Change Latent Heat Research Articles

Supramolecular porous-based phase change material based on cucurbit[7]uril complexed amino-montmorillonite

Under high-temperature conditions, drill bits, downhole guiding tools, etc. are susceptible to failure, so capturing and storing the heat energy generated is critical for efficient drilling. In this work, a porous material of cucurbit[7]uril (CB[7]) complexed amino-montmorillonite (NH2-MT) was proposed for the first time to encapsulate eutectic nitrate (NIT) to alleviate the high-temperature crisis in the drilling process. The results showed that the structural stability and anti-leakage performance of the supramolecular composite phase change materials (MAC@NIT) were significantly improved by the addition 0.25 g of CB[7], and the adsorption of Na+ and K+ by CB[7] was in the form of monolayer adsorption. The phase change temperature and structure of MAC@NIT-2 remained stable after 200 cycles, and the maximum change of latent heat of solid-liquid was only 3.96 J/g. The special cavity structure of CB[7] further enhanced the thermal conductivity of the MAC@NIT, up to 0.48 W/m·k, which was 20 % higher than the MT@NIT. The addition of MAC@NIT improves the stability of the drilling fluids by 15 %. Additionally, the temperature of the drilling fluids could be reduced by 4.3 °C at a low solid-solid latent heat value. In summary, this study provides ideas for developing ultra-deep oil and gas resources efficiently.

Research on dolomite-based shape-stabilized phase change materials for thermal energy storage: Feasibility study of raw and calcined dolomite as skeleton support materials

Mid and high-temperature inorganic salts have limited applications due to their corrosiveness and low thermal conductivity. Shape-stabilized phase change materials (SSPCMs) can ease these issues. This study explored using dolomite, an affordable and abundant mineral with good thermal properties, in SSPCMs. We developed SSPCMs using raw and calcined dolomite at 700 °C, 750 °C, and 800 °C (700Do, 750Do and 800Do) as skeletal support materials (SSMs) and NaNO3-KNO3 as the phase change materials (PCMs). Calcined dolomites had a porous structure compared to raw dolomite (RawDo). 700Do, 750Do and 800Do require a minimum of 50 wt%, 40 wt% and 30 wt% to stabilize the shape of the sintered material by 100 cycles. RawDo was unsuitable for SSM during cycling. XRD confirmed chemical compatibility between components, and SEM showed an even distribution of SSMs within the PCMs. The thermal conductivity of SSPCMs ranged from 1.09 to 1.27 W/(m·K), compared to 0.52 W/(m·K) for NaNO3-KNO3. The NaNO3-KNO3 has a latent heat of about 110.60 J/g and a supercooling degree of 5.84 °C. The latent heat of SSPCMs depends on the proportion of SSMs, slightly lower than the theoretical value by 0.01 %–2.37 %, and the supercooling degree of SSPCMs varies between 4.75 °C and 6.25 °C. The SSPCMs were chemically stable with little latent heat change after 100 cycles, showing excellent cycling stability. The thermal decomposition temperature of SSPCMs containing 700Do was about 580 °C, while SSPCMs containing 750Do and 800Do exhibited a minor decomposition onset at around 380 °C, attributed to Ca(OH)2 decomposition. The best-performing SSPCM contained 30 wt% 800Do, offering a thermal conductivity of 1.27 W/(m·K) and thermal energy storage density of 359 kJ/kg in the 100–320 °C range. In general, calcined dolomite-based SSPCMs offer excellent thermal energy storage performance at a low cost compared to many SSPCMs.

