Model and transient Control strategy design of an Organic Rankine Cycle Plant for waste heat recovery of an Internal Combustion Engine

The multi-sources hybrid polygeneration energy systems are of great interest and topicality as they are one of the most promising technologies in the European’s Green Deal panorama, with the aim of serving users with electrical and thermal energy using a single plant powered by one or more energy sources. In the waste heat recovery field Organic Rankine Cycle (ORC) power plants are becoming increasingly popular, especially for exploiting medium and low temperature heat sources as a micro-small scale power plant. However, the development and diffusion of this technology is still limited due to the high costs and consequently prototype development and experimental assessment of performance is very poor, especially for non-stationary systems. In this work the modelling and validation of a micro-scale waste heat recovery (WHR) plant coupled with a control system is presented. An ORC plant has been modelled through a map-based model approach for the piston pump and the scroll expander while the pipes and the heat exchangers through a 1D thermo-fluid dynamic approach. A preliminary comparison was made between some numerical quantities of the modelled plant and the same experimental quantities in 61 different operating conditions, showing an average error of 50.1%. The model has been calibrated using a vector optimization technique: two calibration parameters of the heat exchangers were calibrated with a genetic algorithm (MOGA II) by reducing the error of 5 quantities obtained from the model with the respective experimental quantities in 15 different operating conditions. The remaining 46 operating conditions were used to evaluate the calibrated model, showing an average error of 3%. Furthermore, in order to provide for the use of the system coupled to highly variable heat sources, such as the exhaust gases of an internal combustion engine, a control strategy has been designed to perform two tasks: leading the ORC performance where the efficiency is higher, acting on the pump speed through a map-based control, implemented by a look-up table control, and protecting the organic fluid from damage caused by high working temperatures through a bypass control system with a PI control, depending on the proportional and integral gains. In order to verify the control strategy behaviour at different thermal transient inputs, a set of simulations has been run, showing a robust and stable manner preserving the organic fluid properties and limiting the superheated steam at expander inlet.