and Exergy Analysis of a Typical LiBr/H 2 O VAR

Transcription

1 CHAPTER - 3 Design and Evaluation of a 3 TR VAR System and Exergy Analysis of a Typical LiBr/H 2 O VAR System 3.1 Introduction The continuous increase in the cost and demand for energy has led to more research and development to utilize available energy resources efficiently by minimizing waste energy. It is important to note that system performance can be enhanced by reducing the irreversible losses in the system by using the principles of the second law of thermodynamics. A better understanding of the second law of thermodynamics has revealed that entropy generation minimization is an important technique in achieving optimal system configurations and/or better operating conditions. Some researchers [1, 2] have used the principles of entropy generation minimization to analyze different systems to improve the systems performance. The absorption refrigeration system (ARS) is becoming more important because it can produce higher cooling capacity than vapor compression systems, and it can be powered by other sources of energy (like waste heat from gas and steam turbines, sun, geothermal, biomass) other than electricity. Furthermore, an ARS does not deplete the ozone layer and hence, it poses no danger to the environment. Theoretical and experimental works on the performance characteristics and thermodynamic 65

2 analysis of ARSs are available in the literature. The absorption cycle uses a heatdriven concentration difference to move refrigerant vapors (ammonia) from the evaporator to the condenser. The high concentration side of the cycle absorbs refrigerant vapors (which, of course dilutes that material). Heat is then used to drive off these refrigerant vapors thereby increasing the concentration again. Water is the most common absorbent used in commercial cooling equipment, with ammonia used as the refrigerant. Smaller absorption chillers sometimes use water as the absorbent and ammonia as the refrigerant. Starting with the evaporator, ammonia is evaporating off the chilled water tubes, thereby, bringing the temperature down being returned from the air handlers to the required chilled ammonia supply temperature. This ammonia vapor is absorbed by the concentrated water solution due to its hygroscopic characteristics. The solution is then pumped to the concentrator at a higher pressure where heat is applied to drive off the ammonia and thereby re-concentrate the water. The ammonia driven off by the heat input step is then condensed, collected, and then flashed to the required low temperature to complete the cycle. Since ammonia is moving the heat from the evaporator to the condenser, it serves as the refrigerant in this cycle. In the recent years, the interest in absorption refrigeration system is growing because these systems have environmental friendly refrigerant and absorbent pairs. Due to this fact, wide spread efforts are currently underway to utilize available energy resources efficiently by minimizing energy consumption, besides developing alternative for refrigerants which are environmental friendly and do not harm the ozone layer. The absorption refrigeration cycle is similar to that of a vapor compression cycle 66

3 which employs a volatile refrigerant. Usually, ammonia or water which alternately vaporizes under low pressure in the evaporator by absorbing latent heat from the material being cooled and condenses under high pressure in the condenser by surrendering the latent heat to the condensing medium. In the absorption cycle, vapor compressor employed in the vapor compression cycle is replaced by an absorber and generator. Also the energy input required by the vapor compression cycle is supplied by the mechanical work of the compressor while the energy input in the absorption cycle is in the form of heat supplied directly to the generator. The source of heat supplied to the generator is usually low grade energy such as waste heat, Renewable energy etc. As the ammonia-water and LiBr-H 2 O mixtures are environmental friendly, are the most commonly used for refrigeration purposes in absorption systems, and despite of the new mixtures under investigation, the ammonia-water mixture is the only one which has clear future [3]. The principle of absorption is providing the necessary pressure difference between the vaporizing and condensing processes, which alternately condenses under high pressure in the condenser by rejecting heat to the environment and vaporizes under low pressure in the evaporator by absorbing heat from the medium being cooled. Ammonia water absorption chillers have been widely used for different occasions [4]. Amount of work associated with theoretical and experimental analysis of the commercial absorption chillers, using ammonia water as working fluid is available in the literature [5, 6]. However, most of the research is carried out with commercially fashioned chillers that have been specially designed as an air cooled system. 67