Investigating the thermal state of permafrost with Bayesian inverse modeling of heat transfer

Abstract. Long-term measurements of permafrost temperatures do not provide a complete picture of the Arctic subsurface thermal regime. Regions with warmer permafrost often show little to no long-term change in ground temperature due to the uptake and release of latent heat during freezing and thawing. Thus, regions where the least warming is observed may also be the most vulnerable to permafrost degradation. Since direct measurements of ice and liquid water contents in the permafrost layer are not widely available, thermal modeling of the subsurface plays a crucial role in understanding how permafrost responds to changes in the local energy balance. In this work, we first analyze trends in observed air and permafrost temperatures at four sites within the continuous permafrost zone, where we find substantial variation in the apparent relationship between long-term changes in permafrost temperatures (0.02–0.16 K yr−1) and air temperature (0.09–0.11 K yr−1). We then apply recently developed Bayesian inversion methods to link observed changes in borehole temperatures to unobserved changes in latent heat and active layer thickness using a transient model of heat conduction with phase change. Our results suggest that the degree to which recent warming trends correlate with permafrost thaw depends strongly on both soil freezing characteristics and historical climatology. At the warmest site, a 9 m borehole near Ny-Ålesund, Svalbard, modeled active layer thickness increases by an average of 13 ± 1 cm K−1 rise in mean annual ground temperature. In stark contrast, modeled rates of thaw at one of the colder sites, a borehole on Samoylov Island in the Lena River delta, appear far less sensitive to temperature change, with a negligible effect of 1 ± 1 cm K−1. Although our study is limited to just four sites, the results urge caution in the interpretation and comparison of warming trends in Arctic boreholes, indicating significant uncertainty in their implications for the current and future thermal state of permafrost.

Effect of paraffin wax with carbon nanotube on melting latent heat under different pressures: A molecular research

Composite organic phase change materials (PCMs) with nanoparticles have attracted considerable interest in various fields. Latent heat and thermal conductivity are the critical properties of composite organic PCMs. To improve thermal conductivity and prevent leakage, composite PCMs are often encased in containers. However, during the phase transition, the pressure of the PCM changes due to a volume change. The change in latent heat caused by pressure can greatly impact the performance of PCMs and has not been investigated. We simulated the melting process of paraffin wax with carbon nanotubes under different pressures and investigated the relationship between pressure and latent heat. The results show that a small change in pressure does not significantly affect the latent heat, but a large increase in pressure will inhibit the release of latent heat. The decrease in latent heat is mainly due to the difference in total enthalpy between solids and liquids. Solids can build up more energy at high pressure, but the atomic structure of liquids fails to maintain the same energy. As a result, the increase in energy of solids and liquids do not match, leading to a decrease in latent heat. Our study aims to provide a reference for future PCM applications.

Influence of operating conditions on in-tube vapour condensation heat transfer characteristics in the presence of noncondensable gases at high pressure

The in-tube vapour condensation heat characteristics in the presence of noncondensable gas at high pressures are experimentally validated with an error of 20%. Numerical simulations are then conducted to explore heat transfer characteristics at different operating pressures, wall surface temperatures and mass flow rates. The flow and heat transfer characteristics are thoroughly discussed and the buoyancy effects induce a secondary circulation altering flow patterns, changing the water film and heat transfer coefficient distribution. In addition, increasing operating pressures is found to reduce the wall heat transfer rate. Even though the flow pattern is significantly different caused by the increased gas mixture density, the total condensate flow rate is approximately the same at different operating pressures due to the same inlet conditions and temperature difference. The difference in the wall heat transfer rate is attributed to the change of latent heat of vapourisation and falling film at different pressures and is evident at high pressures. At the high temperature difference, a nonuniform film height towards the bottom is emerged, sweeping the condensate upward. Similarly, increasing the mass flow rate can also increase the total wall heat transfer rate and total condensate flow rate.

A combined multi-criteria decision-making approach for the selection of carbon-based nanomaterials in phase change materials

