Science Bulletin 62 (2017) 231–233
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News & Views
Challenges in various thermal energy storage technologies Chenzhen Liu, Zhonghao Rao
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School of Electrical and Power Engineering, China University of Mining and Technology, Xuzhou 221116, China
With the worsening of energy shortage and environmental pollution, energy storage has received much attention in the fields of renewable renewable and sustainable sustainable energy in recent recent years. years. It is widely widely known that energy storage is a technique storing excess energy in one form and converting it back to the same form or another when necessary [1] [1].. Especially energy storage occupies an important position in solar energy, wind energy and ocean energy. Generally, energy storage methods include mechanical energy storage, electrical energy storage and thermal energy storage (TES) [2] [2].. Due to the use of natural natural resources, resources, mechanical mechanical energy storage has advantages of environmental protection, large scale, long life cycle and low operation costs. And electrical energy storage has advantages tages in some areas like convenient convenient application, application, low pollution pollution and high conversion efficiency. Mechanical energy storage usually can be subdivided into pumped-hydro energy storage, compressed air energy storage, flywheel energy storage, etc. Electrical energy storage can be divided into battery energy storage, flow battery energy energy storage, storage, supercond superconductin ucting g magnetic magnetic energy energy storage storage and super capacitor energy storage, etc. In the face of various energy storage technologies, how will thermal energy storage accelerate its development and application? According to the material property, thermal energy storage can be classified as sensible heat storage, thermochemical heat storage and latent heat storage [3] [3].. The classification of thermal energy storag storage e materi materials als is sho shown wn in Fig. Fig. 1. Sensi Sensible ble heat heat storag storage e is achieved achieved by changing changing the temperature temperature of the storage material without changing its phase. The storage performance of a storage system depends on the specific heat and density of the storage material. material. Sensible Sensible heat storage material material can be liquid liquid materials materials (water, oil, etc.) or solid materials (rock, soil, sand, etc.). Liquid materials can be used for storage and as a transport medium in thermal energy field. Water is one of the most commonly used thermal thermal storage material for low temperature, temperature, which is widely widely used in solar energy system. In the intermediate and high temperature ranges, molten salt and oil are candidate sensible storage materials. They are widely used in power tower systems and metallurgical industries as thermal energy storage and heat transport fluid. Solid media storage avoids the drawbacks of high vapor pressure and other limitations of liquid. However, the disadvantages of large size and temperature swing are inherent in most sensible heat storage. Especially, a large size thermal storage system fea⇑
Corresponding author. E-mail address:
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[email protected] (Z. (Z. Rao).
tures tures large large space, space, high high cost cost and and great great therma thermall losses losses.. Furthe Furtherr research is needed to reduce the size of thermal storage system by increasing thermal capacity. Thermochem Thermochemical ical heat storage storage can absorb absorb and release release large amounts amounts of thermal thermal energy energy when the molecular molecular bonds of substance are broken and reformed in a completely reversible chemical reaction. The thermal storage performance of thermochemical heat storage system is determined by working conditions, dynamic character characteristic isticss and reversibil reversibility ity of the material. material. In recent recent years, years, many many resear researche chess on thermo thermoche chemi mical cal materi materials als (such (such as salt salt hydrates, methane, carbonates and metal hydride) have exhibited its great prospects of application in thermal energy storage. Especially, salt hydrates are commonly used for thermochemical heat storag storage, e, which which have have high high therma thermall capaci capacity ty and sma small ll volume volume changes during melting and relatively high thermal conductivity. However, there are some serious problems, such as phase segregation, supercooling and decomposition. Therefore, the applications of salt hydrates require the use of thickening and nucleating to minimize minimize supercooling supercooling and phase segregation. segregation. In addition, addition, the application and chemical reaction of thermochemical heat storage are so complex that it is very difficult to conduct them. Strict operation condition, huge instrument investment, short life of energy storage system, high corrosive of energy storage material to equipment remain to be solved. Therefore, further research should be focused on the design of appropriate thermochemical heat storage system which has good reversibility, low corrosion and long service life. If these problems of thermochemical heat storage system can can be solv solved ed prop proper erly ly,, it will will be mu much ch more more prom promis isin ing g in application. Latent heat storage is realized by storing or releasing thermal energy energy through through the phase phase change change process process of thermal thermal mediums mediums which is called phase change materials (PCMs) [4] [4].. Compared with sensible sens ible heat storage storage materials materials,, PCMs have advantages advantages of high energy storage density and thermal energy storage at a nearly constant stant temperatur temperature. e. The forms of phase phase change change are solid–solid, solid–solid, solid–liqu solid–liquid, id, solid–gas solid–gas and liquid–ga liquid–gas. s. Solid–gas Solid–gas and liquid–gas liquid–gas have high latent heat compared with others [5] [5].. However, they have great difficulty in practical thermal energy storage application, such as large volume change during solid–gas or liquid–gas phase transition. The solid–solid phase change has the advantage of small volume change during phase transition, but its latent heat is usually much smaller than the others. Solid-liquid transformation has comparativ comparatively ely larger latent latent heat than solid–soli solid–solid d and smaller volume change than solid–gas and liquid–gas. Therefore,
http://dx.doi.org/10.1016/j.scib.2017.01.019 2095-9273/ 2017 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.
