An autonomous solar ammonia-water refrigeration system

Solar Energy Vol. 36, No. 2, pp. 115-124, 1986 Printed in the U.S.A.

0038-092X/86 53.00 + .00 © 1986Pergamon Press Ltd.

AN AUTONOMOUS SOLAR AMMONIA-WATER REFRIGERATION SYSTEMt M. D. STAICOVICI Solar Energy Laboratory of ICSITEE, Bucharest, Romania (Received 12 January 1983; revision received 4 October 1984; accepted 9 October 1984)

refrigeration is especially attractive in isolated regions. The Danube Delta needs ice for fish preservation. This paper describes an intermittent single-stage H20-NH3 solar absorption system of 46 MJ/cycle. Solar collectors heat the generator. Installation details and experimental results are presented. The system coefficient of performance (COP)system varies between 0.152 and 0.09 in the period of May-September. Solar radiation availability and the theoretical (COP), also applicable to the Trombe-Foex system, are assessed. Reference is made to evacuated solar collectors with selective surfaces. Actual (COP)system values of 0.25-0.30 can be achieved at generation and condensation temperatures of 80"C and 24.3°C respectively. For bigger capacities of 450-675 MJ/day, the pay-off period is estimated to be 6 and 4 years respectively and the life-time to 15-18 years. Abstract--Solar

l. INTRODUCTION

water-cooled this combination has (COP) values higher than the CaCIE-NH3 system,J3], and it varies much less with the evaporation temperature variation than methanol-bromide combinations, [ll]. Moreover, the risk of crystallisation (NaSCN-NH3,[12], LiBr-CH3OH,[10] is avoided, and the system can work at temperatures below 90°C, requiring minor rectification only. The CaCI2-2CH3OH combination has (COP) values 1.3 times higher than those of CaC12-NH3, therefore it is comparable to the H 2 0 - N H 3 system. However it presents the disadvantage of desorption at high temperatures and low absorbability of the absorbent,[10]. For big cooling loads intermittent cycles suffer from the problem that good high temperature collectors such as evacuated tubes containing selective surfaces cannot be good heat rejectors. For such loads the continuous Rankine cycle is preferable,[13]. The purpose of this report is to present a possible solution to this problem, without involving auxiliary power consumption. The laboratory model has been designed for a cooling load of 46 MJ/cycle. The experimental results will be examined to assess the feasibility of greater capacities.

Trombe and Foex,[l], have presented an intermittent single-stage ammonia-water absorption system using exclusively solar energy for ice production. This paper describes such a system with a few differencestt. The system described here has been designed for short-term preservation of fish in the Danube Delta. The Danube Delta is an isolated area, and ice is needed from early in the morning. It is for this reason that the intermittent absorption cycle has been chosen. Recent papers,[2-5], testify to increasing interest in the intermittent cycle as a result of its feasibility and despite a slightly lower (COP) as compared to the continuous cycle. Ammonia NH3 in combination with fluid absorbents such as H20, NaSCN,[6], or solid absorbents such as CaC12, SRC12,[3, 7, 8], is the most usual refrigerant used to achieve temperatures below 0°C. Methanol CH~OH is also used in combination with active carbon and especially in the very promising binary and ternary combinations with LiBr and/or ZnBr2,[9-11]. In association with these bromides CH3OH has higher (COP) values than other working fluid combinations, and the same applies to the CaC12-CH3OH combination as compared to the CaCI2-NH3 system,J10]. The sytem performance is highly dependent upon environmental factors such as cooling water temperature, air temperature and solar radiation. In consideration of the wide variation of these factors as well as of the (COP) values calculated under these conditions, the classic H 2 0 - N H 3 system proves to be the most efficient combination of the above-mentioned ones. When the condenser is

I. THE INTERMITTENT

t This work was financed by the National Center of Science and Technology. t t Patent application No. 80966 t'fled in the S.R.R. by ICSITEE and the author.

