Solar operated water-Amonia absorption heat pump for air-conditioning: Modelling and simulation

Applied Energy 14 (1983) 131-142

Solar Operated Water-Ammonia Absorption Heat Pump for Air-conditioning: Modelling and Simulation

E. Brousse, B. Claudel a n d J. P. M a r t i n e Laboratoire de Cin6tique et G6nie Chimiques, 404 INSA F. 69621, Villeurbanne, France

S UMMA R Y This study is devoted to the mathematical modelling of an ammoniawater absorption heat pump adapted to solar air-conditioning and including refrigerant and solution storages. Mathematical simulation permits the prediction of the performance of the system during a 24-h period and allows one to estimate its energetic performance, which is compared with those of systems using water-lithium bromide.

NOMENCLATURE A A AS C E G HE h~t S SS U X W

Absorber. Heat exchange area. Ammonia store. Condenser. Evaporator. Generator. Heat exchanger. Mass flow rate. Surface area of the collector. Solution store. Heat exchange coefficient. Mass fraction. Power. 131 Applied Energy 0306-2619/83/0014-0131/$03.00 © Applied Science Publishers Ltd, England, 1983. Printed in Great Britain

E. Brousse, B. Claudel, J. P. Martine

132

0~

q,

Loading factor. El~ciency of collectors. Transmittance. Incident flux density. Transmitted flux density ~, = z~b.

Subscripts The numerical subscripts refer to the locations on Fig. 1. The capital subscripts refer to the constitutive parts. a Ambient. f Fluids. max Maximum value. n, n + 1 Time steps.

INTRODUCTION Conversion of solar energy is especially attractive for cooling needs since the former is abundant precisely when the latter is wanted. However, storage devices remain necessary to obviate the effects of the sun's disappearance due to clouds or nightfall. Of the cooling systems available, the absorption heat pump has the advantage of requiring a minimal amount of energy other than that of the sun. 1 Many refrigerant-absorbent couples have been proposed,2 but only two, the water-lithium bromide and the ammonia-water systems, have so far met with significant commercial s u c c e s s . 3 In our laboratory an ammonia-water absorption machine has been built, allowing a refrigeration power of 12kW for a boiler temperature lower than 70°C. This heat pump is therefore well adapted to air-conditioning with fiat plate solar collectors. The purpose of the present paper is to report the possibilities of a storage system adapted to this heat pump.

THE STORAGE SYSTEM In an absorption machine, storing the refrigerant provides a convenient latent heat storage, the refrigerant being accumulated during the sunny hours and released into the evaporator when the insolation ceases. A solution store has to be associated with the absorber in order to avoid

Solar operated water-ammonia absorptio~ heat pump

133

large concentration variations. The basic scheme of the installation, already described in references 1, 4 and 5, is shown in Fig. 1. d)

The flat plate solar collectors are supposed to be immobile and typically consist of a transparent plate and an absorbing plate in which a fluid circulates. The incident radiant flux density, 4', is an experimental datum which is given as a function of time by a curve like that of Fig. 2. The power, We, delivered by the collector is given by: w c = nz s o = u s e

Where S is the surface area of the collector, taken equal to 74 m 2, r/ is the etficiency--a function of (T/- T,,)/@and z is the transmittance of the glass plate and is a function of the incidence angle. Further details of both r/ and ¢ can usually be found in the directions for use of the collector issued by the manufacturers. Figure 3 gives the function r / [ ( T / - Ta)Ab]for the collector which has been taken as a reference ('MISOLEC', with a selectively absorbing layer). Wc

W solar

. . . . . . . . . . . . . . . . . . . . .

Building

',

(co.a,,., ~lAmm°'l

~--.,,

~

:I

...WE./

,,,.

,

/

',

..a,

W solar

0 "-J"

I°

~L . . "~-'7

Fig. 1.

E

~ i It ' I~

~, I e x c h o n g e r r i - ~ L~ ~

: I:;~h'6~

u~1T '': I ~ ' 1 1,1 1 I

o ,'I'P R ,~

_

6 ~ -I ~'~ ~ 1

I.~ :~li

I!

"r I I-.-T. I1~ ~ I-<-' HE I I:a°tutl°n~ '. I . ~ Store V

,

,': w,

~11 ~ ~ ' i

-

A m m o n i a - w a t e r absorption heat pump for solar air-conditioning with refrigerant and solution storages.

E. Brousse, B. Claudel, J. P. Martine

134

( w i n -2 ) $00 700 600 500 Z,O0 300 200 100 t ( hours ) 0

i

I /

l

I

I

I

I

I

I

2

/,

6

8

10

12

1/,

16

18

F i g . 2,

S o l a r f l u x d e n s i t y as a f u n c t i o n

\

I

20

of time.

