Latent Heat Thermal Storage Unit Research Articles

Analysis of the discharging process of latent heat thermal energy storage units by means of normalized power parameters

Many efforts are being made to mitigate the main disadvantage of most phase change materials – their low thermal conductivities – in order to deliver latent heat energy storage systems (LHESS) with adequate performance. However, the effect of applied methods is difficult to compare as they are mostly tested for different storage types and sizes and/or different boundary and initial conditions, which hinders rapid progress in the optimization of these approaches. In this work, a previously developed method for comparing the performance of LHESS is applied to experimental results of different storage systems under different conditions and subsequently analyzed and further refined. The main idea of the method is to normalize the power with the volume and a reference temperature difference and compare its mean value plotted over the normalized mean capacity flow of the heat transfer fluid (HTF). This enables the presentation of the results in a compact and easily comparative way. Attention has to be paid when it comes to the choice of the reference temperature difference, the reference volume and the method for calculating the mean value. Two variants of calculating the mean value (time-weighted and energy-weighted) and two variants of reference temperatures for determining the temperature difference to the inlet temperature of the HTF (initial temperature and melting temperature) are applied and discussed in detail. While the method significantly increases the comparability of results, none of the options listed above are without drawbacks. Approaches are shown to reduce or eliminate these drawbacks in the future. The recommendation for comparing different LHESS under different conditions is to use the method described here and clearly state the chosen reference temperature, reference volume and method for calculating the mean value.

Enhancing the performance of a greenhouse dryer with natural dolomite powder-embedded latent heat thermal energy storage unit and air-to-air heat recovery system

Greenhouse drying systems (GDSs) are grouped in direct-type solar dryers and are widely utilised because of their easy applicability, high product drying capacity and cost-effectiveness. In the present work, it is aimed to improve the drying performance of a GDS by using natural dolomite powder-embedded latent heat thermal energy storage unit (LHTESU) and air-to-air heat recovery system (HRS). In this context, three different types of GDSs have been designed and manufactured including a GDS with paraffin-based LHTESU, a GDS with natural dolomite powder-embedded LHTESU and a HRS-assisted GDS with natural dolomite powder-embedded LHTESU. Designed and manufactured GDSs have been experimentally analysed under the same environmental conditions. According to the experimentally achieved findings, combined utilisation of HRS and dolomite powder in the LHTESU reduced the drying time approximately as 36.36%. Moreover, highest instantaneous outlet air temperature was achieved as 51.4 °C in the system that used natural dolomite-embedded LHTESU and HRS. The exergetic efficiency of the GDS was upgraded as 46.21% by using the mentioned modifications in comparison to the base case (the system contains only paraffin-based LHTESU). In addition, specific moisture extraction rate for the analysed GDSs was attained between 0.96 and 1.52 kg.kWh−1.

Enhanced heat transfer in a latent heat thermal energy storage unit using a longitudinal fin with different structural parameters

The latent heat thermal energy storage (LHTES) unit can be used to resolve the imbalance between energy supply and demand. To boost heat storage efficiency, a model with longitudinal fins was developed to accelerate the melting process. Based on the finite volume method (FVM), the effect of fin angle and number on melting performance was evaluated. Further, the response surface methodology (RSM) was applied to optimize the longitudinal fin by maximizing its melting rate. Results demonstrate same heat transfer capacity at the fin angles θ and θ1(θ1=90−θ). The complete melting time at a θ values of 0°was reduced by 46.66%, compared with a θ value of 45°. The optimized case (θ = 0°, N =16 fins) reduced the total melting time by 62.59%. The melting rate of the upper region of the LHTES unit was higher than that of the lower region in all cases. Further, the study quantified the temperature uniformity of the system using mathematical methods, which showed that the integrated mean value of temperature uniformity at a θ value of 0° was the smallest (12.51°C), suggesting excellent temperature uniformity at a rotation angle of 0°.

