Phase Change Material Slab Research Articles

Design optimization of the cooling systems with PCM-to-air heat exchanger for the energy saving of the residential buildings

Driven by a rapid surge in cooling demand in buildings, the energy consumption dedicated to cooling has experienced remarkable growth. To address this challenge, the adoption of latent heat thermal energy storage utilizing phase change materials (PCM) has gained significant momentum in recent years. This paper presents the design and evaluation of an integrated latent heat thermal energy storage (ILHTES) system tailored for residential buildings. This system integrates a PCM-to-air heat exchanger (PAHX) with an air conditioning unit. Modelica language is utilized to develop a numerical model for the ILHTES system. The heat transfer model of the PAHX is developed and validated using existing literature data. To simulate the dynamic behavior and energy consumption of the residential building, the open-source library AixLib is adopted. The developed ILHTES system model is used for the optimization of key design variables, including the PCM slab thickness and air flow rate, based on the results of long-term simulations covering the entire cooling season. Evaluation of the energy saving potential of the optimized ILHTES systems is carried out in comparison to conventional air conditioning systems, considering various climatic conditions in five European cities. The results highlight the profound impact of PCM types on the Energy Saving Ratio (ESR) throughout the entire cooling season. Among the four commercially available PCMs examined—RT27, RT25, RT20, and RT18—RT25 consistently outperforms the others. Across all five cities investigated, using RT25 leads to a minimum ESR of 16% in Catania and a maximum ESR of 44.7% in Stockholm for the entire cooling season.

Thermal Energy Storage Heat Exchanger Design: Overcoming Low Thermal Conductivity Limitations of Phase-Change Materials

Abstract Recently, there has been a renewed interest in solid-to-liquid phase-change materials (PCMs) for thermal energy storage (TES) solutions in response to ambitious decarbonization goals. While PCMs have very high thermal storage capacities, their typically low thermal conductivities impose limitations on energy charging and discharging rates. Extensive research efforts have focused on improving PCM thermal conductivity through the incorporation of additives. However, this approach presents challenges such as achieving uniform mixtures, maintaining high latent heat, and cost. Alternatively, it has been demonstrated that, in this study, reducing the length scale of the PCM-encasement thickness can eliminate the low thermal conductivity effect of PCMs. To illustrate this concept, a one-dimensional PCM slab was numerically simulated. The thickness of the slab was varied to represent dimensions found in flow passages of compact heat exchangers, and the heat transfer coefficient of the heating fluid was varied to represent lower and upper bounds while also including nominal values encountered in air-to-air heat exchangers. The thermal conductivity was parametrically varied from the natural value of the PCM to simulated enhanced values (potentially achieved through additives) of up to 400 times larger. Results show that reducing the PCM-encasement thickness yields substantially better performance than by improving the thermal conductivity, thereby demonstrating the potential for compact heat exchanger design to overcome the PCM thermal conductivity limitations.

Performance Improvement of The Domestic Refrigerator Using Phase Change Materials

A growing environmental Impact of global warming and rapidly increasing cost of the electrical energy cause researchers in the field of refrigeration and air conditioning to develop the sustainable cooling technologies. This study investigated the influence of latent heat storage materials on the power consumption and the temperature distribution of a commercial domestic refrigerator. The 2 kg PCMs (Phase change materials) slab of thickness 5 mm, which consisted of a 2:1 by weight mixture of refined paraffin wax and kerosene oil, were located on the back side of the roll-bond evaporator and the copper coated steel tubing coils of the hot-wall condenser, in order to improve the higher performance of refrigerator and to decrease the operating cycle time of the compressor. The experimental and CFD (Computation fluid dynamics) simulation results indicated that the refrigerator equipped with the PCMs storage unit showed a significant enhancement of the system performance and a reduction of the temperature fluctuations in the refrigerator compartment compared to a conventional system. In this investigation, the electric power consumption and the compressor on-time ratio were reduced by 15.69% and 12.13%, respectively. Moreover, the COP (Coefficient of performance) of the refrigeration system was increased by 9.53%.

