The absorption heat transformer (AHT) is a promising system for recovering waste heat. It can effectively recover about 50% of the waste heat and reuse it in industrial processes. However, there exists a need for identifying suitable working fluid combinations and for evaluating their relative performance characteristics. As an initial step, this paper presents a comparative performance study for the absorption heat transformer with H2O/LiBr, TFE(2,2,2-trifluoroethanol)/NMP(N-methy1-2-pyrrolidone), TFE/E181(dimethylether tetraethylene glycol) and TFE/PYR(2-pyrrolidone). The results show that the four working fluid combinations are all suitable for absorption heat transformers. H2O/LiBr is suitable at lower operating temperatures, while TFE/NMP, TFE/E181 and TFE/PYR are suitable at higher operating temperatures.
In recent years, large quantities of heat from industries have been rejected to the atmosphere as waste water or waste steam which not only wastes energy but also pollutes the atmosphere. The absorption heat transformer can effectively recover about 50% of the waste heat and reuse it in industrial processes [1].
Even though the absorption heat transformer has been reported to be a promising candidate for boosting the thermal level of waste heat sources, suitable working fluid combinations need to be identified and their relative performance characteristics need to be evaluated. A comparative thermodynamic analysis is the initial step. Such a study is presented here using H2O/LiBr, TFE(2,2,2-trifluoroethanol)/NMP(N-methy1-2-pyrrolidone), TFE/E181(dimethylether tetraethylene glycol) and TFE/PYR(2-pyrrolidone) as the working fluid combinations. The main advantages of H2O/LiBr are: high enthalpy of evaporation, high heat and mass transfer, non-toxicity, no need for rectification apparatus, etc. The main disadvantages are its corrosiveness and crystallization at high temperatures. The main advantages of TFE/NMP, TFE/E181 and TFE/PYR are: high thermal stability, high output heat temperature, flat vapor pressure curve and strong negative deviation from Raoult's law. The main disadvantages are their toxicity, since they all have the same toxicity as ammonia.
The results in Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9 and Fig. 10 show that the absorption heat transformer systems with H2O/LiBr, TFE/NMP, TFE/E181 and TFE/PYR are all suitable. The variations of all the indices relative to the five factors considered in the study are very similar. COP and ηex for H2O/LiBr are higher than those for TFE/NMP, TFE/E181 and TFE/PYR. However, GTL for H2O/LiBr is lower than that for TFE/NMP, TFE/E181 and TFE/PYR. In addition, H2O/LiBr has the disadvantage of corrosiveness and crystallization at higher temperatures, while TFE/NMP, TFE/E181 and TFE/PYR have high thermal stability. So TFE/NMP, TFE/E181 and TFE/PYR can operate over a wider temperature range than H2O/LiBr. The performance characteristics of the absorption heat transformer with TFE/NMP, TFE/E181 and TFE/PYR are very similar, especially for TFE/NMP and TFE/PYR. Since the difference between the TFE and NMP boiling point temperatures is not very great, the TFE/NMP system needs a rectifier which adds to the system’s complexity. Before making a final selection between these four combinations, other practical considerations should be considered. For instance, H2O/LiBr is a commonly used solution, whereas TFE/NMP, TFE/E181 and TFE/PYR are all recent developments. In addition, water is safer and cheaper than TFE.
In general, H2O/LiBr is superior to TFE/NMP, TFE/E181 and TFE/PYR when the output temperature is below 150°C, which is the maximum output temperature for H2O/LiBr because of corrosiveness and crystallization problems. TFE/NMP, TFE/E181 and TFE/PYR are all stable at higher temperatures, up to 200°C. The thermodynamic analysis also shows that H2O/LiBr is suitable at lower operating temperatures, while TFE/NMP, TFE/E181 and TFE/PYR are suitable at higher operating temperatures. Considering these conclusions, a system might even include a two-stage heat transformer with H2O/LiBr for the first stage and TFE/NMP, TFE/E181 or TFE/PYR for the second stage [6].
