Thermodynamic assessment of micro-ORC systems with internal vapor-driven pumping for low-grade energy harvesting

In this study, the thermodynamic effect of implementing in Organic Rankine Cycles (ORC) a pumping phase fully sustained via direct organic-vapor expansion is analysed and reviewed from a purely theoretical perspective and through a parametric numerical evaluation. In particular, in the conceived system, the pumping unit serving the ORC is directly driven by a dedicated expansion machine, mechanically coupled to the ORC pump and operated with a fraction of the process vapor. This concept, already implemented in large power systems, represents instead a novel integration strategy for micro- and small-scale heat recovery systems, aimed at reducing auxiliary electrical consumption and enhancing the net power output of micro-ORC units equipped especially with volumetric machines. A comparison between the proposed cycle setup and the standard ORC pumping arrangement, in which the pump is driven by an external electric motor, is presented. A systematic analysis is carried out on different organic fluids and cycle parameters for low/medium-grade and micro-ORC heat recovery applications, in order to characterize the achievable performance and to optimize the system design, under the subcritical cycle assumption. The key conditions allowing for a power gain via the proposed ORC variant are identified. The required vapor for running the ORC pump in the dedicated expander can range from 5 % to 30 % of the operating fluid flow, depending on the thermodynamic and design conditions. In the most promising case of low-grade heat application, a variable increase in power can be achieved with vapor-driven pumping, as quantified in this comprehensive study, up to 15–20 % of the base ORC power. The outperforming results can be obtained with vapor expander and pump efficiency values in line with current micro-ORC systems, and under the hypothesis of medium-range efficiency for the electric auxiliary machines in the reference ORC (i.e., the generator and the motor driving the pump).