Thermal performance of phase change material–based heat sink with hybrid fin‐metal foam structure under hypergravity conditions

The thermal management systems or heat sinks are an integral part of electronic devices for aerospace applications. The heat transfer characteristics of the widely used phase change material (PCM)-based heat sinks under hypergravity conditions are essential in the designation of the heat sink. The current study conducts a systematic evaluation on the thermal characteristics of PCM coupled with hybrid fin-metal foam (FMF) structure under variable hypergravity conditions. A validated two-dimensional transient melting heat transfer model of the designed system is proposed, and the influences of enclosure orientation, fin height, PCM slab thickness, and the distribution of fin and metal foam under different hypergravity conditions are investigated. Results show that with the decrease in inclination angle, the melting time and the heating wall temperature become lower, and the optimal inclination angle is 0°. As the PCM slab thickness gets larger, natural convection is enhanced, while the heating wall temperature and melting time become larger. With the rise of hypergravity value, the effect of inclination angle strengthens, while the influence of PCM slab thickness weakens. Furthermore, with the rise of the copper foam fraction in the hybrid FMF structure, the melting time and the heating wall temperature firstly decrease and increase afterward. The optimal copper foam fraction is 58.62%. This study guides the optimal design of the PCM-based heat sink in aerospace applications.

A complete numerical model for low temperature composite form-stable phase change material slab based on dynamically simplified temperature transforming method

ABSTRACT Phase change material (PCM) that is used in the roof top slab can minimize the heat infiltration into the living space from the building envelope and increase the indoor thermal comfort level. The major disadvantage of PCM is leakage at liquid state. Expanded graphite (EG) is used here as supporting material for PCM to prevent the leakage and maintains the form stability. The composite PCMs are prepared by adding the EG with various mass ratios, i.e. 3%, 6%, and 9%. The thermal properties of all composite PCMs are determined by DSC and thermal conductivity analyzer. The optimum PCM composition was found at 9% of EG. In this study, the shape-stabilized PCM-integrated roof top was analyzed by simplified temperature transforming (TT) numerical model. The main advantage of this model is that it is used to combine the equivalent enthalpy and heat capacity. The numerical results are validated with experimental values and show the lower mean absolute percentage error (MAPE), i.e. 6.91%. This indicates that present model is well suitable to predict the thermal performance of composite PCM incorporated roof top. The effect of various parameters like thickness of the PCM slab, wind velocity, percentage of EG mass ratio, and location of PCM are investigated by simplified TT model. The results showed that the maximum inner surface temperature reach up to 53°C for BG model (concrete and gypsum) by distributing constant heat flux, i.e. 1,000 W/m2 for 180 min. At this same condition, the inner surface temperature of other PCM models PBG (PCM, concrete, gypsum) and BPG (concreate, PCM, Gypsum) reaches to 50°C and 41°C, respectively. PBG model exhibits excellent thermal performance compared to BPG model. The thermal performance of PCM slab is also investigated by varying the thickness of the PCM layer and found that time lag is raised with the thickness of the PCM slab.

Mitigation of thermal curling of concrete slab using phase change material: A feasibility study

In this study, the effects of phase change material (PCM) on the thermo-mechanical behaviors of concrete pavement slabs are investigated so as to test the feasibility of utilizing PCM for mitigation of thermal curling of pavement. The PCM is elaborately stabilized in porous lightweight aggregate then loaded into concrete, without compromising strength associated with PCM incorporation. Thermal analysis indicates that the PCM introduces a latent heat capacity of about 21 J/g to the pavement slab. Two emerging fiber-optic monitoring techniques, i.e., truly distributed strain/temperature sensors and a high-resolution inclinometer, are employed to continuously monitor distributions of the temperature, strain, and tilting angle of the slabs subjected to heating and cooling, respectively. Validated by the obtained experimental results, a finite element simulation framework is established to further investigate the temperature gradient distributions, slab deformations/curvatures, and thermal strain and stress inside the slabs. The results suggest that the PCM-incorporated slab has a higher top surface temperature, a larger top-vs-bottom surface temperature difference, and thereof a larger curvature. However, the temeprature gradient reveals that heat accumulates at the upper layer before PCM melts completely, thus accounting for most of the temerpature difference. Moreover, regardless of the larger top-vs-bottom strain difference and stress nonlinearity, the linear strain of the PCM slab is smaller due to its lower coefficient of thermal expansion, and the later stage stress at the middle of the PCM slab center is smaller. Based on the findings, strategies of using PCM to control thermal curling of pavement slabs are proposed.

