lable at ScienceDirect

Energy 68 (2014) 862e869

Contents lists avai

Energy

journal homepage: www.elsevier .com/locate/energy

Optimal ammonia water absorption refrigeration cycle withmaximum internal heat recovery derived from pinch technology

S. Du, R.Z. Wang*, Z.Z. XiaInstitute of Refrigeration and Cryogenics, Key Laboratory for Power Machinery and Engineering of M.O.E, Shanghai Jiao Tong University, 800 DongchuanRoad, Shanghai 200240, China

a r t i c l e i n f o

Article history:Received 17 November 2013Received in revised form13 February 2014Accepted 17 February 2014Available online 13 March 2014

Keywords:AmmoniaewaterAbsorption refrigeration cycleInternal heat recoveryPinch technologyPerformance analysis

* Corresponding author. Tel.: þ86 21 34206548.E-mail addresses: rzwang@sjtu.edu.cn, rzwang@ma

yahoo.com (R.Z. Wang).

http://dx.doi.org/10.1016/j.energy.2014.02.0650360-5442/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

Absorption refrigeration technology has attracted more and more interests due to its advantages such asmaking good use of low grade thermal energy and using environmental friendly refrigerants. The in-ternal heat recovery capacity of an ammonia water absorption refrigeration system has significant in-fluence on the system performance. According to cascaded utilization of energy to reduce the internalirreversible loss, this paper presents the optimal cycle with maximum internal heat recovery which isderived from a comprehensive method of pinch technology. The derivation of the optimal cycle isintroduced. The internal integration is clearly shown in a temperatureeheat load diagram. The optimalcycle derived from this method when there is a temperature overlap between the absorption andgeneration processes is exactly the GAX cycle. Performance analysis is carried out to discuss the per-formance improvement of the optimal cycle. The results show that the performance of the optimal cycleis enhanced significantly by 20% at least compared with a traditional one under common operatingconditions. The performance improvement of the optimal cycle is more significant at a lower evaporationtemperature and a higher generation temperature while it has a maximal value with the coolant tem-perature increasing.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Due to the serious energy and environment problems, absorp-tion refrigeration technology has attractedmore andmore interestsdue to its advantages such asmaking good use of lowgrade thermalenergy and using environmental friendly refrigerants. Ammoniaewater absorption cycle is well accepted for thermal driven refrig-eration below 0 �C. Moreover, it is an alternative option for air-cooled absorption system [1,2]. However, a main drawback of anammonia water absorption refrigeration system is its low COP(coefficient of performance). The internal heat recovery capacity ofa system has a significant effect on the system performance. Hence,enhancing the internal heat recovery capacity is an effective way toimprove the system COP. Usually, a SHE (solution heat exchanger)and a RHE (refrigerant heat exchanger) are used for the internalheat recovery in a traditional system. In addition, the rectificationheat can be recovered by the strong solution [3]. However, if the

il.sjtu.edu.cn, wangruzhusjtu@

total strong solution is used, the recovered heat does not increasemuch due to the temperature lift at the cold end of SHE. Thus thesystem performance is not improved obviously. Kang et al. [4]summarized the researches about the internal heat recovery ofammonia water absorption systems including condensation heatrecovery such as a multi effect cycle [5] and absorption heat re-covery such as a GAX (generator-absorber heat exchange) cycle [6].Saghiruddin et al. [7] analyzed an ammonia water cycle with a heatrecovery absorber. The strong solution was preheated by a part ofthe absorption heat. This method has the same drawback withrectification heat recovery by the total strong solution. Neverthe-less, these cycles were originally proposed for the recovery of onetype of the heat. There is no comprehensive planning in the internalheat recovery issue of a whole system. A common optimizationmethod for internal heat recovery is needed.

