Dynamic Simulation of an Absorption Cooling System with Different Working Mixtures

High consumption of electricity represents an economic and social problem in warm places, caused by the massive use of cooling machines. Absorption systems are a sustainable method for air conditioning applications. However, environmental conditions should be analyzed to avoid crystallization problems of the working mixture. This article presents a thermal analysis of a solar absorption cooling system in dynamic conditions using NH3-H2O, H2O-LiBr, NH3-NaSCN, NH3-LiNO3, and H2O-LiCl working mixtures using Equation Engineering Solver (EES) and TRaNsient SYstem Simulation (TRNSYS) software. A solar collector area of 42.5 m2 was selected to carry out the thermal analysis. The results showed that H2O-LiCl obtained the maximum solar (0.67) and minimum heating (0.33) fraction. However, it obtained the maximum lost heat fraction (0.12), in spite of obtaining the best coefficient of performance (COP) among the other working mixtures, due mainly to a crystallization problem. The gain fraction (GF) parameter was used to select the adequate solar collector number for each working mixture. NH3-LiNO3 and NH3-H2O obtained the highest GF (up 6), and both obtained the maximum solar (0.91) and minimum heating (0.09) fraction, respectively, using 88.8 and 100.4 m2 of solar collector area, respectively.


Introduction
Cities located in warm places have a high consumption of electricity, especially in the summertime. This is a socio-economic and environmental problem that is due to the use of air conditioning and refrigeration. They are largely based on the consumption of non-renewable primary resources.
Solar-powered absorption cooling systems use solar heat to drive an absorption chiller and produce a cooling effect in a sustainable way; however, some working mixtures have crystallization problems depending on the changes in environmental temperature, and this is the main problem for these devices because it blocks the internal pipes and stops the chiller operation [1].
In the literature, there are some studies about absorption cooling systems (ACS) in dynamic conditions: Ahmed et al. [2] carried out a simulation based on a performance analysis of a solar assisted single effect absorption cooling system for two system configurations in TRaNsient SYstem Simulation (TRNSYS). In configuration 1 (C-1), the working fluid returning from the generator of the absorption chiller flows toward the hot storage tank. In configuration 2 (C-2), the working fluid returning from the absorption chiller may become isolated from the collector-storage tank loop if the fluid temperature in the storage tank is inferior to the required temperature and is directly fed to the auxiliary boiler. The results showed that C-2 resulted in higher energy saving than C-1 for the whole summer season.
Stanciu et al. [3] analyzed an NH 3 -H 2 O absorption cooling system driven by solar energy. A fully mixed hot water storage tank was used to provide the thermal energy to the generator from the heat collected by a fixed oriented parabolic trough collector. According to the author's results, for the longest continuous operation of the NH 3 -H 2 O cooling system (from 09:00 to 17:10) 10 m × 2.9 m parabolic trough collector (PTC) aperture dimensions with a 0.16 m 3 storage tank volume were calculated at 10 • C of evaporator temperature. In the same year, Stanciu et al. [4] simulated a solar driven cooling system consisting of a single effect H 2 O-LiBr absorption cooling system, a parabolic trough collector, and a storage tank module was analyzed for one full operation day. The simulation was constrained to operate at the maximum exergetic efficiency of the absorption system, under a time-dependent cooling load. A constant evaporator of 10 • C was considered. To set up the solar assembly, two commercial PTCs were selected, namely PT1-IST and PTC 1800 Solitem. The results showed that the best choice for the studied case was the use of two commercial PTC 1800 collectors connected in series with a modular storage tank with capacities of 90 and 140 kg. The simulation incorporated the fact that the system operates continuously between 09:00 to 18:00 on a calm day (0.2 m/s wind velocity). On a windy day (considering a wind speed of 5 m/s), the computations revealed that the operation time interval was reduced to 09:00 to 17:40 due to important energy losses in the PTC.
Sokhansefat et al. [5] carried out a parametric analysis to find the optimum configuration for a 5-ton capacity solar absorption system in transient conditions using TRNSYS software in Tehran. The results indicated that the performance of the setup is capable of 28% enhancement. The optimum values were a collector area of 55 m 2 , storage tank volume of 1 m 2 , temperature set point of the auxiliary boiler of 77 • C, solar collector mass flow rate at 1000 kg/h, and collector slope of 33 • .
Antonopoulos and Rogdakis [6] studied the performance of absorption heat pumps using NH 3 -LiNO 3 and NH 3 -NaSCN in Atenas, Greece. The results showed that NH 3 -LiNO 3 was better than NH 3 -NaSCN for heating because it provides a higher heat-gain factor and useful thermal power. For cooling the NH 3 -LiNO 3 system provides a higher cooling power, while NH 3 -NaSCN achieves a higher coefficient of performance (COP).
Florides et al. [7] presented an H 2 O-LiBr solar absorption cooling system simulation in TRNSYS software under typical conditions in Nicosia, Cyprus. That study consisted of optimizing the storage tank size, slope, and area of the parabolic solar collector. The results showed that the ideal size of the solar collector was 15 m 2 with an inclination of 30 • and 600 L of the storage tank for the generation of 84,240 MJ. Arzoz et al. [8] presented a simulation of a double effect absorption cooling of 3 kW with NH 3 -LiNO 3 using flat solar collectors in Madrid, Spain. The results showed that the equipment could operate about 6 hours of solar cooling at −15 • C, obtaining a COP of 0.3 at a condensation temperature of 40 • C of condensation temperature. The thermal fluid reached a maximum temperature of 100 • C with a solar efficiency of 0.27.
De Francisco et al. [9] carried out a test for the NH 3 -H 2 O absorption refrigeration system at 2 kW. The generated heat was dissipating by natural convection in the absorber and condenser components and the system was manually controlled. The results showed that the COP obtained was very low (0.05); however, the authors concluded that it could be improved using forced convection.
Assim et al. [10] modeled an absorption chiller operated by hot water using an evacuated tube collector in TRNSYS software. It was found that the use of a hot water storage tank and a 12 m 2 collector area were sufficient to maintain the room temperature at 26 • C or below during the cooling season. Agrouaza et al. [11] studied a solar cooling system in several cities of Morocco and it was found that Errachindia city obtained the best average annual solar fraction (30%) and a COP value of 0.33. The COP values varied from 0. 12  higher COP than the NH 3 -H 2 O system, though it has crystallization problems. The NH 3 -LiNO 3 and NH 3 -NaSCN systems are a suitable alternative to NH 3 -H 2 O and both have the advantage of working either refrigeration or air conditioning [12], even though the values of COP are a little higher than the NH 3 -H 2 O system at a high generator temperature. Recently Gogoi and Knowar [13] carried out an energetic and energetic comparison with H 2 O-LiCl and H 2 O-LiBr systems under a similar working condition. They concluded that H 2 O-LiCl obtained a higher COP than the H 2 O-LiBr system. However, H 2 O-LiCl has crystallization problems that depend on environmental temperature changes. Table 1 shows a comparison of working mixtures. Most papers published about the absorption cooling system mainly use only one working mixture in dynamic conditions. The objective of this paper was to carry out a thermodynamic analysis of an ACS using conventional (NH 3 -H 2 O and H 2 O-LiBr) and unconventional (NH 3 -LiNO 3 , NH 3 -NaSCN, and H 2 O-LiCl) working mixtures based on the COP and the limitation on the crystallization of the solution for dynamic conditions.

