit made of a two-stage phase change material (PCM) can decrease the mismatch between supply and demand of energy. The cascaded phase change thermal storage stores and utilizes energy hierarchically. A two-stage cascaded thermal storage unit based on multichannel flat tubes and compact fins is developed in this study. The device uses water as the heat transfer fluid (HTF). Melting/solidification time, charging/discharging power, thermal efficiency, and exergy efficiency are evaluated for different HTF inlet temperatures and PCM volume flow rates. At the charging conditions of 70 °C and 1.5 L/min, the melting time for paraffin and lauric acid are 21and 20 min, respectively. The thermal efficiencies of the first and second stages of the thermal storage unit are 70.62 % and 70.40 %, respectively, and the exergy efficiencies are 81.25 % and 81.19 %. Given the small pressure loss of the pump and the high heat transfer performance of the heat storage system, using 70 °C and 1.5 L/min as experimental conditions for HTF is the key to obtaining short PCM melting times and high power.

Experimental study of a latent heat thermal energy storage system using erythritol for medium temperature applications

Using solar energy and waste heat for medium-temperature thermal applications depends on efficient and economical heat storage development. Developing a compact thermal energy storage system is essential to use excess thermal energy from a source in a process or to shift the utilisation time of solar thermal energy or recovered waste heat. Phase change materials are commonly used for energy storage applications globally. In this work, a latent heat thermal energy storage (LHTES) system with 2.37 kg of Erythritol as the phase change material (PCM) has been studied. A shell and tube heat storage system is selected due to its effectiveness in heat transfer. Synthetic silicone oil Hi-Tech Therm-60 was used as heat transfer fluid. Thermal stability tests in a differential scanning calorimeter (DSC) for a small sample mass and a specifically designed experiment for a larger sample mass were performed to assess the stability of the PCM. The time-dependent temperature distribution inside the storage system and the effect of heat transfer fluid (HTF) inlet temperature on the charging behaviour were studied during the charging process. Charging time and melt fraction have been calculated. No significant change in melting temperature and a 10% reduction in latent heat of PCM was observed in the thermal stability test for a small sample mass in DSC. The charging time of the storage device was observed to be 91–97 min and 195 min for 140 °C and 130 °C HTF inlet temperatures, respectively. The total stored energy in the system was 922 kJ with a Stefan number of 0.59. Temperature variation during the charging process inside the storage also suggests a buoyancy effect, leading to increased natural convection during the melting process.

Multi-factor analysis and optimization design of a cascaded packed-bed thermal storage system coupled with adiabatic compressed air energy storage

In an adiabatic compressed air energy storage (A-CAES), one of the key components is the heat storage system, in which the packed bed filled with encapsulated phase-change capsules has been widely investigated because of its excellent thermal performance. In this paper, a packed-bed thermal energy storage model with three layers of phase change materials (PCM) is proposed in the context of an A-CAES. Four factors primarily affecting the thermal performance of the packed bed thermal storage system, namely the mass flow rate of heat transfer fluid (HTF), the inlet temperature of HTF, the cascade situation of PCM, and the arrangement of particle sizes, are critically analyzed by conducing the orthogonal experiments. On this basis, the machine learning method is applied to predict and optimize the thermal performance of the packed bed aiming to find the optimal results. The comparison of results between the optimized and original models expressively shows a reduction of 8.46% in PCM mass, an increase of 92.18%/116.82% in the overall heat storage/release quantity, and an increase of 19.23% in the overall efficiency. This work outlooks a guideline for the potential application of packed bed in A-CAES.

