Theoretical study of an auto-cascade high-temperature heat pump using vapor injection and parallel compression techniques for steam generation

Utilizing high-temperature heat pump (HTHP) technology to recycle waste heat for steam generation is energy-efficient and environmentally friendly. To enhance the thermodynamic performance of steam generation, vapor injection, and parallel compression techniques were incorporated into an auto-cascade HTHP system. Additionally, dual-stage condensation enabled the simultaneous production of two sets of steam at different temperature levels. Through Python-based models, a comparison of various low-GWP zeotropic mixtures was conducted. The improved auto-cascade cycle exhibited a COP increase of 18.17%-37.4% compared to the basic cycle and 6.3%-21.2% compared to the basic auto-cascade cycle. The two-stage condensation technology enhanced the temperature-matching capabilities of the condensers. Among the zeotropic mixtures, R1234ze(E)&R1336mzz(Z) exhibited the highest thermodynamic effectiveness. The heat transfer degradation of zeotropic mixtures led to a more than 15% increase in the condenser heat transfer area compared to pure refrigerants. Parametric analysis indicated that adjusting key design parameters, such as evaporator outlet superheat and cascade heat exchanger outlet pinch temperature difference, can achieve economical operation while maintaining optimal thermodynamic performance.