0.8

0.6

0./,

0.2

v

0.05

0.1

0.15 T f - T a ~P

Fig. 3.

Collector efficiency (from the ' M I S O L E C ' notice).

Solar operated water-ammonia absorption heat pump

135

(2) The building cooling load is characterised by the power W Ewhich (3)

has to be extracted by the evaporator represented by Fig. 4 with a maximum value of 11.6 kW at 17 h and linear variations with time. The heat pump has the characteristics of the machine already built in our laboratory: the temperature both of the condenser and the absorber is 30°C, the pressures are, respectively, 12.5bar and 6-5 bar. The temperature at the evaporator is 11 °C. The temperature difference between the heat transport fluid and the generator is supposed to be 5 °C. The two stores are at ambient temperature, 30 °C. The mass flow rate of the rich solution is 470 kg h - 1 and the UA product of the sensible heat exchanger, HE, is 2060 kWh. All these data are supposed to be kept constant. Moreover, it will be assumed that all the parts of the system are perfectly insulated from the surroundings and that the liquid-gas equilibrium is reached in the boiler for the poor solution. THE M O D E L

The day will be divided into small time intervals, At, each of a quarter of an hour, during which a stationary state is supposed to be realised (which is true if the response time of the system is small in comparison with At). W ( k c a t h-I )

WG

30000

c

20 000

10000

0

i

i 2

Fig. 4.

i

i /.

l 5

8

10

12

I/.

16

18

i

[ 20

I

I 22

J

I

,~

t ( hours )

Simulation results for the different exchanged powers as functions of time.

136

E. Brousse, B. Claudel, J. P. Martine

An iterative procedure is adopted in which the final values of the parameters at the end of an interval, AT,, are the initial ones for the interval At,+ 1. BASIC E Q U A T I O N S (a) Generator Mass balance: 2~1 -- g~f11(X11 -- Xlo) 1 - XlO

(1)

Enthalpy balance: WG = )~fl(H1 - Hlo) +/~11(H1o - HI 1)

(2)

Hence"

W G = /~rl I(X11 - Xl°) (H 1 -- H10) +/~fll(H10 - H l l ) (3) 1 - Xlo In the latter equation, Xll and M l l are given, the main unknown is X10, the mass fraction of the weak solution. Thermodynamic data between the parameters are taken from the enthalpic diagram H 2 0 - NH 3 of Landolt-Bornstein. 6 The relation H 11(X11, Tll) is obtained by parabolic interpolation between the three nearest neighbours of X 11, on the T 11 isotherm. The relation Hlo(Xlo ) is obtained in the same way on the isobaric boiling curve (for P = PG)- The relation H I(X 1o) is obtained by linear interpolation on the isobaric dew curve. (b) Heat exchanger, H E The heat exchanger is a countercurrent exchanger, supposedly perfectly insulated thermally. 1

/1~,o// Tlo +

1 --exp ~

~kg~fi 0

T8 =

1

~

T9

(4)

/~fll UA (]~11 ) 1 - ~ o l o exp ~ \/l~lo- 1 TI 1 -

311o(Tlo - T s ) + 3111 T9 /0'11

(5)

In these equations, the heat capacities are assumed equal for both flows.

Solar operated water-ammonia absorption heat pump

137

(c) Absorber, A The mass fraction of the fluid leaving the absorber is given by:

X7 =

X l o M l o + 0~j~r5 max /~fl0 + 0~/~r5 max

(6)

J~rmax being the m a x i m u m mass flow rate of a m m o n i a leaving the evaporator, and ~ the loading factor. The mass flow rate leaving the absorber is given by: ]~f7 : J~rl0 + ~ M 5 max

(7)

(d) Stores In the a m m o n i a store, the mass of a m m o n i a after the time interval At is given by MAS,n+ 1 ---- MAS,n + (31rl -- 3]¢5) At

(8)

For the solution store: MSS,n+ 1 = Mss,n + (-A~¢7- )t~'l 1) At XSs,nMss,n "~- (X7./~r7 - Xl lj~rl 1)At XSS,n + 1 --

MSS,n "~- ()~r 7 -- AI11) At

(9) (10)

(e) Exchanged powers At the condenser: W c = ( H 1 - H2))Q" 1

(11)

At the evaporator: WE = (x(n5 - n4.)l~r5 max

(12)

At the absorber: WA

= ~h4t5 maxH5 + A~rl0H6 - M'TH7 = t2/~t 5 max(H5 - H 7 ) + / ~ f l o ( n 6 -- HT)

(13)

The global structure of the c o m p u t a t i o n p r o g r a m m e resulting from these equations is given by Fig. 5.

