From solar, geothermal and waste energy, ultra-low-grade heat of 45-60 degrees C is large in scale, and is still interesting challenges in converting into useful refrigeration of 7 degrees C. In this study, a novel cascade refrigeration system was developed to utilize 45-60 degrees C heat. It is composed of a novel two-stage vapor absorption refrigeration (VAR) subsystem with LiBr/H2O working pairs and a conventional vapor compression refrigeration (VCR) subsystem. The VCR subsystem employs potential R1234yf and R1234ze(E) refrigerants. The novel VAR subsystem operates by utilizing ultra-low-grade heat of renewable energy and recycling condensing heat of the VCR subsystem. Based on thermodynamic models of the novel system, its operating mechanism was theoretically investigated and discussed. The results indicate that the novel cascade system can effectively refrigerate by utilizing 45-60 degrees C ultra-low-grade heat. Its operation is mainly determined by the novel VAR subsystem. There are optimal T-m and T-g1 to maximize its performance. T-g2 has a greater impact on the optimal T-m and little effects on the optimal T-g1. Furthermore, it implies that an available temperature span could be obviously enlarged from 90 degrees C down to 45 degrees C. And, progress of utilizing ultra-low-grade heat of renewable energy could be promoted, beneficially. (C) 2020 Elsevier Ltd. All rights reserved.