Power Maximization of a Heavy Duty Diesel Organic Rankine Cycle Waste Heat Recovery System Utilizing Mechanically Coupled and Fully Electrified Turbine Expanders

Abstract

This paper presents steady state power optimization for an Organic Rankine Cycle (ORC) waste heat recovery (WHR) system in heavy-duty diesel (HDD) applications and examines attainable power levels. Both recirculated and tail pipe exhaust gas streams are utilized as heat sources via a parallel evaporator configuration. A dynamic, physics-based ORC model is utilized to determine optimized net power production. The optimization variables include: working fluid pump speed, mass flow distribution ratio for the parallel evaporators, turbine speed, and the pump speed of condenser cooling fluid. A sensitivity analysis is conducted for all optimization variables. Working fluid pump speed and expansion turbine speed are demonstrated as the most sensitive variables. Sensitivity results indicate increasing system power production as the working fluid mass flow rate increases. A map is developed to identify optimal and safe ORC operating regions for a given engine cycle. Based on the sensitivity analysis working fluid pump speed is selected as the variable of interest in the steady state optimization and results indicate that system power generation could be improved by as much as 25%. Analysis of two quasi steady operational cycles, a multi-mode and a constant speed variable load are conducted. ORC power results indicate that a speed-constrained, mechanically coupled turbine expander operates in its peak efficiency zone to produce maximum power output for the steady-state cycles tested. These steady state power optimization results can be used for future power optimization during engine transients.