Experimental characterization of a scroll expander under two-phase conditions within a micro PE-ORC system

This study presents the first systematic experimental characterization of a scroll expander under two-phase conditions in a partial evaporation ORC (PE-ORC) using R1233zd(E). A structured operating procedure was developed and validated, including a methodology for the real-time determination and regulation of the inlet vapor quality, employing a two-phase-rated Coriolis flowmeter to directly measure vapor density. A detailed analysis, using data from heat-balance calculations as a reference baseline, demonstrates the suitability of the proposed methodology for such applications while highlighting the need for calibration of the Coriolis device due to a systematic deviation between the measured and actual density values in the two-phase region. In total, 181 steady-state operating points were analyzed over a range of expander speeds, pressure ratios, and inlet vapor qualities, with single-phase operation used as a benchmark. The results show that isentropic efficiency decreases significantly with decreasing vapor quality, with maxima occurring at lower rotational speeds and at pressure ratios close to the built-in volume ratio. Volumetric efficiency is primarily affected by rotational speed and deteriorates in two-phase operation, although it is only weakly dependent on vapor quality. At the system level, two-phase operation reduces net thermal efficiency, however, higher net power outputs are achieved at very high vapor qualities (≈0.95) due to increased mass flow rates, highlighting the potential of PE-ORC systems for waste heat recovery applications.