THERMODYNAMIC ANALYSIS OF A MODIFIED RANKINE CYCLE USED FOR POWER GENERATION

Abstract

In this study the modified Organic Rankine Cycle focuses the performances of IHE (Internal Heat Exchanger) in ORC (Organic Rankine Cycle) systems. Although previous studies hold multitudinous opinions, this study gives clear statements of IHE in both subcritical and supercritical ORC systems by setting a new model taking pressure drop in loops and pinch point into consideration. Commonly used working fluids R123 and R600 are chosen for subcritical and supercritical cases separately. The temperature of the heat source applied is 200 0C and the mass flow rate of it is 1 kg/s. The analysis is accomplished by the software Engineering Equation Solver (EES). A modified method of calculating maximum heat exchange in IHE is given when modelling a supercritical cycle, because of the momentously changing specific heat near the critical point. Besides, a new approach is put forward to calculate the outlet temperature of the heat source and find the location of pinch point in supercritical cases. The results provide that IHE is beneficial to a subcritical case, but it improves system performance only in part of the low pressure stage in a supercritical case. Moreover, after the temperature Tad is taken into account, it is found that IHE is able to enlarge the maximum system net output in a subcritical case. And in a supercritical case, the original evaporation pressure which does not conform to the rule Th,out > Tad is available now. It is revealed that the utilization of IHE will strengthen the applicability of the system. In subcritical cycle, it is observed that power output is not affected by IHE and maximum power obtained is 11.3 KW at an evaporator pressure of 1374 KPa and the thermal efficiency is 40.15% at Pevap = 2350 KPa. In supercritical cycle, the maximum power output is 11.55 KW at an evaporator pressure of 5080 KPa and an thermal effiency of 45% .