Description of the System
The present study consists of the cooling of a building via a single stage absorption cooling system using evacuated solar collectors (SC) and an auxiliary system (AS), as shown in Figure 1. The auxiliary system is used to control the temperature coming from the solar collectors to the generator component (GE). COP than the NH3-H2O system, though it has crystallization problems. The NH3-LiNO3 and NH3-NaSCN systems are a suitable alternative to NH3-H2O and both have the advantage of working either refrigeration or air conditioning [12], even though the values of COP are a little higher than the NH3-H2O system at a high generator temperature. Recently Gogoi and Knowar [13] carried out an energetic and energetic comparison with H2O-LiCl and H2O-LiBr systems under a similar working condition. They concluded that H2O-LiCl obtained a higher COP than the H2O-LiBr system. However, H2O-LiCl has crystallization problems that depend on environmental temperature changes. Table 1 shows a comparison of working mixtures. Most papers published about the absorption cooling system mainly use only one working mixture in dynamic conditions. The objective of this paper was to carry out a thermodynamic analysis of an ACS using conventional (NH3-H2O and H2O-LiBr) and unconventional (NH3-LiNO3, NH3-NaSCN, and H2O-LiCl) working mixtures based on the COP and the limitation on the crystallization of the solution for dynamic conditions.

Description of the System
The present study consists of the cooling of a building via a single stage absorption cooling system using evacuated solar collectors (SC) and an auxiliary system (AS), as shown in Figure 1. The auxiliary system is used to control the temperature coming from the solar collectors to the generator component (GE).

Simulation of the Solar Cooling System
The TRNSYS [14] software was used to simulate the solar absorption system for the cooling of a building located in Temixco City, Mexico. Figure 2 shows the schematic diagram of the process. There are two main circuits: solar collector (red line) and cooling (blue line). In the solar collector circuit, heating fluid (water) is pumped (Type 3d) from a storage tank (Type 4a) into the evacuated solar collector (Type 1b) and returned to the tank. This pump is controlled by Type 2b and it is turned on when the output is higher than the input temperature of the solar collector; otherwise, it is turned off. In the cooling circuit, a heating fluid is sent to the auxiliary system (Type 66a) to control the supply temperature and enter towards to the generator component of the absorption cooling system (Type 66a-2). Finally, it is pumped by Type 3d-2 to the tank. The pump Type 3d-2 is controlled by Type 2b-

