## 2 State of the Art
To this end, thermography of  
Figure 3: Generations of district heating systems. heat networks is performed once every 2 years to identify leaks and heat losses at the most significant nodes. The diagnostics are carried out along with the reconstruction of the pipeline DHS. The energy saving measures have halved the consumption of thermal energy for heating 1 m\({}^{2}\) of living space over the past 30 years. Losses of thermal energy from the source to consumers do not currently exceed 3-5% in the main heat networks [53]. In Finland, where the DHS corresponds to 3 GDH, losses in heat networks also do not exceed 1%, and there is 100% heat network redundancy even between neighboring cities. This can be achieved when the distances between cities are small [54; 55]. Not all the countries with district heating have the systems corresponding to 3 GDH. In China and Russia, accidents in the DHS, namely in heat networks, are more frequent. There are examples of pipeline breaks in heat networks, accompanied by human casualties [56]. As the DHS transitions to a more advanced generation, the frequency of accidents goes down, with a decline in the heat transmission losses and a reduction in the specific thermal energy consumption for heating. A high accident rate is observed in DHSs with worn-out and extended heat networks, especially during peak heat loads, when heat networks operate to the limit. The need to combat global climate change and address environmental degradation demands effective ways to reduce the consumption of various forms of energy derived from hydrocarbon fuels. The transformation of electric power and heating systems is greatly influenced by these factors, which in turn are reflected in the ESG standards [57]. These standards play a crucial role in determining the investment attractiveness of energy companies, which encourages the use of appropriate technical solutions when designing strategies for the long-term development of the companies. The distribution diagram of CO2 emissions in different regions of the world (Figure 4) shows the dynamics of their changes over a more than 30-year period [58]. Between 2019 and 2022, the countries of Europe and North America saw a reduction in emissions by 1-2%, which was achieved, in particular, through the measures aimed at enhancing energy efficiency in production and energy conservation in the thermal energy transmission and consumption. There are two ways to boost the energy efficiency of heating systems. These are the introduction of technologies increasing the efficiency of hydrocarbon fuels and the involvement of renewable energy resources in the production of thermal energy [59; 60]. These technologies can reduce the carbon footprint and can serve as a basis for the expansion planning of heating systems [42; 61]. Various types of heat pumps are widely used to enhance the energy efficiency of heat production. Back in the 1980s, the Empire State Building in New York was equipped with  
Figure 4: Dynamics of changes in CO2 emissions from 1990 to 2022.  
a heat pump. Heat pumps are energy-efficient devices for heating and cooling buildings. These pumps utilize renewable energy resources, i.e., heat from soil, water, and air. This solution is a promising avenue to effectively combat global climate change. In the United States, Japan, and some other countries, reversible air-to-air heat pumps are widely used for heating and air conditioning. Europe predominantly uses water-to-water and water-to-air heat pumps. Millions of heat pumps are currently operating in the Unites States, of which more than half are installed for domestic consumers [62; 63]. In Scandinavian countries, large-scale heat pumps are widely used in DHSs. Heating systems for residential and public buildings with low- and high-level ground-water heat sources are developing rapidly [64]. Sweden's experience serves as a clear example, with heat pumps generating around 50% of thermal energy.