## 2 State of the Art
A similar thermal energy storage facility operates in the city of Soro (Denmark), which returns 41-58% of the stored 18 MWh to the city's DHS in the form of thermal energy, with 30-41% used to generate electricity [78]. Figure 6 demonstrates thermal energy storage facilities based on various technologies, which are widely used in DHSs [64]. The technologies used in the heating systems of domestic consumers provide off-season storage of thermal energy for its subsequent use. The advancement of thermal energy storage technologies significantly expands the ranges of capacities and speeds with which thermal energy can be collected and distributed to consumers. These features are determined by the choice of storage technology and the design of thermal energy storage. Table 1 shows the main technical characteristics of some types of thermal energy storage units used by household consumers [64]. Figure 6: Thermal energy storage technologies and prospects for their development. The use of electric boilers utilizing electricity to generate thermal energy is a promising solution for DHSs. However, this requires the availability of a sufficient amount of inexpensive electricity produced primarily from renewable energy resources. Electricity is also required in air conditioning and ventilation systems to cool buildings and premises, although absorption chillers that consume thermal energy are also widely used for these purposes. Despite the achievements of technical progress in the field of heating technology, heating systems in the Netherlands, France, Finland, Germany, Italy, China, and other countries still belong to the third generation. This is not so much because of the sufficient hydrocarbon fuels, which is typical of China and Russia, but because of a lack of economic incentives and technical requirements aimed at increasing the energy efficiency of heating systems. These countries make energy conservation a priority in order to reduce thermal energy consumption and minimize losses during its transmission to consumers. This strategy has beneficial effects, including environmental ones, with a decrease in the volume of combustion of hydrocarbon fuels. While the focus on this strand of energy policy is indeed important, it is crucial to recognize the significant impact that can be achieved by simultaneously improving the energy efficiency of thermal energy production. By doing so, it is possible to magnify the positive environmental effects even further. To this end, attention should be paid to the wider use of cogeneration and trigeneration technologies in DHSs of the residential sector. The flexibility of heating systems refers to the interchangeability of primary energy resources utilized to generate thermal energy, types of energy converted into thermal energy, and thermal energy sources that produce and supply thermal energy to the DHS [79; 80; 81; 82]. The introduction of electric boilers, heat pumps [83], and thermal energy storage units has significantly enhanced flexibility of DHSs. Previously, boiler houses and CHPPs running on various types of hydrocarbon fuels were used as sources of thermal energy. The use of seasonal thermal energy storage devices during the non-heating period with a low heat load for residential consumers allows for increasing not only the flexibility of the DHS but also the efficiency of the CHPP operation [84; 85]. This is due to the unique features of CHPPs where the fuel efficiency factor declines significantly and the specific fuel consumption for electricity production increases without the heat cogeneration [86; 87]. The use of thermal energy storage devices enables the production of affordable electricity from CHPPs all year round, which is economically feasible to use for the generation of thermal energy in electric boilers or for power supply to heat pumps. In order to keep buildings and premises cool during the non-heating period, electricity is primarily consumed by air conditioning and ventilation systems [88; 89].