Analysis of a novel gravity driven organic Rankine cycle for small-scale cogeneration applications

A naturally-controlled organic Rankine cycle (ORC) without negative work is proposed. Compared with conventional ORCs, the novel system uses gravity instead of a pump to pressurize the working fluid, and hence the problems associated with the pump can be avoided. A possible application of the gravity driven ORC for combined heat and power generation is presented to outline the feasibility. Mathematical models are built. The required height for pressurization and the cycle efficiency varying with the condensation and evaporation temperatures are analyzed on using R123, R245ca, R113, PF5050 and PF5060. The results reveal that on the same condition of operating temperature, the required height for pressurization may vary drastically when the working fluid is different. Fluid of high liquid density and low saturation pressure offers a low height. The power efficiency of the novel ORC is adequate for low temperature applications. With an expander efficiency of 75%, evaporation temperature of 100°C and condensation temperature of 40°C, the highest efficiency is about 10.2% with fluid of R123, which is 0.9% higher than that of the pump driven ORC. And the lowest required height is only about 20.9m with fluid of PF5060. The proposed ORC has both cost and efficiency advantages.