Performance research of self regenerated absorption heat transformer cycle using TFE-NMP as working fluids

International Journal of Refrigeration 24 (2001) 510±518

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Performance research of self regenerated absorption heat transformer cycle using TFE-NMP as working ¯uids Xu Shiming *, Liu Yanli, Zhang Lisong Department of Power Engineering, Dalian University of Technology, Liao Ning, 116024, PR China Received 26 January 2000; received in revised form 8 August 2000; accepted 8 August 2000

Abstract A heat transformer is proposed in order to upgrade low-temperature-level energy to a higher level and to recover more energy in low-temperature-level waste heat. It is dicult to achieve both purposes at the same time using a conventional heat transformer cycle and classical working pairs, such as H2O±LiBr and HN3±H2O. The new organic working pair, 2,2,2-tri¯uoroethanol (TFE)-N-methylpyrolidone (NMP), has some advantages compared with H2O± LiBr and NH3±H2O. One of the most important features is the wide working range as a result of the absence of crystallization, the low working pressure, the low freezing temperature of the refrigerant and the good thermal stability of the mixtures at high temperatures. Meanwhile, it has some negative features like NH3±H2O. For example, there is a lower boiling temperature di€erence between TFE and NMP, so a recti®er is needed in refrigeration and heat pump systems. Because TFE±NMP has a wide working range and does not cause crystallization, it can be used as the working pair in the self regenerated absorption heat transformer (SRAHT) cycle. In fact, the SRAHT cycle is the generator±absorber heat exchanger (GAX) cycle applied in a heat transformer cycle. In this paper, the SRAHT cycle and its ¯ow diagram are shown and the computing models of the SRAHT cycle are presented. Thermal calculations of the SRAHT cycle under summer and winter season conditions have been worked out. From the results of the thermal calculations, it can be found that there is a larger temperature drop when the waste hot water ¯ows through the generator and the evaporator in the SRAHT cycle but the heating temperature can be kept the same. That means more energy in the waste heat source can be recovered by the SRAHT cycle. # 2001 Elsevier Science Ltd and IIR. All rights reserved. Keywords: Refrigerating system; Absorption system; TFE; Absorbent; Operation; GAX; Heat recovery

Etude sur la performance d'un cycle aÁ absorption de transformation de chaleur avec autoreÂgeÂneÂration utilisant le TFE et le NMP comme ¯uides actifs ReÂsume Les auteurs ont deÂveloppe un transformateur de chaleur concu pour reÂcupeÂrer de la chaleur aÁ basse tempeÂrature a®n de fournir de l'eÂnergie aÁ une tempeÂrature plus eÂleveÂe. Il est dicile d'accomplir cela aÁ l'aide d'un cycle aÁ transformation de chaleur et des couples actifs classiques tels que le H2O-LiBr et NH3-H2O. Le nouveau couple actif 2,2,2-tri¯uoroeÂthanol (TFE)-N-meÂthylpyrrolidone (NMP) posseÁde un certain nombre d'avantages par rapport aÁ H2O-LiBr et NH3-H2O. Une * Corresponding author. E-mail address: [email protected] (X. Shiming). 0140-7007/01/$20.00 # 2001 Elsevier Science Ltd and IIR. All rights reserved. PII: S0140-7007(00)00071-2

