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Annual Review of Heat Transfer, Vol. 15, p.131-177 https://doi.org/10.1615/AnnualRevHeatTransfer.2012004651 3. Storage of latent heat Materials for the storage of latent heat are also named phase change materials (PCM), because of the fact that they change their physical phase from solid to liquid and vice versa. The phase change is always coupled with the absorption of heat when the solid melts — heat of melting — and a heat release when the liquid solidifies. The phase change occurs at the melting temperature Tm. At that temperature, these materials melt when heat is added, but do not exhibit a rise in temperature. Thus, the heat added cannot be sensed and appears to be latent. The most prominent advantage of storage concepts using the latent heat is the option to store energy within a narrow temperature range close to the phase change temperature. A change of one crystalline form into another without a physical phase change may also be considered as storage of latent heat. The heat of solid-solid phase changes (rearrangement of crystalline forms) is typical smaller compared to the heat of melting and solidification. Although enthalpies are large, the liquid-gas phase change is not utilized for latent heat storage due to the large volume of the gas phase. Hence, in most cases the heat of the solid- liquid phase change is utilized. The heat (or enthalpy at p = const.) stored with a phase change amounts to qlatent, where Δhm is the latent heat (enthalpy) of melting (Equation 6). qlatent hm (6) A rough estimate of the heat of fusion of the various phase change materials can be obtained from the following formulas. For many semiconductors, eutectic, and inorganic compounds, the molar heat of melting Δhm, mol is within the range given in Equation 7, where the melting temperature Tm is given in Kelvin and R is the molar gas constant. 2.5RTm hm, mol 5RTm (7) This equation shows that the heat of fusion increases with the phase change temperature and that compounds with a low molar mass are favorable in terms of large heat of fusion values in the relevant unit J/g. When a material is heated from an initial temperature T1 to the melting temperature Tm and the melt is further heated to T2, the total heat stored is given by Equation 8 and 9, where cp is the specific heat capacity in the solid and liquid phase. Qtotal = Qsensible,solid Qlatent Qsensible,liquid = qtotal m (8) Q mc T Tmh mc T T (9) The heat of fusion (or heat of melting) is much greater than the sensible heat. Therefore, phase change materials have a larger volumetric energy storage capacity than sensible storage materials, if smaller temperature intervals are considered. It is important to note that the melting temperature is material-specific and the material can be selected accordingly. Books on the subject of latent heat storage or phase change materials include the following (Humphries 1977, Lane 1983, Lane 1986, Mehling 2008). Review articles have been presented by the following authors (Abhat 1983, Farid 2004, Zalba 2003, Sharma 2005, total p,solid m 1 m p,liquid 2 mPDF Image | Annual Review of Heat Transfer
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