High‐temperature latent heat storage technology to utilize exergy of solar heat and industrial exhaust heat

Latent heat storage (LHS) using phase change materials is quite attractive for utilization of the exergy of solar energy and industrial exhaust heat because of its high‐heat storage capacity, heat storage and supply at constant temperature, and repeatable utilization without degradation. In this article, general LHS technology is outlined, and then recent advances in the uses of LHS for high‐temperature applications (over 100 °C) are discussed, with respect to each type of phase change material (e.g., sugar alcohol, molten salt, and alloy). The prospects of future LHS systems are discussed from a principle of exergy recuperation. In addition, the technologies to minimize exergy loss in the future LHS system are discussed on the basis of the thermodynamic analysis by ‘thermodynamic compass’. Copyright © 2016 John Wiley & Sons, Ltd.     

Exergetic and performance analyses of two-layered packed bed latent heat thermal energy storage system

In concentrating solar power (CSP) plant, a novel method involving the use of thermocline can be employed to augment the capability of the thermal energy storage system (TES). The rate of thermocline degradation can be reduced by packing encapsulated phase change material (PCM) in the TES. The thermal performance of the packed bed latent heat thermal energy storage system (PBTES) can be further enhanced by employing different diameters of PCM capsules arranged in multiple layers. In this paper, the thermal and exergetic performance of single-layered and two-layered PBTES is evaluated for varying mass flow rate, PCM capsule diameter and bed height of larger PCM capsules using a dynamic model based on simplified energy balance equations for PCM and heat transfer fluid (HTF). The single-layered PBTES has a lower TES latent charging rate than the two-layered PBTES. The charging efficiency and charging time of two-layered PBTES are increased by 15.85% and 16.85%, respectively for reducing the HTF mass flow rate by 14.29%. A higher stratification number can be achieved by using a two-layered PBTES instead of a single-layered PBTES filled with the corresponding larger diameter PCM capsules. The second law efficiency of the two-layered PBTES is found to be less than that of the single-layered PBTES. A decrease in the bed height of larger PCM capsules decreases the exergetic efficiency of the two-layered PBTES by 3.27%. The findings from this study can be used in further designing and optimising the multi-layered PBTES.     

Enrichment of heat transfer in a latent heat storage unit using longitudinal fins

The charging and discharging rates of a phase change material (PCM) in a horizontal latent heat storage unit (LHSU) is largely influenced by the lower thermal conductivity of the PCM. In the present research, four different configurations of longitudinal fins are proposed to augment the heat transfer in horizontal shell and tube type LHSUs. Numerical investigations are reported to establish the thermal performance augmentation with rectangular, triangular, and Y‐shaped (bifurcated) fins. From the results, it has been inferred that all fin configurations provide a faster charging and discharging rate. In the present set of geometric dimensions of LHSU considered, a reduction in charging time of 68.71% is evaluated for case III (three rectangular fins with one fin positioned in the area of the heat transfer fluid [HTF] surface) and case V (two bifurcated fins with one fin positioned in the area of the HTF surface). Moreover, overall cycle (charging + discharging) time is reduced by 58.3% for case III. Employment of fins results in a faster rate of absorption and extraction of energy from the PCM.     

Numerical heat transfer analysis of the packed bed latent heat storage system based on an effective packed bed model

Due to the complexity of the fluid flow and heat transfer in packed bed latent thermal energy storage (LTES) systems, many hypotheses were introduced into the previous packed bed models, which consequently influenced the accuracy and authenticity of the numerical calculation. An effective packed bed model was therefore developed, which could investigate the flow field as the fluid flows through the voids of the phase change material (PCM), and at the same time could account for the thermal gradients inside the PCM spheres. The proposed packed bed model was validated experimentally and found to accurately describe the thermo-fluidic phenomena during heat storage and retrieval. The proposed model was then used to do a parametric study on the influence of the arrangement of the PCM spheres and encapsulation of PCM on the heat transfer performance of LTES bed, which was difficult to perform with the previous packed bed models. The results indicated that random packing is more favorable for heat storage and retrieval as compared to special packing; both the material and the thickness of the encapsulation have the apparent effects on the heat transfer performance of the LTES bed.     