Phase change materials (PCMs) provide renewable and sustainable energy by storing non-continuous renewable energy sources and enabling them to be used on demand. PCMs used in thermal energy storage must provide certain properties to efficiently charge and discharge energy when needed. Because of their unique properties, carbon-based nanomaterials are of interest in improving the properties of PCMs. There are critical parameters for the selection of carbon-based nanomaterials as a PCM additive. At this point, Multi-Criteria Decision Making (MCDM) methodology is very useful. In this study, Multi-Objective Optimization based on Ratio Analysis plus full multiplicative form (MULTIMOORA) and Weighted Aggregated Sum Product Assessment (WASPAS) as the MCDM methods were used for the rating of carbon-based additives in PCMs. Seven evaluation criteria were determined, namely melting point temperature change, latent heat change, thermal conductivity enhancement, leakage, greenhouse gas, cost, and agglomeration. The criterion weights were determined by objective and subjective methods, namely entropy and ranking methods, respectively. According to the results accomplished by the entropy method, the greenhouse gas was determined as the most important criterion, while the most important criterion according to the ranking was thermal conductivity. With each of these criteria weights, the alternatives were ranked by using MULTIMOORA and WASPAS methods. Expanded graphite was found to be the most suitable material to be added to PCMs according to MCDM results. In addition, a sensitivity analysis was carried out to see the effect of changes that may occur between the criteria determined to define each alternative, and according to the results of the sensitivity analysis, expanded graphite was found to be the best alternative.

Experimental study of a gas-injection refrigeration system using refrigerant mixtures to replace R404A

• The performances of a gas injection system with refrigerant mixtures are studied. • R448A and R32/R744/R1234yf mixtures with various compositions are compared. • Optimal composition ratio of the mixture is proposed based on system performances. • The life cycle climate performance (LCCP) of tested refrigerants is investigated. In this study, the performance characteristics of a gas-injection refrigeration system using ternary zeotropic mixtures of R32/R744/R1234yf were investigated to replace R404A. The performance of the gas-injection refrigeration system using R32/R744/R1234yf was measured and analyzed at various composition ratios. The test condition was set based on the refrigeration capacity B condition by the AHRI Standard 1251 (Indoor −23 ℃, outdoor 15 ℃). The variation in the mass fraction of R32 affected the changes in latent heat and temperature glide. In addition, the injection pressure increased with an increase in the mass fraction of R744 owing to the low normal boiling point of R744. Based on the test results, the optimal composition ratio of R32/R744/R1234yf was determined to be 52/1/47 wt% considering the coefficient of performance (COP), refrigeration capacity, and global warming potential. R32/R744/R1234yf (52/1/47 wt%) showed a 9.56% increase in COP and a 9.8% decrease in life cycle climate performance (LCCP) compared with those of R448A owing to the lower compressor work and indirect emissions. The performance of the gas-injection refrigeration system using the mixture could be further improved by optimizing the system configuration.

Performance of Nanocomposites of a Phase Change Material Formed by the Dispersion of MWCNT/TiO2 for Thermal Energy Storage Applications.

Thermal energy storage technology is an important topic, as it enables renewable energy technology to be available 24/7 and under different weather conditions. Phase changing materials (PCM) are key players in thermal energy storage, being the most economic among those available with adjustable thermal properties. Paraffin wax (PW) is one of the best materials used in industrial processes to enhance thermal storage. However, the low thermal conductivity of PW prevents its thermal application. In this study, we successfully modified PW based on multi-walled carbon nanotubes (MWCNT) with different concentrations of TiO2—3, 5 and 7 wt.%. The morphology of PCM and its relationship with the chemical structure and stability were characterized using scanning electron microscopy (SEM), Fourier transform infrared (FTIR) and Thermogravimetric analysis (TGA). As a result, the composites achieved a highest latent heat enthalpy of 176 J/g, in addition to enhanced thermal stability after 15 thermal cycles, and reliability, with a slight change in latent heat observed when using a differential scanning calorimeter (DSC). The thermal conductivity of the composites could significantly be enhanced by 100%. Compared to pure paraffin, the PCM composites developed in this study exhibited an excellent preference for thermal energy storage and possessed low cost, high reliability, and phase change properties.

