Thermal Charging Optimization of a Wavy-Shaped Nano-Enhanced Thermal Storage Unit

A wavy shape was used to enhance the thermal heat transfer in a shell-tube latent heat thermal energy storage (LHTES) unit. The thermal storage unit was filled with CuO–coconut oil nano-enhanced phase change material (NePCM). The enthalpy-porosity approach was employed to model the phase change heat transfer in the presence of natural convection effects in the molten NePCM. The finite element method was applied to integrate the governing equations for fluid motion and phase change heat transfer. The impact of wave amplitude and wave number of the heated tube, as well as the volume concertation of nanoparticles on the full-charging time of the LHTES unit, was addressed. The Taguchi optimization method was used to find an optimum design of the LHTES unit. The results showed that an increase in the volume fraction of nanoparticles reduces the charging time. Moreover, the waviness of the tube resists the natural convection flow circulation in the phase change domain and could increase the charging time.     

Effects of Total Thermal Balance on the Thermal Energy Absorbed or Released by a High-Temperature Phase Change Material

Front tracking and enthalpy methods used to study phase change processes are based on a local thermal energy balance at the liquid–solid interface where mass accommodation methods are also used to account for the density change during the phase transition. Recently, it has been shown that a local thermal balance at the interface does not reproduce the thermodynamic equilibrium in adiabatic systems. Total thermal balance through the entire liquid–solid system can predict the correct thermodynamic equilibrium values of melted (solidified) mass, system size, and interface position. In this work, total thermal balance is applied to systems with isothermal–adiabatic boundary conditions to estimate the sensible and latent heat stored (released) by KNO3 and KNO3/NaNO3 salts which are used as high-temperature phase change materials. Relative percent differences between the solutions obtained with a local thermal balance at the interface and a total thermal balance for the thermal energy absorbed or released by high-temperature phase change materials are obtained. According to the total thermal balance proposed, a correction to the liquid–solid interface dynamics is introduced, which accounts for an extra amount of energy absorbed or released during the phase transition. It is shown that melting or solidification rates are modified by using a total thermal balance through the entire system. Finally, the numerical and semi-analytical methods illustrate that volume changes and the fraction of melted (solidified) solid (liquid) estimated through a local thermal balance at the interface are not invariant in adiabatic systems. The invariance of numerical and semi-analytical solutions in adiabatic systems is significantly improved through the proposed model.     

复合相变储能材料的制备及应用研究进展

系统概述了复合相变储能材料的制备方法及其研究进展。特别介绍了固-液复合相变储能材料的制备方法,如熔融浸渍混合法、溶胶-凝胶法、静电纺丝法、真空渗入法和超声波法等。并结合实例探讨了复合相变储能材料在太阳能利用、建筑节能和纺织行业等领域的应用,在此基础上对其研究方向进行了展望。     

一种致密的相变储能微胶囊的制备与表征

制备了以聚脲为第一壁材、苯乙烯-二乙烯苯为第二壁材,以相变点在16℃左右的石蜡为芯材的相变储能微胶囊。采用红外光谱、差示扫描量热分析、热重分析测试技术表征了制备的相变储能微胶囊的结构组成以及热性能;采用溶剂淋洗法研究了影响包覆率的因素。结果表明,相变储热微胶囊是复合相变材料,微胶囊的热稳定性好,致密性优良;通过对水油比、乳化剂及苯乙烯-二乙烯苯用量等各因素对微胶囊包覆率影响的讨论,得出在水与油质量比3.2,乳化剂相对于水的质量分数为2%时,加入苯乙烯与二乙烯苯质量比为10∶1混合液的质量分数为6.0%时,其包覆率达81.14%;制备的微胶囊能耐较高温度,在150℃以下无质量损失,且微胶囊储热能力高达80J/g。同时发现,储热能力与芯壁比有关,比值越大储热潜能越高。     

Phase-Transition Thermal Charging of a Channel-Shape Thermal Energy Storage Unit: Taguchi Optimization Approach and Copper Foam Inserts

