A solar-powered solid-absorption refrigeration system

A solar-powered solid-absorption refrigeration system

A solar-powered solid-absorption refrigeration system P. Wors~e-Schmidt Un syst&me frigorifique solaire absorption par solide fonctionna# avec des c...

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A solar-powered solid-absorption refrigeration system P. Wors~e-Schmidt

Un syst&me frigorifique solaire absorption par solide

fonctionna# avec des capteurs solaires ~)plaques planes.

On met au point actue//ement un syst~me frigorifique solaire ~)absorption par solide ~ utiliser dans des r6gions od les syst~mes frigorifiques usuels sont inapplicables mais od I'dnergie solaire existe en abondance. Une recherche initiale a montr~ qu" un processus d"absorption par solide prdsentait le meilleur compromis entre la simplicitd du systdme et un coefficient de performance raisonnable Iorsqu'il

On a construit, en rant qu'installation de ddmonstration, une petite fontaine r6frigdrde avec une surface de capteur de 4 m 2. A u Danemark, par temps clair, on a obtenu un coefficient de performance global de 10%, ce qui correspond ~ une production de glace de 6 kg m -2 de surface de capteur. L "installation de ddmonstration est en cours d"essai ~ I'universitd de Khartoum.

A solar-powered solid-absorption refrigerating system is being developed for use in areas where the complexity of conventional refrigerating systems is impractical, but solar energy is abundant. An initial investigation showed that a solid absorption process offered the best compromise between simplicity of the system and a reasonable coefficient of performance when operated with flat-plate solar collectors.

desorption of ammonia in calcium chloride and strontium chloride was carried out to establish suitable design parameters.

An experimental investigation of the absorption and In the wake of the energy crisis in 1973, the utilization of solar energy has attracted much attention for many purposes including refrigeration and, in particular, air conditioning. There is, however, also a need for solar-powered refrigeration that does not arise from increasing oil prices and the more acute awareness of the limited reserves of fossil fuels. In many developing countries the lack of infrastructure severely limits the application of refrigeration for storage of perishable foodstuffs which is essential for developing the potential for increased production that exists in many areas. In such countries solarpowered refrigerating systems which are of simple design and do not depend either on other energy sources, or on the availability of trained servicemen, may be attractive. The crucial question is whether the systems can be made sufficiently reliable at a reasonable price. The author is at the Refrigeration Laboratory. The Technical University of Denmark, DK-2800. Lyngby. Denmark Based on papers presented at IIR Commissions B1, B2 and E1 Meeting 1977-4, Belgrade (Yugoslavia) Nov. 16-18, 1977, and at The 1978 International Conference on the Application of Solar Energy, Haifa. Israel, Sept 3-7. 1978

Volume 2 Number 2 March 1979

On a effectud une recherche expdrimentale sur I"absorption et la ddsorption d"ammoniac dans du chlorure de calcium et du chlorure de strontium afin d'dtablir les paramdtres de la conception approprids.

To serve as a demonstration plant a small drinkingwater cooler with a 4 m 2 collector area was built. On a clear day in Denmark an overall coefficient of performance of 10% was obtained, corresponding to an ice production of 6 kg m -2 of collector area. The demonstration plant is presently being treated at the University of Khartoum. Surveys of the state of the art have been presented by Swartman 1 and Eggers-Lura et al 2. Interest has been concentrated on absorption systems and here mainly on the 'wet' type. The absorption process is well suited for the intermittent operation which comes naturally when the energy supply is limited to 6-8 h during the day. Furthermore, the process can be carried out by means of an extremely simple system, and the temperatures needed for desorption are within the reach of flat-plate collectors. For a Rankine-type cycle with mechanical compression much higher collector temperatures are needed as shown byTabor 3

S e l e c t i o n of a b s o r b e n t / r e f r i g e r a n t combination As a preliminary step in the present project the merits of various absorbents and refrigerants were examined 4. For evaporation temperatures of -10°C and below ammonia was found to be unsurpassed as the refrigerant. Three liquid absorbents (H20, NaSCN, LiNO3) and two solid absorbents (CaCI2, SrCI2), all with

0140-7007/79/020075-10 $02.00 © 1979 IPC Business Press Ltd.

