# Fig nbsp xA Schematic diagram of the

Fig. 8. Schematic diagram of the practical configuration for cascaded thermal storage.Figure optionsDownload full-size imageDownload as PowerPoint slide
4.3.1. PCM selection for cascaded heat-work conversion
For the cascaded heat storage, the HTF temperature is higher than the PCM temperature, both of which are greater than the environment temperature. In this LY2409881 example, HTF (Na–K(22.2–77.8 wt.%) [31]) flows through a pipe with inner radius 0.05 m and length 10 m. The density, thermal conductivity and specific heat capacity of Na–K are 719 kg/m3, 26.2 W/(m K) and 870 J/(kg K), respectively. The inlet temperature and the velocity of HTF are T0 = 873.15 K and u = 0.2 m/s, respectively. The convective heat transfer coefficient inside the pipe is nephron h = 5000 W/(m2 K).
According to Eqs. (15) and (17), the optimal phase change temperature distribution of PCMs can be calculated and PCMs with phase change temperature near the optimal value can be selected for application. Since the phase change temperatures of practical PCMs often slightly differ from their optimal values, the practical exergy is always less than the maximum exergy. To estimate the exergy discount resulting from the practical PCM selection, the relative error was proposed, which was defined as the difference between maximum total exergy and practical total exergy over the maximum total exergy, as shown in Eq. (26)equation(26)E=Extot,max−Extot,prExtot,maxwhere the practical value of total exergy can be calculated withequation(27)Extot,pr=∑i=1nQi,pr(1−TeTmi,pr)