Abstract:

On the bases of an existing water-source heat pump water heater with CO₂ as a refrigerant, the optimal charge was calculated by different theoretical methods. The changes of system performance were studied with various refrigerant charges by a series of experiments and the optimal value was used to verify the accuracy of the theoretical one. The results indicated that there existed an optimal charge in a water-source heat pump water heater with CO₂ as a refrigerant where the system yielded the best COP_heat and the value was 270 g in the experiment system. If the refrigerant charge was reduced to 89% (230 g), COP_heat would reduce by 10.8%, while 111% (300 g) charge reduced COP_heat (coefficient of heating performance) by 2.6% and it could be concluded that the performance of such system was particularly sensitive to the insufficient charge. Moreover, the hot water yield increased with the increase of refrigerant charge, but the effect was not obvious when charge exceeded the optimal value. Comparing with the experiment result, it was found that both the experimental data method and the rated operating method could be applied in the studied water-source heat pump water heater system with the maximum error of 3.7%. This study may provide theoretical guidance on how to determine optimal refrigerant charge and how to maintain efficient operation of a small CO₂ transcritical system.

Key words: optimal charge, carbon dioxide, transcritical, theoretical calculation, experimental validation, enthalpy

摘要：

基于一套已有的小型CO₂水源热泵热水器实验台，运用不同的理论方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，并利用实验结果对理论计算结果的准确度进行验证。研究结果表明：小型CO₂水源热泵热水器存在着最佳充注量，在此充注量条件下，系统的COP_heat最大，实验系统的最佳充注量为270 g。当充注量减小为最佳值的89%时（230 g），COP_heat降低10.8%，增大为最佳值的111%时（300 g），COP_heat降低了2.6%，即充注量不足时，COP_heat对充注量的变化更为敏感。此外，充注量增加会提高系统热水的出水量，但超过最佳值后，效果不明显。实验数据法和额定工况法均适用于本文所研究的CO₂水源热泵热水器系统，最大误差不超过3.7%。本研究可以为小型CO₂跨临界系统最佳充注量的确定及如何维持系统高效运行提供理论指导。

关键词: 最佳充注量, CO₂, 跨临界, 理论计算, 实验验证, 焓