Performance of a cold storage air-cooled heat pump system with phase change materials for space cooling

This paper studies the performance of a cold storage heat pump system integrated with phase change materials (PCMs) for space cooling. An air-cooled heat pump unit comprising of two constant rotation speed compressors is selected. The PCM is an organic material that is commercially available, and has a phase change temperature of 10–12 °C. A coupled computational heat transfer model of the heat pump unit, fan coil units, and a cold storage tank filled with PCM slabs is developed. The simulations are performed under the climatic condition of Fuzhou, China. The operational behaviors of the cold charging and discharging processes of the system are analyzed. Furthermore, the overall electrical energy and electricity charge consumptions at various indoor temperature set points are compared. The results indicate that, the electrical energy consumption of the cold storage heat pump system increases as the indoor temperature increases. However, there is very little difference in the electrical energy consumption of the conventional heat pump system and the cold storage system. While under the demand tariff, the electricity charge saving ratio of the cold storage system over the conventional system is 9.07%-11.28%. In general, as the indoor temperature set point reduces, the electricity charge saving ratio increases. Moreover, the effect of cold charging temperature is also discussed. It is found that higher cold charging temperature consumes less electrical energy, however, takes longer to store the same amount of cold, and the differences relies on the required amount of cold.

Exergy and economic analyses of nanoparticle-enriched phase change material in an air heat exchanger for cooling of residential buildings

This paper studies the energy, exergy, and economic analyses of phase change material (PCM)-air heat exchangers filled with SiO2 based Nano-PCM of RT25 on enabling storage of night-time free-cooling from outdoor air or indoor air during off-peak hours, numerically. The stored cooling is subsequently utilized during a day-time or peak hours as auxiliary cooling, resulting in a reduction of the day-time refrigeration requirements. A computer model for seven days from June 22 to June 28 has been created using ANSYS FLUENT 18.2, to simulate the operation of PCM-air heat exchanger for domestic application. The setpoint temperature is considered at 23–25 °C, 23–26 °C, 24–25 °C, and 25–25 °C. The inlet airflow rate is varied between 2.5 m/s and 7.5 m/s. The PCM-air heat exchanger system is studied by varying the PCM thickness (10, 20, and 30 mm) and slab length (250, 500, and 750 mm). The nanoparticles volume fraction is from 0 to 5%. The maximum cooling power, exergy efficiency, and exergy destruction increase for the nanoparticles volume fraction of 5% is 9.4%, 4.1%, and 6.3%, respectively. To obtain the best balance between the exergy destruction cost rate and the capital cost rate, the exergoeconomic factor is defined for different cases.

Analytical modeling and thermal performance analysis of a flat plate latent heat storage unit

The thermal performance of a flat plate latent heat storage unit (LHSU) consisting of parallel flat plate slabs of phase change material (PCM) was investigated with analytical techniques. The approximate analytical expressions of the heat transfer fluid (HTF) transient temperature distribution and the time wise solid-liquid PCM interface location were developed by solving the energy conservation equations and were validated by comparing the present results with fully converged numerical predictions and analytical solutions in published literature. An index of effective latent heat storage ratio, Er, which was defined as the ratio of the actual amount of available latent thermal energy before HTF outlet temperature reaches a specified value to the total latent heat storage capacity, was proposed to evaluate the thermal performance of a LHSU. The effects of geometric parameters and the thermal conductivity of PCM (kp) on Er were investigated. The results show that the analytical model is valid as the Stefan number is less than 0.15. Moreover, Er increases with the rise of the given dimensionless HTF outlet temperature and the enhancement of kp, and decreases linearly with the increase of HTF channel height and PCM slab thickness, with an abrupt change in the derivative of Er at an inflection point. A dimensionless criterion was proposed to help design thermally efficient flat plate LHSU. Additionally, we found that increasing PCM mass by lengthening the PCM slab could significantly improve Er, while thickening the PCM slab would lead to the opposite trend. This investigation provides guidelines for the optimal design of a flat plate LHSU.