Numerical energy and exergy evaluation for a multiple-layer latent heat storage unit enhanced with nanoparticles under different seasons

Latent heat thermal energy storage unit alleviates the mismatch between energy supply and demand. However, the sole melting temperature of a phase change material in a single-layer heat storage unit does not adapt well to the environment. Therefore, the mechanisms associated with the better environmental adaptability of a multiple-layer heat storage unit are explored in this study based on energy and exergy evaluations via comparison with single-layer units. An innovative methodology is proposed to discuss transient thermal energy and exergy, and exergy distribution of these heat storage units in different seasons. Results show that the multiple-layer units address the limitations of single-layer units by exhibiting excellent heat storage capacity and quality, as well as uniform melting and exergy distribution. Compared with the single-layer unit with a melting temperature of 49 °C, the multiple-layer unit exhibits a 56.64% higher energy in the winter. Moreover, the exergy is 57.14% higher than that of the single-layer unit with a melting temperature of 33 °C in the summer. This study reveals the mechanism of excellent thermal energy storage performance of the multiple-layer latent heat storage unit in different seasons.

Design of combinational fins for a vertical shell-tube latent heat thermal energy storage unit

The employment of annular fins and longitudinal fins in latent heat thermal energy storage unit can significantly improve its thermal performance. However, the annular fins can greatly suppress the convection and longitudinal fins lead to severe non-uniform melting. To address this issue, three different types of combinational fins were proposed and applied in a vertical shell-tube storage unit in this study. The numerical model was first built with the adoption of enthalpy-porosity method. The simulation results were then verified with experimental data. Results showed that combinational fins leads to faster melting thorough enhancing both natural convection and conduction compared with only longitudinal or annular fins. This advantage in melting enhancement is even greater with the increase of fin volume. Moreover, the enhancement effect of combinational fins is better with the same longitudinal and annular fins volume and a higher longitudinal fin at the same longitudinal fin volume. The optimum combinational fins in present work has a melting time reduction of 31.0% and 21.2% compared with only longitudinal and annular fins, respectively. The results can provide important reference for the fins design in vertical shell-tube thermal energy storage units.

Dynamic discharging performance of a latent heat thermal energy storage system based on a PID controller

A PID controller is introduced into a latent heat thermal energy storage unit to compose a coupling system in order to control the discharging performance. Outlet temperature of the working fluid can be precisely regulated by means of its inlet velocity variation based on the PID controller. The theoretical model is built through combining heat transfer of the latent heat thermal energy storage unit with the PID controller. Experimental results are used to validate the proposed theoretical model. PID control analysis indicates that its parameters have obvious effects on performance of the coupling system. Kp of −0.02 m/(s·K), Ki of −0.15 m/(s2·K) and Kd of −0.001 m/K are further optimized according to dynamic response of the transient outlet water temperature. System parameter analysis exhibits that higher target temperature produces larger heat discharging rate, which then reduces the outlet water flow rate accordingly. The discharging rate and total discharging energy of the latent heat thermal energy storage unit decrease with augment of the target outlet temperature. Increase of PCM melting point, thermal conductivity and latent heat is favorable of elevating the working fluid velocity. The discharging rate is improved by the PCM melting point, thermal conductivity and latent heat. The latent heat thermal energy storage units can separately obtain maximum discharging rates of 257.691, 294.437 and 257.603 W at 200 min for PCM melting point of 327.15 K, PCM thermal conductivity of 0.8 W/(m·K) and PCM latent heat of 250 J/g. It is apparent that PCM melting point, thermal conductivity and latent heat are conducive to enhancing the total discharging energy. The largest discharging energy at 200 min is respectively determined as 1280.409, 1060.105 and 974.153 kJ. Temperature and phase change contours can also be substantially affected by the system parameters. In conclusion, the built coupling system is beneficial to efficiently regulate and control discharging performance of latent heat thermal energy storage units.

Heat transfer enhancement of latent heat thermal energy storage with nanoparticle by bubble-driven flow

This study investigated the heat transfer enhancement of phase change material (PCM) with nanoparticles by bubble-driven flow. The addition of nanoparticles can improve conduction heat transfer, while bubble-driven flow can improve convection heat transfer. The 5 wt% nanoparticles increased the thermal conductivity by 6.4 % and viscosity 205 % of phase change material, while decreased the convective flow accordingly. However, bubble-driven flow strengthened the flow velocity by 8.3 times. Since nanoparticles tend to sediment over time in PCM, this issue of sedimentation should be overcome before they are applied in practice. The particle concentration in the vertical direction was analyzed by laser transmission experiment, and as expected, the concentration was the highest at the bottom. The bubble-driven flow uniformized the particle concentration in the vertical direction, which could solve the sedimentation problem. The combined application of nanoparticles and bubble-driven flow to phase change material both improved conductive and convective heat transfer, and provided uniform distribution of particle concentration. A latent heat thermal energy storage (LHTES) unit was designed for combined application study, and a charging performance experiment was conducted. Nanoparticle only improved the average charging power by 1.09 times, while bubble-driven flow only improved it by 1.27 times. Bubble-driven flow and nanoparticles applied altogether increased the average charging power by 1.41 times. Therefore, it was confirmed that the combined application of nanoparticles and bubble-driven flow can effectively improve the heat transfer performance of the latent heat thermal energy storage. Thus, this method is expected to be applied to a wide range of industries.

