Experimental Investigations on a Novel Expansion Engine for Waste Heat Recovery

Waste heat recovery in medium-power systems below 400 kW waste heat power asks for a novel expansion engine concept for water-based Rankine steam cycles. The aim is to combine the advantages of reciprocating piston engines and of turbines at reasonable costs. The so-called rotational wing-piston expander uses two pivoting shafts, each holding two wing-like pistons within one housing, that perform a cyclic movement relative to each other. Thus, four working chambers with varying volumes are shaped, each experiencing repetitive compression and expansion. This solution offers the possibility of sealing the lubricated gearbox against the steam-flooded section containing the working chambers with rotational seals.
For the development of the expansion engine, starting with an initial approach for a functional prototype, experimental investigations are carried out. Motored tests are performed in order to scrutinize kinematics and mechanics. Tests with pressurized air for enhanced load on the components - without applying the corrosion and thermal stress of hot steam - are assessed. The structural problems at the piston mount during the test runs reveal improvement potential and lead to its highly effective redesign. The occurring leakage at the rotational and piston seals and the high friction demand for improvements in further investigations.
This paper treats the design and layout of the novel expansion engine concept, the special challenges of test bench investigations, especially concerning measurement equipment and test bench requirements, and shows the most important findings and insights gained during the experimental investigations of the engine, as well as possibilities for improvement.