Performance analysis of a bio-diesel fired engine bottoming with micro-ORC

The purpose of this work is to investigate the performance in terms of electric power and electric efficiency of a 11kW bio-diesel internal combustion engine (ICE) coupled with a micro Organic Rankine Cycle (ORC) both in design and off-design conditions. A zero dimensional (0D) thermodynamical engine model has been developed to predict the electric and thermal power with 100% biodiesel fuel (B100). B100 has been selected for the performance analysis of the integrated system due its lower environmental impact. For the ORC system, a subcritical thermodynamic model has been used with various working fluid (R245fa, R1233zd(E), R134a, R1234ze(E) and R1234ze(Z)). A plate heat exchanger (PHEX) has been adopted as evaporator of the organic cycle to directly recover the thermal power of the exhaust gas. Both models have been validated using experimental data from literature. Two different expander configurations, dynamic and volumetric, has been investigated. Model validations show good agreement with the experimental and literature data, respectively. Moreover, the results highlight that the micro-ORC could achieve a maximum electric efficiency of about 7% at full load with R1234ze(Z). Although at part load the dynamic expander show better performance, the volumetric expander has been selected for the system analysis due to its real suitability for small and micro scale ORC. Combined system results show a maximum enhancement of engine efficiency of about 5%, and better results, in terms of good compromise between electric efficiency and operating range width, has been shown for R1233zd(E). Furthermore, the direct coupling of the PHEX with the exhaust gases allows to have heat exchange evaporator areas below 2 m2 for all analysed fluids.