Design of air cooling unit with PCM

In this paper a heat absorption unit has been designed to cool the entering hot air. For this aim, five slabs of PCM (Phase change material) have been used inside the cooling unit. The velocity and temperature values of entering hot air respectively are 0.1 m/s and 50°C. Three different cases for PCM design were investigated: in the first case RT 44HC, RT 35HC, RT 31, RT 28HC and RT 25HC as phase change materials were used in such a way that the PCM with higher number gets contact with the hot air first. In the second and third cases only one PCM type were used for the five slabs inside the cooling unit. They are RT 44HC and RT 25HC. Analysis performed for 10h. Results show that although maximum heat absorbed for the first 8h is in the case of only RT 25HC used in the cooling system, after 8h the cooling unit with mixed PCM shows a better performance and minimum exit air temperature has been observed for this case at the end of 10h.

Validation and integration of a latent heat storage model into building envelopes of a high-order building model for Modelica library AixLib

Efficient integration of phase change materials (PCMs) into buildings’ envelopes requires the mathematical modeling of PCMs and whole building systems, which is essential for the optimal design and material selection. Therefore, building performance simulation (BPS) is a cost-effective and time-efficient solution. However, currently, only a few BPS tools embed comprehensively verified and validated PCM models. This study presents a procedure of verification and validation of PCM models for building’s envelopes developed in Modelica employing an approach similar to ASHRAE Standard 140, consisting of empirical validation, analytical verification, and comparative testing.In order to characterize specific enthalpy changes and specific heat capacities of PCMs, three temperature-dependent continuous functions were proposed and validated with experimental adiabatic scanning calorimetry (ASC) data for a paraffin based PCM - RT27. Among the models, the arctangent function model behaved accurately across empirical data. Based on the model, the PCM heat transfer model for a semi-infinite PCM slab was verified analytically using the one-phase Stefan problem, where temperature evolutions at various depths of the slab were compared. For a wall with more realistic geometries and boundary conditions, a dynamic wall simulator for a multi-component wall integrating a microencapsulated PCM layer was selected, where simulated heat fluxes on inner surfaces of the wall were compared to the results of a validated numerical model. Lastly, the accuracy of the PCM model was tested on the whole-building level, thereby the variation of the average inner surface temperature of a building’s wall was compared to measured data.

Optimization and sensitivity analysis of design parameters for a ventilation system using phase change materials

Ventilation system with thermal energy storage (TES) using phase change materials (PCMs) can be employed to save energy in buildings, which stores outdoor coldness in the PCMs at night and releases this energy to cool down the fresh ventilation air during the daytime. However, its performance depends on the design parameters. This paper presents a detailed parametric analysis to address the separate effect of each design parameter on the cooling energy supply and net electricity saving of the TES system against a conventional ventilation system for the climate of Beijing, by using a computational heat transfer model. A genetic algorithm (GA) is used to optimize the design parameters for maximizing the net electricity saving in four cities of China. The decision variables are related to the PCM melting temperature, PCM slab thickness and cold charging airflow rate. The results show that the saving-optimal solution is not unique and depends on the climate. GA optimization increases the net electricity saving by 10%–54%, with a mean value of 31%. Sensitivity analysis of net electricity saving to the above three variables is carried out. Likewise, the sensitivity of each variable is not unique and depends on the climate.

Indoor thermal comfort assessment using PCM based storage system integrated with ceiling fan ventilation: Experimental design and response surface approach

The integration of Phase Change Materials (PCMs) in buildings as a potential method to improve indoor thermal comfort can be achieved via free cooling/heating approach. This paper presents the results of a study on indoor thermal comfort and energy efficiency regarding the PCM's positive role when applied to the new constructive solutions, inside a building with a ceiling fan-assisted ventilation system. The scope was driven to investigate the potential of the solution for overheating/overcooling mitigation. The thermal performance of the proposed system was experimentally evaluated by comparing the behaviour of a prototype test room (including PCM panel), with the behaviour of a similar prototype test room, in which no PCM was added. First the experiments are conducted in an artificial climate inside a laboratory environment chamber which was controlled by harmonic and linearly rising/falling temperature changing processes. Experiments were performed based on 5 level RSM CCD method to quantify the potential effects of using PCM panels and to determine individual/interactive effect of parameters on thermal discomfort index, PPD. Results indicate that minimum discomfort level can be achieved when inlet air temperature and humidity, fan rotating speed and PCM slab height and thickness were set to 29, 48%, 115 rpm, 31 cm and 2.6 cm, respectively. PPD was obtained 4.1% in the optimized condition. Then optimised tests rooms were subjected to realistic daily temperature profiles during winter and summer. In summer case, the experiments proved that the PCM application in one of the rooms lead to an overheating reduction of 13.83% representing a PCM efficiency of 56%. Under the winter condition during two months, the experiments showed proposed hybrid system can averagely reduce discomfort level around 2.61% corresponding a PCM efficiency of about 35.49%. Based on the results, proposed system have the potential to shift cooling/heating energy demand away from peak hours.

