Thermodynamic comparison of organic Rankine cycles employing liquid-flooded expansion or a solution circuit

Abstract

Two modifications to a conventional Organic Rankine Cycle (ORC) are investigated: an Organic Rankine Cycle with Liquid-Flooded Expansion (ORCLFE), and an Organic Rankine Cycle with Solution Circuit (ORCSC). The ORCLFE involves “flooding” the expansion device with a liquid that is in thermal equilibrium with the primary working fluid, while simultaneously expanding the primary working fluid through the same device. The ORCSC employs a zeotropic mixture consisting of two components with a large boiling point difference. The more volatile component in the vapor phase is separated from the absorbent in the liquid phase; the vapor then flows through the expansion device, whereas the liquid absorbent gives rise to a regenerative solution circuit.A thermodynamic model is used to compare these modified ORCs with conventional ORC technology for a range of working fluids including ammonia, water, CO2, acetone, pentane, R134a and R245fa. The working fluid pairs considered for the ORCSC are ammonia–water and CO2–acetone. Based purely on thermodynamic considerations, the conventional ORC using water as the working fluid is found to be more efficient than ORCs that use other working fluids. It yields almost 65% of the Carnot efficiency for a source and sink temperature of 200 °C and 20 °C, respectively. However, the use of water requires low expander exhaust quality, large pressure ratios, and a large expander due to its low density at the expander exhaust. Thus, the practical challenges of using water as a working fluid at typical ORC input temperatures may make its use prohibitive. The ORCLFE always leads to improved cycle efficiency when compared to an ORC for a given working fluid, but it requires the use of a positive displacement expander. The ORCSC shows the lowest efficiencies for the working fluid pairs studied. There are significant practical advantages intrinsic to both the ORCLFE and the ORCSC. For example, the more isothermal expansion of the ORCLFE eliminates the concern of low expander exhaust quality for wet working fluids. The ORCSC provides the ability to use the two-phase temperature glide to match source and sink temperature profiles, facilitates intrinsic capacity control, and can have significantly lower working pressures than an ORC. Ultimately, the overall best choice of cycle and working fluid is highly application-specific and should balance the tradeoffs between efficiency and practical concerns. The thermodynamic analysis presented in this work represents a step toward identifying an optimal ORC solution for an application where the temperature glides of the source and sink fluids are sufficiently small to approximate a thermal energy reservoir.