Comparison of the performance of single-effect, half-effect, double-effect in series and inverse and triple-effect absorption cooling systems operating with the NH3–LiNO3 mixture
Introduction
Conventional cooling systems demand huge amounts of electric energy to operate. It has been estimated that around 15% of the electricity produced worldwide is used for air-conditioning and cooling [1]. In order to reduce this consumption, absorption cooling systems have gained a lot of research interest since they may operate with solar thermal energy or any other thermal energy such as residual heat from industries. Currently the research on absorption systems is being focused on the study of new working fluids and the development of advanced cycles.
Up to now, water–lithium bromide (H2O–LiBr) and ammonia–water (NH3–H2O) have been the most used mixtures in cooling systems, however, due to their well-known disadvantages, new working mixtures such as ammonia–lithium nitrate (NH3–LiNO3) have gained the interest of researchers. Some of the most relevant studies regarding the ammonia–lithium nitrate mixture are described in the following paragraph.
In 1991 Best et al. [2] reported the theoretical coefficients of performance against the main operating temperatures for a single-stage absorption cooling system. They found that the coefficients of performance could be as high as 0.7. In 1995, Antonopoulos et al. [3] compared the performance of solar absorption systems operating for cooling and heating using the NH3–LiNO3 and NH3–NaSCN mixtures. The results showed that the highest coefficients of performance for heating were obtained with NH3–LiNO3, meanwhile for cooling purposes the NH3–LiNO3 mixture reached higher cooling loads, but the highest COP was obtained with NH3–NaSCN. In 1996 and 1997, Ayala et al. [4], [5] simulated and tested a prototype of a NH3–LiNO3 hybrid absorption-compression refrigeration system. They demonstrated that efficiencies increase up to 10% compared to the ones reached using compression or absorption systems individually. In 2003, Rivera et al. [6] modeled an intermittent solar absorption refrigeration system operating with NH3–LiNO3 using a cylindrical parabolic collector as a generator–absorber, the results showed that it was possible to produce up to 8 kg of ice per day with the proposed system. In 2010, Ventas et al. [7] numerically modeled a hybrid cycle based on the single-effect absorption system integrated with a booster compressor between the evaporator and the absorber using the NH3–LiNO3 solution. They found that this cycle allows lower working temperatures compared with the conventional single-effect cycle and has low electricity consumption. In 2011, Zacarías et al. [8] reported the experimental assessment of NH3 adiabatic absorption into NH3–LiNO3 solution using a flat fan nozzle and an upstream single-pass subcooler. The authors obtained correlations for the equilibrium factor and Sherwood number which can be used for the design of adiabatic absorbers. In 2011 Rivera et al. [9] evaluated an intermittent solar refrigeration system for ice production operating with the NH3–LiNO3 mixture. The results showed that evaporator temperatures as low as −11 °C could be reached with COP up to 0.08. In 2012, Acuña et al. [10] conducted a comparative analysis for a diffusion absorption cooling system using lithium nitrate, sodium thiocyanate and water as absorbent substances and ammonia as the refrigerant. They concluded that the NH3–LiNO3 mixture was more efficient than the other two mixtures requiring lower generator temperatures than NH3–H2O for specific conditions. In 2013, Vasilescu et al. [11] reported a theoretical study of a solar driven parallel double-effect absorption system working with NH3–LiNO3 under Mediterranean summer conditions and considering the usage of parabolic trough solar collectors. The results showed that by using 1000 m2 of solar collector a cooling load up to 600 kW could be obtained.
Regarding the study or development of advanced absorption systems, Kaita, in 2002 [12] analyzed three types of triple-effect absorption systems using a new simulation program, triple-effect in parallel, in series, and inverse. The results showed that triple-effect in parallel may reach higher COP, meanwhile the triple-effect inverse system may operate with the lowest temperatures. In 2006, Wan et al. [13] proposed a new two-stage solar absorption refrigeration cycle using the water–lithium bromide mixture; they proposed to mix lithium bromide solution from a high pressure generator with solution from a low pressure absorber, in order to increase lithium bromide concentration at the high pressure generator and decrease pressure at the high pressure absorber. The theoretical results showed that the highest COP reached was 0.605 and the highest available temperature difference goes up to 33.5 °C at temperatures from 75 °C to 85 °C. Kilic et al., in 2007 [14] conducted a performance analysis, through a mathematical model using thermodynamics first and second law for a single-stage refrigeration cycle using water-lithium bromide; results showed that COP increased as generation and evaporation temperatures increased, while the COP decreased as condensation and absorption temperatures increase. In 2010, Gebreslassie et al. [15] made an exergy analysis for absorption systems working with water-lithium bromide and concluded that the highest exergy destruction occurs at the absorbers and generators, especially at higher heat source temperatures. Kaushik et al. [16] developed a computational model which compares single-effect system against double-effect system in series.
As can be noticed from the literature review, there is not a study about the comparison of the performance of single-effect, half-effect, double-effect in series and inverse and triple-effect absorption cooling systems operating with the NH3–LiNO3 mixture. In the actual paper theoretical coefficients of performance against the main operating temperatures of the systems are reported and discussed for each one of the systems separately and then the comparison is made among them describing their advantages, disadvantages or limitations.
Section snippets
Single-effect
A single-effect absorption cooling system consists of a generator (G), an absorber (A), a condenser (C), an evaporator (E), a heat exchanger (HE), two valves, and a pump as can be seen in Fig. 1. The cycle has two circuits: the refrigerant circuit (1–4) and NH3–LiNO3 solution circuit (5–10). An amount of heat is supplied to the generator to separate part of the ammonia from the solution at high pressure and temperature; once the ammonia is evaporated, it is conducted to the condenser where is
Mathematical model
In order to analyze the performance of the different absorption cooling systems, the following assumptions have been made in the development of their mathematical models with reference to Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5. The physical and thermodynamic properties for the NH3–LiNO3 were obtained from Infante Ferreira [17].
- i.
There is thermodynamic equilibrium throughout the entire systems.
- ii.
The analysis is made under steady state conditions.
- iii.
A rectifier is unnecessary since the absorbent does
Results
In order to analyze the behavior of each of the systems, separate graphs of their coefficient of performance were plotted against their main operating temperatures and then a comparison made among all of them describing their advantages, disadvantages, and or limitations.
Conclusions
From the analysis carried out it is concluded that the lowest evaporator temperatures can be achieved with half-effect systems at the lowest generator temperatures (starting from 50 °C), but with coefficients of performance around 0.30. The single-effect system is the simplest configuration since it requires fewer components in comparison to the other systems. Its coefficients of performance are almost twice higher than those with half-effect systems but requiring higher generator temperatures.
Acknowledgements
The authors thank to the projects SENER-CONACyT 117914 and CONACyT 154301 for the economical support given for the development of this study.
Nomenclature
- A
- absorber
- C
- condenser
- CG
- condenser–generator
- COP
- coefficient of performance (dimensionless)
- E
- evaporator
- G
- generator
- h
- enthalpy (kJ/kg)
- HE
- solution heat exchanger
- m
- mass flow rate (kg/s)
- P
- pressure (kPa)
- Q
- heat load (kW)
- RF
- flow ratio (dimensionless)
- T
- temperature (°C)
- Wp
- pump work (kW)
- X
- solution concentration (%)
Subscripts
- A
- absorber
- C
- condenser
- CG
- condenser–generator
- E
- evaporator
- G
- generator
- h
- high
- HE
- solution heat exchanger
- l
- low
- m
- medium
Greek
- η
- efficiency (dimensionless)
- ν
- specific volume of solution (m3/kg)
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