相变微胶囊悬浮液传热特性实验研究

相变微胶囊(microencapsulated phase change material,MPCM)在建筑节能领域应用广泛,为研究其传热特性,搭建了以水为换热流体(heat transfer fluid,HTF),微胶囊悬浮液为储能介质的潜热储能(latent thermal energy storage,LTES)系统。在实验过程中,通过改变换热流体的进口初始温度以及搅拌器的搅拌速率,获得了MPCM悬浮液的温度变化规律并计算了MPCM悬浮液的平均充放冷速率。实验结果表明:在充冷过程中,MPCM相变时温度变化速率减缓,相变温度区间较大,而在放冷过程中,MPCM相变时温度保持恒定,相变温度区间较小;未搅拌时,MPCM悬浮液中温度梯度较大,传热能力较差;搅拌时,MPCM悬浮液混合均匀,其温度梯度很小,传热能力较强;增加搅拌器的搅拌速率及水与相变微胶囊悬浮液的温差均可以提高MPCM的充放冷速率。     

带相变蓄热小型热泵冷凝热回收性能实验研究

通过实验初步研究了采用光管螺旋相变蓄热器替代传统水蓄热器的小型家用热泵冷凝热回收系统的性能,对相变蓄热和水蓄热的冷凝热回收过程进行了对比实验,分析并得到了两类冷凝热回收系统的性能参数及综合能效系数。实验数据表明:与水蓄热系统相比,光管螺旋相变蓄热器体积减小,并且系统运行较传统水蓄热工况下更加平稳,但存在传热效果较差、热回收率低的缺点,回收率仅为15%,系统综合能效系数2.9。可知,相变蓄热器的内部结构对系统综合能效系数影响很大。     

高温相变蓄热的研究进展

从如下两个方面总结了高温相变蓄热的研究现状：①在高温相变材料(PCM)方面，重点介绍了高温相变材料的一些重要性能及其测量，高温相变材料的封装，高温复合相变材料及高温相变材料的应用；②在传热分析方面，主要介绍了相变过程的数值模拟和相变蓄热系统(LTES)的热力学优化。     

A study on latent heat storage exchangers with the high‐temperature phase‐change material

This paper presents a theoretical analysis and an experimental test on a shell‐and‐tube latent heat storage exchanger. The heat exchanger is used to recover high‐temperature waste heat from industrial furnaces and off‐peak electricity. It can also be integrated into a renewable energy system as an energy storage component. A mathematical model describing the unsteady freezing problem coupled with forced convection is solved numerically to predict the performance of the heat exchanger. It provides the basis for an optimum design of the heat exchanger. The experimental study on the heat exchanger is carried out under various operating conditions. Effects of various parameters, such as the inlet temperature, the mass flow rate, the thickness of the phase‐change material and the length of the pipes, on the heat transfer performance of the unit are discussed combined with theoretical prediction. The criterion for analyzing and evaluating the performance of heat exchanger is also proposed. Copyright © 2001 John Wiley & Sons, Ltd.     

Experimental study on performance of square tube absorber with phase change material