Pinch technology is a graphical method for efficient use of en-ergy [8]. The streams of a system are planned for maximum heatexchange. It is applicable for the internal heat recovery issue of anammonia water absorption system. However, it should be notedthat the generated vapor contains a certain amount of waterbecause the partial pressure of water cannot be neglectedcompared with ammonia [9]. If water flows into the evaporator, the

Nomenclature

ABS absorberCGCC column grand composite curveCOP coefficient of performanceCON condenserCSC cold stream curveCSCC cold stream composite curveD mass flow rate of the distillate, kg/sDC distillation columnF mass flow rate of the feed, kg/sGAX generator-absorber heat exchangeGEN generatorHSC hot stream curveHSCC hot stream composite curveL mass flow rate of the liquid, kg/sm mass flow rate, kg/sQ heat load, kWq parameter of feed conditionRHE refrigerant heat exchangerSHE solution heat exchangerT temperature, �CV mass flow rate of the vapor, kg/s

x mass fraction of the ammonia in the liquidy mass fraction of the ammonia in the vaporx* equilibrium mass fraction of ammonia in the liquid

phasey* equilibrium mass fraction of ammonia in the vapor

phase

Greekε performance improvement ratio

Subscriptsa absorberc condenserC coolingCGCC column grand composite curveD distillatedeficit deficite evaporatorF feedg generatorH heatingL liquidmin minimum

Fig. 1. Illustration of hot streams and the composite curves in a TeQ diagram.

S. Du et al. / Energy 68 (2014) 862e869 863

evaporating temperature will rise and the cooling effect will bereduced. Therefore, vapor purification is necessary. A distillationcolumn is commonly used for the vapor generation and purifica-tion. Commonly, themixturewith component separation cannot beconsidered as a stream directly in a pinch point analysis. Likewise,the process in the distillation column should also follow thatprinciple. As a result, a quasi-equilibrium process curve called CGCC(column grand composite curve) is applied to express the process.Dhole et al. [10] presented a method to calculate CGCC based on aPNMTC (practical near minimum thermal condition). PNMTC is aminimum loss condition after considering the inevitable lossescaused by feed condition, pressure drop, sharp separation andconfiguration selection. Bandyopadhyay et al. [11] presented twomethods to calculate CGCC and modified it at the feed stage basedon relative volatility. The CGCC is commonly used in the chemicalindustry field. But it has not been found in the research of ammoniawater absorption refrigeration systems.

Pinch technology is used in many applications such as thechemical industry field [12,13], heat exchanger network designs[14,15] and compound systems [16]. But few researches are foundin ammonia water absorption refrigeration systems. Hanna et al.[17] and Jawahar et al. [18,19] analyzed their proposed cycles withpinch technology. However, they analyzed the internal heat re-covery according to the conditions of the state points which arecalculated by a cycle simulation rather than derived the optimalcycle directly from pinch technology. And the processes withcomponent separation are not considered. In this paper, an optimalcycle withmaximum internal heat recovery is derived directly frompinch technology according to the operating conditions. The in-ternal heat recovery is planned as a whole and the heat integrationof the cycle is obtained. A performance analysis is carried out versusdifferent operating conditions.

2. Pinch technology

Pinch technology is a graphical method for process integration.Generally, a heat exchange process contains many hot and coldstreams which can be distinguished by whether they are to be

cooled or heated. The thermal characteristic of a stream can beexpressed well in a temperatureeheat load (TeQ) diagram as a HSC(hot stream curve) and a CSV (cold stream curve). An illustration ofhot streams is shown in the left of Fig. 1.

The heat load versus the temperature interval of a stream isshown. In the diagram, the value of the abscissa axis represents theheat load and the value of the vertical axis represents the tem-perature. The difference of the value of the abscissa axis is the heatload of the stream versus the temperature interval. Hence, a TeQcurve can be moved horizontally or mirror imaged with respect tothe vertical axis without affecting the temperature and the heatload of the stream. According to the thermal characteristics of thestreams, the hot and cold streams are combined to be a HSCC (hotstream composite curve) and a CSCC (cold stream composite curve),respectively. The composite process of the hot streams can be foundin Fig. 1. And the CSCC is obtained in the same way. Lines 1 and 2represent the HSCC and CSCC, respectively. The overlapped partbetween the HSCC and CSCC is the integrated process. The

Fig. 3. The schematic diagram of a traditional ammonia water absorption refrigerationsystem.

