Energy Conversion and Management 88 (2014) 545–553 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman Energetic and exergetic study of a 10RT absorption chiller integrated into a microgeneration system A.A.V Ochoa a,b,⇑, J.C.C Dutra b, J.R.G Henríquez b, J Rohatgi b a b Federal Institute of Technology of Pernambuco, Av Prof Luiz Freire, 500, CEP 50740-540 Recife, PE, Brazil Federal University of Pernambuco, Av Prof Moraes Rego, 123, CEP 50670-901 Recife, PE, Brazil a r t i c l e i n f o Article history: Received 29 May 2014 Accepted 27 August 2014 Keywords: Cogeneration Exergy Chiller COP a b s t r a c t This paper shows a thermodynamic cogeneration model (single-effect LiBr/H2O absorption chiller coupled to a 30 kW microturbine, a cooling tower and a heat exchanger) and analyzes energetic and exergetic behavior of the system A computational algorithm was developed on the EES-32 platform to evaluate the influence of the main operating parameters of the cogeneration system The effect of parameters such as hot water temperature, the microturbine load, the ambient temperature, etc on the cooling capacity and the COP (Coefficient of Performance) of the chiller are analyzed The balance equations are based on the principles of conservation of energy, mass and species The total exergy of the working fluids was calculated taking into account the physical and chemical characteristics In the cogeneration system, the greatest irreversibility was found in the microturbine with 52.88 kW and the least in the absorption chiller with 1.78 kW The analysis was performed by varying the load of microturbine and its influence on the COP and the cooling capacity The COP values are based on the first and second law of thermodynamics Due to the load variation of the microturbine, and its influence on inlet and outlet temperatures of the water, the COP values were obtained 0.74 and 0.24, respectively Finally the overall energy and exergy efficiencies of cogeneration was determined, these values were about 50% and 26%, respectively Ó 2014 Elsevier Ltd All rights reserved Introduction The term cogeneration produces electric and thermal energies by using a fuel as a prime source [1] The cogeneration reuses the waste energy of a process and as such the overall efficiency increases The waste energy can be in the form of heat, cold or in any other form and as such saves fossil fuels and also reduces the operation and maintenance costs of the process Many authors have investigated cogeneration ‘processes in different ways [2,3] Kong et al [4] presented a study on the problem of energy management and operation of cogeneration system by integrating cooling, heating and power production It was shown that energy management and optimal operation of the micro cogeneration system is dependent on the load conditions On other hand, using an internal combustion engine of kW output, the authors in the reference [5] investigated the electric load of a domestic non-HVAC operation in real time for an Italian multi-family house The results showed a ⇑ Corresponding author at: Federal Institute of Technology of Pernambuco, Av Prof Luiz Freire, 500, CEP 50740-540 Recife, PE, Brazil Tel.: +55 81 9976 4266; fax: +55 81 2125 1750 E-mail address: ochoaalvaro@recife.ifpe.edu.br (A.A.V Ochoa) http://dx.doi.org/10.1016/j.enconman.2014.08.064 0196-8904/Ó 2014 Elsevier Ltd All rights reserved reduction of carbon dioxide emissions by about 2% Vandewalle ´ Haeseleer [6] showed that using manufacturer’s generic data and D could overestimate the impact on the micro cogeneration system, and also use of the thermal storage tank leads to lower peak demand of the gas and hence a lower impact on the gas distribution network In the same context, the work cited in the reference [7] considered the effect of the hot water storage tank on the modeling of a cogeneration system and its influence on the economic feasibility using the first and second law of thermodynamics Many authors are working on the sustainable cogeneration systems wherein the economic and financial feasibility studies are being conducted [8,9] In a study, the unit exergy cost and CO2 emissions of the electricity generated in Brazil are evaluated using electricity mix [10] Taking economic aspect of cogeneration systems, it is important to calculate financial feasibility of the projects [11,12] The study cited in the reference [12] presented an economic study of a cogeneration system consisting of power, heating and cooling The system designed improves the energy efficiency of a large liquefied natural gas plant in the Persian Gulf The economics of this system validated the feasibility of this system payback in 2.7 years and net present value of about USD 158 