Organic Rankine Cycles with Dry Fluids for Small Engine Exhaust Waste Heat Recovery

Engine manufacturers are considering the implementation of thermodynamic cycles for Waste Heat Recovery (WHR) in order to increase Internal Combustion Engine (ICE) system thermal efficiency. For these secondary cycles, the literature illustrates the preference of Organic Rankine Cycles (ORC's) due to its simplicity and efficient recovery of the medium grade waste heat found in engine exhaust. This paper simulates the heat recovery capacity of eight dry fluids (butane, pentane, hexane, cyclopentane, benzene, toluene, R245fa, and R123) for an ORC based on the exhaust from a single-cylinder diesel engine-generator operating under five different loading conditions. The model, developed using REFPROP and the Matlab Optimization Toolbox, represents the physical components using isentropic pump and expander efficiencies, along with two-zone heat exchangers. All fluids present cycle efficiencies between 10-15%, with the heaviest hydrocarbons generating the largest amount of work. Of these fluids, pentane is the ideal candidate for an ORC. This fluid illustrates an approximate 10% improvement to the engine-generator efficiency across all conditions. Moreover, the simulation predicts low peak pressures and expansion ratios that are compatible with available displacement expanders. In addition, this effort compares evaporator flow configurations (counter vs. parallel), along with condenser air flow rates. For the evaporator, results illustrate that a parallel flow evaporator arrangement can transfer more heat in phase change applications by setting the high temperature difference in the area of low heat transfer coefficients. Different air flow rates across the condenser show little variation in cycle efficiency with the consumption of fan power at high rates reducing the overall power generated.