Characterization of Fly Ash Cenosphere – Capric acid composite as phase change material for thermal energy storage in buildings

ABSTRACT Energy demand in buildings has been steadily increasing in the recent past. The majority of energy is used to heat and cool the building envelope to maintain a conducive thermal environment for habitation. Integration of Phase Change Materials (PCM) in buildings has improved the structure’s energy efficiency by absorbing and discharging energy as latent heat. In this work, Cenosphere–Capric Acid (CeCA) composite PCM was developed by the technique of impregnating capric acid (CA) into the hollow core of the cenosphere (Ce) under vacuum followed by sealing it with SiO2. The morphology of the prepared CeCA composite was investigated by FESEM. Mineralogical characterization by XRD suggested that the mineral structure of the CeCA composite has remained unaltered. Phase change temperature and latent heat of CeCA composite were evaluated as 27.60°C and 69.07 J/g for melting and 23.16°C and 71.30 J/g for freezing, respectively. Thermo-Gravimetric Analysis (TGA) of CeCA indicated the enhanced thermal stability of the composite. CeCA composite indicated good chemical compatibility between Ce, CA, and SiO2; hence, the composite is chemically stable, as revealed by FTIR analysis. Accelerated thermal cycling of CeCA composite indicates a relatively modest change in phase change temperature of 0.6°C for melting and 0.1°C for solidification. Further characterization by TGA and FTIR, suggested that the CeCA composite is reliable for the long term. It implies that the CeCA composite has significant potential as a PCM for thermal energy storage application in construction materials and structures.

Numerical study on the climatic effect of the Aral Sea

Lakes play an important role in regulating the regional climate, especially the large one in the arid region. The Aral Sea located in Central Asia, was once the fourth largest lake in the world, but gradually shrunk and lost more than 80% of its surface. As the Ara Sea dries up in last 60 years, the role of Aral Sea in regulating the regional climate dampens. However, how the Aral Sea regulates the regional climate and its mechanisms remains less understand. The climatic effect of the Aral Sea on regional climate is quantified by comparing two ideal simulations, with the Aral Sea included or excluded, using ALARO-SURFEX regional climate model under 4-km horizontal resolution. The model's performance in reproducing the 2-m temperature, precipitation and surface energy fluxes was evaluated firstly. The Aral Sea decreases (increases) the 2-m temperature in spring-summer (autumn-winter) over the Aral Sea and ambient land, which causes higher (lower) sea level pressure in spring-summer (autumn-winter) corresponding to enhanced the divergence (convergence) in spring-summer (autumn-winter). The lake-effect penetrates to the atmosphere, cools (warms) the daytime air temperature by 2.03 °C (0.07 °C) below (above) the height of 1.4-km during May to September but warming the nighttime air temperature by 0.42 °C below 0.6-km, leads to enhancing descent (ascent) motion in daytime (nighttime), which suppress (trigger) development of local convection in daytime (nighttime). The presence of Aral Sea increases specific humidity below 0.8-km in both daytime and nighttime. As a result, the annual precipitation, especially convective precipitation is increased by 7.08 mm and mainly occurs in nighttime-morning and open water period, but reduced precipitation occurs in 13:00–17:00 and ice frozen period. The presence of Aral Sea substantially reduces the sensible heat, enhances the latent heat, increases ground heat fluxes, but these effects depend season and diurnal variation. The temperature change induced by the Aral Sea is most contributed by the change of latent heat (ground heat) in open water period (ice frozen period), in diurnal timescale, the ground heat contributes most in both daytime and nighttime temperature change.

Analysis of dynamic moisture movement within bio-based earth mortars

Understanding moisture movement within building materials is critical for the understanding of how they dynamically respond to their hygrothermal environment. Utilising low embodied energy, hygrothermal materials such as bio-fibers and earth will enable a ‘green solution’ to the current climate change issues surrounding carbon intensive materials such as concrete. Within this research an earth mortar with bio-fibres of 2 types of sheep wool: Wool 1 (W1), Wool 2 (W2) and a Saw Mill Residue (SMR) in addition to a plain, control sample (PL) were cast. Experimentation was completed on these materials to identify Water Vapour Permeability (WVP), Sorption Isotherms and latent heat of sorption. Results demonstrated that for WVP, SMR has the greatest value of WVP (2.2 x10−11 kg.m−1.s−1.Pa−1). PL also had the lowest value (1.4 x10−11 kg.m−1.s−1.Pa−1). For sorption isotherm, over the course of the experiment it can be demonstrated that the incorporation of fibres increases the rate in which the mortars adsorb water molecules. This is particularly highlighted as above 30% RH sample begin to increase in moisture content with this rapidly increasing from approximately 65% RH. Finally, latent heat of sorption analysis demonstrated that a dynamic change in latent heat under differing hygorthermal conditions can be indentified; for all samples, even after 21, 24 h cycles. However, SMR had the lowest quantity of latent heat even by comparison to the PL sample as there is a sustained difference of approximately only 0.5 °C between samples. This establishes that the SMR sample requires less heat energy in order for the water to change to and from vapour and liquid state.

