Thermodynamic study on absorption refrigeration systems using ammonia/ionic liquid working pairs
- Autores: Hifni Mukhtar Ariyadi
- Directores de la Tesis: Alberto Coronas Salcedo (dir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2016
- Idioma: español
- Tribunal Calificador de la Tesis: José S. Urieta Navarro (presid.), Daniel Salavera Muñoz (secret.), Pascal Tobaly (voc.)
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX hdl.handle.net hdl.handle.net
- Resumen
Absorption refrigeration system is a good known technology. The absorption system uses a heat source to produce cooling. It can utilize renewable energy such as solar and geothermal energy or waste heat sources, thus carries a primary energy saving and emission reduction. In addition, the absorption system has no vibration and noise and contains non-HFC and therefore, environmentally friendly and becomes a competitive alternative to the conventional mechanical-driven vapour compression refrigeration systems.
The working fluid for absorption refrigeration systems consists of refrigerant and absorbent. One most common working fluid in absorption refrigeration cycle is ammonia/water. In this working pair, the characteristics of water is volatile, thus as absorbent, it is necessary to add an additional component, rectifier, to minimize the amount of water coming to the condenser. Hence, to overcome these drawbacks, finding a suitable working pair is inevitable.
As a new type of fluids with a great solvent character and other interesting properties such as a good thermal stability and very low vapour pressure, ionic liquids can be good candidates as absorbents for absorption systems to overcome drawbacks associated with the conventional working pairs. Nevertheless, research on both thermodynamic properties and application of ammonia/ionic liquid mixtures for absorption cycle applications is still remains scarce. Research of this thesis was aimed to analyse the feasibility and the performance of ionic liquids as absorbents for ammonia refrigerant in absorption refrigeration systems. Ionic liquids, novel and tailor-made absorbents, can be used with ammonia as working pairs for absorption refrigeration cycles and give some advantages such as elimination of the rectification process in ammonia/water systems.
Although the properties of ammonia/ionic liquid working pairs have been gradually examined in recent years, the information related to the thermophysical properties of the mixtures of ammonia/ionic liquid systems are still remains scarce. Furthermore, investigations related to the application of ammonia/ionic liquid working fluids in absorption refrigeration systems are so far limited to the theoretical studies and computational simulation. In addition, the simulation results were undoubtedly dependent to the thermodynamic model and the availability of thermophysical property data. Therefore it is necessary to select an appropriate model to describe the vapour-liquid equilibrium properties ammonia/ionic liquid mixtures.
Among four different models studied in this research, both NRTL and RK-Soave model show their ability to calculate the vapor-liquid equilibrium of ammonia/ionic liquid mixtures with high accuracy. Although the calculation results using NRTL model were slightly less accurate than those of RK-Soave model, NRTL model is considered as the simplest model in comparison with other thermodynamic models. Subsequently, NRTL model is chosen to evaluate the performances of absorption refrigeration system using ammonia/ionic liquid mixtures available in the literature. Calculation and simulation works were carried out using commercial software ASPEN Plus. Among five ammonia/ionic liquid working fluids studied, at certain operation conditions the ammonia/[emim][NTf2] working fluid presented the highest COP than other ammonia/ionic liquid mixtures. However, although the COP of the system working with ammonia ammonia/[emim][NTf2] was higher than the systems with other working fluids, its circulation ratio was the highest among other working fluids and thus the solution mass flowrate per unit of cooling load (R) was also the highest among other working fluids. It means that at the same ionic liquid mass flow rate and at the same operation conditions, the systems working with ammonia/[emim][NTf2] mixture produces lower cooling capacity. On contrary, the COP of the system working with ammonia ammonia/[bmim][BF4] was lower than the systems with other working fluids and its f and R values were the lowest among other working fluids. However, value was the highest among other working fluids, which means that at the same ionic liquid mass flow rate and at the same operation conditions, the systems working with ammonia/[bmim][BF4] mixture can produce higher cooling capacity.
Apart of ammonia/ionic liquid mixture available in the literature, new proposed ammonia/ionic liquid mixtures working pair for absorption refrigeration applications are theoretically studied and analysed, and compared with the performance of ammoni/LiNO3 working pair. Among all of new ammonia/ionic liquid working fluids studied only [N1113][NTf2] presented higher COP than that of ammonia/LiNO3 at certain operation conditions and the highest circulation ratio among other working fluids. The circulation ratios (f) and R values of the absorption systems working with ammonia/ionic liquid working fluids at same operation conditions were somehow higher as compared with that of ammonia/LiNO3. Finally the viscosities of ammonia/ionic liquids mixtures were generally higher than that of ammonia/LiNO3 however, surprisingly the viscosity of ammonia/[N111(2OH)] [NTf2] was lower than that of ammonia/LiNO3, which may be a competitive absorbent for water absorbent substitution for ammonia-based absorption refrigeration systems in comparison with LiNO3.
In addition to the simulation and theoretical investigation, a measurement setup to study the absorption capacity of the ammonia vapor in ionic liquids in a pool type absorber was also developed and studied to find the most suitable ionic liquid as an absorbent for ammonia refrigerant. Furthermore, it is also important to find the most suitable absorber configuration for the proposed ammonia/ionic liquids absorption systems. Among all measured ionic liquids, [EtOHmim]+ based ionic liquids shows higher absorption capacity than [emim]+ based ionic liquids, which means that the OH structure in the cation may improve the absorption capacity of ammonia. In addition [BF4]- anion shows slightly higher absorption capacity than other anions with same cation. However, in the beginning of the process [emim]+ based ionic liquids show higher absorption capacity than [EtOHmim]+ based ionic liquids.
In the terms of absorption thermal load, heat flow peak of ionic liquids with [NTf2]- anion is higher as compared to those ionic liquids with other anion which means that ionic liquids with [NTf2]- anion release more heat as compared to other ionic liquids. As a results, the ionic liquids having [NTf2]- anion shows higher total thermal load than ionic liquids. Similarly, the total heat released per unit refrigerant mass of ionic liquids with [Ntf2]- anion is higher as compared to those ionic liquids with other anion and same cation. However, in the terms of same anion, ionic liquids having [emim]+ cation give higher total heat released per unit refrigerant mass as compared to other cation with the same anion as as ionic liquids with [emim]+ cation absorbed less refrigerant than other ionic liquids with [EtOHmim]+ cation.
The ammonia/ionic liquid working fluid can provide competitive performance in comparison with conventional absorbent for ammonia refrigerant. However, some drawbacks are still remains to be solved such as relatively low solubility of ammonia into ionic liquids which affects to the solution circulation mass flow ratio and relatively high viscosity of ionic liquid in comparison with other conventional absorbent which may affects to the performance of absorber and solution pump.