S. Du et al. / Energy 68 (2014) 862e869864

integration will be enlarged with moving one of them towards theother horizontally. When the two curves are overlapped to a point,the heat exchange reaches maximum. The coincident point is thepinch point where the temperature difference for heat transfer iszero. Actually, the heat transfer area will be infinite if the temper-ature difference is zero. Therefore, a minimum temperature dif-ference for heat transfer should be determined and the pinch pointof a real heat exchange process is where the minimum temperaturedifference appears. In this case, the overlapped part between theHSCC and CSCC represents the maximum internal heat exchangecapacity. The residual parts of the HSCC and CSCC are cooled andheated to the target temperatures by cooling and heating utilities,respectively.

When there are many streams, problem table algorithm isemployed [8]. In a problem table, the temperatures of the streamsshould be adjusted to ensure the temperature difference for heattransfer. Usually, the temperature of each cold stream is raised by 1/2dT and the temperature of each hot stream is reduced by 1/2dT. Thegraphical result of the problem table is the GCC (grand compositecurve) of the process [8]. A GCC for example is shown in Fig. 2.

Each point on the curve represents the net heat fluxwhich is theheating load or cooling load versus the temperature level. Thepinch point where the heat flux versus the temperature interval iszero can be easily obtained. The curve below the pinch point rep-resents a cooling process while the above represents a heatingprocess. The top of GCC represents the minimum heating loadwhile the bottom represents the minimum cooling load. The GCC isuseful to estimate the energy-saving target.

The processes of an ammonia water absorption refrigerationsystem can be considered as streams and illustrated in TeQ dia-grams for analysis with pinch technology.

3. Optimal cycle derivation

3.1. Description of a traditional system in a TeQ diagram

There are six primary processes of a single stage ammoniawaterabsorption refrigeration system including generation, rectification,condensation, expansion, evaporation and absorption. The sche-matic diagram of a traditional ammonia water absorption refrig-eration system is shown in Fig. 3.

In Fig. 3, GEN, CON, EVA and ABS represent the generator,condenser, evaporator and absorber, respectively. A DC (distillationcolumn) is used for the vapor purification and generation. A

Fig. 2. Illustration of GCC in a TeQ diagram.

refrigerant heat exchanger (RHE) and a solution heat exchanger(SHE) are employed for the internal heat recovery.

The primary processes of the system can be considered as hotand cold streams and expressed in a TeQ diagram. Among them,the process in DC is complex. It cannot be considered as a streamdirectly due to component separation. Therefore, the CGCC isemployed to describe the process. The CGCC is calculated from thetop to the bottom of the column by solving the mass and energybalance equations based on PNMTC with the method presented inRef. [11]. But the CGCC at the feed stage is modified by the feedequations rather than the relative volatility in this paper whichmakes it more simple and convenient. The minimum vapor flowrate and liquid flow rate and the heat deficit are obtained in thecalculation. For a binary mixture like ammoniaewater, the modelfor calculating CGCC is shown in Fig. 4.

According to the control volume as the dashed line showing, themass and energy balance equations are listed as follows:

Mass balance equations:

Vmin þ F ¼ Lmin þ D (1)

Vmin,y*þ F,xF ¼ Lmin,x*þ D,xD (2)

Energy balance equation:

Vmin,Hv þ F,HF þ Qdeficit ¼ Lmin,HL þ D,HD (3)

where Qdeficit represents the heat deficit. The vapor liquid equilib-rium equation is obtained from Ref. [20].

y* ¼ f ðp; x*Þ (4)

Solve the Eqs. (1)e(4), the CGCC is expressed as Eq. (5).

Fig. 4. The calculation model for CGCC of a traditional complete condensation distil-lation column.

Fig. 5. The illustration of CGCC with a minimum reflux ratio and saturated feedcondition.

Table 1The given operating conditions.