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caracteÂristique importante du nouveau couple actif est son large eÂventail graÃce aÁ l'absence de cristallisation, une pression de fonctionnement basse, un point de congeÂlation du frigorigeÁne bas et une bonne stabilite des meÂlanges aÁ des tempeÂratures eÂleveÂes. Cependant, tout comme le NH3-H2O, le nouveau couple actif posseÁde eÂgalement certains deÂsavantages. Par exemple, la di€eÂrence entre les points d'eÂbullition de TFE et NMP est moins importante et il est neÂcessaire d'avoir recours aÁ un recti®cateur dans les systeÁmes frigori®ques et aÁ pompe aÁ chaleur. GraÃce au large eÂventail de fonctionnement et l'absence de cristallisation, le couple actif TFE-NMP peut eÃtre utilise dans un cycle aÁ absorption de transformation de chaleur avec autoreÂgeÂneÂration (SRAHT). En reÂaliteÂ, le cycle SRAHT est un cycle GAX applique dans un cycle de transformation de chaleur. Dans cette communication, on montre le cycle SRAHT et des modeÁles de calcul. On preÂsente eÂgalement des calculs thermiques du cycle SRAHT sous des conditions rencontreÂes en eÂte et en hiver. A partir des reÂsultats des calculs thermiques, on a vu que baisse de tempeÂrature est plus importante lorsque l'eau chaude de reÂcupeÂration est en eÂcoulement dans le geÂneÂrateur et l'eÂvaporateur dans le cycle SRAHT en maintenant la meÃme tempeÂrature de chau€age. Cette approche permet de reÂcupeÂrer davantage d'eÂnergie aÁ partir d'une source de chaleur reÂcupeÂreÂe. # 2001 Elsevier Science Ltd and IIR. All rights reserved. Mots cleÂs : systeÁme frigori®que ; systeÁme aÁ absorption ; TFE ; absorbant ; fonctionnement ; conception ; GAX ; reÂcupeÂration de chaleur

Nomenclature cp f g h L m p q rth t T
t x
x y

heat capacity (kJ/kg ) solution circulation ratio (kg/kg TFE) vapor ¯ow ratio (kg/kg TFE) enthalpy (kJ/kg TFE) potential heat (kJ/kg TFE) mass ¯ow rate (kg/s) pressure (kPa) quantity of heat exchange (kJ/kg TFE) theoretical re¯ux ratio (kg/kg TFE) temperature ( C) temperature (K) temperature di€erence ( C) mass concentration of TFE in solution (%) mass concentration di€erence (%) mass concentration of TFE in vapor (%)

1. Introduction Absorption heat pump technology, which can utilize all kinds of waste heat emitted into the environment, is an e€ective method to reduce CO2 discharge and to protect our environment. However, to date, the absorption heat pumps using NH3±HO2 and H2O±LiBr as working pairs have some defects. In NH3±HO2 absorption refrigeration/heat pump systems, ammonia is toxic and can cause irritation, and it also corrodes copper. When it is used in heat transformers as a working ¯uid, the application range is limited because of its high working pressure and poor safety level. In H2O±LiBr absorption refrigeration/heat pump systems, a higher evaporation temperature, strong corrosion at high temperatures and the problem of crystallization also restrict its applications. Therefore, many researchers have been

Subscripts 1±9 AHXA AHXG ATA AHT c e G HEG L r SRAHT SHE V w wh

status points in Fig. 2 absorbing heat exchange absorber absorbing heat exchange generator absorbing temperature ampli®er absorption heat transformer condenser evaporator generator heat exchange generator liquid rich solution self-regenerated absorption heat transformer solution heat exchanger vapor weak solution waste heat

actively studying and developing various types of absorption heat pump systems using other working ¯uids [1±7] and have made some progress [8±10]. The investigation and development of the absorption refrigeration/heat pump system with the working pair of TFE±NMP is concentrated upon due to its advantages such as the wide working temperature range, lower working pressure, good safety level and so on. TFE has some shortcomings, such as a certain toxicity and low potential heat by contrast with water and ammonia, and the low coecient of performance (COP) value by using the refrigeration/heat pump systems of a conventional single stage cycle. However, TFE±NMP has a large concentration di€erence, so the COP value or the temperature lift amplitude of absorption refrigeration/heat pump systems can be improved by a GAX cycle, regenerative absorption cycle (RA), SRAHT cycle and so forth.

