## 2 State of the Art
As a result, it is possible to reduce the amount of hydrocarbon fuel combustion emissions into the atmosphere by almost 400 thousand tons/year. The most powerful (320 MW) Stockholm heat pump utilizes the water of the Baltic Sea as a source of low-grade heat. This heat pump is located on barges moored to the shore and can cool the seawater from 4 to 2 \({}^{\circ}\)C in winter. The cost of thermal energy production from this heat pump is 20% lower than the cost of its production from heating boilers [65; 66]. It is important to emphasize that this technology is quite expensive, and therefore, the mass introduction of heat pump systems needs appropriate measures of financial support. The main incentives for the use of heat pumps in many countries are an effective system of penalties for CO2 emissions from hydrocarbon fuel combustion and rewards for the use of low-grade heat sources for heating purposes [67; 68]. Thermal energy storage units play a special role in improving the energy efficiency of DHSs [64]. What makes them so appealing is their ability to shift the processes of thermal energy generation and consumption in time [69; 70]. This feature enables the storage of thermal energy across various time spans from hours to weeks or longer periods. Thermal energy storage enables the integration of stochastic renewable energy resources into heating systems, resulting in the production of environmentally friendly heat [71; 72]. To date, the total energy capacity of thermal storage units installed in the world has reached 199 GWh. According to expert estimates, by 2030, the thermal storage market may grow several times, reaching more than 800 GWh [64; 73]. DHSs account for the major share of the installed capacity of thermal energy storage. In 49% of DHSs, where this technology has been introduced (Europe, Canada, China), thermal energy storage serves as the interseasonal energy storage (Figure 5) [64]. The use of thermal energy storage contributes to the decarbonization of the thermal power industry. For example, Finland has put into service a large sand-based thermal energy storage that converts wind energy into thermal energy. An integrated system with heat pumps, electric boilers, and a thermal energy storage unit reduced CO2 emissions by  
Figure 5: The use of thermal energy storage in district heating systems. 30% through the replacement of natural gas-fired boilers. At the same time, the issue of storing surplus electricity generated by wind and solar power plants is addressed. With an increase in the share of stochastic generation in the energy balance, the use of thermal energy storage devices in the electrical-to-thermal energy conversion system enables an elimination (minimization) of limitations on electricity generation from renewable energy resources. A study of the Pennsylvania-New Jersey-Maryland energy market in the USA suggests that the use of hydrocarbon fuels can be reduced by 50-90% through the utilization of either heat pumps with thermal energy storage or resistance heaters with thermal energy storage [63]. Heating systems for domestic consumers started using thermal energy storage in the early 2000s. For example, one of the thermal energy storage units in the UK receives thermal energy from solar collectors for its subsequent storage in eighteen 100 m boreholes to use it for heating in the winter [74]. In Canada, in 2007, a system of solar collectors was installed on the roofs of garages to generate thermal energy and store it in a grid of 35 m wide and 35 m deep boreholes for heating during the cold season [75]. In the city of Braedstrup (Denmark), about 8000 m\({}^{2}\) of solar collectors are used to generate up to 4 million kWh of thermal energy per year and store it in an extensive network of 50 m boreholes for later use [76]. Siemens-Gamesa has built a basalt-based thermal energy storage facility with a temperature of 750 \({}^{\circ}\)C, a capacity of 1.5 MW, and an energy capacity of 130 MWh near the city of Hamburg (Germany) [77].