# Heat Storage as a Way to Increase Energy Efficiency and Flexibility of NPP in Isolated Power System
## 2 Materials and Methods
### PCM Selection  
The second stage of this work is PCM selection. A PCM was selected based on the required thermophysical properties. The main parameter was the melting point of the PCM [70; 71]. In this way, the melting temperature of the selected PCM should be in the range between the temperatures of a hot heat transfer fluid and a cold heat transfer fluid. Visually, the temperature regimes of a hot heat transfer fluid, a cold heat transfer fluid, and the PCM in the charge and discharge modes (when superheated steam is used as a hot heat transfer fluid and water as a cold heat transfer fluid) are shown in Figure 2. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Unit of Measurement** & **Number** \\ \hline Thermal power of the reactor & MW & 175 \\ \hline Steam generator performance in nominal mode & t/h & 261 \\ \hline Steam pressure from steam the generator & MPa & 3.8 \\ \hline Steam temperature from steam the generator & \({}^{\circ}\)C & 295 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Nominal technical characteristics of the RITM-200 [37]. Figure 2: Temperature regimes of the hot and cold heat transfer fluids; PCM in the modes of charge and discharge of the heat accumulator with a PCM. In this paper, a PCM was selected based on the following parameters of the hot and cold heat transfer fluid (according to the parameters of the nuclear power plant RITM–200 and the steam turbine unit TK–35/38–3.4):* The hot heat transfer fluid is superheated steam with a temperature of 295 \({}^{\circ}\)C and a pressure of 3.8 MPa;
* The cold heat transfer fluid is feed water with pressure of 3.8 MPa with nominal temperatures at the inlet and outlet of the accumulator: T\({}_{\rm{inp}}\) = 140 \({}^{\circ}\)C, I\({}_{\rm{out}}\) = 170 \({}^{\circ}\)C (nominal temperatures of feed water at the inlet of HPH1). Consumption of the heated heat transfer fluid: 261 t/h (100% of the steam generator's capacity in nominal mode). The accumulator time in the discharge mode is assumed to be 1600 s.  
Among the PCMs that correspond to the required temperature regime, it is advisable to select a PCM with the following characteristics [71; 72]:  
* High heat of phase transition;
* High heat capacity;
* High thermal conductivity (in solid and liquid phases);
* High density;
* Stability of thermophysical properties during multiple heating-cooling cycles, melting, and solidification cycles;
* Chemical stability and neutrality with structural materials of container construction;
* Low coefficient of thermal expansion and density change during the phase transition;
* Low cost and availability. As a heat storage PCM, the following are used depending on the melting temperature:  
* Metals (most widely available metals have melting points of 400...1100 \({}^{\circ}\)C) [73; 74];
* Salts and their eutectic mixtures (melting points 200...1000 \({}^{\circ}\)C) [70; 75; 76];
* Salt hydrates (melting points 5...120 \({}^{\circ}\)C) [77; 78; 79];
* Organic compounds: paraffins, fatty acids, etc. (melting points 5...120 \({}^{\circ}\)C [70; 75]. Some of the applied heat storage PCMs with thermophysical characteristics are presented in Table 2.