Simulation of the Solar Cooling System
The TRNSYS [14] software was used to simulate the solar absorption system for the cooling of a building located in Temixco City, Mexico. Figure 2 shows the schematic diagram of the process. There are two main circuits: solar collector (red line) and cooling (blue line). In the solar collector circuit, heating fluid (water) is pumped (Type 3d) from a storage tank (Type 4a) into the evacuated solar collector (Type 1b) and returned to the tank. This pump is controlled by Type 2b and it is turned on when the output is higher than the input temperature of the solar collector; otherwise, it is turned off. In the cooling circuit, a heating fluid is sent to the auxiliary system (Type 66a) to control the supply temperature and enter towards to the generator component of the absorption cooling system (Type 66a-2). Finally, it is pumped by Type 3d-2 to the tank. The pump Type 3d-2 is controlled by Type 2b-2 and it is turned on when the building cooling load (Type 56) was activated to keep an indoor temperature below 25 • C. Type 66a and Type 66a-2 are programmed in Equation Engineering Solver (EES [15]) and both are coupled with the TRNSYS model for an hourly simulation. 2 and it is turned on when the building cooling load (Type 56) was activated to keep an indoor temperature below 25 °C. Type 66a and Type 66a-2 are programmed in Equation Engineering Solver (EES [15]) and both are coupled with the TRNSYS model for an hourly simulation. The TRNSYS software has incorporated modules (types) to model a single-effect absorption chiller from normalized data from catalogs of the manufacturer. However, NH3-LiNO3, NH3-NaSCN, and H2O-LiCl are still not available. Therefore, it was necessary to develop the mathematical model for each working mixture to simulate the ACS (Appendix A presents the basic equations, suppositions, and validation). The interaction between the Type 66a-2 and Type 56 consisted in: the instant cooling capacity of the building is equal to the evaporator component of the ACS. The operation of the ACS incorporated a heat transfer in the generator component of ∆T = 5 °C between the supply and generator temperatures (TSUPPLY = TGE + 5 °C) using water as the heating fluid. The heat transfer between the environment, and the condenser and absorber components involved a ∆T = 10 °C (TCO = TAB = TENV + 10 °C) cooled by air.
Type 66a is the auxiliary system and it was used to control the supply temperature to the ACS by using a heating (QHS) or dissipating (QDS) system.
The solar thermal collector efficiency (Type 1b) was calculated using a quadratic or linear efficiency equation according to ASHRAE or European standards. The solar collector is an HTP-Evacuated Tube HP-30SC [16] with a net aperture area of 3.86 m 2 . The cover is made of a glass vacuum tube and the absorber material is made of tube-copper and steel/plate-glass, with a tilt angle of 19° (corresponding to the location latitude). The solar thermal collector efficiency is presented in Equation (1) [16], where TIN and TENV are the input temperatures of the solar collector and environment temperature, respectively, and QRAD is the global radiation incident (W/m 2 °C). The Type 56 is a multi-zone building with 4 windows and only one zone, with some dimensions and main building characteristics shown in Table 2. Four occupants in a rest position and three computers turned on from 08:00 to 18:00 were considered as heat generation. The TRNSYS software has incorporated modules (types) to model a single-effect absorption chiller from normalized data from catalogs of the manufacturer. However, NH 3 -LiNO 3 , NH 3 -NaSCN, and H 2 O-LiCl are still not available. Therefore, it was necessary to develop the mathematical model for each working mixture to simulate the ACS (Appendix A presents the basic equations, suppositions, and validation). The interaction between the Type 66a-2 and Type 56 consisted in: the instant cooling capacity of the building is equal to the evaporator component of the ACS. The operation of the ACS incorporated a heat transfer in the generator component of ∆T = 5 • C between the supply and generator temperatures (T SUPPLY = T GE + 5 • C) using water as the heating fluid. The heat transfer between the environment, and the condenser and absorber components involved a ∆T = 10 • C (T CO = T AB = T ENV + 10 • C) cooled by air.
Type 66a is the auxiliary system and it was used to control the supply temperature to the ACS by using a heating (Q HS ) or dissipating (Q DS ) system.
The solar thermal collector efficiency (Type 1b) was calculated using a quadratic or linear efficiency equation according to ASHRAE or European standards. The solar collector is an HTP-Evacuated Tube HP-30SC [16] with a net aperture area of 3.86 m 2 . The cover is made of a glass vacuum tube and the absorber material is made of tube-copper and steel/plate-glass, with a tilt angle of 19 • (corresponding to the location latitude). The solar thermal collector efficiency is presented in Equation (1) [16], where T IN and T ENV are the input temperatures of the solar collector and environment temperature, respectively, and Q RAD is the global radiation incident (W/m 2 • C). The Type 56 is a multi-zone building with 4 windows and only one zone, with some dimensions and main building characteristics shown in Table 2. Four occupants in a rest position and three computers turned on from 08:00 to 18:00 were considered as heat generation. The Type 4a is a stratified storage tank. The properties and working conditions supplied were: a heat capacity of 4.19 kJ/kg K, a height of 1 m, an initial temperature of 70 • C, and a loss coefficient = 0.75 W/m 2 K. The characteristics of the thermal insulation are: density = 16 kg/m 3 , thickness = 0.05 m and, thermal conductivity = 0.05 W/m K.

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The pressure drops and heat losses are negligible for each pipeline.

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Fraction pump power (The energy converted to fluid thermal energy, fpar) was 0.05.

