Shape-stabilized Phase Change Materials Research Articles

Enhanced heat storage and heat transfer performance of wood-based biomass carbonized skeleton loaded with polyethylene glycol phase change material by surface modification

Thermal energy storage technology can shift the peak and fill the valley of heat, which lays the foundation for realizing the goal of “emission peak and carbon neutrality”. Among various thermal energy storage techniques, the latent heat storage technology based on composite phase change materials can provide large storage capacity with a small temperature variation, and shows great potential in solving the intermittency issue of renewable energy. As a sustainable and renewable material, natural wood has the advantages of a unique anisotropic three-dimensional structure, perfect natural channel, low price, and rich resources. Therefore, the carbonized wood obtained from high-temperature carbonization of natural wood is an excellent choice as a supporting skeleton of composite phase change materials. On the other hand, polyethylene glycol is widely used in energy storage because of its suitable phase transition temperature (46–65℃), high latent heat (145–175 J/g), and stable performance. In this study, carbonized bamboo is prepared at high temperatures. To improve heat storage, thermal conductivity, and photo-thermal conversion properties, the carbonized bamboo is functionalized by graphene oxide and reduced graphene oxide, respectively. Finally, polyethylene glycol is implanted into modified carbonized bamboo to form shape-stabilized phase change materials. Their microstructures, morphologies, and thermophysical properties are characterized. The experimental results show that graphene oxide and reduced graphene oxide can change the surface polarity of carbonized bamboo, thus reducing the interfacial thermal resistance between the carbonized bamboo skeleton and polyethylene glycol, and improving the encapsulation ratio, thermal conductivity, and photo-thermal conversion efficiency without affecting the crystallization behavior of polyethylene glycol. The encapsulation ratio of carbonized bamboo/reduced graphene oxide/polyethylene glycol ternary phase change material is as high as 81.11% (only 4.67% lower than the theoretical value), its latent heat of melting and solidification are 115.62 J/g and 104.39 J/g, its thermal conductivity is greatly increased to 1.09 W/(m·K) (3.7 times that of pure polyethylene glycol), accompanied by substantial growth in its photo-thermal conversion efficiency, reaching 88.35% (3.1 times that of pure polyethylene glycol). This research develops a biomass-derived porous composite phase change material with high heat storage density, high heat transfer rate, and high photo-thermal conversion ability.

Wood and Delignified Wood for Shape-Stabilized Phase-Change Materials in Application to Thermal Energy Storage

Wood and Delignified Wood for Shape-Stabilized Phase-Change Materials in Application to Thermal Energy Storage

Shape–stabilized phase change materials composed of polyethylene glycol and ordered mesoporous silica synthesized from fly ash

Aiming at the problems of easy leakage and low thermal conductivity of solid–liquid phase change materials (PCM), and the recycle of fly ash, ordered mesoporous silica (OMS) was synthesized from fly ash to prepare the shape–stabilized PCM (SSPCM) with various polyethylene glycol (PEG) mass fractions. Aminoated multi–walled carbon nanotubes (MWNT–NH2) were added into SSPCM for enhancing the heat transfer rate. The structure of PEG remains intact after blending with OMS. The shape–stabilization capacity of OMS to PEG was up to 90 wt%. The leakage problem of solid–liquid PCM can be effectively solved owing to the interactions between OMS and PEG, such as surface adsorption, mesoporous confinement and hydrogen bonding. The melting enthalpy of PEG/OMS SSPCM with 90 wt% PEG reaches up to 133.7 J g−1, and its heat storage efficiency is 90%. SSPCM exhibits excellent thermal stability and reliability. Its thermal conductivity is increased by 191.3% upon the addition of 5 wt% MWNT–NH2 comparing with PEG.