Radial integral boundary element method for simulating phase change problem with mushy zone

A radial integral boundary element method (BEM) is used to simulate the phase change problem with a mushy zone in this paper. Three phases, including the solid phase, the liquid phase, and the mushy zone, are considered in the phase change problem. First, according to the continuity conditions of temperature and its gradient on the liquid-mushy interface, the mushy zone and the liquid phase in the simulation can be considered as a whole part, namely, the non-solid phase, and the change of latent heat is approximated by heat source which is dependent on temperature. Then, the precise integration BEM is used to obtain the differential equations in the solid phase zone and the non-solid phase zone, respectively. Moreover, an iterative predictor-corrector precise integration method (PIM) is needed to solve the differential equations and obtain the temperature field and the heat flux on the boundary. According to an energy balance equation and the velocity of the interface between the solid phase and the mushy zone, the front-tracking method is used to track the move of the interface. The interface between the liquid phase and the mushy zone is obtained by interpolation of the temperature field. Finally, four numerical examples are provided to assess the performance of the proposed numerical method.

Upper‐level trajectories in the prototype problem for tropical cyclone intensification

AbstractData from an idealized three‐dimensional numerical simulation of tropical cyclone intensification are used to calculate Lagrangian air parcel trajectories emanating from the inflow layer that develops beneath the upper‐tropospheric outflow layer. It is found that about half of these trajectories end up in the outflow layer itself. The other half slowly subside to the mid‐ to upper troposphere, below the outflow layer, and drift slowly outwards as a result of a relatively weak overturning circulation in that region. Calculations show that pseudo‐equivalent potential temperature is not approximately conserved along the air parcel trajectories, indicating that the turbulent diffusion of heat and moisture and/or the latent heat changes by freezing or melting along the trajectories is appreciable in the mid‐ and upper troposphere.

Real-time monitoring of water and ice content in plant stem based on latent heat changes

With the development of water detection instruments, it is feasible to detect the stem liquid water content. However, real-time and non-destructive monitoring of liquid water and ice content of plant stems in winter remains challenging. Here, we developed a living wood freeze–thaw detection (LWFTD) sensor to detect the liquid water and ice content of plant stems in situ, in real time, and micro-destructively. First, based on the latent heat effect and using a retractable ring-type shrapnel probe, we monitored the freeze–thaw front of the stem water in real time and micro-destructively. Second, using calibration data and infrared detection data, the reliability of the LWFTD sensor and the feasibility of stem freeze–thaw detection based on the latent heat effect were proven. Finally, by simulating a freeze–thaw cycle and analyzing the changes in the stem liquid water content and stem temperature, an ice content model was constructed to calculate the ice content and freeze–thaw fronts. In field experiments, we recorded stem water content and freeze–thaw data of Pachira glabra, Populus tomentosa, and Lagerstroemia indica during an overwintering period. The results showed that the LWFTD sensor can effectively detect the changes in water-related physiological parameters during plant freeze–thawing in real time and that the ice content in the stem volume exhibits diurnal changes, with a single-peak, single-valley wave pattern. Our study provides a reference for evaluating the effects of freeze–thaw on plant metabolism and vitality. Furthermore, this study provides an advanced technical tool for in situ, real-time, and micro-destructive monitoring of the water and ice content in plant stems, which will help further our understanding of the water transport process and freeze–thaw-induced embolism of woody plants during winter.