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2. SRAHT cycle In fact, SRAHT cycle is the GAX applied in heat transformer cycle and was originally introduced in 1989 [1]. It uses some heat energy of the absorption process as heat generation by taking advantage of some features that the solution of TFE±NMP has no limitation of crystalline and larger solution concentration di€erence in the cycle. Thus, the temperature lift amplitude in heat transformers can be raised and more heat energy will be recovered from the waste heat source. As shown in SRAHT cycle ¯ow diagram (Fig. 1), the absorber consists of three parts: (1) a solution temperature ampli®er (STA) in which refrigerant vapor is absorbed by the super-cooling solution compressed by solution pump (p2) after leaving the generator and the solution gets a raise in temperature; (2) an absorbing temperature ampli®er (ATA) in which the heating medium has a temperature lift by receiving absorption heat; (3) an absorbing heat exchange absorber (AHXA) in which absorption heat is taken as generation heat. The generator is made up of two parts; one is the heat exchange generator (HEG) heated by an outer heat source, and the other is an absorbing heat exchange generator (AHXG) heated by absorption heat. Because there is only 129 C di€erence in standard boiling points

between TFE and NMP, the rectifying unit is needed in the cycle. The refrigerant, which is condensed liquid in condenser (COND), is compressed by a pump (p1) to the solution heat exchanger (SHE). It exchanges heat with a high temperature rich solution at the end of absorption process and then enters the evaporator (EVAP). Passing through the throttle valve, the rich solution at a decreased temperature ¯ows into the recti®er (REC). Due to the larger temperature lift obtained by the SRAHT cycle, the waste heat is reused as much as possible by reducing the temperature of the waste heat as low as possible through the SRAHT cycle. According to the characteristics of the SRAHT cycle, waste hot water ¯ows in series connection. The waste hot water with a higher temperature goes in HEG ®rst and releases some quantity of heat, then it enters the evaporator to give o€ further heat in order to recover as much waste heat as possible. In the ®gure of p±T (Fig. 2), the rich solution at absorber outlet reaches status (1) from status (7) by lowering the temperature and pressure, then it goes to status (2) through being heated by an external heat source. Because the heat exchange between AHXA and AHXG needs a certain temperature di€erence, the temperature (t7) of the rich solution at the absorber outlet

Fig. 1. SRAHT cycle ¯ow diagram. Fig. 1. ScheÂma du cycle SRAHT.

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Fig. 2. SRAHT cycle in p±T diagram. Fig. 2. Cycle SRAHT et diagramme p±T.

must be higher than that (t2) of the solution heated by the external heat source. The solution in AHXG receives the heat from AHXA, then its temperature increases continually and the concentration decreases and ®nally it comes to status (3) at the generator outlet. After the weak saturate solution at the generator outlet ¯ows through the solution pump, it is in a super-cooling status because it is basically unchanged in temperature and pressure lift. Then the super-cooling solution with a strong absorption capacity enters the STA and its temperature and concentration increases by absorbing refrigerant vapor from the evaporator. If the pump power is ignored, theoretically the enthalpy value of the solution just leaving the STA can reach status (5), whose enthalpy value is the sum of the weak solution enthalpy value at status (3) and the enthalpy value vapor (g5) from the evaporator. Obviously, status (4) cannot occur in the SRAHT cycle. In the ATA, the heating medium receives the heat energy released by the solution in the process of vapor absorption and has a temperature lift. The temperature (t6) of status (6) must be larger than temperature (t3) of status (3) due to the solution heat transfer requirement.

heat transfer. For the binary working pairs, if two independent thermodynamic property parameters such as temperature and pressure are known, the others can be worked out through thermodynamic property relations of the binary working pairs. Therefore, the solution thermodynamic property parameters of status (7), (1) and (2) can be ascertained. To determine status (3) of the weak solution at the generator outlet, the weak solution concentration can be obtained by assuming the cycle concentration di€erence between the rich and weak solutions. The thermodynamic computation of the cycle can be conducted after obtaining parameters of these status points. If mass concentration of the vapor at the recti®er outlet is equal to 100% in the condition of ideal recti®cation, the solution circulation ratio of the SRAHT cycle is [11]:

3. The computerized models of SRAHT theoretical cycle

qHEG ˆ hV 9 ‡ …f ‡ rth

The condensation temperature (tc) and evaporation temperature (te) of the refrigerant can be obtained according to the temperature of the cooling water, the temperature of the waste heat and the temperature difference of the heat transfer. For pure refrigerants, the condensation pressure (pc) and evaporation pressure (pe) can be determined in accordance with the condensation and evaporation temperature. The pressure drop in the ¯owing process can be ignored for an ideal cycle, and so the temperatures t2 and t7 can be preliminarily de®ned on the basis of the temperature di€erence demand of

f ˆ …1

xw †=…xr

xw †

…1†

It is dicult to get solution status parameters at the recti®er outlet, so REC and HEG are combined as a black box model to do the heat balance calculation. The quantity of heat exchange in HEG is as follows:

g2 ˆ …rth ‡ f†…x1 g3 ˆ 1 ‡ rth

x2 †=…1

g2

g2 †
hL2 x2 †

g3
hV3

f
hL1

…2† …3† …4†

As Eq. (1), under an ideal recti®cation condition, the theoretical re¯ux ratio in the recti®er is [11]: rth ˆ …1

yr †=…yr

xr †

…5†

Without enthalpy computing relations of the mixture vapor of the binary working pairs, the mixture vapor

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from the generator can be regarded as the pure refrigerant vapor in a superheated state. In view of the small di€erence in potential heat between TFE and NMP, and very little content of NMP compared with TFE in mixture vapor, the computation error with this method is very small. hV2 ˆ hV9 ‡ cpV ‰…t1 ‡ t2 †=2

t9 Š

…6†

hV3 ˆ hV9 ‡ cpV ‰…t2 ‡ t3 †=2

t9 Š

…7†

The quantity of heat exchange between AHXG and AHXA is expressed as: qAHXG ˆ …f

1†
hL3 ‡ g3
hV3

qAHXA ˆ …f

g7 †
hL6 ‡ g7
hV8

g7 ˆ 1

g6

g5

…f ‡ rth

g2 †
hL2

f
hL7 ˆ qAHXG

…8† …9† …10†

where Eq. (9) is an implicit function and can be solved by iteration. The heat balance equation of STA is: g5
hV8 ‡ …f g5 ˆ …f

1†
hL3

1†
…x5

…f

xw †=…1

1 ‡ g5 †
hL5 ˆ 0 x5 †

g7 †
hV8 ‡ …f

…12†

1†
hL3

…f

g7 †
hL6

…13†

or qATA ˆ hV8 ‡ …f g6 ˆ …f

1†
hL3

1 ‡ g5 †…x6

f
hL7

x5 †=…1

qAHXA

x6 †

qe ˆ hV8

hL9

hL1 †

…17†

…18†

The coecient of performance of the SRAHT cycle is COP ˆ qATA =…qe ‡ qHEG †

…19†

4. Thermodynamic property relation formula of TFE± NMP According to Ref. [12], the thermodynamic property relations of the binary working pair TFE±NMP are as follows. The relation between the saturated pressure (p), temperature (T) and mass concentration (x) of TFE±NMP solution is: log10 …p† ˆ A ‡ B=…T

43:15†

…20†

4 X bi
xi

…22†

Where Aˆ

4 X ai
xi

;

Bˆ

iˆ0

iˆ0

Values of coecient ai and bi are showed in Table 1. The relation among the liquid enthalpy (h), temperature (T) and mass concentration (x) is:  D
t2 E
t3 ‡ hL ˆ 4:1868
C
t ‡ 2 3

 F ‡ 100

…23†

Where Cˆ

4 X ci
…1

x†i ;

Dˆ

iˆ0

The quantity of condensation heat is qc ˆ hV9

f
…hL7

qe ‡ qHEG ˆ qc ‡ qATA

…14† …15†

hL9

The systematic heat balance of the cycle is

…11†

The quantity of heat provided for heat user by ATA is as follows: qATA ˆ …1

The quantity of evaporation heat is

Eˆ

…16†

4 X ei
…1

4 X di
…1

x†i ;

iˆ0

x†i ;