million with an internal rate return of 39% 546 A.A.V Ochoa et al / Energy Conversion and Management 88 (2014) 545–553 Nomenclature a,b,c,d h s Cp M T _ m y Q_ _ W p w COP ex _ Ex Id R LHV products mol specific enthalpy (kJ kgÀ1) specific entropy (kJ kgÀ1 KÀ1) specific heat (kJ kgÀ1 KÀ1) massa molar (kg kmolÀ1) temperature (°C) mass flow rate (kg sÀ1) mole fraction heat flow (kW) work flow (kW) pressure (kPa) relative humidity (%) coefficient of performance specific exergy (kJ kgÀ1) total exergy (kW) exergy destruction (kW) gas universal constant (kJ kmolÀ1 KÀ1) lower heat value (kJ kgÀ1) Greeks letters energetic efficient standard chemical exergy (kJ kmolÀ1) W exergetic efficient b air ratio g ~e Superscripts st standard The use of cogeneration technology is strongly linked to the absorption systems that use thermal energy to produce cooling [13] This is generally used for air-conditioning and as such large amount of electric energy is saved and may help the economy and environment, as it could be seen in reference [8] This has led to use new thermal systems with absorption chillers [2,3,14] Despite the large scale use of absorption chillers coupled to the cogeneration systems, many studies are still being conducted to improve the energetic and exergetic efficiency of these systems [15,16] Absorption chiller systems have advantage when used in an indirect way, i.e., using waste heat from other processes to drive it [17] There are large number of studies that target single-, double- and even triple-effect chilling by absorption equipment; these analyses are limited to the stationary conditions of the process [18–23] However, in commercial equipment, it is necessary to check the transition condition of the absorption chiller, and these have been cited in references [24–29] The requirement to validate these models, and to understand the real operation of these systems, it was necessary to realize experimental analysis in laboratories and/or in real systems [30–32] Some analyses have been carried out using characteristic equations of single- and double effect absorption chillers by means of multivariable regressions [33–35] The mechanical cooling can be substituted by cogeneration absorption chillers [36–38] The references investigate parameters such as the overall efficiency of the cogeneration systems, and economic/financial viability Another reference [39] compares the energetic and economic improvement of an oil and gas plant to a single-effect LiBr–H2O absorption chiller The concept of exergy aims to find the availability of the energy for any process The exergetic methodology involves the Subscripts amb ambient Sol solution LiBr lithium bromide water H2O sol solution abs absorber condenser pro combustion products gen generator eva evaporator ng natural gas ctp cold tower pump fan cold tower fan sp solution pump ph physical ch chemical in in out out cv control volume i component q hot fluid f cold fluid o reference tot total rel relative j Environmental x, y, z, w molar fraction fuel combination of the first- and second law of the thermodynamics This methodology has been used by various authors to identify and quantify the available energy of the streams of refrigeration, cogeneration, and chemical systems [40–44] The main purpose was to find out what components of the system has higher exergy destruction value In reference [45] is presented an energetic and exergetic study on the Chinese urban residential sector between 2002 and 2011 This methodology demonstrated the large differences of the overall energetic and exergetic efficiencies A modification on this methodology is known as advanced exergy analysis that includes an unavoidable and avoidable exergy destructions [46,47] The unavoidable exergy destruction fraction aims the technical and economical restraints and therefore could not be improved and the remaining part (avoidable) represents a potential for improvement In any thermal system use of efficient energy is most important goal To realize this, mathematical methods are used where important variables are optimized so that the performance of system as a whole is efficient [48–53] These studies involve exergetic analysis of absorption and cogeneration systems In reference [54] is presented an improvement of the efficiency of the cogeneration system by using a real-time operation strategy; whereas in reference [55] is proposed a method to calculate hourly energy demand for the whole year In the same context applying the exergoeconomics method as an optimization tool where maximum values of the cogeneration systems [56–59] The present work develops a tool for numerical simulation where an energetic and exergetic analysis is made of a microgeneration system (single-effect absorption chiller of indirect heating with a 10 TR LiBr–H2O capacity coupled to a 30 kW gas microturbine, a cooling tower and a compact heat exchanger Using data from a manufacturer’s manual and the balance A.A.V Ochoa et al / Energy Conversion and Management 88 (2014) 545–553 547 equations of conservation of energy, mass and species for each component of the cogeneration system, a mathematical model is developed that calculates energy and exergy The model also takes into account physical and chemical properties to calculate the total exergy of the working fluids for each component and for the entire system This model was implemented computationally on the EES32 platform, and enables a calculation to be made of the influence of the main operating parameters of the cogeneration system, such as the microturbine load, the ambient temperature, the hot water temperature, power requirements for the chiller, and also evaluates the functioning and performance of the chiller Methodology The system (Fig 1) consists basically of a microturbine [60] with a nominal capacity of 30 kW, a compact heat exchanger, a single-effect absorption chiller with a cooling capacity of 10TR, a circulation pump and a cooling tower The main idea of the micro-cogeneration is to generate electricity by recycling hot gases rejected into the environment by the microturbine, which are the main source for heating the water and it is this which permits the absorption chiller to operate, and produce chilled water for the air-conditioning process The cogeneration process is started with the burning of the mixture of air and natural gas, in the micro_ alt ) The rejected heat in the turbine, which generates electricity (W combustion gases is recycled by using a compact counter-current heat exchanger The energy contained in the combustion gases is recovered when heating hot water This hot water circuit enters the absorption chiller via the generator in order to heat the LiBr– H2O, and to generate steam that will be liquefied in the condenser It is then cooled by throttling in an expansion valve, and the low temperature because of the heat exchange in the evaporator produces chilled refrigeration water The internal cooling of the chiller is conducted through the cooling tower, in the evaporative process – air–cold water, by removing heat in the absorber and condenser The chilled water produced in the evaporator can reach values between and 12 °C (for this type of chiller), depending on the temperature of the hot water supplied Fig Control volume used in the simulated cogeneration system 2.1 Energetic modeling of the micro-cogeneration system The whole models is based on the first and second laws of the thermodynamics applied on the cogeneration systems shown in the Fig Fig shows the exactly volume control used in the cogeneration models developed Table indicates the names of the streams used in the cogeneration model developed 2.1.1 Microturbine A microturbine burns natural gas and generates electrical power, and some part is rejected as the heat energy in the unused gases Using data from the manufacturer of the microturbine [60] it was possible to adjust its characteristic curves and obtain power Fig Simulated cogeneration system for a WFC – SC10 absorption chiller [61] 548 A.A.V Ochoa et al / Energy Conversion and Management 88 (2014) 545–553 Table Streams of the cogeneration systems Table Molar composition of the natural gas Streams Description Energy Exergy Elements CH4 C2H6 C3H8 CO2 N2 Natural gas (fuel of the microturbine) Air going to the microturbine combustion chamber Combustion products from the microturbine Combustion products from the heat exchanger Hot water inlet (going to the absorption chiller) Hot water outlet (going to the heat exchanger) Chilled water outlet (out as final product) Chilled water inlet (going to the absorption chiller) Cold water outlet (going to cold tower) Cold water inlet (going to the absorption chiller) Air going to the cooling tower Air outgoing from the cooling tower Water make up going to the cooling water h1 h2 ex1 ex2 (%) 83,22 11,11 0,53 3,03 2,11 h3 h4 h5 h6 h7 h8 ex3 ex4 ex5 ex6 ex7 ex8 10 11 12 13 h9 h10 ex9 ex10 h11 h12 h13 ex11 ex12 ex13 Table Chemical exergy patterns of lithium, bromide and water [65] ð1Þ X yi cpi ð2Þ i PMpro ¼ X yi PMi Water ~est H yi ¼ The molar composition of the natural gas is provided by de Gas company of the state of Pernambuco (COPERGAS) as shown in Tables and To determine the specific heat (cppro) and molar mass (PMpro) of the products, the partial fractions of the components of the product were considered: cppro ẳ Bromide ~ estBr2 ẳ 101200kJ kmol1 ị 2O ẳ 900kJ kmol ị where yi is the molar fraction of the component