Thermal performance of phase change material energy storage floor for active solar water-heating system

The conventional active solar water-heating floor system contains a big water tank to store energy in the day time for heating at night, which takes much building space and is very heavy. In order to reduce the water tank volume or even cancel the tank, a novel structure of an integrated water pipe floor heating system using shape-stabilized phase change materials (SSPCM) for thermal energy storage was developed and experimentally studied in this paper. The thermal performances of the floors with and without the SSPCM were compared under the intermittent heating condition. The results show that the Energy Storage Ratio (ESR) of the SSPCM floor is much higher than that of the non-SSPCM floor; the SSPCM floor heating system can provide stable heat flux and prevent a large attenuation of the floor surface temperature. Also, the SSPCM floor heating system dampens the indoor temperature swing by about 50% and increases the minimum indoor air temperature by 2°C–3°C under experimental conditions. The SSPCM floor heating system has a potential of making use of the daytime solar energy for heating at night efficiently.     

Thermal characteristics of shape-stabilized phase change material wallboard with periodical outside temperature waves

Thermal characteristics of shape-stabilized phase change material (SSPCM) wallboard with sinusoidal temperature wave on the outer surface were investigated numerically and compared with traditional building materials such as brick, foam concrete and expanded polystyrene (EPS). One-dimensional enthalpy equation under convective boundary conditions was solved using fully implicit finite-difference scheme. The simulation results showed that the SSPCM wallboard presents distinct characteristics from other ordinary building materials. Phase transition keeping time of inner surface and decrement factor were applied to analyze the effects of PCM thermophysical properties (melting temperature, heat of fusion, phase transition zone and thermal conductivity), inner surface convective heat transfer coefficient and thickness of SSPCM wallboard. It was found that melting temperature is one important factor which influences both the phase transition keeping time and the decrement factor; for a certain outside temperature wave, there exist critical values of latent heat of fusion and thickness of SSPCM above which the phase transition keeping time or the decrement factor are scarcely influenced; thermal conductivity of PCM and inner surface convective coefficient have little effect on the phase transition keeping time but significantly influence the decrement factor; and the phase transition zone leads to small fluctuations of the original flat segment of inner surface temperature line. The results aim to be useful for the selection of SSPCMs and their applications in passive solar buildings.     

Performance of shape-stabilized phase change material wallboard with periodical outside heat flux waves

Numerical simulations were carried out to investigate the performance of shape-stabilized phase change material (SSPCM) wallboard with sinusoidal heat flux waves on the outer surface and compared with traditional building materials – brick, foam concrete and expanded polystyrene (EPS). One-dimensional enthalpy equation was solved using control volume-based implicit finite-difference scheme. Time lag (φ), decrement factor (f) and phase transition keeping time (ψ) of inner surface were applied to analyze the effects of PCM thermo-physical properties, inner surface convective heat transfer coefficient and thickness of SSPCM wallboard. The results showed that for SSPCM, there exist two flat segments within one wave length period of inner surface heat flux lines and it has larger time lag and lower decrement factor than those three ordinary building materials. It was found that melting temperature and thermal conductivity of SSPCM have little effects on φ, f and ψ, which is different from the case of temperature waves; for a certain outside heat flux wave, there exist critical values of latent heat of fusion and thickness of SSPCM above which the heat flux wave amplitude can be diminished to zero; inner surface convective heat transfer coefficient is one important factor which significantly influences the decrement factor; and the phase transition zone leads to small fluctuations of the original flat segments of inner surface heat flux line.     

