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.     

The thermodynamic effect of thermal energy storage on compressed air energy storage system

With the increasing penetration of renewable energy into energy market, it is urgent to solve the problem of fluctuations of renewable energy sources (RES). Energy storage technology is regarded as one method to cope with the unstable nature of RES. One of these technologies is compressed air energy storage (CAES), which is a modification of the basic gas turbine technology. Electric power supplied by CAES can meet peak-load requirement of electric utility systems. Because there is heat waste in the existing CAES systems during compression process, fossil fuels are used to improve the expansion work to generate peak power. In order to avoid the use of fuels and keep high efficiency of system, CAES system with thermal energy storage (TES) is designed to capture and reuse the compressed air heat. This paper uses a thermodynamic model of a CAES system with TES to analyze the effect of TES on system efficiency. Besides, this paper evaluates the influence of temperature and pressure on the utilization of heat in TES. Results show that even when power efficiency reaches maximum, there is still a proportion of thermal energy left in TES for other use. Meanwhile, the utilization of heat in TES can be affected by pressure in the air storage chamber. With appropriate selection of pressure limits, the utilization of compressed air heat can be optimized.     

Operation strategies guideline for packed bed thermal energy storage systems

Packed bed thermal energy storage (TES) systems have been identified in the last years as one of the most promising TES alternatives in terms of thermal efficiency and economic viability. The relative simplicity of this storage concept opens an important opportunity to its implementation in many environments, from the renewable solar‐thermal frame to the industrial waste heat recovery. In addition, its implicit flexibility allows the use of a wide variety of solid materials and heat transfer fluids, which leads to its deployment in very different applications. Its potential to overcome current heat storage system limitations regarding suitable temperature ranges or storage capacities has also been pointed out. However, the full implementation of the packed bed storage concept is still incomplete since no industrial scale units are under operation. The main underlying reasons are associated to the lack of a complete extraction of the full potential of this storage technology, derived from a successful system optimization in terms of material selection, design, and thermal management. These points have been evidenced as critical in order to attain high thermal efficiency values, comparable to the state‐of‐the‐art storage technologies, with improved technoeconomic performance. In order to bring this storage technology to a more mature status, closer to a successful industrial deployment, this paper proposes a double approach. First, a low‐cost by‐product material with high thermal performance is used as heat storage material in the packed bed. Second, a complete energetic and efficiency analysis of the storage system is introduced as a function of the thermal operation. Overall, the impact of both the selected storage material and the different thermal operation strategies is discussed by means of a thermal model which permits a careful discussion about the implications of each TES deployment strategy and the underlying governing mechanisms. The results show the paramount importance of the selected operation method, able to increase the resulting cycle and material usage efficiency up to values comparable to standard currently used TES solutions.     

Economic analysis of heat storage in energy systems

Thermal energy storage (TES) enables more efficient use of conventional energy conversion plants and enhances the exploitation of renewable energy sources. Storage primarily promotes the replacement of heating oil with less expensive fuels, for instance biomass, coal or industrial waste heat. In this paper, some applications of heat storage for large-scale and centralized residential heating systems are presented. Analysis methods to estimate the economy of storage in these energy-systems are also derived. It was estimated that the maximum market potential for short- and long-term heat storage in the present Finnish heat production system would correspond to about 1–1.3 TWh of supplied heat per annum.     

Experimental investigation on a combined sensible and latent heat storage system integrated with constant/varying (solar) heat sources

The objective of the present work is to investigate experimentally the thermal behavior of a packed bed of combined sensible and latent heat thermal energy storage (TES) unit. A TES unit is designed, constructed and integrated with constant temperature bath/solar collector to study the performance of the storage unit. The TES unit contains paraffin as phase change material (PCM) filled in spherical capsules, which are packed in an insulated cylindrical storage tank. The water used as heat transfer fluid (HTF) to transfer heat from the constant temperature bath/solar collector to the TES tank also acts as sensible heat storage (SHS) material. Charging experiments are carried out at constant and varying (solar energy) inlet fluid temperatures to examine the effects of inlet fluid temperature and flow rate of HTF on the performance of the storage unit. Discharging experiments are carried out by both continuous and batchwise processes to recover the stored heat. The significance of time wise variation of HTF and PCM temperatures during charging and discharging processes is discussed in detail and the performance parameters such as instantaneous heat stored and cumulative heat stored are also studied. The performance of the present system is compared with that of the conventional SHS system. It is found from the discharging experiments that the combined storage system employing batchwise discharging of hot water from the TES tank is best suited for applications where the requirement is intermittent.     