Vapor flow rate at the top of the column, m (kg/s) 0.034Temperature of the generated vapor, Tg (�C) 160Condensation temperature, Tc (�C) 34Temperature of the absorber outlet, Ta (�C) 34Evaporation temperature, Te (�C) �30Absorption starting temperature, Ta,start (�C) 62.1Generation starting temperature, Tg,start (�C) 114.8

S. Du et al. / Energy 68 (2014) 862e869 865

QCGCC ¼ QC þ Qdeficit (5)

At the feed stage, the feed equations are as follows.

y ¼ q,x=ðq� 1Þ � xF=ðq� 1Þ (6)

q ¼ ðHV � HFÞ=ðHV � HLÞ (7)

Hence, the value of the CGCC at the feed stage can be obtained.Then the complete CGCC with modification at the feed stage isachieved. Above the feed stage, the feed parameters are neglectedin the calculation. With assuming an initial value of QC, the result iscalculated by iteration until QCGCC is zero. It represents a minimumreflux ratio condition. According to the calculation above, theCGCCs with different feed conditions can be obtained. A CGCC witha minimum reflux ratio and saturated feed condition is illustratedin Fig. 5.

The point of intersection A between the vertical axis and thecurve is the pinch point of the column. Here, the net heat flux iszero which indicates that there is no need for heating or cooling.The point on the curve represents the heating or cooling loadversus the temperature level. Above the pinch point, the curve ABrepresents a process needed to be heated which can be consideredas a cold stream. Similarly, below the pinch point, the curve ACrepresents a process needed to be cooled which can be consideredas a hot stream. The terminal points B and C indicate the minimumheating and cooling loads, respectively. It should be noted thatneither the heating load nor the cooling load is necessary to beadded or removed at the highest or lowest temperatures. A part ofthe heating load can be added at a lower temperature level thus theheating process can be probably integrated with the other steams

which increases the internal heat recovery. It can be found fromFig. 5 that the cooling process is divided into two parts: region 1represents the rectification process and region 2 represents thecondensation process.

The residual processes are called background process andconsidered as cold or hot streams. Among them, the cold streamsinclude the evaporation process, the ammonia vapor from EVA andthe strong solution from ABS while the hot streams include theweak solution from GEN, the liquid ammonia from CON and theabsorption process. The heat characteristics of the streams areeasily obtained when the operating conditions of the cycle aredetermined. It should be noted that component separation alsooccurs in the absorption process as well as the distillation columnprocess. It cannot be considered as a stream directly. Therefore, thecurve of the absorption process is obtained by a calculation with anassumption of quasi equilibrium process. According to the calcu-lation above, the column process curve is expressed. And the heatload versus the temperature interval of each stream of the back-ground process is also obtained. Then the cycle can be analyzed.

It is necessary to know the operating conditions when a prac-tical analysis is performed with pinch technology. Therefore, theoperating conditions of a 40 kW cooling capacity gas driven ice-maker are used as a typical example to explain the method in anammonia water absorption refrigeration system. The traditionalsystem configuration is shown in Fig. 3. The operating conditionsare shown in Table 1. The saturated temperatures are obtained fromthe equations in Ref. [20].

According to the operating conditions, the heat load versus thetemperature interval of every stream of the traditional system isdetermined with a cycle simulation. The streams of the traditionalammonia water absorption cycle are illustrated as shown in Fig. 6.The minimum temperature difference for heat transfer is set as10 �C.

The dash lines represent the cold streams and the full linesrepresent the hot streams. The CGCC can be described as two

Fig. 6. A traditional ammonia water absorption cycle in a TeQ diagram.

Table 2The streams of an ammonia water absorption cycle.

Number Stream Type Initialtemperature(�C)

Targettemperature(�C)

1 Vapor generation process Cold 114.8 1602 Strong solution from ABS Cold 34 114.83 Vapor from EVA Cold �30 62.14 Evaporation process Cold �30 �305 Weak solution from GEN Hot 160 62.16 Absorption process Hot 62.1 347 Liquid ammonia from CON Hot 34 �308 Rectification process Hot 114.8 349 Condensation process Hot 34 34

