相变蓄热型空气源热泵系统的模拟

为提高空气源热泵系统的低温适应性,将相变蓄热技术与空气源热泵结合,实现白天蓄热、夜晚放热的运行模式。本文通过模拟研究了相变蓄热型空气源热泵系统的蓄放热特性和应用多级相变材料对系统的影响。结果表明：系统蓄热量随时间线性增大,380 min时达到18.94 kW·h,平均COP为2.51;系统放热量先随时间线性增大,随后增大速率有所减缓,180 min时达到13.58 kW·h。放热过程进口水温为35℃时,系统经过6次蓄放热循环后达到稳定运行状态,蓄放热效率为99.06%。相比单级PCM,应用三级PCM的系统蓄热时间缩短9.60%,COP提高3.97%,总■效率提高4.84%,其中降低过冷区PCM熔点对提高系统性能起到关键作用。     

Parametric analysis of domestic refrigerators using PCM heat exchanger

In the present study, numerical simulation of refrigeration cycle incorporated with a PCM heat exchanger is carried out. To this end, the refrigeration cycle without PCM has been simulated and then, the performance coefficients of the refrigerator in either with and without PCM are evaluated. The PCM heat exchanger is located in the refrigeration cycle, at a location after the condenser and before the expansion valve. The utilised PCM is N-Octadecane with fusion temperature of 27.5 °C. The simulation of heat exchanger is based on computational fluid dynamics (CFD) in which the flow inside the pipe is considered one-dimensional in the axial extension and PCM surrounding it, is considered two dimensional. Numerical simulation is carried out using MATLAB software. Simulation results show that utilizing PCM in refrigeration cycle of a refrigerator causes an improvement in the convection procedure and results a 9.58% increase in performance coefficient of refrigerator.     

Ice-cream storage energy efficiency with model predictive control of a refrigeration system coupled to a PCM tank

A model predictive controller (MPC) was developed in order to optimise energy management of an ice-cream warehouse refrigeration system coupled to a phase-change material (PCM) tank. The controller's internal model was based on a steady-state refrigeration cycle model and energy balances on the PCM tank and warehouse. Considering crystal growth in ice-cream, the quality evolution was based on Ostwald ripening equation. The product temperature field was determined by a 1-D finite difference equation. An original PCM model was developed for better performance of Levenberg–Marquardt's optimisation algorithm which was modified in order to take into account control variables' bounds. Simulation of the ice-cream storage for a period of 90 days lead to good results on the optimised control sequence with efficient energy management thanks to the PCM tank. Ice crystals size remained below the defined target: 26 μm. This study showed the great potential of MPC to reduce energy consumption and guarantee food quality.     

A methodological concept for phase change material selection based on multiple criteria decision analysis with and without fuzzy environment

Selection of proper phase change material (PCM) plays an important role towards the development of a latent heat thermal energy storage system. Selection of the phase change material is a difficult and restrained task due to the immense number of different available materials having different characteristics. One has to select such PCM which will give the desired thermal performance at minimum cost. This study deals with two Multiple Attribute Decision-Making (MADM) methods to solve PCM selection problem. These two methods are technique for order preference by similarity to ideal solution (TOPSIS) method and fuzzy TOPSIS method that uses linguistic variable presentation and fuzzy operation. Both the methods use an analytic hierarchy process (AHP) method to determine weights of the criteria. TOPSIS and fuzzy TOPSIS methods are used to obtain final ranking. A problem to evaluate the best choice of PCM used in solar domestic hot water system is considered here to demonstrate the effectiveness and feasibility of the proposed model. Empirical results showed that the proposed methods are viable approaches in solving PCM selection problem. TOPSIS is suitable for the use of precise performance ratings while the fuzzy TOPSIS is a preferred technique when the performance ratings are vague and inaccurate.     