Thermal performance of phase change material energy storage floor for active solar water-heating system

The conventional active solar water-heating floor system contains a big water tank to store energy in the day time for heating at night, which takes much building space and is very heavy. In order to reduce the water tank volume or even cancel the tank, a novel structure of an integrated water pipe floor heating system using shapestabilized phase change materials (SSPCM) for thermal energy storage was developed and experimentally studied in this paper. The thermal performances of the floors with and without the SSPCM were compared under the intermittent heating condition. The results show that the Energy Storage Ratio (ESR) of the SSPCM floor is much higher than that of the non-SSPCM floor; the SSPCM floor heating system can provide stable heat flux and prevent a large attenuation of the floor surface temperature. Also, the SSPCM floor heating system dampens the indoor temperature swing by about 50% and increases the minimum indoor air temperature by 2°C–3°C under experimental conditions. The SSPCM floor heating system has a potential of making use of the daytime solar energy for heating at night efficiently.     

Preparation,characterization and permeation kinetics description of calcium alginate macro-capsules containing shape-stabilize phase change materials

Macro-capsules containing shape-stabilize phase change materials (SSPCM) which have 50 wt% of n-octadecane (OD) and 50 wt% of high-density polyethylene (HDPE) were prepared by using a traditional coating pan with calcium alginate (CA) as the shell material. The surface morphologies and construction, wall permeability and the kinetic release parameters of OD in a solvent of petroleum ether along with the thermal properties of the materials were investigated using a scanning electron microscope (SEM), thermal cycles, the extraction release kinetics, and differential scanning calorimeters (DSC), respectively. The results show the wall thickness of the macro-capsules was about 30–50 μm under the experimental conditions. The surface of the SSPCM after the application of chromic acid is rough and littered with numerous, microscopic holes measuring about 3 μm in diameter. From this one may conclude that either the hydrophilicity of the SSPCM surface or the permeability of the prepared macro-capsules was altered during the process and thus differed from the unmodified samples. In addition to this, the weight loss percentage (WLP) of the macro-capsules was approximately 1.5 times in the unmodified capsules, and 3 times in the modified SSPCM. The addition of the plasticizer glycerin into the wall significantly decreased the impermeability of the macro-capsules. From the parameters of the Power exponent, there are two different release mechanisms, Fickian/quasi-Fickian diffusion and anomalous transport for modified and unmodified SSPCM.     

Numerical analysis of effect of shape-stabilized phase change material plates in a building combined with night ventilation

Effect of shape-stabilized phase change material (SSPCM) plates combined with night ventilation in summer is investigated numerically. A building in Beijing without active air-conditioning is considered for analysis, which includes SSPCM plates as inner linings of walls and the ceiling. Unsteady simulation is performed using a verified enthalpy model, with time period covering the summer season. Effects of the following factors on room air temperature are investigated: the thermophysical properties of the SSPCM, the thickness of SSPCM plate and air change per hour (ACH) at both nighttime and daytime. The results show that the SSPCM plates could decrease the daily maximum temperature by up to 2 °C due to the cool storage at night. The appropriate values for melting temperature, heat of fusion, thermal conductivity and thickness of SSPCM plates need to be considered and calculated according to the climate conditions and building structure. The ACH at night needs to be as high as possible but the ACH at daytime should be controlled.     

Energy performance of a hybrid space-cooling system in an office building using SSPCM thermal storage and night ventilation

Thermal performance of a hybrid space-cooling system with night ventilation and thermal storage using shape-stabilized phase change material (SSPCM) is investigated numerically. A south-facing room of an office building in Beijing is analyzed, which includes SSPCM plates as the inner linings of walls and the ceiling. Natural cool energy is charged to SSPCM plates by night ventilation with air change per hour (ACH) of 40 h⁻¹ and is discharged to room environment during daytime. Additional cool-supply is provided by an active system during office hours (8:00-18:00) necessary to keep the maximum indoor air temperature below 28 °C. Unsteady simulation is carried out using a verified enthalpy model, with a time period covering the whole summer season. The results indicate that the thermal-storage effect of SSPCM plates combined with night ventilation could improve the indoor thermal-comfort level and save 76% of daytime cooling energy consumption (compared with the case without SSPCM and night ventilation) in summer in Beijing. The electrical COPs of night ventilation (the reduced cooling energy divided by fan power) are 7.5 and 6.5 for cases with and without SSPCM, respectively.     