Thermal energy storage is a technique that has the potential to contribute to future energy grids to reduce fluctuations in supply from renewable energy sources. The principle of energy storage is to drive an endothermic phase change when excess energy is available and to allow the phase change to reverse and release heat when energy demand exceeds supply. Unwanted charge leakage and low heat transfer rates can limit the effectiveness of the units, but both of these problems can be mitigated by incorporating a metal foam into the design of the storage unit. This study demonstrates the benefits of adding copper foam into a thermal energy storage unit based on capric acid enhanced by copper nanoparticles. The volume fraction of nanoparticles and the location and porosity of the foam were optimized using the Taguchi approach to minimize the charge leakage expected from simulations. Placing the foam layer at the bottom of the unit with the maximum possible height and minimum porosity led to the lowest charge time. The optimum concentration of nanoparticles was found to be 4 vol.%, while the maximu possible concentration was 6 vol.%. The use of an optimized design of the enclosure and the optimum fraction of nanoparticles led to a predicted charging time for the unit that was approximately 58% shorter than that of the worst design. A sensitivity analysis shows that the height of the foam layer and its porosity are the dominant variables, and the location of the porous layer and volume fraction of nanoparticles are of secondary importance. Therefore, a well-designed location and size of a metal foam layer could be used to improve the charging speed of thermal energy storage units significantly. In such designs, the porosity and the placement-location of the foam should be considered more strongly than other factors.     

Thermal Energy Storage and Heat Transfer of Nano-Enhanced Phase Change Material (NePCM) in a Shell and Tube Thermal Energy Storage (TES) Unit with a Partial Layer of Eccentric Copper Foam

Thermal energy storage units conventionally have the drawback of slow charging response. Thus, heat transfer enhancement techniques are required to reduce charging time. Using nanoadditives is a promising approach to enhance the heat transfer and energy storage response time of materials that store heat by undergoing a reversible phase change, so-called phase change materials. In the present study, a combination of such materials enhanced with the addition of nanometer-scale graphene oxide particles (called nano-enhanced phase change materials) and a layer of a copper foam is proposed to improve the thermal performance of a shell-and-tube latent heat thermal energy storage (LHTES) unit filled with capric acid. Both graphene oxide and copper nanoparticles were tested as the nanometer-scale additives. A geometrically nonuniform layer of copper foam was placed over the hot tube inside the unit. The metal foam layer can improve heat transfer with an increase of the composite thermal conductivity. However, it suppressed the natural convection flows and could reduce heat transfer in the molten regions. Thus, a metal foam layer with a nonuniform shape can maximize thermal conductivity in conduction-dominant regions and minimize its adverse impacts on natural convection flows. The heat transfer was modeled using partial differential equations for conservations of momentum and heat. The finite element method was used to solve the partial differential equations. A backward differential formula was used to control the accuracy and convergence of the solution automatically. Mesh adaptation was applied to increase the mesh resolution at the interface between phases and improve the quality and stability of the solution. The impact of the eccentricity and porosity of the metal foam layer and the volume fraction of nanoparticles on the energy storage and the thermal performance of the LHTES unit was addressed. The layer of the metal foam notably improves the response time of the LHTES unit, and a 10% eccentricity of the porous layer toward the bottom improved the response time of the LHTES unit by 50%. The presence of nanoadditives could reduce the response time (melting time) of the LHTES unit by 12%, and copper nanoparticles were slightly better than graphene oxide particles in terms of heat transfer enhancement. The design parameters of the eccentricity, porosity, and volume fraction of nanoparticles had minimal impact on the thermal energy storage capacity of the LHTES unit, while their impact on the melting time (response time) was significant. Thus, a combination of the enhancement method could practically reduce the thermal charging time of an LHTES unit without a significant increase in its size.     