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C. Liu, Z. Rao / Science Bulletin 62 (2017) 231–233
Thermal energy storage materials
Thermochemical
Latent heat storage
Sensible heat
energystorage materials
materials
storage materials
Ammonia
Solid-Gas Solid-Liquid Solid-Solid Liquid-Gas Liquids
Solids
Salt hydrates Organics
Inorganics
Carbonates
Paraffins
Salts
Metal hydride
Esters
Metals
Metallic oxide
Fatty acids
Alloys
Water
Rocks
Moten
Metals
Methane salts
Soil
Oil based fluids
Glycols Fig. 1. Classification of thermal energy storage materials.
solid–liquid PCMs are commonly used in the applications of thermal energy storage. The main selection principles of PCMs are as follows [6]: high latent heat, desirable phase change temperature, high thermal conductivity, small volume changes, little or no supercooling, good chemical stability, no chemical decomposition and corrosion resistance to container. Because of the increasing people’s living standard and decreasing energy supply, thermal energy storage can be applied in the fields of solar energy utilization, intelligent building, thermal regulating fabric, waste heat recovery, battery thermal management system, cool storage air-conditioning technology, etc. [7,8]. Consequently, the development of different thermal energy storage technologies will be urgently required. In the context of different applications, thermal energy storage plays different roles. And it can be used as thermal energy storage bodies or auxiliary thermal buffer for heat dissipation. Currently, PCMs are the most widely used thermal energy storage materials, and their state can be stationary or flowing when applied. In solar energy utilization and industrial waste heat recovery, PCM store solar energy in the form of latent heat and release the stored thermal energy when necessary. This solves the problem of low density, instability and discontinuity of solar radiation with time of the day and the day of the year when utilizing solar energy, or makes a balance between the supply and demand of waste heat in time and space. Different PCMs have different types in solar energy utilization and industrial waste heat recovery. Organic PCMs are mainly used for low temperature utilization of solar energy and industrial waste heat recovery, while inorganic salt, metals and alloys are mainly used for high temperature utilization of solar energy and industrial waste heat recovery. Application of thermal energy storage technology in solar energy utilization and industrial waste heat recovery effectively alleviates environment pollution and increases the energy utilization. However, in the field of either solar energy utilization or industrial waste heat recovery, thermal energy storage technology has not yet been put into widespread use. It has many technical problems in practical application, such as high cost, short life of energy storage system, low thermal conductivity of PCMs and energy storage material’s high corrosion to equipment. Therefore, the application of thermal energy storage technology in the fields of solar thermal utilization and industrial waste heat recovery can be optimized by improving the thermal conductivity of PCMs, optimizing the energy storage system and reducing the cost in future works.
PCMs have been gradually applied in intelligent building since 1970s. The thermal energy storage technology used for intelligent building is impregnating the PCMs into construction materials, including concrete, plaster, gypsum board or wall surface coating [9]. The PCMs can store surplus cold or heat which is produced by air conditioning, heater and ambient natural environment. When the indoor temperature of the building is too high or too low, the PCM relieves the stored cold or heat to indoor ambient. Thereby, the indoor comfort is improved by reducing the fluctuation of indoor temperature. In addition, the PCM can be used to store cold energy converted from electric energy by refrigerating machine in the night and then release cold energy during middle-of-the-day peak period [10]. The application of PCMs in building has good economy for users especially in the areas where peakvalley electricity price exists. But there are three factors that restrict the wide application of PCMs in the field of intelligent building. Firstly, the reversibility and stability of PCM/construction material composites need to be improved. Secondly, the PCM is liable to leak from construction material after long term thermal cycling. Thirdly, the thermal stress produced during the phase transformation and corrosion of PCMs leads to the destruction of the construction material. Hence the compatibility, stability and durability of PCMs and construction materials need to be further improved by packaging technology of microcapsule and improvement of compound technology of PCM and construction material. Thermal regulating fabric is an innovative product which consists of fabric and PCM, and can be used in many specific working environments like underground coal mining. This technology is adding microencapsulated PCM which is encapsulating PCM in a microcapsule to prevent their leakage during solid–liquid phase change process to fabrics. When the environment temperature reaches the melting point of the PCMs, the thermal regulating fabric start to absorb the heat and the reverse process takes place when the temperature falls below the melting point. Therefore, PCMs used in fabrics have significantly improved thermal comfort of users due to their high thermal storage capacity. In addition, the thermal regulating fabric can be maintained at an appropriate temperature for a longer time than traditional fabric at high or low temperature environment. However, after fabric is compounded with phase change material, it feels worse, and long-term use will cause phase change material leakage, environmental pollution and decrease of heat storage capacity. These problems affect their practical application. Therefore, further research is needed to investi-