75

ammonia as the refrigerant, were compared on the basis of a simple, intermittent process. For the NHa/H20-system rectification of the vapour was included whereas heat exchange between the weak and the strong solution was not considered for any of the 'wet' systems. With a liquid absorbent the coefficient of performance of the process (defined as the ratio of

v,

g

c e

Fig. 1 Typical lay-out of a small experimental absorption cooling system of the 'wet' type. The system shown works with H20 and NH3 s but similar systems have been built for NaSCN and NH3 e7 a - 3 mm glass sheets, b - collector frame, c - glass wool insulation, d - lower and upper headers, e - return line, frectifier, g - condenser-cum-evaporator, VA, VB, VC - valves; P~, P2 - pressure gauges Fig. I Schema de m o n t a g e typique d'une petite instal~at/on f r i g o r i f i q u e exp#r/mentale ~ a b s o r p t i o n du type "hum/clef Le syst#me pr#sent# f o n c t i o n n e avec H 2 0 et NH3 5 mais des installations semblables ont #t# construites p o u r NaSCN et NH3 6,7. a - plaques de verre de 3 m m d~paisseur, b - m o n t u r e du capteur, c - ~so~at/on d e / a / n e de verre, d co//ecteurs /nf#r/eur et sup#r/eur, e - eonduite d'aspiration, f - reetificateur, g c o n d e n s e u r #vaporateur, VA, VB, Vc rob~nets, P}, />2 manometres

0.4

the net amount of cooling to the total amount of energy supplied to the desorber) depends critically on the final temperature that can be reached during desorption. As the final temperature is raised more refrigerant is driven off the solution, ie the concentration difference between the strong and the weak solution ('Entgasungsbreite') increases. As a consequence the amount of solution - the heating of which to the desorption temperature constitutes a significant loss- decreases, and the COP goes up. In a continuous system this loss may be reduced considerably by regenerative heat exchange between the strong and the weak solution. However, for an intermittent system - and particularly one without a solution pump - it is not always possible, and at best difficult, to arrange for reasonably effective heat exchange between the strong and the weak solution (see Fig. 1 ). For a given final desorption temperature the lowest obtainable evaporation temperature is determined by the minimum temperature during absorption. The more the evaporation temperature is lowered, the smaller the'Entgasungsbreite' becomes and, hence, the larger the amount of solution that is needed. This explains why the COP drops off rapidly belowa certain evaporation temperature. For the same reason the 'wet' system is also rather sensitive 'to changes in the condensing temperature. These characteristics are illustrated in Figs 2 and 3 for a H20/NH3-system. The LiBr/NH3 and the NaSCH/NH3-systems behave similarly, the COP of the former being somewhat higher, that of the latter slightly lower than the value found for the H20/NH3-system. A larger'Entgasungsbreite' can be obtained without increasing the final desorption temperature by employing two-stage desorption at the expense, however, of a lower coefficient of performance. Such a system has been built for a dairy project in Afghanistan bya group at the Federal Institute of Technology in Zurich 8.

Evaporation temperature ,°C

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I 130 Final desorption temperature OC

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Fig. 2 Coefficient of performance of i n t e r m i t t e n t H 2 0 / N H 3 absorption cooling system Absorber mass corresponding to 10 kg per kg NH 3 generated, condensing temperature 40°C, evaporahon temperature, °C Fig. 2 Coefficient de performance de systemes frlgorifiques absorption intermittente de H20 /NH3. Masse d'absorbeur correspondant ~ 10 kg par kg de NH3 produit, temp#rature de condensation 40°C, temp#rature d'#vaporation, °C

76

5O°C Condensing temperature

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150 o 80

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Final desorption temperature ,°C

Fig. 3 Coefficient of performance o f m t e r m i t t e n t H 2 0 / N H 3 absorption cooling system Absorber mas corresponding to 10 kg per kg NH3 generated Fig. 3 Coefficient de performance de systemes frlgorlhques b absorption/nterm~ttente de H20 /NH 3 g a s d'absorbeur correspondant a 10 kg par kg de NH3 prodult

International Journal of Refrigeration

With a solid absorbent the problems are rather different. Absorption and desorption occur in one or more steps at equilibrium temperatures which are determined solely by the pressure in the system. Calcium chloride absorbs a total of eight moles of ammonia according to the following reaction equations: CaCI2 + 2NH3~_CaCI2 2NH3 + Lo 2

(1)

CaCI2 2NH3 + 2NH3~CaCI2 4NH3 + L2-4

(2)

CaCI2 4NH3 + 4NH3+-~CaCI2 8NH3 + L4-2

(3)

