Thermodynamic performance of new thermofluidic feed pumps for Organic Rankine Cycle applications

This study develops thermofluidic pump technology that is powered by heat, rather than by electrical or mechanical power. The objective is to improve the performance of heat-recovery by Organic Rankine Cycles, by using a recently-proposed thermofluidic pump. The thermofluidic pump promises low-cost, high-reliability, and, since it does not consume any of the power produced by the expander, improved return on investment. No performance data for the new thermofluidic pump have been reported previously, therefore a thermodynamic model is derived and used to evaluate performance metrics that characterise pump operation and its impact on the overall cycle efficiency. Improved pump configurations are then developed and analysed. A two-stage pump configuration is presented that enhances the thermal efficiency of the cycle. An economiser is also proposed in order to obtain boiler efficiencies similar to those for mechanical feed pumps. It has been shown that the cycle efficiency with the two-stage pump is maximum when there is no net heat input in the intermediate evaporator. The resulting thermal efficiency exceeds the best-possible efficiency that is obtainable with an ideal mechanical pump. The relative improvement in cycle efficiency achieved with the two-stage thermofluidic pump is greatest for low-temperature cycles operating below 100°C, for which the back work ratio is usually higher and the efficiencies of electro-mechanical feed pumps are poorer – yielding a relative increase of the cycle efficiency by up to 30%.