Correlation between the local climate and the free-cooling potential of latent heat storage

The natural cooling of energy-efficient buildings using latent heat thermal energy storage (LHTES) that is integrated into the building services makes possible energy savings and improved thermal comfort. In this article, studies of the free-cooling potential for different climatic locations are presented. Six cities from around Europe with a wide range of climatic conditions were selected. The size of the LHTES was optimized on the basis of the calculated cooling degree-hours. First, we analysed the influence of the width of the phase change temperature range and determined the optimal melting temperature of the phase change material (PCM). Then, the optimal LHTES was selected, based on the ratio of the mass of the PCM and the volume flow rate of air ventilating the building. We found that the optimum PCM has a melting temperature that is approximately equal to the average ambient air temperature in the hottest month, and that the free-cooling potential is proportional to the average daily amplitude of the ambient air's temperature swings. For all the analysed climatic conditions the PCM with a wider phase change temperature range (12 K) was found to be the most efficient. The optimal size of the LHTES for the free cooling of buildings is between 1 and 1.5 kg of PCM per m³/h of fresh ventilation air.     

Energiespeicherbeton – ein Beton mit integriertem Latentwärmespeichermaterial

Energy storage concrete – a concrete with integrated latent heat storage material. In times of solar architecture and increased utilisation of renew‐able energy, building components with high thermal heat storage capability are becoming increasingly important. One of the areas future research in building physics and building services will focus on is the development of energy storage units. A particularly interesting research area is latent heat storage. This essay describes the development of a concrete with integrated latent heat storage material (phase change material, PCM) as part of a thesis. This innovative concrete offers significantly improved thermal characteristics. For example, it was possible to more than double the heat capacity within a temperature range of 10 Kelvin around the melting point of the PCM. The PCM has a melting point between approx. 22 °C and 35 °C, depending on application. A highly versatile material such concrete offers a wide range of application options. In principle, it is possible to use latent heat storage concrete to supplement heating systems, to extend the scope of passive solar systems, or to protect against overheating in summer.     

相变通风屋顶的供热节能优化设计研究

提出太阳能相变屋顶系统(主要由太阳能空气集热系统、相变通风屋顶组成),将两种相变材料(PCM1、PCM2,PCM1用于供冷期蓄冷,相变温度在35℃左右。PCM2用于供暖期蓄热,相变温度在18℃左右)及风道(预制在钢筋混凝土板内,供冷期利用夜间低温空气冷却屋顶与PCM1,供暖期利用太阳能空气集热器出口热空气加热屋顶与PCM2)预制在屋顶内,形成相变通风屋顶(由上至下的基本结构为保护层、防水层、找坡层、保温层、找平层、PCM1、钢筋混凝土板),实现供冷期夜间蓄冷日间吸热、供暖期日间蓄热夜间放热。针对供暖工况,采用模拟方法,结合评价指标,对相变通风屋顶中相变材料(由于供暖工况PCM1不发生相变,因此研究对象为相变材料PCM2)的相变温度、结构(即相变材料位置)、相变材料厚度进行优化选取。A型相变通风屋顶将PCM2设置在PCM1与钢筋混凝土板之间,B型相变通风屋顶将PCM2设置在钢筋混凝土板下面,C型相变通风屋顶将PCM2设置在预制风道外圈。PCM2的最佳相变温度为18~20℃,最优结构为B型相变通风屋顶,PCM2最佳厚度为30 mm。与无相变通风屋顶(将B型相变通风屋顶中的30 mm厚PCM2相变材料替换成相同厚度的水泥砂浆,保留预制风道,其他各层材料及厚度均保持不变)相比,最佳相变通风屋顶(PCM2相变温度为18~20℃、厚度为30 mm的B型相变通风屋顶)的各项评价指标均更优。     

Dynamic performances of solar heat storage system with packed bed using myristic acid as phase change material

This paper is aimed at analyzing the thermal characteristics of packed bed containing spherical capsules, used in a latent heat thermal storage system with a solar heating collector. Myristic acid is selected as phase change material (PCM), and water is used as heat transfer fluid (HTF). The mathematical model based on the energy balance of HTF and PCM is developed to calculate the temperatures of PCM and HTF, solid fraction and heat release rate during the solidifying process. The latent efficiency, which is defined as the ratio between the instantaneous released latent heat and the maximum released heat, is introduced to indicate the thermal performances of the system. The inlet temperature of HTF (50 °C), flow rate of HTF (10 kg/min) and initial temperature of HTF (66 °C) were chosen for studying thermal performances in solar heat storage system. The influences of inlet temperature of HTF, flow rate of HTF and initial temperatures of HTF and PCM on the latent efficiency and heat release rate are also analyzed and discussed.     

