Improving the part-load performance of turbines in rankine cycle based automotive waste heat recovery systems

This paper presents two control strategies that improve the part-load performance of turbine-generators in Rankine cycle based automotive waste heat recovery systems. The first strategy concerns regulation of the turbine rotational speed by applying a varying current to the generator. A static feedforward controller is derived and experimentally demonstrated. Simulations indicate this controller to be over 99% optimal in terms of electric energy production. A comparison with tracking of the optimal turbine speed shows the implemented controller to be favorable. The second strategy concerns the admission of steam into the turbine by periodically opening and closing an on/off valve. By using this technique, the pressure ratio over the turbine is increased in a cost effective manner. On an experiment with a low heat input, the electric energy production increased by 18% due to the pulsating flow.