Abstract A novel pump-free organic Rankine cycle (PORC) is proposed for producing more power output and reducing greenhouse gas (GHG) emissions of a life cycle in utilizing unsteady low-grade heat. Thermal energy of high-temperature and high-pressure vapor (HTHPV) is efficiently recovered at the novel pump-free processes. Its theoretical model is firstly constructed from the Fourier's law of one-dimensional infinite plate, and demonstrated using experimental comparisons. Six potential working fluids (R1234yf, R1234ze(E), R236ea, R245ca, R245fa and R365mfc) are comprehensively evaluated from the perspectives of thermal efficiency, exergy efficiency, environmental friendliness and economy. From comparisons with conventional PORCs, the proposed PORC takes superiority of 19.4% in thermal efficiency. Compared with an organic Rankine cycle (ORC), it brings over 23.1%. R1234ze(E) at evaporation temperature range of 70°C–110 °C and R245ca at 110°C–200 °C respectively show the highest exergy efficiency of 59.6% and 61.9% and the lowest GHG emissions of 17.9 and 16.1 ton/kW. The new PORC affords higher and more stable performance under the unsteady operational conditions. For R1234yf, its thermal and exergy efficiencies are increased by 34.8% and 37.3%, and variation rates reduced by 10.8% and 11.3%, respectively. Its maximum GHG emission is reduced by 34.8%. Furthermore, from levelized cost of electricity (LCOE), the new PORC is 0.0927 $/kWh and reduced by 18.7%. These results indicate that the new PORC with R1234yf, R1234ze(E) or R245ca has comprehensive advantages in power generation with the utilization of unsteady low-grade heat at 70°C–200 °C.