i, defined as: _ alt ) as a function of fuel used (m _ ng ), the amount of prodoutput (W _ pro ), excess air used and the ucts from the combustion process (m exhaust temperature of the gases (Tpro), as seen in Table The validation of these correlation was done by using four statistical parameters (regression coefficient (r2), mean bias error (MBE), root mean square error (RMSE), chi-square (v2)), shown in Table To determine the heat energy in the combustion gases, the Law of the Conservation of Species was used, taking into account the amount of each hydrocarbon element present in the gas However, the phenomenon of chemical dissociation was not considered The following equation shows the overall balance for calculating the chemical composition of combustion products: Cx Hy Ow Nk þ bðO2 þ 3; 76N2 Þ ! aCO2 þ bH2 O ỵ cO2 ỵ dN2 Lithium ~est ị Li ¼ 393000ðkJ kmol ð3Þ i ni : ntotal ð4Þ The calculation of thermal waste was made by the equation of energy balance (control volume), where the difference of enthalpy was calculated using the specific heat of the combustion products as per Eq (2) The expression for (Q_ pro ) is calculated from: _ pro cppro ðT À T Þ Q_ pro ¼ m ð5Þ The efficiency (g) of the microturbine may be expressed as: g¼ _ alt Energia produced W ẳ _ ng LHV ng ị Energia consumed m ð6Þ 2.1.2 Heat exchanger The compact heat exchanger used in the system was developed by [62], and the calculation of the heat exchanger were entered into the model to calculate efficiency, and the temperature of the outgoing water heated by the exchanger In [62] it is shows the design and validation of the compact heat exchanger used in the cogeneration model developed 2.1.3 Absorption chiller The simulation model of the chiller was developed by [61] and it was incorporated into the cogeneration system The validation of the absorption chiller is shown in [52] To calculate the physical properties of the LiBr–H2O solution, equations of conservation of energy, mass and species were used The simulation of the chiller enables the determination of Coefficient of Performance (COP) of the chiller Table Adjust curves from the data of the microturbine manufacturer Parameter Correlation Conversion Factor (CF) Exhaust temperature (Tpro) _ 0:000000288356W _ ỵ 0:000033674111W _ 0:001914791577W _ ỵ 0:058327647816W _ alt ỵ 0:024407012072 F ẳ 0:000000000945W alt alt alt alt _ pro ) Products flow (m _ 0:0000141024W _ ỵ 0:0014927857W _ 0:049999397W _ ỵ 0:654716288W _ ỵ 0:981013265W _ alt T pro ¼ À0:0000000471W alt alt alt alt alt ỵ160:5419410412 _ 0:01339646W _ ỵ 0:66556383W _ alt ỵ 5:50983020 _ pro ẳ 0:000191683W m _ ) Alternator power (W alt _ alt ẳ loadịf0:0000024563T 0:0001603025T 0:0028108726T ỵ 0:0308182205T amb þ 30:21948g W amb amb amb 100 alt alt Table Statistical parameters used to validate the curves of micro-turbine Par Conversion Factor (CF) Exhaust temperature (Tpro) Products flow (mpro) Alternator power (kW) r MBE RMSE 0.99904 À0.000250185 0.008911747 8.42619EÀ05 0.99972 À0.003620698 0.0471376056 0.0277744233 0.9996417 À1.49108EÀ05 0.063825469 0.004526323 0.9965 2.21174EÀ05 0.0201094044 0.055143838 v2 A.A.V Ochoa et al / Energy Conversion and Management 88 (2014) 545–553 _ ev a ðh8 À h7 ị Q_ ev a ẳ m 7ị _ gen h5 h6 ị Q_ gen ẳ m 8ị The COP of the chiller is defined as the ratio of the energy removed in the evaporator (Q_ ev a ) to the energy supplied to the chil_ sp ): ler (Q_ gen ỵ W COP ẳ Q_ ev a _ sp Q_ gen ỵ W 9ị According to [63], Eq (16) is divided into two parts; the first represents the chemical exergy of the pure components (lithium bromide and water), and the second part represents dissolution due to the changes in the concentration of LiBr–H2O 2.2.3 Standard chemical exergy This exergy is referred to the pure exergy of the components lithium, bromide and water To calculate the chemical exergy of lithium bromide as a pure component, the procedure is [63]: ~esti ẳ Dg~stf ỵ 2.1.4 Cooling tower The relation (L/G) represents the water/air flow mass fraction into the cold tower The energy balance of the cooling tower is: h12 ¼ h11 ỵ cpwater L=GịT T 10 ị 10ị _ dry _ 12 ẳ ỵ w12 ịm m 11ị _ 13 ẳ m _ water m 12 air _ water Àm 11 ð12Þ The powers of the solution pump, cooling tower pump and the fan are 0.00073 kW, 0.9619 kW, 0.7984 kW, respectively [61] The heat dissipated of the absorption chiller water (Q_ cold;water ) is: _ cold;water h9 h10 ị Q_ cold;water ẳ m 13ị 2.1.5 Overall efficiency of the cogeneration system This efficiency may be defined as the ratio of the sum of the _ alt ), and cooling electric power generated in the microturbine (W power of the chiller (Q_ ev a ) to the energy of the burnt fuel _ ng LHV ng ): (m _ W ỵ Q_ ev a gov erall ¼ _ alt mng LHV ng ð14Þ The exergy