S. Du et al. / Energy 68 (2014) 862e869866

sections including a cold stream of generation process and a hotstream of rectification and condensation process. Lines 1, 2, 3 and 4represent 1.the vapor generation process, 2.the strong solutionfrom ABS to DC, 3.the vapor ammonia from EVA to ABS and 4.theevaporation process, respectively. Similarly, lines 5, 6, 7, 8 and 9represent 5.the weak solution from GEN to ABS, 6.the absorptionprocess curve, 7.the condensation process, 8.the liquid ammoniafrom CON to EVA and 9.the rectification process, respectively.Actually, the cooling effect is the profit and it should not be inte-grated with the hot streams. Moreover, the temperatures of stream3 and stream 8 are lower than the condensation temperature. Thetwo streams have nothing to do with the cooling utility and shouldbe considered independently. An RHE is suitable for heat recoverybetween the two streams. A solution heat recovery process isexpressed by line 2 and line 5. It can be found that the rectificationprocess is not integrated with the internal cold streams. Therectification heat is removed by the condensate directly. It isequivalent to cooling the rectification process by a coolant withconstant temperature. The temperature difference for heat transferis large. Actually, the rectification heat could be used to heat thecold streams and better internal heat integration could be obtained.It can be found that the minimum temperature difference exists atthe cold end of the solution heat exchanger. Line 6 represents aquasi-equilibrium absorption process curve thus line 6 and line 5are not connected end to end. It indicates that the weak solution isnot necessary to be cooled so much. The location of the minimumtemperature difference could be lifted to the absorption startingtemperature. Therefore, the strong solution could be heated by apart of the absorption heat. And the integration between the hotand cold streams could be improved. Moreover, it is learnt fromFig. 5 that the strong solution is heated to the overheated state bythe weak solution. The temperature difference increases after thestrong solution is overheated because the latent heat is large. Thisindicates that the grade of the weak solution is not made good use.From the above, the internal heat recovery of a traditional ammoniawater absorption refrigeration system is simple and not properlyarranged. Although there are some measures to improve it, themeasures are various and focus on one type of the heat usually.Therefore, the internal heat recovery should be planned as a wholefor improving the system performance.

3.2. Optimal cycle derivation

There is an optimal cycle with maximum internal heat recoveryof a single stage ammoniaewater absorption refrigeration system.

The optimal cycle can be derived from Pinch analysis. It is the keyissue to obtain the pinch point. The pinch point of the optimal cyclecan be obtained by problem table algorithm. It should be noted thatthe heat capacity flow rate of streams are usually constant whenproblem table algorithm is applied. However, the generation, ab-sorption and rectification processes cannot be considered asstreams with constant heat capacity flow rate. But the problemtable algorithm can be used if the change of heat capacity flow ratedoes not affect the position of the pinch point which has beenderived from the streamswith constant heat capacity flow rate. Thetemperature difference of the pinch point is also set as 10 �C. And asaturated feed condition is assumed due to the minimum heat andmass transfer loss. The streams of an ammonia water absorptioncycle are listed in Table 2. And the problem table is shown inTable 3.

In Table 2, the temperature interval where the heat flux is zero isthe pinch point of the cycle. And the heat input of the cycle can bealso obtained directly from the table. The difference between theheat input and heat output is due to the errors of calculation such asrounding error. After the pinch point of the cycle is obtained, theinternal heat integration of the cycle should be known to determinethe cycle configuration. Due to the streams of the cycle are notcomplex, the integration can be obtained from the problem tabledirectly according to the stream distributions within the temper-ature intervals. It can be found from Table 2 that the streams whosetemperatures are lower than the condensation temperature are thesame as a traditional system. Therefore, only the streams whosetemperatures are higher than the condensation temperature areillustrated. In addition, the ammonia vapor from the refrigerantheat exchanger is to be heated to satisfy the assumption in thecalculation of the absorption process curve. However, the sensibleheat of it is very small thus it is neglected. The integration isillustrated in the TeQ diagram as shown in Fig. 7.

For a convenient analysis and a clear graphic representation, theCGCC is shown on the right of the vertical axis and the other pro-cesses can be easily recognized in Fig. 7. Lines 1, 2, 5, 6, 8 and 9represent 1.the generation process, 2.the strong solution from ABS,5.the weak solution from GEN, 6.the absorption process, 8.therectification process and 9.the condensation process, respectively.The heat exchange is clear that the strong solution is divided intotwo branches: one to cool the rectification process and the other toexchange heat with the hot streams including the weak solutionand the absorption process. And then the two branches are mixedand continue to be heated by the weak solution to the saturatedstate for feeding. The hot stream at the right of the vertical axis isthe high temperature weak solution which is integrated with thegenerating process. It reduces the heat input thus improving thecycle COP. In Fig. 7, Qg is the minimum external heat input of gen-eration process. Qc1 and Qc2 are the minimum absorption heat andcondensation heat, respectively. The sum of the overlapped parts

Table 3The problem table for the energy target of the system.