Fˆ

iˆ0

4 X fi
xi iˆ0

Table 1 Values of coecient ai and bi in Eq. (22) Tableau 1 Valeurs des coecients ai et bi dans l'Equation 22 i ai bi

0

1

2

3

4

6.81990E+0 2.07689E+3

5.75253E-3 1.52768E+0

1.10093E-4 2.58101E-2

1.84889E-6 7.32551E-4

7.87523E-9 7.19646E-6

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2

5. Calculation examples

Values of coecient ci, di, ei and fi are shown in Table

According to the parameters given in Fig. 1, the SRAHT cycle is calculated respectively in the light of summer and winter working conditions. In accordance with the characteristics of the SRAHT cycle, waste hot water goes into HEG ®rst in which its temperature decreases. Then it ¯ows into EVAP in which its temperature decreases continually, so that the heat energy as much as possible can be obtained from waste heat source. Under the summer condition, the waste hot water at 80 C ¯ows into HEG of the cycle in which its temperature falls to 75 C and then passes through EVAP with a continual temperature drop to 63 C while the cooling water at 32 C goes into the condenser in which it is heated to 37 C and exits. The temperature di€erence demand in the actual heat transfer process can be assumed as follows: Condensation temperature tc=t9= 40 C; evaporation temperature te=t8=60 C; the temperature of the solution at HEG outlet t2=75 C; the temperature of the rich solution at absorber outlet t7=87 C; the concentration di€erence between the rich and the weak solutions
x=0.35. Under the winter conditions, there are some changes. The waste hot water temperature drops to 68.7 C in HEG in stead of 75 C and to 43 C in EVAP while the cooling water at 12 C enters the condenser and leaves at 17 C. The temperature di€erence demand in the practical heat transfer process can be considered as follows: condensation temperature tc=t9=20 C; evaporation temperature te=t8=40 C; the temperature of the solution at HEG outlet t2=75 C; the temperature of the rich solution at absorber outlet t7=85 C; the

In an equilibrium state, the relation between the mass concentration of TFE in vapor and the mass concentration of TFE in solution is: ln…y ‡ 1† ˆ

3 X gi
‰ln…x ‡ 1†Ši

…25†

iˆ0

Values of coecient gi are shown in Table 3. The relation of the potential heat of pure TFE is: R
T2 100:04
P 0 1 7 7 P P 397 ni
Pi 7 n3i
Pi B iˆ1 C iˆ1 C B @ 1512
dT 216
dT A

L ˆ 0:862402797 

…26†

Where ni= 3, 2, 1, 0, 1, 2, 3; R=8.341 J/(K mol) dT=5; Pi ˆ T ‡ ni
dT The relation of the saturated vapor enthalpy of TFE is: hV ˆ L ‡ hL …x ˆ 1:0†

…27†

The relation of the heat capacity of TFE±NMP saturated solution is: cp ˆ 4:1868 C ‡ D
t ‡ E
t2




515

…28†

Table 2 Values of coecient ci , di , ei and fi in Eq. (23) Tableau 2 Valeurs des coecients ci et di , ei et fi dans l'Equation 23 i 0 1 2 3 4

di

ci 3.8411E-1 3.5960E-1 1.2784E+0 1.5295+0 6.3050E-1

ei 1.317E-3 2.813E-3 2.957E-3 1.032E-3 0.0

fi (x40.5) 1.458E-6 2.011E-6 2.840E-6 0.0 0.0

3.97620E-1 4.27500E-1 3.84520E-3 0.0 0.0

fi (x>0.5) 2.75714E+0 5.41428E-1 5.14286E-3 0.0 0.0

Table 3 Values of coecient gi in Eq. (25) Tableau 3 Valeurs des coecients gi dans l'Equation 25 i

0

1

gi (x440%) gi (x>40%)

