Parameters Of Phase Change Material Research Articles

Robust Electrothermal Switching of Optical Phase‐Change Materials through Computer‐Aided Adaptive Pulse Optimization

Electrically tunable optical devices present diverse functionalities for manipulating electromagnetic waves by leveraging elements capable of reversibly switching between different optical states. This adaptability in adjusting their responses to electromagnetic waves after fabrication is crucial for developing more efficient and compact optical systems for a broad range of applications, including sensing, imaging, telecommunications, and data storage. Chalcogenide‐based phase‐change materials (PCMs) have shown great promise due to their stable, nonvolatile phase transition between amorphous and crystalline states. Nonetheless, optimizing the switching parameters of PCM devices and maintaining their stable operation over thousands of cycles with minimal variation can be challenging. Herein, the critical role of PCM pattern as well as electrical pulse form in achieving reliable and stable switching is reported on, extending the operational lifetime of the device beyond 13000 switching events. To achieve this, a computer‐aided algorithm that monitors optical changes in the device and adjusts the applied voltage in accordance with the phase transformation process is developed, thereby significantly enhancing the lifetime of these reconfigurable devices. The findings reveal that patterned PCM structures show significantly higher endurance compared to blanket PCM thin films.

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.

Investigation on the effect of physical parameters of PCMs on the exothermic properties of energy storage units of bilayer tubes

Based on the structure of bilayer tube energy storage unit (ESU), the study investigates the effects of physical parameters and arrangement combinations of phase change materials (PCMs) on the heat storage and exothermic performance of the ESU. The control variable method is used to focus on a single physical parameter of the phase change material to analyze its influence on the heat storage and exothermic performance. Use of experimental and numerical modelling, we firstly analyze the influence of the thermal conductivity of PCMs on the thermal storage and discharge performance, and then study the influence of the latent heat value of PCMs on the thermal storage and discharge performance, and compare and analyze its thermal performance parameters with those of traditional ESUs. Studies have shown that the thermal conductivity of the bilayer ESU can be minimized by up to 26.9% and the heat release time can be increased by up to 23.8% compared with the conventional ESU. The latent heat value of the bilayer ESU can be minimized by up to 28.1% and the heat release time can be increased by up to 35.3% as opposed to the conventional ESU. Obviously, the selection of PCMs must be considered for each parameter, and the selection of PCMs with appropriate parameters plays a crucial role for different energy storage conditions.

Investigation of the Effect of Different Parameters of Phase Change Materials on Heat Exchanger Performance

Technological improvements and increasing energy demand necessitate energy efficient designs for heat transfer systems. The storage and reuse of heat energy plays an important role in the development of energy-efficient systems. Phase change materials (PCMs) are crucial components which increase energy efficiency in heat exchangers as can be applied to many systems. In this study, the heat transfer performance of different types of phase change materials in a regenerative heat exchanger was investigated according to different parameters. Reynolds number depending on the hot fluid velocity (Re=400, 800, 1200, 1600), hot fluid inlet temperature (Tsıcak,giriş=40, 60, 70, 80℃), and different types of phase change materials (RT60, RT100, and SP70) are the parameters used in this study. ANSYS Fluent software was used for computational fluid dynamics analysis. As a result, it has been determined that when the Reynolds number of the hot fluid in the heat exchanger was increased in the range of Re=400-1600, the heat transfer effectiveness increase of 17%; when the hot fluid inlet temperature was increased in the range of Thot,inlet=40-80℃, the heat transfer effectiveness increase of 21%. As regards the effect of different types of phase change materials, the heat transfer effectiveness was 81% for RT60, 79% for SP70 and 76% for RT100. It has been evaluated that, with the results obtained from this study, heat exchangers with higher heat transfer effectiveness and higher energy storage capacity can be designed.