The two moles absorbed initially are so strongly bound that decomposition requires temperatures far in excess of those obtainable in a refrigerating system. The equilibrium temperatures for the other two reactions vary with the pressure shown in Fig. 4. At a pressure of 16 x 10 ~ Nm -2, corresponding to a c o n d e n s i n g temperature of 40°C, the equilibrium temperatures are 106°C for (2) and 93°C for (3). Similarly, at 3 x 10 s Nm -2, corresponding to an evaporation temperature of -10°C, the equilibrium temperatures are 65°C and 52°C, respectively. For the system SrCI2/NH3 the reactions are SrCI2 + NH3~SrCI2 NH3 + Lo-1

(4)

SrCI2 NH3 + 7NH3~-~Sr8NH3 + L1-8

(5)

Again, the one mole of ammonia initially absorbed is very strongly bound. The equilibrium temperatures for the second reaction fall in between those for the system CaCI2/NH3. The heats of reaction for (2), (3), and (5) are nearly equal when referred to a unit mass of ammonia and about twice the latent heat of evaporation. This means that the COP for a refrigerating system can never exceed 0.5. It will, in fact, be somewhat lower because part of the energy supplied goes into heating the absorber from the absorption to the desorption temperature. On the other hand, the COP of a solid-absorption system is not very

sensitive to changes either in condensing temperature or in evaporation temperature. The problem here is whether sufficiently high temperatures can be obtained during desorption and sufficient cooling provided during absorption. The performance of a CaCI2/NH3-system is shown in Figs 5 and 6. For the system SrCl2/NH3 results nearly identical to those for CaCI2/NH3 are found. From Fig. 5, which refers to a system with an absorber mass of 10 kg per kg of ammonia generated, it is seen that the coefficient of performance lies consistently above that of the H20/NH3-system and shows only a slight variation with the condensing and evaporation temperatures. Fig.6 emphasizes the importance of a light absorber design. In fact, according to Professor Rosenfeld ~o the failure of an early Russian attempt (in the thirties) to build solar-powered, solid-absorption refrigerating systems was due to too heavy an absorber design. Later, however, M u r a d o v et al successfully built and tested such a system 111213 For a system which is powered by'free' energy the COP in itself is not particularly significant. In the present context the importance of the COP lies in the fact that it is a measure of the necessary collector area, and the solar collector is by far the most expensive c o m p o n e n t in a solar-powered refrigerating system. However, the necessary collector temperature is equally important because, by and large, it determines the 0.5 Evaporation

temperature, OC

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Fig 4 Temperature-pressure (-1/7", log P) relation forthe CaCI2/NH3 reactions according to Linge 9. a - CaCI2 2NH3 Jr 2NH3~CaCI2 4NH3, b - CaCI2 4NH3 Jr 4NH3~CaCI2 8NH3, c vapour pressure of ammonia

Fig. 4 Relation entre la temperature et la pression (- 1 / T, log P) p o u r les r#actions de CaC/2/NH3 suivant Linge 9 a - CaCI22NH3 Jr 2NH3 ~- CaCI24NH3, b - CaC/2 4NH3 Jr 4 NH3 -~ CaCI2 8NH 3, c - press/on de vapeur d e / ' a m m o n i a c

Volume 2 Number 2 March 1979

I 40

I 45

5O

Condensing t e m p e r a t u r e , °C

[', t I i',l I so

I 35

temperature

I IIO

I 115 for final r e a c t i o n , °C

I

Fig. 5 Coefficient of performance for an intermittent solidabsorption (CaCI2/NH3) coolin¢l system. Absorber mass corresponding to 10 kg per kgNH3 generated

Fig. 5 Coefficient de performance d'un syst#me fngorihque absorption par solide intermittente (CaCI2/NH3). Masse d'absorbeur correspondant a I 0 kg par kg de NH3 p r o d u i t

77

efficiency of the collector. A comparison between different systems should therefore be based on the total COP (net amount of cooling divided by the total amount of radiation hitting the collector), ie the product of the COP for the refrigerating process and the collector efficiency. This has been done in Fig. 7 where the abscissa is the final temperature reached during desorption. For the two solidabsorption systems where the equilibrium temperature is fixed the variation in collector temperature reflects the fact that a certain amount of superheating of the absorbent is needed in order to obtain sufficiently high reaction rates. The calculations were carried out for two different collector designs, one with a selective surface and one with a non-selective surface. The computed efficiences are shown in Fig. 8. The result of the comparison is quite clear: for a simple system such as the one considered here the best overall COP is obtained with a solid absorbent, even when a final super-heating of 30°C above the equilibrium temperature for desorption is assumed. The comparison has been made for an evaporation temperature of -10°C. At lower evaporation temperatures the superiority of the solid-absorption system would have been even more pronounced. Since, moreover, the solid-absorption system is simpler than the 'wet' system and presents no internal corrosion problems it was decided to base the further work on the former, with either calcium chloride or strontium chloride as the absorbent.