3.85471E-5 2.39128E+1

1.99422E+0 1.93108E+1

2

3 0.229852E+0 4.35576E+0

5.10358E-3 3.27379E-1

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X. Shiming et al. / International Journal of Refrigeration 24 (2001) 510±518

concentration di€erence between the rich and the weak solutions
x=0.43. According to the SRAHT cycle computerized model and the thermodynamic property relations of TFE± NMP binary working ¯uids discussed above, the computation program was worked out and the computed results are shown in Tables 4 and 5. 6. Analysis From the calculation results, it can be found that the output temperature (t6, t5) of the SRAHT cycle depends

on the solution concentration di€erence
x of the cycle and the temperature t7 at the absorber outlet when the heat transfer demand of t6>t3 is met. As the concentration di€erence
x and the temperature t7 increase, the output temperature (t6, t5) of the SRAHT cycle rises. However only increasing the solution cycle concentration di€erence
x will lead to temperature t6 approaching to temperature t3 and ®nally the heat transfer demand of t6>t3 cannot be met. In addition, the COP value of the SRHAT cycle decreases rapidly owing to only raising the temperature t7. When the output temperature and the inlet temperature of waste heat are ®xed, the COP value of the

Table 4 Computed results of the SRAHT cyclea Tableau 4 Cycle SRAHT : reÂsultats calculeÂsa Status points

1 2 3 4 5 6 7 8 9 a

Temperature t ( C)

Mass concentration x (%)

Vapor ¯ow (kg/cycle)

Summer

Winter

Summer

Winter

Summer

64.7 75.0 112.0 139.0 130.0 115.4 87.0 60.0 40.0

60.9 75.0 108.5 138.2 129.1 118.2 85.0 40.0 20.0

69.38 61.49 34.38 34.38 40.55 50.17 69.38 100.0 100.0

54.20 42.95 11.20 11.20 20.18 29.49 54.20 100.0 100.0

Enthalpy of vapor hV (kJ/kg)

Enthalpy of solution hL(kJ/kg)

Winter

Summer

Winter

Summer

Winter

0.3853 0.6193

0.4174 0.6347

881.3 902.7

870.1 891.6

0.0908 0.1865 0.7227 1.0 1.0

0.1199 0.1564 0.7237 1.0 1.0

885.4 885.4 885.4 885.4 854.4

854.4 854.4 854.4 854.4 826.9

479.2 493.4 571.9 624.0 601.0 567.0 517.9 525.4 487.4

468.9 497.9 594.7 653.9 621.0 588.4 510.7 487.4 451.8

Thermodynamic parameters at the points in Figs. 1 and 2.

Table 5 Computation results of the SRAHT cycle Tableau 5 Cycle SRAHT : reÂsultats calculeÂs Items

Symbol (unit)

Results Summer

Items Winter

Pressure in condenser

pc (kPa)

20.51

6.64

Pressure in evaporator

pe (kPa)

54.24

20.51

Concentration of rich solution Concentration of weak solution Solution circulation ratio

xr (wt.%)

69.38

54.20

xw (wt.%)

34.38

11.20

Theoretical re¯ux ratio Heat exchange in condenser

rth (kg/kg) qc (kJ/kg)

f (kg/kg)

Symbol (unit)

1.8748

2.0651

0.0046 367.0

0.0521 375.1

Heat exchange in evaporator Heat exchange in HEG Heat exchange in ATA Heat exchange in AHXA or AHXG Heat exchange in SHE Heat balance Coecient of performance

Results Summer

Winter

qe (kJ/kg)

325.4

316.3

qHEG (kJ/kg)

134.2

139.0

qATA (kJ/kg)

92.5

80.2

322.1

353.1

qSHE (kJ/kg)

72.6

86.3

(kJ/kg) COP (%)

0.0 20.15

2.3E-5 17.62

qAHXA (kJ/kg)