Preparation of macro-capsules containing shape-stabilized phase change materials and description of permeation kinetics of its wall

Through in situ polymerization, a kind of macro-capsule was prepared by using silica gel as the shell material and shape-stabilized phase change materials (SSPCM) containing 50 wt% of n-octadecane (OD) of high density polyethylene (HDPE) as the core. The surface and its construction of capsules, the permeability of the capsule wall and the release kinetics parameters of the OD in the system of petroleum ether were experimental investigated by a scanning electron microscope (SEM), thermal cycles and the extraction release kinetics. The results showed that the wall thickness of the macro-capsules was about 20–50 μm under the experimental conditions. The SSPCM surface, modified with chromic acid, is rough, and there are many tiny holes about 3 μm in diameter in it. For these reasons, either the hydrophilicity of the SSPCM surface or the cohesion between the core and the wall have been greatly improved, and then, the weight loss percentage (WLP) of the macro-capsules is decreased by about 1.5 and 2.5 times relative to that of un-modified and modified SSPCM, respectively. Additionally, after the macro-capsules were re-sprayed by using a calcium chloride solution, its wall was more compact. From the fitting parameters of the power exponent, there were two different release mechanisms, quasi-Fickian diffusion and anomalous transport for the un-encapsulated SSPCM and the macro-capsules, respectively.     

Thermal analysis of a direct-gain room with shape-stabilized PCM plates

The thermal performance of a south-facing direct-gain room with shape-stabilized phase change material (SSPCM) plates has been analysed using an enthalpy model. Effects of the following factors on room air temperature are investigated: the thermophysical properties of the SSPCM (melting temperature, heat of fusion and thermal conductivity), inner surface convective heat transfer coefficient, location and thickness of the SSPCM plate, wall structure (external thermal insulation and wallboard material) etc. The results show that: (1) for the present conditions, the optimal melting temperature is about 20 °C and the heat of fusion should not be less than 90 kJ kg⁻¹; (2) it is the inner surface convection, rather than the internal conduction resistance of SSPCM, that limits the latent thermal storage; (3) the effect of PCM plates located at the inner surface of interior wall is superior to that of exterior wall (the south wall); (4) external thermal insulation of the exterior wall obviously influences the operating effect and period of the SSPCM plates and the indoor temperature in winter; (5) the SSPCM plates create a heavyweight response to lightweight constructions with an increase of the minimum room temperature at night by up to 3 °C for the case studied; (6) the SSPCM plates really absorb and store the solar energy during the daytime and discharge it later and improve the indoor thermal comfort degree at nighttime.     

Preparation and properties of solid-solid phase change materials based on polyethylene glycol and EPDM cross-linked resin containing cyclodextrin cavity

β-cyclodextrin with vinyl group (β-CD-MA) was synthesized from β-cyclodextrin (β-CD) and glycidyl methacrylate and identified by FT-IR and ¹H-NMR spectroscopy analysis. A novel adsorption resin containing cyclodextrin cavity moieties was prepared by suspension polymerization technique with ethylene-propylene-diene (EPDM), styrene (St) and β-CD-MA as comonomers in this study. The adsorption resin was used as a solid skeleton for adsorption of phase change material such as polyethylene glycol (PEG) to prepare solid-solid phase change materials (SSPCMs). Structure and phase change behaviors of SSPCMs were studied by FT-IR and DSC, and the phase change process of the SSPCM was observed by using reflection mode of POM. The experimental results showed that the introduction of cyclodextrin cavity structure increased the adsorption capacity of PEG and improved the phase change latent heat of SSPCMs. Otherwise, during the whole process of phase change, the surface morphology of SSPCMs could be observed and appeared bright or dark change. During the transition, the whole PCMs remained in solid state.     

