Performance of high-temperature absorption heat transformers using Alkitrate as the working pair

~

Pergamon

Applied Thermal Engineering Vol. 16, No. 3, pp. 255-262, 1996

1359-4311(95)00069-0

Copyright © 1995ElsevierScienceLtd Printed in Great Britain.All rights reserved 1359-4311/96 $15.00+ 0.00

P E R F O R M A N C E OF HIGH-TEMPERATURE ABSORPTION HEAT TRANSFORMERS USING ALKITRATE AS THE WORKING PAIR C. Z. Z h u o a n d C. H. M. M a c h i e l s e n Laboratory for Refrigerating Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands (Receit~ed 5 March 1995)

Abstract--High-temperature absorption heat transformers with Alkitrate as the working pair have been investigated. Performance parameters of single-stage, double-lift and triple-lift cycles for 1 MW industrialscale absorption heat transformers are calculated by a computer-simulation model, based on the heat and mass balance of each cycle. These parameters are useful for engineering applications. A comparison of Alkitrate cycles with H20 LiBr cycles is illustrated. It is concluded that Alkitrate is especially useful for operating at high temperatures, up to 260°C, the COPs of Alkitrate cycles are the same or better than those of H20-LiBr cycles, under the same operating conditions. However, attention should be paid to the solubility problem of Alkitrate at low temperatures, a condensing temperature of the working fluid (i.e. water) below 50°C is not recommended. Keywords---Absorption cycle, heat transformers, Alkitrate, performance, high temperature. NOMENCLATURE P Q T rh COP

vapour pressure (kPa) heat flow (kW) temperature (°C) mass flow (kg s i ) coefficient of performance, defined as the ratio between the output heat and the input heat

Greek letters

mass fraction of alkali-metal nitrate salts Subscripts

a c e g h

absorber condenser evaporator generator heat exchanger

1. I N T R O D U C T I O N A b s o r p t i o n h e a t t r a n s f o r m e r s are expected to have increasingly wide a p p l i c a t i o n s in industry, due to their a d v a n t a g e s in saving energy, eliminating the use o f o z o n e - d e p l e t i n g refrigerant C F C s a n d H C F C s , reducing the g r e e n h o u s e effect, a n d having a lower o p e r a t i n g cost c o m p a r e d with mechanical compression heat pumps. U n t i l now, only the w o r k i n g p a i r H 2 0 - L i B r has been used in c o m m e r c i a l a b s o r p t i o n heat t r a n s f o r m e r s . T h e a d v a n t a g e s o f this w o r k i n g p a i r are high e n t h a l p y o f e v a p o r a t i o n , g o o d t h e r m o d y n a m i c p r o p e r t i e s , no need for rectification a p p a r a t u s , non-toxic, n o n - f l a m m a b l e , non-explosive, a n d a long p e r i o d o f a p p l i c a t i o n experience in m a n u f a c t u r e a n d o p e r a t i o n for a b s o r p t i o n r e f r i g e r a t i o n machines. The m a i n d i s a d v a n t a g e s are that the t e m p e r a t u r e lift between the waste h e a t a n d the useful h e a t is restricted (usually lower t h a n 50°C) due to the solubility characteristics, a n d the useful m a x i m u m t e m p e r a t u r e is limited (usually lower t h a n 150°C), because the solution is highly c o r r o s i v e at high t e m p e r a t u r e s . Besides, a high v a c u u m with perfect sealing is necessary, otherwise the p e r f o r m a n c e o f the system is b a d a n d serious c o r r o s i o n p r o b l e m s will occur by air leakage. T h e price o f such an a p p a r a t u s is high, because o f the high cost o f c o m p o n e n t materials. 255

