Heat Transfer Enhancement for PCM Thermal Energy Storage in Triplex Tube Heat Exchanger

Thermal energy storage is critical for reducing the discrepancy between energy supply and energy demand, as well as for improving the efficiency of solar thermal energy systems. Among the different types of thermal energy storage, phase-change materials (PCM) thermal energy storage has gained significant attention recently because of its high energy density per unit mass/volume at nearly constant temperature. This study experimentally investigates the using of a triplex tube heat exchanger (TTHX) with PCM in the middle tube as the thermal energy storage to power a liquid desiccant air-conditioning system. Four longitudinal fins were welded to each of the inner and middle tubes as a heat transfer enhancement in the TTHX to improve the thermal performance of the thermal energy storage. The average temperature of the PCM during the melting process in the TTHX with and without fins was compared. The PCM temperature gradients in the angular direction were analyzed to study the effect of the natural convection in the melting process of the thermal storage. The energy storage efficiency of the TTHX was determined. Results indicated that there was a considerable enhancement in the melting rate by using fins in the TTHX thermal storage. The PCM melting time is reduced to 86% by increasing of the inlet heat transfer fluid. The average heat storage efficiency calculated from experimental data for all the PCMs is 71.8%, meaning that 28.2% of the heat actually was lost.     

Thermal Performance Enhancement of Triplex Tube Latent Thermal Storage Using Fins-Nano-Phase Change Material Technique

Latent heat thermal energy storage (LHTES) systems using a phase change material (PCM) can reduce the heat-transfer rates during charging/discharging processes because of their inherently low thermal conductivity. In this study, heat-transfer enhancement using various configurations of longitudinal fins employing both a PCM and a nano-PCM in a large triplex-tube heat exchanger (TTHX) was numerically investigated via the Fluent 15 software. The results showed that the thermal conductivity of the pure PCM (0.2 W/m K) can be observably enhanced by dispersing 10% alumina (Al₂O₃) to 25%. Therefore, the melting time is reduced to 12%, 11%, and 17% for the internal, internal-external, and external fins, respectively, compared with the case of the PCM without nanoparticle. It is concluded that the model of external fins-nano-PCM embedded in a large TTHX is the most efficient model for achieving complete PCM melting in a short time (188 min), where improving the thermal performance to 14% and 11% compared with the TTHX with internal and internal-external fins-nano-PCM, respectively. The simulation results are validated and agree well with experimental results for the PCM and nano-PCM.     

三套管式相变蓄热器强化传热研究

基于三套管式相变蓄热器的特点,提出应用T字形翅片来强化相变蓄热器的传热性能。研究结果表明:添加翅片可有效地降低蓄热器中相变材料的凝固和融化时间,直翅片和T字形翅片的混合强化结构能使凝固过程比未强化结构节省74%的时间,使融化过程节省60%的时间。因此直翅片和T字形翅片的混合使用可以达到进一步强化传热的目的。     

Enhancement of latent heat energy storage using embedded heat pipes

Latent heat thermal energy storage (LHTES) utilizing heat pipes or fins is investigated experimentally. Photographic observations, melting and solidification rates, and PCM energy storage quantities are reported. Heat pipe effectiveness is defined and used to quantify the relative performance of heat pipe-assisted and fin-assisted configurations to situations involving neither heat pipes nor fins. For the experimental conditions of this study, inclusion of heat pipes increases PCM melting rates by approximately 60%, while the fins are not as effective. During solidification, the heat pipe-assisted configuration transfers approximately twice the energy between a heat transfer fluid and the PCM, relative to both the fin-assisted LHTES and the non-heat pipe, non-fin configurations.     