SINGLE-STAGEH20-NH3 SYSTEM

The flow-diagram of the intermittent single-stage ammonia-water system is shown in Figure 1. The working fluid combination is heated in the solar collectors G. The resulting vapour, enriched in ammonia, is subsequently condensed, C and stored, CR. Ammonia absorption in A and the cooling effect in the evaporator E are achieved after the weak solution from the liquid phase separator LPS is cooled, and the absorption circuit pressure is decreased accordingly, by means of the radiator R. The complete cycle of the ammonia-water circulation is achieved as a result of differences in density. The condenser C and the absorber A are

115

M. D. STAICOVICI

16

[

F

F

Fig. 1. Flow-diagram of single-stage intermittent ammonia-water system; G-generator; LPS-liquid phase separator; RF-rectifier; C-condenser; CR-condenser reservoir; E-evaporator; A-absorber; SR-solution reservoir; HE-heat exchanger; R-radiator. cooled by the water on which the pontoon carrying the installation floats. The pontoon ensures easy sun tracking by the solar collector surface in the horizontal plane. Between April and October the cooling water temperature reaches maximum values, i.e. 25°C-28°C, and air temperature ranges between 30°C-32°C in the day time and 22°C-25°C at night. The wind speed is about 5 m/s. A complete working cycle is represented in the log p-1/T diagram, Fig. 2. It is assumed that both the condenser and the absorber operate at a temperature of 24.3°C. At 90°C the dilute ammonia solution has a concentration of 0.36, and the degasing interval achieved is 0.07. The vaporization pressure is defined by the following relation: pv = Pa + Ap, where Pa is the absorption circuit pressure and Ap is the hydrostatic pressure opposed to gaseous ammonia at the absorber inlet (Ap - 0.5 ata). Under these conditions the evaporator operates within a temperature range of - 1 7 ° and -12°C (Fig. 2, points 3 and 4 on the diagram). 3. EXPERIMENTAL RESULTS

The experimental model, Fig. 3, simulates the environmental conditions of normal operation. In the Bucharest area air and cooling water temperatures reach values comparable to those recorded in the Danube Delta and the average wind speed is 3 m/s. The solar collector surface (Ac = 26.9 m 2) is flat and has a fixed orientation fSs-sw = 30°. It consists

of 15 modules with a non-selective absorbing surface, a polyurethane insulating layer 8 cm thick on the back and three transparent shields (one of glass and two of polyethylene film) on the front. The absorber and the evaporator are shown in Figs. 4 and 5 respectively. The heat exchanger is of the tubein-tube type. Typical temperature variations and the insolation during vapour generation and absorption are shown in Fig. 6. The hemispheric solar radiation has been measured and recorded by means of a Kipp and Zonen solar pyranometer. The temperature has been measured and recorded from 12 points simultaneously with thermistors operating within a range of 10°C-100°C (I.F.T.M.). Pressure measurements have been carded out with special pressure gauges for NH3, 0-15 ata (I.M.E). Due to the non-availability of a flowmeter suitable for HzO-NH~ refrigerating units with natural circulation, the flow has been assessed based upon the generator outlet/inlet temperature readings recorded during vapour generation (To). The generator recirculation factor can be defined as: kr = (1/Me)fO rG nms('r)d'r Values of kr close to one are a condition of minimum heat consumption in the generator, and therefore they are preferable. However, the lack of an ammonia-water flow (ms) regulating valve during ini-

Autonomous solar ammonia-water refrigeration system

117

100 80 50 40

20

10

8 4

2

80 °

_~o

-40 °

-20 ° Tempereture

0o

20 °

40 °

60 o

(°C J

Fig. 2. Diagram of the cycle.

Fig. 3. Ice-making pilot with a cooling load of 46 MJ/cycle.

80 ° 100° 120° 140°

---.

's to tO

M. D. STAICOVICI

118

~ ricn sOlut~noutlet

I

I

m

i I i

I

I

I I 1

I t ~

gozeous arnmonr]inlet dilutesolutionInlet Fig. 4. Absorber.

tial experiments resulted in a multiple of circulation, k, = 5-6, and in a low efficiency of the heat exchanger. The values of (COP)system have been derived through the relation: (COP)system =

Qo/(Ac fo ":cHr('r) d't)

regulating valve. For kr - 1 and at the optimum temperature for vapour generation, under condit i o n s similar to those specified in Table l, the (COP)system values obtained are 50% higher than the initial ones. The long-term performance of the solar collector, ~c, can be determined through the integration of instantaneous performance over the ,~ period, but unfortunately the latter is not available:

