Performance analysis and optimization of new double effect lithium bromide–water absorption chiller with series and parallel flowsAnalyse des performances et optimisation d’un nouveau refroidisseur double effet à absorption de bromure de lithium-eau avec des écoulements en série et en parallèle
Introduction
Today, absorption chillers are widely used because of some their advantages such as the possibility of using waste energy from industrial processes, power plants, internal combustion engines, distributed generation, and combined heat and power systems. Also, they can use renewable energy sources such as biomass, geothermal and solar energies.
In addition, another benefits of these systems are regarding to the refrigerants and absorbents that the most common working fluids are LiBr–water and water–NH3, which do not have ozone depletion potential and their global warming potentials are negligible, so they are eco-friendly (Srikhirin and Aphornratana, 2001, Hong et al., 2016, Martínez et al., 2016, Liao and Radermacher, 2007, Chua et al., 2000, Xu et al., 2013).
Absorption chillers with water–LiBr working fluid pair at various sizes from small and medium scales (Güido et al., 2018) to large scales (Jayasekara and Halgamuge, 2014) have been analyzed in different studies.
Also, single effect and double effect absorption chillers with water–LiBr as working fluid have been very usual at these years in different applications (Xu and Wang, 2014).
So far, different studies on the single effect and double effect LiBr–water absorption chillers have been conducted.
Xu and Dai (1997) simulated double effect absorption chillers with parallel flow. They studied the effect of design parameters on COP. Their results showed that with increase in heat recovery ratio in high temperature and low temperature heat exchangers or with reduction of solution circulation ratio, the COP was increased. Also, they illustrated that the effect of pressure was negligible in the high pressure generator, evaporator and condenser on total heat transfer surface area and the COP in the range of design values.
Gomri (2009) compared single and double effect absorption chillers with series cycle according to the first and second laws of thermodynamics and studied the effect of some operating parameters on system performance without consideration of crystallization conditions. He showed that the COP of the cycle was improved with increasing the generator and evaporator temperatures, but it was reduced with condenser temperature rising.
Avanessian and Ameri (2014) studied on three absorption chiller that contains single effect with hot water heat source, hot water double effect series flow and directed-fire double effect series flow in different climates. The crystallization conditions wan not considered in their study. The results showed that with increasing the temperature of generator and evaporator, the energy utilization factor (EUF) was increased, but this factor was decreased with rise of the temperature and humidity of ambient air.
Xu et al. (2015) studied a variable effect H2O–LiBr absorption chiller based on an actual prototype. The effect of heat source temperatures on cooling capacity and COP were estimated for better performance of the system. In addition, the steady state and transient conditions of the system was studied without consideration of crystal formation limits. Furthermore, the impacts of cooling and chilled water temperatures, valve opening and frequency of pump on cooling capacity and COP were analyzed. These results showed that by increasing the generation temperature the COP was increased.
Azhar and Altamush Siddiqui (2017) presented a thermodynamic analysis of single, double and triple effect H2O–LiBr absorption cycles with series arrangements, that the energy sources were compressed natural gas and liquefied petroleum gas. The performance of cycles was simulated and optimum operating temperatures obtained for minimum gas requirement and maximum COP. The simulation results were compared with the experimental and analytical results that mentioned in their literature. They showed that the triple effect cycle had the higher COP and its gas consumption was lower than the other cycles.
Zheng et al. (2016) presented a series flow double effect water–LiBr absorption heat pump with steam as heat source and its performance was studied at part load and nominal operation without consideration of crystal formation conditions. Furthermore, the impacts of operating parameters such as main temperature differences on the total heat transfer areas and the COP at cooling and heating modes of operation were estimated. The mathematical models were compared with experimental data, and showed good agreement at cooling and heating modes. In addition, the life cycle costs (LCC) of the system included the initial investment, the operating and the discard costs and the equivalent uniform annual costs (EUAC) which was a parameter considering the time value of the cost were introduced and a thermoeconomic optimization analysis was performed to minimizing them.
So far, effect of some key parameters such as the temperature of HTG, inlet water temperature to low temperature condenser, absorber, and evaporator, and also the efficiency of high and low temperature heat exchangers on COP have been analyzed in double effect absorption refrigeration cycle (Gomri, 2009; Somers, 2009; Kaushik and Arora, 2009).
Double effect absorption refrigeration cycles by parallel flow against series cycles have different structures. Marcos et al. (2011) analyzed a double effect absorption refrigeration cycle with parallel flow. In their work the effect of some parameters such as the temperatures of low temperature condenser (LTC), absorber and evaporator on COP were studied. They supposed same variation ranges for solution concentration in each generator and estimated the optimum COP based on this consideration. In addition, they validated the results with experimental data.
Ketfi et al. (2017) analyzed performance of small scale water–LiBr absorption chillers in five various climate regions in Algeria. In their study, two commercial water–LiBr absorption chillers including single effect and double effect with parallel flow were selected and their performance was analyzed. Accordingly, a mathematical model was performed based on energy and mass balances of the cycles and a Matlab simulation code was presented for evaluating the operating conditions of the systems. In addition, the impact of heat source temperature on cooling capacity and COP were estimated at two different temperatures of chilled water.
Lizarte and Marcos (2016) studied a triple effect water–LiBr absorption chiller with parallel flow. In their work, a computer simulations were performed for system analysis at different values of condenser and evaporator temperatures and for all generators, the concentration changes were considered with a same value. In addition, the optimum value of COP based on outlet temperature of high temperature generator, at different condensation and evaporation temperatures were estimated and appropriate amounts of solution mass flow rates for each generator were presented.
