Experimental and numerical investigation of direct liquid injection into an ORC twin-screw expander

Abstract

The Organic Rankine Cycle (ORC) is a thermal engine, which is used to convert low temperature heat to electrical power using organic working fluids. It is an established technique for waste heat recovery and for the utilization of renewable heat. This study presents a novel operational strategy of an ORC, which allows for reliable control of process parameters while simultaneously ensuring high power output. Preheated liquid working fluid is injected directly into a volumetric screw expander at an intermediate pressure level. The injected mass flow bypasses the evaporator and can be controlled by a valve. Thus, direct liquid injection into the expander reduces the exhaust temperature, leading to a lower risk of thermal damages in case of a hermetic or semi-hermetic expander. This strategy is analyzed experimentally and compared with a system simulation. The experimental and simulation results show that the exhaust vapor temperature can be reduced by approx. 40 K for the investigated operational conditions. This enables the expander to run at higher live vapor conditions by simultaneously ensuring sufficient cooling of the generator and thus allows for up to 40% higher power production depending on the operational conditions.