256

C.Z. Zhuo and C. H. M. Machielsen

Another working pair, NH3-H20, has been used in absorption refrigeration systems for quite a long time for deep-freeze applications. Although the working pair has good thermodynamic properties, it is not suitable for application in heat transformers, because the working pressure will be too high in the working temperature range. For example, at an evaporating temperature of 100°C, the working pressure will be as high as 60 bar. Such a heat transformer will be too heavy and too expensive, owing to safety protection. Considering the disadvantages of the well known working pairs of H20-LiBr and NH3-H20 , and the strong demand of industry for high-temperature output heat, high-temperature lift and high efficiency, many theoretical and experimental attempts have been made to find new working pairs for heat transformers. A new high-temperature absorption working pair, called Alkitrate, has been proposed in recent years [1, 2]. Alkitrate consists of alkali-metal nitrate salts and water. Thermodynamic property measurements and testing of the working pair have been carried out. The pilot plant testing has confirmed its high-temperature stability, low corrosiveness, and good heat and mass transfer ability. The working pair could achieve a COP as high as H20-LiBr with a higher output heat temperature (up to 260°C). The shortcoming of the working pair is its narrow solubility range at low temperatures, the working concentration of Alkitrate cannot be cooled to the ambient temperature. The absorbent must be diluted with water to about 55% mass fraction salt, in order to remain a liquid during shut-down periods. Since detailed design parameters of absorption heat transformers with Alkitrate as the working pair are still not available in publications, this paper presents such parameters for several absorption heat transformer cycles (single-stage, double-lift and triple-lift cycles) based on the results of computer simulations. 2. SINGLE-STAGE ABSORPTION HEAT TRANSFORMER A schematic drawing of a single-stage absorption heat transformer is shown in Fig. 1. Baseline operating conditions for an industrial-scale heat transformer are selected: the output heat capacity is 1 MW; the output heat temperature is 150°C; the cooling water supply temperature is 42°C; the input waste heat supply temperature is 105°C; the condensing temperature of the working fluid (i.e. water) is 50°C; the evaporating temperature of the working fluid is 100°C; and the heat transfer effectiveness of the mixture heat exchanger is assumed to be 0.85. Based on the calculation of the heat and mass balance of the system by a computer-simulation model [3], the performance parameters of the heat transformer with Alkitrate as the working pair are presented in Table 1. valour

@ heating Qe

@ vapour

I C

i_® I[ ling

Q¢

--@ Fig. 1. Single-stageheat transformer.

-® heat output Qa

Absorption heat transformer performance using Alkitrate

257

Table 1. Single-stage absorption heat transformer Temp. ("C) Temp. ("C) Mass flow rate (kg s" ~) Pressure (kPa) Heat flow (kW)

T1 (steam)

T2

T3

105.0 T9 100.0

105.0 T10 146.8 rh I 7.93 p1 12.3 Q. 1000

42.0 TI1 155.0 rh2 7.45 p2 108.7 Qg 1003

T4 47.0 T12 112.9 th3 0.48

Qc 1198

T5 (steam)

T6

105.0 105.0 TI3 T14 89.2 100.0 rh4 rn5 0.45 0.53 Mass fraction ( k g k g ~) Qc Qh 1195 547

T7

T8 (steam)

150.0 T15 50.0 rh6 0.47 ¢1 0.839

150.0 T16 102.0 m7 57.4 ~2 0.789 COP 0.455

The state points in the table correspond to Fig. 1. As can be seen from the table, using the cooling water supply at 42°C, and the input waste heat supply temperature at 105°C, a COP of 0.455 and temperature lift of 45°C can be achieved. Here the temperature lift is defined as the temperature difference between the output heat temperature and the input waste heat temperature. It should be noticed that the condensing temperature of the working fluid is 50°C, rather high in the design, due to the solubility of the working pair at low temperatures. For absorption heat transformers, the condensing temperature of the working fluid lower than 50°C should not be recommended. 3. DOUBLE-LIFT ABSORPTION HEAT TRANSFORMER For single-stage heat transformers, the temperature lifts are limited, due to the thermodynamic properties of the working pair. With the application of double-lift absorption heat transformer cycles, higher-temperature lifts are possible. Figure 2 shows a schematic drawing of a double-lift absorption heat transformer, which is characterized by the synthesis of two single-stage heat transformers, with common use of the generator and the condenser, with three operating pressure levels. Under the same baseline operating conditions as the single-stage heat transformer described above, except that the output heat temperature is raised to 200°C, the performance of the double-lift cycle is shown in Table 2. Since the output heat at the first absorber A1 is recovered as the input heat to the second evaporator E2, the input heat needed is only for the generator G and the evaporator El. The temperature lift of the cycle is 95°C, and the COP of the cycle is 0.285, 26

