Optimization Study on Fluids for the Gravity-Driven Organic Power Cycle

Abstract

The organic Rankine cycle (ORC) is efficient in using low-grade heat energy, while low pump efficiency and high pump leakage are usually serious problems. A gravity-driven organic power cycle (GDOPC) uses gravity instead of a pump to pressurize working fluid and has the potential to avoid problems associated with the pump. A theoretical method is used to study the performance and suitability of several fluids for GDOPC. The results show that the flow efficiency in high vertical pipes and the pump efficiency determine whether GDOPC gives better performance than ORC or not. When R245fa is selected as working fluid and evaporating temperature is 62 °C, specific energy of GDOPC (flow efficiency is 80%) is 2.5% higher than that of ORC (pump efficiency is 60%). The improvement degree of specific energy and the liquid column height increase with increasing evaporating temperature. R1234yf and R227ea give good performance with specific energy of 4.84 kJ/kg and 4.82 kJ/kg, respectively, while they need a liquid column as much as 76.55 m and 45.65 m, respectively. Although R365mfc and cyclopentane do not give the most excellent cycle performance, they need liquid column height as low as 9.04 m and 10.88 m, respectively. Fluid with low saturated pressure and high density may need low liquid column height and has the advantage to be used in practical applications.