A novel air-cooled based desuperheating approach for maximizing sustainable energy generation potential from ultra-low grade waste heat – Energy, exergy and environmental (3E) evaluation

The growing urgency of climate change, environmental degradation, and energy inefficiency has steered global research efforts towards sustainable energy use. While significant progress has been made in recovering medium to low-grade waste heat, ultra-low-grade waste heat, particularly in the 75-85 °C range, remains underutilized. This study proposes a novel vapor compression cycle (VCC) integrated organic Rankine cycle (ORC) system for ultra-low-grade waste heat recovery from a 35 kW (10-ton) air-conditioning system, wherein the desuperheating zone of the vapor VCC condenser is thermally coupled with the evaporator of the ORC. This integration enables effective utilization of high-quality waste heat for power generation, reducing overall system energy consumption and environmental impact.
The system's performance is evaluated using five refrigerants for the VCC (R290, R404a, R134a, R407c, R410a) and five working fluids for the ORC (R227ea, Butane, R245fa, R141b, R1233zd(e)) through a comprehensive energy, exergy, and environmental (3E) analysis under various operating conditions. Air is employed as the cooling medium. Among all combinations, R410a in the VCC paired with R141b in the ORC yields the highest system performance, achieving a COPsystem of 2.83, ORC thermal efficiency of 7.67%, a power offset of 0.44 kW, and an annual CO₂ emission reduction of 8.71 tons. The R410a-R1233zd (e) and R410a–R245fa pairs also perform well, with thermal efficiencies of 7.49% and 7.43%, power offsets of 0.42 kW and 0.41 kW, and emission reductions of 8.31 and 8.11 tons/year, respectively. The findings demonstrate the feasibility and potential of direct integration of VCC with ORC for ultra-low-grade waste heat recovery, offering a promising pathway towards more energy-efficient and environmentally responsible air-conditioning systems.