1、4733 Simulation and Performance Analysis of Lithium BromideNVater for Absorption Heat Transformer Cycle Systems C.C. Chuang D.C. Sue ABSTRACT Waste heat resources can be used to drive absorption heat pumps (AHP) when operated in the reverse Carnot cycle. An AHPrecovers heat that otherwise wouldbe re
2、jected to the envi- ronment and it can produce cooling energy or recover heat for further use. The absorption heat transformer (AHT) is an inverted AHP; hence, an inverted reverse Carnot cycle can be applied. The AHThas excellentperformance recovering waste energy from the industrial process and inc
3、reasing aportion of that energy to a higher temperature. In the thermodynamics analyses, enem quality is more valuable than energy quantity. The first and second laws of thermodynamics are combined to evaluate the energy and develop guidance for improving the performance of the system. This study is
4、 based on using LiBr/ H20 solution as a working fluid for the performance evalua- tion of a single-effect AHT. Simulation of this AHT system to evaluate the performance at different operating conditions is analyzed in accordance with thefirst and second laws of ther- modynamics. The results can appl
5、y to the optimum operation and fundumentul design of an actual AHT system. INTRODUCTION Many power plants and industry processes release large quantities of low-temperature waste energy to the environ- ment. Not only do larger amounts of waste energy result in lower system efficiencies but also in g
6、reater thermal pollution of the environment. To achieve effective waste energy recov- ery and promote efficient energy utilization in the system, the temperature of at least a portion of the waste energy must be increased to a higher operating temperature. In order to recover the largest portion of
7、the waste energy, several systems are available. The absorption system is one of the best with Y.T. Lin many aL .rantages. For increased temperature application, Chuang et al. 1991). This study uses the thermal efficiency as determined by a second law thermodynamics analyses for an AHT system and ev
8、alu- ates a single-effect absorption heat transformer using a LiBr/ H,O solution as the working fluid. It also utilizes the second law of thermodynamics to analyze the variable data, including temperature of waste heat, temperature of cooling water, mass flow rate of solution, and efficiency and hea
9、t transfer perfor- mance of the solution heat exchanger, as related to efficiency of system performance, variation of heat recovery rate, and thermal efficiency. The results of the analysis can be provided as the theoretical basis for the actual AHT system optimum design and operation. OPERATING CYC
10、LE OF ABSORPTION HEAT TRANSFORMER A single-effect absorption heat transformer system and PTX cycle is shown in Figure 1. The basic AHT is composed High Temperature High Temperature Evaporator Absorber 13 19 ill Solution Heat I- ! LawTempemture Y ! 23-cct, 24 21 Figure 2 Double-effect absorption heat
11、 transformer system configuration. of generator, condenser, evaporator, absorber, and solution heat exchanger. In Figure 1, the waste energy from an indus- trial process is the heat source for the generator (G). The waste heat evaporates water from the dilute LiBr/H,O solution, increasing its concen
12、tration. The produced vapor passes from the generator to the condenser (C), where it is condensed by the shell side cooling water, forming condensate water. The condensate water is pumped to the evaporator (E) where it is converted to vapor using additional waste energy from the same source as the g
13、enerator. The vapor so formed passes over to the absorber. In the meanwhile, the concentrated LiBr/H,O solution in the generator is pumped to the solution heat exchanger (SHX) before entering the absorber (A). In the absorber, the concentrated solution absorbs and condenses the vapor from the evapor
14、ator, diluting the LiBr/H20 solution. The diluted solution is passed through the solution heat exchanger and enters into the generator, completing the cycle. The solution of LiBr/H,O in the absorber is heated by the condensing vapor releasing its heat of vaporization as well as the exothermic proces
15、s of water going into solution. The absorber reaches the highest temperature of the system as a result of these two effects. A separate fluid is circulated through the absorber to remove this recovered heat at as high a temperature as possible for further use. To achieve a higher final temperature,
16、a double-effect absorption heat transformer can be installed (Grossman 1985; Ziegler and Alefeld 1987). The double-effect absorption heat transformer is an expansion of the single-effect AHT system shown in Figure 1. The double-effect AHT is composed of two sets of absorbers, two sets of evaporators
17、, two sets of solution of heat exchangers, and one generator and one condenser, as 41 O ASHRAE Transactions: Research High Temperme High Tempmahire Evapornt or Abso the low- temperature absorber and low-temperature evaporator operate at an intermittent pressure, while the generator and condenser are