4 To investigate the system characteristics being consistent with the designed values, an experimental system is designed and fabricated, using mass and energy balance equations. This study deals with the analysis of an AAR system with options to operate with a low-temperature heat source. Application of absorption refrigeration devices has a large potential for decreasing the consumption of primary energy sources and for reducing environmental pollution. In the last few years, the quest for new cycle models for these devices has become one of the important subjects in the investigation of absorption refrigerators. Aqua-ammonia absorption refrigeration (AAR) system is preferred to classical vapor compression when a heat source is available cheaply. 3.2 Aqua-ammonia absorption refrigeration cycle The basic cooling cycle is the same for the absorption and electric chillers. Both systems use a low-temperature liquid refrigerant that absorbs heat from the water to be cooled and converts to a vapor phase (in the evaporator section). The refrigerant vapors are then compressed to a higher pressure (by a compressor or a generator), converted back into a liquid by rejecting heat to the external surroundings (in the condenser section), and then expanded to a low-pressure mixture of liquid and vapor (in the expander section) that goes back to the evaporator section and the cycle is repeated. The basic difference between the electric chillers and absorption chillers is that an electric chiller uses an electric motor for operating a compressor used for raising the pressure of refrigerant vapors and an absorption chiller uses heat for compressing 68

5 refrigerant vapors to a high-pressure. The rejected heat from the power-generation equipment (e.g. turbines, microturbines, and engines) may be used with an absorption chiller to provide the cooling in a combined heat and power (CHP) system. The basic absorption cycle employs two fluids, the refrigeratnt and the absorbent. The most commonly fluids are ammonia as the refrigerant and water as the absorbent. These fluids are separated and recombined in the absorption cycle. In the absorption cycle the low-pressure refrigerant vapor is absorbed into the absorbent releasing a large amount of heat. The liquid refrigerant/absorbent solution is pumped to a high-operating pressure generator using significantly less electricity than that for compressing the refrigerant for an electric chiller. Heat is added at the high-pressure generator from the heating element. The added heat causes the refrigerant to leave the absorbent and vaporize. The vapors flow to a condenser, where heat is rejected and condense to a high-pressure liquid. The liquid is then throttled though an expansion valve to the lower pressure in the evaporator where it evaporates by absorbing heat and provides useful cooling. The remaining liquid absorbent, in the generator passes through a valve, where its pressure is reduced, and then is recombined with the low-pressure refrigerant vapors returning from the evaporator, so the cycle gets completed and can be repeated. A simple ammonia water VAR system was designed and fabricated to study the performance of this system. The cycle consists of four main components, namely the condenser, evaporator, absorber and generator. A schematic diagram and a photograph of the experimental apparatus are shown in Figs. ( ).The mixture of refrigerant-absorbent (NH 3 -H 2 O) is heated in the generator by an electric heater, which 69

6 is equipped with single electric resistance of 1.5 kw capacity. The heater can be controlled manually by on and off switch, the pressure gauge is installed on the generator outlet and the discharge pressure under normal condition is 10.7bar. The ammonia vapor along with fraction of water vapor enters an air cooled condenser where it rejects heat and is condensed to a liquid before expanding in the expansion device. The low pressure and low temperature refrigerant enters the evaporator. During the cooling process, the liquid ammonia vaporizes and the transport fluid (water) absorbs the vapor to form a strong ammonia solution in the absorber. After leaving the evaporator, the refrigerant vapor enters the absorber where the temperature rises due to mixing of refrigerant vapors with weak solution coming from the generator through pressure regulating device. The mixture of vapor and weak solution cools down by air or other cooling medium and converts it into liquid (strong solution). The strong solution is pumped to the generator by solution pump and the cycle is completed. Water and ammonia properties are obtained from standard properties of pure substances table in the ASHRAE [7]. 70

7 Fig.-3.1 Layout of a basic 3 TR Ammonia-water VAR system Fig.-3.2: Front view of the vapor absorption system 1-Condenser, 2-Evaporator, 3-Pump, 4-Generator, 5-Expansionn Valve, 6-Absorber 71

