OFF-DESIGN BEHAVIOR OF A REGULATED ORC-BASED SYSTEM FOR EXHAUST HEAT RECOVERY ON ICE

Transportation sector is living a transition era, where conventional powertrain based on Internal Combustion Engine (ICE) is gradually replaced by hybrid and electric ones. However, in vehicles used for people and freight mobility, the ICEs will still play a crucial role, since they represent actually over the 95% of the powertrains. In this device, the efficiency target is gradually increasing, in order to meet the worldwide emissions reduction goals. Hence, among the different opportunities, waste heat recovery on the exhaust gas of an engine is one of the most promising ones, with a high technological readiness and also high potential. In this paper, an ORC (Organic Rankine Cycle) based unit has been considered as exhaust heat recovery system. The ORC system, equipped with volumetric machines, has been developed, tested and represented by a numerical model, so experimentally validated. The model can consider all the different aspects on a real small ORC system. In particular, the relations between the main operating quantities such as maximum plant pressure and working fluid mass flow rate are caught by the model. Hence, exploiting the linear relation found between these two parameters given by the hydraulic plant permeability, a regulating strategy was developed. This is based on pump and expander revolution speed regulation and allows to widen the operating range of the ORC unit. In order to assess the feasibility of the proposed strategy, it was implemented through a dedicated and novel subroutine in the ORC-based unit model. In this way, the benefits related to the ORC control are detected under severe off-design operating condition, i.e. exhaust mass flow rate and temperature, related to the specific ICE operating point.