A Comprehensive Assessment of the Refrigerant Charging Amount on the Global Performance of a Transcritical CO2-Based Bus Air Conditioning and Heat Pump System

Abstract

To mitigate the contemporary environmental challenges and to respect the regulations on the progressive ban of hydrofluorocarbons (HFC), natural fluid CO2 was selected as an ideal refrigerant alternative in the transportation domain. In this study, the optimal CO2 charging amount and the refrigerant distribution in a bus air conditioning/heat pump system were analyzed in detail. The results showed that there was a plateau (so named by the best charging amount) of the CO2 charging amount in which the system performance reached an optimal value and maintained it nearly unchanged during this plateau while the performance declined on both sides of the plateau. In addition, the ambient temperature was found to have little effect on the determination of the refrigerant charging plateau, while the refrigerant distribution was affected by the ambient temperature to some extent. Due to the large thermal load and thermal inertia on a bus, the data and conclusions obtained are different from those of ordinary electric small passenger vehicles. This article aims to discover some quantitative parameters and lay a theoretical foundation in this field which is lacking relevant research. Through the research based on the GT-Suite simulation platform, we simulated the transcritical CO2 cycle applied on a bus, and the performance changes of the bus system (COP 1.2–2.2, refrigerating capacity 9.5–18 kW) under different charging rates (3–8 kg) were obtained.

跨临界二氧化碳热泵研究进展与应用综述

“…超临界 CO2 空调系统的最佳标准化充注量范围设置在 0.111 到 0.321 之间。Song 等 [83] 基于 GT-Suite 模 拟平台的超临界 CO2 循环在公交车上的应用的研究，分析了最佳 CO2 充注量和制冷剂分布对系统性 能的影响。得到了不同充注量(3-8 kg)下公交车系统 COP(1.2-2.2)，制冷量(9.5-18 kW)的性 能变化。 3.2.2. CO2 系统部件研究 为了提高电动汽车 CO2 压缩机的性能，Zheng 等 [84] 数值模拟了电动汽车 CO2螺旋压缩机的流场， 分析了不同工作室中切向泄漏流的发展特征和场量分布。结果表明，切向泄漏流和主流之间的不同 相互作用，吸气室和压缩室中的切向泄漏流具有不同的特征。换热器是 CO2 汽车空调的关键部件， 微通道平行流换热器与传统的翅片管式换热器相比，具有结构紧凑、效率高等优点，在汽车空调行 业得到了广泛的应用。马富芹等 [85] 对 CO2 汽车空调采用紧凑式微通道换热器的设计进行了理论探 讨，预测微通道换热器在汽车空调中会有很好的应用前景。Lei 等 [86] 利用一种空闲的加热芯来增加蒸 发器的换热面积，从而提高超临界 CO2 汽车空调系统性能的技术。除了微通道换热器和加热芯技 术，还有许多其他的传热增强方法，如采用不同形状的管材来改善换热器的传热效果，具体分类如 图 13 [87] 所示。 除了压缩机和换热器，节流装置也是 CO2 汽车空调系统的研究热点。Lee 等 [88] 设计并研制了一种 考虑非平衡状态的 CO2 热泵系统两相喷射器，用以替代系统中的膨胀装置。在此基础上，作者对喷 射器结构尺寸的变化(喷嘴直径、截面直径等)进行了性能试验。试验结果表明，各试验条件下均 存在最佳设计参数，采用喷射器的系统性能系数比传统系统提高了约 15%。Yang 等 [89] 构建了一种用…”

Research progress and applications of transcritical carbon dioxide heat pumps: A review

“…In terms of charge, Yin et al [82] investigated the effect of refrigerant charge on the system performance of CO2 automotive air-conditioning systems and suggested that the optimal standardized charge range for supercritical CO2 air-conditioning systems should be set between 0.111 and 0.321. Song et al [83] studied the application of the supercritical CO2 cycle in buses based on a GT-Suite simulation platform and analyzed the effect of the optimal CO2 charge and refrigerant distribution on system performance. The performance variations of the bus system's COP (1.2-2.2) and cooling capacity (9.5-18 kW) were obtained for different charging volumes (3-8 kg).…”