蓄能保温砖的制备和保温性能实验

基材砖体以粉煤灰和硅酸钠为主要原料制备,砖体内部填充定量的水合盐相变储能包覆材料后,制备了一种新型复合储能建材——粉煤灰胶体保温蓄能砖(PCM砖)。对该砖体进行了48h的仿真环境热学性能试验,与普通红砖进行了对比,试验结果表明,由于储能的作用,PCM砖起到了调节温度热波动的作用。热流的波动幅度被削弱,作用的时间被推后,将温度变化维持在较小的范围内,可作为保温墙体材料进行应用。     

Eutectic mixtures of capric acid and lauric acid applied in building wallboards for heat energy storage

Capric acid (CA) and lauric acid (LA), as phase change materials (PCM), can be applied for energy storage in low temperature. The phase transition temperature and values of latent heat of eutectic mixtures of CA and LA are suitable for being incorporated with building materials to form phase change wallboards used for building energy storage. 120, 240 and 360 accelerated thermal cycle tests were conducted to study the changes in latent heat of fusion and melting temperature of phase change wallboards combined with the eutectic mixtures of CA and LA. Differential scanning calorimetry (DSC) tested the transition temperature and latent heat. The results showed that the melting temperature and latent heat of these phase change wallboards with eutectic mixtures have not obvious variations after repeated 360 thermal cycles, which proved that these phase change wallboards have good thermal stability for melting temperature and variations in latent heat of fusion for long time application. Therefore, they can be used for latent heat storage in the field of building energy conservation.     

Numerical and experimental study on heat pump water heater with PCM for thermal storage

An air source heat pump water heater with phase change material (PCM) for thermal storage was designed to take advantage of off-peak electrical energy. The heat transfer model of PCM was based upon a pure conduction formulation. Quasi-steady state method was used to calculate the temperature distribution and phase front location of PCM during thermal storage process. Temperature and thermal resistance iteration approach has been developed for the analysis of temperature variation of heat transfer fluid (HTF) and phase front location of PCM during thermal release process. To test the physical validity of the calculational results, experimental studies about storing heat and releasing heat of PCM were carried. Comparison between the calculational results and the experimental data shows good agreement. Graphical results including system pressure and input power of heat pump, time-wise variation of stored and released thermal energy of PCM were presented and discussed.     

使用泡沫铜增强相变材料换热性能的实验研究

针对有机相变材料(PCM)导热系数较低的缺点,通过实验研究了添加通孔泡沫铜金属材料增强相变材料导热系数的方法。选择脂肪酸二元低共熔混合物相变材料作为蓄热介质,通过对其进行DSC测试分析,得到其相变温度和相变潜热。对壳管式潜热蓄热系统填充介质为纯PCM与PCM/泡沫铜复合相变材料两种工况下的熔化过程进行对比实验研究。实验数据表明,与纯PCM蓄热系统相比,添加泡沫铜的蓄热系统换热性能得到增强,整个蓄热器内PCM达到相变温度的时间仅为纯PCM系统的22.5%。     

Performance of phase change material boards under natural convection

The high thermal storage capacity of phase change material (PCM) can reduce energy consumption in buildings through energy storage and release when combined with renewable energy sources, night cooling, etc. PCM boards can be used to absorb heat gains during daytime and release heat at night. In this paper, the thermal performance of an environmental chamber fitted with phase change material boards has been investigated. During a full-cycle experiment, i.e. charging–releasing cycle, the PCM boards on a wall can reduce the interior wall surface temperature during the charging process, whereas the PCM wall surface temperature is higher than that of the other walls during the heat releasing process. It is found that the heat flux density of the PCM wall in the melting zone is almost twice as large as that of ordinary wall. Also, the heat-insulation performance of a PCM wall is better than that of an ordinary wall during the charging process, while during the heat discharging process, the PCM wall releases more heat energy. The convective heat transfer coefficient of PCM wall surface calculated using equations for a normal wall material produces an underestimation of this coefficient. The high convective heat transfer coefficient for a PCM wall is due to the increased energy exchange between the wall and indoor air.     

Temperature reduction due to the application of phase change materials

Overheating is a major problem in many modern buildings due to the utilization of lightweight constructions with low heat storing capacity. A possible answer to this problem is the emplacement of phase change materials (PCM), thereby increasing the thermal mass of a building. These materials change their state of aggregation within a defined temperature range. Useful PCM for buildings show a phase transition from solid to liquid and vice versa. The thermal mass of the materials is increased by the latent heat. A modified gypsum plaster and a salt mixture were chosen as two materials for the study of their impact on room temperature reduction. For realistic investigations, test rooms were erected where measurements were carried out under different conditions such as temporary air change, alternate internal heat gains or clouding. The experimental data was finally reproduced by dint of a mathematical model.     