T (oC) Streams

Columnheatload(kW)

Background process

Deficientheat(kW)

Accumulated heat heat flux

heatload of

cold stream(kW)

heatload of

hot stream(kW)

heat input(kW)

heat output(kW)

heat input(kW)

heat output(kW)

16522.5 22.5 0 -22.5 71.5 49

15567.9 21.1 46.8 -22.5 -69.3 49 2.2

119.88.1 5.9 2.2 -69.3 -71.5 2.2 0

109.814 33.6 24.8 -5.2 -71.5 -66.3 0 5.2

67.14.4 22 62.1 -44.5 -66.3 -21.8 5.2 49.7

393.5 1.1 25.5 -27.9 -21.8 6.1 49.7 77.6

2935.5 -35.5 6.1 41.6 77.6 113.1

293.9 8.7 -4.8 41.6 46.4 113.1 117.9

-2545 45 46.4 1.4 117.9 72.9

-25

S. Du et al. / Energy 68 (2014) 862e869 867

between the hot and cold streams is the internal recovered heat ofthe cycle. This is the optimal ammonia water absorption cyclederived from pinch technology according the given operatingconditions. In this cycle, the rectification process is cooled by a partof the strong solution. Therefore, the reflux of the condensate is notnecessary thus there is no problem of controlling the reflux ratio. Inaddition, the strong solution can be still heated by the weak solu-tion at the cold end of the solution heat exchanger. This is becausethe minimum temperature difference exists at the hot end. Itsimplifies the heat exchanger network.

Fig. 7. The internal integration of the optimal cycle.

It is concluded from Figs. 6 and 7 that the internal recoveredheat is enlarged due to the integration. The rectification heat re-covery is a main feature. However, it is not necessary under someoperating conditions. When the evaporation temperature rises to2 �C while the other conditions are not varied, the optimal cycle isalso obtained with problem table and illustrated in Fig. 8.

The processes below the condensation temperature are also assame as a traditional system and neglected in Fig. 8. Except theweak solution, a part of the absorption heat is integrated with thegenerating process because there is a temperature overlap between

Fig. 8. The integration of the optimal cycle when the evaporation temperature risesto 2 �C.

Fig. 9. The COP and ε profiles versus the high temperature.

S. Du et al. / Energy 68 (2014) 862e869868

the absorption and generation processes. The pinch point is be-tween the saturated temperature of the strong solution and thecorresponding absorption temperature. In this case, the location ofthe pinch point and the quantity of the integrated heat will not beaffected whether the rectification heat is recovered or not. There-fore, it is not necessary to recover the rectification heat because it isthe low grade heat to be released. Actually, the derived optimalcycle is exactly the GAX cycle which is the optimized cycle pro-posed previously in other literature [21,22].

From the above, it can be known that the configuration of theoptimal cycle is not unique and determined according to theoperating conditions. GAX cycle is the result derived from thismethod when there is a temperature overlap between the gener-ation and absorption processes which occurs under air-conditioning operating conditions usually. It verifies the feasi-bility of the method which is generally applicable for the internalheat recovery issue of an ammonia water absorption system.Therefore, the optimal cycle is determined definitely once theoperation conditions are given.

Fig. 10. The COP and ε profiles versus the mid temperature.

4. Performance analysis and discussions

According to the analysis above, an optimal ammonia waterabsorption cycle with maximum internal heat recovery can beobtained under different given operating conditions. In order toinvestigate the performance of the optimal cycle, the COPs of theoptimal and traditional systems and the ratio of performanceimprovement (ε) are calculated.

COP ¼ Qe=Qg (8)

ε ¼ ðCOPo � COPtÞ=COPt � 100% (9)

COPo and COPt represent the optimal system COP and the tradi-tional system COP, respectively.