138

E. Brousse, B. Claudel, J. P. Martine t=0

J "I

x,~ = Xss

Computation of WG

I !

!"

Computation of ](1o Computation of T10, Ts, TI 1 Introduction of T11in the No J computation of Xlo

1

~Yes I

Computation of ]

~

7"I

. No

Computation of the parameters of the heat pump and of the stores

t = t +0.25 i"

No

Fig. 5. Computation programme.

Introduction of TI in the computation of WG

[ [ i

Solar operated water-ammonia absorption heat pump

139

RESULTS AND DISCUSSION We started, at dawn (5 am) with X 9 -- 0.6 and a mass of solution in the store equal to 1500 kg. The curves of Figs 4 and 6 to 9 give the results of the computations. At dawn, the incident power is insufficient to meet the requirements, and the mass of ammonia in the store, AS, goes on declining until about 7-30 am. At that time ammonia production exceeds its consumption and the store increases up to a maximum at 16h. Of

~MAS( kg )

300

200

100

t ( hours ) 0

I 2

Fig. 6.

I ~,

I 6

I B

I 10

I 12

I lZ,

I 16

I 18

I 20

I 22

I 21.

Mass of ammonia in the refrigerant store, as a function of time.

course, this retention of pure ammonia causes a decrease of its mass fraction in the solutions, as can be seen in Fig. 7. Not unexpectedly, the different temperatures increase (see Fig. 8), the collector efficiency passing through a maximum at about 10 am (see Fig. 9). Consequently, ammonia production at the generator begins to fall off whereas its consumption at the evaporator goes on increasing. The daily average of the coefficient of performance is 0.52 and of the collector efficiency, 0.32. This means that 17 per cent of the incident solar energy is converted into cold. It is interesting to compare the present results with those obtained by Grassie and Sheridan 4 with a water-lithium bromide absorption heat

E. Brousse, B. Claudel, J. P. Martine

140

0.6

0.5

t

°~I

I o

Fig. 7.

L /*

2

( hours )

[

l

I

I

J

I

I

i

I

L

6

8

10

12

1/,

16

18

20

22

24

v

Mass fraction of a m m o n i a in the solutions, as a function of time.

pump. Although the solar radiation intensity corresponds in their case to the 1st of January at Brisbane, Australia, it parallels the curve of Fig. 2. Similarly, the building loads peak at 18 h with a maximum value of about 11 kW. Consequently, Wa, Wc, T10 have the same trends as Qa, Qc and t3, respectively in the preceding work. Grassie and Sheridan claim a coefficient of performance of 0.77 and a J

T (=C)

80

70 60 50

J

/.0

e T 6

r i i

i

30 I

I 6

Fig. 8.

I

I 8

I

I 10

I

I 12

i

I 1/,

I

I

I

L

16

18

t

v

(hours)

System temperatures as a function of time.

Solar operated water-ammonia absorption heat pump

141

0.6

0.5

0./,

0.3

0.2

0~1 6

8

10

12

14

16

t ( hours )

Fig. 9. Collector efficiencyas a function of time. collector efficiency o f 0-40, i.e. an overall yield of 0.31, or 1.8 times larger than ours. This superiority of the water-lithium bromide absorption heat p u m p is counterbalanced by some drawbacks: risk of crystallisation at higher concentrations of LiBr and high temperatures in the generator and collector.

CONCLUSIONS Further study is needed to check experimentally the validity of our working hypotheses and to examine more closely the transitory states, at the start and during a m m o n i a generation breaks due to the passage of clouds. A machine of the same size as that described in the model has already been built in our laboratory, and its performance will be reported in a later paper.

REFERENCES 1. W. E: J. Neal and M. Pabon-Diaz, Solar energy for refrigeration and air conditioning, Refrig. Air Cond. (1978), p. 59. 2. K. Stephan and D. Seher, Arbeitsgemische fiir Sorptionswfirme pumpen, Ki. Klima-Kalte-Heizung, 8 (1980), p. 21. 3. J. P. Chiou, On the study of applications of solar thermal energy for mobile homes, Solar Energy, 19 (1977), p. 449.

142

E. Brousse, B. Claudel, J. P. Martine

4. S. L. Grassie and N. R. Sheridan, Modelling of a solar operated adsorption air conditioner system with refrigerant storage, Solar Energy, 19 (1977), p. 691. 5. N.R. Sheridan and S. C. Kaushik, A novel latent heat storage for solar space heating systems: Refrigerant storage, Applied Energy, 9 (1981), p. 165. 6. Landolt-B6rnstein, Zahlwerte und Funktionen (6th edn), Berlin, 1972, Vol. 4b, p. 199. 7. D. Q. Kern, Process heat transfer, McGraw-Hill Kogakusha, 1950, p. 93.