Enhancing ultra-low-temperature heat pump performance for high-speed trains using mechanism-based model

Abstract

Heat pumps are a promising technique for providing energy-efficient thermal comfort for passengers in high-speed trains, especially in cold winters of northern China. This study investigates the heating capacity and other performance characteristics of an economizer vapor-injection heat pump system, particularly under ultra-low-temperature conditions. A theoretical analysis of the EVI cycle is conducted, comparing ideal and detailed compressors to highlight differences in the variation trends of COP and heating capacity. Subsequently, a mechanism-based model incorporating a detailed scroll compressor and heat exchangers is developed and validated against experimental data. A 40 kW R410A heat pump operating under rated heating conditions is analyzed, and five experimental results with varying injection pressures (ranging from 0.55 MPa to 1.05 MPa) are used to validate the simulation under ultra-low-temperature conditions. The experiment results indicate that vapor injection significantly boosts the heating capacity by 9.8–24.1 % and improves the coefficient of performance (COP) by 0.7–21.6 %. The performance trends of various models emphasize the importance of accounting for injection port geometry and heat transfer within the economizer during simulation. Under ultra-low-temperature conditions, the optimal injection pressure is identified to be approximately 0.95 MPa, resulting in two-phase injection. Furthermore, an updated injection port design featuring an involute geometry is proposed, yielding a 4.6 % increase in maximum heating capacity.