Experimental research on scroll expanders operating in parallel in an organic Rankine cycle system with a biomass boiler

This paper presents the results related to the performance of two expanders running in parallel in a regenerative ORC system. Scroll expanders with nominal outputs of 1 kW(e) were used. The heat source was a prototypical biomass boiler with a nominal output of 40 kW(t). An HFE7100 low-boiling fluid was used as a working medium. The research was conducted at selected flow rates and at various temperatures of the working medium, the glycol solution and the thermal oil. The paper presents the curves of the net electrical power generated, load current and voltage as functions of the rotational speed of the expanders connected in parallel. The total maximum electric power produced by two expanders was 1.4 kW(e). The maximum rotational speeds of the expanders No. 1 and No. 2 were 2760 rpm and 2640 rpm respectively. For expander No. 1, the maximum generated voltage and current were 185 V and 4.0 A, while for expander No. 2 they were 175 V and 3.9 A, respectively. The correlations describing current, voltage and electric power generated by the expanders as functions of the rotational speed and the pressure ratio are included in the paper. The maximum isentropic efficiencies of expanders No. 1 and No. 2 were 83% and 84% respectively. However, the maximum efficiency of the ORC system with HFE7100 as a working medium was 5.5% when the two expanders operated in parallel. The maximum RMS amplitude of the vibration velocity measured on the casing of the first and second expander was approximately 16.71 mm/s and 14.72 mm/s, respectively. The paper includes an economic analysis of the ORC system that operates with one and two scroll expanders.