自然工质CO2具有良好的环境和热力学性能，在小型系统中得到了越来越广泛的应用。小型系统的充注量决定系统性能优劣，存在着一个最佳值。基于一套小型CO2水源热泵热水器实验台，选用6种典型的空泡系数模型，运用空泡系数法对其最佳充注量进行计算，通过实验研究不同充注量对系统性能的影响，并利用实验结果对各种模型的计算结果进行验证，判断其计算准确度。结果表明，6种模型计算结果均具有较高的准确度，最大误差为9.80%；本系统中，Hughmark模型对于最佳充注量的计算结果误差为2.59%，且这个误差对于系统制热性能系数EERheat几乎没有影响。

The consumption of powerful greenhouse gases should be drastically reduced worldwide due to the serious problem of global warming. Natural refrigerants CO2 is the most promising refrigerants and has been widely applied in small refrigeration systems because of the low global warming potential (GWP) and no ozone depletion potential (ODP). It is universally acknowledged that an optimum refrigerant charge existing in a small refrigerant system can maximize the coefficient of performance (COP) of system. In the present work, the void fraction method with 6 classic models was used to predict the optimum refrigerant charge of a small CO2 water-source heat pump water heater. Based on an existing unit in the lab, the changes of system performance were studied with various refrigerant charges by a series of relevant experiments and the optimum charge was obtained to verify the accuracy of all the void fraction models in the references. The results indicated that each model had a relatively good accuracy in determining the optimum refrigerant charge of the tested unit with the error less than 9.80%. Hughmark model could be recommended to predict the optimum refrigerant charge of the studied system because of the negligible errors by less than 2.59%. Moreover, in the view of heating energy efficiency ratio (EERheat), the value calculated by the Hughmark model was almost the same with that obtained from the experimental measurement. On the basis of a small system with a transcritical cycle, the optimum charge calculated by void fraction method was more accurate than the ones from experimental data method and rated working condition method. The present work can provide the guidelines of predicting the optimum refrigerant charge in small CO2 systems.