Experimental investigation of the absorption and desorption processes The primary goal was to design and build a small demonstration plant to be tested at the University of Khartoum in collaboration with the Institute of Solar Energy, Council for Scientific and Technological Research of the Sudan. It was decided to design the system as a small drinking-water cooler with about 4 m 2 of (non-selective) collector surface The solar collector and absorber/desorber were integrated into a single unit with the absorbent contained in horizontal tubes as shown schematically in Fig. 9. Since hardly any information could be found in the literature on the heat transfer processes during absorption and desorption a small test rig was built in which various electrically heated absorber tubes could be tested (Fig. 10). Three kinds of tests were conducted: desorption with nearly constant heat flux; simulation of the actual desorption process with solar heating; and, absorption with constant tube-wall temperature.

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temperature ,°C

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Equilibrium temperature for final r e o c t i o n , ° C

Fig. 6 Coefficient of performance of interm~ttent sol~d-absorpt~on (CaCI2/NH~) cooling system with different absorber mass ratios. Evaporation temperature-5°C Fig. 6 Coefficient de performance d'un systeme frlgorffique absorption par sohde interm~ttente (CaC/2/NH3) avec dffferents taux de masse d'absorbeur. Temp#rature d'#vaporation - 5°C

78

I I O0

Collector temperature,

I 25

Condensing

90

Fig 7 Total coefficient of performance of solar-powered. intermittent absorption cooling systems. Evaporation t e m p e r a t u r e - l O ° C , condensing temperature 40°C Absorber mass corresponding to 10 kg per kg NH3 generated. Efficiency of solar collectors accordmg to Fig 8 selective surface, - - - - - non selective surface Fig. 7 Coefhcient de performance g/obal de systemes frlgorihques so~aires a absorption zntermittente. Temperature d'evaporation - 10°C, temperature de condensation 40°C. Masse d'absorbeur correspondant a 10 kg par kg de NH3 produit. Rendement des capteurs so~aires d'apres la Fig. 8 --surface s#/ective- -- -- surface non s#lectwe

International Journal of Refrigeration

Details of the test procedure and of the results are reported elsewhere ~4,~5.A summary of the test results is presented in Tables 1-3. Fig. 11 shows howthe surface temperature varies during I.O

0.8 -

"6 0.6 -Selective

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80

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130

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Collector temperature °C Fig. 8 Efficiencies of singly glazed solar collectors with selective coating ( K = 0 . 9 6 , e = 0 . 0 9 ) and ordinary black surface (K= 0.96). Insulation thickness 100 mm, direct radiation 8 9 0 W m -2, diffuse radiation 150 W m -2, ambient temperature 31 °C Fig. 8 Rendements de capteurs so~aires a une seule wtre avec rev#tement s#lectif (K = O, 96, e = O, 0 9 ) et surface no~re ordinaire (~ = O, 96). Epaisseur d'isolation 1O0 ram, r a y o n n e m e n t direct 8 9 0 W m -2, r a y o n n e m e n t diffus 1 5 0 W m -2, temperature ambiante 3 1°C Solar collector/ A

desorpti0n when the heat flux is kept nearly constant. The temperature rises rapidly until the equilibrium temperature for (3) is reached (T1 in Fig. 11 ). At this point there is a sharp break, indicating that generation has started. The temperature continues to rise, but at a much slower rate. When it has reached a value 7-2some degrees above the equilibrium temperature for (2), there is a further reduction in the slope. Finally, when about 90% of the maximum amount of ammonia that may be generated has been driven off, the slope gradually increases again. It should be noted that by the end of the generation period the surface temperature (T3) exceeds the equilibrium temperature by 30-40°C. Part of this temperature difference is caused by the thermal resistance of the absorbent, part of it is the local superheating needed for maintaining a sufficient reaction rate. At temperatures around 130-140°C the efficiency of a flat-plate collector with a non-selective surface is quite low. Itwas therefore decided to design the system for a degree of desorption of only about 70%. A penalty in terms of higher absorber weight per unit mass of ammonia generated is incurred, but the final temperature is lowered considerably. Fig. 12 shows the temperature variation during simulation with a final degree of desorption of 73% The absorber/ . . . . . tar tube and Aboorber/generoll)r tubewith electric the NH3 receiver hsoter and coolingcoil ~sulationlre shown without

