Layout and Mechanics Development of a Novel Piston-type Expansion Engine for Waste Heat Recovery

orc2019 Tracking Number 160

Within the wide range of possible waste heat recovery technologies, the Rankine Cycle showed to be widely applicable and established on the market, but mainly for high power systems; if used for smaller applications, either efficiency or costs of the system are often not satisfying. This is mostly caused by the characteristics of the used expansion engine types, mainly being reciprocating piston engines or turbines. Whereas the first show good part load performance and are well suited for small power, they suffer from lubrication problems and life time issues. Turbines on the other hand are preferably used for high power and can do without elaborate lubrication, but suffer under part load conditions and are quite costly. Especially for medium power systems, i.e. waste heat fluxes between 200 and 400 kW, a novel concept for the expansion engine within a Rankine steam cycle was conceived. The aim was 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 are performing a cyclic movement relative to one another. This way, four working chambers with varying volumes are resulting, each experiencing repetitive compression and expansion. The conversion of the cyclic changing angular velocity to a constant rotation at the output shaft is done via a non-circular gear. This solution offers the possibility of sealing the lubricated gearbox against the steam-flooded section containing the working chambers via rotational seals, being much easier than the sealing within a conventional reciprocating piston engine. This paper treats the design and layout of this novel expansion engine concept, followed by the experimental mechanics development on the test bench. The strategy for creating a robust and effective expansion engine design as well as the most important findings and insights gained during the experimental investigations of the engine are shown.