Monitoring results of an interior sun protection system with integrated latent heat storage

An interior sun protection system consisting of vertical slats filled with phase change material (PCM) was monitored from winter 2008 until summer 2010. While conventional interior sun protection systems often heat up to temperatures of 40 °C or more, the monitoring results show that the surface temperature on the interior side of the PCM-filled slats hardly ever exceeded the PCM melting temperature of 28 °C even in case of long-term intense solar radiation. As long as the PCM is not fully melted, the latent heat storage effect reduces the solar heat gain coefficient (g-value) of the sun protection system to 0.25 for a totally closed blind, and 0.30 for slats set at 45° (the g-values of the same system without PCM are 0.35 and 0.41, respectively). This reduced the maximum air temperature in the offices by up to 2 K in contrast to a reference room with a comparable conventional blind. The sun protection system with PCM therefore considerably improves thermal comfort. In order to discharge the PCM, the stored heat must be dissipated during the night. In climates with sufficiently low outside air temperatures, this is best achieved using a ventilation system in combination with tilted windows.     

Energetic efficiency of room wall containing PCM wallboard: A full-scale experimental investigation

A wallboard new PCM material is experimentally investigated in this paper to enhance the thermal behavior of light weight building internal partition wall. The experiments are carried out in a full-scale test room which is completely controlled. The external temperature and radiative flux dynamically simulate a summer repetitive day. The differential test concern walls with and without PCM material under the same conditions. The PCM allows to reduce the room air temperature fluctuations, in particular when overheating occurs. A numerical modeling has been used to investigate energy storage. Five millimeters of PCM wallboard double the energy that can be stocked, and destocked, during the experiment. The experiments are fully described so that the results can be used for the validation of numerical models dealing with phase change materials.     

Experimental analysis of thermal performance of evacuated tube solar air collector with phase change material for sunshine and off-sunshine hours

An experimental investigation of an evacuated tube solar air collector coupled to a latent thermal energy store for generating hot air when no solar radiation is incident was undertaken. Acetamide was used as a phase change material (PCM). The latent thermal energy store was integrated with the manifold of the solar collector and water was used as the working fluid transferring solar gain to the air being heated. The maximum measured temperature differential between the heated air and the ambient air was 37°C and 20.2°C during conditions of incident and non-incident solar radiation, respectively. This occurred using a circular fin configuration at a flow rate of 0.018?kg?s^?1. The efficiency at low (0.018?kg?s^?1) air flow rates was 0.05–0.50 times less as compared to high (0.035?kg?s^?1) air flow rates. This system has advantages over systems using sensible storage as it can be used after sunset due to better heat storing capacity of the PCM.     

Mathematical modelling of PCM air heat exchanger

In order to cool a room with a cold night air phase change material, PCM, is stored in an air heat exchanger. During night the PCM crystallises, energy is released. During daytime air is circulated in the unit, energy is absorbed and the indoor air is cooled. The characteristic of PCM is that there is an increase of the specific heat over a limited temperature span. This is the principle that is used in the design of the PCM air heat exchanger unit.The action of a PCM storage unit will act differently depending of the thermal properties of the material. In an ideal material the phase transition occurs at a given temperature. On the market, compounds containing PCM are available which, in order to create a suitable melting temperature, are mixtures of different products. In these materials, the transition from liquid to solid takes place over a temperature span, i.e. the specific heat varies with the temperature. This can be represented by a c_P(T) curve, specific heat as a function of the temperature.In this paper, the development of a mathematical model of the PCM air heat exchanger is presented. Considerations are taken to different shapes of the c_P(T) curve. The mathematical model is verified with measurement on a prototype heat exchanger.The development of the equipment is part of the CRAFT project Changeable Thermal Inertia Dry Enclosures (C-TIDE) the possibility of use of phase change materials integrated into a building is explored.     

空气源热泵相变蓄能除霜系统蓄热模式及除霜特性的研究

本文阐述了基于相变蓄热的空气源热泵蓄能除霜系统及其相变蓄热器的设计,提出了3种蓄热模式并搭建了空气源热泵蓄能除霜试验台.通过对实验数据进行分析,认为串联蓄热模式时,系统可有效完成蓄热且相变蓄热器的蓄热速度较快;单独蓄热模式和并联蓄热模式时系统不能有效运行,压缩机的吸、排气温度偏高,而吸、排气压力偏低,功率偏低,相变材料...     

