Conventional and PCM-based heat recovery configurations for hybrid electric heavy-duty vehicles fuelled with fossil or alternative fuels

Hard-to-electrify transportation systems, such as heavy-duty vehicles, still rely on thermal engines because of their high energy demands, which current battery technologies cannot yet meet. In this context, waste heat recovery (WHR), combined with electrification and the adoption of carbon–neutral fuels, represents a key strategy to enhance efficiency and reduce emissions. The IRIDESCENT project (“biodIesel hybRID Electric buS with waste heat reCovEry aNd sTorage”) aims to exploit synergies among hybridization, WHR, and biodiesel use to achieve carbon neutrality and mitigate cold-start emissions in heavy-duty vehicles. Within this framework, the present study proposes a WHR system that integrates Organic Rankine Cycles (ORCs) with Phase Change Materials (PCMs) to buffer thermal fluctuations, thereby enabling extended ORC operation under variable load conditions. The design and off-design analyses of the proposed advanced WHR system are carried out using real-world data. A comprehensive dataset of 28 real driving trips acquired from a heavy-duty vehicle is analyzed to capture the variability of exhaust gas temperature and flow rate under dynamic conditions. A quasi-static engine model combined with a frequency-domain analysis, based on a modified rectangle criterion, is employed for the WHR design. For the ORC, a sensitivity analysis is performed considering different configurations and working fluids. At the same time, a novel optimization method is developed to determine the optimal melting temperature of the PCM, identifying potassium nitrate (KNO3) as the most suitable material. Results show that PCM integration enhances ORC efficiency by up to 18.5% and WHR efficiency by up to 11.3%, while maintaining a utilization factor above 96%, thus confirming its effectiveness under dynamic driving conditions. The regenerative ORC configuration with R245fa and PCM integration achieved the best performance, demonstrating the feasibility of a compact, lightweight WHR system for sustainable heavy-duty transportation.