Potential evaluation of energy flexibility and energy-saving of PCM-integrated office building walls

Building energy flexibility is important for reducing peak-to-valley differences in air-conditioner loads. Integrating phase change material into office building envelopes and precooling can effectively reduce peak load, but precooling may increase total energy consumption. Therefore, it is imperative to investigate its energy flexibility and energy consumption features. In present work, a heat transfer model of phase change material (PCM)-integrated office building walls was established firstly and verified by experiments. Then, the energy flexibility and energy-saving potential of the PCM-integrated wall under different precooling strategies were studied, and the effects of various PCM parameters on energy flexibility and energy-saving potential were evaluated. Finally, the influence of different peak-to-valley electricity tariff differences on electricity costs was analyzed. The results show that the optimized precooling strategy and peak-to-valley electricity tariff difference make the energy flexibility index of the PCM-integrated wall up to 69.7% at a total load reduction of 1.3% and save the electricity cost by 51%. The PCM location and melting point have the most significant effect on the energy flexibility and energy-saving potential of the PCM-integrated wall under precooling operating conditions. This study guides the selection of energy flexibility and energy-saving precooling strategy and PCM parameters for PCM-integrated office building walls and the formulation of time-of-use tariff policies.

The effect of PCM on mitigating thermal runaway propagation in lithium-ion battery modules

The main safety issue of lithium-ion batteries (LIBs) is thermal runaway (TR), which has greatly hindered the application of LIBs. Phase change materials (PCM) have been used in battery modules as a thermal management method due to the heat absorption function during the phase change process. However, the role of PCM in TR propagation mitigation has not been carefully examined. In this paper, a 1D heat transfer model was developed to predict the mitigation effect of PCM on TR propagation of a battery array, which shows the superiority of high simulation speed and accuracy. The effects of thermo-physical parameters of PCM on cascading cell-to-cell thermal runaway propagation behavior were investigated. In addition, we gave an optimal range of key parameters of PCM based on safe regulations of China and the United Nations regarding to the TR in electric vehicle LIBs. It was found that the latent heat, density and heat capacity of PCM have similar effects on TR propagation behavior. With the increase of the three PCM parameters, the time duration between cell-to-cell failure propagation increases linearly, which shows a slower TR propagation rate. The optimal ranges for density, heat capacity and latent heat of PCM are over 2150 kg·m−3, over 4100 J·kg−1·K−1 and over 9 × 105 J·kg−1, respectively. What’s more, the TR propagation will exacerbate with the increase of thermal conductivity of PCM, which reminds us to notice a balance between thermal management and TR mitigation in PCM-based thermal management system. Generally, through analyzing the effect of thermal diffusivity of PCM in TR inhibition, we found higher thermal diffusivity will speed up the propagation of TR. And the TR propagation speed is more sensitive to the change of density of PCM. The thermal diffusivity of PCM should be less than 9 × 10−7 m2·s−1 to satisfy the safety standards. The PCM manifests great performance in retarding TR propagation with the phase transition temperature in the range from 318.15 to 518.15 K. Moreover, we found that there exists a critical PCM thickness over which TR propagation will not occur. As the battery thickness increases from 8.8 to 56.5 mm, the critical PCM thickness shows a non-monotonic variation trend (increasing at first and decreasing later). This study demonstrates the feasibility of using PCM as a thermal runaway mitigation strategy and provides a guideline for the optimization of physical parameters of PCM.

Energy storage potential analysis of phase change material (PCM) energy storage units based on tunnel lining ground heat exchangers

Low geo-temperature geothermal energy in the surrounding rock can be extracted by tunnel lining ground heat exchangers (GHEs) and stored in phase change material (PCM) plates to realize the cold energy utilization. However, the research of the coupling heat transfer of tunnel lining GHEs and PCM plates has been rarely reported. In this study, a 3D coupling heat transfer model of tunnel lining GHEs and PCM plates was developed to investigate the cold energy storage performances of PCM plates under different PCM and tunnel parameters. The circulative iteration calculation is used to solve the coupling model. The results show that the cold energy storage of PCM plates utilizing tunnel lining GHEs for energy storage is feasible. The PCM melt fraction and solid phase PCM proportion within the PCM plates decrease and increase respectively with an increase in the PCM thermal conductivity and melting temperature and a reduction in the surrounding rock temperature. The cold energy storage efficiencies of PCM plates improve by 77.8% and 34.1% as the PCM thermal conductivity and melting temperature increase by 1 W/(m K) and 4 ℃. Moreover, the cold energy storage efficiency of PCM plate enhances by 68.5% as the surrounding rock temperature reduces from 10 to 1 ℃. A larger difference between the surrounding rock temperature and PCM melting temperature is efficient for the cold energy storage of PCM plates, and the cold energy storage time and temperature difference show a power function relationship. Finally, increasing the tunnel lining GHEs length is conducive to enhancing the cold energy storage efficiency of PCM plates, whereas this enhancement is limited. In actual applications, selecting a reasonable tunnel lining GHEs length is necessary.