Investigation of thermal performance and charging/discharging characteristics of finned coaxial evacuated tube collector filled with phase change material

Present numerical model simulates the charging (melting), discharging (solidification) and heat transfer phenomenon of a phase change material (PCM) filled in the finned type coaxial evacuated tube solar collector. Thermal performance of the proposed evacuated tube has been predicted with Paraffin wax as the PCM. Solar radiation data measured on March 13, 2022 at Srinagar (Garhwal), Uttarakhand, India (latitude 30.2247° N and longitude 78.7986° E) has been applied as heat flux (450–965 W/m2) on wall of the evacuated tube. Heat transfer fluid (HTF) at the varying inlet temperature flows through the tube and absorbs heat, consequently its temperature increases. Results predict that during the charging process, 90 % of the PCM was melted by 11:30 AM for 0.0063 kg/s mass flow of the HTF having inlet temperature of 303 K. For the same operating conditions, maximum temperature rise achieved by HTF is ≈ 30 K at 14:00 PM. In the absence of solar radiation, discharging process (during 18:00 to 23:00 PM) of the PCM occurs that liberates the heat which is carried by HTF. Hence, heating of the HTF is continued during this period, however, outlet temperature continuously drops as night progresses. For the considered inlet temperature of the HTF = 303 K, maximum and minimum outlet temperatures were found to be 316.87 K and 307.55 K respectively. Furthermore, mass flow rate of the HTF significantly affects the melting rate of the PCM. At the mass flow rate of 0.0063 kg/s, 99.36 % of the PCM was melted by 12:30 PM. Whereas, till 14:00 PM, only 91.68 % and 70.54 % of the PCM melts when mass flow rate increases to 0.016 kg/s and 0.026 kg/s respectively. Findings also confirm that by increasing the HTF inlet temperature from 288 K to 308 K, melting period of the PCM is reduced by 40 %. Hence, it may be concluded that varying intensity of the solar radiation has significant effect on the performance of the evacuated solar tube. Moreover, the performance also depends on various other operating parameters.

Modelling a latent heat thermal storage demonstrator and identification of the model key-parameters

Latent Heat Thermal Energy Storage (LHTES) has a great potential to reduce CO2 emissions in urban areas, when associated to a district heating network. It facilitates the flexibility of the network by decoupling the production and consumption. Thus, consumption peaks that are often managed with fossil fuels will be managed by unloading the storages of the network. LHTES based on tube and shell is the most widely studied technology, as it combines low cost, high maturity, and high compactness factor. This paper presents a numerical model to simulate the thermohydraulic behaviour of a shell-and-tube LHTES, based on a 1D approach for the heat transfer fluid (HTF) and a 1.5D approach for the Phase Change Material (PCM). The objective is to develop a model representing a whole system, faster than a Computational Fluid Dynamic approach while keeping a sufficient degree of accuracy. The paper focuses on the presentation of the model and its validation against experimental data from a real-scale 180 kWh demonstrator, currently in operation in the substation of the urban heating network of Grenoble, France. The validation against three different charges is presented, two charges at a constant flow rate (0.6 and 1.6 kg/s-1) and one at constant power (60 kW). The model gives results with an accuracy superior to 95 % and >80 % of the points are between the experimental uncertainty curves. It has also been found that the model loses its accuracy when the inlet HTF temperature abruptly decreases and the top collector does not behave as a stratified tank anymore (the relative error on total energy accumulated reaches 35 %). In the second part of the paper, a sensitivity study to the main parameters of the model shows that the model is sensible to the discretization of the collectors, the precision of the Nusselt correlation for convective transfers in the tubes, and the fusion temperature of the PCM. These parameters can induce up to respectively 22 % (0D model for the collectors' volume), 4.5 % to 9 % (considering the annular space as a flat plane or a cylindrical tube) and 3 % to 6 % (error of 0.5 to 1 °C in the PCM fusion temperature) error.

