夹点温度对LNG冷能ORC发电系统热经济性的影响

Effect of pinch point on thermo-economic performance of LNG cold energy ORC power system

中国已成为全球第一大液化天然气(LNG)进口国,但LNG冷能有机朗肯循环(ORC)发电因投资收益率较低,在大陆地区尚无已投产的工程应用项目。以我国东部沿海不同纬度的4个典型城市的海水条件为例,以最大净发电功率为目标函数,计算了4种海水温度下LNG冷能ORC发电系统的热力性能,探讨了3~25℃内冷凝器和蒸发器夹点温度对系统热经济性的影响。结果表明,在北方沿海地区(上海以北)或当海水温度较低(<17℃)时,适合采用降低夹点温度的方式提高系统净发电功率。蒸发器夹点温度每降低1℃,蒸发温度提高约1.1℃,系统净发电功率增加73.6~86.6 kW(1.6%~2.5%)。在夹点温度低于10℃的LNG冷能ORC发电系统中,提高夹点温度有利于缩减系统总换热面积。海水温度越低,系统动态投资回收期对夹点温度越敏感。与冷凝器夹点温度相比,蒸发器夹点温度对系统动态投资回收期影响更大。以最小系统动态投资回收期为评价指标,最佳蒸发器夹点温度为7~11℃,最佳冷凝器夹点温度为13~17℃。

China has become the largest importer of liquefied natural gas(LNG)in the world.Due to the low rate of investment pay-back,there is no LNG cold energy organic Rankine cycle(ORC)plant in service in Chinese mainland.Taking the seawater conditions of four typical cities in different dimensions along the eastern coast of China as examples and the maximum net power generation as objective function,the thermal performance of LNG cold energy ORC power system was calculated,and the effect of condenser and evaporator pinch temperature in the range of 3℃ to 25℃ on the thermo-economic performance of the system was discussed.The results show that in the northern coastal area(north of Shanghai)or when the seawater temperature is low(<17℃),it is more suitable to reduce the pinch temperature to improve the net power generation of the system.When the pinch temperature of the evaporator decreases by 1℃,the evaporating temperature increases by about 1.1℃,the net power generation of the system increases by about 73.6 kW to 86.6 kW(1.6%to 2.5%).In the LNG cold energy ORC power system with small pinch temperature(<10℃),increasing the pinch temperature has greater potential to reduce the total heat exchange area of the system.When the seawater temperature is lower,the system dynamic investment payback period is more sensitive to the pinch temperature.Compared with the condenser pinch temperature,the evaporator pinch temperature has a greater impact on the system dynamic investment payback period.Taking the minimum system dynamic investment payback period as the evaluation index,the optimal evaporator pinch temperature is 7℃ to 11℃,and the optimal condenser pinch temperature is 13℃ to 17℃.