Solvothermal synthesis of porphyrin-ferrocenyl conjugated microporous polymer nanospheres for shape-stable phase change materials with improved latent heat and cyclability

Organic phase change materials (PCMs) have attracted considerable attention for thermal energy storage applications because of their non-toxicity, suitable working temperature range and excellent thermal/chemical stability. However, most traditional organic PCMs have small molecular structures and are prone to leakage during fusion. To address this problem and enhance the shape-stability of organic PCMs, nanosphere-shaped porphyrin-ferrocenyl conjugated microporous polymers (PFCMPs) with high porosity (~ 650 m2/g) were solvothermally synthesized using a Diels-Alder reaction between 1,1′-ferrocenedicarboxaldehyde and pyrrole in the presence of glacial acetic acid and anhydrous Lewis acids (FeCl3, AlCl3, and CuCl2). The PFCMPs were then encapsulated with PCMs, that is, 1-octadecanol (ODA), to prepare the composite materials of ODA@PFCMPs. The optimized composite exhibited a high latent heat (up to 153.8 J/g), excellent reversibility (negligible change in latent heat upon 100 cycles of heating–cooling), good shape stability, and long heat storage durability (425 s), making it a promising candidate for solar thermal energy engineering and management.

The impact of atmospheric moisture transport on winter Arctic warming: Radiation versus latent heat release

AbstractAtmospheric moisture transport (AMT) contributes significantly to the recent accelerated Arctic warming. However, the impact of AMT has not been well quantified, not to mention the relative contribution of its impact on microphysical latent heating (LAH) and longwave radiative heating (LWH). A series of Polar‐WRF model experiments with different magnitudes of AMT are conducted to study the response of winter Arctic temperature to AMT variations. Results show that atmospheric precipitable water is very sensitive to AMT variations and thus can define the changes in surface air temperature by altering surface downward longwave radiation. Additionally, because evaporation and sublimation tend to balance the AMT‐induced moisture changes near the surface, LWH in the lower troposphere is determined by downward longwave radiation and thus can be the dominant factor for temperature variations. However, temperature in the mid‐ and upper troposphere is primarily determined by changes in LAH, because the content of ice‐phase cloud aloft is significantly affected by AMT. In addition, the changes in LWH in the mid‐ and upper troposphere are governed by upward longwave radiation, thus offsetting some of the temperature variations. These findings have implications for the attribution of Arctic current warming and the prediction of its future temperature change.

Thermal performance of PEG-MWCNTs composites as shape-stabilised phase change materials for thermal energy storage

The development of phase change materials (PCM) for thermal energy storage is a promising technology. However, the liquid PCM leaks and low thermal conductivity limit the practical PCM applications. This article aims to solve these problems; it presents the preparation and thermal characterisation of PCM enhanced by carbon-based nanoparticles. The polyethene glycol 6000 (PEG 6000) is used as PCM and multi-walled carbon nanotubes (MWCNTs) as a shell matrix and thermal conductivity enhancer. The sample was prepared by the sonification method under vacuum conditions. Fourier transform infra-red spectroscopy (FT-IR) and thermo-gravimetric analysis (TGA) tested the chemical and thermal compatibility of the prepared samples. The storage performances are tested by modulated differential scanning calorimetry characterisation. The nano-enhanced-PCM (NPCM) with 1 wt% MWCNTs showed excellent shape stability without any liquid leakage when the temperature was about 110 °C for 30 minutes. Drying method has a significant effect on the thermal storage capacity of the NPCM. The melting, solidification points and the latent heats of the NPCM were measured as 61.75, 35.50 °C, and 174.24, 167.84 J g−1, respectively. Meanwhile, the specific heat is 2.63 J g−1°C−1 for the solid-state and 2.14 J g−1°C−1 for the liquid-state. The thermal conductivity of pristine PEG was improved by 49%. Highlights PEG-MWCNTs shape-stabilised phase change materials were prepared through ultrasound-assisted vacuum method for efficient thermal energy storage. The thermophysical properties and thermal stability of the proposed NPCM has been analysed and discussed. The drying method highly influences the phase change temperatures and latent heats of the prepared NPCM. PEG adsorption into MWCNTs reached 99 wt% without leakage, and 1 wt% of MWCNTs enhanced the thermal conductivity by 49% and maintained good latent heat of 175 J g−1. The developed NPCM is an excellent potential candidate for electronic devices cooling applications.