Analysis of a borehole ground heat exchanger with shape-stabilized phase change material backfill

GSHP (ground source heat pump) is a popular green building technology that utilizes renewable energy sources, but its efficiency can decrease significantly due to heat accumulation in soil. The borehole ground heat exchanger (BGHE) with shape-stabilized phase change material (SSPCM) backfill is a novel method to alleviate temperature fluctuation owing to better thermophysical properties. However, relevant experimental studies are still in scarcity and the impacts of backfill material on the soil temperature remain to be explored. This study conducted an experimental measurement on the soil temperature of a BGHE with SSPCM (expand graphite adsorbed paraffin) and sand backfill. A numerical model is established accordingly to be validated by the experimental results, in order to provide additional simulations on the sand-SSPCM mixture to identify the system performance and cost-effectiveness under different mass ratios of SSPCM. The experimental results show that SSPCM can effectively mitigate temperature fluctuations, especially closer to the heat source. The excess temperature difference is up to 10 °C near the heater wall. Compared to sand backfill, the sand-SSPCM backfill area's outside also maintains a steady temperature for a longer time during the cooling process. The numerical model couples the line heat source model of the BGHE with the validated phase change process. The simulation results found that the heat influence radius under the novel backfill was reduced by 16.5% after one operating cycle. Furthermore, after a 14 h shutdown, the novel backfill temperature is higher than the traditional backfill in the backfill area. The impact of various SSPCM mass mixing ratios indicates that a higher mass ratio can help reduce temperature variations, but the larger the mass ratio the less cost-effective it becomes. The optimum mass ratios are dependent on different parameters used as objectives.

Methylated mesoporous silica loaded with 1-octadecanol as a new shape-stabilized phase change material for enhanced thermal energy storage efficiency

Here, a 1-octadecanol (OD)/methylated mesoporous silica (OD/M-MS) shape-stabilized phase-change material (SSPCM) was prepared with enhanced thermal energy storage. The M-MS was synthesized by methylation of low-cost MS with trimethylchlorosilane. Fourier-transformed infrared spectroscopy (FTIR), N2 adsorption–desorption isotherm, and thermogravimetric analysis confirmed successful methylation of MS. OD was then impregnated into the pores of MS and M-MS to form OD/MS and OD/M-MS SSPCMs, and their thermal properties were thoroughly characterized and compared. FTIR result of OD/MS revealed interfacial hydrogen bond (H-bond) interactions between the hydroxyl group (–OH) on the OD molecules and silanol groups (Si–OH) on the MS surface. These interactions prevented the free movement and ordered arrangement of OD molecules for crystallization, thus declining the heat storage capacity. By methylating the MS surface, the H-bond interactions were suppressed and the OD/M-MS could recover the thermal performance. The heat storage capacity increased by 96.8% from 43.5 J g−1 of OD/MS to 85.6 J g−1 of OD/M-MS. Moreover, a test of 400 accelerated thermal cycles proved good thermal reliability for OD/M-MS SSPCM. This work provides an effective strategy to achieve OD/M-MS SSPCM with low cost and high thermal performance, promising for large-scale thermal energy storage applications.

Experimental study on thermal and mechanical properties of tailings-based cemented paste backfill with CaCl2·6H2O/expanded vermiculite shape stabilized phase change materials

Experimental study on thermal and mechanical properties of tailings-based cemented paste backfill with CaCl2·6H2O/expanded vermiculite shape stabilized phase change materials

Shape-stabilized phase change material for thermal energy storage: Sr+2 doped BaCO3 matrix incorporating polyethylene glycol

An Sr2+-doped BaCO3 matrix incorporating polyethylene glycol (PEG-6000) was developed using a simple impregnation process to build a shape-stabilized functional phase transition composite for application in thermal energy storage systems. A direct hydrothermal process was used to prepare a unique anhydrous rhombohedral BaCO3. The developed rhombohedral material, characterized following a hydrothermal reaction in the presence of 5 mol% Sr2+, was used to prepare extremely tiny porous particles (5 % SrBaCO3). The 5 % SrBaCO3 powder was hydrothermally produced and utilized to encapsulate PEG to create a phase change material (PCM) that prevents leakage of PEG during phase change. The shape-stabilized composite PCM (ss-PCM) that was prepared with nano- and micro-sized porous SrO powder and containing pure BaCO3 exhibited consistent and superior performance throughout a significant number of heat cycles. DSC results indicated that the melting temperature of the PEG/5BaCO3 is 60.17 °C, and the latent heat is 148.8 J/g. The R% (Impregnation ratio) and Eeff% (Efficient energy per unit mass of PEG) values of 5 % SrBaCO3 were more than those of 10 % SrBaCO3 and 15 % SrBaCO3, probably due to the presence of micro- and macro-pores and high pore volume. Additionally, it was clear from the XRF, EDX, and XPS data that Ba2+ segregation in 5 % SrBaCO3 was considerably less than that in 10 % SrBaCO3 and 15 % SrBaCO3. Compared to the long-established ss-PCMs, the developed composites exhibited much greater thermal enthalpy, higher thermal conductivity, and lesser super cooling. The presence of OH−1 on the surface of the PCM contributed to a reduction in the super cooling. Furthermore, the supporting materials improved the thermal conductivity of the PEG, overcoming the organic PCM's poor thermal conductivity. As pure PEG is infused in 5 % SrBaCO3, its super cooling effect is diminished by 15.5 %. Overall, the PCM with 5 % SrBaCO3/PEG preserves its capacity to store and release energy without a substantial modification its properties after multiple thermal heating and cooling cycles.