Thermal performance of a solar latent heat storage unit using rectangular slabs of phase change material for domestic water heating purposes

In this paper, the thermal performance of a rectangular latent heat storage unit (LHSU) coupled with a flat-plate solar collector was investigated numerically. The storage unit consists of a number of vertically oriented slabs of phase change material (PCM) exchanging heat with water acting as heat transfer fluid (HTF). During the day (charging mode), the water heated by the solar collector goes into the LHSU and transfers heat to the solid PCM which melts and hence stores latent thermal energy. The stored thermal energy is later transferred to the cold water during the night (discharging process) to produce useful hot water. The heat transfer process was modeled by developing a numerical model based on the finite volume approach and the conservation equations of mass, momentum, and energy. The developed numerical model was validated by comparing the simulation results, obtained by a self-developed code, with the experimental, numerical and theoretical results published in the literature. The numerical calculations were conducted for three commercial phase change materials having different melting points to find the optimum design of the LHSU for the meteorological conditions of a representative day of the month of July in Marrakesh city, Morocco. The design optimization study aims to determine the number of PCM slabs, water mass flow rate circulating in the solar collector and total mass of PCM that maximize the latent storage efficiency. The thermal performance of the LHSU and the flow characteristics were investigated during both charging and discharging processes. The results show that the amount of latent heat stored in the optimum design of the storage unit during the charging process is about 19.3 MJ, 16.54 MJ, and 12.79 MJ for RT42, RT50, and RT60, respectively. The results also indicate that depending on the mass flow rate of HTF, the water outlet temperature during the discharging process varies within the temperature ranges 43.6 °C-24 °C, 51.7 °C-24 °C, and 62.86 °C-24 °C for RT42, RT50, and RT60, respectively.

Numerical investigation of rectangular thermal energy storage units with multiple phase change materials

Latent heat thermal storage (LHTS) using phase change materials (PCMs) is one of the efficient and useful technologies in conservation and retention of thermal energy. One of the main advantages of this technology is the utilization of such systems in cooling/heating applications where the air is the heat transfer fluid (HTF). Generally, the heat transfer rate in a LHTS unit and its efficiency depends on the difference between the melting temperature of the PCM and the HTF temperature. If a single PCM is used, the temperature difference between the PCM and the HTF along the flow direction will be decreased. This leads to a decrease in the heat transfer rate and efficiency. In this case, nearly a constant temperature difference between the PCM and the HTF can be maintained during phase change. Therefore, the heat transfer rate to/from the PCM is constant. This work presents a two-dimensional numerical investigation of the performance of the LHTS unit which is composed of several rectangular PCM slabs. The enthalpy method is used to solve the governing equations for the melting process in PCMs. The convective heat transfer inside the air channels is analyzed by solving the energy equation, which is coupled with the heat conduction equation in the PCM wall. The general equations of temperature and the local liquid fraction are discretized with the finite difference method and are solved by a fully implicit scheme. The effect of geometrical parameters of storage such as the PCM slab thickness and the length as well as the effect of air flow rate in the outlet air temperature of storage are investigated.

Phase Change Material Freezing in an Energy Storage Module for a Micro Environmental Control System

Abstract This study analyzes phase change material (PCM) freezing process in a novel latent heat storage device (LHSD). Heat is removed from the PCM with an embedded evaporator. A mathematical model of freezing in a finite-thickness PCM slab is presented. An experimentally validated reduced-order model (ROM) based on the mathematical model was developed to analyze the heat transfer between the freezing PCM and an evaporating refrigerant flowing inside a flat, microchannel tube coil embedded in the PCM. A detailed finite element model (FEM) of the same device was also developed and employed to verify the validity of the ROM over a wider range of conditions. The freezing times and total “cooling” stored in the PCM computed by the ROM agree very well with those computed by the detailed FEM. The ROM executes in ∼1 min for a full heat exchanger, compared with more than 10 h for the FEM, making the former much more practical for use in parametric analysis and optimization of design alternatives.

