Experimental characterization of irreversibilities in a Worthington-type isobaric expansion engine for low-grade waste heat recovery

Low-grade waste heat recovery technologies are crucial for improving industrial energy efficiency. Among these, Worthington-type isobaric expansion engines (IEEs) offer operational simplicity, yet existing literature focuses almost exclusively on global cycle modeling, leaving their internal thermodynamic degradation poorly understood. To date, no study has performed a Second-Law analysis to pinpoint specific areas for improvement, and an exergy destruction assessment of individual irreversibilities within the expander core remains unaddressed. To bridge this gap, this study presents the first comprehensive experimental characterization of an IEE, introducing a system-wide exergy mapping framework utilizing fast-response intra-chamber transducers to isolate and trace component-level irreversibilities. Results reveal that while increasing hydraulic throttling reduces evaporator exergy destruction by 24.6% (from 734 W to 553 W), it triggers a severe operational trade-off: as operating pressure scales up, internal valve leakage exergy destruction surges from 180 W to 258 W, cannibalizing heat exchanger gains and plateauing net mechanical work around 491 W. Furthermore, unresisted cycle blowdown induced by cylinder dead volumes represents a massive exergy destruction term (up to 387 W) unresolvable via aerodynamic valve resizing. Nevertheless, mitigating the other internal irreversibilities (leakage, friction, suction, and discharge) alongside hydraulic circuit restrictions (104 W) would unlock an ultimate thermal efficiency of 7.6%, inherently elevating operating pressure and diminishing evaporator exergy destruction. Achieving this level proves IEE technology can match conventional micro-ORC net efficiencies through a fundamentally simpler, lower-cost, and more robust mechanical architecture.