相变储能材料的研究进展

综述了相变材料的研究进展状况 ,介绍了相变材料的分类以及各类相变材料的性能、储能机理和优缺点 ,并介绍了一些新型的相变材料 ,指出了该领域中有待解决的问题 ,展望了未来相变材料的发展前景。     

Combination of Passive and Active Solar Heating with Thermal Energy Storage

This study investigated the impact of individual and combination of different sources of heating (passive solar heating, electric oil-heater, and solar air heater) in a pilot-scale building containing phase change material (PCM) for a potential reduction in energy consumption while maintaining thermal comfort. Unlike most of the recent simulations and modelling studies, this impact was tested experimentally using two identical control and test huts located at the University of Auckland. The control hut was equipped with standard gypsum boards while the test hut had gypsum boards containing PCM (PureTemp 20, PT20). The study found that combining both active and passive solar heating with a temperature-controlled electric oil heater demonstrated the ability to provide significant energy savings and maintain thermal comfort in the test hut, most notably overnight. The suggested combination was tested over different weather conditions and with different temperature constraints to maintain thermal comfort and achieve energy savings ranging from 33% to 87.5%.     

石蜡/膨胀石墨/石墨烯复合相变储热材料的制备及性能

以石蜡(PA)作为相变储热材料、 膨胀石墨(EG)作为主导热材料和支撑材料, 石墨烯气凝胶(GA)作为导热增强材料和辅支撑材料制备了PA/EG/GA复合相变材料, 研究了GA添加量对复合相变材料相变温度、 相变潜热、 导热性能以及循环稳定性的影响. 结果表明, 所制备的80%PA-17%EG-3%GA复合相变材料导热性能良好, 循环稳定性出色. 与80%PA-20%EG复合材料相比, 该材料的相变温度、 相变潜热以及循环稳定性无明显变化, 但导热系数由4.089 W/(m·K)提升到了5.336 W/(m·K), 显示出良好的应用前景.     

Characterization of Unripe and Mature Avocado Seed Oil in Different Proportions as Phase Change Materials and Simulation of Their Cooling Storage

Environmental problems have been associated with energy consumption and waste management. A solution is the development of renewable materials such as organic phase change materials. Characterization of new materials allows knowing their applications and simulations provide an idea of how they can developed. Consequently, this research is focused on the thermal and chemical characterization of five different avocado seed oils depending on the maturity stage of the seed: 100% unripe, 25% mature-75% unripe, 50% mature-50% unripe, 75% mature-25% unripe, and 100% mature. The characterization was performed by differential scanning calorimetry, Fourier transform infrared spectroscopy, and thermogravimetric analysis. The best oil for natural environments corresponded to 100% matured seed with an enthalpy of fusion of 52.93 J·g−1, and a degradation temperature between 241–545 °C. In addition, the FTIR analysis shows that unripe seed oil seems to contain more lipids than a mature one. Furthermore, a simulation with an isothermal box was conducted with the characterized oil with an initial temperature of −14 °C for the isothermal box, −27 °C for the PCM box, and an ambient temperature of 25 °C. The results show that without the PCM the temperature can reach −8 °C and with it is −12 °C after 7 h, proving its application as a cold thermal energy system.     

交联型聚氨酯固-固相变材料的相变性能及形态

聚氨酯;聚乙二醇;储能材料;固 固相变材料;相变行为     

Microencapsulation of n-Eicosane as Energy Storage Material

For heat energy storage application, polynrea microcapsules containing phase change material, n-eicosane, were synthesized by using interfacial polymerization method with toluene-2,4-diisocyanate (TDI) and diethylenetriamine(DETA) as monomers in an emulsion system. Poly(ethylene glycol)octyl-phenyl ether (OP), a nonionic surfactant,was the emulsifier for the system. The experimental result indicates that TDI was reacted with DETA in a mass ratio of 3 to 1. FF-IR spectra confirm the formation of wall material, polyurea, from the two monomers, TDI and DETA.Encapsulation efficiency of n-eicosane is about 75%. Microcapsule of n-eicosane melts at a temperature close to that of n-eicosane, while its stored heat energy varies with core material n-eicosane when wall material fixed.Thermo-gravimetric analysis shows that core material n-eicosane, micro-n-eicosane and wall material polyurea can withstand temperatures up to 130, 170 and 250℃, respectively.     