The (COP)system values recorded in the May-September period of 1981 as well as the typical daily values recorded over each of these months for solar radiation (Q,.a), cooling water temperature (t,~.c) and air temperature (to) are shown in Table I. In favourable conditions, specific to the spring to summer transition, when condensation temperatures are low (tw.c = 10°C) and solar irradiance is high (Qt.~ = 523 MJ/day Ac), maximum values of cooling ratio are obtained ((COP)system -- 0.179). Lately the experimental unit has been provided with a flow

-¢~ = FR((~'d), -

Uc((tf,i - t.)/HT))

In Equation (1) the quantity FR has been approximated as FR = F'(1 - exp(-x))/x (2), where x = UeF' fG'~p and Uc are constant. Collector efficiency ~c has been derived from the generator energy balance as: "~c = (QKR + QL + Qc)/(A~ f0 rG Hr(x) d'r)

Table 1.

tw,c:oc)

ta(=C )

Month

Day

Q t : (Nd/daY]

(COP)syste m

Nigh(

V

(I)

162

519

0.152

20B

521

0.128

VI

23O

VII

25.~

22.0

23.9

513

O.f06

VIII

23.2

20.3

23.0

504

~2092

IX

19.8

16.5

18.6

z,61

0.079

(3)

Autonomous solar ammonia-water refrigeration system

119

tstment valve

id ammonia inlet

gazeous ammonia outlet

e

Fig. 5. Evaporator.

forms

120

M. D. STAICOVICI

,oo-

I

1

I

I

T

~

i

I

I

e-Generator temperature o- Solor irradJance o-Ambient temperature. A-Generator pressure A-Vapor,'za tion pressure m-Vapori~fion temperature X-Cooling water temperature

t" 8 0 -

10

+ 60c~

+ 40-" Q;

~ ~ 20-.~

4

201

-

20-

O~

J

I

A~. 10

12

2

I

I

I

I

I

I

I

I

I

4

6

8

10

12

2

4

6

8

10 AN.

I

J

i

i

Day hours Fig. 6. Typical working diagram.

I

100 ~

~00

i

i

I

I

i

i

e-Generator outlet temperature o-Generator thief temperature "-Rectifier outlet temperature A-Solar /rrediance

A-Generator pressure

rJ-Ambient temperature ,r

/

I

i

i

10

/,~/"~

~

--~

~" ~

Dote:04.05.,981

/

I

t~ " ~

t

~oc

80-

60-

l

o-

8

,oc

QJ

40-

200

20"

2 I !

A~.6

~ Heoting I

I

8

I

'.I

10

I

Boiling --~ I

12

I

I

I

2

}--- Cooling I

4

I

I

6

De), hours Fig. 7. Diagram of the refrigerant vapour generation period.

RN.

Autonomous solar ammonia-water refrigeration system

121

The experimental results show that in the areas under study tracking of the sun azimuth (optimum 13 = 45 °) leads to an important energy gain of about 25% as compared to the fixed South orientation (optimum 13 = 25°). In this case a hemispheric insolation of 750 W / m z is available about 7 hrs a day. The additional 5% gain achieved through the zeroincidence angle tracking does not seem to justify the complicated installation which this motion implies. n ' ~ ~ nm,,m(t¢,o(rO) To determine the theoretic (COP)cycnethe followty,i(O))/((1/rG)/ fo ~ (tf,o(r) -- ty,i(r)) dr) ing assumptions have been made, Fig. 1: a) k~= 1; b) M, is stored in the L P S and S R only, the conwhere ms,m = M , / r c is the minimum working fluid tents of G, H E and of the connecting pipes being flow rate (k~ = 1). In relations (1)-(3), ?p and G are negligible; known factors, and F ' can be considered a design c) the generating circuit (G + L P S + H E + S R ) parameter. Thus, the unknown factor U~ can be deloses heat through G only; rived through the trial-and-error method. For the d) the rich and dilute solutions do not mix, and the day under study, for ~. = 0.314, (r--6")~ = 0.556, (tsa heat exchange between them is achieved in the - t , ) / H r = 0.09 Km2/W, G = 2.5 10 -3 kg/sm a, Ep heat exchanger H E only. = 4.7 I0 +3 J/kgK and F ' = 0.968, the U~ value The L P S must contain a minimum quantity of obtained was 2.23 w / m a K , and the instantaneous working fluid: performance of the solar collector was defined by the following relation,[14]: MLeS = (V~ - Va)p~ (5) = M R ( f - ( f - l)(9~/P~)) ~,. = 0.62 - 2((t~,~ - &)/Hr)