Energy storage and solidification of paraffin phase change material embedded with graphite nanofibers

Phase change materials (PCMs) are known to be excellent candidates for thermal energy storage in transient applications. However, enhancement of the thermal conductivity of a paraffin-based PCM is required for effective performance, particularly during solidification where diffusion is the dominant heat transfer mode. This study experimentally examines the effect that graphite nanofibers (GNFs), aspect ratio and power density have on both thermal storage and solidification time of a PCM which is embedded between two sets of aluminum fins. Additionally, a figure of merit is introduced in order to quantify the effectiveness of each of these three parameters with respect to solidification time. GNF enhancement was shown to reduce the maximum temperature in the thermal containment unit (TCU) by 48%. It was also found that for aspect ratios of 1, the GNF enhancement shortens solidification time by as much as 61% over the paraffin samples. This research indicates that GNF impregnation into phase change materials is an effective method for the enhancement of the thermal energy storage and the solidification of paraffin-based phase change materials.     

Analysis of solidification in a finite PCM storage with internal fins by employing heat balance integral method

Here, a simplified analytical model has been proposed to predict solid fraction, solid–liquid interface, solidification time, and temperature distribution during solidification of phase change material (PCM) in a two‐dimensional latent heat thermal energy storage system (LHTES) with horizontal internal plate fins. Host of boundary conditions such as imposed constant heat flux, end‐wall temperature, and convective air environment on the vertical walls are considered for the analysis. Heat balance integral method was used to obtain the solution. Present model yields closed‐form solution for temperature variation and solid fraction as a function of various modeling parameters. Also, solidification time of PCM, which is useful in optimum design of PCM‐based thermal energy storages, has been evaluated during the analysis. The solidification time was found to be reduced by 93% by reducing the aspect ratio from 8 to 0.125 for constant heat flux boundary condition. While, for constant wall temperature boundary condition, the solidification time reduces by 99% by changing the aspect ratio from 5 to 0.05. In case of convective air boundary surrounding, the solidification time is found to reduce by 88% by reducing the aspect ratio from 8 to 0.125. Based on the analytical solution, correlations have been proposed to predict solidification time in terms of aspect ratio and end‐wall boundary condition.     

Experimental research on thermal characteristics of PCM thermal energy storage units

To explore thermal management integration in electric vehicles (EVs), a phase change materials (PCMs) thermal energy storage unit using flat tubes and corrugated fins is designed. The investigation focuses on the thermal characteristics of the PCM unit, such as the temperature variation, heat capacity, and heat transfer time, etc. Meanwhile, the heat storage and release process will be influenced by different inlet temperature, liquid flow rate, melting point of the PCM, and the combination order of the units. Under the same inlet temperature and flow rate condition, the PCM unit with higher melting point enters the latent heat storage stage slowly and enters the phase change melting release stage quickly. Furthermore, the heat storage and release rates increase with increasing liquid flow rates, but the effects are diminishing in the middle and later periods. The multiple PCM units with different melting temperatures are cascaded to help recycle low-grade heat energy with different temperature classes and exhibit well heat storage and release rates.     

The optimum fin length distribution of tabular PCM heat exchanger

The study aims to find the optimal fin length distribution for improved heat transfer during melting and solidification in a tubular phase change material (PCM) heat exchanger (HE) designed for heat storage. Three types of horizontal PCM tabular HEs, all with five longitudinal fins, were studied numerically. While maintaining a constant heat-transfer area, each model depicts a unique fin length distribution design. The first model, which serves as the reference design, has a uniform fin length distribution and each fi\n is 30 mm long. The second model has shorter upper and side fins and longer lower fins. The third model has long lower fins but shorter than that of the second model, with short side fins and no change in upper fin length with reference design. The findings indicate that the second model exhibits the best heat-transfer performance for the melting process, while the first model is most effective for solidification. Interestingly, the third design emerges as the optimum choice for both melting and solidification processes, where for 1 h of melting operation, results obtained 87%, 92%, and 90% for three models, respectively, from the first uniform model to the third model. While for 2 h of solidification the result obtained 11%, 17%, and 13% liquid fraction for the three models, respectively.     

