Thermodynamic analysis and experimental study of two-stage thermal-gravitational pumping ORC under low evaporation temperatures

To improve the performance of organic Rankine cycle (ORC) systems at low evaporation temperatures, an ORC system with a two-stage thermal-gravitational pumping module (TPM) for replacing the electrically-driven pump is established. The system with scroll expander and R245fa as working fluid is investigated by thermodynamic modeling and experiments under evaporation temperatures lower than 75 degrees C. The effects of evaporation temperatures and parameters of TPM on system performance are analyzed. The complete working process consists of discharge process and pressurization process, which induces periodic variations of system temperature, pressure, and shaft power during practical operation. The stability and continuity of the shaft power of this system are improved compared to the existing pumpless ORC system with a minimum power fluctuation of 22.18 %. Experimental results show that the maximum shaft power of 226.1 W is obtained at the evaporation temperature of 62.4 degrees C, and the maximum thermal efficiency and exergy efficiency of 3.78 % and 24.29 % are achieved at the evaporation temperature of 53.9 degrees C. This study demonstrates the feasibility and the superiority of the presented ORC system with TPM for low evaporation temperature situations.