Phase Change Materials For Thermal Energy Storage Research Articles

Nanofluid PCMs for thermal energy storage: Latent heat reduction mechanisms and a numerical study of effective thermal storage performance

Abstract The latent heat of fusion of paraffin-based nanofluids has been examined to investigate the use of enhanced phase change materials (PCMs) for thermal energy storage (TES) applications. The nanofluid approach has often been exploited to enhance thermal conductivity of PCMs, but the effects of particle addition on other thermal properties affecting TES are relatively ignored. An experimental study of paraffin-based nanofluids containing various particle sizes of multi-walled carbon nanotubes has been conducted to investigate the effect of nanoparticles on latent heat of fusion. Results demonstrated that the magnitude of nanofluid latent heat reduction increases for smaller diameter particles in suspension. Three possible mechanisms – interfacial liquid layering, Brownian motion, and particle clustering – were examined to explain further reduction in latent heat, through the weakening of molecular bond structures. Although additional research is required to explore detailed mechanisms, experimental evidence suggests that interfacial liquid layering and Brownian motion cannot explain the degree of latent heat reduction observed. A finite element model is also presented as a method of quantifying nanofluid PCM energy storage performance. Thermal properties based on modified effective medium theory and an empirical relation for latent heat of fusion were applied as model parameters to determine energy stored and extracted over a given period of time. The model results show that while micro-scale particle inclusions exhibit some performance enhancement, nanoparticles in PCMs provide no significant improvement in TES performance. With smaller particles, the enhancement in thermal conductivity is not significant enough to overcome the reduction in latent heat of fusion, and less energy is stored over the PCM charge period. Therefore, the nanofluid approach may not be justifiable for energy storage applications. However, since the model parameters are dependent on the material properties of the system observed, storage performance may vary for differing nanofluid materials.

Thermophysical characterization of Mg–51%Zn eutectic metal alloy: A phase change material for thermal energy storage in direct steam generation applications

The possibility of using magnesium based eutectic metal alloys as phase change material (PCM) for thermal energy storage (TES) in concentrated solar power (CSP) applications is analysed. An extensive thermophysical characterization of the Mg–51%Zn eutectic metal alloy between room temperature and melting temperature has been performed. The results are compared with some available data in the literature, and differences found are discussed. A comparison with pure, binary and ternary inorganic salts used as PCM is presented highlighting the advantages and disadvantages of the different systems. This alloy is proposed not only as a candidate for latent heat thermal energy storage for direct steam generation (DSG) in CSP applications but for high pressure and high energy steam processes as well. Reported data are essential for ensuing modelling and experimental reactor studies employing this alloy as a PCM for TES.

Stability of sugar alcohols as PCM for thermal energy storage

Sugar alcohols as phase change materials (PCM) are very promising due to their high storage capacity, safety and economic reasons. Their phase change temperatures make them suitable for medium temperature storage, which is needed for solar process heat or waste heat recovery applications. To guarantee a long PCM working lifetime they should be tested under a repeated number of freezing/melting cycles to check whether the thermophysical properties remain constant or not. The cycling stability of d-mannitol, myo-inositol and galactitol has been studied with DSC and the chemical stability with FT-IR. Myo-inositol shows a good cycling stability under the chosen measuring parameters. The cycling and chemical stability of d-mannitol up to 20 cycles can be achieved by controlling some parameters. The variable affecting it the most is the amount of oxygen in contact with it. Hence, in the storage unit this could be prevented with an inert or vacuum atmosphere.

Energy management and CO2 mitigation using phase change materials (PCM) for thermal energy storage (TES) in cold storage and transport

Low temperature sensitive products transport and storage is an issue worldwide due to changes of the lifestyle population increase. Thermal energy storage (TES) is nowadays one of the most feasible solutions in facing the challenge of achieving energy savings. Many researchers have investigated energy efficiency of different cold units by applying TES systems using phase change materials (PCM). This paper provides an overview of the existing Spanish and European potential energy savings and CO2 mitigation by incorporating TES systems to cold storage and transportation systems. Data on energy savings were compiled from different case studies. Results depend on the scenarios studied and the extent of TES systems implementation; in the case of Europe for instance, yearly CO2 emissions may be cut down between 5% and 22% in reference to 2008 CO2 emissions from cold production considering that the proposed implementation of PCM TES in the case studies found in the literature is done.

