تجزیه و تحلیل عملکرد از چرخه انتشار تبرید میزان جذب کار با مخلوط TFE-TEGDME

The binary mixture of TFE–TEGDME has good thermo-physical properties and been investigated by many absorption chiller and heat pump researchers. However, few studies have been reported on the diffusion absorption refrigeration (DAR) system using TFE–TEGDME as the working fluid. The present article numerically investigates the potential of TFE–TEGDME used in the DAR system with two cooling mediums, viz. water (32 °C) and air (35 °C). It is found that with the absorber effectiveness of 0.8, the optimum generation temperature for the air-cooled TFE–TEGDME DAR system is around 170 °C, and the corresponding coefficient of performance (COP) is up to 0.45. In comparison, the performance of the water-cooled system is better with a lower optimum generation temperature around 130 °C and a higher COP reaching 0.56. Parametric studies are also conducted to analyze the effects of the cooling medium, generation temperature, evaporation temperature and absorber effectiveness on the system performance. Finally, the performance of the TFE–TEGDME and NH3–H2O DAR cycles is compared in terms of the COP and circulation ratio. Overall, it can be concluded that the TFE–TEGDME mixture is a good working fluid for the DAR cycle.

In recent years, energy consumption in buildings has become a priority issue. The building sector currently accounts for approximately one third of the total primary energy consumption worldwide [1], and this value will be higher in the future as the development of the economy together with the improvement of people's living standard [2]. Meanwhile, the building sector produces large carbon dioxide emissions associated with the use of fossil fuels. For instance, buildings contribute to around 18% of global carbon dioxide emissions in China [3]. With respect to energy savings and emission reduction, the interest in the refrigeration systems driven by low-temperature heat sources, such as solar energy and waste heat, for building cooling is growing [4]. The thermally activated diffusion absorption refrigeration (DAR) cycle introduced by Platen and Munters [5] in the 1920s has been recognized as one of the most promising sustainable technologies for cooling production. The cycle operates at a constant total pressure level and utilizes a refrigerant–absorbent mixture as the working fluid and an inert gas for pressure equalization. Comparing with the conventional absorption refrigeration cycle, the DAR has no solution pump, which is instead of a bubble pump and leads to silent operation [6]. As the growing demand on high quality living condition in residential and commercial buildings in recent years, the advantages make the DAR attract gained attention. The most common DAR system uses NH3–H2O as the working fluid and hydrogen or helium as the auxiliary inert gas, which has been extensively investigated. For example, Zohar et al. [7] and Starace and De Pascalis [8] developed thermodynamic models for the NH3–H2O DAR cycle with hydrogen as the auxiliary inert gas; Chen et al. [9] developed a new generator configuration that increases the COP of the cycle by 50%; Srikhirin et al. [10] carried out an experimental study on an NH3–H2O DAR cycle using helium as the auxiliary gas. Zohar et al. also compared the COP of two cycle configurations with and without condensate sub-cooling prior to the evaporator entrance [11], and investigated the influence of the generator and bubble pump configuration on the cycle performance [12]. Jakob et al. experimentally and theoretically studied a 2.5 kW solar heated NH3–H2O DAR machine with helium as pressure compensating inert gas [13], [14] and [15], which demonstrates that the DAR system can be used for domestic air conditioning. However, the NH3–H2O DAR system has some limitations resulting from the working fluid. It requires a generation temperature above 150 °C; it is a high-pressure system that needs a rectifier; in addition, ammonia is toxic, explosive and corrosive to copper [16]. To overcome the limitations, organic fluids are suggested. Koyfman et al. [17] conducted an experimental investigation on the performance of the bubble pump for the DAR cycle with R22–DMAC as the working fluid, and they indicated that the cycle can be operated at the maximum average generator temperature below 90 °C. Zohar et al. [18] compared the performance of the DAR cycle using five refrigerants (R22, R32, R124, R125 and R134a) in combination with the organic absorbent DMAC with the NH3–H2O system. Ben Ezzine et al. [16] reported that the R124–DMAC DAR system gives a higher COP at lower driving temperatures comparing with the NH3–H2O system; they also experimentally investigated a DAR system using C4H10–C9H20 as the working fluid and helium as the auxiliary gas [19]. Wang et al. [20] investigated a DAR with the binary refrigerants R23/R134a, the absorbent DMF and auxiliary inert gas helium for lower temperature applications with the generating temperature between 110 and 160 °C. These studies concluded the promising prospect of the organic working fluids for the DAR cycle. Among the organic working fluids, the binary organic mixture of 2,2,2-trifluoroethanol (TFE)-tetraethylene glycol dimethyl ether (TEGDME) has many advantages such as complete miscibility, wide working temperature range, low working pressure, no corrosion and good safety level. Also, owing to the great difference of the normal boiling temperature (about 200 °C) between TFE and TEGDME, the system does not need a rectifier, which makes the system simpler and cheaper [21]. The overall performances of TFE–TEGDME as the working pair utilized in the absorption systems (absorption chiller, absorption heat pump and absorption heat transformer) has been widely investigated [21], [22], [23], [24] and [25]. However, to our knowledge, the investigation on the TFE–TEGDME DAR system has not been performed. With regard to the excellent thermo-physical properties, it is believed that a DAR system using TFE–TEGDME as working fluids will have a good performance. Hence, the present study aims to investigate the TFE–TEGDME DAR cycle with helium as the auxiliary gas. The effects of different operating parameters on the system performance are analyzed.

The present article investigates the potential of TFE–TEGDME used as the working fluid in the DAR system. Using the thermodynamic model developed, the numerical simulation shows that with the absorber effectiveness of 0.8, the highest COP of the air-cooled TFE–TEGDME DAR cycle is up to 0.45, and the corresponding optimum generation temperature is about 170 °C; while the water-cooled system has a maximum COP of 0.56, corresponding to a lower optimum generation temperature at 130 °C. Moreover, the results show that the cycle's performance is influenced by the generation temperature, cooling medium and evaporation temperature and absorber effectiveness. The comparison between the TFE–TEGDME and NH3–H2O DAR system shows that although the NH3–H2O system gives better performance when using air as the cooling medium, in the case of water-cooled system, the advantages of the TFE–TEGDME system become obvious. Hence, the TFE–TEGDME mixture can be considered as a good alternative for the DAR cycle, especially for the water-cooled system. The limitation of the present study is that the model developed here is not validated against experimental data. So, future works should be directed to carry out experimental studies on the TFE–TEGDME DAR system and revise the numerical model.