Off-design operation of ORC engines with different heat exchanger architectures in waste heat recovery applications

Abstract

Organic Rankine cycle (ORC) engines in waste - heat recovery applications experience variable heat - source conditions ( i.e. temperature and mass fl ow rate variations ). Therefore maximising the ORC system performance under off - design conditions is of key importance, for the financial viability and wider adoption of these systems. In this paper , the off - design performance of an ORC engine is investigated, with screw expander and two heat exchanger ( HEXs ) architectures, while recovering heat from an internal combustion engine (ICE) . Unlike previous s tudies where the ORC expander and HEX s performance is assumed fixed during off - design operation, in this work we consider the time - varying characteristics of the system components. Firstly, nominal system sizing r esults indicate that the screw expander isentropic efficiency exceeds 80% , while th e plate HEXs (PHEXs) heat transfer area requirements are 50% lower , than the respective ones for double pi pe (DPHEX) design . Next, the ORC engine operation is optimised at part - load (PL) ICE conditions. Although, the HEXs heat transfer coefficients decrease by 30% with part - load, the HEX effectiveness increases by up to 20% , due to higher temperature difference across the working fluids. Findings also reveal that t he PHEX performance is less sensitive to the off - design operation. Optimum o ff - design power output maps indicate that the ORC e ngine PL reduces to 72%, for ICE PL of 60% , while ORC engines with PHEX s generate slightly more power , for the same heat source conditions. Overall, th e modelling tool developed predict s the ORC performance over an operating envelope and allows the selection of optimal designs and sizes of ORC HEXs and expanders . The findings can be used by ORC plant operators to optimise the ORC engine power output, given the varying heat source conditions observ ed on their site , and by ORC vendors to inform HEX and expander design decisions.