Performance analysis of the phase-change heat-storage tank based on numerical simulation

Phase-change heat storage system contributes to the smooth operations of building energy supply and demand. In this paper, the numerical model of the heat-storage tank with phase change material blocks is established. The charge and discharge process of the heat-storage tank is analyzed with the numerical model. The performances of the heat-storage tank with and without phase change material blocks are compared by experiment and numerical simulation. The effects of structural parameters of phase change material blocks on the heat storage and release rate are comprehensively discussed, including the number and size of phase change material blocks. The results indicate that under the experimental conditions, the charge capacity and duration of the tank with phase change material blocks are extended by 7.43% and 8.3% respectively under the condition of replacing 6.75% volume of hot water with phase change material. The thermal stratification phenomenon in the heat-storage tank without phase change material blocks is more obvious. For certain volume of the phase change material, the performance analysis of three cases with different amount and size of phase change material blocks is investigated. The results show that the phase change material blocks with number and size of 24 and 0.45 m × 0.1 m × 0.1 m reduces the charge and discharge duration by 2.47% and 3.78% compared with those of 12 and 0.9 m × 0.1 m × 0.1 m.

Heat transfer in phase change materials for integrated batteries and power electronics systems

An integrated batteries and power electronics system has great potential in improving the compactness, flexibility and multifunctionality in electrical vehicles. However, it needs to overcome thermal management challenges due to different operation temperatures of batteries and power electronics. To tackle this issue, this paper presents the first systematic study on the heat transfer characteristics in phase change materials (PCMs) based thermal management system sandwiched between two significantly different constant heat sources, through an experimentally validated transient three-dimensional heat transfer model. All key parameters of PCMs affecting the system are identified through a new dimensionless formulation, including the ratio of horizontal and vertical thermal conductivity kxy and kz, the aspect ratio (ratio of thickness and length), and Jakob number Ja (ratio of sensible heat and latent heat). Modelling results show that the system operation duration τd increases monotonically with the increase of kxy. However, increasing kz is not always benificial for τd and there is an optimal value for kz when keeping kxy unchanged. In addition, the increase of PCMs thickness can prolong τd monotonically. The reduction of Ja, e.g. increasing latent heat, is always beneficial for τd.

Performance Evaluation and Optimum Design of Ventilation Roofs with Different Positions of Shape-Stabilized PCM

Environmental pollution and energy shortages have become increasingly prominent. Building energy conservation is an important part of a low-carbon strategy. Integrating phase change material (PCM) into a building’s roof is effective in altering the space cooling load, however less effective in reducing it. To reduce the cooling load, a novel ventilation roof with shape-stabilized PCM (VRSP) is introduced. The mechanical ventilation is used at night by embedding ducts in the roof to remove the solidification heat of the PCM. To identify the best position of PCM and an optimum design, the thermal performances of three kinds of VRSPs were compared and investigated through CFD simulation: ventilation roofs with outer-layer shape-stabilized PCM (VRSPO), middle-layer shape-stabilized PCM (VRSPM) and inner-layer shape-stabilized PCM (VRSPI). The effects of PCM and ventilation parameters on the thermal performance of three roofs were analyzed on a typical design day in summer in Wuhan. The results show that for VRSPO, VRSPM and VRSPI, the proper thicknesses of PCM are 35 mm, 25 mm and 15 mm; melting temperatures are 35~37 °C, 33~35 °C and 29~31 °C, respectively; the proper ventilation speeds are 2.5~2.6 m/s; and the optimum cavity radii are all 40 mm. The best performance can be obtained by placing PCM on the outer layer. The PCM of VRSPO has the highest number of days in which the phase change process occurs (specifically, 75 days in the summer). The application of VRSP can effectively reduce the internal surface temperature of the roof, by an average of 1.77 °C. The maximum and average inner surface temperatures of VRSPO in different weather conditions can be calculated using the daily average outdoor sol-air temperature or average dry bulb temperature by fitting equations. The structure can be used as a passive and active envelope in areas with hot and long summers.