where QxR, QL, and Q c stand for the generationrectification heat, the heat lost by radiation and convection to the environment and the circuit caloric capacity respectively. Fig. 7 shows the system temperatures during one of the day under study. The flow rate -G = n-~flAc can he approximately based upon the observation, (Fig. 7), that:

4. INSOLATION AVAILABILITY, THEORETICAL (COP) AND IMPROVEMENTS OF (COP)system

Solar irradiance on horizontal surfaces was recorded in several stations in Romania in the period 1971-1980. Hemispheric and direct monthly daily average hourly radiation data (kJ/hm 2) are available for Bucharest and Constantza-Danube Delta areas (in latitude + 44.3 North). Insolation over tilting surfaces has been determined by the Liu and Jordan method,J14-16]. When tracking the sun azimuth, the sun azimuth angle, to, is correlated to the collector azimuth, y, by the following relation: y=m±w

as:

The mean hourly incidence angle is defined through the integration of the instantaneous incidence angle expression,[14, 15], resulting from the above relation, over a period of an hr: (cos O)h = (2~/24)

to ensure the dilute solution circulation after generation. In Equation (5), MR stands for the distilled refrigerant amount, f = ( ~ R a - - ~ a ) t ( ~ - - ~ta) is the circulation factor,[17], p~ and pa are the densities of the concentrated and dilute solutions respectively considered for tae equal to taa and Vr = Mflpr and Va = (Ms MR)/pa are the volumes of the concentrated and dilute solutions respectively. Based upon assumptions (a)-(d) the generation period can be derived into two distinct phases: 1) an amount MA = MLes of non pre-heated working fluid flows through the generator; 2) the remaining ME = Ms - Ma = M R ( f -- 1)(%/ P,) is pre-heated. The heat values for the two phases are defined

COS('r)d'r

and (QKR)E = ( M e / f ) ( q r R ) E

(6)

where (qxR)a and (qxR)E are unitary heat values (J/ kg refr.) defined by the following relations:

= (2,rr/24)(sin 8 sin ~ cos 13 + 0.9972 cos ~ cos(8 + 13) •cos N + ½ cos 8 sin 13

(QKR)a = (Ma/f)(qKR)a

(4)

•(0.9886(sin ~ - 1) cos 25 + sin4~ + 1)) where U is the sun azimuth at the middle of the one hr interval.

(qXR)a = i~,,~ -- i'KE q" f ( t K" E -- laE) " + r(i'km - i'R)

(qKR)E & = - - l x E + f ( ~ =

"

"

i'

--

i~w) + r(i~,, - i~)

and the rectifying factor r = ( ~ . - ~,,)/(~¢,. ~k) is computed by means of the following relation:

122

M. D. STA|COVlCl where it is considered that the tube temperature is equal to the ambient temperature, t ~ ( K ) . The average plate temperature, t p , , , ( K ) , is assessed in relation to the average fluid temperature, ty.,,, = (t.f,i + ts,o)/2, by the following relation,J14]:

~ = ~7o, - ~lRr(~TCr, -- ~'). When refrigerant subcooling is used,[17], the resulting maximum theoretical (COP) is: (COP),h = Q o / ( ( Q K R ) A + ( Q K R ) e ) -- ~)e)/(1

qo,p((~. --

~, ~ O E JW~i,, ) ~ Km

f ( i'r e -

tv,m

i'AE) + r(i'km -

- (f - l ) ( i ~ - w -

tf,m

Jr Q , , R p - s

(10)

The generator inlet and outlet temperatures, ts,~and ts.o respectively influence each other through the heat exchanger. In stationary conditions, ts.o is given by the relation,[14]:

i'n)

i'AE)(Pr/pa))

An approach to Equation (7) would be possible if solar collectors with concentration were used as generators. F o r a non-volatile absorbent combination (e.g. N a S C N and NH 3) r, ~ E and ~ in the above relations are 0,0 and 1 respectively. Equation (7) is also applicable to the Trombe-Foex system. There is a need for future improvements of the (COP)systemgiven by: (COP)system =

=

(7)

i~e +

- -

ty.o = (tf,i -

t~ -

('rl~)e

x Hr/U~) exp(- U~WF'L/m,Cp) + t, + (xa)~Hr/U,.

