1、EXERGY ANALYSIS OF AN EXPERIMENTAL SINGLE- STAGE HEAT TRANSFORMER OPERATING WITH THE WATER/LITHIUM BROMIDE MIXTURE WITH AND WITHOUT ADDITIVES* W. Rivera1, H. Martnez1, J. Cerezo2, R.J. Romero3 and M. J. Cardoso41Centro de Investigacin en Energa, UNAM, A.P.34, 62580 Temixco, Mor., Mxico2Instituto de
2、Ingeniera, Universidad Autnoma de Baja California, Blvd. Benito Jurez, Baja California 21280, Mxico3Centro de Investigacin en Ingeniera y Ciencias Aplicadas (CIICAp), UAEM. Av. Universidad No.1001 Col. Chamilpa, C. P. 62209, Cuernavaca, Morelos, Mxico.4Instituto de Investigaciones Elctricas, Av. Ref
3、orma 113, Col. Palmira, 62490, Cuernavaca, Mor. Mxico.AbstractSecond law of thermodynamics have been used to analyse the performance of an experimental single-stage heat transformer operating with the water/lithium bromide mixture with and without additives. The system is installed in the Centro de
4、Investigacin en Energa of the Universidad Nacional Autnoma de Mxico. Enthalpy coefficients of performance (COP), external coefficients of performance (COPEXT), exergy coefficients of performance (ECOP) and the irreversibilities of the components have been calculated against the main operating temper
5、atures of the system. The results showed that the highest COP, COPEXT and ECOP values and the lowest irreversibilities are obtained with the use of the 2-ethyl-1-hexanol (400ppm) additive, followed by the 1-octanol additive. The lowest coefficients of performance and highest irreversibilities were o
6、btained by using the water/lithium bromide mixture without additive. Analysing the irreversibilities in each one of the main components of the system, it was found that the use of the additive 2-ethyl-1-hexanol decreases considerably the irreversibility in the absorber increasing the efficiency of t
7、his component and consequently of the entire cycle. Keywords: absorption heat transformers; water/lithium bromide; exergy, additives.* Corresponding author. Tel. +52555629740, E-mail: wrgfcie.unam.mx1. Introduction Theoretical studies based on the first law of thermodynamics of single and advanced a
8、bsorption heat transformers operating with the water/lithium bromide mixture have been studied by Rivera et al., 1-2, Zhao et al. 3 and Barragan et al. 4. Experimental studies have been realized by Barragan et al. 5-8. Rivera et al. 9 reported the experimental evaluation of a single-stage heat trans
9、former operating with the water/carrolTM mixture. Rivera et al. 10 compared the theoretical and experimental performance of a single-stage heat transformer operating with the water/lithium bromide and water/carrolTM mixtures. Rivera and Cerezo 11 published an experimental study of the use of additiv
10、es in the performance of a single-stage absorption heat transformer operating with water/lithium bromide. With regard to second law or exergy analysis of absorption heat transformers, Ishida and Ji 12 analyzed the theoretical performance of a single-stage heat transformer with a graphical exergy met
11、hodology based on energy-utilization diagrams. The analysis provided detailed information on internal phenomena such as the driving forces and the distribution of the exergy loss in each subsystem. Rivero and Le Goff 13 reported the performance criteria of sorption heat pumps and heat transformers.