Basic Equation (Energy and Mass Balances)
This section presents a brief mathematical description [17] for each component model (Types). The pump (Type 3d) computes a mass flow rate using a variable control function (0 ≤ γ ≤ 1) and a fixed (user-specified) maximum flow capacity (m MAX ).
where fpar is the fraction pump power, p is the power (kW), m OUT is the output mass flow rate (kg/s) and, C p is the heat capacity of the fluid (kJ/kg K). The solar collector (Type 1b) uses a general equation for solar thermal collector efficiency (η) and it is calculated via Equations (4) and (5), where Q U is the useful heat of the solar collector (W), A is the gross area (m 2 ) and, Q RAD is the global radiation incident on the solar collector (W/m 2 ). Equation (4) can be rewritten as: where the constants a 0 , a 1 and a 2 are, respectively, the intercept of the collector efficiency, the negative of the first-order coefficient and the negative of the second-order coefficient. The quadratic Equation (5) can be written as a linear Equation (1) with very similar results. The stratified storage tank (Type 4a) can be modeled by assuming that the tank consists of N fully-mixed equal volume segments. An energy balance written in the ith tank segment (i) is expressed as: where m, T, and t are the mass flow rate, temperature and time, respectively. Subscripts h, L, and f are the solar collector, cooling auxiliary circuits and fluid, respectively (see Section 3), α i and β i are a control function, and U is the loss coefficient between the tank and its environment (kW/m 2 • C). The Quantity integrator (Type 24) integrates a series of quantities over a period of time using the following equation, where X i is the ith quantity to be integrated and Y i is the time integral of X i The multizone building (Type 56) models the thermal behavior of a building having multiple thermal zones, with the thermal capacity of the air volume and capacities which are closely connected with the air node (see Figure 3a). The convective heat that flows to the air node is calculated using Equation (8).
where the subscripts surf, inf, vent, ic, af, solar and asr are the energy gain for convective heat due to the: surfaces, infiltration (air flow from the outside only), ventilation (user-defined air flow), internal convective heat (people, equipment, radiator, etc.), the air flow from air node i or boundary condition, the fraction of solar radiation (due to the external windows) and the absorbed solar radiation on all internal shading devices that is directly transferred as a convective gain to the internal air, respectively. The Quantity integrator (Type 24) integrates a series of quantities over a period of time using the following equation, where Xi is the ith quantity to be integrated and Yi is the time integral of Xi The multizone building (Type 56) models the thermal behavior of a building having multiple thermal zones, with the thermal capacity of the air volume and capacities which are closely connected with the air node (see Figure 3a). The convective heat that flows to the air node is calculated using Equation (8).
where the subscripts surf, inf, vent, ic, af, solar and asr are the energy gain for convective heat due to the: surfaces, infiltration (air flow from the outside only), ventilation (user-defined air flow), internal convective heat (people, equipment, radiator, etc.), the air flow from air node i or boundary condition, the fraction of solar radiation (due to the external windows) and the absorbed solar radiation on all internal shading devices that is directly transferred as a convective gain to the internal air, respectively.
(a) Convective heat flows (b) Radiation heat flows The radiative heat flowing through the wall and window is calculated via Equation (13) (see Figure 3b), where Qw,i represents the radiative gains for the wall surface temperature node, Qsurf,w,i represents the radiative air node internal gains received by the wall, Qsolar,w,i represents the solar gains through the zone windows received by the walls, Qlong,w,i represents the longwave radiation exchange between this wall and all the other walls and windows, and Qgain,w represents the user-specified heat flow to the wall or window surface. All of these quantities are given in kW.
The auxiliary system (Type 66a) is used to control the supply temperature (TSUPPLY), when the storage tank temperature (TT) is lower than TSUPPLY (obtained from Table 3); the heating system (QHS) is activated using a simple energy balance (Equation (14)); otherwise, the dissipation system (QDS) is activated and Equation (15) is applied to it: The radiative heat flowing through the wall and window is calculated via Equation (13) (see Figure 3b), where Q w,i represents the radiative gains for the wall surface temperature node, Q surf,w,i represents the radiative air node internal gains received by the wall, Q solar,w,i represents the solar gains through the zone windows received by the walls, Q long,w,i represents the longwave radiation exchange between this wall and all the other walls and windows, and Q gain,w represents the user-specified heat flow to the wall or window surface. All of these quantities are given in kW.
The auxiliary system (Type 66a) is used to control the supply temperature (T SUPPLY ), when the storage tank temperature (T T ) is lower than T SUPPLY (obtained from is activated using a simple energy balance (Equation (14)); otherwise, the dissipation system (Q DS ) is activated and Equation (15) is applied to it: Table 3. Operation range of the supply temperature range for absorption cooling system (ACS). The Differential controller (Type 2b) is an on/off differential controller that, can have a value of 1 or 0. The value of the control signal is chosen as a function of the difference between upper (Th) and lower (Tl) temperatures, and both are compared with two dead-band temperature differences ∆Th and ∆Tl. The new value of the control function depends on the value of the input control function at the previous time step.
The weather data processor (Type 15-2) serves the purpose of reading data at regular time intervals from an external weather data file, interpolating the data at time steps of less than one hour, and making it available for other TRNSYS components.

Numerical Procedure
The TRNSYS library includes many of the components commonly found in thermal and electrical energy systems, as well as component routines to handle input of weather data or other time-dependent forcing functions and output of simulation results. The default solver uses successive substitutions in order to solve the systems equations. The iterative numerical methods are based on a guess or initial values that are used as starting points for the iterations. If the system's equating is very complex it needs a good guess value [17].
EES software uses a variant of Newton's method [18] to solve systems of non-linear algebraic equations. The Jacobian matrix needed in Newton's method is evaluated numerically at each iteration. Sparse matrix techniques are employed to improve calculation efficiency and permit rather large problems to be solved in the limited memory of a microcomputer. The efficiency and convergence properties of the solution method are further improved by the step-size alteration and implementation of the blocking algorithm which breaks the problems into a number of smaller problems which are easier to solve. Several algorithms are implemented to determine the minimum or maximum value of a specified variable.
EES software supports Dynamic Data Exchange (DDE) commands in order to exchange information with other programs. DDE is a method of sending information to other programs through the Microsoft Windows operating system. Type 66 makes use of DDE commands to launch EES, pass parameter and input information to EES, and ask EES to solve and recuperate output information, which is then treated like the output from any other TRNSYS component [17].

