Design parameters and control strategy for different power couplings of an Organic Rankine Cycle Waste Heat Recovery System

A low temperature Waste Heat Recovery Organic Rankine Cycle System converts the thermal energy recovered in the coolant loop of an ICE into mechanical energy. This mechanical energy can either be injected in the power train of the vehicle, or be further converted into electrical energy. As a result of this choice of integration, the Organic Rankine Cycle (ORC) control parameters are impacted by the vehicle driving and ambient conditions through imposed rotation speeds and respectively the capacity of the ORC heat sink.
In this article the different ways of coupling between the Waste heat Recovery System (WHRS) and the vehicle power train system and their influence on the control parameters are described. For a basic system configuration the shaft power of the Expander of the ORC is directly injected in the belt drive of the engine via a pulley with controllable clutch. In this first configuration the Working Fluid (WF) pump is mechanically coupled to the Expander machine and sized for a nominal operating point which is a compromise between different options. In this case the only control parameter is the signal for the clutch.
The second version uses an expander and a WF-pump which are coupled to an electrical alternator/motor with controllable speed. The control parameter is the rpm of compound expander-pump. And finally in the third version the Expander and WF-Pump are coupled individually to a controllable alternator, respectively an electric motor with variable speed. In this case pump rpm and expander rpm can be controlled independently.

The control strategy for each configuration is described and the influence of the different degrees of freedom in terms of control on the OCR net power output is evaluated by simulation for steady state driving conditions.