An analysis of a use of an environmentally friendly natural working fluids mixture based on carbon dioxide (R744) in modern ejector-based refrigeration cycles: simulations data

The synthetic refrigerant replacement by a low-GWP or blend with natural working fluids to reduce GWP [1] is the current challenge, but the use of new refrigerants, i.e., HFO, affects the poisoning of water, e.g. groundwater, and may be harmful to human health [2]. As a result of the harmfulness of synthetic working fluids, it is necessary to use ecological refrigerants in applications characterised by low coefficient of performance (COP), i.e., low and ultra-low freezing in the range of -50 ° C to -20 ° C used in food transport, freeze drying and storage of food products [3]. In Poland, almost 5 million tons of food is wasted annually at the stages of production, processing, distribution and consumption [4]. Therefore, the only safe and necessary solution in the food industry and to reduce losses in the food chain is to use a mixture of natural refrigerants, i.e. R744, or hydrocarbons, due to thermodynamic properties, availability and no negative impact on the environment [1]. The mixture of natural working fluids will expand the scope of application of cooling systems in low- and high-temperature areas, although in order to maintain a high COP, it is required to use state-of-the-art technologies in the refrigeration system, i.e., a two-phase ejector. The main purpose of the ejector operation in the system is to partially recover of the expansion work, which results in a higher efficiency of the entire system (even by 25%), as well as lowering the pressure ratio in the compressor, reducing the total energy consumption at a properly designed new generation ejector [5].

The main aim of the work was to investigate the effect of the use of a two-phase ejector on the flow behavior and thermodynamic effects of environmentally friendly and non-flammable mixtures of natural working fluids based on carbon dioxide and propylene in refr. systems at low temperature processes. As a result, the current limitations of refrigeration cycles based on single-component natural refrigerants will be removed to shorten the food chain, reduce food waste and improve the energy efficiency of food storage processes in the food industry. Minimizing losses in the food chain is essential to reducing the global and local problem of food waste. This design goal is especially important during phasing out of synthetic refrigerants and introducing low-impact working fluids into all refr. systems.

Acknowledgment: Project No. 2021/05/X/ST8/00266 funded by Polish NCN.

References:
[1] N. Abas et al., Nat. & synth. refrigerants, global warming: A rev. Ren. & Sust. En. Rev., 2018
[2] Refolution Industriekälte GmbH, Report and statement of the downsides of HFO refrigerant usage, 2021.
[3] S.J. James, C. James, The food cold-chain and climate change. Food Res. Int., 2010.
[4] Federacja Polskich Banków Żywności, Raport “Nie marnuj jedzenia 2020”, 2020.
[5] J. Bodys et al., Exp. and num. study on the R744 ejector with a suction nozzle bypass, Appl. Therm. Eng., 2021.

以低全球变暖潜势（Global Warming Potential, GWP）制冷剂或天然工质混合物替代合成制冷剂以降低GWP是当前面临的挑战[1]，但新型制冷剂（如氢氟烯烃Hydrofluoroolefins, HFO）的使用会对水体（例如地下水）造成污染，并可能危害人体健康[2]。鉴于合成工质的危害性，在低性能系数（Coefficient of Performance, COP）的应用场景中必须使用生态友好型制冷剂——即应用于食品运输、冷冻干燥与食品储存的-50℃至-20℃低温及超低温冷冻场景[3]。
波兰每年在生产、加工、分销与消费环节产生近500万吨食物浪费[4]。因此，在食品工业中减少食品链损失的唯一安全且必要的解决方案，是使用天然制冷剂混合物（如R744或烃类制冷剂），这得益于其优异的热力学性能、可获取性以及对环境无负面影响[1]。天然工质混合物可拓展制冷系统在低温与高温区域的应用范围，但为维持较高的COP，需在制冷系统中采用前沿技术，即两相喷射器（two-phase ejector）。喷射器在系统中的核心作用是部分回收膨胀功，可使整个系统的效率提升最高达25%，同时降低压缩机内的压比，在经过合理设计的新一代喷射器中可减少总能耗[5]。

本研究的主要目的是探究两相喷射器的使用对低温制冷系统中以二氧化碳和丙烯为基础的环保型、不可燃天然工质混合物的流动特性与热力学效应的影响。据此，当前基于单组分天然工质的制冷循环所存在的局限将被消除，从而缩短食品链、减少食物浪费，并提升食品工业中食品储存过程的能源效率。最小化食品链损失对于缓解全球及区域性的食物浪费问题至关重要。在逐步淘汰合成制冷剂并将低环境影响工质引入所有制冷系统的当下，这一设计目标尤为重要。

致谢：本项目由波兰国家科学中心（National Science Centre, Poland, NCN）资助，项目编号为2021/05/X/ST8/00266。

参考文献：
[1] N. Abas 等. 天然与合成制冷剂与全球变暖：综述[J]. 可再生与可持续能源综述, 2018.
[2] Refolution Industriekälte GmbH. HFO制冷剂使用弊端报告与声明[R]. 2021.
[3] S.J. James, C. James. 食品冷链与气候变化[J]. 食品研究国际, 2010.
[4] 波兰食品银行联合会. 《“勿浪费食物”2020报告》[R]. 2020.
[5] J. Bodys 等. 带吸气喷嘴旁通的R744喷射器实验与数值研究[J]. 应用热工程, 2021.