相变材料在夏热冬冷地区建筑围护结构中应用的性能研究

相变材料(Phase change material,PCM)在建筑围护结构中的应用是一种改善室内热环境和居住舒适度的有效方法。为探索相变材料在夏热冬冷地区应用的节能潜力,对比研究了不同相变材料应用方法的节能效果,以能源使用强度(Energy use intensity,EUI)评价不同应用方法的建筑性能,旨在找出最优的应用方法。所用的相变材料以高密度聚乙烯球封装,并嵌入在XPS保温板中,形成XPSPCM板。研究结果表明:在制冷季,将XPSPCM板安装在建筑物墙体内表面时的EUI比其装在建筑物墙体外表面时降低了0.27~0.66kWh/m2,采暖季的降低幅度为0.68~0.88kWh/m2。综合考虑全年工况时,当XPSPCM板安装至建筑物墙体靠近内表面时EUI值最小。以EUI雷达图对比XPSPCM板集中于建筑不同朝向的应用效果,结果显示对于熔点为25℃的相变材料,相变材料集中布置于西向外墙时时建筑能耗最低。     

Resolution enhancement in a reflection-type confocal microscope with a phase-conjugate mirror

Theoretical and experimental results for a new reflection-type confocal microscope with a phase-conjugate mirror (PCM) are presented. The microscope achieves better lateral and axial resolution than the conventional confocal microscope. The observation volume is reduced considerably (approximately 63%). Owing to the properties of the PCM, the system is self-aligning.     

膨胀石墨/石蜡复合材料的制备及热管理性能

石蜡作为相变材料(PCM),膨胀石墨(EG)为导热增强剂,制备不同EG含量的膨胀石墨/石蜡(EG/PCM)复合材料。采用瞬态热线法测量样品的导热系数;把EG/PCM应用于锂离子电池热管理,研究不同EG含量的EG/PCM热管理性能;采用ANSYS软件分析EG/PCM的导热系数对锂离子电池热管理的影响。结果表明:EG的加入大幅度提高了PCM的导热系数,EG含量≥9%时,EG/PCM的导热系数呈各向异性;锂离子电池表面温度随EG含量增加而减小,EG(12)/PCM(88)表现出优异的热管理性能;适当地提高EG/PCM的径向导热系数,有利于提高它的热管理性能。     

定形相变材料蓄热地板电采暖热性能实验研究

为减小污染和电网峰谷差,提出了一种利用定形相变材料（PCM）蓄存夜间廉价电热的地板电采暖系统,研究了此定形PCM的热性能,搭建了应用此采暖系统的实验房间,测量了系统和室内空气的温度变化,分析了此采暖方式的使用效果。结果表明,此系统的热性能较好,白天的电热负荷转移到了夜间低谷电价时段,在实行峰谷电价的地区,可以节省电费,同时使室内温度分布均匀,是一种节能舒适的采暖方式。     

混合蓄冷空调的试验研究

尝试一种高温相变材料C7与冰相混合的蓄冷方式，并对有高温相变材料凹（混合蓄冷）及无高温相变材料（冰蓄冷）两种方案进行了蓄放冷试验。介绍试验装置及其工作原理，并对试验数据进行分析。结果表明，采用高温相变蓄冷材料的方案将使系统具有节能（系统COP值大大提高）和增能（蓄冷槽蓄冷量及释冷量大大提高）的双重效果。     

Advanced multivariate cumulative sum control charts based on principal component method with application

Existing multivariate cumulative sum (MCUSUM) control charts involve entire associated variables of a process to monitor variations in the mean vector. In this study, we have offered MCUSUM control charts with principal component method (PCM). The proposed MCUSUM control charts with PCM capture the whole process variations using fewer latent variables (principal components) while preserving as much data variability as possible. To show the significance of proposed MCUSUM control charts with PCM, various performance measures are considered including average run length, extra quadratic loss, relative average run length, and performance comparison index. Furthermore, performance measures are calculated through advanced Monte Carlo simulation method to explore the behavior of proposed MCUSUM control charts and to conduct comparative analysis with existing models. Results revealed that proposed MCUSUM control charts with PCM are efficient to detect variations timely by involving smaller number of principal components instead of considering entire associated variables. Also, proposed MCUSUM control charts have the ability to accommodate the features of existing control charts, which are illustrated as the special cases. Besides, to highlight the implementation mechanism and advantages of proposed MCUSUM control charts with PCM, a real-life example from wind turbine process is included.     

Reproducibility of solidification and melting processes in a latent heat thermal storage tank

This study analyzes the reproducibility of solidification and melting tests in a tank containing 181 kg of paraffin for cold storage at around 8 °C. Firstly, an experimental campaign of 10 identical tests was carried out. The performance is practically the same in terms of PCM temperatures and thermal power, with a maximum deviation of 2% in the capacity of all tests. In a second campaign, the impact of the initial conditions was studied. The results indicate that fixing a same mean PCM temperature at the beginning of the tests is insufficient to ensure an accurate reproducibility. Depending on the heat transfer rate during the preparation tests, the capacity differed in up to 33%. In tanks with such quantities of PCM, fixing a uniform initial PCM temperature is hardly possible, thus it is important to prepare the tank with same operation conditions.     