8 Fig.-3.3: Side view of vapor absorption system 6- Absorber, 7- Pressure reducing valve, 8- Fan. 3.3 Design and Experimental Analysis of a 3 TR VAR System The thermodynamic analysis of the vapor absorption cycle is based on the following three equations which can be applied to any part of the system: Principle of Mass balance Principle of Material balance m = 0 (3.1) mx = 0 (3.2) Principle of Energy balance 72

9 mh = Q + 0 (3.3) An experimental set-up of a simple ammonia-water absorption system was designed, fabricated and evaluated. The thermodynamic models of the components have been assembled so as to quantify the simultaneous heat and mass transfer processes occurring within the heat exchangers. The results based on design calculations were compared with experimental data. It shows that heat losses from the generator have a strong effect on the system performance, and the cooling capacity is also limited by the evaporator or absorber mass transfer performance. Thermodynamic equations for the performance evaluation of the ammonia water vapor absorption refrigeration system have been obtained, analyzed and reported in this chapter. The energy, mass and material balance equations have been used for obtaining the coefficient of performance (COP) and compared with the ammonia water absorption system. It has been observed that this refrigerant absorbent pair has a greater COP at low generation temperatures and moderate condenser, absorber and evaporator temperatures Assumptions for Analysis a The vapor leaving the condenser is saturated at condenser temperature. b The strong solution leaving the absorber is saturated at absorber temperature. c The weak solution leaving the generator is saturated at generator temperature. d The strong solution is heated only up to saturation temperature, and no vapor generation takes place in the heat exchanger. 73

10 e The work input for the pump is negligible relative to the heat input in the generator. Therefore, the pump work is neglected for the purpose of analysis Coefficient of Performance The general refrigeration system can be considered as a perfectly reversible system, the net refrigerating effect is the heat absorbed by the refrigerant in the evaporator. The theoretical COP is given by: Q COP = e Q (3.4) g In the absorption refrigeration system, the total energy supplied to the system is the total heat supplied in the generator and work done by the pump. The actual COP of the ammonia-water absorption chiller is calculated from COP Q = e (3.5) Qg + WP The design of 3 TR basic vapor absorption refrigeration system is based on parameters given in Table-3.1 and the temperature, pressure and specific enthalpy of mixture at different state points are tabulated in Table 3.2. The calculated mass flow rate of the refrigerant has been found to be kg/sec. Using mass balance equations the mass flow rate of solution has been found to be kg/sec. The detailed calculations are shown in Appendix -I. Based on the cooling capacity of 3TR, the area of the evaporator has been found to be m 2. The heat rejected by the condenser and the area of the condenser has been found to be 9.66 kw and 0.54 m 2 respectively. The heat supplied to the generator and the collector area required to 74

11 provide same quantity has been found to be kw and m 2 respectively. This is based on the insolation of 600 W/m 2 and collector efficiency of 30%. Table 3.1: Design Parameters for Aqua Ammonia VAR system S. No. Parameters Values 1 Capacity of system 3TR 2 Concentration of NH 3 in refrigerant, X r Concentration of NH 3 in solution, X s Concentration of NH 3 in absorbent, X w Temperature of the Evaporator, T E 2 o C 6 Generator or condenser pressure, P H 10.7 bar 7 Evaporator pressure, P L 4.7 bar 8 Temperature of the Condenser, T C 54 o C 9 Temperature of the Absorber, T A 52 o C 10 Temperature of the Generator, T G 120 o C The heat rejected from the absorber and the COP of the system has been found to be 49.5 kw is respectively. The detailed calculation has been shown in the appendix. Based on the above parameters the system was fabricated and experimental analysis on the actual system was carried out by actually measuring the temperature at various state points and calculation of mass flow rate is done using mass balance equations. 75

