Exhaust Heat Recovery Rankine System for passenger cars: modelling and design

Nowadays, in average, one third of the fuel energy consumed by an engine is wasted
throughout the exhaust gases. The recovery of this amount of energy would enable a
reduction of the fuel consumption. One solution is to integrate an exhaust heat recovery
system based on a Rankine cycle. The first and most crucial step in the design is the
definition of its architecture, the definition of the rated operating conditions and the
sizing of the different components whilst considering the heat recovery over customer
driving cycles.
This paper presents a steady-state model of a steam Rankine cycle built by
interconnecting sub-models of the heat exchangers, the pump and the expander. All the
models were developed under EES (Engineering Equation Solver) environment.
The models of heat exchanger are developed using the ε-NTU method. A heat
exchanger is subdivided into three zones, each of them being characterized by both the
exhaust gases and water side pressure drop as well as heat transfer coefficient. The
evaluation of the two-phase zone pressure drop and heat transfer with respect to the
vapour quality was carried out. The model of rotary piston expander describes the
evolution of the fluid through the device and was split into three global steps: pressure
drop across the supply port, isentropic and expansion at constant volume, internal and
external heat transfers.
The overall simulation model is finally used to predict the performance of the heat
recovery system as the operating conditions and geometrical characteristics of each
component are modified. An optimal sizing of the system is proposed and the
underlying methodology is discussed. The analysis of the results pointed out that the
levels of high/low pressure and the amount of superheat are the main parameters that
impact the performance of the cycle and the sizing of the components.