A simplified dynamic model of building structures integrated with shaped-stabilized phase change materials

The use of phase change materials (PCM) to enhance the building energy performance has attracted increasing attention of researchers and practitioners over the last few years. Thermodynamic models of building structures using PCMs are essential for analyzing their impacts on building energy performance at different conditions and using different control strategies. There are few PCM models of detailed physics providing good accuracy in simulating thermodynamic behavior of building structures integrated with PCM layers. However, simplified models with acceptable accuracy and good reliability are preferable in many practical applications concerning computation speed and program size particularly when involving large buildings or models are used for online applications. A simplified physical dynamic model of building structures integrated with SSPCM (shaped-stabilized phase change material) is developed and validated in this study. The simplified physical model represents the wall by 3 resistances and 2 capacitances and the PCM layer by 4 resistances and 2 capacitances respectively while the key issue is the parameter identification of the model. The parameters of the simplified model are identified using genetic algorithm (GA) on the basis of the basic physical properties of the wall and PCM layer. Two GA-based preprocessors are developed to identify the optimal parameters (resistances and capacitances) of the model by frequency-domain regression and time-domain regression respectively. Validation results show that the simplified model can represent light walls and median walls integrated with SSPCM with good accuracy.     

An assessment of mixed type PCM-gypsum and shape-stabilized PCM plates in a building for passive solar heating

Thermal performance of two phase change material (PCM) composites, mixed type PCM-gypsum and shape-stabilized PCM plates, has been numerically evaluated in a passive solar building in Beijing with an enthalpy model. Effects of the melting temperature and phase transition zone of the PCM are analyzed and a comparison between the two types of PCM composites is performed. The results show that: (1) for the present conditions, the optimal melting temperature is about 21 °C; (2) PCM composites with a narrow phase transition zone provide better thermal performance; (3) both mixed type PCM-gypsum and shape-stabilized PCM plates effectively shave the indoor temperature swing by 46% and 56%, respectively; (4) the shape-stabilized phase change material (SSPCM) plates respond more rapidly than the mixed type PCM-gypsum and prove to be thermally more effective in terms of utilizing the latent heat.     

Recent progress about expanded graphite matrix composite phase change material for energy storage

膨胀石墨基复合相变材料具有导热系数高,储能密度大以及相变过程无液体泄漏等优点,是近年来储能科学领域的研究热点.本文探讨了应用于储热系统的相变材料的性能及分类,并对膨胀石墨及其复合相变材料的制备方法进行了简要分析,最后综述了石蜡类,脂肪酸类,共晶混合物类,聚乙二醇以及乙酰胺等膨胀石墨基复合相变材料的国内外研究进展.     

Recyclable solid-solid phase-change materials cross-linked by reversible oxime?carbamate bonds for solar energy storage

Polymeric solid-solid phase-change materials (SSPCMs) possessing excellent shape stability and adaptability are able to store renewable thermal energy in an economically feasible and environmentally friendly way. Integration of chemical cross-links colorless and recyclability in a single SSPCM is challenging and interesting at present. Herein, the oxime carbamate bond was firstly introduced in chemically cross-linked polyurethanes to prepare recyclable SSPCMs, in which finely dispersed PEG segments function as phase-change components as well as polymeric skeletons. The excellent thermal energy storage capacity (with the enthalpy reached up to 101 J/g) and remarkable thermal stability of the synthesized SSPCMs had been confirmed. Moreover, the as-prepared SSPCMs can be recycled and reprocessed by simple hot pressing because of the reversibility of oxime carbamate bond. This SSPCM combines good thermal storage capacity and appealing recyclability, exhibiting sustained usability for energy storage.     

Application of energy storage enclosure with phase change materials in building energy saving

相变材料具有高效的能量储存功能。相变材料与建材基体结合,制成一种具有储热功能的围护结构。该围护结构可发挥相变储热功能,降低建筑室内温度波动,增强建筑热舒适度,能够更加有效地减少建筑物运行能耗,从而实现建筑节能。本文对当前现有相变材料的优缺点、相变材料在不同围护结构中的应用、相变储能围护结构对室内热环境的影响及对建筑运行能耗的影响、相变储能围护结构应用的经济性等方面进行了分析,提出相变储能围护结构在工程应用中所存在的不足及其发展前景。     