12 Table 3.2: Temperature, Pressure & Specific Enthalpy of mixture at different state points State Temperature Pressure Specific Enthalpy Points ( o C) (bars) (kj/kg) Discussion of Results for the 3TR VAR System The experiment is performed on the 3TR ammonia-water vapor absorption refrigeration system and various sets of observations were recorded. These set of observations include the temperature readings at the various locations of the ammonia-water VAR system. The temperature readings include the generator temperature, condenser temperature, evaporator temperature, temperature of working fluid at pump inlet and outlet and absorber temperature. The temperatures were measured and recorded using an infrared thermometer (non contact type) and are shown in Table

13 From Table 3.3, the average evaporator temperature is 9 o C and the average condenser temperature is 35 o C. Corresponding to these values of T e and T c, the saturated pressures are P g =13.5 bar and P e = 6 bar. Based on the assumption that the concentration of refrigerant solution (X r ) =0.98, concentration of weak solution (X w ) =0.38, conc. of strong solution (X s ) =0.42, the COP for 3 TR ammonia water VAR system = Contrary to this, the design pressures were P g =10.7 bar and P e = 4.7 bar and the design values for temperature were T c = 54 C and T e = 2 C based on the design values the COP was found to be In the present experimental systems, the heat source to the generator is provided externally using an electric heating element of 1.5 kw. Table-3.4 shows the pressure and specific enthalpy values obtained using enthalpy concentration diagrams. Using the data in Tables 3.3 and 3.4 and applying the mass and energy balance equations, the COP is found to be 0.599, which is greater than the designed value and this is due to the fact that both the condenser and evaporator temperatures and pressures are higher as compared to the designed values i.e T e = 9 C, T c = 35 C, P e = 6 bar and P g = 13.5 bar. The calculated cooling capacity increases almost linearly with the generator heat input. When the heat input is lower, the system is not able to produce any cooling capacity. When the heat input increases beyond the minimum value, it can produce cooling capacity since there is enough liquid ammonia throttled and entered the evaporator. Further increase in heat input produces a higher cooling capacity as more pure refrigerant vapor is generated and this causes COP to increase. When the heat input continues to increase, the cooling capacity also increases but the 77

14 rate will depend on the temperature limitations and also on need of rectification which is absent in this system. Table 3.3: Actual Temperature readings at different state points T cond.in C T cond.out C T evap. C T evap.to S. No. T gen. T weak soln. T pump T pump out C abs. to abs. in C C C C The above system can be modified so that it can be used on solar collector based heating sources to make it fully renewable with little electrical consumption of solution pump. The increase in heat input produces a higher cooling capacity as more pure refrigerant is generated this also enhances the COP. The rise in generator temperature above the designed value is due to the fact that automatic temperature controller is not used to maintain the temperature in the generator. 78

15 Table 3.4: Measure of properties of mixture at different state points State points Pressure (bar) Specific enthalpy (kj/kg) Exergy Analysis of LiBr/H 2 O Absorption System In recent years, there has been growing interest in the use of the principles of 2 nd law of thermodynamics for analyzing and evaluating the thermodynamic performance of thermal systems as well as their technologies [8]. 2 nd law analysis is based on the concept of exergy, which can be defined as a measure of work potential or quality of different forms of energy relative to environmental conditions. A Large number of researchers have used 2 nd law analysis for thermodynamic optimization of refrigeration plants based on the theoretical analysis given by Bejan et al. [9]. Szargut [10] presented energy and exergy balance of an NH 3 -H 2 O absorption refrigerator. Kotas [11] has used exergy analysis method in the analysis of thermal systems. Talbi and Agnew [12] carried out exergy analysis of an absorption LiBr- H 2 O.refrigeration cycle. Numerical results for the cycle were tabulated. A design procedure was applied to a lithium-bromide absorption cycle and an optimization 79

