Off-design optimisation of organic Rankine cycle (ORC) engines with different heat exchangers and volumetric expanders in waste heat recovery applications

Organic Rankine cycle (ORC) engines in real applications experience variable heat-source conditions. In this paper, the off-design performance of small- to medium-scale ORC engines recovering heat from stationary internal combustion engines (ICEs) is investigated. Of particular interest are the employment of screw vs. piston expanders, and two heat exchanger (HEX) architectures. Unlike previous studies where the performance of the expander and HEX are assumed fixed during off-design operation, here we consider explicitly their varying and interacting characteristics within the overall system. Nominal sizing results reveal indicated isentropic efficiencies > 80% for twin-screw and > 85% for piston expanders. Following nominal design, the ORC engine operation is optimised for ICE part-load (PL) operation. Although the heat transfer coefficients in the evaporator decrease by up to 30% at PL, the effectiveness in this HEX increases by 20% due to the larger temperature differences across the component. The screw expander efficiency reduces by up to 3% at off-design operation, whilst that of the piston expander increases by up to 16%. Optimised off-design maps indicate that the ORC engine power output reduces to 77% (piston) or 68% (screw) of its full-load value when the ICE operates at 60% PL, and that ORC engines with plate HEXs generate 5-11% more power than those with double-pipe HEX designs. Under variable ICE operation, smaller ORC engines with piston expanders generate more power than larger engines with screw expanders, highlighting the resilient off-design operation of piston machines. The modelling tool developed here can predict ORC performance over a wide operating envelope and provides performance maps that can be used by operators to optimise ORC engine operation in variable conditions and by ORC vendors to inform component design decisions