Operation Range of the Auxiliary System (AS)
An analysis of the environment temperature was carried out in order to select the time of operation of the absorption cooling system (ACS). Type 15-2 was built in the TRNSYS software to simulate a year's environmental conditions obtained from meteorological data in Temixco city, Mexico.  shows the environment temperature and global radiation profiles across a year. The period from March to May (1460-3560 h) was selected to operate the absorption system due to the highest ambient temperature in a year (blue box). The maximum and minimum environment temperatures were 16.3 and 33.5 • C, these conditions will be used to dissipate the heat load in the condenser and absorber components of the ACS. It is a limitation of the working operation because ACS could fall into the crystallization zone or low COP depending on the environmental temperature. COP is defined as the ratio of the energy absorbed in the evaporator to the energy supplied by the generator. ACS could fall into the crystallization zone or low COP depending on the environmental temperature. COP is defined as the ratio of the energy absorbed in the evaporator to the energy supplied by the generator. Figure 5 shows the minimum and maximum condenser and absorber temperatures that ACS will operate across the 3 months at 8 °C of the evaporator temperature. H2O-LiCl obtained the highest COP (around 0.80-0.85) at TCO = TAB = 30 °C and NH3-H2O obtained the lowest COP value (lower than 0.70). On the other hand, H2O-LiBr obtained the highest COP (around 0.73) and NH3-H2O the lowest value (lower than 0.53) at TCO = TAB = 44 °C. It is clear that the increment of TCO and TAB, decrease in the range of operation of ACS, mainly in the case of H2O-LiCl, and range of operation of the generator temperature is from 55 to 69 °C (∆T = 14 °C) and from 86 to 87 °C (∆T = 1 °C) at a TCO = TAB of 30 and 44 °C, respectively.  The NH3-LiNO3 and NH3-NaSCN system obtained similar COP values; however, NH3-LiNO3 has a wider operating range than the NH3-NaSCN system. Both mixtures obtained a COP of around 0.68 to 0.72 and 0.60 to 0.63 at TCO = TAB = 30 and 44 °C, respectively. Figure 6 shows the temperature as a function of the solution mass fraction for the absorber input, which is one of the streams most susceptible to fall in the crystallization zone (Point 4, from Figure  1b   ACS could fall into the crystallization zone or low COP depending on the environmental temperature. COP is defined as the ratio of the energy absorbed in the evaporator to the energy supplied by the generator. Figure 5 shows the minimum and maximum condenser and absorber temperatures that ACS will operate across the 3 months at 8 °C of the evaporator temperature.   The NH3-LiNO3 and NH3-NaSCN system obtained similar COP values; however, NH3-LiNO3 has a wider operating range than the NH3-NaSCN system. Both mixtures obtained a COP of around 0.68 to 0.72 and 0.60 to 0.63 at TCO = TAB = 30 and 44 °C, respectively. Figure 6 shows the temperature as a function of the solution mass fraction for the absorber input, which is one of the streams most susceptible to fall in the crystallization zone (Point 4, from Figure  1b The NH 3 -LiNO 3 and NH 3 -NaSCN system obtained similar COP values; however, NH 3 -LiNO 3 has a wider operating range than the NH 3 -NaSCN system. Both mixtures obtained a COP of around 0.68 to 0.72 and 0.60 to 0.63 at T CO = T AB = 30 and 44 • C, respectively. Figure 6 shows the temperature as a function of the solution mass fraction for the absorber input, which is one of the streams most susceptible to fall in the crystallization zone (Point 4, from Figure 1b) corresponding to the same working condition in Figure 5. Crystallization temperatures are obtained mainly when the generator temperature obtained the maximum values; this means that the limit generator temperature is 82 • at T CO = T AB = 30 • C in the case of the H 2 O-LiBr that corresponds to a 0.67 mass fraction and to 45 • C ( Figure 6) located next to the crystallization line, as shown in Figure 7.
Energies 2018, 11, x FOR PEER REVIEW 9 of 19 a 0.67 mass fraction and to 45 °C ( Figure 6) located next to the crystallization line, as shown in Figure  7.
Note that the mass fraction for H2O-LiBr and H2O-LiCl are defined as the ratio of the mass of the absorbent to the mass of the solution, and NH3-LiNO3 and NH3-NaSCN are defined as the ratio of the mass of the refrigerant (NH3) to the mass of the solution.
The Auxiliary system (Type 66a) controls the supply temperature of the ACS in a dynamic way because the storage tank (T) and environment temperatures change throughout the day. When the supply temperature (see Table 3) was lower than the operation range, a heating system (QHS) is turned on, when it was higher than operation range it is turned off and the dissipating heat system (QDH) is turned on. Table 3 shows the operation range of the supply temperature in order to obtain COP values of up to 0.4 or in order to avoid falling into the crystallization zone for each working mixture.  It was necessary to work at several ranges when working in the generator temperature because a wider temperature range limits its function due to the crystallization (except the NH3-H2O system); besides, 135 °C was fixed as the maximum supply temperature.

Simulation Results
The total cooling energy consumption of the building was 1.43 × 10 7 kJ evaluated during 3 months, fixing a maximum indoor temperature of 25 °C. This value was calculated using Type 24, which integrates the instant cooling capacity over the time interval of the summary. Figure 8 shows the behavior of indoor and environment temperatures and the cooling capacity from 2000 to 2160 h (one week); as can be seen the cooling capacity was turned off when the indoor temperature was lower than 25 °C. a 0.67 mass fraction and to 45 °C ( Figure 6) located next to the crystallization line, as shown in Figure  7.
Note that the mass fraction for H2O-LiBr and H2O-LiCl are defined as the ratio of the mass of the absorbent to the mass of the solution, and NH3-LiNO3 and NH3-NaSCN are defined as the ratio of the mass of the refrigerant (NH3) to the mass of the solution.
The Auxiliary system (Type 66a) controls the supply temperature of the ACS in a dynamic way because the storage tank (T) and environment temperatures change throughout the day. When the supply temperature (see Table 3) was lower than the operation range, a heating system (QHS) is turned on, when it was higher than operation range it is turned off and the dissipating heat system (QDH) is turned on. Table 3 shows the operation range of the supply temperature in order to obtain COP values of up to 0.4 or in order to avoid falling into the crystallization zone for each working mixture.  It was necessary to work at several ranges when working in the generator temperature because a wider temperature range limits its function due to the crystallization (except the NH3-H2O system); besides, 135 °C was fixed as the maximum supply temperature.