Shiue et al. (2018) performed an experimental analysis for a double effect water–LiBr with parallel flow. Also, a set of thermodynamic equations based on energy balance were developed to estimate COP of the chiller. In addition, impacts of effectiveness of heat exchangers, temperatures of absorber, condenser, and high pressure generator, and part load operation of system were investigated. Furthermore, by using direct search method and changes of the mentioned temperatures, the optimum value of COP was estimated by using EES software and appropriate values of operating parameters were presented at this condition.
The crystallization conditions wan not considered in their study.
Arun et al. (2001) performed a thermal analysis of double effect series and parallel flow absorption H2O–LiBr system for a theoretical cycle. In their work the effect of evaporator and condenser temperatures on COP were studied. They illustrated that at conventional range of operating conditions, the maximum COP for the parallel flow system was greater than that for the series flow system. In addition, the results showed that the impact of evaporator temperature was more effective than condenser and absorber temperatures on COP of parallel flow system.
Although, many studies have been done on the analysis and modeling of absorption chillers with series and parallel flow, but comprehensive studies that can perform comparisons of these systems in various operating conditions are few. In the most of previous works, only one type of cycle arrangements (series or parallel) were analyzed and impacts of operating parameters on this series or parallel arrangement were studied. In addition, the crystallization limit have not been investigated in many of the previous studies.
While, in this study addition to analysis of operating parameters on performance of both cycles, the series and parallel cycles have been compared with each other at different conditions and optimum COP of the systems have been investigated by using an evolutionary algorithm. In Table 1, some of main features of previous studies are summarized about modeling and analyzing of water–LiBr absorption chillers.
In this study, two novel arrangements of double effect absorption chillers with series and parallel flow are proposed that have an additional heat recovery heat exchanger in comparison with previous conventional absorption chillers. Also, the effects of some parameters such as temperatures and mass flow rates of inlet vapor of generator and inlet water of absorber on COP are investigated. In this research, the crystallization limit is considered and the operating conditions are such that crystallization does not occur. In addition, the performances of the series and parallel cycles are compared with each other at different conditions and best choices in these cases are proposed. Furthermore, two new parameters based on mass ratio are defined and their effects on the system performance are analyzed. Finally, the Genetic Algorithm as an evolutionary algorithm method has been used to optimization of COP and presentation appropriate parameters at series and parallel flow cycles. Although different works have been done in modeling of these cycles, the present work can be an appropriate complement to them, which provides a comprehensive analysis, and with consideration of the limitation of crystal formation, the results can be applicable in actual cycles.
A brief summary of the main contributions and innovations of this paper in comparison with the previous studies can be presented as:
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Presenting new arrangements for both of series and parallel cycles with an additional heat recovery device.
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Consideration of the crystallization limit in the system modeling and optimizing.
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Investigating the effects of some effective parameters such as temperatures and mass flow rates of inlet vapor of generator and inlet water of absorber on COP for both series and parallel cycles.
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Comparison of series and parallel cycles with each other at different operating conditions and presenting proper values of system performances.
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Presenting two new parameters based on mass ratio and analyzing their impacts on the system performance.
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Optimization of the system COP with Genetic Algorithm method and presenting the appropriate parameters at series and parallel flow cycles.
Section snippets
Double effect refrigeration cycle with series flow
In this paper, a series cycle is designed and modified according to the absorption chiller with series flow diagram of a HVAC company (World Energy Absorption Chiller Catalog). Fig. 1(a) shows the schematic diagram of the series cycle. The main parts of this cycle are a low temperature generator (LTG), a high temperature generator (HTG), an absorber, an evaporator, a condenser, an expansion valve, a heat exchanger, a heat recovery heat exchanger, and diluted (weak) solution pumps. Steam is used
System modeling
The mass, concentration and energy balance equations are used for both series (Fig. 1(a)) and parallel (Fig. 1(b)) cycles.
Crystallization conditions
In the modeling of two series and parallel cycles, three pressure levels, high, medium, and low pressures are considered and the pressure drops in all components of the cycle are neglected. The greatest probability of crystal formation in the lithium bromide–water solution circulation path, located between LTHX and absorber, due to its high concentration and temperature degradation. The highest probability of lithium bromide crystal formation for series cycle is at point 13 (Fig. 1(a)) and for
Optimization
In this section, using the genetic algorithm (GA) optimization method of the MATLAB software the best COP as objective function is calculated with the condition that no lithium bromide crystal is formed.
In the series cycle, the decision variables that can be modified by absorption chiller constructors are chosen as T1, , and , and in the parallel cycle, they are T1, , , and R1.
The condition for stopping the genetic algorithm are 50 consecutive generation without changing the value
Validation
In this paper, two novel arrangements of double effect series and parallel absorption chiller were modeled based on the mass and energy equations and a simulation code was developed via Engineering Equation Solver (EES). The thermophysical properties of mixtures of water and lithium bromide are implemented as an externally compiled EES routine (Engineering Equation Solver; http://fchart.com/ees/libr_help/libr_ashrae/libr_external_routines.htm). This library provides property data for mixtures
Conclusion
In this study, two new arrangements of double effect LiBr–water absorption chillers with series and parallel flow were proposed that had an additional heat recovery heat exchanger compared to previous conventional absorption chillers. The effects of some important parameters such as temperatures and mass flow rates of inlet vapor of high temperature generator and inlet water of absorber on COP were studied. The operating conditions were such that crystallization did not occur. In addition, the
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