# Heat Storage as a Way to Increase Energy Efficiency and Flexibility of NPP in Isolated Power System
## 3 Results  
### Selected PCM  
The PCMs considered as candidates and their characteristics are presented in Table 4.  
The PCMs considered as candidates and their characteristics. The eutectic salt mixture KNO\({}_{3}\)-NaNO\({}_{3}\) named <<solar salt>> has the largest integral coefficient; therefore, this eutectic compound was chosen as the PCM this work. Other PCMs that were also of interest for this work were LiNO\({}_{3}\)-NaCl and LiOH-NaNO\({}_{3}\)-NaOH. LiNO\({}_{3}\)-NaCl is better than solar salt in its physical properties but is significantly worse in cost. LiOH-NaNO\({}_{3}\)-NaOH has good physical properties and cost, but its relatively high melting point can make the charging process difficult. The solution strongly depends on the chosen weighting coefficients. In future work, we plan to check the selected coefficients by simulating operation of the thermal accumulator using various PCMs, as well as technical and economic calculations. The thermophysical properties of Eutectic salt KNO\({}_{3}\)-NaNO\({}_{3}\) are presented in Table 6. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Compound**} & **Melting Point** & **Melting Heat** & **Heat Capacity** & \multicolumn{2}{c}{**Thermal Conductivity**} & **Density** & **Cost** \\ \cline{2-7}  & \({}^{\circ}\)**C** & **kJ/kg** & **J/(kg\(\cdot\)K)** & **W/(m\(\cdot\)K)** & **Kg/m\({}^{-1}\)** & **5/kg** \\ \hline LiOH–LiNO\({}_{3}\) & 183 & 352 & 2000 & 690 & 2124 & 2.4 \\ \hline LiNO\({}_{3}\)–NaNO\({}_{3}\) & 194 & 262 & 1720 & 590 & 2317 & 1.3 \\ \hline LiNO\({}_{3}\)–NaCl & 208 & 369 & 1560 & 630 & 2350 & 2.2 \\ \hline KNO\({}_{3}\)–KOH & 214 & 83 & 1350 & 540 & 1905 & 0.3 \\ \hline KNO\({}_{3}\)–NaNO\({}_{3}\) & 222 & 110 & 1490 & 510 & 2028 & 0.2 \\ \hline LiBr–LiNO\({}_{3}\) & 228 & 279 & 1380 & 570 & 2603 & 2.4 \\ \hline LiOH–NaNO\({}_{3}\)–NaOH & 230 & 184 & 2000 & 670 & 2154 & 0.2 \\ \hline ZnCl\({}_{2}\)–KCl & 235 & 198 & 656 & 800 & 2480 & 0.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The PCMs considered as candidates and their characteristics. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Compound**} & **Melting Point** & **Melting Heat** & \multicolumn{2}{c}{**Heat Capacity**} & \multicolumn{2}{c}{**Thermal Conductivity**} & **Density** & **Cost** & \multicolumn{1}{c}{**Integral Criterion**} \\ \cline{2-7}  & **Co** & **kJ/kg** & **J/(kg\(\cdot\)K)** & **W/(m\(\cdot\)K)** & **Kg/m\({}^{-1}\)** & **5/kg** \\ \hline LiOH–LiNO\({}_{3}\) & 183 & 352 & 2000 & 690 & 2124 & 2.4 \\ \hline LiNO\({}_{3}\)–NaNO\({}_{3}\) & 194 & 262 & 1720 & 590 & 2317 & 1.3 \\ \hline LiNO\({}_{3}\)–NaCl & 208 & 369 & 1560 & 630 & 2350 & 2.2 \\ \hline KNO\({}_{3}\)–KOH & 214 & 83 & 1350 & 540 & 1905 & 0.3 \\ \hline KNO\({}_{3}\)–NaNO\({}_{3}\) & 222 & 110 & 1490 & 510 & 2028 & 0.2 \\ \hline LiBr–LiNO\({}_{3}\) & 228 & 279 & 1380 & 570 & 2603 & 2.4 \\ \hline LiOH–NaNO\({}_{3}\)–NaOH & 230 & 184 & 2000 & 670 & 2154 & 0.2 \\ \hline ZnCl\({}_{2}\)–KCl & 235 & 198 & 656 & 800 & 2480 & 0.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The PCMs considered as candidates and their characteristics. To determine the characteristics of the accumulator and simulate the processes of discharge, the average thermophysical characteristics of solar salt were used, among those presented in the literature (Table 6):  
* Melting point: 222 \({}^{\circ}\)C;
* Melting heat, 105.3 kJ/kg;
* Heat capacity of the solid phase: 1.01 kJ/(kg\(\cdot\)K);
* Heat capacity of the liquid phase: 1.46 kJ/(kg\(\cdot\)K);
* Thermal conductivity of the solid phase: 0.73 W/(m\(\cdot\)K);
* Thermal conductivity of the liquid phase: 0.53 W/(m\(\cdot\)K);
* Density: 1950 kg/ m\({}^{3}\). An important factor when choosing a PCM, which we have not yet considered, is the chemical compatibility of the PCM with the container materials and the heat exchange surface. So, this is maybe a big issue.