Supercharged Expander to Enhance Waste Heat Recovery through ORC-Based Recovery Unit in Vehicle Applications

Waste Heat Recovery is one of the major opportunities to increase the engine efficiency in internal combustion engines (ICE) for the transportation sector and to meet the emissions targets. ORC-based units are widely investigated, in particular for heavy duty vehicles and light commercial ones. However, when a typical operation of the ICE on a vehicle is considered, working temperature and exhaust flow rates are not always suitable for recovery, being characterized by low-grade enthalpy. Volumetric expanders are among the most suitable technological solutions for small scale ORC-based power units, but they can suffer of low efficiency in real operation. A way to improve its performances is represented by a supercharging technique, which involves a further intake port. Indeed, keeping constant the mass flow rate provided by the pump, the dual-intake expander produces a reduction of the intake pressure with a mechanical power similar to the single intake machine, thanks to a higher permeability. This aspect can enhance the expander operability in off design conditions, which is particularly interesting when the hot source is represented by the exhaust gases of an ICE. In fact, the mass flow rate circulating inside the ORC-based recovery unit can increase, in order to recover more thermal power. In fact, keeping constant the intake pressure of the dual-port intake expander, a higher mass flow rate can be elaborated with respect to the single-intake port. In this paper, a combined theoretical and experimental activity has been done, reproducing real ICE operations in a small-scale ORC test bench fed by exhaust gases of a 3L turbocharged diesel engine and prototyping the supercharged expander. In this way, the benefits related to the additional port are assessed in real engine working points, compared to the single port one and introducing further developing paths.