Thermodynamic modeling of organic rankine cycle for dynamic engine exhaust waste heat recovery

Abstract

The diesel engine efficiency can be improved by extracting the waste heat from engine exhaust and coolant. Organic Rankine cycle (ORC) technology, as a bottoming technique, is efficient for waste heat recovery from low-temperature sources. The engine-ORC system's optimization is carried out based on the critical trade-off between parameters such as weight, backpressure, space, and cost versus thermal efficiency and fuel economy. A steady-state and dynamic organic rankine cycle (ORC) model is developed in MATLAB with REFPROP to simulate waste recovery from diesel engine exhaust gas. Ethanol, R123, R152a and R415B are screened based on thermodynamic, environmental, health, and safety concerns. The dynamic performance is analyzed at 3 MPa evaporation pressure and condensation temperature at 290 K and 323 K under fixed thermal inertia of evaporator. R123 shows maximum thermal efficiency at the cost of the superheating issue. A novel approach of embedding phase change material (PCM) into evaporator walls to enhance its thermal inertia to stabilize working fluid performance is studied. LiNO3-KCl is proposed PCM due to superior performance stabilization capability. R123 is selected as working fluid for higher thermal efficiency with an assurance of complete dry expansion without superheating. The performance of the engine-ORC system having evaporator without and with PCM are compared for dynamic analysis. ORC system having evaporator without PCM showed large fluctuations at working fluid side due to low thermal inertia, unstable net power output and thermal efficiency with fluctuations. The PCM-based evaporator system showed stable performance as PCM is maintained in a constant temperature zone throughout the operation, stable net power output and thermal efficiency at 0.65 kW and 12.8% respectively throughout the drive cycle.