Polyols as Phase Change Materials for Low-grade Excess Heat Storage

Polyols are an emerging phase change materials (PCM) category for thermal energy storage (TES), with moderate phase change temperatures and considerable enthalpies. These can be employed in systems for harvesting surplus energy from, industrial and power generation processes. However, knowledge on the properties of polyols in relation to desirable PCM properties is presently sparse and rather inconsistent. This work summarizes a literature review on polyols as PCM for TES. In addition, preliminary T-History characterization of some selected polyols was done. This study is expected to be an initiation of an extensive polyol phase equilibrium evaluation.

New kinds of energy-storing building composite PCMs for thermal energy storage

Energy storing-composite phase change materials (PCMs) are significant means of thermal energy storage in buildings. Although several building composite PCMs (BCPCMs) have been developed in recent years, the additional investigations are still required to enrich the diversity of BCPCMs for solar heating and energy conservation applications in buildings. For this purpose, the present work is focused the preparation, characterization and determination of 10 new kinds of BCPCMs. The BCPCMs were prepared by blending of liquid xylitol pentalaurate (XPL) and xylitol pentamyristate (XPM) esters with gypsum, cement, diatomite, perlite and vermiculite as supporting matrices. The scanning electron microscopy (SEM) and Fourier Transform Infrared (FT-IR) analysis showed that the ester compounds were adsorbed uniformly into the building materials due to capillary forces. The highest adsorption ratio of XPL ester into gypsum, cement, perlite, diatomite, and vermiculite were found to be 20, 19, 71, 52 and 40wt.%, respectively while it was found to be 22, 25, 66, 50 and 41wt.% for XPM ester, respectively. Differential scanning calorimetry (DSC) results indicated that the melting temperatures and latent heat energy storage capacities of the prepared BCPCMs are in range of about 40–55°C and 31–126J/g, respectively. Thermogravimetric (TG) investigations showed that the BCPCMs had good thermal endurance even above their phase change temperatures. The BCPCMs exhibited almost same chemical and phase change characteristics after 1000 thermal cycling test. It can be also concluded that especially the BCPCMs perlite, vermiculite, diatomite content were found to better candidates for thermal energy storage applications in buildings due to the fact that they have relatively higher heat storage capacity.

Improved design for heat transfer performance of a novel phase change material (PCM) thermal energy storage (TES)

This study evaluates numerically various configurations of PCM thermal energy storage devices, e.g., U-tube, U-tube with in-line fins, U-tube with staggered fins and a novel festoon design. The conjugate heat transfer between the heat transfer fluid (HTF) and PCM, which undergoes a cyclic melting and freezing process, is solved numerically using the computational fluid dynamic approach utilizing enthalpy-porosity formulation. The results indicate that our novel festoon channel design yields improved heat transfer rate for both charging and discharging stages. To further enhance heat transfer performance, we introduce use of multiple PCMs with various configurations of different PCM arrangement. Advantages and limitations of each design are evaluated with respect to their heat transfer performance vis-à-vis heat storage capacity. Further R&D and experimentation is needed to arrive at commercially viable designs of PCM heat storage units.

Application of PCM thermal energy storage system to reduce building energy consumption

The building sector is known to make a large contribution to total energy consumption and CO2 emissions. Phase change materials (PCMs) have been considered for thermal energy storage (TES) in buildings. They can balance out the discrepancies between energy demand and energy supply, which are temporally out of phase. However, traditional PCMs need special latent storage devices or containers to encapsulate the PCM, in order to store and release the latent heat of the PCM. The proper design of TES systems using a PCM requires quantitative information and knowledge about the heat transfer and phase change processes in the PCM. In Korea, radiant floor heating systems, which have traditionally been used in residential buildings, consume approximately 55% of the total residential building energy consumption in heating. This article reviews the development of available latent heat thermal energy storage technologies and discusses PCM application methods for residential building using radiant floor heating systems with the goal of reducing energy consumption.

Thermal energy storage system using phase change materials: Constant heat source

The usage of phase change materials (PCM) to store the heat in the form of latent heat is increased, because large quantity of thermal energy is stored in smaller volumes. In the present experimental investigation paraffin and stearic acid are employed as PCMs in thermal energy storage (TES) system to store the heat as sensible and latent heat also. A constant heat source is used to supply heat transfer fluid (HTF) at constant temperature to the TES system. In the TES system PCMs are stored in the form of spherical capsules of 38 mm diameter made of high density poly ethylene (HDPE). The results of the investigation are related to the charging time and recovery of stored energy from the TES system.

