## 5 Waste Heat Recovery (WHR) Based on ORC Technologies  
Compared to traditional steam power plants, ORC systems adopt organic fluids with higher molecular mass and lower boiling points as working fluids. The ORC units are suitable for small-scale applications, with electric power ranging between 1 kW and 10 MW. The main advantage of this technology lies in its ability to exploit low-temperature and waste heat from several sources, such as industrial processes [55; 56], internal combustion engines [57; 58], geothermal energy [59; 60], solar energy [61; 62], biomass [63; 64], and, also, the waste heat generated by data centers [3; 65]. The adoption of ORC for low-temperature WHR has many advantages, like simple mechanical structure, low pressure, convenient maintenance, and remarkable economic benefits [66; 67].  
The increasing interest in small-scale systems for WHR purposes, with time-variable conditions of the heat source and heat sink, raises the issue of off-design plant operation [68; 69; 70]. This area of low-temperature heat sources has been the subject of several comprehensive review papers that delve into a broad range of topics, from technical and energy-related issues to those focusing on the characteristics of the heat source and economic considerations [71; 72; 73; 74; 75].  
Specifically, the study in [75] provides an in-depth review of WHR using ORC, examining various system designs discussed in the literature. The paper covers a variety of ORC configurations, including those used as bottoming cycles, in combined cycles, alongside heat pumps and multi-fluid systems, as well as real-world industrial applications. The conclusions drawn highlight ORC as both technically and economically feasible.  
Typical ORC systems often encounter significant exergy losses in the evaporator due to a mismatch in temperature between the heat source and the working fluid [76; 77; 78]. Trilateral cycle (TLC) and organic flash cycle (OFC) variants are emerging as promising alternatives. These cycles offer improved performance and a better alignment of temperatures between the heat source and working fluid in the evaporator. However, both TLC and OFC are currently undergoing further technical refinement. The unique feature of TLC is its expansion process beginning at the boiling point while the fluid is still in a liquid state, leading to entry into the two-phase region and thus employing a two-phase turbo-expander. On the other hand, OFC represents a middle ground between TLC and the conventional ORC. In OFC, an initial expansion occurs at the boiling point in a two-phase state, followed by a division of the flow rate--one part is brought to saturated liquid conditions via an iso-thermobaric process and then expanded, while the other reaches saturated vapor conditions for subsequent expansion. This approach effectively lowers the latent-to-sensible heat ratio of the working fluid in the ORC heater, cooling the waste heat flux to lower temperatures more efficiently.  
Furthermore, transcritical ORCs are noteworthy due to their superior efficiency over subcritical configurations [79; 80]. The advantages of transcritical systems stem from their ability to offer significant reductions in size and facilitate more effective heat transfer from the heat source to the organic fluid [81; 82]. This efficiency gain is mainly due to minimized irreversibility and reduced energy loss. Comparative analyses of optimized systems have shown that transcritical setups usually have shorter payback periods than subcritical ones across various working fluids, with reductions reaching up to 90%. For instance, a study detailed in the referenced work [81] conducts a techno-economic evaluation of an innovative CHP hybrid biomass/solar transcritical ORC system operating with a medium thermal oil at 350 \({}^{\circ}\)C, demonstrating notable efficiency even under partial load conditions. Similarly, another study examines the transcritical ORC system adoption to harness a high-temperature geothermal source at 230 \({}^{\circ}\)C [82].