16 procedure that consists of determining the enthalpy, entropy, temperature, mass flow rate and heat rate in each component, and coefficient of performance was calculated. Horuz and Callander [13] did an experimental investigation of the performance of a commercially available Vapour Absorption System. The cooling capacity of the plant was 10 kw and used aqua-ammonia solution as the refrigerant. The response of system to variations in chilled water inlet temperature, chilled water level in evaporator drum, chilled water flow rate and variable heat input were presented. Asdrubali and Grignaffini [14] obtained an experimental evaluation of a plant aimed at stimulating and verifying performances of single stage H2O-LiBr absorption machine. Antonio et al [15] studied the employment of an alternative absorbent used in absorption refrigeration cycles to replace the absorbent currently employed in this kind of engines i.e., lithium bromide. The alternative system consists of absorbent (LiBr:CHO2K=2:1by mass ratio) and refrigerant (H 2 O). Lee and Sherif [16] gave the 1 st and 2 nd law analysis of absorption system for cooling and heating applications. Xu et al [17] presented an advanced energy storage system using aqueous lithium bromide as working fluid. The working principle and flow of the variable mass energy transformation and storage system were introduced and the system dynamic models were developed. Tozer and James [18] derived the thermodynamic absorption cycle performance and temperature formulae. Ideal absorption cycle was demonstrated as the combination of a Carnot driving cycle with a Reverse Carnot cooling cycle. Performance and temperature relations of double, triple and multistage cycles were derived. Validation of the fundamental thermodynamics of absorption cycles was presented by applying an exergy analysis. Tozer et al [19] described the use of the T-S diagram of water 80

17 extended with additional curves to represent real and ideal water/libr absorption cycles. An explanation was provided on several methods available, including details of the thermodynamic justification of the method used, to construct the extended diagrams. Extended T-S diagram was provided with the representation of a real singleeffect water/libr absorption refrigeration cycle. Nikolaidis and Probert [20], investigated the behavior of 2-stage compound compression cycle with flash intercooling, using refrigerant R-22, by exergy method. The effects of temperature changes in condenser and evaporator, on the plant s irreversibility were determined. This paper carries the exergy and energy analysis of 496 TR absorption cooling system using LiBr-H 2 O as working fluids. Exergy analysis is done to look for losses with in the systems. The coefficient of performance (COP) under different operating conditions for cooling applications are determined and shown graphically. 3.6 System Description (Double Effect LiBr-H 2 O VAR System) The system to be analyzed is a 496 TR vapour Absorption system and uses saturated steam as heat source, water as refrigerant, lithium bromide as absorbent, produces the chilled water under vacuum conditions for the purpose of air conditioning and technology process. The chiller consists of following main parts: High pressure generator (HP generator),low pressure generator(lp generator),condenser,evaporator, absorber, high temperature heat exchanger, low temperature heat exchanger, and condensate heat exchanger, auxillary generator for high pressure generator, and such auxillary parts such as purging unit, de-crystallisation piping and hermetically sealed pumps(solution pump and refrigerant pump). 81

18 A double-effect chiller is very similar to the single-effect chiller, except that it contains an additional generator. In a single-effect absorption chiller, the heat released during the chemical process of absorbing refrigerant vapor into the liquid stream, rich in absorbent, is rejected to the cooling water. The main objective of a higher effect cycle is to increase system performance when high temperature heat source is available. As shown in Fig.-3.4, high temperature heat from an external source (steam) is supplied to the first-effect generator. Fig.-3.4 Block diagram of the Vapor Absorption Chiller System 82

19 Where, 1 Strong water concentration Li-Br solution leaving absorber 2 Strong water concentration Li-Br solution entering low temp. Heat exchanger 3 Strong water concentration Li-Br solution leaving high temp. Heat exchanger 4 Strong water concentration Li-Br solution entering Generator 1 5 Weak water concentration Li-Br solution leaving Generator 1 6 Weak water concentration Li-Br solution entering Generator 2 7 Weak water concentration Li-Br solution leaving Generator 2 13 Low pressure water entering evaporator 14 Low pressure water vapor entering absorber 15 Steam in 16 Condensate out 17 Cooling water in 18 Cooling water out 19 Chilled water in 20 Chilled water out 21 Cooling water in 22 Cooling water out 8 Weak water concentration Li-Br solution leaving low temp. Heat exchanger 9 Weak water concentration Li-Br solution entering absorber 10 High pressure water vapor entering Generator 2 11 High pressure water vapor entering condenser 12 High pressure water leaving condenser 83