Numerical simulations on performance enhancement of a cross-flow latent thermal energy storage heat exchanger

基于列管式换热器具有传热面积大、结构紧凑、操作弹性大等优点,使其在相变储能领域具有广阔的应用前景。本文建立一种新型列管式相变蓄热器模型,在不考虑自然对流的情况下,利用Fluent软件对相变蓄热器进行二维储热过程的数值模拟。本文主要研究斯蒂芬数、雷诺数、列管排列方式、肋片数以及相变材料的导热系数对熔化过程的影响,并对熔化过程中固液分界面的移动规律进行了分析。模拟结果表明,内肋片强化换热效果明显,特别是对应用低导热系数相变材料[导热系数小于1 W/(m·K)]的列管式蓄热器,相对于无肋片结构,加入肋片（N_fn=2）可缩短熔化时间52.6%。     

Thermal and heat transfer characteristics in a latent heat storage system using lauric acid

The thermal and heat transfer characteristics of lauric acid during the melting and solidification processes were determined experimentally in a vertical double pipe energy storage system. In this study, three important subjects were addressed. The first one is temperature distributions and temporal temperature variations in the radial and axial distances in the phase change material (PCM) during phase change processes. The second one is the thermal characteristics of the lauric acid, which include total melting and total solidification times, the nature of heat transfer in melted and solidified PCM and the effect of Reynolds and Stefan numbers as inlet heat transfer fluid (HTF) conditions on the phase transition parameters. The final one is to calculate the heat transfer coefficient and the heat flow rate and also discuss the role of Reynolds and Stefan numbers on the heat transfer parameters. The experimental results proved that the PCM melts and solidifies congruently, and the melting and solidification front moved from the outer wall of the HTF pipe (HTFP) to the inner wall of the PCM container in radial distances as the melting front moved from the top to the bottom of the PCM container in axial distances. However, it was difficult to establish the solidification proceeding at the axial distances in the PCM. Though natural convection in the liquid phase played a dominant role during the melting process due to buoyancy effects, the solidification process was controlled by conduction heat transfer, and it was slowed by the conduction thermal resistance through the solidified layer. The results also indicated that the average heat transfer coefficient and the heat flow rate were affected by varying the Reynolds and Stefan numbers more during the melting process than during the solidification process due to the natural convection effect during the melting process.     

Approximate analytical model for PCM solidification in a rectangular finned container with convective cooling boundaries

Thermal energy storage units that utilize latent heat storage materials have received increased attention in the recent years because of their relatively large heat storage capacities and isothermal behavior during charging and discharging. In this study, an analytical approach is presented for the prediction of temperature during the solidification in a two-dimensional rectangular latent heat storage using a phase change material (PCM) with internal plate fins. The basic energy equation is formulated accounting for the presence of a heat thermal fluid (HTF) on the walls. A two-dimensional numerical model is developed based on the enthalpy method to predict the distribution temperature of the fin and solid–liquid interface in storage. Results from the analytical solution and numerical model show a good agreement. The developed analytical model estimates satisfactorily the solidification time of PCM in storage, which is useful in the design of PCM-based thermal energy storages and cooling systems.     

Performance enhancement of high temperature latent heat thermal storage systems using heat pipes with and without fins for concentrating solar thermal power plants

A key drawback of using latent heat thermal storage systems for concentrating solar thermal power plants is the low thermal conductivity of the phase change material during the melting and solidification processes. This paper investigates an approach for reducing the thermal resistance by utilising axially finned heat pipes. A numerical model simulating the phase change material melting and solidification processes has been developed. This paper also includes the models of the evaporation and condensation of the heat pipe working fluid. The results show that by adding four axial fins and including the evaporation and condensation, the overall thermal performance of the storage system is enhanced significantly compared to having bare heat pipes. After 3 h a total of 106% increase in energy storage is obtained during the charging process. The results also show that the combined effect of incorporating the evaporation/condensation process and adding the fins leads to a threefold increase in the heat storage during the first 3 h. During the discharge process, there was a 79% increase in energy discharged and also the combined effect of incorporating the evaporation/condensation as well as adding the fins results in an almost four fold increase in the heat extracted within the first 3 h. A parametric analysis has also been carried out to analyse the effect of the finned heat pipe parameters after incorporating evaporation and condensation of the heat pipe working fluid.     