Study of the melting performance of shell-and-tube latent heat thermal energy storage unit under the action of rotating finned tube

Study of the melting performance of shell-and-tube latent heat thermal energy storage unit under the action of rotating finned tube

A critical assessment of nanoparticles enhanced phase change materials (NePCMs) for latent heat energy storage applications

Phase change material (PCM) laden with nanoparticles has been testified as a notable contender to increase the effectiveness of latent heat thermal energy storage (TES) units during charging and discharging modes. In this study, a numerical model is developed and implemented based on the coupling between an advanced two-phase model for the nanoparticles-enhanced PCM (NePCM) and the enthalpy-porosity formulation for the transient behavior of the phase change. Therefore, a porosity source term is added to the nanoparticles transport equation to account for the particles' frozen state in regions occupied by solid PCM. This two-phase model includes three main nanoparticles’ slip mechanisms: Brownian diffusion, thermophoresis diffusion, and sedimentation. A two-dimensional model of a triplex tube heat exchanger is considered and different charging and discharging configurations are analyzed. Compared to pure PCM, results show a substantial heat transfer enhancement during the charging and discharging cycle in which a homogeneous distribution of nanoparticles is considered as the initial condition. For this case, the two-phase model predictions are superior to the ones obtained with the classical single-phase model. In the case of multi-cycle charging and discharging, a significant deterioration of the heat transfer rate is observed using the two-phase model while such assessment is senseless using the single-phase mixture model due to the physical assumptions upon which this model is formulated. The two-phase model results reveal that, for a NePCM with high nanoparticles concentration (> 1%), the melting performance during the second charging cycle is reduced by 50% compared to the first one. This performance degradation is attributed to a noteworthy non-homogeneous distribution of the nanoparticles at the beginning of the second charging cycle. The dominant nanoparticles migration mechanism, in this scenario, is the one resulting from sedimentation effects.

Improving the energy release of a latent heat storage with multiple phase change materials loading and partition shaping

This paper presents a theoretical study of the integration of two selected phase change materials (PCMs) into a vertical shell and tube latent-heat thermal energy storage unit. Various cell shapes and PCM arrangements are analyzed for their heat release characteristics and compared to a reference unit. Using the finite volume method with an enthalpy-porosity formulation, the study simulates conjugate heat transfer between an isothermal wall and PCMs. This approach examines the impact of both the delay of the phase transition and the layout of the PCM cavities on natural convection. The findings indicate that these factors have a significant influence on energy transport. Delaying the solidification process near the cold wall can lead to a substantial reduction in the phase transition time. Furthermore, triangular cross-section cells improve convection and increase the solidification and complete energy discharge times by 42.7% and 25.3%, respectively. The energy charging performance is also enhanced by 39.7%.

Comparative study on heat transfer enhancement of metal foam and fins in a shell-and-tube latent heat thermal energy storage unit

Comparative study on heat transfer enhancement of metal foam and fins in a shell-and-tube latent heat thermal energy storage unit

Dynamic tuning of phase change interfacial thermal energy transport in hybrid nano-enhanced phase change materials by a non-uniform magnetic field

The present study aims to explore the synergistic enhanced phase change heat transfer behavior between magnetic field and hybrid nanoparticles under the consideration of non-Newtonian effects. The visualized experiments are conducted using thermochromic liquid crystal technology to extract the interconnection between dynamic phase change interface evolution and temperature change of hybrid nano-enhanced phase change material (HNEPCM), and thermocouples to measure temperature data inside the latent heat thermal energy storage unit. The magnetic field and hybrid nanoparticles synergistic mechanism is then further revealed based on the analysis of liquid phase fraction, Nusselt number, energy storage and energy storage efficiency. The results show that under magnetic field, the magnetic nanoparticles form a “thermal conductivity layer” at the solid-liquid interface, while the non-magnetic nanoparticles in the liquid phase region drive the phase change heat transfer to proceed. The non-Newtonian effect induced by high concentration of HNEPCM can weaken the convection intensity. The competition between natural convection and magnetic viscous effect under magnetic field is induced at different hybrid ratios of magnetic and non-magnetic nanoparticles. High hybrid ratios (R = 1.0, 1.5 and 2.0) will be more favorable to satisfy the high thermal conductivity requirement in the liquid phase region and the dynamic adjustment condition at the phase change interface. Under a non-uniform magnetic field, 1 wt.% HNEPCM with R = 1.0 is considered as the optimum strategy with better energy storage performance and higher magnetic field effect.