In Eq. (8), Qe is the cooling effect and Qg is the heat input in thegeneration process. It is obtained by the result of the cycle simu-lation for the traditional system and the problem table for theoptimal cycle. A minimum reflux ratio and the same minimumtemperature difference for heat transfer are set in the traditionalcycle simulation. The minimum reflux ratio condition of thedistillation column is the ideal one in a traditional system. And theassumption is identical with that in the optimal cycle which isderived from problem table so that the comparison is fair. In theanalysis, the condensation temperature and the temperature ofabsorption finished are considered as mid temperature. Similarly,the temperature of generation finished is considered as high tem-perature and the evaporation temperature as low temperature.There are two configurations of the optimal cycle as shown inFigs. 7 and 8 according to the operating conditions. The variationsof the COP and ε versus the high, mid and low temperatures areshown in Figs. 9e11, respectively.

It can be found from Fig. 9 that both the COPs of the optimal andtraditional cycles first increase rapidly with the high temperatureincreasing and then keep stable. This is because the deflation ratioincreases with the high temperature increasing and then keepsstable for both the optimal and traditional cycles. It can be foundthat the COP of the traditional cycle decreases slightly with the hightemperature increasing when the COP reaches stable. The reason isthat the rectification heat load increases with the high temperatureincreasing. And the effect of the increase of deflation ratio is nolonger significant. Thus the heat input increases. However, thetrend does not appear in the optimal cycle because the rectificationheat is utilized properly. The variation of ε is similar with the COP of

the optimal cycle. During the stable region of COP, the performanceof the optimal cycle is enhanced significantly more than 20%.

As is shown in Fig. 10, the COPs of both cycles decrease with themid temperature increasing and the variation tends to be intense.This is because the deflation ratios of the cycles decrease with themid temperature increasing. The ε first increases and then de-creases sharply. This indicates there is a critical value of the midtemperature. Below the critical value, the performance of theoptimal cycle is improved more obviously with the mid tempera-ture increasing due to the heat recovery capacity. However, theinfluence of the heat recovery capacity on the performance isweakened above the critical value. The COPs of the cycles come tobe close. There is an optimal value of ε with the mid temperatureincreasing. Similarly, the performance of the optimal cycle isenhanced more than 20% under usual operating conditions.

It can be known from Fig. 11 that the COPs of the optimal andtraditional cycles increase with the low temperature increasing.And after the low temperature rises to a certain value, the COP ofthe optimal cycle increases rapidly. This is because the optimalcycle of GAX cycle is applied in this case. The ε first decreases andthen increases rapidly with the low temperature increasing. It isindicated that the performance improvement of the optimal cycle ismore significant at a lower evaporation temperature. And whenthere is a temperature overlap between the absorption and gen-eration processes, the optimal cycle of GAX cycle performs muchbetter than a traditional cycle. On the whole, the performance

Fig. 11. The COP and ε profiles versus the low temperature.

S. Du et al. / Energy 68 (2014) 862e869 869

improvement is also above 20%. In a real system, the distillationcolumn can be replaced by heat exchangers such as plate heatexchanger so that the side cooling and heating can be achieved. Theoptimal system can be implemented according to the analysis andthe pinch point can be self-adapting and stable. The specific can bediscussed further in the next.

5. Conclusions

This paper presents the optimal ammonia water absorptioncycle for the maximum internal heat recovery with pinch tech-nology which is a comprehensive planning method for heat inte-gration. The derivation of the optimal cycle is introduced and theinternal integration of the optimal cycle is clearly shown in a TeQdiagram. Performance analysis is carried out to discuss the per-formance improvement of the optimal cycle. The results are listedas follows:

1. Pinch technology is generally applicable for the internal heatrecovery issue of an ammonia water absorption system.

2. Under the given conditions, the optimal cycle with maximuminternal heat recovery is derived. When there is a temperatureoverlap between the absorption and generation processes, theoptimal cycle derived from the method is exactly the GAX cycle.

3. A TeQ diagram is clear and convenient to carry out the analysis.4. The performance of the optimal cycle is enhanced significantly

more than 20% compared with a traditional one under widelyoperating conditions.

5. The performance improvement of the optimal cycle underrefrigeration conditions is more significant at a lower evapora-tion temperature and a higher generation temperature while ithas a maximal value with the coolant temperature increasing.

Acknowledgments

This paper is supported by The National Basic Research Programof China (973 project) under the contract 2010CB227301. Thesupport from the Key project of the Natural Science Foundation ofChina for international academic exchanges under the contract No.51020105010 is also appreciated.

