[home] [Personal Program] [Help]

---

Session: Poster Session and Sponsors Exhibition
Session starts: Thursday 22 September, 14:00

---

Mohammed Khennich (Génie mécanique, Université de Sherbrooke)
Nicolas Galanis (Génie mécanique, Université de Sherbrooke)

Abstract:
Subcritical and transcritical Rankine cycles operating between a low temperature heat source (Ts,in = 100, 165 and 230 °C) of fixed volume flowrate (1.2 106 m3/h, idealized as atmospheric air at Ps = 101 kPa) and a fixed temperature heat sink (water at Tp,in = 10 °C) have been analyzed using the principles of classical and finite-size thermodynamics. The model of the system and its validation have been presented elsewhere [1]. Optimum operating conditions (pressure of the working fluid during heat addition, Pev, and temperature difference DT between the working fluid and the two external fluids) and the corresponding values of several system characteristics have been determined for different net power outputs using the variable metric method for each of the following objectives: maximum thermal efficiency, minimum total exergy destruction, minimum total thermal conductance of the two heat exchangers UAt and minimum turbine size SP. Typical results with R134a as the working fluid are presented. For this fluid the cycle is subcritical for Ts,in = 100 °C and transcritical for the other two values of the heat source temperature. At the turbine outlet the fluid is always superheated vapor. The lowest exergy losses as well as the smallest total conductance and turbine are obtained with Ts,in = 100 °C while the highest thermal efficiencies are obtained with Ts,in = 230 °C. The combinations of Pev and DT which maximize the thermal efficiency and minimize the exergy destruction are essentially identical. For these conditions the net power output has no effect on the thermal efficiency. On the other hand the exergy losses as well as the size of the turbine and heat exchangers increase with the net power output, albeit at different rates (the variation of the exergy losses, UAt and SP with the net power output is not linear). In all these cases the pinch in the high temperature heat exchanger occurs at the heat source inlet. The combinations of Pev and DT which minimize UAt and SP are different from each other and from those which maximize the thermal efficiency. The conditions which minimize UAt give turbine sizes not much bigger than the corresponding minimum size; on the other hand, the conditions which minimize SP give a thermal conductance significantly bigger than the corresponding minimum values.