Optimisation of off-design internal combustion-organic Rankine engine combined cycles

Abstract

Organic Rankine cycle (ORC) engines are an efficient means of converting low - to - medium renewable or waste heat to useful power . In practical applications, ORC systems experience varying thermal input profile , due to the dynamic nature of real heat source s . M aximis ing the uptake of this technology requires optim ised ORC design s and sizing to maintain high efficiency and power output, not only at full - load operation, but also under off - design conditions. Key for maintaining the efficient operation of the sys tem is the maximisation of heat extraction from the heat source, in the ORC evaporator. In this paper, the off - design operation of an ICE - ORC combined heat and power (CHP) system is investigated, to optimise the ORC performance under varying ICE load condi tions. First, the ORC engine thermodynamic design is optimised for the 100% load operation of the ICE. Alternative working fluids are inv estigated, including low ODP/ GWP refrigerants and hydrocarbons. The ORC system is then sized using two different heat e xchanger (HEX) architectures; tube - in - tube (DPHEX) and plate (PHEX) design s , at design conditions. The sizing results reveal that the PHEX area requirements are almost 50% lower than the respective ones for DPHEX, while recovering equivalent quantities of heat. Next, the ORC engine operation is optimised at part - load ICE condition s , and the HEX heat transfer coefficients (HTCs) are predicted. R esults indicate that : i) PHEX HTCs are up to 50% higher than DPHEX equivalents ; ii) HTCs decrease at part load f or both HEXs, but because the average temperature difference increases, the overall HEX effectiveness improves; and iii) the ORC system with a PHEX evaporator has slightly higher power output tha n the DPHEX equivalent at off - design operation. Overall, t he modelling tool developed here can predict ORC performance over an operating envelope and allows the selecti on of optimal design s and size s of ORC HEXs.