of a system is the maximum amount of useful energy that can be extracted in the thermal processes [63] The exergy of a system contains four types: physical, chemical, kinetic and potential; however, the present work considers only the physical and chemical exergies 2.2.1 Physical exergy This represents the maximum possible work when a system leaves its original state and reaches equilibrium with the environment The dead point considered was temperature of 20 °C and pressure of atm [63,64] exph ¼ ðh À h0 Þ À T o ðs À s0 Þ ð15Þ 2.2.2 Chemical exergy Chemical Exergy consists of bringing each state of the dead point (T0, p0) to a standard state of the atmosphere (T0, p⁄0Yi), where the term (p⁄0Yi) represents the partial pressure [64] The chemical exergy of pure components [63] can be determined in the following manner: exch ¼ ÉÃ É È ÂÈ ~St yLiBr ~eSt LiBr ỵ yi eH2O ỵ RT yLiBr ln aLiBr ỵ yH2O ln aH2O ị M sol ð16Þ For a substance, such as air, Eq (16) can be written as a function of the air components n oi X hX fyi Á ~esti g þ R Á T Á exch ¼ yi Lnðyi Þ M n X ~estele ð18Þ i¼1 where the formation of lithium bromide from pure lithium and bromide has to be: Li ỵ Br2 ! LiBr 19ị ~estLiBr ẳ Dg~stLiBr ỵ ~estLi ỵ ~estBr 2 20ị Dg~stLiBr ẳ 342; 0kJ kmol ị Tables and shows the exergy standards used in the analysis [65] According to the procedures mentioned in Eqs (19) and (20) [63], and using exergy standards, we have: e_ stLiBr ¼ 10; 1600ðkJ kmolÀ1 Þ 2.2.4 Total exergy of the system The total exergy (extot ) represents the sum of the physical and chemical exergies of the state: extot ¼ exph þ exch 2.2 Exergetic modeling of the micro-cogeneration system ð17Þ 549 ð21Þ 2.2.5 Distribution of exergy in the cogeneration system To analyze the components of the cogeneration system, the exergies of the working fluids in the different states in the cycle were determined Then the combination of the first and second Law of Thermodynamics gives: X X @Ex T0 _ _ cv À p Á @V ỵ _ tot;in Qj W ẳ Ex @t cv @t Tj j in X _ tot;out I_d Ex 22ị out where: _ tot ẳ m _ Á extot Ex ð23Þ This equation enables the destruction of exergy or irreversibility in each component in the cycle As the heat loss to the environment is negligible, and the process is in steady state, the terms P ð1 À T ÞQ_ j and ð@ExÞ of Eq (22) are zero j Tj @t cv The exergy destruction of the cogeneration components (I_d;MT , I_d;HE , I_d;CH , I_d;CT ) given in the Eq (16) are: _ ½1 ỵ Ex _ ẵ2 ị Ex _ ẵ3 ỵ W _ alt ị I_d;MT ẳ Ex 24ị _ ẵ3 ỵ Ex _ ẵ6 ị Ex _ ẵ4 ỵ Ex _ ẵ5 ị I_d;HE ẳ Ex 25ị _ ẵ5 ỵ Ex _ ẵ8 ỵ Ex _ ẵ10 ị Ex _ ẵ6 ỵ Ex _ ẵ7 ỵ Ex _ ẵ9 ị ỵ W _ sp I_d;CH ẳ Ex 26ị _ ẵ9 ỵ Ex _ ẵ11 ỵ Ex _ ẵ13 ị Ex _ ẵ10 ỵ Ex _ ẵ12 ị þ W _ fan þ W _ ctp I_d;CT ¼ Ex 27ị 550 COP ex ẳ _ ev a ex8 ex7 ị DExtot;evaporator m ẳ _ _ sp _ gen ex5 ex6 ị ỵ W DExtot;gen ỵ W sp m ð28Þ 100 90 80 70 60 50 40 30 20 2.3 Overall exergetic efficiency of the cogeneration system 110 100 90 80 70 60 50 40 30 20 Q_gn Ex_gn 20 40 60 80 Ex_gn (kW) 2.2.6 Coefficient of exergetic performance of the chiller and the microgeneration system The exergetic Coefficient of Performance of the cooling system by single-effect absorption may be defined: Q_gn (kW) A.A.V Ochoa et al / Energy Conversion and Management 88 (2014) 545–553 100 load (%) Analysis and discussion of the results 3.1 Energy and exergy in the combustion of natural gas Before presenting the results of the cogeneration system, it was decided to present the variation of the energy and exergy of the natural gas to be burned in the microturbine at various operating loads, and its effect on the energetic and exergetic efficiencies (see Figs and 4) Fig shows that increase in fuel consumption leads to the increase in the power when the turbine is operating without reusing the waste heat Also, the thermal energy generated and the maximum energy available from the natural gas increases Fig shows the energetic and exergetic efficiencies of the microturbine This is due to the fact that the energetic use of the fuel keeps on decreasing while the consumption of natural gas keeps on increasing as a result of the load generated by the microturbine However, the destruction of exergy is not great if only the generation of power is considered This shows that both the energetic and exergetic analyses enable a clear quantification to be made of the efficiency of this component operating without considering cogeneration Since it is considered that the energy lost in the combustion gases is in the microturbine, the energetic and exergetic performance of the combustion products leaving the turbine with the variation of the load could be observed In Fig 5, it can be seen that both streams increase with the load due to the increase in the consumption