18、 at low pressure. The double-effect AHT operating cycle provides the waste energy source to the generator (G) for evap- orating the water out of heating the dilute LiBr/H20 solution. The vapor so formed passes to and is condensed in the condenser (C) by the tube side cooling water, becoming condensa
19、te water. The concentrated LiBr/H,O solution is pumped from the generator to the first solution heat exchanger (SHX2) after which a portion goes to the first absorber (A2) and the remainder goes to the second solution heat exchanger (SHXl) and enters the high-temperature absorber (Al). In the meanwh
20、ile, the condensate water is pumped from the condenser to the two evaporators (El and E2) connected in parallel. Water heat energy is provided to the low-temperature evaporator (E2) to evaporate the condensate water in it, and vapor formed passes to the low-temperature absorber (A2) that is absorbed
21、 by the concentrated solution from generator. The absorption process at the low temperature absorber (A2) produces absorption heat energy, which is recovered by the internal loop and pumped to the high-temperature evaporator (E 1) to evaporate the condensate water from the condenser. The evaporated
22、vapor is connected to the high-temperature absorber (AI), wherein the vapor from the evaporator is combined with the concentrated solution. Therefore, the high- temperature absorber (Al) can obtain a higher increment of temperature. By transferring the waste energy recovered in the low-temperature a
23、bsorber (A2) to the high-temperature evap- orator (EI) and subsequently transferring it to the high- temperature absorber (Al), the recovered heat is at a higher temperature. A triple-effect absorption heat transformer is needed to get an even higher temperature of the recovered energy. The triple-e
24、ffect AHT is composed of three sets of absorbers, evap- orators, and solution heat exchangers and one generator and one condenser. The system flow diagram is shown in Figure 3. The triple-effect AHT basic operation is same as the single- effects and double-effects ofAHT. For a triple-effect AHT, the
25、 waste energy is provided to the generator (G) and low-temper- ature evaporator (E3), and the cooling water is connected to condenser (C). The only difference is the distribution of the concentrated LiBr/H20 solution through the three solution heat exchangers and to the three absorbers, adding the i
26、nter- mediate and high-temperature evaporators (E2 and E i), which utilizes the absorbed energy from low and intermediate temperature absorbers (A3 and A2) via separate internal heat- ing loops, enabling the recovered energy to be increased by three stages. Using this three-stage approach, the high-
27、 temperature absorber (A 1) creates a highest temperature than otherwise would be available for the high-temperature recov- ery fluid. The typical process temperatures of double- and triple- effect AHTs can be achieved with the different systems. THERMODYNAMICS ANALYSES The mass balance and the firs
28、t and second laws of ther- modynamics are used to analyze the thermodynamics of an absorption heat pump. Each component can be considered as an inlet and outlet control volume with steady flow for its contribution to the overall system. The mass balance of the system is the sum of the mass balance o
29、f each element. In the steady flow system, conservation of mass yields the following equations: where m is the mass flow rate of solution, and Cis the concen- tration of LiBr solution. From the first law of thermodynam- ics, the energy balance of each component of the AHT can be calculated as where
30、Q is the heat transfer rate between control volume and surroundings, and + Wis work input in to the system. The coef- ficient of performance (COP) for an absorption system can be derived from the first law. It is defined as the ratio of recovered energy obtained from the system to the total energy i
31、nput to the cycle. ASHRAE Transactions: Research 41 1 RecoveredOupuEnergy Tot a llnpu t Energy copcYcle = Mass Flow Rate Waste energy provided to the I 4.0kgls (4) Operating Temperature Temperature of waste energy I 135C For the AHT, the recovered energy is obtained from the absorber. The COP is obt
32、ained by dividing the recovered energy by the total input waste energy to both generator and evaporator. Therefore, the COP for single-effect of AHT in Figure 1 can be written as follows: Absorber UA Condenser UA Evaporator UA Generator UA Solution Heat Exchanger Eaciency where m is the mass flow ra
33、te of working fluid at each state point, and h is the enthalpy for each state point as shown in Figure 1. The second law is used to determine the efficiency of an absorption system (Abrahamsson et al. 1994). The definition of efficiency is the ratio of the actual cycle COP to the ideal Carnot cycle
