Working fluids for heat transformers

Heat Recovery Systems & CHP Vol. 9, No. 2, pp. 175--181, 1989

0890-4332/89 $3.00+ .00 Pergamon Press pk

Printed in Great Britain

WORKING FLUIDS FOR HEAT TRANSFORMERS K, P. TYAGI Bharat Heavy Electricals Lid, Corporate R and D Division, Vikasnagar, Hyderabad 500 593, India

and SUDHIR MATHUR, AJIT SINGH, A P SRINIVAS a n d GIRIJESH MATHUR Department of Mechanical Engineering, College of Engineering, Osmania University, Hyderabad, India (Received 26 July 1988)

Abetract--A large amount of thermal energy, in the temperature range of 50-90°C, released to the atmosphere by many commercial installations such as agrofeed, paper mills, dairies and process industries, can be upgraded making possible its use in various forms. Many attempts have been made in most industrial sectors to recovery this energy by heat pumps, organic Rankine cycles and heat transformers. Among various possibilities, heat transformers present an attractive solution as low level heat can be transformed into a higher temperature with minimum consumption of external energy. Further transformers do not require high maintenance and operating cost. The theoretical performance characteristics of single stage heat transformers using various promising binary mixtures as working fluids have been discussed in this paper. The coefficient of performance, energy efficiencies and mass circulation ratio have been analysed as a function of heat delivery temperature. The comparison of working fluids has also been discussed.

NOMENCLATURE CR COP h m

Q

t T W X

circulation ratio (mass of weak solution to the mass of the refrigerant) coefficient of performance specific Enthalpy (kJ kg- *) mass flow rate 0cg h - i ) heat (kJ h- i ) temperature (°C) temperature (K) pump work (kJ) concentration (weight of refrigerant/weight of the solution) exergy efficiency

Subscripts

A C d E G H YC HXS PC PS

s VS Ax

absorber condenser delivery temperature evaporator generator heat exchanger between condenser and evaporator heat exchanger between generator and absorber pump between condenser and evaporator pump between generator and absorber source temperature throttling valve between absorber and generator the difference in concentration of strong and weak solution I. I N T R O D U C T I O N

Stephan and Seher [I, 2] have discussed the heat transformer cycles for single-and double-stage processes. Kripalani et al. [3] have studied the performance analysis of a vapour absorption heat transformer with differentworking fluid combinations, i.e.lithium bromide--water, R 2 1 - D M F , R 2 2 - D M F and R 2 2 - D M T E G . Rojey et al. [4] have discussed the present state of new technology for heat transformers.Tyagi [5]has analysed the coefficientof performance, exergy efficiencies,mass circulationratio,pump work, etc.for singleand two stage heat transformers using aqua-ammonia as a binary working fluid. The present paper deals with the analysis of a single-stageheat transformer using the following binary fluids as working fluids: (i) A m m o n i a (NH3)--I,4 butanediol; (ii)Ammonia (NH3)--2,3 175

K.P. TYAGIet al.

176

HEA'r S0gRCE

I

:

n

USEFUL HEAl'

aA I=A/

. s _

' oo 52.

oo,,

Fig. 1. Single stage absorption heating transformer.

butanediol; (iii) Ammonia (NH3)-TEG DME (triethylcneglycol dimethyl ether); (iv) sulphur dioxide (SO2)-DMA (dimethylacetamide). 2. THE SINGLE-STAGE ABSORPTION HEAT T R A N S F O R M E R

2.1. Working principle Figure 1 shows a single-stage heat tranfformer. It consists of generator G, the evaporator E, the absorber A, the condenser C, two heat exchangers HXS and HXC, two fluid pumps PC and PS and throttling valve VS. The generator G and evaporator E receive heat at medium temperature, the absorber delivers heat at higher temperature, whereas part of the heat flowing into the process is rejected by condenser at ambient temperature. In the generator G, some part of the lower volatile component is evaporated and the pure vapour of this component is pre-cooled in the heat exchanger HXC and flows into the condenser C. The condensate is pumped by the pump PC through the pre-heater HXC into the evaporator E, where it is evaporated at the heat source temperature and the vapour flows into the absorber .4. The weak solution leaving the generator is pumped by the pump PS, through the pre-heater HXS, into the absorber, where it absorbs the vapour coming from the evaporator. The heat of absorption is released as useful heat. The resulting strong solution is pre-cooled in the heat exchanger HXS, throttled through VS and enters in the generator. Thus, the cycle is completed. The thermodynamic state points of the working fluid are numbered. 2.2. Salient parameters The performance parameters may be defined as discussed by the author in his earlier paper. (i) Coefficient of performance: COP

=

QA/(Q~ + Qr).

