Permeability effects assessment on recovery performances of small-scale ORC plant

Waste heat recovery (WHR) of internal combustion engine (ICE) exhaust gases through organic Rankine cycle (ORC)-based power units is one of the most effective technological alternatives to increase ICE efficiency, thereby limiting CO2 emissions. Nevertheless, for an optimum design of components and plants, the assessment of intimate ORC-based plant behaviour is a key factor. This is the case of the mass-based permeability of the plant, which represents its attitude to be crossed by the working fluid, defining a specific relation between the mass flow rate and maximum achieved pressure level. Indeed, it was experimentally found that when the pump and expander are volumetric machines, a univocal relation exists between the operating parameters. Thus, the permeability relation defines certain operating paths of the ORC plant, limiting the domain in which the physical quantities can vary and ensuring the prediction of deviations from the ideal design behaviour. The concept of permeability has not been deeply addressed in the literature despite its importance; thus, it was deepened in this study through experimental and theoretical approaches. In particular, the knowledge acquired through a broad experimental campaign allowed us to obtain a physically based relationship, highlighting the main terms which influence this parameter. These terms are then grouped into dimensionless factors that orient the design and outline an easily implementable model-based control of the maximum plant pressure. Theoretical analyses were conducted through a comprehensive mathematical model of the recovery unit validated by an experimental campaign developed on a fully instrumented ORC test bench fed by the exhaust gases of a 3 L turbocharged diesel engine. The results indicate that the maximum values of the recovered power and efficiency are 3 kW and 4.4%, respectively.