34、COP. 1 O.OkW/K 1O.OkWK 15.OkWX 25 .OkW/K 0.8 The ideal AHT cycle operates at temperatures between Te and Tc and Carnot heat machine is operated at temperature of Ta and Tg as a Carnot heat pump. The energy balance and entropy of the AHT cycle can be written as follows: Q,+Qc = Qe+Qg (7) absorber Coo
35、ling water provided to the condenser Waste energy provided to the Waste energy provided to the evaporator generator Solution leaving the generator sge, = Q A-+- Qe Qo+$=, Tg . , Tc provided to the absorber 4.0kg/s Temperature of cooling water 30C provided to the condenser 4.0kgIs Temperature of wast
36、e energy 1 20C 4.0kgls Temperature of waste energy 120C provided to the evaporator provided to the generator 1 .Okg/s where the subscripts of a, e, g, and c represent absorber, evap- orator, generator, and condenser, respectively. In the ideal cycle, all processes are reversible; therefore, in Equat
37、ion 8 entropy generation, Sgen, is equal to zero. In general, the generator and evaporator of AHT utilize the waste energy that comes from the same source. Hence, it can be assumed Tg = T, From Equations 5, 7, and 8 a Carnot COP value for an ideal AHT system can be derived as follows: This COP is ma
38、ximum that can be achieved by an AHT. ASSUMPTIONS FOR THE SIMULATION MODEL The thermodynamic properties of the working fluid have a major effect on the design, operation, and performance of the AHT. A LiBr/H20 solution is frequently used as the working fluid of an AHT. In order to calculate the syst
39、em COP and second law efficiency, we first calculate the thermodynamic properties of the LiBr/H20 solution. At normal AHT operat- ing conditions, the temperature of LiBr/H,O solution, pres- sure, enthalpy, and concentration can be related to each other using the thermodynamic property equations prov
40、ided in the ASHARE Handbook-Fundamentals (2001). lrvine and Liley (1984) developed the approximate equations for simulation of water vapor in the saturated and superheated states. This study developed a computer program to evaluate the variations in AHT performance for different operating conditions
41、. The results are similar to those obtained by other researchers using computer program analyses including Grossman et al. (1994), ORNL (1995), and Garimella et al. (1996). The major param- eters for inputting into this AHT computer program are listed in Tables 1 and 2, which includes the heat trans
42、fer character- istics, mass flow rate, external fluid design temperature, and circulating rate of the solution for sizing the solution heat exchanger. We made the following assumptions to simplify the simulation and analyses: 1. Neglect the heat loss and heat gain between the system and 2. Compared
43、to the quantity of waste heat entering the evap- orator and generator, the pump input work for condensate water and solution circulation can be neglected. The waste heat source supplying the generator and evapo- rator and the outlet fluid from the absorber are hot water at a constant pressure. The f
44、nction and pressure losses in components and piping are neglected. surroundings. 3. 4. Table 1. Single-Effect LiBr/H,O AHT Operating Parameters 41 2 ASHRAE Transactions: Research Table 2. Single-Effect LiBr/H,O Operating Parameters at Each State Point of Figure 1 Mil mUl State Points Wkgl Wsl 1 (Sol
45、ution entering generator) 277.3981 1.078 2 (Solution leaving SHX) 277.3981 1.078 PU1 Tul XVI Wal “Cl I% 16.054 102.8 59.4 104.076 124.7 59.4 3 (Solution leaving absorber and entering SHX) 4 (Solution leaving SHX and entering absorber) 332.7978 I .O78 104.076 153 59.4 329.9273 1 104.076 144.8 64 5 (S
46、olution is pumped to SHX) 270.205 1 1 104.076 112 64 6 (Solution comes from generator) 270.154 1 05 300 a Y 200 8 o d 0.4 16.054 112 64 110 122 133 145 7 (Vapor comes from generator to condenser) 8 (Condensate water is pumped from condenser) Waste Energy Temperature, (c) 2688.293 0.078 16.054 100.5
47、23 1.8397 0.078 16.054 55.4 Figure 4 Eflect of waste energy temperature on COP und Qabr 9 (Condensate water is pumped to evaporator) 10 (Vapor entering absorber) 5. 6. Use UA to represent the heat transfer characteristics of the various heat exchanger components, The mixtures of LiBr/H20 solution an
48、d water vapor are at equilibrium in the generator and absorbers. The AHT at actual operating conditions has heat losses from the components to the surroundings amounting to approximately 3% to 5% of the total heat input while the components have internal friction losses. These losses decrease system
49、 efficiency by increasing irreversibility. This effect is not discussed in the study. By inputting different oper- ating conditions into the computerized simulation, the result- ing performance of the AHT can be evaluated. 23 1.929 1 0.078 104.076 55.4 2676.872 0.078 104.076 100.8 O60 I O0 o 90 5 a O BO 8 O 70 O 60 o 55 r 8 050 l o O 45 O 40 0 50 110 122 133 145 -cop -Qabs 4oo Waste Energy Temperature,(“C) Figures Eficiency Et, and COP vs. waste energy temperature. RESULTS AND DISCUSSIONS Waste Heat Temperature vs. Performance Figure 4 shows the relationship of differen
