Experimental characterization of a hermetic scroll expander operating in an ORC-based power unit bottoming an internal combustion engine

Organic Rankine Cycle systems represent a promising solution for Waste Heat Recovery applications in the transportation sector due to its reliability, flexibility and cost and, primarily, to the fuel saving and CO2 emission reduction potential. Even though this technology is not yet on the market, the number of potential users is huge and the extent of the possible recovery make this opportunity really challenging. In an ORC-based power unit, the expander assumes the most important role being responsible for the power delivery and, ultimately, of the overall performances of the recovery unit. An important feature not frequently considered is that it sets the operating pressure of the cycle when a given working fluid mass flow rate is fixed by the pump, representing a very robust and reliable control variable. The fluid-dynamic permeability of the plant, in fact, is mainly due to the expander, participating all the other components in a weaker way. In this paper a novel scroll expander has been tested inside a test rig which allows evaluating the performance of an ORC-based power unit: the test bench has been extensively used in order to verify the behavior of several expanders as well as of most relevant components (Heat Recovery Vapor Generator (HRVG), pumps, plant layouts). The recovery unit is fed by the exhaust gases of an internal combustion engine represented by an Iveco F1C, a 3L turbocharged diesel engine, with 130 kW maximum power at 3250 RPM. As working fluid R245fa was selected. Expander performances are evaluated measuring the electric power output after an AC/DC conversion system at different rotational speed by means of a variable electric load: this choice was due to the need of accounting also for the efficiency of the electrical conversion system from mechanical to electrical energy. Results related to isentropic-to electric conversion efficiency are presented for different thermal loads recovered from the exhaust gases, corresponding to different engine stationary operating points. Finally, the permeability characteristic of the machine, at different working fluid mass flow rates, was measured allowing to correlate it with the maximum operating pressure of the recovery unit.