Model-driven development of durable and scalable thermal energy storage materials for buildings

The energy impact of integrating phase change materials (PCMs) in buildings for thermal energy storage has been modeled by various whole-building simulation programs, demonstrating that PCM incorporation can reduce energy consumption, provide grid flexibility and resilience, and reduce CO2 emissions. The models assume that the PCMs are in perfect operating condition and underestimate the impact of actual phase change behavior (e.g., enthalpy curve shape) on thermal load shifting in practical deployment. In this paper, we bridge the gap between theory and practice when evaluating the energy impact of PCMs by using a model-driven approach to develop durable thermal energy storage materials with desired phase change properties. For ease of integration, we fabricate shape-stabilized PCMs (ss-PCMs) by encapsulating solid-liquid polyethylene glycol (PEG) consisting of different molecular weights within mesoporous magnesium oxide (MgO) matrices. Learning from the modeling results, we manipulate phase change properties such as peak melting temperature and temperature glide of PEG-MgO ss-PCMs during the synthesis process to achieve desired properties. As such, the energy density is maximized within the optimum operating temperature range, which is critical to boosting energy efficiency. Compared to a case with no PCM, a layer of PEG-MgO ss-PCM integrated into the wall provides a 50% reduction in the peak load and also exhibits a repeatable phase change behavior for up to 1000 thermal cycles without leakage, showing durability of this material. We also show that this lab-scale synthesis process is easy to be scaled up by 100 times for a demonstration of large-scale industrial production. The synthetic tunability of transition temperature of ss-PCMs also extends their applicability beyond buildings.

Design of corn straw/paraffin wax shape-stabilized phase change materials with excellent thermal buffering performance

The introduction of 3D structured supporting materials is reckoned as an effective way to relieve the liquid leakage and improve the shape stabilization of phase change materials (PCMs). However, the complicated preparation process and high cost restrict their large-scale application. Herein, shape-stabilized composite PCMs are designed with cheap corn straw (CS) as 3D porous framework via simple pre-treatments. The loading efficiency of paraffin wax (PW) is up to 99.7%, resulting in superior thermal storage and release performance of CS/PW (CP) composite PCMs with phase change enthalpy above 185 J/g. The shape stability temperature of CP rises to 95 °C due to the capillary force. Excellent thermal buffering effect is proved by five highly consistent thermal cycles and only a little loss appears after 200 cycles. Moreover, CP mat with a diameter of 8 cm shows superior thermal buffering performance to retard the cooling of water and avoid the high temperature of table. As compared to rubber mat, 5.6 °C rise on water temperature and 14.5 °C reduction on table temperature are accomplished by CP mat, demonstrating the great application prospect in the field of thermal buffering materials and laying a foundation for the large-scale application of PCMs.

Additive manufacturing of shape-stabilized composite phase change materials via ultraviolet curing

Material composition can ensure structural strength and avoid leakage of phase change materials. Here, a novel preparation method was present for shape-stabilized composite phase change materials of photosensitive resin and polyethylene glycol via UV curing, in which the resin was cured into the supporting frame and used to encapsulate polyethylene glycol. The thermal and mechanical performance of the samples composited by the proposed method were measured. The sample with PEG of 50 % resin mass fraction melted at 33.16 °C with a latent heat of 11.30 J/g. The stress-strain curves exhibited that the samples produced plastic deformation and ultimate compression strength (σb) and yield strength (σs) increased with the increase in the resin mass fraction. With increasing monolayer curing depth and exposure time during printing, σb first increased and then decreased. In this work, the optimal monolayer exposure time was 13.5 s, and the monolayer curing depth was 40 µm for the sample with a 50 % resin mass fraction.