(11)

and if we take into account assumption (c), the heat balance on the heat exchanger is derived from the following relation:

(8)

(COP)th-~c

(tf,i-

tAE)nms-Cp.r

Maximum theoretical (COP) values are obtained when the generator operates at an optimum temperature for given condensation and vaporization temperatures as well as when internal heat recovery is ensured. F o r high solar collector efficiency it is suggested to use evacuated solar collectors with selective surfaces. The instantaneous performance of the plate-in-tube type is approximated by the following relation:

With sufficient accuracy it can be considered that in Equation (12) ?p,r ~ ?p,~ ~ ?p and tAE ~ t~ so that using Equations (11) and (12) the following can be written:

~c = F'(('rOt)e

ty,o = ta + t*(1 - B / ( 1 + A(1 - B)))

(13)

ty,,, = ta + (t*/2)(2A + 1)/(A + 1/(1 - B))

(14)

-

(tr/Hr)(t~.m -

t4)/(1/ep + 1/% -

I

|

.

1))

= (tS,o -

!

l

I

I

I

L~

0,6

2,6

E ~0,4

0.~

"6 to

0,2

0.2

C..,

O3

I

80

I

(12)

ty.i = ta + t*/(1 + l/A(1 - B))

(9)

u

te)nms'dp,a = K S A t m

I

I

100 ~20 Tempera turo {°C)

I

i

140

Fig. 8. The (COP) and solar collector efficiency function of the outlet generator temperature; [3average collecting area efficiency for evacuated solar collectors with selective surface ((Ta)e = 0.8; ep = 0.1; F ' = 0.87); I-(COP) with heat recovery during the whole generation period (ts,i = 71°C; tc = 24.3°C; 6" = 0.45; ~ka = 0.998) and refrigerant sub-cooling (ty = 15°C); A-(COP) with partial heat recovery similar to the system presented in this paper (relation (7)); O-(COP) without heat recovery but with refrigerant sub-cooling; A-(COP) of the sytem, equation (8).

Autonomous solar ammonia-water refrigeration system where t* = (xet)~Hr/Uc, A = K S / n - ~ , % and B = e x p ( - U ~ W F ' L / - ~ f ~ p ) . The solar collector efficiency is proportional to the working fluid quantity circulated through the generator in the two phases. Then the average solar collector efficiency can be expressed as: ~e = (MA/Ms)(ec)A + ((M~ -- MA)/Ms)(~,.)E

or ~ = (I - (1 - (I/f))(p,/p~)) x ((L.)a - (L.)E) + (~,.)E (15) If appropriate values are available for parameters t*, A, B, then Equations (9), (10), (14) and (15) can be used to compute ~,,. For the first generation phase K = O and consequently A = O. Anyway, for F ' = 0.87,[14], (~-~)~ = 0.8,[18, 19], % = 0.1, % = 0.9, ( i f , i ) a = I a = 300 K , (t/.3e = 344 K (boiling point), Q,flp,f = 2°C, H r = 700 W/m z, ~, is plotted in Fig. 8 in relation to ts.o, through Equations (9), (10), (15). For comparison, the same figure shows the (COP) of cycles with full heat recovery and without heat recovery as a function of if.o, as well as of cycles with partial heat recovery (Equation (7)). The values used in calculation are: t,. = 24.3°C, ~" = 0.45, ~k~ = 0.998, rlRF = 0.83,[17], tAE = 26°C and ty = 15°C. Fig. 8 also shows the (COP)system (Equation (8)) which has been plotted by means of Equation (7) and (15). The maximum values, (COP)system = 0.30-0.33, are obtained at a ts,o temperature below 90°C. 5. CONCLUSIONS The experimental model proves the feasibility of the suggested flow-diagram. The intermittent cycle chosen is capable of working without auxiliary power consumption. At usual condensation temperatures, maximum theoretical values of (COP)system are achieved for generator outlet temperatures below 100°C, the actual maximum values being expected in the range of 0.25-0.30. Subject to a total solar insolation of 750 W/m 2 being available 7 hours daily and assuming that an amount of 450 kJ/kg ice is required, with a (COP)system = 0.25, the resulting solar collector cooling capacity amounts to 10.5 kg ice/m2 collector. Further improvements of the system components will enable the transition to bigger capacities of about 10001500 kg of ice per day, which according to present estimates will pay off in 6-4 years respectively and will have a life time of about 15-18 years. NOMENCLATURE cp cp., cp., F'