Cascaded latent heat storage for parabolic trough solar power plants

The current revival of solar thermal electricity generating systems (SEGS) unveils the still existing need of economic thermal energy storages (TES) for the temperature range from 250 °C to 500 °C. The TES-benchmark for parabolic trough power plants is the direct two tank storage, as it was used at the SEGS I plant near Barstow (USA). With the introduction of expensive synthetic heat transfer oil, capable to increase the operating temperature from former 300 °C up to 400 °C, the direct storage technology became uneconomical. Cascaded latent heat storages (CLHS) are one possible TES alternative, which are marked by a minimum of necessary storage material. The use of a cascade of multiple phase change materials (PCM) shall ensure the optimal utilization of the storage material.This paper reports experimental and numerical results from the investigation of cascaded latent heat storages with alkali nitrate salts like NaNO₃, KNO₃ and others more. The experiments were conducted with vertical shell and tube type heat exchanger devices under realistic operation parameters. The experimental results were used for a numerical model to simulate different CLHS configurations. Dymola/Modelica was used to conduct the simulation. The outcome of this work shows on the one hand, that the design of CLHS for this temperature range is more complex than for the temperature range up to 100 °C. And on the other hand, the low heat conductivity of available PCM is an obstacle which must be overcome to make full use of this promising storage technology.     

Experimental investigation of a new thermal energy storage system using thermo‐sensitive magnetic fluid and porous medium

In this paper, a novel thermal energy storage (TES) system based on a thermo‐sensitive magnetic fluid (MF) in a porous medium is proposed to store low‐temperature thermal energy. In order to have a better understanding about the fluid flow and heat‐transfer mechanism in the TES system, four different configurations, using ferrofluid as the basic fluid and either copper foam or porous carbon with different porosity (90 and 100 PPI, respectively) as the packed bed, are investigated experimentally. Furthermore, two thermal performance parameters are evaluated during the heat charging cycle, which are thermal storage velocity and thermal storage capacity of the materials under a range of magnetic field strength. It is shown that heat conduction is the primary heat‐transfer mechanism in copper foam TES system, while magnetic thermal convection of the magnetic fluid is the dominating heat‐transfer mechanism in the porous carbon TES. In practical applications in small‐scale systems, the 90‐PPI copper foam should be selected among the four porous materials because of its cost efficiency, while porous carbon should be used in industrial scale systems because of its sensitivity to magnetic field and cost efficiency.     

Experimental investigation on a MnCl2–CaCl2–NH3 thermal energy storage system

Thermal energy storage (TES) is regarded as one promising technology for renewable energy and waste heat recovery. Among TES technologies, sorption thermal energy storage (STES) has drawn burgeoning attention due to high energy storage density, long-term heat storage capability and flexible working modes. Originating from STES system, resorption thermal energy storage (RTES) system is established and investigated for recovering the heat in this paper. The system is mainly composed of three high temperature salt (HTS) unit beds; three low temperature salt (LTS) unit beds, valves and heat exchange pipes. Working pair of MnCl₂–CaCl₂–NH₃ is selected for the RTES system. 4.8 kg and 3.9 kg MnCl₂ and CaCl₂ composite adsorbents are filled in the adsorption bed. Results indicate that the highest thermal storage density is about 1836 kJ/kg when the heat charging and discharging temperature is 155 °C and 55 °C, respectively. Volume density of heat storage ranges from 144 to 304 kWh/m³. The highest ratio of latent heat to sensible heat is about 1.145 when the discharging temperature is 55 °C. The energy efficiency decreases from 97% to 73% when the discharging temperature increases from 55 to 75 °C.     