High thermal conductive shape-stabilized phase change materials based on water-borne polyurethane/boron nitride aerogel

Phase change materials (PCMs) applied in energy storage and temperature control system are important for energy conservation and environmental protection. In this work, structure-adjustable water-borne polyurethane (WPU)/boron nitride (BN) aerogels were synthesized via directional freeze-drying method, and used as supporting scaffolds to confine paraffin wax (PW) and obtain composite phase change materials. The three-dimensional (3D) porous thermal conductivity network of BN was derived by the in-situ ice crystal mound in aerogel, which endows the PW/WPU/BN composite PCM-2.5 with high thermal conductivity (0.96 W m−1 K−1) and high energy storage density (140.04 J/g). Shape-stabilized PCMs with high thermal conductivity and excellent electrical insulation prepared by the simple method have great potential for the thermal management of electronic products.

Preparation and characterisation of sodium nitrate/stone-sawing mud shape-stabilized phase change materials for medium-high temperature thermal energy storage

Stone-sawing mud (SSM) is a solid waste generated during stone processing, with a presently estimated global production of 30 million m3. SSM is usually disposed of in the stone yard, occupying a large area of land and causing serious dust and water pollution. In this study, a novel shape-stabilized composite phase change material (ss-CPCM) containing SSM loaded with NaNO3 was prepared using the mixing and sintering method for medium-high temperature thermal energy-storage applications. The ss-CPCM retained 50 wt% NaNO3 without leakage. X-ray diffraction analysis showed good chemical compatibility between the SSM and NaNO3, and microscopy analysis demonstrated that NaNO3 was uniformly distributed within the SSM structure. The compressive strength of the ss-CPCM reached 23.14 MPa. The melting temperature and latent heat of the ss-CPCM are approximately 306.78 °C and 79.86 J·g−1, respectively, and no significant variations are observed after thermal cycling. The energy storage density of the material is 444.86 J·g−1 in the range of 50–400 °C, and its thermal conductivity is 0.696 W (m·K)−1. Furthermore, the thermal conductivity of the composites reached 2.421 W (m·K)−1 with the addition of 10 wt% of graphite. Hence, the NaNO3/SSM ss-CPCM is considered a promising material for waste heat recovery, thereby providing a valorisation method for the resource utilisation of SSM.

Clarification of the Supercooling and Heat Storage Efficiency Calculation Formula for Shape-Stabilized Phase Change Materials.

It is essential to determine the heat storage efficiency of shape-stabilized phase change materials (ss-PCMs). In two published articles, the formula for heat storage efficiency is presented using two distinct equations. Using the two equations, the calculated values for heat storage efficiency revealed significant discrepancies. The outcomes cannot be compared. The evaluation of heat storage efficiency has a substantial impact on the practical application of PCMs and serves as a performance benchmark for the PCM samples that have been tested. In this paper, the correct equations for calculating the efficiency of heat storage are presented. Furthermore, it is essential to note that the methods used to calculate the supercooling value are not straightforward. Consequently, this paper clarified the correct formula and/or method for determining the supercooling value and heat storage efficiency of ss-PCMs.

Anisotropic porous skeleton for efficient thermal energy storage and enhanced heat transfer: Experiments and numerical models

The pore structure of the porous skeleton plays a decisive role in the performance of the shape-stabilized phase change materials (ss-PCM). For example, in the adsorption rate of the phase change material and the effect of heat transfer enhancement. In this paper, the anisotropic porous skeleton was prepared via ice-template method and used as supports for phase change materials (PCM). The coherent hierarchical pore structure not only made the adsorption of PCM more effectively, increasing the pore utilization rate to 98.22 % and realizing a satisfactory heat storage density of 94.17 J/g (50.42 wt%), but also achieved the prominent heat transfer enhancement in axial direction. Owing to this, the prepared anisotropic ss-PCM showed excellent thermal management capability, which reduced the chip surface temperature from 160 °C to 80 °C; even 20 °C lower than using aluminum heat sinks. The numerical model via Lattice-Boltzmann method was used to investigate the mechanism of enhanced heat transfer. It showed that the anisotropic porous skeleton strongly enhanced the convection in the melting process of PCM by reducing the tortuosity, which greatly compensated for the low thermal conductivity of the skeleton and fully demonstrated the importance of the design of the pore structures. Therefore, it provided useful suggestions for the preparation of high quality ss-PCMs.

