Latest research on ammonia-based absorption refrigeration systems
A recent article has reviewed literature on absorption chillers that use ammonia as a refrigerant, along with water (H2O), lithium nitrate (LiNO3), and lithium nitrate + water (LiNO3 + H2O) as absorbent substances.
Absorption refrigeration systems correspond to compression refrigeration systems that use thermal compressors. They offer the advantage of reduced electricity consumption and do not use environmentally unfriendly refrigerants such as CFCs and HCFCs. Despite high capital cost, maintenance costs are relatively low. However, absorption refrigeration systems are heavy equipment that achieve a lower COP (coefficient of performance) than vapour compression systems. In order to improve the efficiency and performance of absorption cooling systems, the choice of working fluids (refrigerant and absorbent) is crucial.
A recent article has reviewed literature on absorption chillers that use ammonia as a refrigerant, and water (H2O), lithium nitrate (LiNO3), and lithium nitrate + water (LiNO3 + H2O) as absorbent substances. The main aspects considered by the authors were the thermodynamic properties of the mixtures, numerical correlations, improvement of heat and mass transfer processes in the thermal compressor, energetic and exergetic studies, active and passive applications using solar energy, and available prototypes and alternative equipment.
Thermodynamic properties of working fluids
The authors found that the commercial mixtures LiBr/H2O and NH3/H2O had several disadvantages such as corrosion, crystallisation problems, low-pressure operation and toxicity. In comparison, NH3/LiNO3 and NH3/(LiNO3 + H2O), showed better results, especially when considering the use of solar energy as the driving source for the system.
The addition of water as an absorbent in the mixture with NH3 allowed a better performance in the absorption and desorption process. The ternary mixture provided even better performance when plate heat exchangers were used, thanks to the reduction in size and cost of the absorption chiller, along with ease of installation.
Furthermore, the authors found studies supporting the use of ionic fluids in the solution to increase the efficiency of the heat and mass transfer process in absorption refrigeration systems.
Overall, thermodynamic studies have been a driving force in the search for alternative working fluids, and for more efficient, compact and flexible absorption refrigeration systems, for residential and industrial applications.
Configurations using solar energy
Due to the lower driving temperature, absorption chillers that use alternative working fluids (NH3/LiNO3 and NH3/LiNO3 + H2O) can be activated with the thermal energy obtained through solar collectors. One of the main advantages is a reduction in electrical consumption. However, the use of thermal storage systems and collection systems becomes mandatory for better efficiency when opting for solar energy in absorption chillers.
This has been solved with alternative configuration systems, such as multiple-effect absorption chillers (double- and triple-effect with higher COP). In addition, ejector components have also been found to improve the thermal and power efficiency in many studies.
When using solar energy as an input source in absorption systems along with alternative working fluids, the authors found COP values comparable to those of conventional single-effect absorption chillers, around 0.6–0.7. Nevertheless, these COP values were obtained with driving temperatures of about 60–90°C, which are lower than the driving temperatures in conventional systems. Such findings support the use of solar energy in advanced modified absorption chillers, for instance in residential applications.
The authors of the review article have identified several studies presenting prototypes that appear to be technically and financially viable. However, to the best of the authors’ knowledge, there is currently no mass marketing of absorption refrigeration systems that use ammonia with different water absorbers.
Recommendations for further research
Based on the current state of the art of absorption refrigeration systems, further research is still needed to improve their performance. The authors suggest the following priorities for future research:
- Reducing the cost and size of absorption chillers, by developing a better configuration of heat exchangers, especially the absorber and generators for the thermal compressor.
- Developing alternative ionic fluids to increase the heat and mass transfer and reduce the amount of the solution carried for the absorption refrigeration system, in order to reduce the size and hence, the cost.
- Investigating more alternative working fluids operating at lower temperatures and using sustainable energy sources, such as solar energy. These systems would provide cooling for the residential sector and small- and medium-capacity buildings.
Lima, A.A.S.; Leite, G.d.N.P.; Ochoa, A.A.V.; Santos, C.A.C.d.; Costa, J.A.P.d.; Michima, P.S.A.; Caldas, A.M.A. Absorption Refrigeration Systems Based on Ammonia as Refrigerant Using Different Absorbents: Review and Applications. Energies 2021, 14, 48. https://doi.org/10.3390/en14010048