Study of a floor supply air conditioning system using granular phase change material to augment building mass thermal storage—Heat response in small scale experiments

We have proposed a new floor supply air conditioning system, using phase change material to augment building mass thermal storage. A scale model was constructed for such a system. Granules containing phase change material (PCM), with a phase change temperature of about 20 °C, were made from foamed glass beads and paraffin waxes. Results from measurements simulating an air conditioning schedule in office buildings indicate that 89% of daily cooling load could be stored each night in a system that used a 30 mm thick packed bed of the granular PCM.     

Stearic acid/silica fume composite as form-stable phase change material for thermal energy storage

The aim of this research is to prepare a novel form-stable composite phase change material (PCM) for the latent heat thermal energy storage (LHTES) in buildings, passive solar space heating by impregnating of stearic acid (SA) into silica fume (SF) matrix through the technique of solution impregnation. The structure, thermal properties, thermal reliability, thermal conductivity and heat storage or release performance of the composite PCM were determined by scanning electron microscope (SEM), Fourier transformation infrared (FTIR), differential scanning calorimetry (DSC) and thermal cycling test analysis technique. The results show that the form-stable composite PCM has the optimal effect, preventing the leakage of SA from the composite, emerges when the SA and SF mass ratio is 1:0.9. The SA loaded on the matrix surface by physical attraction with the mass ratio of 47% during the preparation process. The latent heat of the composite PCM is measured as 82.53 J/g for the melting process and 84.47 J/g for the freezing process, respectively, which indicate the heat storage ability of composite is connected with the mass ratio of SA in composite. The results of DSC, FTIR and thermal cycling test are all show that the thermal reliability of the composite PCM has an imperceptible change. The increase of thermal conductivity was also confirmed by comparing the melting time, freezing time and phase change time of the composite with that of SA. All of the conclusions indicate that the composite has a better thermal conductivity and good thermal and chemical stability.     

Effects of wallboard design parameters on the thermal storage in buildings

The phase change material (PCM) could be added to the wallboard to increase the thermal mass to decrease in indoor temperature fluctuation and improve thermal comfort. In this study, experimentally validated simulation was performed to investigate the effects of various parameters of PCM including the nominal average phase change temperature, its range, the convective heat transfer coefficients and the wallboard thickness on the thermal storage performance of the wallboard such as the thermal energy storage and the time shift.It was found that the average phase change temperature should be close to the average room temperature to maximize the thermal heat storage in the wallboards. The phase change temperature should be narrow to maximize the thermal heat storage in the PCM wallboards. The thermal heat storage increased with the convective heat transfer coefficient, and the optimal average phase change temperature to maximize the storage shifted a bit to a higher temperature with it. The time shift was found to decrease with the convective heat transfer coefficient and the phase change temperature range.     

相变材料应用于建筑外墙的温度与能耗模拟

使用EnergyPlus能耗模拟软件对相变材料作为外墙表面隔热材料的应用效果进行模拟,在小空间和小型办公室的模型上,改变相变材料的相变温度、材料结构和用量等使用条件,并进一步考虑室内热源和不同气候区的影响,对比分析在空调季节里空间内部温度的变化情况和空调节能效果。模拟结果表明:相变温度稍高的相变材料更有利于夜间散热蓄冷,同时,结合双层复合结构可获得更好的温度抑制和节能效果;内热源的存在会提高房间空调能耗的基数,从而使相变材料空调节能率计算值降低,并且在一定程度上掩盖了相变材料对室内平均温度的抑制作用,尽管如此,相变材料在有内热源环境下使用时空调节能量仍与无内热源时相当。     

Preparation and characteristics of n-nonadecane/cement composites as thermal energy storage materials in buildings

n-Nonadecane/cement composites as thermal energy storage materials (TESM) were prepared by absorbing n-nonadecane in porous network of cement. In composite materials, n-nonadecane was used as the phase change material (PCM) for thermal energy storage, and cement acted as the supporting material. Fourier transformation infrared spectroscope (FT-IR), X-ray diffractometer (XRD) and scanning electronic microscope (SEM) were used to determine the FT-IR spectra, the crystalloid phase and microstructure of n-nonadecane/cement composites, respectively. The thermal properties and thermal stability were investigated by a differential scanning calorimeter (DSC) and a thermogravimetric analysis apparatus (TGA), respectively. The SEM results showed that n-nonadecane was well dispersed in the porous network of cement. The DSC results indicated that the n-nonadecane/cement composite material has the melting latent heat of 69.12 kJ/kg with melting temperature of 31.86 °C, and solidifying latent heat of 64.07 kJ/kg with solidifying temperature of 31.82 °C.