Entropy generation analysis of multilayer PCM slabs integrated with fins

In this study, the utilisation of composite phase change materials (PCMs) slabs with fins in a thermal energy storage (TES) system is numerically analysed. The storage system consists of three PCM layers at different melting temperatures. First, the predicted results are verified against the analytical solution that is taken from literature. Afterwards, the influence of the design parameters such as fin length and fin spacing on time for complete melting of the multi-layer TES are examined. Custom field functions are integrated into the ANSYS-FLUENT software to assess the local and volume averaged entropy generation within the system. The results show that at constant fin spacing, that increasing fin length from 20 mm to 80 mm enhances the melting rate up to 50%. In addition, when the fin spacing is doubled, the elapsed time for complete melting decreased about 37%. Moreover, the fin arrangement has a considerable effect on entropy generation.

Entransy dissipation-based thermal resistance optimization of slab LHTES system with multiple PCMs arranged in a 2D array

A two-dimensional (2D) model for slab latent heat thermal energy storage (LHTES) system was developed to predict the melting of multiple phase change materials (PCMs). The performance of the slab LHTES system with multiple PCMs arranged in a 2D array was optimized based on the entransy theory, in which the transient heat conduction in the PCM slab was considered. The formula of the entransy dissipation-based thermal resistance of the slab LHTES system was deduced. A novel optimization criterion was proposed based on the minimum entransy dissipation-based thermal resistance principle, with which the melting temperatures of multiple PCMs arranged in a 2D array were optimized. According to the optimization analysis, the appropriate PCMs from literature were selected and the melting processes of the optimized multiple PCMs were investigated. Furthermore, the effect of the PCM thermal conductivity on multiple PCMs arrangement was studied. The results show that the thermal performance of the slab LHTES system is significantly improved using the optimized multiple PCMs arranged in a 2D array compared to that using single PCM. And the multiple PCMs arranged in a 2D array is preferred when PCM slab conduction thermal resistance and hot fluid convection thermal resistance are at the same level.

Exergoeconomic and environmental analyses of an air conditioning system using thermal energy storage

In this work, a combination of a latent heat thermal storage unit and a refrigeration system is presented as an air conditioning system. Overall system, including charging and discharging processes is analyzed for different phase change materials (PCMs) from exergoeconomic and environmental points of view. Based on comfort condition, three cases are selected with different size of PCM slabs and different PCMs: RT27, S27 and SP25. When the charging process takes place during the whole of off-peak hours the following results are obtained: (1) the system using SP25 has the highest value of the coefficient of performance; (2) the system using RT27 has the highest value of exergy efficiency and (3) the system using S27 has the lowest value of total cost rate. Finally, in order to obtain the best balance between the exergy destruction cost rate and the capital cost rate, the exergoeconomic factor is defined for each component, for different cases when system operates in the best performance conditions.

Experimental investigations on thermal performance of phase change material – Trombe wall system enhanced by delta winglet vortex generators

Experiments have been performed on the thermal behavior of a PCM (phase change material) – Trombe wall system with heat transfer enhancement by DWVGs (delta winglet vortex generators). PCM (CaCl2·6H2O) slabs were attached on the gap – side wall surface to increase the heat storage. Delta winglet VG pairs (attack angle β = 45°, ratio of length to height l/h = 2) were arranged on the surface of PCM panel to enhance the air side convective heat transfer in the gap. The VG height, pitch and locations were also evaluated. The results showed that the delta winglet VG pairs really enhanced the heat transfer rate at the surface of PCM panel during both the charging and discharging processes. Accordingly, the gap air flow rate and heating rate to the room with a single row of delta winglet VG pairs are 28.5% and 39.4% higher than the case without VGs. Delta winglet VG pairs with height of 30 mm, front edge pitch of 2 mm, lateral spacing of 225 mm and vertical spacing of 340 mm give best thermal performance under present conditions. The results indicate the advantages of using longitudinal vortex generators to improve the performance of PCM – Trombe wall systems for passive solar heating.