Thermal energy storage system using stearic acid as a phase change material

The thermal performance and phase change stability of stearic acid as a latent heat energy storage material has been studied experimentally. The thermal performance and heat transfer characteristics of the stearic acid were tested and compared with other studies given in the literature. In the present study, parameters such as transition times, temperature range and propagation of the solid–liquid interface as well as the effect of the heat flow rate on the phase change stability of stearic acid as a phase change material (PCM) were studied. The experimental results showed that the melting stability of the PCM is better in the radial direction than in the axial direction. The variation in the melting and solidification parameters of the PCM with the change of inlet water temperature is also studied. We observed that while the heat exchanger tube is in the horizontal position, the PCM has more effective and steady phase change characteristics than in the vertical position. The heat storage capacity of the container (PCM tube) is not as good as we expected in this study and the average heat storage efficiency (or heat exchanger effectiveness) is 50.3%. This indicates that 49.7% of the heat is actually lost somewhere.     

High temperature latent heat thermal energy storage using heat pipes

A thermal network model is developed and used to analyze heat transfer in a high temperature latent heat thermal energy storage unit for solar thermal electricity generation. Specifically, the benefits of inserting multiple heat pipes between a heat transfer fluid and a phase change material (PCM) are of interest. Two storage configurations are considered; one with PCM surrounding a tube that conveys the heat transfer fluid, and the second with the PCM contained within a tube over which the heat transfer fluid flows. Both melting and solidification are simulated. It is demonstrated that adding heat pipes enhances thermal performance, which is quantified in terms of dimensionless heat pipe effectiveness.     

Enrichment of heat transfer in a latent heat storage unit using longitudinal fins

The charging and discharging rates of a phase change material (PCM) in a horizontal latent heat storage unit (LHSU) is largely influenced by the lower thermal conductivity of the PCM. In the present research, four different configurations of longitudinal fins are proposed to augment the heat transfer in horizontal shell and tube type LHSUs. Numerical investigations are reported to establish the thermal performance augmentation with rectangular, triangular, and Y‐shaped (bifurcated) fins. From the results, it has been inferred that all fin configurations provide a faster charging and discharging rate. In the present set of geometric dimensions of LHSU considered, a reduction in charging time of 68.71% is evaluated for case III (three rectangular fins with one fin positioned in the area of the heat transfer fluid [HTF] surface) and case V (two bifurcated fins with one fin positioned in the area of the HTF surface). Moreover, overall cycle (charging + discharging) time is reduced by 58.3% for case III. Employment of fins results in a faster rate of absorption and extraction of energy from the PCM.     

A numerical investigation into the charge behaviour of a spherical phase change material particle for high temperature thermal energy storage in packed beds

基于高温相变材料,对填充床储热系统中储热单元球体的储热性能进行了模拟研究.研究了不同传热流体温度和球体直径对球体储热性能的影响规律,对导热为主的相变储热过程与导热和自然对流共同作用的相变储热过程进行了比较分析,同时还探讨了高温辐射换热的影响.结果表明,相变时间随球体直径的增大而增大,随传热流体温度的增大而减小.当考虑相变区域自然对流时,总的相变时间显著减少,和单纯导热相比,完全相变时间缩短了近16%.在导热和自然对流的基础上加上辐射传热后可以看出,辐射换热强化了球体内的传热过程,加快了相变材料的熔化速度,强化了自然对流的作用.     

