# Potential Evaluation of Twin-Screw Air Expanders with Dual-Lead Rotors Used in PEMFC Systems
## 1 Introduction
In order to enhance the performance of TSEs, it is essential to elucidate their internal loss mechanisms and operational characteristics. [PERSON] et al. [22] conducted a theoretical analysis of the performance of a twin-screw compressor-expander coupled with a 50 kW PEMFC stack under the impact of geometric parameters such as 1/d-ratio, wrap angle and internal compression ratio. [PERSON] et al. [29] and [PERSON] et al. [30] investigated the p-V diagram of a dry expander without the synchronesh gear. They identified clearance leakage and suction throttling loss as primary loss mechanisms for TSEs and emphasized that optimizing the HP port design was crucial for enhancing the recovery power. Our team's previous studies [31; 32] focused on examining the influence of the HP port area as well as the rotor length on the suction loss and operational characteristics; the results indicated that the suction throttling loss, along with the pre-expansion throttling loss, was major reasons for the low expander efficiency. Additionally, matching the operating parameters between the expanders and compressors was also significant in practical applications. The twin-screw compressor can be regarded as a reverse-rotated TSE due to its similar structure but opposite rotation direction. Currently, twin-screw compressors are typically constructed with a constant rotor lead. However, some researchers have proposed the utilization of a non-constant rotor lead for air and refrigeration twin-screw compressors, which is considered a promising technology for achieving higher isentropic efficiency by providing a larger area of the HP port and reducing the leakage in the latter compression process [33; 34; 35; 36; 37; 38]. A non-constant rotor lead can fall into two categories: continuously variable lead and multiple lead. Among them, dual-lead rotors are comparatively easier to manufacture, and the dual-lead rotor prototype has been successfully produced by TU Dortmund University using a five-axis milling machine [33]. Since 2018, Gardner Denver, Inc. [39] has claimed energy cost savings of up to 35% by adopting continuously variable-lead rotors in their twin-screw cyclo-blower product. A general technique for honing screw rotors with an internal meshing wheel that enables the honing of variable-lead screw rotors was proposed by [PERSON] et al. [40]. According to the aforementioned studies, the implementation of a TSE at the stack outlet has been proven effective in enhancing the efficiency of PEMFCs, particularly when operated under high HP conditions. However, there are two main loss mechanisms that hinder its practical application: throttling loss through the HP port and clearance leakage. Recently, variable-lead rotors have shown promising performance improvements in twin-screw compressors, which has inspired the idea of applying them to TSEs for the performance improvement of TSCs. Nevertheless, this concept has not yet been explored in the field of PEMFCs. The purpose of this paper is to evaluate the performance improvement potential of applying dual-lead rotors in TSEs for PEMFC applications, to fill the gap in the existing literature. Therefore, a mathematical model of a TSE was developed to assess the expander's performance under different dual-lead curves, owing to the easy machining of dual-lead rotors, to identify the potential for performance improvement by using variable-lead rotors.