sensible heat, J/kg K dilute solution sensible heat, J/kg K concentrated solution sensible heat, J/kg K collector efficiency factor

123

FR G Hr i~e

collector heat removal factor specific working fluid flow rate, kg/s m2 hemispheric insolation on the collector, W/m2 enthalpy of concentrated solution at the absorber outlet, J/kg i~¢e enthalpy of dilute solution at the generator outlet, J/kg ik enthalpy of the condensate reflux, J/kg i~w enthalpy of concentrated solution at the heat exchanger outlet, J/kg i~m enthalpy of the vapour at the generator outlet, J]kg K overall heat exchange coefficient of the heat exchanger, W/m2K L maximum length of the solar collector pipe, m Ms concentrated solution mass, kg n number of parallel tubes of the solar collector surface qo.p pure refrigerant vaporization heat, J/kg Q, working heat of the collector, W/A~ Qo heat removed in the evaporator, J/cycle QKR generation-rectification heat, J/cycle Rp-s plate-fluid thermal resistance, Km2/W S thermal transfer area of the heat exchanger, m2 tc condensation temperature, K tE dilute solution temperature at the heat exchanger outlet, K tf refrigerant sub-cooling temperature, K taA dilute solution temperature at the absorber inlet, K taE concentrated solution temperature at the absorber outlet, K U, overall collector heat loss factor, W/mZK W fin width, m 13 collector tilt angle, degrees sun declination angle, degrees ep plate emittance ee shield emittance 0 radiation incidence angle, degrees ~" dilute solution concentration ~" rich solution concentration ~bE solution concentration at the evaporator outlet ~h reflux condensate concentration ~,~ mean vapour concentration at the generator outlet tr Stefan-Boltzmann constant, W/m2 K 4 time, s (rct)e actual transmittance-absorptance product latitude angle, degrees ~Rr rectifier efficiency At,,, mean logarithmic temperature difference in the heat exchanger, K (-) averaged ( )A referring to the generation without pre-heating ( )e referring to the generation with pre-heating

REFERENCES

1. F. Trombe and M. Foex, Production de glace a l'aide de l'dnergie solaire. International Meeting of CNRSFRANCE, Montlouis (1958). 2. N. J. H. Gallo and M. F. de Souza, Solar powered intermittent absorption cooler. Proc. 1.1.R. Session, Israel 1982. 3. I. Pilatowsky and R. Best, Study on the utilisation of CaCI2/NH3 in a solar absorption refrigeration system. Proc. I.I.R. Session, Israel 1982. 4. R. Deigado et al., Study of the intermittent charcoalmethanol cycle for the realisation of a solar-powered ice-maker. Proc. I.I.R. Session, Israel 1982. 5. M. Dupont et al., Study of solar-powered ice conservators using the day-night intermittent zeolite 13Xwater cycle in temperate and tropical climates. Proc. 1.1.R. Session, Israel 1982. 6. R. S. Agarwal and M. K. Agarwal, A thermodynamic

124

M. D. STAICOVICI

study of Sodium thiocynate-ammonia combination for solar-powered refrigeration system. Proc. I.I.R. 16-th International Congress of Refrigeration, Paris, 1983. 7. N. E. Clausen and P. WorsCe-Schmidt, Analysis of ammoniated metal salt suspensions for use in solar refrigeration systems. Proc. 1.1.R. Session, Israel 1982.

