Thermodynamic performance comparison of organic Rankine flash cycle with and without ejector for geothermal power output

Abstract

To improve the matching between heat source and working fluid and alleviate the problem of high pump power and low system efficiency in small-scale organic Rankine cycle (ORC), an organic Rankine flash cycle with ejector (EORFC) was proposed. In this paper, the theoretical analysis of the EORFC is conducted based on one-dimensional global model of the ejector and one-dimensional analytical thermodynamic model of the system. A parametrical analysis on the EORFC with R245fa is conducted, the influences of the entrainment ratio and area ratio on the ejector mechanism and system performance are investigated in turn. Then, a comparative analysis is further carried out, which mainly focus on the comparison between the thermodynamic performance of the EORFC and organic Rankine flash cycle (ORFC) by analyzing the influences of the system operating parameters, including overall efficiency of the pump, flashing temperature, condensing temperature and dryness degree at the evaporator outlet. The results show that with the decrease of the entrainment ratio and the area ratio, the pressure lift and ejector efficiency increase, thereby improving the performance of the EORFC. The temperature at the ejector outlet is also inversely proportional to the entrainment ratio, while it is almost unaffected by the area ratio. Meanwhile, the four system operating parameters have positive influence on the net power output of the EORFC and ORFC. When the overall efficiency of the pump is lower than 30% and the condensing temperature is higher than 30 °C, the EORFC can put more net power. Compared with the ORFC, the EORFC always has higher thermal efficiency and exergetic efficiency due to the heating effect of the ejector, besides the EORFC is superior at low cycle efficiencies.