有机朗肯循环混合工质的热力学分析

有机朗肯循环是利用低品位余热的关键技术之一,在一定程度上可有效缓解能源危机。相比于传统的纯工质,混合工质因其在换热器中具有温度滑移特性,可以更好地与热源进行耦合。因此选用混合工质可以使机组的效率得到更进一步的提升。通过采用二次迭代的方法,热源温度为120℃时,理论分析了以R245fa为参考的6种混合工质对,其混合比例对系统输出功与效率的影响。结果表明:工质吸热量一定的情况下,系统采取混合工质可以获得比纯工质更高的输出功、热效率。当吸热量为197.29 k W时,以纯R245fa为工质的系统最大输出功与热效率分别为11.98 k W与6.07%。存在最佳混合工质对R113/R245fa(质量分数0.7∶0.3),相比于纯工质R245fa,输出功与热效率分别提高了22.87%和22.89%。同时发现,无量纲积分温差ΔT~*与系统净输出功、热效率之间存在反比关系。当工质为R113/R245fa(质量分数0.7∶0.3)时,在最大净输出功工况下,6种混合工质对下的最小值ΔT~*为0.233。

Thermodynamic Analysis of Zeotropic Mixtures used in Organic Rankine Cycle

Organic Rankine Cycle is a key technology to convert low grade waste heat to electric power. The zeotropic mixtures have a better properties than pure organic in the exchangers. Therefore, the zeotropic mixtures will be a better choice to improve the system performance. In this paper, the twice iteration method was employed and R245fa was the reference working fluid. Six different organics were mixed with R245fa to get six different zeotropic mixtures. As the heat source temperature is 120 °C, different system output power and efficiency with different zeotropic mixture ratio was studied. The simulated results showed that under the constant heat source and the working fluid heat absorption, the zeotropic mixtures could improve the power output and thermal efficiency. When pure R245fa was used, the maximum power output and the maximum thermal efficiency were 11.98 kW and 6.07% respectively. But the output power and thermal efficiency were increased by 22.87% and 22.89% respectively, when zeotropic mixtures R113/R245fa (0.7:0.3) were used. Besides, the non-dimensional integration temperature difference had an inverse relationship with the system net power. For example, when zeotropic mixtures R113/R245fa(0.7:0.3) was used, the system maximum net power is 14.72 kW. And the non-dimensional integration temperature difference reached minimum 0.233.