Analysis and optimization of the performance of closed heat pump dryer based on zeotropic mixtures

Compared with traditional electric heating dryers, heat pump dryers are more energy efficient and environmentally friendly. In this work, a novel heat pump dryer with zeotropic mixtures as working fluids and recuperators is studied and compared with the cycle using the corresponding pure working fluid. The effects of the evaporation temperature, and the components and mass fractions of the zeotropic mixtures on the performance of the system are investigated. After comprehensively considering various performance coefficients, this work selects a CO₂/R290 (mass fraction of 20/80) mixture as the optimal working fluid of the heat pump dryer system. Compared with R134a, it significantly improves the coefficient of performance (COP), specific moisture extraction rate (SMER), volumetric heating capacity (VHC), and pressure ratio (PR), with maximum performance improvements of 33.66 %, 41.50 %, 181.44 %, and 46.55 %, respectively. The zeotropic mixture also has significant performance advantages in terms of less charge and exergy destruction. Moreover, the addition of CO₂ as a nonflammable refrigerant to the components reduces the flammability of the working fluid compared with that of pure R290, which improves safety in practical applications.