Thermodynamic performance of cascaded latent heat storage systems for building heating

Decarbonization of building space heating is essential for China to meet its carbon neutrality goal by 2060. Cascaded latent heat storage (CLHS) coupled with electric heating is a promising technology to promote renewable energy consumption, reduce carbon emissions, and save on heating bills. However, few studies have focused on the thorough investigation of the superiority of the CLHS system over a non-cascaded system. This study investigated the effects of heat transfer fluid (HTF) flow rates, HTF inlet temperatures, and number of stages on the thermodynamic performance of non-cascaded and CLHS systems based on energy and entransy analysis. Results showed that, compared with the non-cascaded system, the charged thermal energy and discharged thermal energy of the two-stage CLHS system increased by 26.5%–44.6% and 19.8%–74.5%, respectively, and the entransy increase of cold HTF increased by 20.0%–75.7%; while, in heating systems, it is not guaranteed that thermodynamic performance improves by using more stages in the CLHS system.

Experimental investigation of thermal characteristics of phase change material in finned heat exchangers

A significant challenge to the widespread use of practical latent heat energy storage systems based on phase change materials is the inherent low thermal conductivity of these materials. Inserting fins is considered one of the effective methods of heat transfer enhancement. In the present paper, an experimental investigation of the melting process of PCM in finned and unfinned heat exchangers is conducted. The experimental setup was constructed, and the measurement system was used to monitor the transient progress of the melting process. The heat exchanger comprises annular horizontal concentric double pipes, where the inner tube is heated by flowing water as heat transfer fluid (HTF), and the outer shell is thermally insulated. Four configurations of fins of the same fin/PCM volume ratio are attached to the inner tube. These configurations are longitudinal fin (LF), circular fin (CF), longitudinal perforated fin (LPF), and circular perforated fin (CPF). The effect of inlet HTF temperatures of 60, 70, and 80 °C is tested. The experimental findings indicate that the thermal performance of finned heat exchangers exceeds that of unfinned heat exchangers. Also, the perforated finned heat exchangers showed a higher thermal feature than the unperforated ones. In addition, the CPF achieved the lowest melting time and the highest melting rate, followed by LPF, CF, and LF. The maximum reductions of melting time are 71.1 %, 70.6 %, and 70.2 % due to using CPF HE instead of UF HE for inlet HTF temperatures of 60, 70, and 80 °C, respectively.

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.

Design and thermodynamic analysis of an innovative parabolic trough photovoltaic/thermal system with film-based beam splitter

This study aims to find a feasible photovoltaic/thermal (PV/T) utilization method based on film-based beam splitting technology. In contrast with previous studies, the main novelty of this paper is proposing the design of an innovative PV/T system using both parabolic trough concentrator (PTC) and film-based beam splitter (FBS). The design approach of the PV/T system is presented. A new FBS used for this PV/T system is designed by using the Needle method, and its average reflectivity and transmissivity are 27.2% and 72.8%. By using Ray Tracing and Computational Fluid Dynamics (CFD) simulation methods, the performance evaluation of the PV/T system is conducted. The evaluation results reveal that when the East-West sun tracking error is 0.2°, the optical efficiencies of the photovoltaic module and the solar thermal collector tube (TCT) can be kept at 95% and 93.9%. Moreover, when the heat transfer fluid (HTF) in the TCT is selected to be the ethylene glycol, the operation analysis results demonstrate that the photoelectric efficiency of the photovoltaic module is 25.4% and the thermal efficiency of the PV/T system is 17.24%. The thermal performance of the system can be improved when the HTF flow velocity is increased properly or when the inlet HTF temperature is decreased. The technical feasibility of the proposed PV/T system can be revealed from the analysis results of this study.