20 The chiller is purged from the non-condensable gases and kept under the vacuum conditions. Weak solution from the absorber is pumped in to the HP generator through LT, condensate and HT heat exchangers. It is heated by operating system and concentrated in to intermediate solution, and high temperature refrigerant vapour is produced. Intermediate solution enters LP generator through HT heat exchangers in order to exchange heat with weak solution which is passed through the tubes, and heated by refrigerant vapors from HP generator, concentrated to strong solution, releasing refrigerant vapour at same time. The strong solution passes through the outside tube space of LT heat exchanger, enters absorber, transmitting heat to weak solution from absorber. In absorber the strong solution absorbs refrigerant vapour again. Refrigerant vapour from HP generator is condensed in LP generator to form condensate, which enters the condenser through throttle. Refrigerant vapour formed in the LP generator flows to the condenser to form condensate also. These two parts of refrigerant condensate flows in to the flash chamber through U-pipe. A part of refrigerant vapour is flashed to form vapour, which flows in to the re- absorption chamber at the bottom of absorber, while another part of refrigerant water is cooled, and enters evaporator refrigerant pan. Refrigerant from evaporator refrigerant pan is pumped over the evaporator tubes for the refrigeration effect, and evaporates to form the vapour by absorbing heat of chilled water flowing through tubes. Produced refrigerant vapour enters absorber, and absorbed by strong solution in the absorber. Chilled water is cooled and return to the system of customer. Strong solution is diluted by absorbing refrigerant vapour in absorber and absorbing flashed refrigerant vapour in the re- absorption chamber, then it is transferred by solution pump to HP and LP 84

21 generator for concentration. Heat generated is carried to atmosphere by cooling water. This process is continued and refrigeration effect is repeated. The photographic external view and side view of double-effect, steam-fired absorption chiller is given in fig.-3.5(a) and fig.-3.5(b) respectively. The working fluid used in the above system is LiBr-H2O where refrigerant water is handled from the refrigerant pan of evaporator, and sprayed over the tubes in the evaporator. System water to be chilled in the evaporator gives heat to the refrigerant, and decreases temperature. In the mean time, the refrigerant water gains heat, and evaporates. As absorbent for the chiller, a lithium bromide solution is used. It can be taken as the carrier of the refrigerant water, and functions as to absorb the refrigerant the refrigerant vapour, produced in the evaporator by removing the heat of the chilled water, and carries refrigerant in to HP and LP generators. Weak solution is divided in to water and strong solution under the heat of supplied steam. Then the strong solution returned in to absorber to absorb water vapour, produced in the evaporator. Refrigerant vapor enters condenser to be condensed by dissipating heat into the atmosphere through cooling water. Refrigerant condensate returns in to evaporator to produce cooling effect. 85

22 Fig.-3.5(a): External view of double-effect, steam-fired absorption chiller Fig.-3.5(b): Side view of double-effect, steam-fired absorption chiller 86

23 3.7 Thermodynamic Analysis of LiBr-H 2 O VAR System For the thermodynamic analysis of the absorption system the principles of mass conservation, first and second laws of thermodynamics are applied to each component of the system. Each component can be treated as a control volume with inlet and outlet streams, heat transfer and work interactions. In the system, mass conservation includes the mass balance of total mass and each material of the solution. The governing equation of mass and type of material conservation for steady state and steady flow system are m i m o = 0 (3.6) ( mx ) ( mx ) = 0 i o (3.7) Where m is the mass flow rate and x is mass concentration of LiBr in the solution. The first law of thermodynamic yields the energy balance of each component of the absorption system as follows: ( mh ) ( mh ) o + [ Q i Q o ] + W i (3.8) First Law Analysis (Energy analysis) For the thermodynamic analysis of vapor absorption refrigeration system, the energy balance equations of the various components are given in the Table