Thermal energy storage optimization using composite foam-nano enhanced phase change materials

A design of latent heat thermal energy storage (LHTES) unit for the rapid charging process of the nano-enhanced coconut oil inside an open-cell copper foam was proposed. A stream of hot liquid coconut oil was allowed to enter the thermal energy storage unit from the bottom and leave the unit from the top to accelerate the melting process. A heat transfer tube was placed inside the unit to melt the PCM layer and open a liquid film passage between the inlet and the outlet. The impact of design parameters such as porosity of copper foam, inlet PCM liquid pressure, hot inlet liquid inside the heat transfer tube, and volume fractions of nanoparticles was investigated on the charging time of the unit and its thermal behavior. The formation of the liquid PCM film over the heated tube is the key to fast charging. The key parameter in liquid film formation was the inlet velocity of hot liquid inside the tube. Increasing the inlet velocity by 2.5 times could reduce the thermal charging time by 2.5 times. Moreover, using nano-additives was not beneficial in this design since they could delay the liquid PCM film formation and the charging time.

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

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

Optimization of melting performance of a heat storage tank under rotation conditions: Based on taguchi design and response surface method

The melting performance of a rotating triplex-tube latent heat thermal energy storage unit is studied by numerical simulation method. The Taguchi design and response surface method are applied to its optimized design. Firstly, the Taguchi design method is used to quantitatively reveal the specific effects of fin distribution, fin number, and fin material on the melting performance of the unit. Compared with all-inner tube-fin or all-outer tube-fin structures, the melting time of alternating inside and outside fin structures is shortened by 52.64% and 32.42%, the average heat absorption rate is greatly increased by 105.56% and 47.26%, and the total heat is reduced by 2.64% and 2.17%, respectively. Then, the response surface method is applied to the eight alternating fins obtained by the Taguchi design, and the effects of fin length, width, and rotation angle on the melting time and average heat absorption rate of the unit are studied. Compared with the original structure, the optimal structure reduces the total melting time by 7.37% and increases the average heat absorption rate by 7.23%. The geometric parameters’ interaction with the relevant target response is studied, and the corresponding fluid-structure interaction equation is fitted. Finally, the melt growth phenomenon near the wall is found in the initial melting stage of the optimized model by mechanism analysis.

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

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

Investigation and optimization on melting performance of a triplex-tube heat storage tank by rotational mechanism

Phase change heat storage is the backbone of energy storage technology, but its storage time is affected by the low thermal conductivity of phase change materials. Therefore, the melting performance of a triplex-tube latent heat thermal energy storage unit (T-LHTESU) in a phase change heat storage system is studied in this paper, and the rotation mechanism is applied to the unit. Firstly, a numerical model of the T-LHTESU considering the rotation mechanism is constructed, and the validity of the rotation unit is verified by comparison with experimental data. In this unit, N-eicosane is used as a phase change material for heat exchange. The effects of different rotational speeds on the liquid phase distribution, temperature distribution, flow velocity distribution, total energy storage, and energy storage efficiency of the T-LHTESU are studied. The results show that the melting time of this unit at 0.1 and 1 rpm is 46.98 and 69.35% lower than that of the stationary model, respectively. The total amount of stored heat is decreased by 0.67 and 2.17%, and the heat storage efficiency is increased by 87.34% and 219.19%, respectively. This indicates that the addition of the rotation mechanism greatly increases the heat storage efficiency of the T-LHTESU and reduces its total melting time, while the reduction of the total energy stored in the melting cycle is small. Then it is proved that rotation improves the single heat transfer mechanism of the stationary model and eliminates the thermal deposition caused by natural convection by studying the internal temperature/velocity response of the stationary model and the speed of 0.1 rpm. The related geometric structure of the model is optimized by response surface optimization design based on 0.1 rpm rotation speed. The influence of each variable on the target response is obtained, and compared with the original model, the melting time of the optimized model is reduced by 12.24%. Finally, based on the geometric optimization design, the influence of element physical factors (temperature and material of fin/tube wall) on the related melting properties is studied. This study is helpful to promote the effective use of rotation mechanism in phase change heat storage systems and has a certain guiding role in the structural design.

