Numerical simulation of a multi-layer latent heat thermal energy storage system

A computational model for the prediction of the thermal behaviour of a compact multi-layer latent heat storage unit is presented. The model is based on the conservation equations of energy for the phase change material (PCM) and the heat transfer fluid (HTF). Electrical heat sources embedded inside the PCM are used for heat storage (melting) while the flow of an HTF is employed for heat recovery (solidification). Parametric studies are performed to assess the effect of various design parameters and operating conditions on the thermal behaviour of the unit. Results indicate that the average output heat load during the recovery period is strongly dependent on the minimum operating temperature, on the thermal diffusivity of the liquid phase, on the thickness of the PCM layer and on the HTF inlet mass flowrate and temperature. It is, on the other hand, nearly independent of the wall thermal diffusivity and thickness and of the maximum operating temperature. Correlations are proposed for the total energy stored and the output heat load as a function of the design parameters and the operating conditions. © 1998 John Wiley & Sons, Ltd.     

Review on heat transfer analysis in thermal energy storage using latent heat storage systems and phase change materials

Thermal energy storage (TES) is a technology that stocks thermal energy by heating or cooling a storage medium so that the stored energy can be used later for heating and cooling applications and for power generation. TES has recently attracted increasing interest to thermal applications such as space and water heating, waste heat utilisation, cooling, and air conditioning. Phase change materials (PCMs) used for the storage of thermal energy as latent heat are special types of advanced materials that substantially contribute to the efficient use and conservation of waste heat and solar energy. This paper provides a comprehensive review on the development of latent heat storage (LHS) systems focused on heat transfer and enhancement techniques employed in PCMs to effectively charge and discharge latent heat energy, and the formulation of the phase change problem. The main categories of PCMs are classified and briefly described, and heat transfer enhancement technologies, namely dispersion of low‐density materials, use of porous materials, metal matrices and encapsulation, incorporation of extended surfaces and fins, utilisation of heat pipes, cascaded storage, and direct heat transfer techniques, are also discussed in detail. Additionally, a two‐dimensional heat transfer simulation model of an LHS system is developed using the control volume technique to solve the phase change problem. Furthermore, a three‐dimensional numerical simulation model of an LHS is built to investigate the quasi‐steady state and transient heat transfer in PCMs. Finally, several future research directions are provided.     

Numerical study on performance enhancement of latent heat storage unit

建立了考虑液态相变材料自然对流的壳管式相变蓄热单元的三维模型,数值分析了自然对流对相变蓄热过程的影响.对比研究了外侧强化传热管和双侧强化传热管对相变蓄热单元蓄热性能的强化效果.结果表明,液态相变材料的自然对流,会引起固-液界面分布不均匀现象,采用外翅片管可以有效削弱这一现象;采用外侧强化传热管和双侧强化传热管,都可以缩短相变材料完全熔化以及整个蓄热过程所需时间.与采用光管时相比,采用外侧强化传热管时,完全熔化时间减少了18.0%;采用双侧强化传热管时,完全熔化时间减少52.5%.可见,采用带有外翅片的强化传热管,不仅可以削弱自然对流引起的固-液界面不均匀性问题,而且可以强化相变蓄热单元的蓄热性能.     

潜热储热系统强化传热研究进展

近年来,潜热储热系统在太阳能和工业废能的利用中发挥着极其重要的作用,因此用于潜热储热的相变材料受到普遍关注.文章对国内外潜热储热系统众多强化传热技术进行了综述与讨论.     

Performance enhancement in latent heat thermal storage system: A review

Phase change material (PCM) based latent heat thermal storage (LHTS) systems offer a challenging option to be employed as an effective energy storage and retrieval device. The performance of LHTS systems is limited by the poor thermal conductivity of PCMs employed. Successful large-scale utilization of LHTS systems thus depends on the extent to which the performance can be improved. A great deal of work both experimental and theoretical on different performance enhancement techniques has been reported in the literature. This paper reviews the implementation of those techniques in different configurations of LHTS systems. The influence of enhancement techniques on the thermal response of the PCM in terms of phase change rate and amount of latent heat stored/retrieved has been addressed as a main aspect. Issues related to mathematical modeling of LHTS systems employing enhancement techniques are also discussed.     