Simulation Results
The total cooling energy consumption of the building was 1.43 × 10 7 kJ evaluated during 3 months, fixing a maximum indoor temperature of 25 °C. This value was calculated using Type 24, which integrates the instant cooling capacity over the time interval of the summary. Figure 8 shows the behavior of indoor and environment temperatures and the cooling capacity from 2000 to 2160 h (one week); as can be seen the cooling capacity was turned off when the indoor temperature was lower than 25 °C.  Note that the mass fraction for H 2 O-LiBr and H 2 O-LiCl are defined as the ratio of the mass of the absorbent to the mass of the solution, and NH 3 -LiNO 3 and NH 3 -NaSCN are defined as the ratio of the mass of the refrigerant (NH 3 ) to the mass of the solution.
The Auxiliary system (Type 66a) controls the supply temperature of the ACS in a dynamic way because the storage tank (T) and environment temperatures change throughout the day. When the supply temperature (see Table 3) was lower than the operation range, a heating system (Q HS ) is turned on, when it was higher than operation range it is turned off and the dissipating heat system (Q DH ) is turned on. Table 3 shows the operation range of the supply temperature in order to obtain COP values of up to 0.4 or in order to avoid falling into the crystallization zone for each working mixture.
It was necessary to work at several ranges when working in the generator temperature because a wider temperature range limits its function due to the crystallization (except the NH 3 -H 2 O system); besides, 135 • C was fixed as the maximum supply temperature.

Simulation Results
The total cooling energy consumption of the building was 1.43 × 10 7 kJ evaluated during 3 months, fixing a maximum indoor temperature of 25 • C. This value was calculated using Type 24, which integrates the instant cooling capacity over the time interval of the summary. Figure 8 shows the behavior of indoor and environment temperatures and the cooling capacity from 2000 to 2160 h (one week); as can be seen the cooling capacity was turned off when the indoor temperature was lower than 25 • C.
QtIN is the total energy input to the system due to the solar collectors (QtSC) and heating system (QtHS). QtLH is the lost heat caused by the dissipating heat system (QtDH) and the storage tank (QtT). Figure 9 shows the number of solar collectors as a function of energy fractions, fixing a storage tank volume of 2 m 3 and a flow rate of 1000 kg/h for pumps P1 and P2. A collector area of 42.5 m 2 (11 solar collectors) was selected to compare each absorption system because this is the minimum number to satisfy the adequate relationship between the solar and heating fraction for all working mixtures. Note that the H2O-LiCl system can be satisfied using 30 m 2 of collector area. Moreover, the lost heat fraction (red line) has a constant value around 0.07 for all working mixtures with few solar collector areas. It was caused by the energy loss from the storage tank; however, when the solar collector is increased, the LF increases significantly, mainly for H2O-LiBr and H2O-LiCl systems, and this is because the dissipating heating system is activated.
Qt IN is the total energy input to the system due to the solar collectors (Qt SC ) and heating system (Qt HS ). Qt LH is the lost heat caused by the dissipating heat system (Qt DH ) and the storage tank (Qt T ). Figure 9 shows the number of solar collectors as a function of energy fractions, fixing a storage tank volume of 2 m 3 and a flow rate of 1000 kg/h for pumps P 1 and P 2 . A collector area of 42.5 m 2 (11 solar collectors) was selected to compare each absorption system because this is the minimum number to satisfy the adequate relationship between the solar and heating fraction for all working mixtures. Note that the H 2 O-LiCl system can be satisfied using 30 m 2 of collector area. Moreover, the lost heat fraction (red line) has a constant value around 0.07 for all working mixtures with few solar collector areas. It was caused by the energy loss from the storage tank; however, when the solar collector is increased, the LF increases significantly, mainly for H 2 O-LiBr and H 2 O-LiCl systems, and this is because the dissipating heating system is activated. number to satisfy the adequate relationship between the solar and heating fraction for all working mixtures. Note that the H2O-LiCl system can be satisfied using 30 m 2 of collector area. Moreover, the lost heat fraction (red line) has a constant value around 0.07 for all working mixtures with few solar collector areas. It was caused by the energy loss from the storage tank; however, when the solar collector is increased, the LF increases significantly, mainly for H2O-LiBr and H2O-LiCl systems, and this is because the dissipating heating system is activated.     Table 4 shows a resume of energy fractions evaluated over 3 months. The numbers that appear on the left side mean the best (1) and the worst (5) condition values. H 2 O-LiCl obtained the maximum solar (0.67) and minimum heating (0.33) fraction; however, it obtained the maximum (0.12) lost heat fraction. This last value means that 12% of the energy supplied (Q IN ) was lost in the storage tank and the dissipating heat system. Note that SF + HF = 1.  Figure 10 shows the COP behavior for each working mixture across one week from using 11 solar collectors (42.5 m 2 ); as can be seen the H2O-LiCl system obtained the highest COP value (around 0.78-0.86), H2O-LiBr obtained 0.70-0.83 and the NH3-LiNO3, NH3-NaSCN and NH3-H2O systems obtained around 0.50-0.71.  Table 4 shows a resume of energy fractions evaluated over 3 months. The numbers that appear on the left side mean the best (1) and the worst (5) condition values. H2O-LiCl obtained the maximum solar (0.67) and minimum heating (0.33) fraction; however, it obtained the maximum (0.12) lost heat fraction. This last value means that 12% of the energy supplied (QIN) was lost in the storage tank and the dissipating heat system. Note that SF + HF = 1.   Table 5 shows the total energy of the solar collector (Qt SC ), heating (Qt HS ), storage tank (Qt LH,T ) and dissipation heat (Qt LH,D ) evaluated during 3 months for each working mixture using 42.5 m 2 of collector area. The NH 3 -LiNO 3 system obtained the maximum value of energy from solar collectors (2.13 × 10 7 kJ) and the H 2 O-LiBr system was the worst (2.04 × 10 7 kJ). It was caused because the H 2 O-LiBr system obtained a better COP value than the NH 3 -LiNO 3 system; therefore, the storage tank temperature starts to increase, reducing the solar collector efficiency. Table 5. Total energy using 11 solar collectors (42.5 m 2 ).
The lost energy was caused mainly by the storage tank. The NH3-LiNO3 system obtained the lowest value of lost energy in the storage tank (2.14 × 10 6 kJ) and H2O-LiBr obtained the highest (2.42 × 10 6 kJ), caused by the highest storage tank temperature. The lost energy due to the dissipating system was only activated for the H2O-LiCl and H2O-LiBr systems. Figures 11 and 12 show the effect volume and the thickness of the insulator of the storage tank on the solar and heating fraction, respectively. The increment of the tank volume slightly lowers the values of SF, which positively affects the HF values.

