Thermodynamic and technical criteria for the optimal selection of the working fluid in a mini-ORC

Abstract

Waste energy recovery (WER) is a suitable solution to improve the fuel utilization of Internal Combustion Engines (ICEs) by producing an eco-friendly electrical power from an energy source currently wasted. Organic Rankine Cycle (ORC) technology has been developed in the past few years to generate electric power from medium temperature (500 K – 800 K) ICE wasted thermal sources. Working fluid selection represents the first step in the design of an ORC. At the state of the art, authors where not able to select a single optimal organic fluid. This is mainly because of the different thermodynamic conditions of the heat sources which offer wasted thermal energy. This paper proposes a procedure for the ORC system preliminary working fluid selection, which takes into consideration thermodynamics and design parameters of the system components. The study is applied to WER systems specifically designed as bottoming cycles to ICE for transport applications. However, the method is quite general and makes the model easily adaptable to different heat sources. A steady state thermodynamic model of the system is developed via the software MATLAB. A wide variety of organic fluids (OF), such as R245fa, Solkatherm (SES36) and hexane have been investigated to identify the candidate which offers the best recovery opportunity. Regeneration is also included in this work. Results show that recover thermal energy in the regenerator is an essential method to improve power recovery when applying ORC to WER systems. The effect of superheating on the system power output has been investigated as well. It is capable to increase the cycle power output only when coupled with regeneration. The paper shows that the addition of a bottoming ORC to the ICE is convenient both in terms of recovered electric power (up to 14% of the engine nameplate power) and heat source utilization rate (up to 11 % heat source conversion into electricity). In addition, it is shown that water offers lower performance with respect to organic fluids when considering single stage radial expanders.