Experimental Investigation with Steady-State Detection in a Micro-ORC Test Bench

The exploitation of low grade thermal sources is recognized as a feasible strategy in order to pursue the primary energy saving target worldwide. This concept, adaptable to a number of different applications, is aimed at exploiting low-value heat fluxes that would be wasted otherwise; additional useful electric power can be produced locally, with ORC energy systems; this is one of the most promising heat recovery solutions. In particular, the paper refers to the test bench developed in the laboratories of the University of Bologna; a prototypal micro-ORC energy system is here investigated. The micro-ORC system presents a reciprocating three-piston expander operated with refrigerant fluid. Heat is provided to the ORC from via hot water at low temperature, in order to simulate a constant low-enthalpy heat recovery process. The system rejects unused heat via a water-cooled condenser, dependent on the external ambient conditions. The test bench layout and the real-time data acquisition system, developed in the LabVIEW environment, are here described. In particular, the paper focus is on the system steady-state detection methodology. Starting from an experimental campaign, steady-state operational points are identified through an appropriate literature approach. The measured quantities and calculated performance have been post-processed in order to evaluate the influence on steady state detection, of different hot source temperature set points. Moreover, the selected steady-state detection method is suitable for real-time implementation, due to its simple formulation and the low number of variables required to be stored at time step of acquisition.