## 3 IPTMS Architecture Design
### IPTMS with Energy Recovery  
With energy recovery, an IPTMS converts thermal energy to other secondary powers, such as mechanical power and electrical power. A simple IPTMS with energy recovery can be assumed as an integration of an APU (auxiliary power unit) and ECS (environmental control system), consisting of a compressor, a turbine, and a generator, as Figure 3 shows. In this kind of IPTMS, fluids (i.e., gas and coolant) expand within the turbine to achieve a low temperature for cooling. Through the expansion, the turbine output power can drive the compressor and the generator. Thereby, the IPTMS can have heat-cooling and power-generation functions. The IPTMS achieves ECS functions by implementing thermodynamic cycles, such as the Brayton cycle and Rankine cycle. The ECS with reversed Brayton cycle principle is used widely and has become commercially mature [37]. Generally, the working fluid of this type of ECS is air, resulting in low weight, high operating reliability, and low operating cost [59, 60]. Compared with the ECS based on the Brayton cycle, the ECS based on reversed Rankine cycle can eliminate the use of bleed air. Additionally, it can have better efficiency, but it is heavier and less commercially mature [37]. A prominent example of IPTMS based on the Brayton cycle is the IPTMS developed by Honeywell [13, 61]. This IPTMS integrates four conventional aircraft system functions: the auxiliary power unit (APU), emergency power unit (EPU), environmental control system (ECS), and thermal management system (TMS). The system has four operating modes: the Self-Start Mode, the Main-Engine Start, the Cooling Mode, and the Emergency Power Mode. The functions of these modes are as follows:  
Self-Start Mode: In this mode, the battery initially provides power for the starter/generator of the IPTMS to drive the turbomachinery. When the shaft speed of the turbomachinery increases to an initial value, the IPTMS achieves self-sustaining operation and transitions to the Main-Engine Start Mode. Main-Engine Start: In this mode, the system plays the role of APU. The IPTMS can provide power to start the main engine without the use of the aircraft battery. Cooling Mode: In this mode, the IPTMS is powered by the bleed air from the main engine instead of the combustor of the IPMTS. The IPTMS plays the roles of roles of ECS and TMS, implementing active air cooling with a reversed Brayton cycle. The turbomachinery of the IPTMS functions similarly to a conventional air cycle machine, with the added capability of driving a generator to provide power, as Figure 3 shows. The IPTMS generates cooled air which not only serves the purpose of air conditioning but also aids in dissipating heat from avionics, flight-critical electronics, and oil. Emergency Power Mode: When the main engine fails, the system transitions to the Emergency Power Mode. In this mode, the IPTMS plays the role of EPU, and is powered by the combustor of the IPTMS instead of the bleed air. The IPTMS-driven generator provides power for critical inflight electronics or for the main engine restart. Based on this milestone set by the IPTMS of Honeywell, several studies have attempted to improve the IPTMS performance by reducing overall system weight, increasing energy conversion efficiency, and extracting more power. [PERSON] et al. [62, 63] proposed a unit,  
Figure 3: Simple IPTMS with energy recovery. named the Integrated Power and Cooling Unit, for the IPTMS. This unit simplifies the system by reducing the number of recirculation compressor components and limiting the use of bleed and ram air. Although this IPTMS design decreases the system's weight and volume and minimises the negative impact of bleed and ram air, it was found that the fluid efficiency of phase-change cooling and the recovery effectiveness are low. [PERSON] et al. [64] integrated a closed-loop air cycle system with an open-loop gas generator cycle to cool the gas turbine engine heat loads and provide sufficient power for auxiliary power loads.