The size-dependent SHC of NPs has also been observed from previous studies [17, 18]. This increment of SHC with reducing particle size could be explained by the Debye model of heat capacity of solids, wherein the heat capacity increases

as the Debye temperature CH5183284 concentration reduces [18]. The Debye temperature decreases with reducing particle size [17], resulting in an increased SHC. Figure 4c shows the SHCs of solid salt and solid salt doped with 13-nm and 90-nm alumina NPs at 0.9, 2.7, and 4.6 vol.%, respectively (measured using model 7020 of EXSTAR). The effect of NP concentration on the SHC of the solid salt doped with NPs is not significant whereas the SHC decreases with increasing NP size. The NP-size-dependent SHC might be due to the fact that the larger NPs have a smaller SHC (see Figure 4b). Nevertheless, the effect of NP addition on the SHC of the

nanofluid is pronounced (see Figure 4a). The effects of size and concentration of the NPs on the SHCs of the nanofluids are illustrated in Figure 5. The temperature-averaged SHCs of nanofluids between 290°C and 335°C were taken to evaluate the effectiveness on the energy storage Ro 61-8048 of the nanofluids in the temperature range. The cross mark data at 0 vol.% in Figure 5 is the SHC of the molten salt without doping with NPs (measured using model Q20 of TA). The red solid squares and blue open squares are the experimental results of the temperature-averaged SHCs of the nanofluids having 13-nm and 90-nm alumina NPs at 0.9, 2.7, and 4.6 vol.%, respectively. A reduced SHC of the nanofluid as compared to that of the base fluid is observed, and the SHC Phosphoribosylglycinamide formyltransferase of the

nanofluid decreases with increasing NP concentration, which is similar to previous studies [6–10]. Furthermore, the SHC of the nanofluids is particle-size-dependent. The SHC decreases with reducing particle size, in contrast to the trend observed in the solid salt doped with NPs (see Figure 4c). The particle-size-dependent SHC in nanofluids had never been reported before and could not be explained by the size-dependent SHCs of alumina NPs since smaller NP has a larger SHC (see Figure 4b). Figure 5 Effects of NP size and concentration on the SHC of the nanofluid. The cross mark at 0 vol.% is the SHC of the molten salt without doping with NPs (measured using model Q20 of TA). The red dash and blue dash-dot lines show the model prediction using Equation 1 for 13- and 90-nm alumina NPs at various volume fractions. The red solid squares and blue open squares are the experimental results of the SHCs of the nanofluids having 13- and 90-nm alumina NPs at 0.9, 2.7, and 4.6 vol.%, respectively. The red solid line and blue dash line are the model predictions considering the nanolayer effect on the SHC of the nanofluid (Equation 5). The theoretical prediction using Equation 1 is also shown in Figure 5, where the values of c p,np are obtained from the temperature-averaged (290°C to 335°C) SHCs of the 13- and 90-nm alumina NPs shown in the Figure 4b (i.e., 1.30 and 1.