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SRAHT cycle is related to the waste heat temperature at the evaporator outlet. The higher the temperature of the waste heat at the evaporator outlet or the higher the evaporation temperature, the larger the COP value of the cycle. However with the rapid decrease in the recovered waste heat due to the evaporation temperature rising, the heat output of the SRAHT cycle decreases relatively despite the larger COP value obtained. Consequently, the design target of the SRAHT cycle is not to make the COP value maximal but to maximize the heat output of the cycle when the quantity of the waste heat is given. The SRAHT cycle can recover a larger quantity of waste heat. In this computing example with the SRAHT cycle, there was a 17 C temperature drop of waste heat from 80 to 63 C under the summer condition but a 37 C temperature drop from 80 to 43 C under the winter condition. Although the COP values of the SRAHT cycle were 20.15 and 17.62% due to the small temperature di€erence between ambient condition and waste heat and to a high temperature lift, respectively, the total heat output was greatly increased because more waste heat was recovered in the cycle. If the working pair of H2O±LiBr is used in the conventional absorption heat transformer (AHT) cycle, generally there will be a temperature drop of 710 C. If the AHT cycle using H2O±LiBr as working pair, in which the temperature drop of waste heat is 7 C under the summer condition and 10 C under the winter condition, is used to produce an equal amount of heat to that of the SRAHT cycle under the same heating temperature condition, it requires that the COP value of the AHT cycle should be higher. We can calculate by following: qSRAHT ˆ qATA ˆ COPSRAHT …qe ‡ qHEG † ˆ COPSRAHT mcp;wh
tSRAHT

…29†

qAHT ˆ COPAHT …qe ‡ qG † ˆ COPAHT mcp;wh
tAHT

…30†

where
tSRAHT and
tAHT are the temperature drops of waste hot water that ¯ow through the SRAHT system and the AHT systems, respectively. If the mass ¯ow rate of waste hot water and the heat output in the SRAHT system are equal to those in the AHT system, there will be: COPAHT ˆ COPSRAHT


tSRAHT
tAHT

…31†

When the data above are put in Eq. (31), the COP of the AHT cycle obtained will be 48.94 and 65.19%,

517

respectively. Evidently, it is very dicult for the AHT cycle with H2O±LiBr to get such large COP values under the big temperature lifts condition, so it is clear that the use of the SRAHT cycle can realize the targets of recovering more waste heat, reducing energy consumption and protecting the environment. At the same time, we must point out that the larger COP values cannot be expected in the SRAHT cycle while recovering more waste heat and raising heating temperature as much as possible. Actually, the GAX e€ect on the SRAHT cycle is to increase the temperature lift but not to raise the COP values. This is di€erent from on the GAX cycle. According to the second law of thermodynamics, it is known that the lower the mean temperature of the waste hot water ¯owing through the SRAHT cycle and the higher mean heating temperature under the condition of a constant cooling water temperature, the lower the COP value of the cycle[13]. As a result, the SRAHT cycle still can provide a great amount of the heat output because more heat energy is recovered from the waste heat resource, although the COP values in the calculation example above are not large. 7. Conclusions Based on the calculation and analysis above, some conclusions can be drawn as follows: 1. When the heating output temperature (t5) is determined, the solution concentration di€erence
x and the temperature t7 need to be adjusted simultaneously in order to meet the heat transfer demands (t6>t3, t7>t2) and to maximize the COP value of the cycle. 2. Under the condition of the constant output temperature and inlet temperature of waste heat, the COP value increases and the heat output decreases with increasing temperature of the waste heat at the evaporator outlet or the higher evaporation temperature. So, when designing the SRAHT cycle, we should not pursue the maximal COP value but the largest quantity of heat output of the cycle. 3. Although the COP value of the SRAHT cycle is not large, it can recover more waste heat and has a larger temperature lift compared with the conventional AHT cycle using H2O±LiBr. 4. It is recommended that the waste hot water should pass through HEG ®rst and then ¯ow into the evaporator. Thus, the maximum working pressure in the cycle can decrease to some extent and this will be bene®cial for the working pairs with high working pressure, such as NH3±H2O, to be applied in the SRAHT cycle.

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Acknowledgements This project is ®nancially supported by Chinese NSFC (59876006).

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