# Heat Storage as a Way to Increase Energy Efficiency and Flexibility of NPP in Isolated Power System
## 1 Introduction  
The development of fuel and energy as well as mineral and raw materials complexes facilities in the Arctic zone of the Russian Federation [1, 2], eastern Siberia [3, 4, 5], and the Far East [6, 7] has great potential and strategic importance for Russia. In these facilities, a wide variety of mineral, raw materials, and fuel resources are mined, such as hydrocarbons, minerals, precious and rare metals, etc. [8, 9, 10]. The development of these facilities should be based on the principles of sustainable development [11, 12], which should provide indicators of energy supply such as environmental friendliness, reliability, safety, and efficiency [13, 14, 15]. Energy consumers of such objects, in combination with connected networks and power supply sources, are isolated energy systems [16]. In such energy systems, a very actual problem is the balance of generated and consumed electric power in the conditions ofuneven daily schedules of electricity consumption [16; 17]. The problem of the balance between generated and consumed power in the power system is the need to maintain equality in generated and consumed power in the power system at every moment (taking into account losses in the networks). Ignoring this creates significant risks for the operation of the power system [18]. There are several ways to solve the problem of power balance in the power system: flexibility of power supply sources, application of network energy storage devices, demand response, and expansion of the power system (inclusion of several time zones and various consumers in the power system) [19; 20; 21]. Each of these methods is characterized by advantages and disadvantages. There is no one universal solution to the problem of the balance between generated and consumed power in the power system. Demand response for industrial enterprises has a wide potential [22]. However, today, industrial consumers have little interest in participating in demand response. This is due to the technical risks of disrupting the production process, costs, and low savings from stimulating demand management [19]. The expansion of a distant power system seems to be hardly possible in conditions of geographical distances from other large energy facilities and power systems [20]. At least, if the goal is to manage load schedules in the power system, the investment in such a solution is unreasonably high. The application of network energy storage devices is a promising, actively developing direction. It has many achievements and implementations. At the same time, the application of network energy storage devices also has several disadvantages and limitations: (1) electro-chemical accumulators: the problem of utilization [23], and cost; (2) pumped storage power plants: problems related to building (flooding of territories, capital investments) [23; 24]; (3) low efficiency, limited capabilities, or low availability of technology--hydrogen storage technologies, condenser batteries, mechanical accumulators, compressed air storage, gravity storage, etc., [25; 26; 23]. The use of maneuverable capabilities of energy sources in power systems is most widespread today [27; 28]. So, today in the Russian energy system, traditional fossil fuel power plants fulfill most of the requirements for regulating the frequency of current and power, and most of these power plants operate in semi-peak mode [29]. The obvious advantage of this method is the simplicity of the organization in comparison with other methods (central regulation, in which one or some sources adapt to the needs of the power system due to technical maneuvering capabilities). This method is limited by the technical capabilities of maneuvering the power plant [30]. In addition, the operation of most conventional power plants in maneuverable modes is associated with a decrease in the capacity factor, increased wear, reduced efficiency, and increased risks of premature equipment failure [31].