of fuel in the combustion chamber, which results in an increase in the temperature of the 30 30 25 25 Ψ_mt (%) This energetic–exergetic model was developed and implemented computationally on the EES-32 platform to simulate the cogeneration system with complete combustion in the steady state, due to the complexity of modeling in a transient stage However, the model enables calculation of the energetic and exergetic efficiencies of the different components of the system, and also to evaluates the points of greatest exergy destruction of each component and their relationship with the overall energetic and exergetic efficiency of the system The tool enables the influence of the main operating parameters of the cogeneration system to be evaluated such as the microturbine load, the ambient temperature, the hot water temperature (the driving force for operating the chiller), generating capacity of electric power, and evaluation of the functioning and performance of the absorption chiller It also enables the identification of what is the best combination of thermal power generation (fully produced by the waste heat from the microturbine, and/or the inclusion of another source of heat) to meet the demand for chilled water from the absorption chiller η_mt (%) ð29Þ 20 20 Energetic efficiency Exergetic efficiency 15 15 10 10 5 0 20 40 60 80 100 120 load (%) Fig Energetic and exergetic efficiency as a result of the microturbine load Q_pro (kW) wov errall _ alt ỵ Ex8 Ex7 ị W ¼ _ ng Ex Fig Energy and exergetic fuel flow due to the load of the microturbine 45 40 35 30 25 20 15 10 Q_pro EX_pro 75 80 85 90 95 100 45 40 35 30 25 20 15 10 105 Ex_pro (kW) The overall efficiency of the absorption chiller integrated with the cogeneration system may be defined as the ratio of exergetic _ alt ỵ Ex8 Ex7 ị) to the fuel burnt by the microturflow produced W _ ng ), [63]: bine (Ex load (%) Fig Energetic and exergetic flow of the products from combustion due to the microturbine load products of the combustion gases However, the energetic flow contained in the combustion gases is higher than that of the exergetic flow, and by definition, exergy represents the maximum useful heat available This means that this difference in the heat of the gases that will be rejected has lower work capacity if compared to the energetic flow of electric power produced but it represents a source of high potential for cogeneration In this case, activating the chiller by means of recycling the exergy of the gases 3.2 Energetic and exergetic analysis in the co-generation system Data used in the simulation: It was established from the manufacturer catalogs that the chiller would operate when inlet water is in the range of 70 and 95 °C [66] Then the simulation of the system was conducted using the data shown in Table [60,66] However, in our model the energy supplied by the microturbine and recovered in the heat exchanger only allowed a range of 70–80 °C The cold water and chilled water temperatures of the chiller were fixed and represented the effect of a constant thermal load It is worth noting that, according to the manufacturer, the outflows of the water circuits of the system did not vary 551 Streams m (kg/s) T (°C) Hot water inlet Cold water outlet Chilled water inlet Cold tower air inlet Cold tower air outlet Make up water Microturbine air inlet 2.39 5.08 1.52 3.37 3.3974 0.0274 0.2934 70–80 35.00 12.50 30.00 31.00 25.00 25 (1 atm) COP Table Data for simulating the cogeneration system 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 COP COP_ex 75 80 85 90 95 100 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 105 COP_ex A.A.V Ochoa et al / Energy Conversion and Management 88 (2014) 545–553 load (%) It can be seen in Fig that as the load generated in microturbine increases, there is an increase in the (energetic and exergetic) COP This is due to the fact that the higher the load is generated, higher fuel consumption is required, and consequently there is an increase in the temperature of the exhaust gases and therefore this results in an increase in the temperature of the hot water Comparing the COP of an absorption chiller and a mechanical one, it can be said that in the absorption chiller, the hot water energy that reaches the generator is already of low exergy, that the chiller works in a vacuum and there is a significant destruction of exergy in the absorber due to the fact that the liquefaction of the steam occurs by the strong LiBr–H2O solution, an exothermic reaction As the rate of absorption of steam by the solution decreases when the temperature increases, it is necessary to remove heat from the solution and discard it into the environment via the cooling tower This loss of exergy has a part