Heat transfer intensification in horizontal shell and tube latent heat storage unit

Employment of latent heat storage unit (LHSU) utilizing phase change material (PCM) in a substantial scale is constrained by the poor thermal conductivity of PCMs. Future utilization of LHSU will therefore to a great extent rely on the heat transfer intensification techniques. Present research is on enhancement techniques in which heat transfer mechanism is altered without altering the mass of PCM and heat transfer surface area. The intensification mechanisms considered in the present research include imparting eccentricity to heat transfer fluid (HTF) pipe, imparting rotation to the LHSU and providing multi HTF tube. Numerical investigations are reported here towards comparative evaluation of the thermal characteristics associated with such intensification mechanisms for horizontal LHSU. In the present study stearic acid (melting point 55.7–56.6?°C) is used as PCM and water is used as HTF. Results infer that all the three mechanisms offer quicker melting rate. For the geometric configuration of LHSU considered in the present research, a reduction in melting time of 47.75% is evaluated for rotating LHSU. The rate of energy storage is higher for both eccentric and rotating LHSU. Solidification process is however not accelerated by such techniques. On the contrary, eccentric and multi HTF tube LHSU takes more time for solidification.     

Numerical analysis on the thermal behavior of high temperature latent heat thermal energy storage system

High temperature latent heat thermal energy storage technology is a promising option for future cost reduction in parabolic trough or tower power plant. However, low thermal conductivity of phase-change material (PCM) is the major shortage of latent heat thermal energy storage. This paper proposed a new thermal energy storage system (TESS) that metal foam and fins were used to enhance the effective conductivity of PCM. Three-dimensional physical model was established for representative element extracted from TESS. Considering the natural convection in the liquid part of PCM, volume-averaged mass and momentum equations were employed with the Brinkman–Forchheimer extension to Darcy law to simulate the porous resistance. A local thermal equilibrium model was developed to obtain temperature field. The governing equations were solved with finite-volume approach and enthalpy method was employed to account for phase change. The model was firstly validated against low temperature experiments from the literature and then used to predict the charging and discharging behavior of the present TESS. The position of solid/liquid interface was explored and the effects of design parameters, including that of metal foam pore density and porosity, configuration of fin and Rayleigh number, on melting and solidifying rate and energy stored in each time step were revealed and discussed. The results indicate that metal foam and fins can effectively improve the heat transfer performance for thermal storage system and decrease charging and discharging time.     

Experimental and numerical investigation of thermal energy storage with a finned tube

A latent heat thermal energy storage system using a phase change material (PCM) is an efficient way of storing or releasing a large amount of heat during melting or solidification. It has been determined that the shell‐and‐tube type heat exchanger is the most promising device as a latent heat system that requires high efficiency for a minimum volume. In this type of heat exchanger, the PCM fills the annular shell space around the finned tube while the heat transfer fluid flows within the tube. One of the methods used for increasing the rate of energy storage is to increase the heat transfer surface area by employing finned surfaces. In this study, energy storage by phase change around a radially finned tube is investigated numerically and experimentally. The solution of the system consists of the solving governing equations for the heat transfer fluid (HTF), pipe wall and phase change material. Numerical simulations are performed to investigate the effect of several fin parameters (fin spacing and fin diameter) and flow parameter (Re number and inlet temperature of HTF) and compare with experimental results. The effect of each variable on energy storage and amount of solidification are presented graphically. Copyright © 2005 John Wiley & Sons, Ltd.     

外翅片强化PV/PCM系统热管理性能研究

以光伏/相变材料冷却系统（PV/PCM）为研究对象,数值研究翅片间距、厚度及外翅片长度对PCM换热性能影响。结果表明,翅片间距降低和外翅片长度增加可强化光伏组件冷却,提升电池发电性能,但翅片厚度对PCM换热性能影响较小。此外,选取无外翅片（方案1）和有外翅片（方案2,间距7.3 mm、厚度2 mm和长度25 mm）做对比研究,相对于方案1,方案2融化时间延长34.6%,电效率下降速度减缓68%且180 min时电池温度降低约30 ℃。