Off-Design Optimization of Engine-Organic Rankine Cycle System for Low-Temperature Waste Heat Recovery

Abstract This article presents the design and off-design performance analysis of the organic Rankine cycle (ORC) based on the low-temperature waste heat recovery from the engine jacket cooling water. The modeling approach considers the real-world variability of heat input conditions, particularly due to variations in coolant mass flowrate. To analyze this effect, the present analysis considers engine-coolant heat load between 2 and 20 kW, corresponding to a heat source mass flowrate variation from 0.024 to 0.24 kg/s, while maintaining a fixed inlet engine-coolant temperature of 90 °C and a return temperature of 70 °C for nominal engine operation. This distinct strategy enables comprehensive evaluation and optimization of system behavior under both design and off-design conditions, emphasizing the integrated relationship between the evaporator and the expander. In the initial sizing phase, a compact heat exchanger is designed to recover low-temperature heat with three different refrigerants: R245fa, R123, and R600a. Unlike previous work, this model captures the real performance of the expander across varying operating conditions instead of assuming constant turbine efficiency. The dual expander and sliding pressure control strategies have been implemented within the off-design model to enhance the overall system performance. The maximum work output achieved from a simple cycle is 0.83 kW with a cycle efficiency of 6.02%. Introducing a dual expander operating mode further increased the power output to 0.93 kW and improved cycle efficiency by 21.74%.