跨临界CO2热泵的热气旁通除霜方法及除霜时间分析

Hot-Gas Bypass Defrosting Method and Analysis of Defrosting Time for Transcritical CO2 Heat Pump

空气源热泵系统在低温工况下运行时,存在蒸发器表面结霜、系统性能恶化等问题.针对传统除霜方法在跨临界CO2热泵系统应用中的局限性,对热气旁通除霜方法进行实验研究.在搭建的空气源跨临界CO2热泵系统实验平台上,以外径12.7 mm的旁通铜管作为节流机构,对除霜过程中的动态参数变化以及环境温度对除霜时间的影响进行分析,并绘制除霜不同时刻的系统压焓图.实验结果表明:热气旁通除霜过程较为稳定,各测点参数变化较为平缓.结合实验数据,可以发现,采用热气旁通除霜方法可明显提高蒸发器进口温度至30℃左右,缩短除霜时间;而除霜时间受除霜稳定期影响较大,环境温度降低或环境湿度增大均会延长系统除霜时间,除霜能耗比值与除霜时间比值的变化趋势基本一致.对热气旁通除霜效率进行计算,其值为46.5%,与其他热气除霜方法相比,效率增长33.62%,除霜时间缩短100 s,说明热气旁通除霜方法更适用于空气源跨临界CO2热泵系统.

When the air-source heat pump system is operated under low temperature conditions, there exists some problems such as frost formation on the evaporator and deterioration of system heating performance. Considering the limitations of traditional defrosting methods applied in the transcritical CO2 heat pump system, the hot-gas bypass defrosting method was experimentally investigated. The platform of air-source transcritical CO2 heat pump system was designed and a copper bypass tube with an outer diameter of 12.7 mm was used as the expansion device. The platform was tested under various conditions to analyze the dynamic parameters during defrosting process and the effect of ambient temperature on the defrosting time. Meanwhile, the defrosting process at different times was depicted in the pressure-enthalpy diagram. The experimental results show that the hot-gas bypass defrosting process is relatively stable, and the parameters of each measuring point change relatively gently. According to the experimental data, it can be found that the hot-gas bypass defrosting method can significantly increase the evaporator inlet temperature to about 30 ℃, effectively shortening the defrosting time. The defrosting time was greatly affected by the defrosting stability period. The decrease of environmental temperature or the increase of environmental humidity would extend the defrosting time of the system. The change trend of defrosting energy consumption ratio is basically consistent with the defrosting time ratio. The defrosting efficiency is calculated to be 46.5% and 33.62% higher than that of other defrosting methods and the defrosting time is shortened by 100 s, which indicates that the hot-gas bypass defrosting method is more suitable for the air-source transcritical CO2 heat pump.