8. N. E. Clausen and P. WorsCe-Schmidt, Solar absorption refrigeration utilising suspended solid absorbents.

Proc. 1.I.R. 16-th International Congress of refrigeration, Paris, 1983. 9. P. D. Iedema and C. H. M. Machietsen, Fundamental equations for the free enthalpy of LiBr and ZnBr2 solutions in methanol. Proc. 16-th International Congress of Refrigeration, Paris, 1983. 10. E. R. Grosman et al., Methanol as a working medium in sorption type thermal converters. Proc. 16-th International Congress of Refrigeration, Paris, 1983. 11. A. AUoush and W. B. Gosney, An absorption system using methanol plus lithium and zinc bromides for refrigeration using solar heat. Proc. 16-th International Congress of Refrigeration, Pads, 1983. 12. C. A. Infante Ferreira, Thermodynamic and physical properties of the NaSCN/NH3 system. Laboratory for Refrigerating Engineering, Delft University of Technology. 13. K. Sherwin, Study of solar-powered refrigerator with thermally powered vapour compression system. Proc. 16-th International Congress of Refrigeration, Paris, 1983. 14. J. A. Duffle and W. A. Beckman, Solar Energy Thermal Processes, John Wiley and Sons, New York, 1974. 15. F. Kreigh and I. Kreider, Principles of Solar Energy Engineering, McGraw-Hill Book Co., New York, 1978. 16. S. A. Klein, Calculation of monthly average insolationa on tilted surfaces. Solar Energy 19(4), 325-329, 1977. 17. W. Niebergall, Sorptions Kiiltemaschienen, Springer Verlag, Berlin. 18. D. S. Ward et al., Integration of evacuated tubular

solar collectors with lithium bromide absorption cooling systems, Solar Energy 22(4), 335-342, 1979. 19. J. D. Felske, Analysis of an evacuated cylindrical solar collector. Solar Energy 22(6), 567-570, 1979. R~sum6e--Le refroidissement solair est particuli~rement utilis6 darts des lieux isol6s. Le Delta du Danube a besoin de la glace pour la conservation du poisson. On d6crit une installation ~ absorption intermitente, mono6tag6e, syst6me HEO/NH3, avec la capacit6 de 46 M J/cycle. Les capteurs solairs constituent le g6n6rateur. On pr6sente des details de construction et des r6sultats exp6rimentales. Le (COP)systeme vade entre 0.152 et 0.079 dans la p6riode mai-septembre. On 6value la disponibilit6 de l'insolation en cas de la suite de I'azimut du soleil et de m6me le (COP) th6orique, valable aussi pour l'installation Trombe et Foex. On consid6re aussi l'utilisation des capteurs solairs vid6s b. surfaces absorbantes s61ectives. Les valeurs r6elles du (COP)systeme, de 0.25-0.30, on peut les obtenir aux temp6ratures de g6n6ration et de condensation de 80°C et 24.3°C respectivement. On estime que pour des capacit6s plus grandes, de 450-675 M J/cycle, la dur6e de r~.cup6ration est de 6-4 ann~.es, respectivement, et celle de vie de 15-18 ann~.es. Resumen--E1 resfriamiento solar es atractivo especialmente para lugares aislados. El Delta del Danubio necesita hielo para conservar el pez. Se describe una instalati6n con absorci6n intermitente en un paso, un sistema H20/ NH3 de capacidad 46 MJ/cyclo. Los captadores son el generador. Se presentan detalles constructivos y resultados experimentales. El sistema (COP) varia entre 0.152 y 0.079 en el periodo mayo-septiembre. Se estiman la disponibilidad de la instalaci6n en el caso de perseguir el acimut del sol y (COP) te6rico, tambi6n v,-tlido para la instalaci6n Trombe y Foex. Se examinan tambi6n los captadores solares arrugados con superficies absorbantes selectivas. Valores reales del sistema (COP) de 0.25-0.30 se pueden obtenir con las temperaturas de generati6n y condensaci6n de 80°C y 24.3°C respectivamente. Para capacidades mayores de 450-675 MJ/cyclo el perfodo de recuperati6n se estima a ser 6, respectivamente 4 afios, yel de vida entre 15 y 18 afios.