Response of Vertical Velocities in Extratropical Precipitation Extremes to Climate Change

AbstractPrecipitation extremes intensify in most regions in climate model projections. Changes in vertical velocities contribute to the changes in intensity of precipitation extremes but remain poorly understood. Here, we find that midtropospheric vertical velocities in extratropical precipitation extremes strengthen overall in simulations of twenty-first-century climate change. For each extreme event, we solve the quasigeostrophic omega equation to decompose this strengthening into different physical contributions. We first consider a dry decomposition in which latent heating is treated as an external forcing of upward motion. Much of the positive contribution to upward motion from increased latent heating is offset by negative contributions from increases in dry static stability and changes in the horizontal length scale of vertical velocities. However, taking changes in latent heating as given is a limitation when the aim is to understand changes in precipitation, since latent heating and precipitation are closely linked. Therefore, we also perform a moist decomposition of the changes in vertical velocities in which latent heating is represented through a moist static stability. In the moist decomposition, changes in moist static stability play a key role and contributions from other factors such as changes in the depth of the upward motion increase in importance. While both dry and moist decompositions are self-consistent, the moist dynamical perspective has greater potential to give insights into the causes of the dynamical contributions to changes in precipitation extremes in different regions.

Preparation and thermal properties of microencapsulated paraffin with polyurea/acrylic resin hybrid shells as phase change energy storage materials

Microencapsulated paraffin with polyurea/acrylic resin hybrid shells as phase change energy storage materials was obtained in situ by combining interfacial polymerization and suspension-like polymerization. Glycerin (GC) acts as a cross-linking agent to modify the shells. The morphologies, particle size distributions, thermal storage properties, thermal stabilities and thermal reliabilities of microencapsulated phase change materials (MicroPCMs) were determined by scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and thermal gravimetric analysis (TG). The temperature regulation performance of the foam with MicroPCMs was investigated by an infrared thermal imager. DSC results showed that MicroPCMs with polyurea/butyl methacrylate (PU/BMA) possess an improved heat ability and thermal reliability compared to MicroPCMs with polyurea/methyl methacrylate (PU/MMA). The incorporation of GC to shell-forming composition led to an enhancement in thermal storage capacity of the MicroPCMs. The MicroPCMs with GC-modified PU/BMA hybrid shell has the highest PCMs content by as much as 82.6 mass%. The change in latent heat of MicroPCMs with GC-modified PU/BMA hybrid shell was very small of less than 4 mass% after 500 thermal cycles. The infrared thermography indicated that the PU foam incorporating the MicroPCMs with GC-modified PU/BMA hybrid shell has better temperature-regulated property. In conclusion, the MicroPCMs with PU/BMA hybrid shells, especially with GC-modified PU/BMA hybrid shell, possess a promising prospect applying in energy-conserving building materials and thermal control system of shipping packages.

Development of paraffinic phase change material nanoemulsions for thermal energy storage and transport in low-temperature applications

In this study, new phase change material nanoemulsions (PCMEs) were designed and characterized as possible storage and heat transfer media for low-temperature thermal uses. Water- and (ethylene glycol + water)-based emulsions with fine droplets of n-heptadecane and RT21HC commercial paraffin were produced by a solvent-assisted emulsification method. No phase separation or significant growth in PCM drops were observed for the prepared emulsions through storage, after freeze–thaw cycles and under mechanical shear. Phase change transitions were characterized and a significant sub-cooling was observed, with solidification temperatures up to 13 K below the melting point. One pure alkane and two commercial paraffin waxes with higher melting points were considered as nucleating agents to reduce sub-cooling effect. Although the emulsions exhibited diminutions in thermal conductivity up to 9% with respect to the carrier fluids used as base fluid, enhancements in energy storage capacity (considering an operational temperature interval equal to the sub-cooling) reached 26% in the case of RT21HC nanoemulsion based on the (ethylene glycol + water) mixture that contained 10% in mass of paraffin. In addition, the thermal reliability of the nanoemulsions was verified analyzing the changes in latent heat after storage and throughout 1000 thermal cycles.