The Journal of Energy: Engineering & Management74-40Vol. 2, No. 4, Winter 1391, P.

Transient Characteristics of a Single-EffectAbsorption Refrigeration Cycle

A. Iranmanesh1, M.A. Mehrabian2*

1 Department of Mechanical Engineering (MSc student), Shahid Bahonar University of Kerman, Kerman, I.R. Iraniranmanesh.aghil@gmail.com

2 Department of Mechanical Engineering (Professor), Shahid Bahonar University of Kerman, Kerman, I.R. Iranma_mehrabian@uk.ac.ir

Received: 20 August 2011 Revised: 17 February 2012Accepted: 7 July 2012

AbstractThis paper deals with a lumped-parameter dynamic simulation of a single-effect LiBr-H2O absorption chiller. In many studies thethermodynamic properties of LiBr-H2O solution were taken from some approximate relations causing the results to be somewhatinaccurate. These relations were used to solve simultaneous differential equations involving the continuity of species constituting theLiBr-H2O solution, momentum equations, and energy balances. To diminish the effect of these approximate relations on the results,in this study the thermodynamic properties were taken from the EES software. By making a link between EES and MATLABsoftwares, the simultaneous differential equations were solved in MATLAB environment and this process was continued until theconvergence criterion was satisfied. Moreover, this study considers the effect of quality on the concentration of solution at the exitsof generator and absorber. This effect was ignored in the previous works. In other words, the concentrations of solution at thegenerator and absorber were not assumed to be equal to the corresponding concentration at the exit of those components in thismodel. Furthermore, a transient analysis of exergy is accomplished. As time passes, both the coefficient of performance and exergeticefficiency decrease, approaching the steady-state values. The results deduced from the dynamic simulation are compared with thosein steady-state condition. The comparison shows that the transient simulation predictions are in close agreement with steady-stateresults.

Keywords: Absorption chillers, LiBr-H2O, Single-effect, Transient behavior.

* Corresponding Author

41 Transient Characteristics of a Single-Effect…

1. IntroductionNowadays, researchers are prompted to reconsider the

absorption refrigeration systems because of theincreasing anxiety on the depletion of ozone layer andglobal warming. Absorption systems use the workingfluids not dangerous to the environment and the inputenergy to the absorption chiller is heat, rather thanmechanical work. Different heat sources can be appliedin the absorption chillers such as solar and geothermalenergy, as well as waste heat released from industrialmachines.

Many studies were conducted on the steady-state anddynamic simulation of absorption chillers so far. Jeong,et al. [1] developed a numerical model for the dynamicsimulation of a steam-driven LiBr-H2O absorption heatpump for recovering low-grade waste heat. Storage termswith thermal capacities and solution mass storage wereassumed to be constant in this model. The solution andvapor mass flow rates were determined using the pressuredifferences between the vessels. Time step and heattransfer coefficients in the simulation were assumed to beconstant. Sugano, et al. [2] developed a dynamic modelfor the absorption chiller driven by hot water and thismodel was used for controlling the system in part loadapplications. Cai, et al. [3] studied the transient behaviorof a single-effect absorption chiller using ionic liquids.Several relations were used to predict the thermodynamicproperties of ammonia-water solution causing the resultsto be erroneous. Furthermore, the concentrations ofsolution at the exit of the absorber as well as thegenerator were assumed to be equal to the concentrationinside the absorber and generator, respectively.Kohlenbach and Ziegler [4] developed a dynamic modelfor a small single-effect absorption chiller on the basis ofexternal and internal steady-state enthalpy balances. Theresults obtained from the simulation were compared withthe experimental data. Zinet, et al. [5] developed adynamic model for the simulation of a single-effect LiBr-H2O absorption chiller. The machine was driven by twodistinct heat sources. In contrast to previous works, thefocus was on the detailed physical modeling of the heatand mass transfer phenomena occurring in theevaporator–absorber and in the desorber. The purpose ofthat approach was to reduce the dependence of the modelon empirical and global parameters. An absorption chillermodel used in an existing solar cooling system wasdeveloped by Marc, et al. [6] and presented the solarcooling plant, the metrology, and the control strategy.Secondly, the experimental results were analyzed and thesteady-state chiller model and also the identificationmethod were developed. Afterwards, the simplex methodwas used to determine the design parameters of themachine. An improved control algorithm that was provento have high control performance was presented by Seo etal. [7]. Improving control performance was an importantarea for more complicated systems, such as triple-effectchillers or solar hybrid chillers. It was concluded thatlevel control performance could be improved to varywithin a narrower band by adding an additional levelswitch. Thermodynamic modeling of a double-effect

LiBr-H2O absorption refrigeration cycle wasaccomplished by Iranmanesh and Mehrabian [8].Conductance of all components was evaluated based onthe approach temperatures assumed as input parameters.The effect of input data on the cycle performance and theexergetic efficiency were also investigated.

This study deals with the dynamic analysis of asingle-effect absorption chiller by considering the effectof quality on the solution concentration. Dynamicapproach can be applied for controlling purposes. Thefourth order Range-Kutta method is selected to solve thesimultaneous equations deduced from continuity,momentum, and energy balances. The results inferredfrom the dynamic analysis are compared with the datafrom the steady-state condition. Obtaining majorparameters such as coefficient of performance, secondlaw efficiency, etc. as a function of time are the otherobjectives of this research.

2. Formulation of the dynamic model

2.1 Governing equationsThe schematic diagram of a single-effect LiBr-H2O

absorption chiller is illustrated in Fig. 1. Each maincomponent is characterized by a single pressure,temperature, and concentration. Several assumptions aremade to simplify the simulation:

The amount of heat diffusion in the flow direction isnegligible.Neither the generator releases any heat to thesurroundings nor does the evaporator receive any heatfrom the surroundings.The refrigerant at the exit of the evaporator is assumedto be at the state of saturated vapor.The refrigerant at the exit of the generator is assumedto be at the state of superheated vapor.The concentration of solution leaving the generator andabsorber is different from the concentration inside thegenerator and absorber respectively.The input work of the pump is negligible.There is no pressure loss in the pipes.The fluid temperature at the exit of each component isthe same as the temperature inside that component.

The governing equations for the dynamic analysis are:

Mass continuity:

outini mm

dtMd )( (1)

Continuity of LiBr and water species:

1146)( XmXmZMdtd

AA

(2)

4413)( XmXmZMdtd

GG

(3)