Progress in hydrated salt based composite phase change materials

水合盐相变储能材料具有相变温度适中、导热系数大、潜热值高、价格低廉等优点,因而具有广阔的使用前景。然而,过冷、相分离、循环稳定性差等诸多问题限制了水合盐的实际应用。许多学者将水合盐与其它材料结合,构成复合相变材料,成功地解决了以上问题。前人对水合盐复合相变材料的研究以解决水合盐在使用过程中的上述问题居多。然而近年来,有研究者制备复合相变材料以改善水合盐的热物性,如相变温度、导热系数、潜热值等,取得了一定成果,但这一研究思路仍需进行进一步探索。文章将制备水合盐复合相变材料的目的作为线索,总结了水合盐复合相变材料的研究思路,详细介绍了国内外相关的研究工作和研究成果,并指出了今后水合盐复合相变材料的研究重点。     

Synthesis of solid–solid phase change material for thermal energy storage by crosslinking of polyethylene glycol with poly (glycidyl methacrylate)

Polyethylene glycol (PEG10000)/poly (glycidyl methacrylate) (PGMA) crosslinked copolymer as a novel solid–solid phase change material (SSPCM) was successfully synthesized through the ring-opening crosslinking reaction of end-carboxyl groups in carboxyl polyethylene glycol (CPEG) and epoxy groups in PGMA. Fourier transform infrared spectroscopy (FT-IR), polarizing optical microscopy (POM), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and thermogravimetry (TG) were employed to study the chemical structure, crystalline properties, phase transition behaviors and the thermal stability of the copolymer, respectively. The results from WAXD patterns and POM images show that the crystalline form of the copolymer is similar with that of pure PEG, and the PEG soft segment phase transition between crystalline and amorphous states results in heat storage and release of the copolymer. Due to the crosslinking network restricted the free movement of the soft segments, at temperature above the PEG phase melting transition, the copolymer was still solid. The DSC results indicate that the copolymer imparts balanced and reversible phase change behaviors at the temperature range of 25–60 °C, and it has high latent heat storage capacity of more than 70 J/g. The TG results suggest that the copolymer had a much broader applicable temperature range compared with pure PEG.     

相变储能建筑材料的研究进展

相变储能建筑材料是相变材料与建材基体复合制备的一种新型储能建筑材料。本文分析了相变材料的筛选和改进方法及其封装技术的研究现状,介绍了相变材料与建材基体复合工艺,系统阐述了相变储能建筑材料的作用机理和应用现状,并指出了相变储能建筑材料在实际应用中存在的一些问题,最后展望了相变储能建筑材料的发展前景。     

Performance of a hybrid heating system with thermal storage using shape-stabilized phase-change material plates

Performance of a hybrid heating-system, combined with thermal storage using shape-stabilized phase-change material (SSPCM) plates, is investigated numerically. A direct gain passive solar house in Beijing is considered, which includes SSPCM plates as the inner linings of walls and the ceiling. Unsteady simulation is performed using a verified enthalpy model, with a time period covering the winter heating-season. Additional heat supply is employed during load hours at late night and early morning (23:00–07:00 in Beijing) or during the whole day necessary to keep the minimum indoor air temperature above 18 °C. The results indicate the thermal storage effect of SSPCM plates, which improves the indoor thermal comfort level and saves about 47% of normal-and-peak-hour energy use and 12% of total energy consumption in winter in Beijing.     

硬脂酸丁酯/多孔石墨定形相变材料的实验研究

本文利用多孔石墨的毛细管作用吸附硬脂酸丁酯制成了一种定形相变材料,利用DSC研究了硬脂酸丁酯质量含量不同的定形相变材料的相变温度、相变潜热和热稳定性,得出硬脂酸丁酯含量的临界值。研究结果表明:硬脂酸丁酯和纳米多孔石墨形成的定形相变材料相变温度合适、相变潜热较大、热稳定性好,是适合于在建筑墙体中使用的相变材料。