vapour

25 ~ - - a ~

14 27 heatlr 13

23 - - - ~

22 3 Fig. 2. D o u b l e - l i f t h e a t t r a n s f o r m e r .

12

258

C. Z. Zhuo and C. H. M. Machielsen

Temp. (°c)

Table 2. Double-liftabsorption heat transformer T2 T3 T4 T5 T6 (steam) 105.0 42.0 47.0 105.0 105.0 T10 Tll T13 TI4 T15 (steam) 200.0 100. 141.0 195.4 205.0 T19 T20 T21 T22 T24 113.5 87.1 100.0 50.0 102.0 rnl m3 rn5 rh7 tn9 0.972 120.0 0.591 45.0 0.514 thl8 tn21 m24 rh26 rh27 10.45 1.01 0.52 0.52 0.49 p21 p27 p26 Mass fraction ~11 12.3 108.7 363.1 (kgkg-I) 0.840 Q~ Q¢ Qcl Q~I Qa2 2505 1326 1201 1000 1201

T1 (steam) 105.0 T9

Temp. (°C) 200.0 Temp. TI8 ('C) 148.2 Mass flow rate (kgs i) Mass flow rate (kg s-J ) Pressure (kPa) Heat flow (kW) 2180

T7

T8

143.2 T16

149.4 TI7

155.3 145.0 T26 T27 140.2 102.2 rhll rnl5 9.44 9.96 415 418 0.796 0.758 Q~2 COP 0.285

when the cooling water supply temperature is 42°C and the input waste heat supply temperature is 105°C. In c o m p a r i s o n with the single-stage cycle, the output heat temperature has been increased f r o m 150 to 200°C, and the temperature lift has been raised from 150 to 200°C, but the C O P has decreased from 0.455 to 0.285 and three extra heat and mass transfer units are needed for the double-lift cycle. 4. T R I P L E - L I F T

ABSORPTION

HEAT TRANSFORMER

The synthesis o f three single-stage heat transformers can form a triple-lift heat transformer, as shown in Fig. 3. The cycle has 11 heat and mass transfer units with a c o m m o n generator and condenser, with four operating pressure levels. Such a cycle is capable o f achieving 250°C

13

vapour

9 -~

E3f]

'

12 ~ . . ~

heat

QO3

~ 3 t 1 1 ~ 1 8 ~ 1 7

[ 32vapour ~

I , 7

Qh3 I~x~l 19~

- ~--~ ['10,---~20

30 vapour ~ ~

] ~

k

'~

22

6

I ~--~-"]-

5

- 7 "''~24

vapour ~

4

Qhll Xxl 25 |

27 2

.

~

~

Fig. 3. Triple-lift heat transformer.

A b s o r p t i o n heat t r a n s f o r m e r p e r f o r m a n c e using A l k i t r a t e

259

Table 3. Triple-lift absorption heat transformer Temp.

T1 (steam)

T2

T3

T4

T5 (steam)

T6

("C) Temp.