The intermittent nature of solar radiation has decreased the performance efficiency of solar heaters. Integrating the solar heater with thermal energy storage component could increase its performance effectively. In this article, an investigation on the effect of phase change material (PCM) as the thermal energy storage component on the performance of square aluminum tube was carried out experimentally. In the first phase, the temperature behavior of square aluminum tube with two types of PCM, namely, generic plant-based PCM (A2) and paraffin wax (A3), was compared with square aluminum tube without PCM (A1). In the second phase, the performance of square aluminum tube was investigated with different paraffin wax masses of 38 g (B1), 48 g (B2), and 58 g (B3). Based on the result, the A3 tube configuration performed better than A1 and A2 tube configurations with higher heat gain rate (0.08°C/s) and lower heat discharge rate (−0.04°C/s). The B2 tube configuration was found to have maximum heat gain of 3.73 kJ with higher heat discharge rate as compared with other square tube configurations. The average temperature difference between internal and external surface tube of B2 was lower (4.3°C) leading to higher average temperature difference at ambient temperature of 25.3°C. Instantaneous efficiency of the tube B2 is higher than the B1 and B3 tube configurations by 16% and 26%, respectively. The result suggests that the insertion of paraffin wax inside the square absorber tube improves the temperature response of the absorber in the situation of intermittent solar radiation.     

Medium‐ and high‐temperature latent heat thermal energy storage: Material database,system review,and corrosivity assessment

Latent heat thermal energy storage refers to the storage and recovery of the latent heat during the melting/solidification process of a phase change material (PCM). Among various PCMs, medium‐ and high‐temperature candidates are attractive due to their high energy storage densities and the potentials in achieving high round trip efficiency. Although a few review studies on high‐temperature PCMs have emerged in the past few years, the quantity, completeness, and accuracy of the presented data are relatively poor. Also, an efficient indexing methodology for retrieving useful PCM data is missing in the open literature. In this article, we created an up‐to‐date PCM database following a holistic review of the PCMs in medium‐ and high‐temperature applications over a temperature range of 100°C to 1680°C. Such effort then allows us to develop an accurate indexing tool for the fast selection of suitable PCM candidates and extraction of the related property data. More specifically, the created PCM database covers 496 entries of PCM materials, which are extracted from the scattered research works published during the year 1956 to 2017. The collected information includes both the basic thermo‐physical properties of PCMs (eg, melting temperature, heat of fusion, and thermal conductivity) and crucial design factors during construction and engineering phases (eg, energy storage density, volume expansion, liquid/solid densities, and cost). The reviewed PCMs comprise a wide variety of materials, including fluorides, chlorides, hydrates, nitrates, carbonates, metals and alloys, and other uncommon compounds and salts. In addition, the current work presents a brief review on high‐temperature latent heat thermal energy storage systems categorized into metallic and non‐metallic systems. The corrosivity and stability of PCMs, which are commonly ignored in previous studies, are also examined.     

Fatty acid/poly(methyl methacrylate) (PMMA) blends as form-stable phase change materials for latent heat thermal energy storage

Fatty acids such as stearic acid (SA), palmitic acid (PA), myristic acid (MA), and lauric acid (LA) are promising phase change materials (PCMs) for latent heat thermal energy storage (LHTES) applications, but high cost is the most drawback which limits the utility area of them in thermal energy storage. The use of fatty acids as form-stable PCM will increase their feasibilities in practical LHTES applications due to reduced cost of the energy storage system. In this regard, a series of fatty acid/poly(methyl methacrylate) (PMMA) blends, SA/PMMA, PA/PMMA, MA/PMMA, and LA/PMMA were prepared as new kinds of form-stable PCMs by encapsulation of fatty acids into PMMA which acts as supporting material. The blends were prepared at different mass fractions of fatty acids (50, 60, 70, 80, and 90% w/w) to reach maximum encapsulation ratio. All blends were subjected to leakage test by heating the blends over the melting temperature of the PCM. The blends that do not allow leakage of melted PCM were identified as form-stable PCMs. The form-stable fatty acid/PMMA (80/20 wt.%) blends were characterized using optic microscopy (OM), viscosimetry, and Fourier transform infrared (FT-IR) spectroscopy methods, and the results showed that the PMMA was compatible with the fatty acids. In addition, thermal characteristics such as melting and freezing temperatures and latent heats of the form-stable PCMs were measured by using differential scanning calorimetry (DSC) technique and indicated that they had good thermal properties. On the basis of all results, it was concluded that form-stable fatty acid/PMMA blends had important potential for some practical LHTES applications such as under floor space heating of buildings and passive solar space heating of buildings by using wallboard, plasterboard or floor impregnated with a form-stable PCM due to their satisfying thermal properties, easily preparing in desired dimensions, direct usability without needing an add encapsulation and eliminating the thermal resistance caused by shell and thus reducing cost of LHTES system.     