Performance assessment of the hybrid PV-MCHP-TE system integrated with PCM in all-day operation: A preliminary numerical investigation

Regarding the existing research on photovoltaic (PV) and thermoelectric generator (TE) integrated systems mostly focus on their daytime behaviors, a solar system that can achieve continuous electrical supply throughout the day, namely the PV-TE system with the micro-channel heat pipe (MCHP) (the PV-MCHP-TE system), is proposed in this paper. The implementation method is to use the phase change material (PCM) which releases the heat it harvests during the day to maintain the uninterrupted electrical output of TE at night, on the basis of the system generating electricity through PV and TE by day. Considering that the system performance may change with the difference in cooling effects, three PV-MCHP-TE systems adopting different cooling methods are designed and their performance changes during a continuous working week are analyzed. The results show that the daytime average electrical efficiencies of PV in the systems using free cooling, forced air cooling, and water cooling are 9.33%, 11.85%, and 13.10%. The maximum electrical efficiencies of TE in the three systems are 0.47%, 1.48%, and 1.87%, and the nighttime average efficiencies are 0.13%, 0.15%, and 0.16%, respectively. Additionally, a comparative analysis is also performed to compare the performance of the considered systems under different PCM parameters.

Inversion of extinction coefficient and refractive index of variable transparency solid–solid phase change material based on a hybrid model under real climatic conditions

The application of phase change materials (PCMs) in transparent building envelopes has received extensive attention. However, most of the studies have simplified the optical parameters of the PCMs in their models. Because the temperature of PCMs cannot be controlled when using the spectrophotometer, it is difficult to measure the changing trend of optical parameters versus temperature. In this study, an inverse approach was developed for the first time, based on a hybrid model, to fit the accurate function expressions between the extinction coefficient and the refractive index of PCM and temperature. The hybrid model was a combination of experimental data-driven, mathematical models, and multi-objective optimization. Firstly, a test platform of the variable transparency solid–solid PCM (VTSS-PCM) window was constructed and tested last for a year to collect the datasets. Secondly, the photo-thermal coupling model of the window was established, in which the hysteresis and the total internal reflection phenomena were considered. Then, taking the unknown coefficients in the function expressions as decision variables, a bi-objective optimization model was built, based on two error statistics of experimental values and simulated values, and solved by genetic algorithm. Finally, the inversion dataset and validation dataset were used to prove the reliability of the inversion results. The results showed that the refractive index and extinction coefficient of the VTSS-PCM were 1.11 and 25.73 m−1 in the transparent phase, and 5.33 and 152.82 m−1 in the opaque phase. Within the phase change temperature range, the function expressions between the refractive index and extinction coefficient and the temperature were obtained respectively. The verification results showed that the function curves obtained by inversion were reliable. No matter whether the inversion dataset or validation dataset was used as input, the values of RMSE and CV(RMSE) met the ASHRAE Guidelines. The inversion method is meaningful to get the optical parameters of PCMs, to accurately evaluate the performance of PCM-built envelope.

Parameter optimization and sensitivity analysis of a Lithium-ion battery thermal management system integrated with composite phase change material

In order to ensure the performance and to extend the life cycle of power battery module within electric vehicle, a battery thermal management system (BTMS) integrated with composite phase change materials (PCM) was proposed. The heat dissipation performance of BTMS using paraffin/expanded graphite (EG) composite PCM with different mass fraction was investigated. The results showed the optimal heat dissipation performance was observed using composite PCM with 20 wt% EG. The combined effect of liquid volume fraction and thermophysical parameters of composite PCM on heat dissipation performance of battery module was revealed. The phase transition temperature of composite PCM was found to have the largest impact on the maximum temperature and temperature uniformity of battery module, followed by latent heat and thermal conductivity of composite PCM. A 10 % decrease in the phase transition temperature led to a 7.9 % in the maximum temperature of battery module. Both the thermal conductivity and latent heat growth contributed to the reduction in the maximum temperature of battery module. Composite PCM with the thermal conductivity of 3 W m−1 K−1, phase change temperature of 38 °C, and latent heat of 220 J g−1, was found to achieve the optimal heat dissipation performance of BTMS and a high utilization rate of 97.3 % for composite PCM. The findings of this work could be beneficial for future development of highly-efficient and economical composite PCM for BTMS.

