Closed-Loop PI Control of an Organic Rankine Cycle for Engine Exhaust Heat Recovery

The internal combustion engine (ICE) as a main power source for transportation needs to improve its efficiency and reduce emissions. The Organic Rankine Cycle (ORC) is a promising technique for exhaust heat recovery. However, vehicle engines normally operate under transient conditions with both the engine speed and torque varying in a large range, which creates obstacles to the application of ORC in vehicles. It is important to investigate the dynamic performance of an ORC when matching with an ICE. In this study, the dynamic performance of an ICE-ORC combined system is investigated based on a heavy-duty diesel engine and a 5 kW ORC with a single-screw expander. First, dynamic simulation models of the ICE and the ORC are built in the software GT-Power. Then, the working parameters of the ORC system are optimized over the entire operation scope of the ICE. A closed-loop proportional-integral (PI) control together with a feedforward control is designed to regulate the operation of the ORC during the transient driving conditions. The response time and overshoot of the PI control are estimated and compared with that of the feedforward control alone. The results based on the World Harmonized Transient Cycle (WHTC) indicate that the designed closed-loop PI control has a shorter response time and a better trace capacity during the dynamic processes. The average output power and thermal efficiency during the WHTC cycle are improved by 3.23% and 2.77%, respectively. Compared with the feedforward control alone, the designed PI control is more suitable for practical applications.