Analysis of solidification in a finite PCM storage with internal fins by employing heat balance integral method

Here, a simplified analytical model has been proposed to predict solid fraction, solid–liquid interface, solidification time, and temperature distribution during solidification of phase change material (PCM) in a two‐dimensional latent heat thermal energy storage system (LHTES) with horizontal internal plate fins. Host of boundary conditions such as imposed constant heat flux, end‐wall temperature, and convective air environment on the vertical walls are considered for the analysis. Heat balance integral method was used to obtain the solution. Present model yields closed‐form solution for temperature variation and solid fraction as a function of various modeling parameters. Also, solidification time of PCM, which is useful in optimum design of PCM‐based thermal energy storages, has been evaluated during the analysis. The solidification time was found to be reduced by 93% by reducing the aspect ratio from 8 to 0.125 for constant heat flux boundary condition. While, for constant wall temperature boundary condition, the solidification time reduces by 99% by changing the aspect ratio from 5 to 0.05. In case of convective air boundary surrounding, the solidification time is found to reduce by 88% by reducing the aspect ratio from 8 to 0.125. Based on the analytical solution, correlations have been proposed to predict solidification time in terms of aspect ratio and end‐wall boundary condition.     

Comparative study based on exergy analysis of solar air heater collector using thermal energy storage

This communication presents the comparative experimental study based on energy and exergy analyses of a typical solar air heater collector with and without temporary heat energy storage (THES) material, viz. paraffin wax and hytherm oil. Based on the experimental observations, the first law and the second law efficiencies have been calculated with respect to the available solar radiation for three different arrangements, viz. one arrangement without heat storage material and two arrangements with THES, viz. hytherm oil and paraffin wax, respectively. It is found that both the efficiencies in case of heat storage material/fluid are significantly higher than that of without THES, besides both the efficiencies in case of paraffin wax are slightly higher than that of hytherm oil case. Copyright © 2011 John Wiley & Sons, Ltd.     

太阳能单罐相变蓄热系统实验测试和数值模拟研究

太阳能的高效利用是解决能源短缺和环境污染问题的主要手段之一。文章基于太阳能相变蓄热系统,建立带有辅助热源的太阳能单罐相变蓄热系统实验台,并利用该实验台对相变材料(融点为48~50℃的石蜡)进行热能存储实验。实验结果表明:加热12 min后,顶层石蜡温度达到140℃以上,并且融化完全;加载完成后,中层石蜡的最高温度为63℃;静置2 h后,储能罐中的石蜡开始放载,最后均匀混合,顶层、中层和底层石蜡的温度均达到42℃。文章在实验结果的基础上,利用Fluent软件对太阳能单罐相变蓄热系统内相变材料(石蜡和熔融盐)的换热过程进行数值模拟,模拟结果表明,在相同时间内,当采用石蜡作为相变材料时,太阳能单罐相变蓄热系统的换热效率较高。     

Parametric study of cascade latent heat thermal energy storage (CLHTES) system in Concentrated Solar Power (CSP) plants