Evaluating and optimizing the energy saving benefits of latent heat in phase change materials with new indices

Phase change material (PCM) is of potential to reduce energy consumption in buildings. When applying PCM into envelopes, both thermal storage and thermal insulation bring energy saving benefits. Few studies have clarified the respective benefit brought by thermal storage or thermal insulation. In this work, novel indices are proposed to distinguish benefits from the two energy-saving mechanisms of PCM, and assess the energy-saving improvement from latent heat. PCM is applied on the roof during summer in Guangzhou. Based on the novel indices, two parameters of PCM, thickness and latent heat, are discussed comprehensively. In combination with climate conditions, the reason why latent heat saving or wasting energy are analyzed. Results show that as latent heat increases, the total saved, total wasted and net saved energy from latent heat all increase. Thickness has different impacts on the total saved or total wasted energy from latent heat, which reaches the optimum at 60 mm and 10 mm, respectively. The net saved energy from latent heat reaches the maximum at 50 mm. When exceeding 50 mm, the energy-saving benefit from latent heat reduces. The recommended PCM thickness according to latent heat energy saving ratio (LHESR) is from 20 to 40 mm. The optimum LHESR is less than 40%, which illustrates that the energy-saving benefit of PCM in this work is mainly from thermal insulation. Researchers can utilize the newly-proposed indices to evaluate and strengthen the energy saving benefit from latent heat under different meteorological environments.

A comprehensive investigation of PCM based annular thermoelectric generator for energy recovery: Energy conversion characteristics and performance evaluation

Annular thermoelectric generators (ATEG) are increasingly attracting attention and are pivotal in thermal energy recovery due to their unique characteristics, which can be perfectly adapted to cylindrical heat sources. Considering the efficient thermal management and power generation of thermoelectric generator (TEG), phase change material (PCM) can be integrated with the cold side of TEG as a favourable cooling medium to meet its functional requirements. However, previous studies have focused on the intrinsic connection between flat-plat TEG and PCM, while the impact of PCM on ATEG has not been systematically explored. In the present study, an annular thermoelectric generator with PCM (ATEG-PCM) under a pulsed heat source is proposed, and its dynamic characteristics and energy distribution are evaluated. The effects of PCM parameters such as height, latent heat and melting temperature are comprehensively explored from multi-index, including PCM temperature, liquid fraction, temperature difference, output power, energy and exergy efficiencies. Furthermore, a comparative investigation of ATEG-PCM, PCM-ATEG where PCM is applied on the hot side, and ATEG without PCM is further carried out to clarify the coupled mechanism of PCM on thermal management and power generation enhancement of ATEG. The results indicate that the energy efficiency of ATEG-PCM has saturated at PCM height of 10 mm to 12 mm, while the exergy efficiency merely slightly increases. Especially, the PCM melting temperature merely affects the time at which PCM initiates to change from solid state to liquid state, without affecting the total energy efficiency of ATEG-PCM. Furthermore, the comparison analysis determines that ATEG-PCM deserves more attention, judging by thermal management and power generation. This work not only contributes to the understanding of the coupling mechanisms between PCM and ATEG but also provides a research direction for energy recovery in PCM based ATEG hybrid system.

Melting performance of a cold energy storage device filled with metal foam–composite phase-change materials

Performance prediction of cold thermal energy storage (CTES) devices is an important step in guiding their design and application. However, related studies are limited, and some do not consider the influence of structural parameters. In this study, a CTES with metal foam–composite phase-change materials (PCMs) was built, and the influence mechanism of the physical parameters of PCMs and structural parameters of metal foam on the melting of composite PCMs was studied through experiments, theory, and numerical simulation. Results show that the porosity, the material of the metal foam, and the thickness of the metal foam are the main factors affecting the melting rate and heat transfer strength of PCMS, whereas the effect of pore density can be ignored. Three general correlations about the liquid fraction; Nusselt number with Fo, Ste, and Ra; and dimensionless structural parameters of metal foams were analyzed and obtained. The results will provide data and theoretical basis for the design of CTES using metal foam–composite PCMs.