Pressure~(~switches

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T eeder valve

t

valve

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&Doublepipe condenser -~Pulsation ~ ~IF |damping II // ~ illing valve Stagnant water conaenser Fig.9 Schematic representation of a solld-absorption refrigerating system (a small drinking-water cooler). The c o m b i n e d solar collector and a b s o r b e r / d e s o r b e r consists of a number of horizontal tubes containing the absorbent, connected by straight fins, The system goes through a full cycle every 24 h with absorption {and cooling) during the night, whereas desorption and condensation of ammonia vapour takes place during the 5-7 h around noon. The liquid ammonia is stored in the receiver at the bottom of the water basin until the temperature of the collector and. hence, the pressure in the system starts decreasing. The check valve then closes, and the liquid a m m o m a is transferred to the insulated receiver and surge drum. By continued cooling of the absorber the pressure is lowered further and evaporation of ammonia commences Fig. 9 Representation sch#matique d'un systeme fmgorifique a b s o r p t i o n par sohde tune petite fonta/ne r#fr/g#r#e) La c o m b i n a i s o n du capteur solmre et d e / ' a b s o r b e u r - d # s o r b e u r consiste en un certain n o m b r e de tubes horizontaux contenant I'absorbant, reh#s p a r des ailettes droites. Ce systems a c c o m p h t un cycle c o m p l e t en 2 4 h a v e c absorption (et refroidlssement) au cours d e / a nuit, tandis que /a d#sorptlon et la condensation d e / a vapeur d ' a m m o m a c se font en 5 a 7 h vers m~d/. L "ammomac hqu~de est stock# dons le reservoir au f o n d du bass~n d ' e a u / u s q u 9 ce que la temperature du capteur eL p a r su/te, /a press/on du syst#me c o m m e n c e n t a s'abatsser. Le c/apet de non r e t o u r se forms a/ors et /'ammoniac hquide est transfer# au r~servoir isol# et au bal/on basse pression. Par refro~d~ssement continu d e / ' a b s o r b e u r / a pression est encore abaissee et ~'evaporation d e / ' a m m o n i a c c o m m e n c e

Volume 2 Number 2 March 1979

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Fig. 10 Schematic diagram of test rig Fig. 10 Sch#ma du banc d'essai

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79

for a location c o r r e s p o n d i n g to that of Khartoum on M a r c h 21. Two layers of glazing and an insulation c o r r e s p o n d i n g to 100 mm of mineral wool were assumed. It is seen that the surface t e m p e r a t u r e n o w does not exceed 11 5°C. In Fig. 13 the course of the surface t e m p e r a t u r e has been traced in a d i a g r a m s h o w i n g the heat absorbed by the

c o l l e c t o r at various c o l l e c t o r t e m p e r a t u r e s This plot gives a direct measure of the fraction of the total insolation that is utilized for generabon of a m m o n i a : the area below the intensity curve represents the total insolation, and the area b e l o w the actual operation curve the utilized energy. The ratio between these two areas is the average c o l l e c t o r e f f i c i e n c y for the particular day. In the s i m u l a t i o n tests the influence of design p a r a m e t e r s like tube d i a m e t e r and tube spacing was also investigated. Table 3 summarizes the results obtained. From these it appears that a tube d i a m e t e r close to 36 mm and a tube spacing of 100 mm are near the o p t i m u m values. With a tube d i a m e t e r of 26 mm and a spacing of 50 mm the same overall c o e f f i c i e n t of p e r f o r m a n c e is obtained, but with this design both material costs and in p a r t i c u l a r the labour cost (welding) will be c o n s i d e r a b l y higher.

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A b s o r p t i o n tests were carried out with a test tube fitted with a c o o l i n g coil w h i c h permitted a c o n s t a n t surface t e m p e r a t u r e to be held during absorption. The pressure was maintained at a value c o r r e s p o n d i n g to an e v a p o r a t i o n t e m p e r a t u r e of - 1 0 ° C . The results are summarized in Table 2. They i n d i c a t e that an average surface t e m p e r a t u r e of a p p r o x i m a t e l y 4 0 ° C is needed for the a b s o r p t i o n to be c o m p l e t e d w i t h i n the t i m e available during the night.