24 Table-3.5: Energy balance equations of various components in the absorption System S. No. Component Energy balance equation 1 Evaporator = m ( h ) Q e h 20 2 Condenser = m ( h ) Q c h 17 3 Generator-I Q = m ( h ) g I h 16 4 Generator-II Q = m ( h ) g II 6 6 h 7 5 Absorber = m ( h ) Q a h 21 6 Pump ( P P ) W p = m 1 g ρ a The refrigeration system can be considered as a perfectly reversible system and the net refrigerating effect is the heat absorbed by the refrigerant in the evaporator and therefore the theoretical COP is given by Q Q e COP = (3.9) g Whereas in case of absorption refrigeration system, the total energy supplied to the system is the total of the heat supplied in the generator and work consumed by the pump. The actual COP of absorption chiller is calculated from the equation below COP = Q e ( Q + W ) g I P (3.10) 88

25 3.7.2 Second law analysis (Exergy Analysis) Second law analysis is a relatively new concept, which has been used for understanding the irreversible nature of real thermal processes and defining the maximum available energy. The second law analysis is based on the concept of exergy, which can be defined as a measure of work potential or quality of different forms of energy relative to the environmental conditions. In other words, exergy can be defined as the maximum theoretical work, derivable by the interaction of an energy resource with the environment. Exergy analysis applied to a system describes all losses both in the various components of the system and in the whole system. With the help of this analysis, the magnitude of these losses or irreversibilities and their order of importance can be understood. With the use of irreversibility, which is a measure of process imperfection, the optimum operating conditions can be easily determined. The advantage of exergy analysis based on thermo-economic optimization is that the different elements of the system could be optimized independently. It is possible to say that exergy analysis can indicate the possibilities of thermodynamic improvement of the process under consideration. The physical exergy component is associated with work obtainable in bringing a stream of matter from initial state to a state that is in thermal and mechanical equilibrium with the environment. Mathematically, physical exergy is expressed as [13]: E x [( h h ) T ( s s )] = m (3.11) o o o Where, E x is the exergy of the fluid at temperature T. The terms h and s are the enthalpy and entropy of the fluid, whereas, h o and s o are the enthalpy and entropy of the fluid at environmental temperature T o. 89

26 3.8 Discussion of Results for LiBr-H 2 O VAR System To carry out the comparative study of the LiBr-H 2 O vapor absorption system the real time data was measured and the calculations were made using simple excel sheet. The temperature, pressure and mass flow rate were measured using sensor based thermocouple, pressure meter and flow meter, respectively, at different state points. The basic properties such as entropy, enthalpy, were calculated assuming the steady state operation. The exergy loss (irreversibility) and other performance parameters were calculated using a simple Excel sheet and the results are shown in Table-3.6 & 3.7 respectively. The losses due to mixing are because of evaporation of the refrigerant in the generator from a strong solution and this required large amount of heat as compared to refrigerant in pure state. Due to this there is large exergy loss in the generator. In the present system, the exergy loss for the condenser and absorber are same. However, usually the exergy losses in the absorber are more as compared to condenser and this may be due to the fouling of the absorber heat exchanger which has led to the increase in the exergy loss of generator. The exergy efficiency can be enhanced by optimum matching of the heat source with the temperature of the working fluid in the generator. The exergy loss in the evaporator is due to temperature difference between the environment and the refrigerant temperature. 90

27 Table-3.6: Exergy losses for absorber, evaporator, generator-i, condenser and exergy efficiency S. No Exergy Loss (absorber) (kw) Exergy Loss (evaporator) (kw) Exergy Loss (generator-i) (kw) Exergy Loss (condenser) (kw) Exergy Efficiency

