Design and performance analyses for a novel organic Rankine cycle with supercritical-subcritical heat absorption process coupling

Organic Rankine cycle (ORC) is a promising heat-power conversion technology for renewable energy development and waste heat recovery. This study proposes a novel ORC that can couple supercritical and subcritical heat absorption processes. The proposed dual-pressure (supercritical + subcritical) heat absorption cycle can potentially better enhance the heat-power conversion efficiency compared with the existing cycle types. For the dual-pressure (supercritical + subcritical) heat absorption cycle, the heat absorption pressures and vapor generator outlet temperature were optimized to achieve the maximum net power output, and its thermodynamic and exergy performance was studied for various heat source temperatures. The thermodynamic performance superiorities of the proposed novel cycle over those of other cycle types were also quantitatively analyzed. Results show that the dual-pressure (supercritical + subcritical) heat absorption cycle has advantages of substantially increasing the system efficiency compared to the conventional subcritical and dual-pressure evaporation cycles, and remarkably increasing the heat absorption capacity compared to the transcritical cycle. For R1234ze(E), the maximum net power output of the dual-pressure (supercritical + subcritical) heat absorption cycle is the largest for heat source temperatures above approximately 135 °C, and it can increase by 19.9%, 49.8%, and 20.4% at most compared with those of the conventional subcritical, transcritical, and dual-pressure evaporation cycles, respectively. The external exergy efficiency is 92.4–94.9% for heat source temperatures of 150–200 °C which indicates that the heat source fluid and working fluid can achieve an excellent temperature match. For other five working fluids, the maximum net power outputs of the dual-pressure (supercritical + subcritical) heat absorption cycle are also the largest.