The impact of initial working fluid charge on performance of Organic Rankine Cycle using zeotrope mixture under design condition

Mass-based analysis of thermodynamic cycle can be a link to realize the matching between the fluid properties, components performance, and operation parameters. The present paper proposes a mass-based analysis for ORC systems using zeotropic fluid mixture, focusing on evaporator and condenser as the phase-change heat transfer components. This proves to be an effective way to investigate the impact of composition shift and the initial working fluid charge on system design performance. Results indicated that liquid-phase zones in both heat exchangers account for about 60 % of total mass. Since the largest composition shift occurs in condensation zone, this relation affects the masses and composition shifts in other zones as well as the overall system. Meanwhile, the relationship between the initial charge and composition shift are analyzed. It is found that the increase of total mass leads to a decrease of mass in two-phase zones and an increase of mass in single-phase zones. It therefore causes a reduction of the average composition shift. Moreover, analysis shows the highest net output work of 2.34 kW, which occurs when the initial working fluid charge rises to 130 % of the design value. However, further increases in mass will have a negative impact on system performance.