H 2 O-LiBr NH 3 -H 2 O NH 3 -NaSCN NH 3 -LiNO 3 H 2 O-LiCl
Collector area = 42.5 m 2 Figure 11. The energy fraction as a function of the volume tank using different working mixtures. The increment of the insulation thickness of the tank positively affects the solar fraction and negatively affects the heating fraction. Moreover, when the insulation thickness was higher than 0.08 m, the energy fractions were slightly affected by it. Figure 13 shows that the increments of the solar collector area, the solar and lost heat fraction increase too. The maximum SF obtained was around 0.90 for the H2O-LiBr and H2O-LiCl systems using 77.20 m 2 of collector area and was 0.94 for the other working mixtures using 108.1 m 2 of collector area. After these values, the increment of the solar collector area will affect softly to the SF because the dissipation heat system is activated to avoid crystallization problems or COP values lower than 0.4 and because the generator temperature was limited to 135 °C.
The solar fraction shows a linear behavior from 15 to 50 m 2 of collector area for all working mixtures, after which the increment shows a logarithmic behavior because the energy lost (LF) starts to increase due to the dissipation system. In order to select the adequate number of solar collectors for each working mixture, a maximization of the gain fraction (GF) parameter was carried out. This parameter represents the best relationship between the solar energy and the energy that is "not desirable" (the heating system and lost heat from the storage tank and dissipation system). Each working system obtained a maximum value of GF of 2.3, 3.8, 4.7, 6.1 and 6.6 for the H2O-LiCl, H2O-LiBr, NH3-NaSCN, NH3-H2O and NH3-LiNO3 systems, respectively as is shown in Figure 14. Table 6 shows the number of solar collectors for each working mixture system and the energy fractions based on the GF. The H2O-LiCl system obtained the minimum solar collector area (54.0 m 2 ), however, it obtained the maximum lost heat fraction (0.20) and the lowest solar fraction value (0.78). It was caused by the short working operation due to the crystallization problem. Meanwhile, the NH3-LiNO3 and NH3-H2O systems obtained the maximum solar (0.91) and minimum heating (0.09) fraction, using an area solar collector of 88.8 and 100.4 m 2 , respectively. This means that this system could operate at a higher solar collector number with low lost energy. The increment of the insulation thickness of the tank positively affects the solar fraction and negatively affects the heating fraction. Moreover, when the insulation thickness was higher than 0.08 m, the energy fractions were slightly affected by it. Figure 13 shows that the increments of the solar collector area, the solar and lost heat fraction increase too. The maximum SF obtained was around 0.90 for the H 2 O-LiBr and H 2 O-LiCl systems using 77.20 m 2 of collector area and was 0.94 for the other working mixtures using 108.1 m 2 of collector area. After these values, the increment of the solar collector area will affect softly to the SF because the dissipation heat system is activated to avoid crystallization problems or COP values lower than 0.4 and because the generator temperature was limited to 135 • C. The increment of the insulation thickness of the tank positively affects the solar fraction and negatively affects the heating fraction. Moreover, when the insulation thickness was higher than 0.08 m, the energy fractions were slightly affected by it. Figure 13 shows that the increments of the solar collector area, the solar and lost heat fraction increase too. The maximum SF obtained was around 0.90 for the H2O-LiBr and H2O-LiCl systems using 77.20 m 2 of collector area and was 0.94 for the other working mixtures using 108.1 m 2 of collector area. After these values, the increment of the solar collector area will affect softly to the SF because the dissipation heat system is activated to avoid crystallization problems or COP values lower than 0.4 and because the generator temperature was limited to 135 °C.
The solar fraction shows a linear behavior from 15 to 50 m 2 of collector area for all working mixtures, after which the increment shows a logarithmic behavior because the energy lost (LF) starts to increase due to the dissipation system. In order to select the adequate number of solar collectors for each working mixture, a maximization of the gain fraction (GF) parameter was carried out. This parameter represents the best relationship between the solar energy and the energy that is "not desirable" (the heating system and lost heat from the storage tank and dissipation system). Each working system obtained a maximum value of GF of 2.3, 3.8, 4.7, 6.1 and 6.6 for the H2O-LiCl, H2O-LiBr, NH3-NaSCN, NH3-H2O and NH3-LiNO3 systems, respectively as is shown in Figure 14. Table 6 shows the number of solar collectors for each working mixture system and the energy fractions based on the GF. The H2O-LiCl system obtained the minimum solar collector area (54.0 m 2 ), however, it obtained the maximum lost heat fraction (0.20) and the lowest solar fraction value (0.78). It was caused by the short working operation due to the crystallization problem. Meanwhile, the NH3-LiNO3 and NH3-H2O systems obtained the maximum solar (0.91) and minimum heating (0.09) fraction, using an area solar collector of 88.8 and 100.4 m 2 , respectively. This means that this system could operate at a higher solar collector number with low lost energy.   The solar fraction shows a linear behavior from 15 to 50 m 2 of collector area for all working mixtures, after which the increment shows a logarithmic behavior because the energy lost (LF) starts to increase due to the dissipation system. In order to select the adequate number of solar collectors for each working mixture, a maximization of the gain fraction (GF) parameter was carried out. This parameter represents the best relationship between the solar energy and the energy that is "not desirable" (the heating system and lost heat from the storage tank and dissipation system). Each working system obtained a maximum value of GF of 2.3, 3.8, 4.7, 6.1 and 6.6 for the H 2 O-LiCl, H 2 O-LiBr, NH 3 -NaSCN, NH 3 -H 2 O and NH 3 -LiNO 3 systems, respectively as is shown in Figure 14. Table 6 shows the number of solar collectors for each working mixture system and the energy fractions based on the GF. The H 2 O-LiCl system obtained the minimum solar collector area (54.0 m 2 ), however, it obtained the maximum lost heat fraction (0.20) and the lowest solar fraction value (0.78). It was caused by the short working operation due to the crystallization problem. Meanwhile, the NH 3 -LiNO 3 and NH 3 -H 2 O systems obtained the maximum solar (0.91) and minimum heating (0.09) fraction, using an area solar collector of 88.8 and 100.4 m 2 , respectively. This means that this system could operate at a higher solar collector number with low lost energy.

