## 2 Thematic Review and Analysis
### The Distribution Network
#### 2.1.3 Cascading Applications  
In DH systems, cascading involves the sequential use of heat by multiple units or subbranches, where each unit or cluster utilizes the thermal energy from the return medium of the preceding unit. This arrangement can occur at any level--substation, network, or both--enhancing the discharge of supplied heat potential and lowering the overall return temperature by further utilizing the residual thermal energy in subsequent units operating at lower temperatures (see Figure 2 for a network cascading example). This approach maximizes the thermal potential of the supplied heat by ensuring that the return medium, instead of being directly recirculated to the heat source, is further utilized in another heat demand sector before being discharged. Cascading is particularly effective in low-temperature DH systems, where multiple stages of heat extraction align with varying temperature requirements across different consumer types. For successful implementation, cascading requires precise hydraulic balancing and advanced control mechanisms to ensure stable temperature gradients and avoid unintended thermal mixing. Smart temperature control valves and differential pressure regulation are essential for maintaining optimal heat distribution and preventing excessive cooling at the final stages of the cascade [1; 11; 76]. Renovations enhancing a building energy efficiency and upgrades to substations in high-temperature DH systems form the condition with varying operational temperatures across sub-networks. This allows for a cascaded network configuration, where the return line from a higher-temperature sub-network can supply a lower-temperature sub-network within the same DH system [77; 78]. [PERSON] et al. [78] illustrate a network cascading application where a high-temperature DH network (Hoje Taastrup) supplies a low-temperature DH network (Sonderby). In this setup, the high-temperature supply and return mediums are mixed at an \"area-substation\" designed for cascading, adjusting the temperature for the low-temperature network. For instance, with a 90 \({}^{\circ}\)C supply and a 52 \({}^{\circ}\)C return from the high-temperature network, the low-temperature network receives a supply, resulting a final return temperature at 26 \({}^{\circ}\)C. This arrangement allows the high-temperature DH to meet 80% of Sonderby's heat demand, with the remaining 20% covered by mixed supply. The authors suggest that such cascading applications can enhance heat utilization and significantly lower return temperatures in existing DH systems [30; 60; 78]. [PERSON] [79] presents a cascading concept in large-scale DH systems with multiple sub-networks. The primary sub-network operates at 90/70 \({}^{\circ}\)C, serving high-demand consumers such as hospitals, baths, and industries. The secondary sub-network, which receives its supply from the return of the first sub-network, operates at 70/50 \({}^{\circ}\)C. This sub-network serves the majority of consumers and includes traditional radiators, air handling units, and booster heaters for domestic hot water. The tertiary sub-network, supplied by the return from the second sub-network, functions as a modern low-temperature network with temperatures of 50/40 \({}^{\circ}\)C and relies on additional water heaters for domestic hot water. It is also noted that peak load stations and local heat pumps should be considered at the connections between sub-networks. The primary advantage of network cascading is the reduction in network heat loss by lowering operational temperatures in the sub-networks when feasible. The 'URBANcascade' project by the Austrian Institute of Technology [80] aims to enhance DH systems through network cascading.