Fig. 12 Collector temperatures and generahon of ammoma as found by simulation test for March 21 at a location corresponding to that of Khartoum {test series II, test tube 'A', tube spacing 100 mm) Fig. 12 Temperature du capteur et production d'ammomac d'apres I'essaJ de simulation le 21 mars bun emplacement correspondant a ce/ui de Khartoum (essai sefle //, tube d'essal ",4~espacement des tubes 1O0 ram) Table

1. Summary

Tableau

of desorption

1. R ~ c a p i t u l a t i o n

tests

des essais de d#sorption Measured

Test tube

Test no

A A A A A A A A B C E E

3G 6G 7G 8G 10G 11G 12G 13G 1G 2G 1G 2G

Table

2. Summary

Tableau A A A

GI/

G2/

Gma×

Gmax

0 0 0 0 0

0.84 0.86 0.84 0.89 0.89

0 0 0.11 0

0.81 0.84 0.86 0.85

T2,°C

T3, °C

12.7 14.1 13.8 13.8 11.2 13.8 13.6 16.4 14.1 12.7 14.1 13.8

109 111 109 109 107 111 107 116 107 108 111 106

136 139 132 136 135 147 128 140 138 138 127 122

of absorption

2. R ~ c a p i t u l a t i o n

49A 50A 67A

P, 105 ,'7/G/I"A, TII,° C Nm -2 h

0 0 0.37

0.69 0.87 0.94

4.60 4.74 6.50 5.51 4.70 3.30 8.50 4.90 4.50 4.45 3.60 6.33

85 89 88 88 83 88 87 94 89 80 88 88

Computed

TmG/TmA,°C TmG/TmA, °C

Discrepancy, °C

113.6 116.4 112.7 114.4 111.6 119.2 108,8 119.6 115.9 113.6 107.9 103.8

112.8 115.1 108.2 113.2 1 13.2

-0.8 -1.3 -4.5 -1.1 1.6

117.4 112.4 106.3 103.8

1.5 -1.2 -1.6 0

44.5 40.0 40.0

44.8 39.6 37.8

0.3 -4.0 -2.2

tests

des essais 3.0 3.0 3.0

d'absorption

10.5 10.5 10.0

Test tube characteristics Test tube Diameter. od/id, mm Tube length, m Internal NH3 distribution tube Absorbent Specific absorber volume. I (kg NHs) -1

80

A

B

C

D

E

42/36 2.0 Yes CaCI2/cem 2.90

42/36 0.7 Yes SrCI2/cem 2.87

42/36 0.7 Yes Pure CaCI2 3.18

32/26 2.0 No CaCI2/cem 2.78

32/26 2.0 Yes CaCI2/cem 3.14

I n t e r n a t i o n a l J o u r n a l of Refrigerahon

Since the compacting of the salt impedes the diffusion of ammonia the process is self-sustaining and may eventually create extremely hard and virtually impervious chunks of absorbent.

In addition to the tests mentioned above a longterm test of the mechanical stability of the absorbent was carried out. From the application of the solid absorption system for household refrigerators before World War II it is known 16 17 that migration of the absorbent is liable to occur after repeated cycling unless special precautions are taken. The reason for this phenomenon is that calcium chloride and strontium chloride swell considerably when absorbing ammonia. If - as is usuallythe case- the absorption does n o t o c c u r uniformly t h r o u g h o u t the absorber the salt will be displaced, and in some places it will be compacted.

I.I

-

60

0.8

70 80 9() I00

., o.7 E 0.6 =

-

During the first 100 cycles the temperature measurements on the test tube indicated no tendency to migration. From then on, however, first the reading of one and later of more of the thermocouples showed that some migration did in fact take place. When after 232 cycles the test tube was dismantled and cut in various places the absorbent was found as a fine powder, not filling the tube where the thermocouple reading had been high and compacted where the reading had been low. That very high pressures had prevailed during the compacting was evidenced by the fact that a central distribution tube for the ammonia vapour, an 8 x l mm steel tube, had been flattened against the wall of the absorber tube in several places.

40°C

I.O 0.9

In order to prevent this phenomenon the absorbent was mixed (in a wet state) with 1 5% of Portland cement and baked at 300°C. This procedure produced a highly porous mass which could be broken into granules of a suitable size (6-10mm), apparently possessing a reasonably good mechanical strength.