Shape-Stabilized Phase Change Materials of Stearic Acid Confined in Fumed Silica

Stearic acid (C18OOH) as a phase change material (PCM) possesses high potential for thermal energy storage applications. To prevent leakage of C18OOH in liquid state, a porous support is required to form shape-stabilized PCMs (SSPCMs) by impregnating C18OOH into the porous support. The SSPCMs were carefully investigated to demonstrate how the nanoconfinement in the porous support affects their thermal performance. In this work, a series of stearic acid/fumed silica (C18OOH/FS) SSPCMs with various C18OOH contents was fabricated, and their thermal characteristics were extensively examined. The FS provided an adequate inner void to confine up to 70 wt % C18OOH owing to its high porosity (88%). Meanwhile, C18OOH was well confined within the FS pore structure owing to hydrogen-bonding (H-bonding) interactions, surface tension, and capillary force. The SSPCM with an optimal content of 70 wt % C18OOH exhibited the melting and crystallization temperatures of 66.0 and 63.1 °C, respectively, which are slightly lower than those of pure C18OOH owing to the confinement effects. It also possessed good leakage resistance, a high crystallization fraction of 93.1%, and high thermal energy storage (TES) capacity of 146.3 J/g. Additionally, the composite exhibited excellent thermal cycling stability after 1000 melting/crystallization cycles. The thermal conductivity of the 70 wt % C18OOH/FS SSPCM (0.150 W/m·K) was lower than that of pure C18OOH (0.224 W/m·K), indicating that the SSPCM retards heat transfer during the heat charging and discharging processes. Generally, C18OOH/FS SSPCMs exhibit excellent TES capacity, high thermal cycling stability, and heat transfer retardation properties, with a facile preparation process at a low cost. Therefore, they are ready for industrial-scale applications in the fields of thermal insulation and thermal protection.

Fabricated Polyethylene glycol/ hydroxylated carbon nanotubes shape-stabilized phase change materials with improving thermal conductivity

To improve the thermal conductivity and shape-stability of polyethylene glycol (PEG), a series of novel PEG based phase change materials (PPCs) have been successfully synthesized by grafted polymerization, in which PEG as the phase change ingredient, hydroxylated carbon nanotubes (HCNTs) as the thermal conductive filler and γ-glycidyl ether oxypropyl trimethoxysilane (KH-560) as the bridge. The results indicate that the HCNTs does not agglomerate during phase transition since the HCNTs are introduced into the molecular structure of the PEG by KH-560. So, it can maintain good thermal conductivity and thermal stability after 100 thermal cycles. As the result, the latent heat of phase change for PPC is in the range of 119.9 ∼ 127.9 J·g−1, and the phase transformation temperature is in the field of 311.7 ∼ 330.9 K. The thermal conductivity of the PPC is 0.7081 W·m−1·K−1 as adding 9.0 wt% HCNTs, which is 2.2 times that of pure PEG. The temperature regulation experimental results reveal that PPC5 has an obvious buffering effect in the temperature ranges of 311.7 ∼ 330.9 K during the heating and cooling process. It shows good potential for application in thermal energy storage and temperature regulation.

Experimental study on preparation and thermal storage properties of expanded graphite/paraffin wax as a shape-stabilized phase change material

As an organic phase change material (PCM), paraffin wax (PW) has the problem of low thermal conductivity. It has been applied in the heat storage engineering. To improve the thermal performance of paraffin wax adopted in heat storage, it was mixed with expanded graphite (EG) via melt-blending method. The EG/PW was compressed into a shape-stabilized phase change material (SSPCM). The DSC curve was measured and the relationship between pressure and density of the SSPCM was tested. The SSPCM was vacuum encapsulated in aluminum bags and placed in a packed bed heat storage system to measure the thermal storage properties. The results showed that the thermal conductivity of the SSPCM was 7.06 W/(m °C), and the maximum density of the SSPCM was 1.07 g/cm 3. The average thermal efficiency of the heat storage system was 95.3%. The number of heat storage units was positively correlated with the heat release power. This study explored the feasibility of a simplified heat storage system.