(1)

(ii) Exergy efficiency: r/=

QA(I - Tc/T,,)

(2)

W+ (QG+ Q E ) ( T S T : 9 " (iii) Circulation ratio: CR

= m,/m~2,

(3)

where QA = 1 kW = 3600 kJ h-I

(4)

Qc = ml ht + m7h7 - m6h6

(5)

Working fluids for heat transformers 8INRRY

A A

FLUID-NH~-I.4

*.-SEe.

8UTRNEDIOL

~c-3El:

Q

•

mmm

,s-S~,

, ~.2SC

000

•~s-5BC. * ~-30C

I.B

18

18

B.8

14

~.7 .COP [L O U

18

B.5 m

0 0

"" C 0

B.9

Exerg~l E f f l c l e n c g

12 u

177

6

9" ID

~,. hJ

B.4

I) I. $ X t*J

B.3

I. LI

-8.2

-8.1

8

I

I

I

I

2

4

6

8

A x ocross fh.

1

IB

l

12

I

14

I

16

18

ebsoa-ber(Z)

Fig. 2. Performanceofsinglestage heattrans~rmer.

QL = m,,(hl,_ -- h i , )

(6)

W = ml Wps + m~ W ? ( .

(7)

3. R E S U L T S AND D I S C U S S I O N S The following range of operating parameters for binary fluids of single stage absorption heat transformers were considered: (I) the heat source temperatures, t,, of 50"C and 6@C; (2) the heat sink temperatures, t,, of 25 C and 3 0 C .

3.1. Performance characteristics of ammonia-l,4 butanedioi single state absorption heat transformer The circulation CR, COP and exergy efficiency are plotted in Fig. 2 as a function of Ax. For low values of Ax, the circulation ratio CR increases sharply, hence more power to drive the fluid and more heat exchanger area required. (ii) There is no significant change in the values of COPs with the change of Ax. (iii) The exergy efficiency q increases with increase of Ax. (i)

178

K . P . T'/AGI et al.

BINRRY F L U I D - N H s . - 2 . 3

n

,.

,-,

*,=See.

BUTRNEnIOL

* c?2S~: I.B

o o o

*= "5BC, *c"30C

10

B.9

115

it.8

14

il.7

12

it.6

(:Of' U

u

0.5

lg t

0 0

9" @

q,. b./

B.4

:) O~ L. 0 X bJ

~.3

U k

il.2

-0.1

2

4

6

A x o~oss

0

1B

12

14

16

'6"~ o b s o r b . r t g }

Fig. 3. Performance of single stage heat transformer.

(iv) By decreasing the heat sink temperature tc while keeping the same heat source temperature ts the circulation ratio CR and COP decreases but exergy efficiency increases. (v) By decreasing the heat source temperature t~, while keeping the heat sink temperature to, the circulation ratio CR increases. The change in the values of COP and F/are very small. 3.2. Ammonia-2,3 butanediol single stage absorption heat transformer The performances characteristics have been plotted in Fig. 3. The trend in variation of these characteristics is of a similar pattern to the one observed in Section 3.1. 3.3. Ammonia-TEG, DME, single-stage absorption heat transformer In Fig. 4 the performance characteristics have been plotted. The variations of circulation ratio

CR, COP and exergy efficiency with Ax were observed with similar trends as Fig. 2. 3.4. Sulphur dioxide-dimethyl acetamide single-stage absorption heat transformer Figure 5 shows the performance characteristics of sulphur dioxide-dirnethyl acetamide single stage absorption heat transfer with similar trends as Fig. 2.

179

Working fluids for heat transformers BINRRY FLUID-NH3-TEGDME

A A

* ~ - S K , *c-~'C

a = D

~cs'=6BC, * ¢"25C

2¢.

I.B o o o

*s'SEC-

• c ' 3 O r" Exerg~

18

Elf I c I encu

g.9

16

B.8

14

B.7 .

o q* o 0c

12

la.S

1B

IL5

8

B. 4

q-

@

-

S

g.3

I. LJ

8.2

B.1

ot

!

i

1

i

2

4

8

8

Z~ x

~ r ~ s

i

IB

i

12

i

14

i

16

18

"~ho absorbs/X)

Fig. 4. Performance of single stage heat transformer.

4. C O N C L U S I O N S At source temperature ts = 60°C and sink temperature, tc = 30°C the coefficients of performance

COP, circulation ratio CR, heat delivery temperatures ts and exergy efficiencies have been tabulated in Table l, Table 2, Table 3 and Table 4, respectively. The following observations have been made: (i) (ii) (iii) (iv) (v) (vi)

The circulation ratio, CR, is at a minimum for NH3-1,4 butanediol binary mixture, hence less power is required for driving the fluids. Heat delivery temperature td is at a maximum for ammonia-TEG-DME. Exergy efficiency t/and COP are maximum for ammonia-water mixtures. The rectifying column is only needed for the aqua-ammonia system. Amm0nia-l,4 butanediol and sulphur dioxide-dimethyl formamide systems seem to be quite attractive. Working fluids have to be selected after optimising the heat transformer cycles considering pump work, heat delivery temperature and the efficiency of the cycles.