0.5

I

0.4

,,o 130

0.3 0.2

0.1

5

6

7

8

9

I0 II

Despite the negative outcome of the stability test the demonstration plant was charged with calcium chloride prepared as described above, because the development of a better solution to this problem could be expected to take some time. In order to keep the distribution tube in place and reduce the rate of migration, washers were inserted every 200 mm in the absorber tubes.

12 15 14 15 16 17 18 19 20 Local time

Fig. 13 Operation curve for the collector as found by the simulation test of Fig. 12, plotted in a diagram showing the collector Characteristics computed by a mathematical model. The curve marked H" shows the total insolation, the other curves t h e net heat absorbed at the temperatures indicated

Design and performance of the demonstration plant

Fig. 13 Courbe de fonctionnement du capteur d'apr#s I'essai de simulation de la Fig. 12, reproduite dans un diagramme montrant les caract#ristiques du capteur calcul~es a/'aide d'un modele mathematique. La courbe marqu6e H"montre /'insolation totale, les autres courbes la chaleur nette absorb6e aux temperatures indiqu~es

The demonstration plant was designed for a net amount of cooling of approximately 8 000 kJ per day. The combined solar collector/absorber was divided into two units, each with an effective

Table 3. Summary of simulation tests Tableau 3. R~capitulation des essais de simulation Test series

I

It

Date

Tube diam-

Tube

eter do. m

spacing w. m

Insolation

H". W h m

-2

Condens-Maximum , Refrigeraingtem- tubetem-Degree°T tionoutperature, • Tc. °C

Heat of Coeffic-, genera- ~entoT

erature generat,on put per m z tion per Pmax °C ,

Pmerfore

Average Overall,, II r coeH~cc o ecto ~fentmOafP:r" efficiency

G2/Grnax

Qo". Whm. 2

m 2 Qo". Whm J

Z=Qo,,'/Q',E=Q"/H"coP=ZE

March 21

0.036

0.07 0.10 0.20

7908

( 3 0 - ) 3 7 105 ( 2 8 - ) 3 8 115 ( 2 9 - ) 3 7 137

0.566 0.716 0.940

809 718 472

2890 2521 1655

0.280 0.285 0.285

0.365 0.319 0.209

0.102 0.091 0.060

June 21

0.036

0.07 0.10 0.20

6992

(30-) 37 98 ( 3 1 - ) 3 8 110 ( 3 0 - ) 3 7 124

0.455 0.609 0,789

650 610 397

2588 2232 14,01

0.251 0.273 0.283

0.370 0.319 0.200

0.093 0.087 0.057

March 21

0.026 0.036

0.05 0.10

36 36

113 114

0.873 0.731

803 733

2686 2541

0.299 0.289

0.340 0.321

0.102 0.093

June 21

0.026 0.036

0.05 0.10

6 992

36 36

108 107

0.693 0.638

637 638

2290 2261

0.278 0.282

0.328 0.323

0.091 0.091

0.036

0.078

7908

36

90

0.667

1 270

3188

0.385

0.403

0.155

Theor. a March 21

7908

a - Theoretical results refer to generation at the equilibrium temperature for (3)

Volume 2 Number 2 March 1979 I t R. 2/2--1~

81

Table 4. Summary of data from test on September 19 Tableau 4. R # c a p i t u l a t / o n des r#su/tats de ressai du 19 septembre A m o u n t of NH3 generated

area of 0.46 m 2. The evaporator coil upon which ~ce is frozen is made from a 1 7 . 2 / 1 5 . 0 mm stainless steel tube with a length of 4.7 m. giving a heat transfer area of 0.24 m 2. The coil is placed in a 60 I water tank, also made from stainless steel. 7 25 kg

Enthalpy difference (sat'd vapour at - 10°C -sat'd liquid at 18°C) Gross a m o u n t of cooling 7.25 x 1 1 6 9 =

1 169 kJ kg -~ 8 4 8 0 kJ

Cooling of surge drum

3 6 4 kJ

Heat infiltration

226 kJ

Net a m o u n t of cooling

7 . 8 9 0 kJ

Expansion due to freezing of ice Amount of ice frozen

2.22 1/0.917-1

2 22 I 2 4 5 kg =

Latent heat of fusion

3 3 3 kJ kg -~

Net a m o u n t of cooling 24.5 x 3 3 3 : Discrepancy 100 (8 160-7 8 9 0 ) / 8

8 . 1 6 0 kJ 160 =

3.3%

M e a n insolation (total) 8 : 3 0 - 1 6 : 0 0

762 W m-2

Total insolation 7.5 x 3 6 0 0 x 762 =

83.300 kJ

COP~ot= 8,025/83.300 = Thermal capacity of collector/absorber (Tf,nal - T,n,t,aOmean: 96-27 =