Recently, Concentrated Solar Power (CSP) is attracting numerous research attentions, and thermal energy storage (TES) system filled with energy storage media is a critical component in all CSP plants. To realize a high energy storage efficiency ($\xi$) and exergy efficiency ($\eta$), a comprehensive study to the cascade latent heat thermal energy storage (CLHTES) system is necessary from the perspective of heat transfer. In this study, a dimensionless parametric study was presented using an enthalpy-based 1D transient model for energy storage/extraction in CLHTES system. A dimensionless parameter space (${\tau }_r,H_{CR},St{f}^{\text{*}}$) was constructed by considering $\xi$ and $\eta$ as the objective functions to explore the effects from dimensionless material properties (such as latent heat, specific heat at solid and liquid phases) and dimensionless operational parameters (such as charging/discharging time period, TES tank height and diameter). It is recommended that when $H_{CR}$<0.5 and ${\Pi }_c/{\Pi }_d$<1.0, the system performance is very sensitive to $H_{CR}$ and ${\Pi }_c/{\Pi }_d$, furthermore for the same TES tank volume (H/D = D/H = 1.0), the sensitivity by varying its diameter alone is double than that from changing its height. The novelty of this study is to provide the design criteria for the CLHTES system, so that it can easily be designed and its efficiencies can be competitive to sensible heat thermal energy storage (SHTES) system. The results from this parametric study and sensitivity analysis are expected to benefit the solar thermal research and industry community to design the CLHTES system.     

Experimental study of the phase change and energy characteristics inside a cylindrical latent heat energy storage system: Part 2 simultaneous charging and discharging

The time mismatch between energy availability and energy demand with solar domestic hot water (SDHW) systems is often solved using energy storage. Energy storage systems typically employ water for thermal energy storage, however, water storage takes up considerable space and weight due to the large volumes required under certain conditions. A latent heat energy storage system (LHESS) may provide a valuable solution to the space and weight issue, while also correcting the energy mismatch by storing energy in phase change materials (PCMs) when it is available, dispensing energy when it is in demand, and acting as a heat exchanger when there is supply and demand simultaneously. PCMs are advantageous as energy storage materials due to their high energy density which reduces the space requirements for energy storage. However, heat transfer problems arise due to the inherently low thermal conductivity of PCMs. Simultaneous charging and discharging has not been addressed in literature making questionable the ability of a LHESS to operate as a heat exchanger during the mode of operation. The main objective of this research is to study the heat transfer processes and phase change behavior of a PCM during simultaneous charging and discharging of a LHESS.In Part 2 of this paper, experiments are performed using a vertical cylindrical LHESS which is charged and discharged simultaneously to replicate latent heat energy storage paired with a SDHW system with simultaneous energy supply and demand. Dodecanoic acid is used as the PCM. Experimental results for simultaneous operations are presented, under various scenarios and flow rates for both the hot and cold heat transfer fluids. The ability of the system to directly transfer heat between the hot and cold heat transfer fluids is studied, and the results found during consecutive, or separate, charging and discharging, presented in Part 1 of this paper, are compared to the results found during simultaneous charging and discharging. It was found that natural convection in the melted PCM clearly provides an advantage towards direct heat exchange between the hot and cold heat transfer fluid; while the low thermal conductivity of solid PCM provides a barrier to this direct energy exchange.     

Study on design and thermal characteristics of vacuum tube solar collector intubated with heat storage tube

In this work, a heat storage vacuum tube solar collector intubated with heat storage tube is designed, which consists of solar vacuum tube, phase change material insert tube, and heat holding cover. The internal energy conversion, transmission, and storage theory are established based on the structure of the heat storage vacuum tube. The parallel and series‐parallel solar air collector system prototype consisting of nine heat storage solar vacuum tube solar collectors is designed and tested. The test results showed that the daily average conversion efficiency of the parallel and series‐parallel prototype reached 56.9% and 48.46%, respectively. Compared with nonheat storage prototype, the heat storage parallel and series‐parallel prototype had higher conversion efficiency by, respectively, 10.9% and 7.8%, longer effective heating time, and better heating stability and practicability. At the same time, the heat storage solar collector has compact structure, which is convenient to use.     

