Energy, exergy and environmental analysis of a low-grade heat-driven organic Rankine cycle integrated with an ultra-low temperature auto-cascade refrigeration cycle

This research examines a system combining the Organic Rankine Cycle (ORC) with the Auto-cascade Refrigeration Cycle (ARC), driven by a low-grade heat source, to produce ultra-low temperatures. In the ARC, a zeotropic mixture of R170 and R290 is used to achieve temperatures ranging from −65 °C to −45 °C. In the ORC, to ensure efficient operation, various dry working fluids such as R245fa, R245ca, RE245cb2, RE245fa2, R1224yd (Z), R1233zd (E), and R1336mzz (Z) were tested. A comprehensive thermodynamic model was developed to assess system performance. A subsequent parametric study was conducted to investigate the effect of condensing parameters on these performances. Variations in the COP, exergy destruction, and global exergy efficiency were evaluated. The results clearly show that low-grade thermal energy can be used to achieve ultra-low temperatures. Increasing the mass fraction of R170 (from 0.3 to 0.7) results in a 22.22% improvement in the COPARC, mainly due to a decrease in the compression ratio, thereby increasing the refrigerating capacity. The R245ca provides the highest mechanical power (21.3 kW) and cooling capacity (6.53 kW), whereas the RE245fa2 achieves the best exergy efficiency (∼9.5% at 30 °C). Furthermore, Low-GWP fluids (R1224yd(Z), R1233zd(E), R1336mzz(Z)) display very low TEWI values of less than 0.2 TonCO2-eq. Increasing condensing temperature from 30 to 45 °C reduces the efficiency of the ORC by approximately 31%. Under extreme conditions (−65 °C, 40 °C), R1336mzz (Z) is recommended for its optimal performance/environmental impact trade-off (COP≈0.03; TEWI = 0.188 Ton CO2-eq.). Exergy analysis shows that exergy loss is mainly attributed to the compressor, boiler, and evaporator.