Experimental and numerical evaluation of phase-change material performance in a vertical cylindrical capsule for thermal energy storage

• Laboratory study of melting in a vertical cylinder heated from the bottom/side. • Directly visualized experimental results of convective and close-contact melting. • Original in-house numerical model encompassing all basic physical phenomena. • Melting patterns in a vertical cylindrical enclosure under realistic conditions. • Conclusions relevant for latent heat thermal energy storage units design. Thermal energy storage (TES) is considered vital for the advancement of renewable energy solutions. Latent heat thermal energy storage (LHTES) captures the thermal energy via a solid-liquid phase transition that occurs in phase-change materials (PCM). The PCM is usually encapsulated in some way. In this study, we consider PCM melting in a vertical cylindrical enclosure, that is a prototype of a capsule used in a future storage system. Moreover, we achieve the highly desirable close-contact melting (CCM), which speeds up the charging process significantly. First, different types of experiments are conducted to elucidate the melting behavior in an original experimental device that allows different types of melting in such configuration, demonstrating the prominent role of close-contact melting. The device main features are transparency of the melting chamber, separate heating methods for the periphery and the bottom of the chamber, and axisymmetric melting. To further investigate the heat transfer modes in the system, an in-house numerical model for combined convective and close-contact melting in an axisymmetric cylindrical geometry is validated with the experimental results. Then, this original model is used to simulate a vertical cylindrical shell in a practical configuration, namely, when an enclosure is exposed to fluid flow normal to its bottom. The results clearly illustrate the importance of close-contact melting for the overall melting process under practical conditions. The conclusions from this work would aid the design of LHTES installations which involve macro-encapsulated PCM.

Low-Temperature Industrial Waste Heat (IWH) Recovery Using a New Design for Fast-Charging Thermal Energy Storage Units

The dynamic melting of CuO–coconut oil was addressed in a latent-heat thermal energy storage unit loaded with copper foam. In a new design, the thermal storage unit is made of a shell-tube-shaped chamber, in which a liquid flow of hot phase-change material (PCM) is allowed to enter the chamber from a port at the bottom and exit at the top. A fin is mounted in the chamber to forward the entrance PCM liquid toward the solid regions. The control equations were solved using the finite element method. The impact of foam porosity, inlet pressure, fin length, and the concentrations of CuO nanoparticles on the thermal charging time of the chamber was investigated. A fast-charging time of 15 min with a foam porosity of 0.95 was achieved. A porosity of 0.95 can provide a maximum thermal charging power of 15.1 kW/kg. The inlet pressure was a significant parameter, and increasing the inlet pressure from 0.5 kPa to 4 kPa reduced the melting time by 2.6 times. The presence of the fin is not advantageous, and even a long fin could extend the thermal charging time. Moreover, dispersed nanoparticles were not beneficial to dynamic melting and extended the thermal charging time.

Assessment on the effect of longitudinal fins upon melting process in a latent heat thermal energy storage unit

Latent heat thermal energy storage(LHTES) unit are an effective technical means to balance the demand and supply of energy. To enhance the heat transfer efficiency of the phase change material(PCM) in the LHTES unit, a series of novel three-tube heat exchangers with longitudinal fins was designed. The study was carried out in terms of the number, length, thickness, and arrangement of fins. Results demonstrate that the thickness change of fins has a significant effect on reducing the melting time of the whole PCM. The thin long fins have a higher melting rate. The complete melting time of the thin long fins was 10.6 % faster than that of the short thick fins. By studying the effect of fin length on the heat storage performance, it was found that the structure appeared to be more reasonable at L = 30 mm and the heat storage time was shortened by 18.0 %. In addition, the use of the lower long and upper short fin arrangement can strengthen the heat transfer. Considering that the amount of heat storage capacity decreases as the number of fins increases, N = 20 is considered as the best fin number. Therefore, we give an optimal combination of fin design parameters (N = 20, L = 30, W = 1), which shortens the total melting time of PCM by 71.2 %. To be conclusive, it is more beneficial to add fins to heat transfer enhancement than to have no fins, saving more energy charging time.