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12 H. Zhang et al./Progress in Energy and Combustion Science 53 (2016) 1–40 Table 5 Inorganic substances/compounds for potential use as PCM [34,36,41,42]. Inorganic compounds Mg(NO3)2.2H2O Hitec XL: 48% Ca(NO3)2—45%KNO3—7%NaNO3 Hitec: KNO3—NaNO2—NaNO3 LiNO3—NaNO3 KNO3/NaNO3 eutetic NaNO3 65.2%NaOH—20%NaCl—14.8%Na2CO3 KNO3 22.9% KCl—60.6% MnCl2—16.5% NaCl KOH MgCl2/KCl/NaCl Na2CO3—BaCO3/MgO Li2CO3 Sb2O3 MgCl2 80.5% LiF—19.5% CaF2 eutetic LiF Na2CO3 K2CO3 NA, not available. Melting point (°C) 130 140 142 195 223 307/308 318 333/336 350 380 380 500–850 618 652 714 767 850 854 897 Heat of fusion Density (kJ/kg) (kg/m3) 275 NA NA 1992 84 1990 NA NA 105 NA 74 2260/2257 290 2000 266 2110 215 2250 150 2044 400 1800 NA 2600 NA 2091 387 5670 542 2140 790 2100/2670 811 NA 276 2533 236 2290 Specific heat at the melting point (kJ/kg K) NA 1.44 1.34 NA NA NA 1.85 NA 0.96 NA 0.96 NA 2.07 0.43 NA 1.97/1.84 NA NA NA Thermal conductivity (W/mK) NA 0.519 0.6 NA NA 0.5 1.0 0.5 0.95 0.5 NA 5 NA NA NA 1.7/5.9 NA 2 2 experimented for solar plant applications, but it has not yet been commercially used [45,46]. The development of high temperature thermal energy storage using PCMs is of increasing interest since they are fairly cheap, have a high energy density, can be available in large quantities, and are able to store and release thermal energy at a constant temperature. In addition to adapting PCM melting points to a high temperature range, their heat conduction charac- teristics need to be improved in order to increase the efficiency of the charging and discharging processes. Although PCMs are ex- pected to have a potential advantage towards energy storage in comparison with sole sensible heat storage, Fig. 4 illustrated that the efficiency of sensible heat storage can in some cases exceed the contained sensible and latent heat of the PCM. Molten salts gen- erally offer significant advantages. To counteract the slow charging/ discharging rates of PCMs, the integration of high conductivity structures has been investigated. Figs. 11 and 12 illustrate some of the experimental results [27]. The experimental results illustrate the effect of different param- eters. The temperature of phase change should be a constant 230 °C, Fig.11. Temperaturevs.timecoolingoftheE-PCM(nitrates)[27].Air-coolingofliquid PCM (1) no inserts; (2) metallic sponge; (3) metallic foam. Air-cooling of solid PCM (4) no inserts; (5) metallic sponge; (6) metallic foam; water-cooling of (7) liquid PCM + foam; and (8) solid PCM + foam. as obtained when a pure substance solidifies. With mixtures, the solidification takes place over a range of temperatures, sometimes referred to as the “mushy region” between the solid and liquid zones [36]. The effect of inserted metallic foam or metallic sponge is evident, with a significantly shorter discharge rate as a result. The curve with forced water cooling (hence with a far higher external heat transfer coefficient) is very steep, and the temperature de- creases from 270 °C to 160 °C in less than 700 s (against 2500– 3700 s when air cooling is applied). This external heat transfer coefficient is an important parameter of the Biot-number, the di- mensionless group that rates convection and conduction. The date treatment can be performed by different methods: (i) Usingananalytical,unsteady-stateconductionapproach,in- volving a material temperature to be dependent on both time and spacial co-ordinates, with solutions provided by Carslaw and Jaeger [47] and Smith [48]. The solution of the equa- tions results in the prediction of the temperature at a given point in the body at time t measured from the start of the cooling or heating operation, for given values of Fig. 12. T vs. cooling time for E-PCMs [27].PDF Image | Thermal energy storage: Recent developments
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