三套管式相变蓄热器强化传热研究

基于三套管式相变蓄热器的特点,提出应用T字形翅片来强化相变蓄热器的传热性能。研究结果表明:添加翅片可有效地降低蓄热器中相变材料的凝固和融化时间,直翅片和T字形翅片的混合强化结构能使凝固过程比未强化结构节省74%的时间,使融化过程节省60%的时间。因此直翅片和T字形翅片的混合使用可以达到进一步强化传热的目的。     

Enhanced thermal performance analysis of buried heat transfer tubes thermal storage system filled with microcapsule particles

The heat transfer enhancement performance of a phase change buried tubes thermal storage system is influenced by major parameters such as arrangement of heat transfer tubes, fin structure and fin geometry size. We developed a three-dimensional numerical model with two different arrangements and five different enhanced heat transfer structures respectively. For the sake of analysis the effects of arrangement of heat transfer tubes, fin structure and fin geometry size. In addition, we applied the enthalpy-transforming model to obtain the liquid fraction and location of the solid-liquid interface at different time in the phase change process. The numerical results show that the melting time of the thermal storage system model with a triangle arrangement is about 6.1% longer than that of the model with a square arrangement. Besides, the melting time of the model with 55 mm tube pitch is about 16.7% shorter than that of tube pitch with 60 mm. Moreover, the buried tube thermal storage system models with circle fins have the shortest melting time, which is 18 seconds. Melting time of the model with circle fins is about 40% shorter than that of the model with smooth tube. In addition, the melting time of the model with 3 mm fin thickness is 10 seconds, which is the shortest. The model with thicker fins means the shorter time of melting process. Moreover, the melting time of the model with 10.5 mm fin spacing is about 23.5% shorter than that of the model with 12.5 mm fin spacing, which is 13 seconds. In conclusion, the main factor of the melting time is the heat transfer area. It provides a guidance for the design and reconstruction of the type of heat storage structure.     

复合相变储能材料的传热性能研究进展

总结近年来国内外相变储能材料的研究状况,包括相变储能材料的制备、传热性能、相变过程数值模拟和应用等,并对复合相变储能材料的传热性能研究方法的前景作了展望。     

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 semi-analytical solution for time-varying latent heat thermal energy storage problems

Latent heat thermal energy storage (LHTES) problems include a lot of boundary conditions that could not be solved by exact solution, so new approaches to solving such problems could revolutionize the advanced energy storage devices. This paper focuses on reformulating the generalized differential quadrature method (GDQM) for a one-dimensional solidification/melting Stefan problem as a fundamental LHTES problem and solves some practical cases. Convergence and comparisons demonstrate that the proposed approach is sufficiently reliable. By checking the accuracy of the proposed approach for the LHTES problem (where Stefan number is below 0.2), it was demonstrated that for all Stefan numbers, the maximum error is less than 3.81% for temperatures. As the usual range of thermal energy storages, for Stefan numbers up to 0.2 the solution yields errors less than 0.2%. Then, the proposed approach is very ideal for such applications. In comparison, GDQM has a more accurate response than an integral solution for Stefan numbers less than 0.2. When this priority of GDQM comes with its low computational cost, it would undoubtedly be preferable.     

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.     

Channel formation and visualization of melting and crystallization behaviors in direct-contact latent heat storage systems

Thermal storage systems are an essential component for increasing the share of renewable energies in residential heating and for the valorization of waste heat. A key challenge for the widespread application of thermal storage systems is their limited power-to-capacity ratio. One potential solution for this challenge is represented by direct-contact latent heat storage systems, in which a phase change material (PCM) is in direct contact with an immiscible heat transfer fluid (HTF). To demonstrate the applicability of the direct-contact concept for domestic hot water production, a PCM with a phase change temperature of 59°C is chosen. To enable cost-efficient implementation of the storage system, a eutectic mixture of two salt hydrates, magnesium chloride hexahydrate and magnesium nitrate hexahydrate, is chosen as the PCM. One key aspect for the direct-contact concept is that, during discharge, the HTF channels in the PCM do not become clogged during the solidification of the PCM. In this study, the formation and topology of the channels in direct-contact systems under an optimized flow condition are investigated via visual observation and X-ray computed tomography. The elucidation of the channel structure provides information on the melting and crystallization behaviors of the PCM, which are shown schematically.     