105.0 T9

105.0 TI0

42.0 TI1

47.0 T12 (steam) 250.0 T20 190.3 T28 50.0 m5 0.651 rn24 11.69 p30 108.7 Qol 1461 Oc3 1241

105.0 TI3

105.0 T14

179.8 T21 187.8 T29 102.0 rh7 50.0 rn27 1.56 p27 12.3 Qal 1230 Qa3 1000

100.0 T22 142.2 T30 102.0 rh9 50.0 rh29 1.06 Mass fraction (kg kg-t ) Mass fraction (kg kg -t )

(°C) 182.8 Temp. TI7 ('C) 243.8 Temp. T25 (~C) 113.6 Mass flow rate (kgs -t ) Mass flow rate (kgs -t ) Pressure (kPa) Heat flow (kW) Heat flow (kW)

188.6 250.0 TI8 TI9 255.0 196.8 T26 T27 82.5 100.0 ml m3 1.517 185.0 thl8 rh21 10.69 11.19 p13 p32 997.6 305.5 Q~ Q~ 3403 3864 Qe2 Qa2 1230 1241

T7

T8

137.2 TI5

142.9 T16

135.8 T23 138.0 T31 134.2 trill 0.571 th30 0.497 ~14 0.840 ~21 0.760

180.0 T24 142.2 T32 134.2 rnl4 10.13 rh31 0.56 ~18 0.796 ~24 0.728 COP 0.206

output-heat temperature, under the same baseline operating conditions described earlier. The calculation results in Table 3 show that the temperature lift of 145°C can be realized with a COP of 0.206. A new triple-lift cycle is proposed here, as shown in Fig. 4, which is conceived on the basis of the rules for the design of multi-stage absorption machines [4]. The cycle is characterized by the synthesis of two single-stage heat transformers, with a common condenser, with three operating pressure levels. The absorption heat of the first absorber A1 is recovered as the input heat for the second generator G2 and the second evaporator E2 at the same time. The new triple-lift cycle is composed of only nine heat and mass transfer units. Giving the baseline condition for the new triple-lift cycle: an output heat capacity of 1 MW; an output heat temperature of 250°C; a cooling water supply temperature of 67°C; an input waste-heat supply temperature of 120°C; a condensing temperature of the working fluid of 75°C; an evaporating temperature of the working fluid of 115°C; and the heat transfer effectiveness of either the first or the second mixture heat exchanger assumed to be 0.85, the performance of the cycle is calculated. It can be seen from Table 4 that the output heat 1 MW at 250°C has been achieved, with a temperature lift of 130°C. However, the COP of such a triple-lift cycle is low (COP = 0.162). It should be noticed that the baseline operating conditions of the cycle are not the same as the 30 vap~ur [

12

h

output

I -11

]

,

I ~ A:

Qa2

lm

16

- 9 "-"'~23 Qhl [~

26

vat)our 20

18 t

valour

Q~21

+

15 ~

Fig. 4. Triple-lift heat t r a n s f o r m e r with nine heat a n d m a s s transfer units.

C. Z. Zhuo and C. H. M. Machielsen

260

Table 4. Triple-lift absorption beat transformer with nine heat and mass transfer units Temp.

T1

T2

T3 (steam)

T4

T5

T6

T7 (steam)

(~C) Temp.

159.9 T9

155.9 T10

120.0 Tll

120.0 T12

67.0 TI3

CC) Temp. ((C) Temp. (~C) Mass flow rate (kgs ~ ~ Mass flow rate (kgs L) Pressure (kPa) Heat flow (kW) Heat flow (kW)

