# Potential Evaluation of Twin-Screw Air Expanders with Dual-Lead Rotors Used in PEMFC Systems
## 1 Introduction  
The challenges surrounding environmental degradation and energy crises have gained significant attention due to the ongoing advancement of modern industrial society [1]. As a result of minimal environmental pollution, remarkable energy conversion efficiency, low operating noise levels and high power density, the polymer electrolyte membrane fuel cell (PEMFC) has emerged as a highly preferred alternative for transportation, and distributed and portable power generation [2, 3]. The air supply subsystem is a vital component of PEMFC systems to supply electrochemical reactions with filtered, pressurized and medium-temperature air as an oxidant [4, 5], whose typical flowchart is shown in Figure 1, including air filter, compressor, intercooler, humidifier and expander. Several benefits can be obtained from using compressed air as an oxidant, such as increased stack output power, better water management, quicker dynamic response and smaller stack volumes [6, 7]. However, it consumes approximately 20%-30% of the overall output power generated by the stack [8]. Thus, cutting the air supply system's power consumption has emerged as a major obstacle in the development of PEMFCs. The air supply pressure is gradually rising due to technological advancements in PEMFCs and air compressors. It usually ranges from 2 to 3 bar [9], but can occasionally reach 3 to 4 bar or even higher in distributed power production applications and heavy-duty trucks [10]. The high-pressure (HP) operation of PEMFC systems is a potential direction for future research. An expander, usually of the same kind as the air compressor, at the downstream of the stack output can efficiently recover the remaining energy from exhaust air and further increase the PEMFC's efficiency at higher air supply pressures than 2.5 bar [11], as displayed in Figure 1.  
To date, various types of expanders, such as turbine type [12; 13; 14; 15], roots type [16; 17; 18], scroll type [19; 20] and twin-screw type [21; 22; 23], have been investigated to meet the requirements of PEMFC applications. Studies [24; 25; 26] have conducted comparative analysis of various compressor-expander combinations. It can be concluded that the twin-screw expander (TSE) is an ideal choice for energy recovery in PEMFC systems due to its easy control, high reliability and liquid tolerance, but is also limited by its larger weight, size and noise in comparison to turbines. As a result, the TSE is not ideal for use in PEMFC systems in passenger cars, but can be used in stationary applications in which the air supply pressure is greater than 3.5 bar [27]. Considering the growing significance of HP PEMFCs and distributed power generation in future developments, implementing a TSE is emerging as an exceptionally promising choice. Considerable efforts have been dedicated to the design and performance estimation of TSEs in previous research. [PERSON] [21] demonstrated that the compressor input power may be recovered by 30-35% via employing a TSE, thereby reducing the power output, weight and size of the motor correspondingly. Additionally, he presented various approaches for integrating the expander with a compressor and drive motor. [PERSON] et al. [23] discovered that integrating a TSE into a Mercedes-Benz A-Class car could recover shaft power by approximately 2 kW when operating at full load. [PERSON] et al. [28] presented an innovative screw compressor-expander that combined compression and expansion functions into a single set of rotors. They also conducted theoretical simulations to determine how the system's efficiency and combined compressor-expander's efficiency were impacted by the compressor and expander efficiencies. These studies effectively demonstrate the feasibility of employing TSEs in PEMFC systems. Nevertheless, the low efficiency of contemporary TSEs still limits their practical application. Figure 1: Schematic flowchart of a typical air supply subsystem in PEMFC systems.