Experimental analysis of a small-scale scroll expander for low-temperature waste heat recovery in Organic Rankine Cycle

Highlights: • A commercial scroll compressor converted into an expander has been investigated. • The expander overall efficiency achieved a peak of 45% for expansion ratios of 1.9–2.1. • The overall efficiency of the expander is not affected by the superheating degree. • The expander mechanical output was about 650 W for expansion ratios of 2–2.2. • The ORC plant mechanical efficiency reached 3% for expansion ratios of 2.2–2.5. -- Abstract: In this work a small-scale non-regenerative Organic Rankine Cycle test rig has been developed to experimentally assess the performance of the scroll compressor SANDEN TRS090 converted into an expander. The performance of the expander has been evaluated using R245fa as working fluid in terms of overall efficiency and filling factor under variable flow rates, expansion ratios, rotational speeds and superheating temperatures. A peak isentropic efficiency of 45% has been obtained for an expansion ratio of 1.9–2.1 whilst the rotational speed has a minor effect. The highest values of expander overall efficiency have been achieved for a filling factor ranging from 2.5 to 3 which correspond to rotational speeds higher than 1000 rpm. During the tests the expander has achieved a mechanical power output of about 650 W for an expansion ratio in the range 2–2.2 whilst the peak values of the mechanical efficiency of the plant were >3% (self-consumption of the pump and auxiliaries excluded) for an expansion ratio of 2.2–2.5. Eventually, it was proved that the machine has no advantages in terms of overall efficiency when the fluid is superheated but the whole ORC system has a higher performance with a superheating grade in the range 10–30 K.