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Sustainability 2018, 10, 191 11 of 32 where ρf, Af, and cp,f are the density, flow area, and specific heat of the fluid. The equation and boundary conditions for PCM storage can be simplified for particular cases. It has been shown that axial conduction during flow is negligible, and if the fluid capacitance is small, Equations (13) and (14) become [35] ∂u = NTU(ts − tf) (15) ∂Θ ∂tf = NTU(tf − ts) (16) ∂(x/L) where ratio Θ = τmcp,f/ρsAL and effectiveness NTU = UPL/(mcp,f). As depicted in Figure 1, the phase change process takes place in different modes: solid–solid, liquid–gas, and solid–liquid. In the first case, heat is stored by transition between different kinds of crystallization forms. For liquid–gas systems, latent heat is very high, but there are problems in storage control due to the high volume variations during phase change. The most widespread are the solid–liquid PCMs, which have a limited volume variation during latent heat exchange (generally less than 10%) and a fairly high melting latent heat. Melting processes involve energy densities of 100 kWh/m3 (e.g., ice) compared to a typical 25 kWh/m3 for SHS options. PCMs can be used for both short-term (daily) and long-term (seasonal) energy storage, using a variety of techniques and materials. Possible applications of PCMs are as follows: − − − − 1. 2. 3. implementation in gypsum board, plaster, concrete, or other wall covering material being part of the building structure to enhance the thermal energy storage capacity, with main utilization in peak-load shifting (and shaving) and solar energy [38] (in this application, typical operating temperature is 22–25 ◦C, but it can vary as a function of climate and heating/cooling loads); cold storage for cooling plants (operating temperature 7–15 ◦C) [26]; warm storage for heating plants (40–50 ◦C) [26]; hot storage for solar cooling and heating (80–90 ◦C) [26]. Any latent heat energy storage system therefore possesses at least following three components: a suitable PCM with its melting point in the desired temperature range, a suitable heat exchange surface, and a suitable container compatible with the PCM. 4.1. Characteristics Proprieties of PCMs PCMs have been used in thermal applications for a few decades. PCMs have − thermo-physical properties (latent heat of transition and thermal conductivity should be high, and density and volume variations during phase-transition should be, respectively, high and low in order to minimize storage volume), − kinetic and chemical properties (super-cooling should be limited to a few degrees, and materials should have long-term chemical stability, compatibility with materials of construction, no toxicity, and no fire hazard), and − economic advantages (low cost and large-scale availability of the PCMs are also very important). LHS materials are broadly classified based on their physical transformation for heat absorbing and desorbing capabilities. As seen from Figure 6, wide classifications of solid–liquid PCMs, which are further classified into organic, inorganic, and eutectic materials, are presented. PCMs are classified as different groups depending on the material nature (paraffin, fatty acids, salt hydrates, etc.). A few advantages and disadvantages of organic and inorganic PCMs and their influence on solar cooling application are listed in Table 4 [39].PDF Image | Comprehensive Review of Thermal Energy Storage
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