Meshless analysis of piezoelectric devices

 The sensor and actuator properties of piezoelectric materials make them well suited for applications in a variety of microelectromechanical systems (MEMS). Simulating the response of piezoelectric devices requires solving coupled electrical and mechanical partial differential equations. In this paper, we have implemented a meshless point collocation method (PCM) to solve the governing equations. Interpolation functions are constructed from a reproducing kernel approximation, and the governing equations are discretized using a collocation approach. PCM is implemented using either a relaxation algorithm or a fully-coupled algorithm. Comparisons between the two algorithms are given. To demonstrate the performance of PCM, the behavior of two static single-layer problems and a piezoelectric bimorph have been modeled. The bimorph analysis is extended to model a prototype MEMS device. Received: 6 June 2000     

Performance study on air-cooled household refrigerator with cold storage phase change materials

The air-cooled frost-free refrigerator has become the main power consumer among household appliances. This study presents a new air-cooled household refrigerator that uses PCM in the fresh food and freezing chambers. Operational characteristics and performance test results are provided in the conditions of off-peak refrigeration with PCM and conventional refrigeration with/without PCM. Results show that refrigeration off-peak modes resulted in the peak temperature difference between M-packs and maximum temperature increase of M-packs in the freezing chamber. The energy consumption of the refrigerator is relatively increased. The energy consumption of the refrigerator prototype with PCM under the original control mode is reduced by 18.6% and the compressor ON-time ratio is reduced by 13.6% compared with the refrigerator prototype without PCM. Frost-free refrigerator with PCM under original control type exhibits advantages in terms of energy consumption and food quality.     

Multi-walled carbon nanotube laden with D-Mannitol as phase change material: Characterization and experimental investigation

In this paper, multi-walled carbon nanotubes (CNT) were dispersed in D-Mannitol to prepare enhanced thermal conductive nanocomposites phase change materials (PCM) with 0.1% and 0.5% weight fraction. The PCM were tested for 100 thermal cycles and characterized by using techniques such as Differential Scanning Calorimetry (DSC), Thremogravimetric analyser (TGA) and Fourier Transform Infrared (FT-IR). The effect of adding CNT on energy storage/release performance of DM was experimentally studied. Maximum thermal conductivity enhancement was found to be ～7% and ～32% respectively for 0.1–0.5?wt.% DM-CNT composites. It was observed from the crystallization kinetics study of DM that addition of CNT resulted in lowering the crystallization of DM. However after thermal cycling, the latent heat capacities decreased yet showed a high latent heat enthalpy of 241.16?kJ/kg. Experimental results showed that the total time for complete cycle reduced by ～25.7% for 0.5?wt.% DM-CNT. The analysis of the experimental results indicate that the proposed PCM nanocomposites exhibit excellent thermal and chemical stability with enhanced heat transfer characteristics.     

超声辅助溶胶凝胶法制备硬脂酸/SiO2定形相变储能材料

在超声波的促进作用下, 以正硅酸乙酯为溶胶前驱体、 硬脂酸为相变组分, 经溶胶-凝胶过程制备了硬脂酸/SiO₂复合定形相变储能材料。为了研究硬脂酸/SiO₂复合定形相变储能材料的制备工艺, 考察了不同超声功率对材料制备过程中加强两相传质作用的辅助效果的影响以及不同种类催化剂对材料制备过程中溶胶-凝胶过程的影响, 最后确定出具有良好定形效果下材料的最大硬脂酸质量分数为60%。超声辅助酸碱复合催化法制备硬脂酸/SiO₂材料具有无需添加助溶剂和表面活性剂、 凝胶速度快、 方法简便的优点。对硬脂酸/SiO₂复合定形相变储能材料进行了FTIR结构表征和DSC、 TG热性能及热稳定性测试, 产品的相变焓值达91.46 J/g, 在低于175℃具有良好的热稳定性。      

Performance evaluation of a combined variable refrigerant volume and cool thermal energy storage system for air conditioning applications