Preparation of stearic acid/halloysite nanotube composite as form-stable PCM for thermal energy storage

A novel form-stable composite as phase change material (PCM) for thermal energy storage was prepared by absorbing stearic acid (SA) into halloysite nanotube (HNT). The composite PCM was characterized by TEM, FT-IR and DSC analysis techniques. The composite can contain SA as high as 60 wt% and maintain its original shape perfectly without any SA leakage after subjected to 50 melt–freeze cycles. The melting temperature and latent heat of composite (SA/HNT: 60/40 wt%) were determined as 53.46°C and 93.97 J g−1 by DSC. Graphite was added into the SA/HNT composite to improve thermal storage performance, and the melting time and freezing time of the composite were reduced by 65.3 and 63.9%, respectively. Because of its high adsorption capacity of SA, high heat storage capacity, good thermal stability, low cost and simple preparation method, the composite can be considered as cost-effective latent heat storage material for practical application. Copyright © 2011 John Wiley & Sons, Ltd.

Preparation and characterization of fatty acid ester/building material composites for thermal energy storage in buildings

In this study, fatty acid ester/building material composites as novel form-stable phase change materials (PCMs) were prepared by absorbing liquid fatty acid esters into porous network of conventional building materials. In the composite erythritol tetrapalmitate (ETP) and erythritol tetrastearate (ETS) esters were used as PCM for thermal energy storage, and cement and gypsum used as supporting material. The composite PCMs were characterized using scanning electron microscope (SEM) and Fourier transformation infrared (FT-IR) analysis technique. The SEM results showed that the esters were confined in the porous network of the cement and gypsum. Thermal properties and thermal stabilities of the composite PCMs were determined by using differential scanning calorimetry (DSC) and thermal gravimetric (TG) analysis techniques. DSC results showed that the melting temperatures and the latent heats of the prepared composite PCMs were in range of 21.6–32.3°C and 35.9–43.3J/g, respectively. TG analysis indicated that the composite PCMs had good thermal stability. The thermal cycling test including 1000 heating and cooling process was conducted to determine the thermal reliability of the composite PCMs and the test results revealed that the composite PCMs have good thermal reliability and chemical stability.

Analysis and Design of a Paraffin/Graphite Composite PCM Integrated in a Thermal Storage Unit

The addition of latent heat storage systems in solar thermal applications has several benefits including volume reduction in storage tanks and maintaining the temperature range of the thermal storage. A phase change material (PCM) provides high energy storage density at a constant temperature corresponding to its phase transition temperature. In this paper, a high temperature PCM (melting temperature of 80°C) made of a composite of paraffin and graphite was tested to determine its thermal properties. Tests were conducted with a differential scanning calorimeter and allowed the determination of the melting and solidification characteristics, latent heat, specific heat at melting and solidification, and thermal conductivity of the composite. The results of the study showed an increase in thermal conductivity by a factor of 4 when the mass fraction of the graphite in the composite was increased to 16.5%. The specific heat of the composite PCM (CPCM) decreased as the thermal conductivity increased, while the latent heat remained the same as the PCM component. In addition, the phase transition temperature was not influenced by the addition of expanded graphite. To explore the feasibility of the CPCM for practical applications, a numerical solution of the phase change transition of a small cylinder was derived. Finally, a numerical simulation and the experimental results for a known volume of CPCM indicated a reduction in solidification time by a factor of 6. The numerical analysis was further explored to indicate the optimum operating Biot number for maximum efficiency of the composite PCM thermal energy storage.

Preparation, thermal properties and thermal reliability of microencapsulated n-eicosane as novel phase change material for thermal energy storage

This study deals with preparation, characterization, thermal properties and thermal reliability of n-eicosane microcapsules as novel phase change material (PCM) for thermal energy storage. The microcapsulated PCMs were prepared by coating n-eicosane with polymethylmethacrylate (PMMA) shell. Fourier transform infrared (FT-IR), scanning electron microscope (SEM) and particle size distribution (PSD) analysis were used to characterize the PMMA/eicosane microcapsules as microcapsulated PCMs. The PSD analysis indicated that the average diameter of microcapsules was found to be 0.70 μm under the stirring speed of 2000 rpm. Thermal properties and thermal reliability of the microcapsules were determined using differential scanning calorimetry (DSC) and thermo gravimetric analysis (TGA) methods. From DSC analysis, the melting and freezing temperatures and the latent heats of the microcapsules were measured as 35.2 °C and 34.9 °C, 84.2 and −87.5 J/g, respectively. TGA analysis indicated that PMMA/eicosane microcapsules degrade in three steps at considerably high temperatures. Accelerated thermal cycling tests have been also applied to show the thermal reliability of the microcapsules. All results showed that thermal properties make the PMMA/eicosane microcapsules potential PCM for thermal energy storage.

