过冷度对地热发电非共沸工质有机朗肯循环热力性能的影响

Effect of Subcooling on Thermal Performance of Organic Rankine Cycle With Zeotropic Mixture in Geothermal Power Generation

有机朗肯循环(organic Rankine cycle,ORC)是利用中低温地热能(<150℃)发电的主要途径,在实际运行中,非共沸工质往往会冷凝至过冷状态。分析了冷凝过冷度对非共沸工质ORC热力性能的影响,建立了ORC、内回热(internal heat exchanger,IHE) ORC的热力学模型,以净输出功最大为目标函数优化了工质的蒸发压力,并开展了系统的?分析。结果表明:过冷度影响了工质与冷源换热流体间的温度匹配特性,受夹点温差的限制,随着过冷度的增加,工质的冷凝压力上升;过冷度亦改变了预热器和蒸发器的热量分摊,随着过冷度的增加,最佳蒸发压力亦上升。混合工质异丁烷/异戊烷的质量配比为0.4:0.6时,净输出功受过冷度的影响最大,当过冷度为2℃时,净输出功下降了4.36%。IHE回收膨胀机排汽的余热,提高了预热器入口温度,可提高过冷ORC系统净输出功0.55%。过冷度增大了冷凝器的?损失;采用内回热冷凝器的?损失降低了24.7%。

Organic Rankine cycle(ORC) is the main way to generate electricity by using medium-low temperature(<150 ℃) geothermal heat. In actual operation, zeotropic mixture tends to condense to the subcooling state. This paper analyzed the effect of subcooling on t hermal performance of ORCs using zeotropic mixture as working fluid. The mathematics model of the ORC and the ORC with internal heat exchanger(IHE) were established, and then the evaporation pressure was optimized to maximize the net power output. Moreover, the system exergy analysis was also conducted. The results show that the condensation subcooling affects the temperature profile match between the working fluid and the heat sink. Limited by the pinch point temperature difference, the condensation pressure rises with the subcooling increasing. The subcooling also changes the heat allocation between the preheater and evaporator, the optimal evaporation pressure also increases as the subcooling increases. When the subcooling is 2 ℃, the net power output can be reduced by 4.36% for isobutene/isopentane(0.4:0.6) which is the most affected by the subcooling. The IHE recovers the waste heat of expander exhaust to preheat the working fluid, as a result, the net power output of the subcooled ORC system can be increased by 0.55%. The subcooling causes a significant increase in the condenser exergy destruction that can be reduced by 24.7% using the recuperative ORC system.