28 Table-3.7: Energy analysis of Li-Br/H 2 O Absorption System S. No Q e (kw) Q g-i (kw) Q a (kw) Q c (kw) W P (kw) COP

29 Figure 3.6 shows the variation of the COP of the absorption system with the evaporator temperature. As can be seen from the figure, the COP of the system increases slightly as the evaporator temperature increases and this is due to the fact that with change in heat extracted by the evaporator the enthalpy difference between the chilled water inlet and outlet of evaporator also varies and finally shows the variation in COP of the system. Unlike COP, the exergy efficiency of the system decreases with increase in evaporator temperature and the same is shown in Fig COP EVAPORATOR TEMPERATURE/ o C Fig.-3.6: Variation of COP with evaporator temperature Fig.-3.7 shows the variation of COP with generator-i temperature. As can be seen from the figure, COP of the system increases with increase in generator-i temperature upto a certain level and further increase will have adverse effects because of increased rate of heat input which leads to higher heat transfer losses at higher temperatures and this is why the performance of the absorption system largely depends on the operating parameters of the system. However, the COP shows a 93

30 downward trend at increased generator temperatures because of larger heat transfer losses at higher temperature. Figure 3.11 shows the effect of generator-i temperature on exergy efficiency and it is found that it does not show the same trend as that for COP of the system with generator temperature COP GENERATOR-I TEMPERATURE/ o C Fig.-3.7: Variation of COP with generator I temperature COP GENERATOR -II TEMPERATURE/ o C Fig.-3.8: Variation of COP with generator II temperature 94

31 COP CONDENSER TEMPERATURE/ o C Fig.-3.9: Variation of COP with condenser temperature 0.35 EXERGY EFFICIENCY EVAPORATOR TEMPERATURE/ o C Fig.-3.10: Variation of exergy efficiency with evaporator temperature Also, Fig shows the variation of exergy efficiency of the system with generator temperature and it is evident that the exergy efficiency decreases with the increasing 95

32 generator temperature. This is due to the fact that the system operating on higher temperature can improve the COP of the system but the more input exergy is supplied to the system and more exergy losses occur in the generator during the heat transfer process. Fig shows the comparative exergy loss in each component. EXERGY EFFICIENCY GENERATOR-I TEMPERATURE/ o C Fig.-3.11: Variation of exergy efficiency with generator I temperature 300 GENERATOR 250 EXERGY LOSS/kW EVAPORATOR CONDENSER ABSORBER 0 Fig.-3.12: Comparative exergy Loss of different components of Li-Br-H 2 O absorption system 96

33 3.9 Conclusions A simple vapor absorption system was designed and fabricated to analyze the performance of the system. The system is tested with heat input from an electric heating element of 1500 watts capacity for a pressure of 13.5 bar. The COP is found to be and the increase from the designed value is because of higher generator temperature. A more efficient thermal system should have higher COP and lower total entropy generation. Comparison between actual and calculated values shows that heat loss from the generator greatly affects the system performance. The cooling capacity is limited because of limitations temperature and need of rectification which is absent in the current system. Further analysis to this system should involve the entropy generation to identify and quantify performance degradation of the system. The COP can be increased further by using a heat exchanger between the absorber and generator as well as between the condenser and pressure reducing valve. The various components of 3TR aqua ammonia vapor absorption system were fabricated using mild steel due to the corrosive nature of ammonia on copper, brass etc. The thermodynamic analysis of absorption system using LiBr-H2O as working fluid has been presented. The irreversibility rate in generator is found to be the highest while it is found to be the lowest in the condenser and absorber. It is found that the irreversibility rate in the generator is more because of increase rate of heat transfer in the generator, also the exergy losses are more in generator because of heat of mixing in the solution, which is not present in pure refrigerant/fluids. 97

34 Results show that as expected the COP of the system increases minutely as the generator temperature is increased but the exergy efficiency of the system drops with the increase in generator temperature. It is also found that the COP of the system increases with increase in evaporator temperature this largely depends on the enthalpy difference between the chilled water at inlet and outlet of evaporator. However, it is reverse in case of exergy efficiency. The results with respect to exergy losses in each component and exergy efficiency are very important for the optimization of absorption system. These results are helpful for designers to bring changes in the actual system for optimum performance and less wastage of energy. 98

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