六羟基化合物为骨架的高分子固-固相变材料的合成与性能

分别使用三种含6个羟基的化合物(山梨醇、双季戊四醇和肌醇)作为分子骨架,聚乙二醇(PEG)作为相变功能链,4,4'-二苯基甲烷二异氰酸酯(MDI)为交联剂,合成了3种具有不同交联结构的新型固-固相变储能材料。通过傅里叶变换红外光谱(FT IR)、X-射线衍射(XRD)、偏光显微镜(POM)、示差扫描量热法(DSC)和热重量分析法(TG)分别对合成材料的分子结构、结晶性能、相变行为和热稳定性进行了研究。结果显示,所制备的材料在30~70℃温度范围内具有典型的固-固相变特性,其升温和冷却过程的相变焓最高可达107.5J/g和102.9J/g。此外,通过热重分析发现所合成材料具有较好的可重复使用性和热稳定性。因此,合成的新型固-固相变材料在热能储存和控温领域具有巨大的应用潜力。     

A Review on Polymeric-Based Phase Change Material for Thermo-Regulating Fabric Application

Abstract

Phase Changing Materials (PCM) portrays proficiency to liberate perceptible amount of latent heat on the course of phase transformation between liquid-solid or solid-liquid, thereby creating momentary warmth or cooling effect. PCM has been utilized in garments for introducing thermoregulating effect to diminish thermal discomfort of clothing. Assimilation of thermal energy by PCM causes delay in upsurge of microclimate temperature and results in substantial diminution of moisture release from skin thereby leading to inhibition of heat stress conditions and enhancement of thermo-physiological wearing comfort. Simultaneously, the insulating characteristic of such garment can also avert wearer from certain pivotal corollaries like hypothermia or heat syncope, keeping the individual in consolation owing to their automatic acclimatizing attribute in accordance with body and ecological temperature. As the assimilation of PCM into various textile materials have been extensively studied by researchers, an attempt has been made to explicate the recent existing literatures that have successfully integrated and implemented PCM in textile, concentrating on characteristics of PCMs integrated into fibers, and fabrics for potential industrial applications. Finally, various methodologies like coating, spinning & lamination being utilized for applying PCMs onto textiles for developing thermoregulated clothing have been discussed & concludes with challenges & future prospects.     

煤基碳泡沫/聚氨酯相变复合材料的制备及储热性能

以煤基碳泡沫(CCF)作为骨架材料来封装改性固-固相变材料聚氨酯(PU),并实现其功能化应用。使用场发射扫描电子显微镜(SEM)、透射电子显微镜(TEM)、X射线粉末衍射仪(PXRD)、傅里叶变换红外(FT-IR)光谱仪、热重分析仪(TGA)、差示扫描量热(DSC)分析仪、热导测试仪对所得到的复合材料(PU@CCF)进行结构和特性表征。结果显示,聚乙二醇(PEG-6000)与异氰酸酯(HDI)反应制备聚氨酯的最优摩尔比例为1:2,煤基碳泡沫可以很好地阻止聚氨酯从复合材料中泄露出来。相较于聚乙二醇,复合材料的导热率上升了54%,经过200次热循环,复合材料保持了良好的稳定性,而且其相变主体材料PU的过冷度降低了10℃以上。基于碳泡沫骨架良好的导电性,加载高于0.8V的低电压就可实现聚氨酯电热相变储能,在1.1V电压驱动下,其电热转换效率可达75%。该工作是目前报道的最低电压下可实现电热相变转换的复合功能材料,为实现低成本相变复合材料的制备与功能化提供重要参考。     

Confined‐Volume Effect on the Thermal Properties of Encapsulated Phase Change Materials for Thermal Energy Storage

We have encapsulated the heat exchange material, n‐docosane, into polyurethane capsules of different sizes. Decreasing the size of the capsules leads to changes of the crystallinity of phase‐change material as well as melting/crystallization temperature. The novelty of the paper includes 1) protection of the nanostructured energy‐enriched materials against environment during storage and controlled release of the encapsulated energy on demand and 2) study of the structure and surface‐to‐volume properties of the energy‐enriched materials dispersed in capsules of different sizes. The stability of energy nanomaterials, influence of capsule diameter on their energy capacity, homogeneity and operation lifetime are investigated.     