Using demolition wastes from urban regeneration as sensible thermal energy storage material

Efficient use of solar energy in industrial applications calls for a cost‐effective thermal energy storage (TES) system. Packed bed is a viable technology for high‐temperature TES applications. The packing material acting as the TES material has to be sustainable with favorable thermal properties and compatible with the heat transfer fluid. Demolition wastes—leftovers from urban regeneration projects—in many countries are a big burden economically and environmentally. This paper aims to investigate the potential of using demolition wastes as sensible thermal energy storage (STES) material in packed bed column for industrial solar applications below 300°C. STES material samples have been prepared using binding additives with demolition waste dust. Chemical composition, mechanical strength, and thermal analysis tests have been carried out to determine suitability of STES samples. The DSC results showed that new STES samples had average specific heat capacity of 1000 to 1460 J/kg C in temperature range of 100°C to 500°C. The samples were thermally stable until 750°C under TGA analysis. These results showed that demolition wastes are potential low‐cost sensible heat storage material for applications up to 750°C. Furthermore, valorization of demolition wastes as sensible heat storage material is a sustainable approach in reducing fossil fuel consumption of high‐temperature industrial applications and avoiding the use of natural resources as packing material.     

Development of graphite foam infiltrated with MgCl2 for a latent heat based thermal energy storage (LHTES) system

Thermal energy storage (TES) systems that are compatible with high temperature power cycles for concentrating solar power (CSP) require high temperature media for transporting and storing thermal energy. To that end, TES systems have been proposed based on the latent heat of fusion of the phase change materials (PCMs). However, PCMs have relatively low thermal conductivities. In this paper, use of high-thermal-conductivity graphite foam infiltrated with a PCM (MgCl₂) has been investigated as a potential TES system. Graphite foams with two porosities were infiltrated with MgCl₂. The infiltrated composites were evaluated for density, heat of fusion, melting/freezing temperatures, and thermal diffusivities. Estimated thermal conductivities of MgCl₂/graphite foam composites were significantly higher than those of MgCl₂ alone over the measured temperature range. Furthermore, heat of fusion, melting/freezing temperatures, and densities showed comparable values to those of pure MgCl₂. Results of this study indicate that MgCl₂/graphite foam composites show promise as storage media for a latent heat thermal energy storage system for CSP applications.     

Research and application of molten salts for sensible heat storage

熔融盐具有液体温度范围宽,黏度低,流动性能好,蒸汽压小,对管路承压能力要求低,相对密度大,比热容高,蓄热能力强,成本较低等诸多优点,已成为一种公认的良好的中高温传热蓄热介质.本文对熔融盐显热蓄热技术原理和发展现状进行了简要概述,包括熔融盐的种类,熔融盐显热蓄热技术的原理,关键技术,研发现状及其在太阳能热发电和间歇性余热利用中的应用.认为开展高温熔融盐传热蓄热介质制备,热性能表征和熔融盐流动与传热性能研究,进而完善整个熔融盐蓄热系统,提高蓄热效率,降低管路腐蚀性,提高系统可靠性仍将是未来熔融盐蓄热技术的研究重点.     

Comparative study on adsorbent characteristics for adsorption thermal energy storage system

The adsorption performance of the thermal energy storage (TES) system changes depending on the material properties of the adsorbent itself, but the change of the hardware structure can also substantially change the adsorption characteristics. In this study, a laboratory‐scale adsorption‐based TES system was constructed, and the adsorption performance of three adsorbents was evaluated in the same system to compare the adsorption performance between adsorbents. The adsorption characteristics of silica gel, zeolite 13X, and 4A, which are the most preferred adsorbents in the physical adsorption‐based TES system, were selected for evaluation. Experiments with each adsorbent were performed, including heat recovery to evaluate the heat transfer effect and the amount of heat recoverable in the actual TES system. Experimental results have identified several key characteristics of the adsorption and performance of each adsorbent in the TES system, as well as operating parameters that determine the influence of adsorption performance on the TES system. The actual energy storage density of the adsorbent is affected not only by the enthalpy of adsorption of the material itself but also by other factors. These factors include the difference in thermal conductivity that causes a difference in temperature distribution and the magnitude of mass transfer resistance due to the shape of the adsorbent particle and the actual TES system reactor structure. If the reaction heat generated during the adsorption reaction cannot be effectively released, the adsorption performance is significantly lowered due to the increased temperature of the reactor inside. This phenomenon was commonly observed in adsorbents examined in the present study. The uptake amount, X [g/g], was increased by allowing the inside of the reactor to be maintained at a lower temperature through heat recovery. In case of silica gel, the temperature rise during adsorption reaction is not high due to the difference of isotherm characteristics compared with zeolites, but it is possible to absorb more amount of adsorbate and to recover heat for a longer time. The energy storage density is affected by the temperature increase effect and the uptake amount of adsorbate during the adsorption reaction. The experimental results show that the energy storage density of zeolite 13X is 15% and 28.7% higher than that of silica gel and 4A, respectively, and the temperature rise due to heat generation during adsorption reaction is also high, which is advantageous in adsorption TES system performance.     