12、The authors proposed new parameters for exergy analysis including the improvement potential used in the present work. Lee and Sherif 14 utilized the second law to theoretically analyze the performance of multi-stage water/lithium bromide absorption heat transformers. The results provided theoretical
13、 basis for the optimal operation and design of absorption heat transformers. Zhao et al. 15 theoretically studied the performance of a double-absorption heart transformer using TFE-E181 as working fluids. The results showed that the new solution cycle has not only wider operating range of absorber t
14、emperatures but also a higher coefficient of performance, higher available energy or exergy efficiency and more heat released in the absorber per unit mass of TFE leaving the generator than that of the water/lithium bromide mixture. Sozen 16 studied the irreversibilities in a single-stage heat trans
15、former used to increase a solar ponds temperature. The results showed that the absorber and the generator need to be improved thermally in order to increase the efficiency of the system. Fartaj 17 compared the energy, exergy and entropy balance methods for the analysis of a double- stage absorption
16、heat transformer cycle. The results obtained show the influence of irreversibilities of individual components on deterioration of the effectiveness and the coefficient of performance of the system. Sozen and Arcacklioglu 18 proposed the artificial neural networks technique to determine the exergy lo
17、sses for each one of the main components of an ejector-absorption heat transformer. The results showed good accuracy between the training data and the output results. Martnez and Rivera 19 analyzed the theoretical performance of a double-absorption heat transformer reporting values of exergy coeffic
18、ients of performance and irreversibilities for the whole system and the main components. As can be seen from de literature review, it is clear that there are not exergy studies based on the experimental performance of absorption single-stage heat transformers operating with additives, because of thi
19、s, in the present study a detailed analysis is presented for the experimental heat transformer operating with the water/lithium bromide mixture using the 1-octanol and 2-ethyl-1-hexanol additives. 2. Thermodynamic cycleA single-stage heat transformer (SSHT) consists of an evaporator, a condenser, a
20、generator, an absorber, and an economiser. Figure 1 shows a SSHT in a plot of temperature against pressure. A quantity of waste heat QGE is added at a relatively low temperature TGE to the generator to vaporise the working fluid from the weak salt solution containing a low concentration of absorbent
21、. The vaporised working fluid flows to the condenser delivering an amount of heat QCO at a reduced temperature TCO. The liquid leaving the condenser is pumped to the evaporator in the higher pressure zone. The working fluid is then evaporated by using a quantity of waste heat QEV which is added to t
22、he evaporator at an intermediate temperature TEV. Next, the vaporised working fluid flows to the absorber where it is absorbed by the strong salt solution containing a high concentration of absorbent from the generator delivering heat QAB at a high temperature TAB. Finally, the weak salt solution is
23、 returned to the generator to preheat the strong salt solution in the economiser before repeating the cycle again. genratorcondesrevaporatr absorber QABQCOQEVQGECOPEVPGETEV ABTCOT TPeconmiserFigure 1. Schematic diagram of an absorption heat transformer in a pressure-temperature plane.3. Experimental
24、 system The experimental single-stage heat transformer of 2 kW of power supplied into the generator and evaporator, and about 1 kW of output power in the absorber was constructed entirely in stainless steel 316. The system operated firstly with the water/lithium bromide mixture without additives and
25、 then the additives 2-ethyl-1-hexanol (400ppm) and 1-octanol (400ppm) were added separately to the water/lithium bromide mixture. The generator and evaporator are of the stagnant pool type where heat is supplied by means of electrical heaters immersed in the solution and water respectively. The cond
26、enser is a tank with a coil inside which condenses the water vapour coming from the generator. The absorber is of a vertical falling film type where oil circulates inside the tubes to remove the heat at the higher temperature produced by the absorption of the water vapour into the strong solution. T
27、he economiser is a concentric tube heat exchanger. The vessels of the system were designed following the ASME code section VIII division I. Three auxiliary systems were connected to the heat transformer: (i) the cooling system, (ii) the vacuum system, and (iii) the heat-recovery system. These system
28、s served to condense and vaporise the working fluid, and to recover the heat delivered to the absorber, and also to generate vacuum in the components and take samples from the absorber and generator. Gear pumps with velocity controllers were used to pump the working fluid in the system. The heat inp
29、ut was controlled by two variable transformers, each with a maximum power of 2 kW. Copper piping was used in the auxiliary cooling and vacuum systems, and neoprene hosepipes were used to connect the main components. The entire system was adequately insulated. Temperatures, pressures, mass-flow rates
30、, concentration and electrical power were measured in the system utilizing similar instrumentation and control devises described by Rivera et al 9. Figure 2 shows a photograph of the experimental system.Figure 2. Experimental single-stage absorption heat transformer.4. Mathematical model As it is we