154.8 T 17 255.0 T25 111.7

159.9 T 18 174.2 T26 115.0 rnl 60.0 thl5 26.0 p20 38.5 Qg~ 2586 Qc2 1598

159.9 T 19 150.9 T27 75.0 m3 1.175 rhl7 26.71 p26 38.5 Q~ 5159 Q~2 1028

153.6 T20 154.8 T28 117.0 m5 245.6 rh20 0.709 p29 180.3 Q~I 3574 Qa2 1000

250.0 T21 115.0 T29 117.0 th7 1.623 rh21 41.0 p30 484.2 Q~ 2626

72.0 TI4 (steam) 250.0 T22 154.0 T30 150.6 th9 120.0 th23 42.4 Mass fraction ( k g k g i) Mass fraction ( k g k g i)

T8

120.0 TI5

120.0 TI6

154.8 T23 160.9

240.0 T24 123.8

roll 60.0 rh26 2.153 ~ 15 0.912 ~21 0.757

~h13 0.571 rn28 0.709 ¢ 17 0.887 ~23 0.732 COP 0.162

former cycles, the condensing temperature of the working fluid has been changed from 50 to 75°C, and the evaporating temperature of the working fluid has been changed from 100 to 115°C, because of the solubility problem of the working pair. The new cycle of Fig. 4 needs a wider concentration range than the cycle of Fig. 3, but the concentration range of Alkitrate is limited at low temperatures. Crystallization of the Alkitrate solution may occur when the condensing temperature of the working fluid is 50°C, which unfortunately limits the high-temperature lift ability of the cycle. 5. COMPARISON WITH H20-LiBr CYCLES Under the same baseline operating conditions, the performances of the single-stage heat transformer and double-lift heat transformer with H20-LiBr as the working pair have been calculated, as shown in Tables 5 and 6, respectively. From a comparison of Table 1 with Table 5, it can be seen that the performance parameters of these two cycles are almost the same, except that the working concentration of Alkitrate cycle is much higher than that of the H20-LiBr cycle, the concentration (expressed in mass fraction) difference between the poor solution and the rich solution for the Alkitrate cycle is higher than that of the H20-LiBr cycle, therefore, for reaching the same output heat capacity, the internal solution flow rates of the Alkitrate cycle are lower than those of the H20-LiBr cycle. A comparison of Table 6 with Table 2 indicates that the Alkitrate double-lift cycle has a higher COP, a slightly lower cooling water supply and waste heat supply, higher solution concentrations and lower solution flow rates than those of the double-lift H20-LiBr cycle. The working pair Alkitrate can be used at high operating temperatures (up to 260°C), and the operating temperatures for the working pair H: O-LiBr are normally limited to below 150°C. With newly developed corrosion inhibitors, high operating temperatures, up to 200°C, for the working pair H20-LiBr have been achieved recently [5]. The performances of triple-lift H20-LiBr cycles to achieve 250°C output heat, as with triple-lift Alkitrate cycles, are not calculated, because available thermodynamic properties of H20-LiBr are limited to temperatures below 200°C [6, 7], Table 5. Single-stage H20-LiBr heat transformer Temp.

T1 (steam)

(°C) 105.0 Temp. T9 (°C) 100.0 Mass flow rate (kgs I) Pressure (kPa) Heat flows (kW)

T2

T3

T4

105.0 TI0 146.8 rhl 13.0 pI 12.3 Qa 1000

42.0 TI1 155.0 m2 12.5 p2 108.7 Qs 1003

47.0 TI2 111.6 rh3 0.48

Qc 1202

T5 (steam)

T6

105.0 105.0 TI3 TI4 96.2 100.0 m4 m5 0.45 0.53 Mass fraction ( k g k g -~) Qc Qh 1199 1107

T7

T8 (steam)

150.0 TI5 50.0 rh6 0.47 ~1 0.613

150.0 TI6 102.0 n~7 57.5 ¢2 0.590 COP 0.454

261

Absorption heat transformer performance using Alkitrate Table 6. Double-lift H20-LiBr absorption heat transformer Temp.

TI (steam)

(~C) Temp.

105.0 T9

("C) 200.0 Temp. T18 ("C) 153.2 Mass flow rate (kg s ~) Mass flow rate (kgs ~) Pressure (kPa) Heat flows (kW)

T2

T3

105.0 42.0 TI0 TI1 (steam) 200.0 100.0 T19 T20 112.1 94.8 rnl m3 1.028 1 2 5 . 7 rhl8 rh21 21.71 1.06 p21 p27 12.3 108.7 Qg Qc 2306 2625

T4

T5 (steam)

