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Thermodynamic Model of a Single Stage H2O-Libr Absorption Cooling

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E3S Web of Conferences 234, 00091 (2021) https://doi.org/10.1051/e3sconf/202123400091 ICIES 2020 Thermodynamic model of a single stage H2O-LiBr absorption cooling Siham Ghatos1,*, Mourad Taha Janan1, and Abdessamad Mehdari1 1Laboratory of Applied Mechanics and Technologies (LAMAT), ENSET, STIS Research Center; Mohammed V University in Rabat, Morocco. Abstract. Due to the dangerous ecological issues and the cost of the traditional energy, the use of sustainable power source increased, particularly solar energy. Solar refrigeration and air conditioner such as, absorption solar systems denote a very good option for cooling production. In present work, an absorption machine that restores the thermal energy produced by a flat plate solar collector in order to generate the cooling effect was studied. For the seek of good performances, it is necessary to master the modeling of the absorption group by establishing the first and second laws of the thermodynamic cycle in various specific points of the cycle. The main objective was to be able to finally size different components of the absorption machine and surge the coefficient of performance at available and high heat source temperature. A model were developed using a solver based on Matlab/Simulink program. The obtained results were compared to the literature and showed good performances of the adopted approach. 1 Introduction reinforcement techniques (cold or hot). The work of Gomri et al. [4] has shown that when the condenser- This nowadays, air conditioning is generally provided by absorber and the evaporator temperatures are changed classic mechanical compression machines that consume a from 33°C to 39°C, 4°C to 10°C, respectively, the COP considerable amount of electrical energy in addition to the values of the single, double and triple effect absorption toxic fluids used inside. An alternative is to use the solar- system are arranged between 0.73–0.79, 1.22–1.42, 1.62– powered refrigeration cycles instead. Among the possible 1.90, respectively. The exergetic efficiency values for solutions, absorption cycles are the most technologically single, double and triple effect absorption cycle are in the mature and use natural refrigerants such as water. In range of 17.7–25.2%, 14.3–25.1%, 12.5–23.2%, addition, this technology only requires thermal solar respectively. Dynamic simulation of an absorption panels because of the low temperature required. refrigeration endo-reversible system is established to Mechanical compression machines often have a higher minimize the time optimization and reach a maximum coefficient of performance (COP) compared to the solar value of the chiller efficiency using a method that absorption systems, this issue is overcome by the long- combines heat and mass transfer principles [5]. A term efficiency and the pollution elimination. dynamic model of a single-effect Absorption The literature reveals several studies that have been Refrigeration System (ARS) using LiBr–H2O as work conducted on absorption refrigeration systems. Gutiérrez- fluid , to evaluated the influence of thermal masses of each Urueta et al. [1] treated conventional absorption models component, the method is solved by the fourth-order of H2O/LiBr. The characteristic equations used in this Runge–Kutta and the results showed that the thermal load study were developed by Hellmann et al. (1998) and of condenser and generator are reliant on the condenser represent an experimental evaluation of chiller thermal mass. However, the thermal load of the absorber performance. Karamangil et al. [2] have presented a and evaporator are hardly influenced by thermal masses thermodynamic analysis of single-stage absorption [6]. Assilzadeha et al. [7] presented a comparison between applying various refrigerant–absorbent pairs. The conventional mechanical compression and solar comparison between different theoretical system absorption air conditioning systems, they showed that this performances have proved that the coefficient of latter has more advantages. Balghouthi et al. [8] have performance values decreases as the absorber and shown that the high use of conventional air conditioning condenser temperatures decrease, and increase as the technologies has led to increasing environmental evaporator and condenser temperatures increase. A trial problems in addition to the cost of fossil fuels while the work of solar cooling absorption cycles executed in solar resource is free. As for any thermal system, it is Reunion Island was devoted by Marc et al. [3], the necessary to study the evolution of the process and objective is to attain an effective cooling of classrooms determine the main intervening factors that contribute to using a solar-powered cooling machine without any its performance. Ketfi et al. [10] have described the * Corresponding author: [email protected] © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). E3S Web of Conferences 234, 00091 (2021) ICIES’2020 https://doi.org/10.1051/e3sconf/202123400091 ICIES 2020 thermal behavior of different elements of a single-stage • The evolutions in the four primary exchangers solar absorption machine functioning with LiBr-H2O (generator, condenser, evaporator and absorber) was (YAZAKI WCFSC5) with 17.6 kW cool operating in assumed adiabatic. Algerian climate. Ghatos et al. [11] is preferred to work • The refrigerant came out of the condenser were with a generator temperature where the coefficient of considered as saturated liquid at the corresponding performance reaches its maximum value. temperature and pressure. The present work aims to establish and solve a detailed • The heat exchanged with the surroundings and pressure model of a single effect absorption refrigeration system drops were neglected. working with H2O-LiBr binary pair. A simulation • The refrigerant at the outlet of the evaporator was a program has been realized to describe the thermodynamic saturated steam at the temperature and pressure of analysis of different components in a typical single stage evaporation. solar absorption machine. The thermal and physical properties of the binary mixture are calculated in each 2.1 Formulation of analysis model stage of the absorption cycle in order to determine the coefficient of performance, and therefore, improve the The mass and energy balance equations for each heat working conditions of the system. exchanger can be expressed as follows: mw mmm6 78 2 Thermodynamic model XXw 6 XXs 8 2.1 System description mX66 m 7 mX 88 As shown in figure 1, the ARS contains a condenser (c), Q mh mh mh expansion valve (V1), evaporator (e), absorber (a), pump g 77 88 66 (1) (p), reducing valve (V2), a solution heat exchanger (SHE) and generator (g). The terms Qg, Qc, Qa and Qe denote the Qg being the thermal power activation and m the mass heat transfer rate from the generator, condenser, absorber, flow rates. and evaporator, respectively. In the machine operation, With Xs and Xw representing the concentrations of the the water vapor leaving the evaporator is mechanically strong and weak LiBr solution, respectively. h the absorbed by the strong bromide lithium solution (high enthalpy of the solution at different levels of the concentration) in the absorber. The weak solution (low generator. concentration) is pumped from the absorber to the mmmr 71 generator in which the H2O/Libr refrigerant-absorber pair is separated, the superheated refrigerant (water vapor) Qc mh11 h 7 (2) leaves the generator to the condenser and the strong solution goes down to the absorber by a reducing valve. Qc being the heat flux of the condenser With mr the In the condenser, the generated steam is condensed and refrigerant mass flow rate. the water at the high pressure passes via an expansion mmmmm valve to evaporate in the lower pressure in the evaporator r 3217 where the whole process starts again. The heat exchanger Q mh h (3) e 13 2 between the absorber and generator is regularly applied to increase the COP by preheating the weak solution going Qe being the cooling capacity. up to the generator. mmmrsw 0 mXss m ww X 0 Qa mh4 4 mh 3 3 m 10 h 10 (4) Qa being the heat flux of the ab. T95 TE T 8(1 E) m8 h6 h 5 hh 89 (5) m5 Fig. 1. Single effect absorption cooling cycle. With E the thermal efficiency of the solution heat exchanger. The heat exchanger modeling is taken into The specific solution flow rate f is defined as the ratio consideration using the pair (H2O/LiBr) as the operating fluid under the following considerations [12]: of the weak solution mass flow rate ( ) on the 2 E3S Web of Conferences 234, 00091 (2021) ICIES’2020 https://doi.org/10.1051/e3sconf/202123400091 ICIES 2020 refrigerant masse flow rate ( ) generated in the generator [14]: The coefficients Ui are given in table 1 below: Table 1. Coefficients for the calculation of mX f ws (6) m XX i U r sw 0 -7,85823 1 1,8399 The strong and weak solutions concentrations are given 2 -11,781 by Lansing [13] as: 3 22,6705 4 -15,9393 49,04 1,125.T T X ab e (7) 5 1,77516 d 134,65 0,47.Tab The specific heat of the LiBr solution with concentration X is given by [14]: 49,04 1,125.Tgc T X c (8) 134,65 0,47.Tg 2 Cp X 2,01. X00 5,15. X 4,23 Xs: The strong solution concentration. X X 0 (13) Xw: The weak solution concentration. 1 0 0 The coefficient of performance (COP) defined by the ratio The enthalpy of liquid water with temperature T, in the of the amount of heat absorbed by the evaporator on the case of the condenser outlet, is given by [14]: amount of heat supplied to the generator (plus the pumps work if not neglected): hTliq 4.185 T (14) Qe COP (9) The enthalpy of the water vapor, in the case of the Q W gp evaporator, is given by the following empirical equation [14]: Qe: Amount of heat absorbed by the evaporator.
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