# Heat Storage as a Way to Increase Energy Efficiency and Flexibility of NPP in Isolated Power System
## 2 Materials and Methods
### PCM Selection
\begin{table}
\begin{tabular}{c c c c c} \hline
**Name/Composition** & **Type** & **Melting Point, \({}^{\circ}\)C** & **Melting Heat, KJ/kg** & **Source** \\ \hline ZnCl\({}_{2}\):3H\({}_{2}\)O & Salt hydrate & 4 & 109 & [76] \\ \hline Formic acid & \begin{tabular}{c} Fatty acid \\ (organic) \\ \end{tabular} & 8 & 277 & [75] \\ \hline LiClO\({}_{3}\):3H\({}_{2}\)O & Salt hydrate & 8 & 155–253 & [78] \\ \hline CaCl\({}_{2}\)-6H\({}_{2}\)O & Salt hydrate & 29 & 191 & [76; 78] \\ \hline Paraffins & \
\begin{tabular}{c} Paraffins \\ (organic) \\ \end{tabular} & \(0...90\) & 150-250 & [75; 80; 81] \\ \hline Palmitic acid &
\begin{tabular}{c} Fatty acid \\ (organic) \\ \end{tabular} & 61 & 222 & [75] \\ \hline Beeswax & Organic & 60...68 & 145...395 & [82; 83] \\ \hline LiNO\({}_{3}\)–MgNO\({}_{3}\)-(H\({}_{2}\)O)\({}_{6}\) & Salt hydrate & 72 & 180 & [75] \\ \hline MgCl\({}_{2}\)-6H\({}_{2}\)O & Salt hydrate & 117 & 167 & [76; 78] \\ \hline KNO\({}_{2}\)–NaNO\({}_{3}\) & Eutectic salt & 149 & 124 & [75] \\ \hline LiNO\({}_{3}\)–KC1 & Eutectic salt & 160 & 272 & [75] \\ \hline Hydroquinone & Hydrocarbon & 172 & 258 & [75] \\ \hline Li & Metal & 180 & 432 & [73] \\ \hline LiOH–LiNO\({}_{3}\) & Eutectic salt & 183 & 352 & [84] \\ \hline LiNO\({}_{3}\)–NaNO\({}_{3}\) & Eutectic salt & 194 & 262 & [84] \\ \hline \end{tabular}
\end{table} 
Table 2: Heat storage PCMs. In this work, the most attention was paid to salts and their eutectic compounds due to the suitability of their melting points to the present case, as well as their relatively low cost and availability. The choice of a PCM was made based on the weighted sum method (WSM). We considered PCMs (Table 2) with melting temperatures from 180 to 235 \({}^{\circ}\)C. The lower limit of this range was determined by the maximum temperature of the cold heat transfer fluid (feed water temperature at the outlet of the high-pressure heater = 170 \({}^{\circ}\)C) + 10 \({}^{\circ}\)C. The upper limit of this range was determined by the minimum temperature of the hot heat transfer fluid (condensation temperature of fresh steam = 245 \({}^{\circ}\)C) - 10 \({}^{\circ}\)C.  
Each PCM was evaluated using the following criteria: melting point, melting heat, heat capacity, thermal conductivity, density, and cost. Each criterion was evaluated on a scale from 0 to 5. Each criterion was assigned weighting coefficients, and the sum of the all-weighting factors is 1. The optimal PCM was determined using the integral criterion:  
\[F=\sum y_{i}x_{i} \tag{1}\]  
where \(y_{i}\) is the weighting coefficient; \(x_{i}\) is the value of the criterion. Melting point was the most significant criterion because it is this value that determines the normal operation of the heat accumulator in charge and discharge modes. The weight coefficient of this criterion was taken to be 0.3. The value of this criterion was determined by its proximity to the ideal value. The optimal value of the PCM melting point was determined based on the charge and discharge time of the accumulator. Thus, the accumulator operating in discharge mode for one cycle charge/discharge is 1600 s. If we assume that the accumulator is used twice a day to work in daily peaks of the load, then the accumulator charge time will be several hours during the minimums of the load. Thus, the heat transfer power in the discharge mode should be several times higher than the power in the charge mode. In turn, the heat exchanger power was determined by the temperature difference between the PCM and heat transfer fluid. Thus, the temperature difference between the hot heat transfer fluid and the PCM should be lower than that between the cold heat transfer fluid and the PCM. The optimal ratio of these values is taken to be 1/3 in this work. Thus, the optimal melting point of the PCM is 221.25 \({}^{\circ}\)C. If the PCM melting point = 221.25 \({}^{\circ}\)C, then this criterion = 5; alternatively, if the PCM melting point is equal to the boundaries of the considered range (180 and 235 \({}^{\circ}\)C), then this criterion = 0.