Performance analysis of a dual-loop organic Rankine cycle system for waste heat recovery from engine coolant and exhaust of a heavy-duty truck

Abstract

• Heat exchanger employed influences the system efficiency. • Significant heat loss observed from engine block at part-load. • Variable expander-to-engine speed gear ratio gives a small gain in power. • Performance comparison for unique Scandinavian motorway data. Strict legislations over emissions from heavy-duty trucks are pushing the manufacturers towards improving their brake thermal efficiency, thereby mitigating fuel consumption and reducing CO 2 emissions. Organic Rankine cycle-based waste heat recovery system has proven to be an inevitable technology in boosting the engine performance besides the other methods and approaches. In addition to recovering the exhaust heat from the engine, recovering heat from the engine coolant is also beneficial, provided its nominal temperature is raised. Thus, this work investigates the scope of improvement in performance of a heavy-duty truck engine when integrated with a dual-loop organic Rankine cycle system for recovering heat from its exhaust and coolant, simultaneously. The analysis has been performed as a simulation study on GT-SUITE using the one-dimensional model of the engine with its organic Rankine cycle heat recovery setup. The model was developed based on the components of a real commercial truck engine connected with a simple exhaust waste heat recovery system. For the work described in this paper, model of the single-loop exhaust waste heat recovery system was modified to a dual-loop circuit comprising of two scroll expanders and R1233zd(E) as the working fluid. Notably, performance investigation was carried out using a unique Scandinavian motorway road data retrieved from the truck. This paper addresses the challenges associated with simultaneous heat recovery from engine exhaust and coolant. Performance comparison at four steady-state engine operating points reveal that the low temperature radiator installed for the indirect condensation of the working fluid is the major influencing component on system efficiency. At higher engine loads, the overall system efficiency considerably decreased due to limited heat rejection capacity of this radiator. Moreover, on assessing the scope of improving the system’s performance by having variable gear ratio between the engine and the expanders, 1.6% points (0.12 kW) gain in power was observed by having the high-pressure expander’s speed fixed and the low-pressure expander’s speed varied. Furthermore, elevating the engine coolant temperature from 120 °C to 140 °C, significant heat loss from the coolant to the engine surroundings resulted in a substantial drop in net power at lower part loads although it had improved at higher engine operating points.