蓄热调温石蜡相变微胶囊的制备及性能

采用界面聚合法,以甲苯2,4-二异氰酸酯和哌嗪为反应单体、30号相变石蜡为芯材,制得了一种智能纺织品用蓄热调温相变微胶囊。通过红外光谱、扫描电镜、差示扫描量热仪对微胶囊的化学组成、形貌和蓄热性能进行了表征,测试了其耐热和耐溶剂性。结果表明:所得微胶囊主要为球形,表面光滑,平均粒径为10.6μm,对w=0.40的NaOH溶液、w=0.60的H2SO4溶液、无水乙醇、丙酮稳定,能被甲苯、二甲基甲酰胺、乙醚破坏。相变潜热为118 J/g,石蜡在微胶囊中的质量分数为84%。     

Improvement of Phase Change Materials (PCM) Used for Solar Process Heat Applications

The high intermittency of solar energy is still a challenge yet to be overcome. The use of thermal storage has proven to be a good option, with phase change materials (PCM) as very promising candidates. Nevertheless, PCM compounds have typically poor thermal conductivity, reducing their attractiveness for commercial uses. This paper demonstrates the viability of increasing the PCM effective thermal conductivity to industrial required values (around 4 W/m·K) by using metal wool infiltrated into the resin under vacuum conditions. To achieve this result, the authors used an inert resin, decoupling the specific PCM material selection from the enhancement effect of the metal wools. To ensure proper behavior of the metal wool under standard industrial environments at a broad range of temperatures, a set of analyses were performed at high temperatures and an inert atmosphere, presenting a thorough analysis of the obtained results.     

正十六烷聚脲微胶囊化相变材料

用界面聚合法，合成了直径大约2.5 μm可用于热能储存含相变材料的聚脲包覆微胶囊.在含乳化剂的水溶液中，将溶有芯材正十六烷的有机相乳化成微米级油性液滴，随后加入的水溶性单体二胺与甲苯2,4-二异氰酸酯在胶束界面相互反应形成囊壁.分别用乙烯二胺，1,6-己二胺和它们的混合物作为水溶性单体进行了研究.并用红外光谱和热分析分别考察了不同胺类对微胶囊化学结构和热性质的影响.红外谱图显示合成了聚脲微胶囊，热重曲线表明含正十六烷的聚脲微胶囊能够耐受大约300 ℃高温，差示扫描量热测试表明所有样品均具有合适的相转变热，冷热循环实验揭示微胶囊能够维持储热容量不衰减.研究表明微胶囊化的正十六烷作为相变储热材料具有良好的应用前景.     

Development of Novel Polypropylene Syntactic Foams Containing Paraffin Microcapsules for Thermal Energy Storage Applications

polypropylene (PP) syntactic foams (SFs) containing hollow glass microspheres (HGMs) possess low density and elevated mechanical properties, which can be tuned according to the specific application. A possible way to improve their multifunctionality could be the incorporation of organic Phase Change Materials (PCMs), widely used for thermal energy storage (TES) applications. In the present work, a PCM constituted by encapsulated paraffin, having a melting temperature of 57 °C, was embedded in a compatibilized polypropylene SF by melt compounding and hot pressing at different relative amounts. The rheological, morphological, thermal, and mechanical properties of the prepared materials were systematically investigated. Rheological properties in the molten state were strongly affected by the introduction of both PCMs and HGMs. As expected, the introduction of HGMs reduced both the foam density and thermal conductivity, while the enthalpy of fusion (representing the TES capability) was proportional to the PCM concentration. The mechanical properties of these foams were improved by the incorporation of HGMs, while they were reduced by addition of PCMs. Therefore, the combination of PCMs and HGMs in a PP matrix generated multifunctional materials with tunable thermo-mechanical properties, with a wide range of applications in the automotive, oil, textile, electronics, and aerospace fields.