References

[1] Lin P, Wang RZ, Xia ZZ. Numerical investigation of a two-stage air-cooledabsorption refrigeration system for solar cooling: cycle analysis and absorp-tion cooling performances. Renew Energy 2011;36(5):1401e12.

[2] Du S, Wang RZ, Lin P, Xu ZZ, Pan QW, Xu SC. Experimental studies on an air-cooled two-stage NH3-H2O solar absorption air-conditioning prototype. En-ergy 2012;45(1):581e7.

[3] Chua HT, Toh HK, Ng KC. Thermodynamic modeling of an ammoniaewaterabsorption chiller. Int J Refrig 2002;25(7):896e906.

[4] Kang YT, Kunugi Y, Kashiwagi T. Review of advanced absorption cycles: per-formance improvement and temperature lift enhancement. Int J Refrig2000;23(5):388e401.

[5] Garimella S, Lacy D, Stout RE. Space-conditioning using triple-effect absorp-tion heat pumps. Appl Therm Eng 1997;17(12):1183e97.

[6] Jawahar CP, Saravanan R. Generator absorber heat exchange based absorptioncycleda review. Renew Sustain Energy Rev 2010;14(8):2372e82.

[7] Saghiruddin, Siddiqui MA. Economic analyses and performance study of threeammonia-absorption cycles using heat recovery absorber. Energy ConversManage 1996;37(4):421e32.

[8] Kemp IC. Pinch analysis and process integrationIn A user guide on processintegration for the efficient use of energy. 2nd ed. Elsevier; 2006.

[9] Fernández-Seara J, Sieres J. The importance of the ammonia purificationprocess in ammoniaewater absorption systems. Energy Convers Manage2006;47:1975e87.

[10] Dhole V, Linnhoff B. Distillation column targets. Comput Chem Eng 1993;17:549e60.

[11] Bandyopadhyay S, Malik RK, Shenoy UV. Temperatureeenthalpy curve forenergy targeting of distillation columns. Comput Chem Eng 1998;22(12):1733e44.

[12] Ficarella A, Laforgia D. Energy conservation in alcohol distillery with the appli-cation of pinch technology. Energy Convers Manage 1999;40(14):1495e514.

[13] Panjeshahi MH, Langeroudi EG, Tahouni N. Retrofit of ammonia plant forimproving energy efficiency. Energy 2008;33(1):46e64.

[14] Lara Y, Lisbona P, Martínez A, Romeo LM. Design and analysis of heatexchanger networks for integrated Ca-looping systems. Appl Energy2013;111:690e700.

[15] Wang YF, Feng X, Cai Y, Zhu MB, Chu KH. Improving a process’s efficiency byexploiting heat pockets in its heat exchange network. Energy 2009;34(11):1925e32.

[16] Domenichini R, Gallio M, Lazzaretto A. Combined production of hydrogen andpower from heavy oil gasification: pinch analysis, thermodynamic and eco-nomic evaluations. Energy 2010;35(5):2184e93.

[17] Hanna WT, Wilkinson WH, Saunders JH, Philips DB. Pinch-point analysis: anaid to understanding the GAX absorption cycle. ASHRAE Trans 1995;101(1):1189e98.

[18] Jawahar CP, Raja B, Saravanan R. Thermodynamic studies on NH3eH2O ab-sorption cooling system using pinch point approach. Int J Refrig 2010;33(7):1377e85.

[19] Jawahar CP, Saravanan R. Experimental studies on air-cooled NH3-H2O basedmodified GAX absorption cooling system. Int J Refrig 2011;34(3):658e66.

[20] Ziegler B, Trepp Ch. Equation of state for ammonia water mixtures. Int J Refrig1984;7(2):101e6.

[21] Engler M, Grossman G, Hellmann HM. Comparative simulation and investi-gation of ammonia-water: absorption cycles for heat pump applications. Int JRefrig 1997;20(7):504e16.

[22] Srikhirin P, Aphornratana S, Chungpaibulpatana S. A review of absorptionrefrigeration technologies. Renew Sustain Energy Rev 2001;5(4):343e72.