T6

T7

47.0 TI3

105.0 T14

105.0 TI5

146.5 TI6

151.7 T17

145.2 T21 100.0 rn5 0.588 rn24 0.577 p26 398.4 Qcl 1320

196.0 T22 50.0 rh7 60.0 rh26 0.577 Mass fraction (kgkg ~) Q~t 1332

156.8 T26 143.5 roll 20.65

146.0 T27 102.0 rhl5 21.23

~15 0.597 Qc2 1332

~18 0.583 COP 0.276

205.0 T24 102.0 rh9 0.514 rh27 0.483 ¢11 0.613 Qa2 1000

T8

and the operation of a H 2 0 - L i B r cycle at temperatures higher than 200°C has been impossible until now. It should be noticed that Alkitrate has solubility problems at low temperatures, even worse than those of H20-LiBr. For a single-stage Alkitrate heat transformer, when the generating temperature is 100°C, the condensing temperature of the working fluid (H20) should not be lower than 50°C, while for a H 2 0 - L i B r single-stage heat transformer, with the same generating temperature, the condensing temperature of the working fluid could be as low as 40°C. Therefore, the temperature of the cooling water supply could be lower for a H 2 0 - L i B r heat transformer than for an Alkitrate heat transformer. It has been known that, under identical operating conditions, the lower the temperature of the cooling water supply, the higher the value of the COP for an absorption heat transformer. 6. C O N C L U S I O N S Absorption heat transformers with Alkitrate as the working pair have been investigated. The working pair Alkitrate is especially useful for absorption heat transformers working at high temperatures, up to 260°C, however, attention should be paid to the solubility problem of Alkitrate at low temperatures. A condensing temperature of the working fluid (i.e. water) lower than 50°C is not recommended. Performance parameters of single-stage, double-lift and triple-lift cycles are calculated. Under the same waste heat supply and cooling water supply, to achieve a high-temperature lift and high output-heat temperature, a low value of the COP is unavoidable. Therefore, to design an appropriate heat transformer, the COP and the temperature lift of the cycle should be taken into consideration simultaneously. A comparison of Alkitrate cycles with H2 O-LiBr cycles is illustrated. It is concluded that the COPs of Alkitrate cycles are the same or better than those of H 2 0 - L i B r cycles, under identical operating conditions. A new triple-lift heat transformer cycle is proposed. It is composed of only nine heat and mass heat transfer units, while a normal triple-lift cycle needs 11 heat and mass transfer units. The cycle can upgrade waste heat at 120°C to useful output heat at 250°C with a COP of 0.162 for the working pair Alkitrate, when the cooling water temperature is 67°C.

REFERENCES 1. W. F. Davidson and D. C. Erickson, 260°C aqueous absorption working pair under development, lEA Heat Pump Centre Newsletter 4(3), 29-31 (1986). 2. L. A. Howe and D. C. Erickson, 260°C absorption working pair ready for field test. lEA Heat Pump Centre Newsletter 8(4), 7-9 (1990). 3. C. Z. Zhuo and C. H. M. Machielsen, Thermodynamic assessment of an absorption heat transformer with T F E - P y r as the working pair. Heat Recovery Systems & CHP 14(3), 265-272 (1994). 4. G. Alefeld, Regeln Fiir den etwurf yon mehrstufige absorbermaschinen. Brennstoff-Wdrme-Kraft 34(2) 64-73 (1982). 5. S. Inoue, S. Yamamoto, T. Furukawa, Y. Wakiyama and K. Ochi, Improvement of high-temperature applicability and compactness of a unit of an absorption heat pump. Heat Recovery Systems & CHP 14(3), 305-314 (1994).

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C.Z. Zhuo and C. H. M. Machielsen

6. L. A. McNeely, Thermodynamic properties of aqueous solutions of lithium bromide. A S H R A E Trans 85, 413~134 (1979). 7. G. Feuerecker, J. Scharfe, I. Greiter, C. Frank and G. Alefeld, Measurement of thermodynamic properties of aqueous LiBr-solutions at high temperatures and concentrations. The Int. Absorption Heat Pump Conf. "94, New Orleans, USA. ASME, AES-Volume 31, pp. 493 500 (1994).