Energetic and exergetic analysis of waste heat recovery from a microturbine using organic Rankine cycles

This article examines the exhaust waste heat recovery potential of a microturbine (MT) using an organic Rankine cycle (ORC). Possible improvements in electric and exergy efficiencies as well as specific emissions by recovering waste heat from the MT exhaust gases are determined. Different dry organic working fluids are considered during the evaluation (R113, R123, R245fa, and R236fa). In general, it has been found that the use of an ORC to recover waste heat from MTs improves the combined electric and exergy efficiencies for all the evaluated fluids, obtaining increases of an average of 27% when the ORC was operated using R113 as the working fluid. It has also been found that higher ORC evaporator effectiveness values correspond to lower pinch point temperature differences and higher exergy efficiencies. Three different MT sizes were evaluated, and the results indicate that the energetic and exergetic performance as well as the reduction of specific emissions of a combined MT-ORC is better for small MT power outputs than for larger MTs. This article also shows how the electric efficiency can be used to ascertain under which circumstances the use of a combined MT-ORC will result in better cost, primary energy consumption, or emission reduction when compared with buying electricity directly from electric utilities. Copyright (c) 2012 John Wiley & Sons, Ltd.