Experimental study on latent heat storage characteristics of W/O emulsion -Supercooling rate of dispersed water drops by direct contact heat exchange-

Recently, much attention has been paid to investigate the latent heat storage system. Using of ice heat storage system brings an equalization of electric power demand, because it will solved the electric -power-demand-concentration on day-time of summer by the air conditioning. The flowable latent heat storage material, Oil/Water type emulsion, microencapsulated latent heat material-water mixture or ice slurry, etc., is enable to transport the latent heat in a pipe. The flowable latent heat storage material can realize the pipe size reduction and system efficiency improvement. Supercooling phenomenon of the dispersed latent heat storage material in continuous phase brings the obstruction of latent heat storage. The latent heat storage rates of dispersed water drops in W/O (Water/Oil) emulsion are investigated experimentally in this study. The water drops in emulsion has the diameter within 3 ～ 25μm, the averaged water drop diameter is 7.3μm and the standard deviation is 2.9μm. The direct contact heat exchange method is chosen as the phase change rate evaluation of water drops in W/O emulsion. The supercooled temperature and the cooling rate are set as parameters of this study. The evaluation is performed by comparison between the results of this study and the past research. The obtained experimental result is shown that the 35K or more degree from melting point brings 100% latent heat storage rate of W/O emulsion. It was clarified that the supercooling rate of dispersed water particles in emulsion shows the larger value than that of the bulk water.     

Experimental study on the phase change behavior of phase change material confined in pores

An experimental study on the phase change behavior of organic phase change materials (PCMs) in porous building materials is reported. Three kinds of porous materials and two kinds of PCMs were used. The phase change behavior of organic PCMs and phase change composites was measured by means of differential scanning calorimetry (DSC). The pore structure of the porous materials was characterized by means of mercury intrusion porosimetry (MIP). X-ray fluorescence spectrometry (XRF) and Fourier transformation infrared spectroscopy (FTIR) were used to characterize the chemical properties of porous materials and phase change materials. Quite different phase change behaviors were found for these two kinds of PCMs in porous materials. For capric acid with a functional group of –COOH, a remarkable elevation of melting temperature was found when confined in porous materials. But for paraffin with only inactive functional groups of –CH₂ and –CH₃, no elevation or depression of the melting temperature was found when confined in the porous materials. The interaction between functional groups of PCM molecules and alkaline spots on the inner pore surface of the porous materials and the Clapeyron equation were used to explain the different shift of the phase change temperature of capric acid and paraffin in porous materials.     

Time and spatial heat transfer performance around an isothermally heated sphere placed in a uniform, downwardly directed flow (in relation to the enhancement of latent heat storage rate in a spherical capsule)

The aim of this study is to reveal the temporal and spatial heat transfer performance of an isothermally heated sphere placed in a uniform, downwardly directed flow using a micro-foil heat flow sensor (HFS). A HFS, whose response time is about 0.02 s, was pasted on the surface of a heated copper sphere. Experiments were carried out using air with a Grashof number of 3.3 × 10⁵ and with several Reynolds numbers (Re) up to 1800. Three flow patterns appeared: a chaotic flow at Re<240; a two-dimensional steady separated flow at 240 Re<500, and a three-dimensional unsteady separated flow at Re 500. In addition, the instantaneous and time-averaged heat transfer performance around the sphere in each of the three regions was clarified. Next, enhancement of the latent heat storage rate of a solid phase change material (PCM) in a spherical capsule was performed. The flow around the spherical capsule, in which the solid PCM was filled and placed in a heated, upwardly directed flow, is the approximate adverse flow phenomenon around the heated sphere which was placed in a downwardly directed flow. In other words, the buoyant flow and the forced flow are in the opposite directions in these two cases. Tests of latent heat storage were run for two Reynolds numbers which represented different flow characteristics in the heat transfer experiments, Re=150 and 1800. Furthermore, copper plates were inserted into the solid PCM, of which thermal conductivity was considerably low, to enhance the latent heat storage rate for the two Reynolds number flows.