In recent years, the growing needs for fulfilling the comfort cooling requirements with efficient refrigerating and air conditioning systems have gained impetus. In this work, the performance of a combined variable refrigerant volume (VRV) and cool thermal energy storage (CTES) air conditioning (A/C) system was experimentally investigated for summer and winter design conditions. Experimental results suggest that the VRV-CTES A/C system (combined A/C system) precisely maintained the indoor temperature at 24 °C for year-round operational conditions. The PCM being applied has exhibited good latent heat capacity (~160.81 kJ kg⁻¹) with congruent phase transition characteristics (~9.92 °C), even after 1000 repeated thermal cycles. Furthermore, the reduced cooling capacities and reduced combined power consumption were attributed to the improved performance of the combined A/C system. In total, this system would be beneficial in terms of accomplishing comfort cooling needs for conditioning applications without sacrificing energy efficiency.     

Phase change material for the thermal protection of ice cream during storage and transportation

Ice cream is a very temperature sensitive product and temperature variations during the storage and distribution steps may result in a reduction of quality. It is possible to improve the ice cream storage and transportation conditions by using an additional packaging with a low thermal diffusivity. This paper studies a phase change material (PCM) packaging and compares its performance to a polystyrene packaging configuration. The impact on temperature fluctuations and ice crystal size distribution was characterized experimentally during long term storage and temperature abuse. The results show that the use of an additional PCM packaging has a significant impact on the final quality of the product by keeping ice cream temperature stable and close to the phase change temperature of the PCM material. These results were compared with the insulation material results and discussed, showing that a material with a buffering heat capacity can be more efficient to reduce temperature fluctuations than a low conductivity material, and that the same results can be usually obtained with a much thinner layer of material.     

Performance enhancement of a household refrigerator by addition of latent heat storage

This paper studies the effect of adding a phase change material (PCM) slab on the outside face of a refrigerator evaporator. A dynamic model of the vapour compression cycle including the presence of the phase change material and its experimental validation is presented. The simulation results of the system with PCM show that the addition of thermal inertia globally enhances heat transfer from the evaporator and allows a higher evaporating temperature, which increases the energy efficiency of the system. The energy stored in the PCM is yielded to the refrigerator cell during the off cycle and allows for several hours of continuous operation without power supply.     

Use of PCM Boards for Solar Cell Cooling

This contribution explains the use of fractal theory to describe thermal properties of materials. The basic idea is rooted in the theory of fractal fields defined in E-dimensional Euclidian space. Generic equations describing heat distribution are then specialized to describe the changes in heat transfer as a response to step-wise increases in the amount of heat added to the system. This model was then applied to the study of properties of a systems consisting of solar cells attached on a phase-change material (PCM) back sheet board. The aim of this study is to evaluate the ability of PCM boards to decrease the working temperature of solar cells and thus to increase the efficiency of the cells. Regression of experimental data was used to obtain model parameters. The parameters obtained this way were the thermal diffusivity, thermal conductivity, and specific heat, as well as parameters of the heat source and parameters related to the heat losses of the system. The method was then verified against parameters of the system based on poly-methyl-methacrylate and then applied to a PCM with a phase-change temperature of 25 °C. The values of the thermal parameters were determined at temperatures where both components of the PCM composite (Micronal^® and gypsum wall) were solid and below the phase-change temperature and then again at temperatures where one of the components (gypsum) was still solid, while the other one was already liquid (wax). The attempt to determine the parameters during the phase change was not made due the physicochemical processes taking place which would alter the measured data.     

空气源热泵系统无霜化及除霜方法概述

空气源热泵因性能稳定、使用方便等特点在房间供暖与空调等领域得到广泛应用,冬季室外换热器表面结霜和除霜是影响其系统性能的主要因素。本文概括了空气源热泵系统的结霜条件、无霜化以及各类热力与非热力除霜方法,重点阐述了采用相变蓄热装置的各种热泵系统及除霜方法,对研究报导中应用的相变材料、蓄热换热器结构进行综述。最后对实现空气源热泵产品的无霜化或除霜高效化和持续供热的研究方向进行展望:可靠的吸附材料及其再生方法研发是通过干燥方法破坏除霜条件的技术需求;高压电场和超声波除霜方法需进一步完善,提高稳定性和经济性;蓄热除霜有很大的市场潜力,但需优化系统及蓄热换热器设计,并深入相变材料的研究。