Variable geometry turbine for organic rankine cycle

Internal combustion engines (ICEs) generally convert only approximately up to 40% of the fuel energy into useful power and discharge the remaining energy as waste heat to the atmosphere.

The project's overall aim was to improve the performance of waste heat recovery technologies for heavy-duty truck/automotive applications.

The transportation sector alone was responsible for 25.8% of the EU-28 greenhouse emissions, out of which 70% was from road transport. Organic Rankine Cycle-based, Waste Heat Recovery (ORC - WHR) systems can be used to convert this untapped heat source and convert it into mechanical/electrical power thus enabling a reduction of fuel consumption and CO2 emissions by as much as or more than 15%. The technology readiness level (TRL) for automotive applications is still low mainly because of the ORC system’s lack of performance at part load/off-design conditions and control complexities.

The aim of “SuperVGE” was to develop a novel turbine equipped with a variable geometry turbine expander (VGE) nozzle design suitable for supersonic flow and a wide range of operations. In addition, appropriate control schemes were developed to allow high efficiency and power to be generated throughout its dynamic operating range.

A mathematical model of an ORC-based power system was developed based on the exhaust heat profile of a heavy-duty truck. The cycle was optimized to provide boundary conditions for turbine inlet/outlet conditions are peak and part-load conditions. A 1D turbine design code was developed in python programming environment linked to CoolProp property database. The code could generate rotor, stator and volute geometry data for working fluid of choice. A novel mechanical design of stator was developed to control the throat area and flow angle of stator exit. FEA was performed to evaluate stresses and flow conditions. The design geometry was optimized for manufacturability. The high-fidelity simulation results revealed that the proposed stator with variable geometry can improve the performance of the turbine in part load operation. The turbine performance curve was used in the dynamic model to optimize the superheat control at the turbine inlet using state of the art proportional integral and derivative based controller.

The research results have demonstrated that the variable geometry turbine can improve the part-load performance of the turbomachine by up to 15%, which can be reflected in an ORC efficiency improvement up to 5 efficiency points for the mini-scale <20kW ORC. These numbers are, however, dependant on the scale, conditions and operating point of the system.

Grant Agreement number: 844023 — SuperVGE | MSCA Programme: Marie Skłodowska-Curie Individual Fellowships