to play in reducing the energetic COP of the chiller because in the heat supplied by the hot water, there needs to be an extra amount of heat to replenish this loss However it should be borne in mind that the energy that drives the absorption chiller is obtained from thermal waste In the mechanical chiller, the compression process is adiabatic, leaving the compressed refrigerant gas under high pressure, with a large exergy The destruction of exergy in the process of condensation is very small In the isenthalpic expansion process, the drop in pressure and temperature is of low exergetic destruction, thus enabling removal of greater heat in the evaporator than the energy ceded to compress the refrigerant That is why there is higher value of the COP, when compared to that of the absorption chiller 3.4 Analysis of the co-generation system In the cogeneration system there is an increase in the useful energy generated, given that besides the generation of electricity, there is a recovery of thermal energy from the combustion gases, which is used to generate chilled water for air-conditioning The results generated by the model made it possible to analyze the behavior of the integrated cogeneration system in function of the load Fig shows the energetic and exergetic efficiency of the cogeneration system In it what can be identified is that despite the increase in the energetic and exergetic efficiencies of the system, with the increase of the electricity generated by the microturbine and the cool air generated by the chiller, the effect of the destruction of exergy in the microturbine, in the absorption chiller and cooling tower, appears in the overall exergetic efficiency of the 60 60 50 50 40 40 eta_overall psi_overall 30 30 20 20 10 10 75 80 85 90 95 100 psi_overall (%) 3.3 Analysis of the chiller as a result of the generation in the turbine Fig Variation of the (energetic and exergetic) COP due to the microturbine load η_overall (%) It was found that the microturbine ought to operate at 80% of the full load [61] to generate combustion gases with sufficient energy to heat hot water, and thus to reach this minimum operating temperature (see Fig 6) From the simulation model we obtained the Coefficient of Performance of the chiller and the overall coefficient of the cogeneration system 105 load (%) Fig Overall energetic and exergetic efficiency of cogeneration due to the microturbine load 3% 6% 15% Microturbine Exchanger Heat 76% Chiller Cooling Tower Fig Destruction of the exergy of each component of the system cogeneration system, thus showing that there was an increasing destruction of exergy when the load generated was varied 3.5 Comparative overview of the destruction of exergy in the components of the cogeneration system With data generated by the model it was possible to put the percentages of exergy destruction of the cogeneration system, separately, see Fig It can be seen that the microturbine operating without reusing the energy contained in the combustion gases has an exergy destruction of 52.88 kW (76%) due to the reduction of exergy in the conversion of thermal energy into electrical energy, and in the combustion chamber, compressor and the turbine The component that supplies the least exergy destruction is the absorption chiller with 1.78 kW (3%), which operates below atmospheric pressures and there are some losses due to friction, and to the environment However, using cogeneration (coupling the microturbine to the absorption chiller), part of the exergy which was destroyed or not used in the microturbine is converted into a form of thermal exergy and used in heating water to drive the chiller This destruction of exergy due to the low COP of the singleeffect absorption systems end up increasing energetic losses 552 A.A.V Ochoa et al / Energy Conversion and Management 88 (2014) 545–553 Conclusions The model showed that it is‘ an important tool for simulating cogeneration systems It enables modeling to be done either by inclusion or exclusion of components and estimate the performance of each component of the system, besides the overall efficiency of the system, Moreover, the model allowed an exergetic analysis of the components to be made and of the system, and enabled to identify the largest amounts of exergy destruction, and the reasons of such destruction From the results, the causes of exergy destruction could be traced and associated to the system components The exergy recovered in the exchanger is sent on in the form of hot water to drive the chiller, thereby causing the destruction of exergies of the cogeneration system In the chiller, the exergetic COP obtained was quite low, which showed that the 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