Influence of finned charges on melting process performance in a thermal energy storage

• Performance of the melting process of PCMs (thermal charge period) inside the enclosure with and without fins attached to thermal chargers is studied numerically. • Enthalpy-porosity method is used for numerical simulation. • The effects of fins’ number and fins’ location on the charging process have been scrutinized. • Optimal location of the charger with necessary fins’ number for effective charging period has been found. Thermal energy storages based on the phase change materials are considered as bright technologies because they have more advantages than other energy storage methods. In addition to the thermal parameters of phase change materials, the performance of the thermal energy storage system using latent heat depends on the heat exchanger properties. Here, several different parameters related to the use of fins in the thermal energy storage devices have been studied. Using enthalpy-porosity method, the process of phase-change material melting into a square cavity is evaluated. Cylindrical thermal chargers with a circular cross-section that transfer heat energy to the phase change material are installed inside the chamber. Longitudinal fins are placed on the heat charger surface to assist the heat charging process. By increasing the fins number to four compared with two fins, the melting process shows a significant increase for the position where the heat charger is located in the center and at the bottom of the cavity. Also for these conditions, higher temperature gradients and a thinner boundary layer are observed at the melting point. Also, the heat transfer rate increases and the melting process is accelerated.

Preparation, performance study and application simulation of gypsum-paraffin/EG composite phase change building wallboard

The performances and application parameters of phase change materials (PCM) are crucial for integration with construction in practice. In this paper, composite PCMs (CPCM) were prepared by paraffin/expanded graphite (EG) and applied to phase change gypsum board (PCGB). The composite PCM was characterized by SEM, DSC, leakage ratio and thermal conductivity tests to determine the optimal ratio of EG and paraffin; The thermal storage and release properties of PCGB were experimentally investigated by two building cabin models. The results showed that the CPCM obtained 12 wt% EG exhibited excellent thermal conductivity (2.32 W/mK) and latent heat (103.9 J/g). The PCGB prepared with above CPCM has excellent thermal conductivity (0.3968 W/mK) and latent heat (42.2 J/g). When the EG content exceeded 12%, the thermal properties and leakage ratio of the CPCM stabilized after 200 thermal cycles despite the increase of EG. The phase change cabin exhibited much lower heating and cooling rates than that of the pure gypsum cabin, with a 1.14 °C decrease in peak internal surface temperature and a 3.4 °C decrease in temperature fluctuation. In addition, the room temperature of the established reinforced concrete wall model with PCGB was simulated based on the hourly variation pattern of the winter-summer temperature in Urumqi, China. The results show that in summer, the PCGB with phase change temperature of 29–31 °C and thickness of 20 mm possesses a larger heat storage capacity, with a smaller average internal surface temperature (28.63 °C) and a higher average phase change utilization (61.5%), which can reduce the cooling and heating load of the wall and save energy effectively.

Thermal performance analysis of lightweight building walls in different directions integrated with phase change materials (PCM)

Phase change materials (PCM) can improve the thermal performance of lightweight building walls (LBW), but the effect is not only dependent on the PCM parameters but also high on the heat exchange between the surface and the ambient environment. For actual buildings, there are differences in the outdoor thermal environment of the walls in different directions, especially the solar radiation intensity. Therefore, a small-scale lightweight building was made in this study and its thermal environment in different directions were tested. Then, the influence rules and suitability of PCM parameters on the thermal performance of LBW in different directions were analyzed by developing a typical two-dimensional heat transfer model. The results show that the application effects and suitable parameters of PCM in walls in different directions are different. PCM location (phase-transition temperature) is adapted to the middle (20–30 °C) for the east and south-facing wall, while the inside (18–28 °C) and outside (24–34 °C) is optimal for the west and north-facing wall. The thermal performance improvement with the most noticeable in the east and west-facing walls at suitable PCM parameters and their peak and average heat flux can be reduced by 62.8–66.4% and 28.2–29.5%, and delay time increased by 5–5.34h compared to reference wall.

Numerical study and parametric analysis on performance enhancement of a latent heat storage unit with fractal fins

Adding fractal fins is an approach to improve the heat storage performance. In this paper, a new type of fractal fins is established, and a new evaluation index is putting forward in order to analyze the performance of the heat storage unit from the view of temperature uniformity. The advantages of fractal fins and the optimal number of fins are discussed by temperature uniformity factor. Finally, the influence of angle ratio, length ratio and phase change material (PCM) parameters on heat storage units are discussed by dimensionless criterion number. The results show that the application of fractal fins can improve the performance of heat storage unit and the temperature uniformity factor can be reduced by 131.5 % at most. The heat storage unit with six fractal fins has the smallest temperature uniformity factor. The optimal angle ratio and length ratio are 2/3 and 0.5 respectively. The change of PCM parameters has different effects on different melting periods. This research has a further guiding role for the development of fractal finned latent heat storage units.