Approximately 45% of power generated by conventional power systems is wasted due to power conversion process limitations. Waste heat recovery can be achieved in an Organic Rankine Cycle (ORC) by converting low temperature waste heat into useful energy, at relatively low-pressure operating conditions. The ORC system considered in this study utilises R-1234yf as the working fluid; the work output and thermal efficiency were evaluated for several operational pressures. Plate and shell and tube heat exchangers were analysed for the three sections: preheater, evaporator and superheater for the hot side; and precooler and condenser for the cold side. Each heat exchanger section was sized using the appropriate correlation equations for single-phase and two-phase fluid models. The overall heat exchanger size was evaluated for optimal operational conditions. It was found that the plate heat exchanger out-performed the shell and tube in regard to the overall heat transfer coefficient and area.

Use of ORCs in waste heat recovery is widely seen as a viable and promising solution for increasing energy efficiency and emission reduction efforts, with "on-board" vehicular concepts becoming increasingly popular. In this study, the potential of an ORC harnessing exhaust energy from a diesel generator is considered. Preliminary fluid selection was based on satisfactory thermodynamic performance, and expander size requirement as the limiting parameter. Both simple and recuperative ORC systems were modelled. The effect of the exhaust temperature and the high operational pressure of the ORC model were evaluated in terms of energetic and exergetic performance. For the toluene ORC, moderate pressure values were dictated by the expander size limitation, yet this can be alleviated by high exhaust temperatures. Simple ORCs required a larger heat input and had lower exergetic efficiency. Recuperative ORCs showed better thermal efficiency and lower overall exergy destruction. The expander efficiency was identified as a vital parameter for cycle design and thermodynamic performance.

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