Thermal performance augmentation of parabolic trough solar collector using nanomaterials, fins and thermal storage material

Investigating and evaluating parabolic trough collector's (PTC) thermal performance have been shed out numerically by many researchers. Only a few researchers have focused their attention on using experiments to study the PTC's thermal performance. Consequently, the objective of this paper is to utilize an experimental method to put more shed on the PTC performance using active and passive methods. This aim is achieved by developing an experimental setup with modifications in reference parabolic trough collector (RPTC) and employing different heat transfer fluids. RPTC modifications include adding fins to the outer tube surface (FPTC), coating the absorber with Copper Oxide nanoparticles (CuO) combined with black paint (NCPTC), and filling the annulus with graphite (GPTC). Heat Transfer fluid (HTF) is changed by inserting CuO nanoparticles into water (NFPTC). All experiments are performed during August 2021 in Kafrelsheikh University. Data such as: inlet and exit temperatures of HTF, air temperature, air speed, solar irradiation, and rate of mass flow of HTF are recorded hourly for RPTC, NFPTC, NCPTC, NF-NCPTC, FPTC, and GPTC. Based on these data, thermal efficiency is calculated. FHT mass flow rate is controlled and typical values of 8, 10, 14, 16, 20, and 26 l/h are employed with RPTC in purpose to explore the optimum value based on practical considerations, HTF temperature difference, and higher efficiency. This value is found to be 20 l/h and is used in all modified collectors. All modified collectors' thermal performance is studied and compared with the traditional collector. The main finding in present study indicates that there is an augmentation in the thermal efficiency by 19.18 %, 26.23 %, 37.76 %, 44.43 %, and 54.82 % for NFPTC, NCPTC, NF-NCPTC, FPTC, and GPTC respectively when compared with RPTC.

Numerical investigation of a latent cold storage system using shell-and-tube unit

The use of cold thermal storage systems in low-temperature industrial applications is considered one of the most promising ways of improving energy efficiency and reducing the use of power during peak periods. In this study, the thermal performance of a shell-and-tube cold storage system under realistic operating conditions is investigated numerically. The proposed model is developed based on energy balances and then validated using existing experimental data from the literature. Glycol/water is used as the heat transfer fluid (HTF) and the phase transition phenomena in the phase change material (PCM) is simulated using the enthalpy–porosity approach. The influence of several design and operating parameters, including the HTF mass flow rate, HTF temperature, PCM type, and volume, on the cold storage performance during the crystallization process is presented and analyzed. The numerical results show that increasing the HTF mass flow rate accelerates the PCM crystallization process. However, the delivery periods of constant thermal power and constant HTF outlet temperature are reduced. The HTF inlet temperature has a significant effect on the cold storage performance, and the complete charging period is reduced by approximately 37% when the HTF inlet temperature is reduced from −4 °C to −7 °C. Increasing the number of tubes in the cold storage unit is concluded to significantly improve the thermal performance of the system, and using water/ice as a cold storage medium is more suitable than using the commercial PCMs RT2-HC and RT4-HC. Finally, the proposed numerical model for cold storage systems can be successfully used to design and simulate their realistic operation under different conditions.

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

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

Numerical analysis of latent thermal energy storage system under double spirally coiled tube configuration with two separated paths

The current research attempts to illustrate results of a reliable 3-D numerical simulation to scrutinize the charging process of paraffin as the phase change material (PCM) in an innovative heat exchanger called double spirally coiled tube heat exchanger (DSCTHE) with two separated paths. The water as a heat-transfer fluid (HTF) streams through the double spiral tubes embedded throughout the latent thermal energy storage (LTES) system. The PCM melting characteristics under three different geometries are investigated. After validating the proposed numerical simulation model, the impacts of operational and geometrical factors including inlet temperature of HTF and radius of the inner spiral tube on the behavior of the LTES system in the course of the melting process are investigated in detail. In order to achieve better insight into practical aspects of the presented LTES system, the transient behavior of a PCM for all three considered cases under various working conditions is discussed. Moreover, the 3-D temperature and liquid fractions contours at various time frames are obtained. The findings reveal that a decrement in the radius of the inner spiral tube resulted in a lower capability to melt PCM at the commencement of the charging procedure but more efficiency at the final stages. Generally, the overall time essential for melting of 95 wt% PCM in the storage diminishes up to 126 % by reducing the radius of the inner spiral tube from 90 mm to 50 mm.