Review of latent thermal energy storage systems for solar air-conditioning systems

Solar air conditioning is an important approach to satisfy the high demand for cooling given the global energy situation. The application of phase-change materials (PCMs) in a thermal storage system is a way to address temporary power problems of solar air-conditioning systems. This paper reviews the selection, strengthening, and application of PCMs and containers in latent thermal storage system for solar air-conditioning systems. The optimization of PCM container geometry is summarized and analyzed. The hybrid enhancement methods for PCMs and containers and the cost assessment of latent thermal storage system are discussed. The more effective heat transfer enhancement using PCMs was found to mainly involve micro-nano additives. Combinations of fins and nanoadditives, nanoparticles, and metal foam are the main hybrid strengthening method. However, the thermal storage effect of hybrid strengthening is not necessarily better than single strengthening. At the same time, the latent thermal storage unit has less application in the field of solar air-conditioning systems, especially regarding heat recovery, because of its cost and thermal storage time. The integration of latent thermal storage units and solar air-conditioning components, economic analysis of improvement technology, and quantitative studies on hybrid improvement are potential research directions in the future.     

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.     

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.     

Analytical solution of heat transfer in a shell-and-tube latent thermal energy storage system

An analytical solution of a latent heat storage unit (LHSU) consisting of a shell-and tube was obtained by using the Exponential Integral Function and the variables separation technique. The working fluid (water) circulating by forced convection inside the inner tube charges and discharges the storage unit. The comparison between analytical predictions and experimental data shows good agreement. Extensive parametric studies were conducted in order to examine the effect of the pertinent parameters (such as natural convection, mass flow rate of HTF, outer tube radius, pipe length etc.) on the melting and solidification processes of paraffin as a PCM. In order to provide guidelines for system performance and design optimisation, unsteady temperature distributions within PCM during melting/solidification, energy stored, position of moving interface and thermal efficiency have been obtained by a series of numerical calculations and represented graphically.     

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。     

Recapitulation on latent heat hybrid buildings

In the existent paper, the performance of thermal storage hybrid buildings exploiting the latent heat of phase change materials (PCMs) for thermal refrigeration and heating of the contemporary period has been investigated. The conventional buildings consume a large amount of electricity, primarily for the heating and cooling applications. Electricity generation primarily relies on coal-based thermal power plants. The emissions from these establishments pose a serious threat to the environment. Moreover, conventional heating/cooling units rely on exorbitant energy cost. The usage of any kind of thermal storage system is an efficacious way of stockpiling thermal energy and utilizing it when needed. This paper gives a comprehensive overview of the available thermal storage units incorporating PCMs. The various segments of the buildings, viz, ceiling, window, wall, and floor have been analyzed in details. The results are quite promising in terms of load reduction and overall energy saving. Indoor surface temperature reduction of up to 7^oC has been achieved. The energy saving of up to 40% can be realized by employing PCM. A comprehensive list of the PCMs is also tried to build up for end users according to their temperature requirement.     

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.     

Enhancing the thermal response of latent heat storage systems

Latent heat storage systems especially those employing organic materials have been reported to exhibit a rather slow thermal response. This is mainly due to the relatively low thermal conductivities of organic latent heat materials. This paper reports experiments carried out to investigate methods of enhancing the thermal response of paraffin wax heat storage tubes by incorporation of aluminium thermal conductivity promoters of various designs into the body of the wax. Heating and cooling runs were carried out and phase change times determined. It was found that the phase change time reduced significantly by orders of up to 2·2 in energy storage (heating) and 4·2 in energy recovery (cooling). Internal fins performed much better than the star matrices and expanded aluminium performed better than promoters made from aluminium sheet metal in both storage and recovery of heat. © 1997 John Wiley & Sons, Ltd.