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gate microencapsulated PCM with good dispersion, uniform particle size and no leakage for thermal regulating fabric. The high temperature of battery during charge and discharge will lead to the decrease of battery performance including efficiency, cycle life, reliability, security, etc. [11]. Therefore, battery thermal management is particularly important. The battery thermal management system based on PCM has been a novel technology in recent years. The thermal management of battery is achieved by latent heat of PCM storing the heat which is produced by battery during charge and discharge. In the selection of PCMs, the melting temperature range between 25 and 40 C is appropriate for lead acid, Ni-MH and Li-ion battery thermal management systems. The battery thermal management system based on PCMs has many advantages compared with air and liquid, such as none parasitic power consumption, low cost, easy maintenance and long life. However, there are still some problems in the battery thermal management by using PCMs, such as leakage and low thermal conductivity of PCMs and increasing the weight of battery thermal management system. Therefore, improving thermal conductivity, preventing the leakage and lightweight design of PCMs are key points of researches in battery thermal management technology. Thermal energy storage technology has a long history, and it has broad application prospect in various fields. Thermal energy storage materials play a crucial role in the thermal energy storage technology. Among various thermal energy storage materials, PCMs are the most promising materials for thermal energy storage. However, no matter in which area PCMs are applied, some problems of PCMs used for thermal energy storage technology have appeared, such as leakage during phase change process, low thermal conductivity, flammability, decomposition, corrosion, supercooling, phase separation, etc. To a large extent, these shortcomings restrict the popularization of thermal energy storage technology in various fields. In order to overcome these shortcomings, heat transfer enhancement and encapsulation of PCMs should be carried out. In heat transfer enhancement, there are various methods, including inserting metal fins, dispersing high thermal conductivity materials (i.e. Cu particle, Al particle, expanded graphite, graphene, carbon nanotubes) and inserting porous materials (i.e. copper foam, aluminum foam, nickel foam, carbon foam) into PCMs. Encapsulation of PCMs, putting PCM itself in a capsule to form the shell-core composite material, has a significant effect on improving leakage, decomposition, corrosion, supercooling, phase separation. In addition, the development of new thermal storage materials with high heat storage density, low cost, environmental friendliness and long cycle life is of great significance to the ther-
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mal energy storage technology. Moreover, the practical application of the thermal energy storage technology has many limitations, such as compatibility of thermal energy storage material with heat reservoir, optimal heat transfer of heat reservoir, cost and safety. Meanwhile, the large-scale application and popularization of thermal energy storage will be an important direction in the further research. These are the new challenges of thermal storage technology in the new period. Only research and exploration from two aspects of thermal energy storage material and system are the possible ways to solve the above problems and realize the popularization and application of the thermal energy storage technology. Conflict of interest The authors declare that they have no conflict of interest. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. U1407125) and the Key Disciplines Fund of China University of Mining and Technology (No. XZD201601). References [1] Mahlia TMI, Saktisahdan TJ, Jannifar A, et al. A review of available methods and development on energy storage; technology update. Renew Sust Energ Rev 2014;33:532–45. [2] Kousksou T, Bruel P, Jamil A, et al. Energy storage: applications and challenges. Sol Energy Mater Sol Cells 2014;120:59–80. [3] Lee KS. A review on concepts, applications, and models of aquifer thermal energy storage systems. Energies 2010;3:1320–34. [4] Agyenim F, Hewitt N, Eames P, et al. A review of materials, heat transfer and phase change problem formulation for latent heat thermal energy storage systems (lhtess). Renew Sust Energ Rev 2010;14:615–28. [5] Farid MM, Khudhair AM, Razack SAK, et al. A review on phase change energy storage: materials and applications. Energy Conv Manag 2004;45:1597–615. [6] Ge Z, Li Y, Li D, et al. Thermal energy storage: challenges and the role of particle technology. Particuology 2014;15:2–8. [7] Liu CZ, Rao ZH, Zhao JT, et al. Review on nanoencapsulated phase change materials: Preparation, characterization and heat transfer enhancement. Nano Energy 2015;13:814–26. [8] Sharma A, Tyagi VV, Chen CR, et al. Review on thermal energy storage with phase change materials and applications. Renew Sust Energ Rev 2009;13:318–45. [9] Wang X, Zhang Y, Xiao W, et al. Review on thermal performance of phase change energy storage building envelope. Chin Sci Bull 2009;54:920–8. [10] Oro E, de Gracia A, Castell A, et al. Review on phase change materials (PCMs) for cold thermal energy storage applications. Appl Energy 2012;99:513–33. [11] Zhang Q, Huo Y, Rao Z. Numerical study on solid–liquid phase change in paraffin as phase change material for battery thermal management. Sci Bull 2016;61:391–400.