K. P. TYAG1 et al.

180

BINRRY

FLUID-S(Z-DIMETHYL 2

= = ,-.

2~

RCETRHIDE

-~s-80C, * c-25C 1.0 •

o o o

•

.ts,.SSC.

"t c - ~ C

16

0.9

18

0.8

14

0.7

12

0.6

B.5 0 ,p m

~"

8

ua

B.4

X

0,3

,p @

bJ

g. U

g.2

4

B. 1

I

!

I

I

I

2

4

6

8

10

x ocross

I

12

'I

!'I'

14

16

*he obsor'b*r'tg)

Fig. 5. P e r f o r m a n c e o f single stage heat t r a n s f o r m e r .

Table I. Coefficients of performance of binary fluids source temperature, t, = 60"C; condensation temperature, l~ : 30"C Coefficient of lx'rformaace Ax %

NH3-TEG, DME

NH~-2.3 butan¢diol

NH~-! ,4 butanediol

NH3-H20

SO2-DMA

2 4 6 8 I0 12 14 16

0.507 0.500 0.497 0.494 0.492 0.490 0.488 0.487

0.515 0.509 0.506 0.505 0,503 0.501 0.500 0.498

0.519 0.514 0.511 0.509 0.508 0.507 0.505 0.503

0.620 0.617 0.607 0.600 0,598 0.596 0.594 0.593

0.578 0.569 0.563 0.559 0,564 0.55 0.546 0.541

18

Working fluids for heat transformers

181

Table 2. Circulation ratio of binary fluids source temperature, t~ ~ 60°C; sink temperature, t~ = 30°C Circulation ratio, CR Ax%

NH3-TEG, DME

NH3-2,3 butancdiol

NH3-1,4 butanediol

NH3-H20

SO2-DMA

2 4 6 8 10 12 14 16

22.57 10.78 6.86 4.90 3.71 2.93 2.37 1.95

17.06 8.03 5.02 3.51 2.61 2.01 1.58 1.26

15.92 7.46 4.64 3.23 2.38 1.28 1.41 1.11

19.1 9.84 6.20 4.40 3.1 2.63 2.12 1.93

16.103 7.55 4.701 3.276 2.421 1.851 1.443 1.137

Table 3. Heat delivery temperature td of binary fluids, source temperature, I, = 60°C; sink temperature, tr = 30°C Ax %

NH3-TEG , DME

NH3-2,3 butanediol

NH3-1,4 butanediol

2 4 6 8 10 12 14 16

103.30 99.90 96.7 93.6 90.8 88. I 85.5 83.1

95.9 93.1 90.4 87.8 85.3 82.6 80.7 78.5

93.0 90.7 88.7 86.7 84.9

83.3 81.8 80.4

SO2-DMA 100.3 96.5 93. I 89.7 86.5 80.5 77.5 77.1

NH3-H20 91.4 89.0 87.0 85.0 82.8 80.4 78.6 76.0

Table 4. Exergy efficiency of binary fluids, source temperature, t~ = 6ff~C, sink temperature, t~ = 30°C Exergy efficiency Ax %

NH3-TEG, DME

NH3-2,3 butancdiol

NH3-1,4 butanediol

NH3-H20

SO:-DMA

2 4 6 8 10 12 14 16

0.517 0.664 0.717 0.735 0.734 0.725 0.710 0.693

0.539 0.669 0.713 0.724 0.721 0.709 0.693 0.674

0.532 0.662 0.708 0.725 0.728 0.723 0.717 0.708

0.999 1.010 0.995 0.969 0.941 0.911 0.880 0.85

0.571 0.733 0.789 0.802 0.795 0.776 0.752 0.723

REFERENCES I. K. Stephan and D. Seher, Heat transformer cycles---l. One and two stage processes, J. Heat Recovery Systems 4, 365-369 (1984). 2. K. Stephan and D. Seher, Heat Transformer eyclcs--II. Thermodynamic analysis and optimisation of single stage absorption heat transformer, J. Heat Recovery Systems 4, 371-375 (1984). 3. V. M. Kripalani, S. Srinivasamurthy and M. V. Krishnamurthy, Performance analysis of a vapour absorption heat transformer with different working combinations, J. Heat Recovery Systems 4, 129-140 (1984). 4. A. Rojey, G. Cohen and J. P. Cariou, Heat transformers: present state of new technology, Proc. Inst. Mech. Engrs 197A, 71 (1983). 5. K. P. Tyagi, a q u a - a m m o n i a heat transformers, J. Heat Recovery Systems 7, 423-433 (1987).

HRS 9/2--F