31、ll known, analysis based on the first law of the thermodynamics gives information about the amount of energy entering and leaving of each one of the components as well as the entire system, however, it does not give information about the energy quality neither the irreversibilities in the components
32、 and in entire system. Because of this, an exergy analysis is important (based on the second law of thermodynamics) to determine a precise behaviour of systems. The exergy is defined as the maximum possible reversible work that can be produced by a stream or system in bringing the state of the syste
33、m into equilibrium with a reference environment. Exergy is conserved in an ideal process and destroyed during a real process. Neglecting nuclear, magnetic, electric and chemical effects, the exergy for a specific state with reference to the environment can be written as:(1)00sThmxEWhere h0 and s0 ar
34、e evaluated at the reference environment temperature T0 = 298.15 K. In steady state conditions and neglecting the kinetic and potential energies by means of an exergy balance in an open system, the exergy destruction or the irreversibility equation can be written as:(2)WxExQT1I OUTiiINiijjj0 From eq
35、uations (1) and (2) and realizing energy and exergy balances for each one of the main components of the single-stage heat transformer with reference to Figure 3 the following equations can be obtained:Generator(3)1011IN,GEhmhQ(4)GEXT,IV(5)1100 xExIXTGE Condenser(6)121IN,COhmQ(7)178OHEXT, Tp(8)1271xE
36、xICEvaporator(9)1341IN,EVhmQ(10)EVXT,I(11)143,0xIXTEV Absorber(12)541IN,ABhmhQ(13)16OILIEXT, TCp(14)54164xExI Economiser (15)2375hmhm(16)372xExEICPumps(17)fPvWGEABOH21(18)mixture(19)21Wp(20)P213wExxEI With the above equations for each one of the main components of the system it is now possible to de
37、termine the coefficients of performance, the exergy coefficient of performance, the irreversibility and the improvement potential which are the most important parameters to evaluate and design a single-stage heat transformer based on the first and second law of thermodynamics. SamplepointSamplepoint
38、expansionvalveEvaporatorpumppumpAbsorberCondenser GeneratorEconomiser18 1716 1511121314182945QGE,EXTQCO,EXTQEV,EXT QAB,EXT36710Figure 3. Schematic diagram of the absorption heat transformer.The coefficient of performance (COP) represents the efficiency of an absorption heat transformer. It is define
39、d as the heat delivered in the absorber per unit of heat load supplied to the generator and the evaporator plus the work done by the pumps. (21)PEVGABINTWQCOPThe external coefficient of performance is:(22)PEXTVEXTGABEXTWQCOP,The exergy coefficient of performance (ECOP) it is defined as the maximum e
40、xergy obtained from the systems to the exergy supplied in the generator and the evaporator.(23)PEXT,V0EXT,G0165WQQ1xECOP The irreversibility of the entire cycle is given by(24)WECABEVCOGECYL IIII The physical and thermodynamic properties for the water/lithium bromide mixture were taken from McNeely
41、20. The entropy values of the water-lithium bromide mixture were obtained from the work of Kaita 21. The physical and thermodynamic properties for water were taken from the data published by Irvine and Liley 22. 5. ResultsIn order to evaluate the single-stage heat transformer operating with the wate
42、r/lithium bromide mixture with and without additives, more than forty test runs were carried out at different temperatures and solution concentrations, however just fifteen of these reached the steady state condition. The steady state condition was considered when the system temperatures varied less
43、 than 1C over a period of about 2 hours. Table 1 shows the temperatures, mass flow rates, concentrations and pressures for thefifteen test runs for the experimental single-stage heat transformer.Figures 4-7 show the coefficient of performance (COP), the external coefficient of performance (COPEXT),
44、the exergy coefficient of performance (ECOP) and the cycle irreversibility (ICYCLE) against the absorber temperature for the heat transformer operating with the water/lithium bromide without and with the 1-octanol and 2-ethyl-1-hexanol additives. In Figure 4 it can be seen that the coefficient of pe
45、rformance decreases with an increment of the absorber temperature for the three cases. Also it can be observed that the coefficient of performance values are almost the same for the system without additive and with the 1-octanol additive, however, the coefficients of performance obtained by using th
46、e 2-ethyl-1-hexanol additive are higher varying from 0.48 to 0.49. 0.450.460.470.480.490.500.510.5275 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95Absorber temperature, TAB (C) COP(dimensionless) G2 -ethyl-1-hexanol (400pmm)without additive1-octanol (400pmm)Figure 4. COP against TAB fo
47、r the heat transformer operating with the water/lithium bromide mixture with and without additives.In Figure 5 it can be seen that the external coefficients of performance decrease with an increment of the absorber temperature. The COPEXT vary from 0.39 to 0.13. Also it can be observed again that th
48、e coefficients of performance are almost the same for the system without additive and with the 1-octanol additive, however, the external coefficients of performance obtained by using the 2-ethyl-1-hexanol additive are considerably higher. For absorber temperatures higher than 83C the external coeffi
49、cients of performance using the 2-ethyl-1-hexanol additive are approximately 100% higher than those obtained without additive. 0.000.100.200.300.400.5075 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95Absorber temperature, TAB (C) COPEXT(dimensionless) G 2 -ethyl-1-hexanol (400pmm)without additive1-octanol (400pmm)Figure 5. COPEXT against TAB for the heat transfor