Sustainable thermal energy storage technologies for buildings: A review

Energy management in buildings is indispensable which would control the energy use as well as the cost involved while maintaining comfort conditions and requirements in indoor environments. Energy management is intensely coupled with energy efficiency and increasing of which would provide a cost-effective pathway for reducing greenhouse gas emissions. In recent years, the magnitude of energy consumption in buildings seems to crest from the normal demand and that has to be carefully addressed through implementing energy conservative and energy management techniques. In the class of having several energy efficient schemes, thermal energy storage (TES) technologies for buildings are increasingly attractive among architects and engineers. In the scenario of growing energy demand worldwide, the possibility of improving the energy efficiency of TES systems can be achieved from break-through research efforts. The prime intention of this paper is to review the potential research studies pertaining to a variety of latent heat energy storage (LHES) and cool thermal energy storage (CTES) systems solely dedicated for building heating, cooling and air conditioning (A/C) applications. Technical revelations regarding the integration and performance evaluation of heat storage materials in building fabric elements as well as using separate heat storage facility to satisfy the space thermal load demand have been gleaned from numerous research contributions and presented. Emphasis is also given on advanced heat storage materials produced using micro and nanoparticles to realize their improved heat transfer characteristics which would eventually enhance the overall performance of these TES systems. Furthermore, the sustainable aspects of these TES systems to gain the Leadership in Energy and Environmental Design (LEED) credentials for low carbon/high performance buildings are signified.     

Thermodynamic analysis and the design of sensible thermal energy storages

Thermal energy storage (TES) is an important technology for effective and efficient energy management. The proper design and operation of a TES require an understanding of its behavior and characteristics. Here, the transient behavior during charging and discharging of a fully mixed, open TES is modeled and analyzed. Included are developments and analyses of the charging temperature function and the maximum charging temperature of the TES, the charging energy flow function and the maximum heat flow capacity of the TES, the discharging temperature function and the minimum charging temperature of the TES, the discharging energy flow function and the maximum heat flow capacity of the TES, and the expression for one cycle of the TES. The impact of various factors on charging and discharging are investigated. The results show that, by increasing the input energy flow rate, the charging temperature of the TES is raised, and that an increase in the input energy flow rate raises the discharging temperature of the TES in the early stage of discharging, while a decrease in the outlet energy flow rate increases the discharging temperature of the TES in the late stage of discharging. Copyright © 2016 John Wiley & Sons, Ltd.     

Potential applications of thermal energy storage in electric power generation sector (Ⅱ)

简要介绍了储热技术在电力系统中最有潜力的四种具体应用技术,包括太阳能热发电,压缩空气储能,深冷储能,热泵储电,指出了太阳能热发电储热技术在短期内具有很大发展潜力,目前双罐式液体显热储热(储冷)具有较好的整体效率,将在电力系统储热技术中发挥重要的作用.     

Experimental analysis of hydrogen storage performance of a LaNi5–H2 reactor with phase change materials

Energy storage, especially thermal energy storage, has an important place in terms of efficient use of energy. Systems in which phase change materials (PCMs) are used are among the thermal energy storage (TES) options, thanks to their advantages such as energy storage at almost constant temperature. The use of PCM as a TES material in the metal hydride (MH) reactor is an influential method to store the heat released by the exothermic reaction occurring in the hydrogen charging process and to recover this heat with the endothermic reaction occurring in the hydrogen discharge process. In the present study, hydrogen charge and discharge processes in a LaNi₅–H₂ reactor were experimentally investigated and compared with and without PCM. Therefore, a hybrid system was designed by integrating PCM around the cylindrical MH reactor filled with LaNi₅ alloy. The hydration process was carried out at both constant pressure and variable pressure. The temperature changes on the reactor surface and inside the PCM were measured over time. In experiments to determine the change in the amount of hydrogen stored in MH reactors over time, it was determined that the hydrogen storage pressure and reactor design significantly affect the hydrogen charge-discharge rate. Considering the use of MH reactors in transportation vehicles such as automobiles and submarines, designing a hybrid MH-PCM storage system is promising for the development of hydrogen storage technologies and transportation technologies.     

