Dynamic modelling and development of a reliable control strategy of organic Rankine cycle power systems for waste heat recovery on heavy-duty vehicles

Organic Rankine cycle is a promising technology for waste heat recovery on heavy-duty vehicles. However, the waste heat from internal combustion engines of heavy-duty vehicles is characterized by large fluctuations in mass flow rate and temperature, which may lead to failures of the organic Rankine cycle system. Therefore, it is of crucial importance to develop a suitable control strategy ensuring safe operation and high performance of the organic Rankine cycle system. The objective of the present paper is to develop a simple and robust proportional integral derivative controller for an organic Rankine cycle system recovering the waste heat on a long-haul truck. The exhaust data considers 45 minutes operation of a 450 hp diesel engine of a long-haul truck. A dynamic model was developed in the object-oriented language, Modelica. The dynamic response of the organic Rankine cycle unit for open loop operation was analysed to develop the input and output data for the tuning of the controller. Two different control strategies ensuring the superheat at the expander inlet was developed: i) manipulation of the pump speed and an engine exhaust gas valve, and ii) manipulation of the pump speed and a valve located at the inlet of the expander. The controllability and performances of the controllers were evaluated using the exhaust data. The results indicate that both of the control strategies ensure at all times the superheat at the expander inlet. The second control strategy results in 10.8% more net power output and 12.7% higher thermal efficiency compared with the organic Rankine cycle unit employing the first control strategy. Furthermore, the efficiency of the truck engine is increased by 2.34% for organic Rankine cycle unit employing the first controller and 3.82% employing the second controller.