# Heat Storage as a Way to Increase Energy Efficiency and Flexibility of NPP in Isolated Power System
## 5 Conclusions  
The application of a heat accumulator in the NPP scheme makes it possible to use the accumulated thermal energy of excess waste steam to increase electricity generation during a sharp increase in demand for electricity in power systems. The increase in power in the case under consideration is provided during the estimated time of operation of the accumulator (1600 s) and can be carried out without involvement in the regulation of the power of the active zone. The increase in power in the accumulator discharge mode for the case under consideration was 3 MW or 5.8% of the nominal power of NPP. For the considered small nuclear power plant with an electric capacity of 52 MW, the characteristics of the accumulator are presented below. The number of heat exchange elements (heat exchange tubes, the area around which is filled with the PCM) is 120 units. The inner and outer diameters of the heat exchange tubes are 50 and 56 mm, respectively. The length of each heat exchange element is 120 m, when executed in the form of tube coils, the height of each straight section is 4 m, and the number of straight sections is n = 30 units. The dimensions of the accumulator with a staggered arrangement of the heat exchange surfaces are 6.63 \(\times\) 5.76 \(\times\) 4.0 m, excluding the wall of the accumulator container. The volume of the heat accumulator (for the PCM and heat exchange surfaces) is 153.24 m\({}^{3}\). The volume of the area with the PCM is 116.7 m\({}^{3}\), and the mass of the PCM is 222.6 tons. The volume of the PCM is 76 percent of the accumulator. To increase this value, it is advisable to use methods of heat exchange intensification from the PCM side. This method of application of a heat accumulator considered in this work is a way to reduce the negative impact of maneuvering modes on the equipment of the NPP core. This method of application of the heat accumulator provides an increase in the maneuverability and energy efficiency of the NPP source. This application of a heat accumulator is especially relevant for remote power systems, in which the problem of uneven daily energy consumption is especially acute. The process of accumulator discharge with a PCM is characterized by non-stationary heat exchange. For this reason, the temperature of the heat transfer fluid at the outlet of the accumulator is not a constant value (it decreases during the discharge of the accumulator). Therefore, the accumulator must be selected (designed) in such a way that initially a power reserve is provided, and at the end of the discharge process, the minimum required power is provided. Changes in power during accumulator discharge must be adjusted by the control system. This paper presents a method for determining the approximate dimensions of an accumulator with a PCM, the amount of the PCM, and the heat exchange power at each time. An experimental verification of the model was carried out, and a good convergence of the results was obtained. This method is iterative, that is, using the presented model, the method of successive approximations is determined by: (1) the length of the heat exchange elements; (2) the number of heat exchange elements for heating the required heat transfer fluid flow; and (3) the area filled with the PCM around each direct section of the heat exchange surface. This method can be used for economic justification, pre-design decisions on the development of accumulators with a PCM, and evaluation of the effectiveness of the application. To determine the approximate dimensions of the accumulator with a PCM, the amount of the PCM, and the heat exchange capacity at each moment, according to the proposed method, it is necessary to use a model based on the finite difference method. It is necessary to calculate heat exchange power along the entire length of the heat exchange surface. It is acceptable to use a calculation in which each straight section (or part of each straight section, for example, 1 m) is calculated sequentially one after another. It is acceptable to use a one-dimensional model.