## 2 Integrated Power and Thermal Management in Electrified Propulsion Aircraft
### Thermal Management System  
In TMS, three key elements should be considered: heat acquisition, heat transport, and heat rejection [26]. TMS absorbs the heat from heat sources (heat acquisition) and then transfers the heat to coolants. Finally, through the coolants of cooling loops (heat transport), the heat is rejected to terminal heat sinks (heat rejection). As discussed in Section 2.2, because of additive electric components, the considerations of TMS for conventional aircraft and electrical propulsion aircraft (EPA) are different. The TMS design of a conventional aircraft typically does not consider turbine blade cooling and relevant mechanisms [27]. Therefore, the heat loads considered by the TMS for a conventional aircraft include accessory gearboxes, engine gearboxes, engine-driven generators, bearings of shafts, oil pumps, etc., [27; 28; 29]. Compared with conventional aircraft, the additive heat loads of EPA involve converters (including inverters and rectifiers), batteries, electric bus/feeders, motors, and generators. These additive heat loads cause challenges for system weight, efficiency, power management, and thermal management. Generally, there are three active TMS cooling strategies to transfer heat: (1) air cooling, (2) liquid cooling, and (3) vapour cycle system (VCS)/phase-change cooling. The most common method of cooling is air cooling, which is mature and widely used in civil aircraft [30]. Many coolants for liquid cooling have been studied, including the coolants that have been applied in aircraft such as fuel, oil, water/glycol mixture, polyalphaolephin (PAO), etc. [31], as well as coolants for potential application in aircraft such as supercritical carbon dioxide (sCO\({}_{2}\)), therminol fluids, etc. [32; 33; 34; 35]. Compared with air cooling and liquid cooling, the coolants of VCS cooling are two-phase. Because of the phase-change coolants, VCS cooling has better efficiency but is heavier and less commercially matured [36; 37]. Active air cooling is usually the lightest, because it is open-loop and does not require heat exchangers, coolant pumps, and tanks. However, under high-heat-load conditions, the ram air drag, the weight, and the power due to the air-cooling fan could be considerable. In this case, air cooling is no longer suitable. In terms of liquid cooling, the IPTMS design should consider the coolant performance in thermal and power domains. When a water/glycol mixture is selected as the coolant, the use of propylene glycol can increase the freezing point of the coolant but can increase the viscosity of the coolant, which causes more cooling power for overcoming the pressure drop. Additionally, the IPTMS design should consider how different coolants impact the system weight, power requirements, and induced drag under high- and low-temperature conditions. As three active TMS cooling strategies have their pros and cons, the IPTMS design should compare their performance at the system level and obtain the optimal strategy. The heat sinks for aircraft typically involve fuel and bleed/ram air. Using fuel as the terminal heat sink for an aircraft can improve its engine efficiency as the fuel is preheated. Additionally, unlike other coolants, the coolant of fuel does not require additive components (e.g., coolant pumps and tanks). However, one should be careful that the fuel is not overheated and that the flow rate of fuel is sufficient for heat transfer. This is why HEAusually use ram air as the terminal heat sink instead of fuel. The heat sink of air is low-cost, simple, and effective [33; 35], but it induces drag [36; 38]. Passive cooling can be implemented in a TMS. Compared with active cooling, passive cooling does not require any power for cooling. Generally, the passive cooling could include natural air convection, radiation, a heat acquisition technique [39; 40], and aircraft skin heat exchangers [41; 42; 43; 44].