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18 H. Zhang et al./Progress in Energy and Combustion Science 53 (2016) 1–40 Table 11 Costs of candidate salts for CSP [92]. Composition (wt %) 40% KNO3 + 60% NaNO3 KNO3 + LiNO3 + NaNO3 KNO3 + NaNO2 + NaNO3 KNO3 + NaNO2 + LiNO2 + NaNO3 KNO3 + LiNO3 + NaNO3 + MgK LiNO3 + NaNO2 + NaNO3 + KNO3 LiNO3 + NaNO2 + NaNO3 + KNO2 + KNO3 Melting point (°C) 222 117 99 79 101 99 95.7 Heat capacity (kJ/kg K) 1.539 2.32 1.462 1.505 1.579 1.557 1.546 Energy density (MJ/m3) 756 1524 1080 1073 1181 1114 1110 Salt price (US$/kg) 1.080 2.206 1.266 1.928 1.537 1.809 1.797 Two tank system cost/stored energy (US$/kWhth) 31.21 14.66 15.87 19.11 16.15 18.27 18.23 the melting temperatures of some lithium compounds are around 10 °C, 30–35 °C and 70–80 °C which are useful for cooling, compa- rable with Glauber salt and useful for domestic hot water respectively [91]. Regarding their heat of fusion, lithium composites at 100 kJ/ kg are considered the lowest to be commercially feasible and those with a latent heat in excess of 300 kJ/kg are considered of high po- tential towards different applications [91–94]. Lithium compositions for high temperature TES are illustrated in Fig. 20. The advantage of using Li-based PCMs is illustrated in Table 12, where important properties are compared with a common NaNO3/ KNO3 molten salt. Table 12 shows that a lithium salt mixture has the advantage of a lower point of fusion and twice the sensible heat compared with the current solar salt. The density values are quite similar, but the price of the Li mixture is more than two times that of solar salt. Re- garding physical properties as point of fusion, sensible heat and density, a reduction in the size of the molten salt tanks, a reduc- tion of heat tracing parasitics and a reduced amount of material can be obtained. These improvements will reduce the final prices of in- vestment and cost of energy. 3.4.4.3. Lithiummarket. Asfarastheavailabilityofthemetaliscon- cerned, the worldwide reserves of lithium are estimated in 36.72 million tonnes, with Bolivia, Chile, Argentina and China having es- timated reserves of 8.9, 8.04, 7.09 and 5.15 million tonnes respectively. Other countries in Europe, Australia, Africa, Russia and Canada represent reserves of 0.7–1.5 million tonnes each. As illus- trated in Table 12, the cost of Li salts is about 2–3 times the cost of common alkali-nitrates. 3.5. Mechanismstoimprovephasechangematerialapplications 3.5.1. Generalprinciples To improve the efficiency of the charging and discharging pro- cesses of PCMs, the most relevant parameter to be studied is their thermal conductivity. In order to increase the thermal conductiv- ity of PCMs, several heat transfer enhancement techniques have been studied [14,27,34], such as the use of metal carrier structures made of steel or stainless steel; a dispersion of high conductivity mate- rial i.e. copper, silver or aluminium particles, within the PCM; the impregnation of high conductivity porous materials, either as a metal foam (copper, steel or aluminium) or as porous material like graph- ite; the use of high conductivity, low density materials such as carbon fibres and paraffin composites; and the micro-encapsulation of PCMs using graphite [48], polymers, or the nickel film coating of PCM copper spheres [95]. The general review of these techniques was undertaken by Agyenim et al. [96], and the encapsulation of PCM in shells was determined as being one of the most promising and suitable approaches at the present stage of the investigations. The reasons behind this selection are the result of the detailed assess- ment, which summarizes as given below. Fig. 19. Lithium compositions used in building applications (adapted from Ref. 91). Potential lithium compositions for solar energy applications (adapted from Refs. 91–94). Fig. 20. Table 12 Comparison of Li-based PCM and solar salt [93,94]. Mixture Solar salt (60% NaNO3 + 40%KNO3) 25.92%LiNO3 + 20.01%NaNO3 + 54.07%KNO3 Point of fusion (°C) 222 117 Thermal stability (°C) 588.51 Not defined Sensible heat (kJ/kg K) 1.54 2.32 Density (kg/m3) 2192 1720 Price (2014) (US$/tonne) 716.9 1684.4PDF Image | Thermal energy storage: Recent developments
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