Synthesis, thermal energy storage properties and thermal reliability of some fatty acid esters with glycerol as novel solid–liquid phase change materials

The synthesis, thermal energy storage properties and thermal reliability of some fatty acid esters with glycerol as novel solid–liquid phase change energy storage materials were investigated. The esters were synthesized by means of the Fischer esterification reaction of the glycerol with myristic, palmitic and stearic acids. The chemical structures of esters were proven by FT-IR and 1H NMR techniques. The melting temperatures and latent heats of the synthesized esters were found in the range of 31–63 °C and 149–185 J/g, by DSC method. The results showed that the esters as phase change materials (PCMs) had good thermal reliability with respect to the 1000 thermal cycles. TGA analysis was performed to determine thermal stability of the esters. The thermal conductivity of the PCMs was also improved significantly by adding 5 wt% expanded graphite. Based on all results it can also be concluded that the synthesized esters can be considered as potential PCMs for thermal energy storage.

Preparation, thermal properties and thermal reliability of palmitic acid/expanded graphite composite as form-stable PCM for thermal energy storage

This study is focused on the preparation and characterization of thermal properties and thermal reliability of palmitic acid (PA)/expanded graphite (EG) composite as form-stable phase change material (PCM). The maximum mass fraction of PA retained in EG was found as 80 wt% without the leakage of PA in melted state even when it is heated over the melting point of PA. Therefore, the PA/EG (80/20 w/w%) composite was characterized as form-stable PCM. From differential scanning calorimetry (DSC) analysis, the melting and freezing temperatures and latent heats of the form-stable PCM were measured as 60.88 and 60.81 °C and 148.36 and 149.66 J/g, respectively. Thermal cycling test showed that the composite PCM has good thermal reliability although it was subjected to 3000 melting/freezing cycles. Fourier transformation infrared (FT-IR) spectroscopic investigation indicated that it has good chemical stability after thermal cycling. Thermal conductivities of PA/EG composites including different mass fractions of EG (5%, 10%, 15% and 20%) were also measured. Thermal conductivity of form-stable PA/EG (80/20 w/w%) composite (0.60 W/mK) was found to be 2.5 times higher than that of pure PA (0.17 W/mK). Moreover, the increase in thermal conductivity of PA was confirmed by comparison of the melting and freezing times of pure PA with that of form-stable composite. Based on all results, it was concluded that the form-stable PA/EG (80/20 w/w%) has considerable latent heat energy storage potential because of its good thermal properties, thermal and chemical reliability and thermal conductivity.

Heat transfer enhancement of fatty acids when used as PCMs in thermal energy storage

Phase change materials (PCM) used in latent heat storage systems usually have very low thermal conductivities. This is a major drawback in maintaining the required heat exchange rate between PCM and heat transfer fluid. This paper investigates the enhancement of the heat transfer between PCM and heat transfer fluid, using high thermal conductivity as additives like stainless steel pieces, copper pieces and graphite–PCM composite material. In the experiments, palmitic–lauric acid (80:20) (PL) and stearic–myristic acid (80:20) (SM) were used as PCMs. Test results show that heat transfer enhancement of copper pieces was better at 0.05 Ls−1 flow rate compared to 0.025 Ls−1. Using copper as an additive increased the heat transfer rate 1.7 times for melting and 3.8 times for freezing when flow rate was 0.050 Ls−1. Decreasing the flow rate from 0.050 to 0.025 Ls−1, increased the melting times 1.3 times and freezing times 1.8 times, decreasing heat transfer rates accordingly. The best result of heat transfer enhancement was observed for the PCM–graphite composite. However, changing the flow rate did not affect the heat transfer rate when graphite was used as additive. Copyright © 2007 John Wiley & Sons, Ltd.

Energy analysis of space solar dynamic heat receivers

Energy analysis of space solar dynamic heat receivers employing solid–liquid phase change storage is developed. The heat receiver is a critical component of a solar dynamic system. Phase change thermal energy storage is used in the heat receiver. The energy analysis presented here can be used to understand the energy transfer in the heat receiver and thermal energy storage in phase change materials (PCM). The heat receiver cavity radiation mathematical model and the working fluid tube heat model are established. Energy loss, energy absorbed by gas, the latent and sensible thermal energy storage in PCM, maximum tube temperature, gas outlet temperature and liquid PCM fraction were calculated. The results are analyzed and could be used in heat receiver design.

Exergetic Analysis of Energy Storage using Multiple Phase-Change Materials

This paper presents an exergetic analysis of the charge process in energy storage using multiple phase-change materials (PCMs). Thermal storage using two, three, five, as well as an infinite number of PCMs is analyzed with a distributed model for the heat transfer fluid. Analytical results show the relative merits of using multiple PCMs compared with a single PCM for thermal energy storage. Sample results are presented and discussed.