储能技术在冷热电三联供系统中的研究现状与应用

冷热电三联供（CCHP）系统是利用一次能源或可再生能源发电,并通过多种余热回收设备高效利用余热,建立在能源的综合梯级利用基础上的产能系统。用户负荷动态变化及可再生能源输出不稳定会导致冷热电联供系统供、需侧能量不匹配,储能技术可有效解决该问题。本文总结了CCHP系统中储能技术类型及其研究现状,阐明了CCHP系统中电能储存和热能储存技术的应用方式。指出在传统能源与可再生能源相结合、供能系统越发复杂化的能源发展态势下,系统特性、配置优化和对不同场景制定出运行策略是储能技术与CCHP集成系统未来的研究方向。     

State of the art on high temperature thermal energy storage for power generation. Part 1—Concepts, materials and modellization

Concentrated solar thermal power generation is becoming a very attractive renewable energy production system among all the different renewable options, as it has have a better potential for dispatchability. This dispatchability is inevitably linked with an efficient and cost-effective thermal storage system. Thus, of all components, thermal storage is a key one. However, it is also one of the less developed. Only a few plants in the world have tested high temperature thermal energy storage systems. In this paper, the different storage concepts are reviewed and classified. All materials considered in literature or plants are listed. And finally, modellization of such systems is reviewed.     

Review on solar thermal energy storage technologies and their geometrical configurations

Because of the unstable and intermittent nature of solar energy availability, a thermal energy storage system is required to integrate with the collectors to store thermal energy and retrieve it whenever it is required. Thermal energy storage not only eliminates the discrepancy between energy supply and demand but also increases the performance and reliability of energy systems and plays a crucial role in energy conservation. Under this paper, different thermal energy storage methods, heat transfer enhancement techniques, storage materials, heat transfer fluids, and geometrical configurations are discussed. A comparative assessment of various thermal energy storage methods is also presented. Sensible heat storage involves storing thermal energy within the storage medium by increasing temperature without undergoing any phase transformation, whereas latent heat storage involves storing thermal energy within the material during the transition phase. Combined thermal energy storage is the novel approach to store thermal energy by combining both sensible and latent storage. Based on the literature review, it was found that most of the researchers carried out their work on sensible and latent storage systems with the different storage media and heat transfer fluids. Limited work on a combined sensible-latent heat thermal energy storage system with different storage materials and heat transfer fluids was carried out so far. Further, combined sensible and latent heat storage systems are reported to have a promising approach, as it reduces the cost and increases the energy storage with a stabilized outflow of temperature from the system. The studies discussed and presented in this paper may be helpful to carry out further research in this area.     

Thermal performance of a small oil-in-glass tube thermal energy storage system during charging

A very small oil-in-glass tube thermal energy storage (TES) system is designed to allow for rapid heat transfer experiments. An electrical hot plate in thermal contact with a steel spiral coil (SSC) is used to charge the TES system under different hot plate temperatures and under different average charging flow rates. Thermal performance during charging is presented in terms of the axial temperature distribution, the axial degree of thermal stratification, the total energy stored and the total exergy stored. The energy and exergy delivery rates of the energy delivery device (EDD) are also evaluated in relation to the thermal performance of the storage system. Results of charging the storage system under different hot plate temperatures indicate that there is an optimal charging temperature for optimal thermal performance. The results also indicate that exceeding this optimal temperature leads to a degradation of the thermal performance due to increased heat losses. Charging at the same temperature conditions under different flow rate regimes suggests that there is an optimal charging flow rate. This optimal flow rate is a compromise between achieving a greater heat transfer rate in the EDD and achieving a greater degree of thermal stratification in the TES system.