IR-stealth-gel based on low IR emissivity aluminum powder and high latent heat shape stabilized phase change material

• The latent heat is 140.6 J/g, and only 24.4 J/g loss compared to raw material. • IR emissivity in the 3–5 µm and 8–14 µm bands is 0.257 and 0.408, respectively. • Under heating at 100 °C, the radiation temperature is below 60 °C for 120 min. In this work, Infrared (IR)-stealth-gel based on low IR emissivity and high latent heat of melting shape-stabilized phase change material (PCM) were developed. The relevant tests showed that the IR-stealth-gel has good shape stability and its backbone material can load up to 322.15% of PCM. Further tests show that the latent heat of melting is 140.6 J/g, which has good heat absorption ability and only 24.4 J/g loss compared to pure PCM. Meanwhile, its average emissivity in the 3–5 µm and 8–14 µm bands is only 0.257 and 0.408, respectively. IR thermography shows that its radiation temperature can maintain below 60 °C in 120 min under continuous heating at 100 °C, reflecting a very good IR stealth effect.

Novel properties of stearic acid / MXene - Graphene oxide shape - Stabilized phase change material: Ascended phase transition temperature and hierarchical transition

To develop novel materials with active photothermal conversion and latent heat storage properties, the shape-stabilized phase change material was fabricated via self-assembly method employing stearic acid (SA) as heat storage medium, graphene oxide and MXene as hybrid matrix (GM) and photothermal conversion agent. The morphology, structure, properties and phase transition behavior were investigated by various techniques. The results shown that SA/GM has sandwich-like structure and SA is confined between matrix nanosheets in multiple crystallographic forms. Due to the superposition of host-guest interaction and intrinsic van der Waals force between nanosheets, the primary melting temperature of SA/GM climbs 27.7 °C approximately than that of SA, as well as the total latent heat decreases by 28.55%–30.85% compared with theoretical values. The confinement effect increases the crystallization activation energy of confined SA, and leads to the formation of non-phase transition layer, transitional confinement layer and proximate bulk layer. However, the above observations are applicable to fatty acids/GM, only the suppressed latent heat occurs in all GM-matrix composites. In addition, the SA/GM7 has excellent photothermal conversion efficiency, as high as 93.34%, and gradient latent heat storage capacity, 45.96 J g −1 at 74.43 °C and 91.19 J g −1 at 92.37 °C. Stearic acid/Mxene - graphene oxide (SA/GM) composite was fabricated though self-assembly method. The confined SA exists in multiple crystal forms and shows abnormal properties, including the ascended phase transition temperature, hierarchical phase transition and the formation of confined layer. The SA/GM presents excellent photothermal conversion efficiency and gradient heat storage/release performances. • SA/GM was self-assembled by the inherent forces of matrix nanosheets and host-guest interaction. • The main melting temperature of SA/GM increased by 27.7 °C approximately. • Non-phase change layer, transitional confinement layer and proximate bulk layer exist in SA/GM. • Ascended transition temperature and stratified phase transition are applicable to fatty acids/GM.

Double-skeleton based shape-stabilized phase change materials with excellent solar-thermal energy conversion and shape memory performance

Polyethylene glycol (PEG) is widely used for thermal energy storage, but its practical application is limited by its strong rigidity, poor shape stability. In this work, a kind of melamine foam/graphene aerogel (MF/GA) double-skeleton was prepared, then PEG was stably encapsulated into MF/GA by vacuum melt impregnation method to obtain MF/GA@PEG shape-stabilized phase change materials (SSPCMs). Structural characterization and performance tests were performed on MF/GA@PEG SSPCMs, including morphological observation, latent heat, thermal conductivity, shape stability, and photothermal conversion capability. The experimental results show that the MF/GA@PEG SSPCMs show high energy storage density (168.3 J/g), good shape stability, shape memory performance and excellent solar-thermal energy conversion capability with a high efficiency of 84.8%, the thermal conductivity of MF/GA@PEG SSPCMs is 161% higher than that of pure PEG. The MF/GA@PEG SSPCMs with comprehensive properties have promising application in solar-thermal energy storage and thermal management in microelectronic devices.