Table 2. Thermophysical properties of proposed PCM candidates-sugar alcohol, molten salt, and metallicwith phase-changing point over 100°C.from steelworks 2) has more enthalpy at lower temperature but more exergy at a higher temperature over 100°C. Table 3 lists frequently selected keywords from the collected papers. Papers on LHS and PCM frequently used general keywords such as "phase-change material", "PCM", "thermal energy storage", and "latent heat". Apart from these, interestingly, "solar energy" was also frequently used; these papers dealt with PCMs that can store solar energy through melting for various utilities. Keywords such as "phase-change composites", "thermal conductivity", "shape-stabilized" should be noteworthy in that these are related to general problems encountered during the use of PCMs, that is, capsulation and low thermal conductivity.
Recent Advancements in LHS Technology
PCM CapsulesGenerally, because LHS mainly utilizes the phase change between solid and liquid, the encapsulation of the PCM is necessary to avoid the leakage of a liquid PCM. In addition, Regin et al. 70) noted the functions and requirements of PCM containment: (i) meeting the requirements of strength, flexibility, corrosion resistance, and thermal stability; (ii) acting as a barrier to protect the PCM from harmful interactions with the environment; (iii) providing a sufficient surface for heat transfer, and (iv) providing structural stability and easy handling. PCM capsules are classified into macroand microcapsules.Macrocapsules are the most conventional PCM capsules, and many papers have reported various shell materials such as metal and plastics, and various shapes such as spherical [71][72][73][74][75] and cylindrical. 72,[76][77][78] In contrast, micro-encapsulation of PCM has recently attracted considerable attention [79][80][81][82][83][84][85] because it reduces the reactivity of the PCMs with the outside environment, increases the heat transfer area of the PCMs, and enables the core material to withstand frequent changes in the volume of the storage material during phase change. Table 4 lists trends in manufacturing methods for PCM capsules. Many papers have reported the production of various microcapsules, and the manufacturing methods, core PCM materials, shell materials, thermophysical properties, and capsule sizes of the same have been studied. Micro-interfacial polymerization, 80) in-situ polycondensation, [81][82][83][84][85] and complex coacervation 79) are the most popular microen- ISIJ International, Vol. 50 (2010), No. 9
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1232© 2010 ISIJ proposed the encapsulation of spherical metal PCMs such as Cu and Pb with an electroplated Ni layer to recover industrial hightemperature waste heat such as combustion offgas. The PCM spherical capsule with Ni coverage provided hybrid functions of both heat storage and catalysis. Nickel worked well as a catalyst of the gas phase reaction CH 4 ϩH 2 Oϭ 3H 2 ϩCO at 1000°C.In summary, the encapsulation of low-temperature PCMs is a state-of-the-art technology; in p...
