### Seasonal thermal storage  
In recent years, energy storage has become an important part of renewable energy technology systems [6; 16]. Thermal energy storage (TES) is a technology that stores energy by heating or cooling a storage medium so that the stored energy can later be used for heating and cooling, as well as for power generation [17]. The advantages of using the thermal energy storage in the energy system include an increase in overall efficiency and greater reliability, as well as reduced investment and operating costs [18]. Due to the enormous scale of thermal energy consumption, any improvements in thermal energy management practices could bring significant benefits to society [19]. Seasonal thermal energy storage (STES) is a highly efficient energy use system that uses heat carriers to store and use thermal energy in cycles, which is key to achieving low and zero carbon emissions [12; 20; 21; 22; 23]. One of the criteria for dividing seasonal heat storage is the division according to the type of heat storage medium. Sample systematics of the seasonal thermal energy storage methods are presented in Figure 1. Thermal energy storage methods include the following:  
* Aquifer Thermal Energy Storage (ATES) [1];
* Borehole Thermal Energy Storage (BTES) [2,5];
* Cavity Thermal Energy Storage (CTES) [16,17];
* Tank Thermal Energy Storage (TTES) [2]. The thermal energy that can be stored in the warehouse may come from a solar installation and also from the so-called waste heat that is generated, e.g., in manufacturing processes. Then, the effect of heat storage will be much greater (even twice) and fundamentally more economical. Moreover, the use of underground thermal energy storage systems (UTESs) enables the collection and storage of excess heat generated in summer and then its recovery and use for heating in winter. In this way, the efficiency of using the solar systems increases significantly. According to the literature data, the optimal working medium used in the seasonal thermal energy storage should have the following parameters [18]:  
* high heat capacity;
* high energy stability (stored thermal energy);
* high chemical stability (in the case of water);
* low operating costs;
* environmental neutrality. Energy storage can be characterized by the following parameters [19]:  
* capacity, which determines the amount of energy that can be stored. This depends on the storage method (technology), the medium used, and the size of the system;
* storage unit power, i.e., quantity determining how quickly the storage unit can be charged or discharged;
* system efficiency, i.e., the ratio of energy supplied to the user to the energy needed to charge the warehouse--this value takes into account energy losses during storage as well as losses during charging and discharging the battery;
* storage period;  
Figure 1: Division of the methods for seasonal thermal energy storage. * charging and discharging time;
* costs related to efficiency--this value depends on capital expenditure and operational life of the system. The current geopolitical situation (Russia's suspension of energy supplies to Finland, which was a consequence of this country's declaration of willingness to join NATO) and the above-mentioned geopolitical factors have resulted in Finland taking various innovative initiatives in the energy sector. The project realized in Finland tried to provide citizens with enough heat and light. One of the ideas was to use sand as an energy store. Finland has introduced the world's first commercial \"sand battery\". The installation operates in the city of Kankaanpaa, approximately 230 km north-west of Helsinki. Houses, offices, and a public swimming pool are heated there with low-emission thermal energy, stored in a steel container 4 m wide, 7 m high, filled with 100 tons of sand. The advantage of a sand bed is that it is a durable, inexpensive material and it can store heat at a temperature of about 500-600 \({}^{\circ}\)C for many months.