## 2 State of the Art
The cost of electricity produced by CHPPs remains cheaper than that produced using renewable \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Type of Thermal** & **Technology** & **Capacity** & **Power** & \begin{tabular}{c} **Operating** \\ **Temperature**, \({}^{\circ}\)C \\ \end{tabular} & \
\begin{tabular}{c} **Efficiency, \%** \\ **Time** \\ \end{tabular} &
\begin{tabular}{c} **Storage** \\ **Time** \\ \end{tabular} & **Lifetime** \\ \hline \multirow{3}{*}{Sensible} & WTES * & 1 kWh-1 GWh & 1 kW-10 MW & 10–90 & 50–90 & 15–40 years \\  & UTES ** & MWh-GWh & 1–100 MW & 5–95 & \(>\)90 & 50 years \\  & Solid state & 10 kWh-GWh & 1 kW–100 MW & 160–1300 & \(>\)90 & 5000 cycles \\ \hline \multirow{3}{*}{Latent} & Low-temperature PCM *** & \multirow{3}{*}{1 kWh–100 kWh} & \multirow{3}{*}{1–10 kW} & \multirow{3}{*}{\textgreater{}120} & \multirow{3}{*}{\textgreater{}90} & 300–3000 \\  & High-temperature PCM *** & & & & & & \\ \hline Thermo-chemical & Salt hydration & 10 kWh–100 kWh & – & 30–200 & 50–60 & 1000 years \\ \hline \hline \end{tabular}  
* WTTES—water tank thermal energy storage. *** UTES—underground thermal energy storage. *** Low-temperature PCM—low-temperature phase-change material. *** High-temperature PCM—high-temperature composite phase-change material.  
\end{table} 
Table 1: Technical characteristics of thermal energy storage units. energy resources. By directing this electricity to power household air conditioners and ventilation systems of public buildings, their operating efficiency can be increased. The use of absorption chillers, however, is even more effective, since they can utilize the thermal energy produced from the CHPP. The use of trigeneration helps boost the DHS efficiency. The use of thermal energy storage devices, heat pumps, and electric boilers and the transition to trigeneration enhance the flexibility of DHSs [90; 91]. At the same time, conditions are created to improve the CHPP efficiency during the non-heating period, which increases the capacity factor, i.e., enables the generation of a larger volume of affordable thermal and electrical energy [92; 93]. The presence of a large number of various thermal energy sources also augments the flexibility of the DHS [94; 95; 96]. Figure 7 shows a DHS scheme where along with the CHPP and heating boiler houses, there are electric boilers and heat pumps that generate thermal energy. Furthermore, to increase the efficiency of electricity production, thermal energy storage facilities, and absorption chillers are used together with the CHPP. The dotted lines in Figure 7 show the electric power system within the DHS, indicating the directions of power flows. In addition, the figure indicates the connection of the electric power system with the external electrical network, which receives the surplus electricity produced by the CHPP, and which delivers electricity in the event of its undersupply [97; 98]. The DHS illustrated in Figure 7 is highly environmentally friendly, which is crucial in the current context. This factor enhances the appeal of DHSs for investment and fosters a positive perception of their development within society. According to the classification, the system in Figure 7 corresponds to generation four of DHSs. Nowadays, many countries have the necessary resources, technological capabilities, and economic potential to transition from current DHSs, providing heat to households, to more advanced fourth generation of the systems.