Conclusions
This paper presented a thermal analysis of the absorption cooling system with different working mixtures in a dynamic condition for conditioning a building located in Temixco, Mexico using TRNSYS and EES software. In this study, we analyzed the behavior of the system as a function of the number of evacuated solar collectors and the volume of the storage tank. The following conclusions are presented: • The COP of the absorption cooling system analyzed the condenser and absorber temperatures at 30 and 44 °C at a fixed evaporator temperature of 8 °C, in order to select the operation range of the generator temperature to avoid obtaining values lower than 0.4 for the COP and crystallization zone, after which different condenser and absorber temperature ranges were set. • A solar collector area of 42.5 m 2 was selected to carry out the thermal analysis because it presented the adequate relationship between the energy fraction of the heating system and the solar collectors. The H2O-LiCl system obtained the maximum solar fraction (0.67) and the minimum heating fraction (0.33); however, it obtained the maximum lost heat fraction (0.12), in spite of obtaining the best COP among the other working mixtures. The NH3-LiNO3 system obtained the maximum value of energy from solar collectors (2.13 × 10 7 kJ) and the H2O-LiBr system was the worst (2.04 × 10 7 kJ) because it obtained a better COP than the NH3-LiNO3 system. Therefore, the storage tank temperature starts to increase, reducing the solar collector efficiency. • The increment of the tank volume decreases the values of SF slightly, which positively affects

Conclusions
This paper presented a thermal analysis of the absorption cooling system with different working mixtures in a dynamic condition for conditioning a building located in Temixco, Mexico using TRNSYS and EES software. In this study, we analyzed the behavior of the system as a function of the number of evacuated solar collectors and the volume of the storage tank. The following conclusions are presented:

•
The COP of the absorption cooling system analyzed the condenser and absorber temperatures at 30 and 44 • C at a fixed evaporator temperature of 8 • C, in order to select the operation range of the generator temperature to avoid obtaining values lower than 0.4 for the COP and crystallization zone, after which different condenser and absorber temperature ranges were set. • A solar collector area of 42.5 m 2 was selected to carry out the thermal analysis because it presented the adequate relationship between the energy fraction of the heating system and the solar collectors. The H 2 O-LiCl system obtained the maximum solar fraction (0.67) and the minimum heating fraction (0.33); however, it obtained the maximum lost heat fraction (0.12), in spite of obtaining the best COP among the other working mixtures. The NH 3 -LiNO 3 system obtained the maximum value of energy from solar collectors (2.13 × 10 7 kJ) and the H 2 O-LiBr system was the worst (2.04 × 10 7 kJ) because it obtained a better COP than the NH 3 -LiNO 3 system. Therefore, the storage tank temperature starts to increase, reducing the solar collector efficiency.

•
The increment of the tank volume decreases the values of SF slightly, which positively affects the HF values. On the other hand, the increment of the insulation thickness of the tank positively affects the solar fraction and negatively affects the heating fraction. Moreover, when the insulation thickness was higher than 0.08 m, the energy fractions were slightly affected by it.

•
The maximum solar fraction obtained was around 0.90 for the H 2 O-LiBr and H 2 O-LiCl systems using 77.20 m 2 of collector area and was 0.94 for the other working mixtures using 108.1 m 2 of collector area. This is because the dissipation heat system was activated to avoid crystallization problems or COP values lower than 0.4 and because the generator temperature was limited to 135 • C. The H 2 O-LiBr thermodynamic properties were obtained from the publication of Patek and Klomfar [20] programed as a library routine in the EES software. The COP obtained in this study was compared with that of the Gogoi and Konwar [13] simulation at different temperatures conditions as shown in Table A1. The COP difference percent was lower than 2.6% between the present simulation and the bibliography.

. H 2 O-LiCl System
The H 2 O-LiCl thermodynamic properties were obtained from the publication of Patek and Klomfar [21] (heat capacity) and Conde [22] (concentration, density, and enthalpy). The COP obtained in this study was compared with that of the Gogoi and Konwar [13] simulation at different temperatures conditions as shown in Table A2. The COP difference percent was lower than 10% between the present simulation and the bibliography. The NH 3 -LiNO 3 thermodynamic properties were obtained from the publication of Infante Ferreira [23]. Table A3 shows the values of COP obtained from the Sun simulation [12] and from the present work at an evaporator temperature = −5 • C and condenser and absorber = 25 • C. The COP difference percent is lower than 8%.

-NaSCN System
The NH 3 -NaSCN thermodynamic properties were obtained from the publication of Ferreira [23]. Table A4 shows the values of COP obtained from the Sun simulation [12] and the present work at an evaporator temperature = −5 • C and condenser and absorber = 25 • C. The COP difference percent is lower than 8%.

. NH 3 -H 2 O System
The NH 3 -H 2 O thermodynamic properties were obtained from the publication of Ibrahim and Klein [24] programmed as an EES Library routine in the EES software. Table A5 shows the values of COP obtained from the Sun simulation [12] and the present work at an evaporator temperature = −5 • C and condenser and absorber = 25 • C. The COP difference percent is lower than 13%.