0.096 1O0 kJ°C-~ 69°C

Average heat of reaction

2300 kJ kg-~

Sensible heat 69 x 100--

6 900 kJ

Total heat of reaction 7 25 x 2 3 0 0 =

16 7 0 0 kJ

Total a m o u n t of heat absorbed

23 6 0 0 kJ

COPprocess = 8 . 0 2 5 / 2 3

600 =

Average collector efficiency 23 6 0 0 / 8 3 3 0 0 =

034 028

Prior to shipment to Khartoum the demonstration plant was tested at the Technical University of Denmark in Lyngby (Fig. 14). The test period lasted from the middle of August until late September. During the first two weeks the weather was reasonably good with maximum air temperatures around 25°C and only slight cloudiness or haze. However, a perfect day with a clear sky did not occur until September 19. The demonstration plant was provided with t h e r m o c o u p l e s for recording the temperature of the collector plates (ten on each), of the cooling water and of the second receiver and the water tank, The pressure in the system was measured with a pressure transducer. A sight glass on the second receiver permitted determination of the amount of ammonia generated during the day. In addition, the amount of ice frozen during the night could be determined directly by measuring the expansion accompanying the formation of ice. This was done by connecting the water in the tank to a 70 mm plexiglass tube and observing the rise in the water level. Climatic data (air temperature, wind speed and direction) and the total and diffuse radiation were provided by the Laboratory of Thermal Insulation which operates a test facility for solar collectors. During the tests in Lyngby the collectors were raised to an angle of 45 ° corresponding to the maximum solar height in late August. On reasonably good days in August the ice production was around 20 kg. However, on September 1 9, 7.25 kg of ammonia was generated and during the following night 24.5 kg of ice was frozen. The temperatures and the pressures recorded are shown on Fig. 1 5, and in Table 4 a summary of the test results is given. Note h o w t h e temperature at

collector area of 2.05 m 2. . The collector p l a t e / a b s o r b e r was made from ten 4 0 / 3 6 mm drawn steel tubes, separated by 68 mm wide steel fins. The collector plate was covered by two layers of glazing ( o r d i n a r y 3 mm w i n d o w pane) and painted with a matt black paint. On the back and along the edges the collectors were insulated by 100 mm of mineral wool. Cooling of the absorber during the absorption period presented a problem due to the heavy insulation. Calculations of the amount of heat that could be rejected at night showed that without special precautions it would not be possible to operate at an average surface temperature of 40°C. The solution chosen was to design the unit such that the entire back-side insulation could be opened.

F g. 14 Demonstration plan[ ourmg preliminary testing on the campus of The Technical Unwerslty of D e n m a r k m L y n g b y

In the stagnant-water condenser six 95 mm ( 3 / 8 i n ) steel tubes give a heat transfer area of 0.32 m 2. In addition the receiver at the bottom has a surface

Fig. 14 /nsta//at/on de d#monstratlon au cours des essa/s pr #hmmai r es au campus de I~co/e po/ytechnique du Danemark b Lyngby

82

International Journal of Refrigeration

I0 9 8 7

Pressure 120

/"

ur)

o

l

/

I

E Z

top

6 5

I00

t t

/

80 c)

60 o

4 3

i

Surge drum

Water basin

40

20 0

I1|1

0

-20

I 06

08

I

I

I

I

t

l

I

1

I

I

I

IO

12

14

16

18

20

22

24

02

04

06

Time

,

h

Fig. 15 Pressure and temperatures as recorded during test on September 19-20. The back-side insulationwas closed at 08:30 and opened again at 16:00. At 17:20 the insulation was closed in order to prevent the evaporation temperature from dropping below - 10°C Temperature at the bottom of the surge drum (lower curve) corresponds to the evaporation temperature Fig. 15 Pression et temperature enregistr#es au cours de I'essai des 19-20 septembre. L "isolation du fond a ete ferm#e a 8 h 3 0 et rouverte ~) 16h. A 17h20 I'isolat~on a #t~ ferrnee pour eviter que la ternp#rature d'evaporat~on ne descendft au-dessous de - 10°C La temperature au fond du ballon basse pression (courbe du bas) correspond #/a temperature d'~vaporation

the bottom of the collector consistently lies below that at the top, an indication of convection between the collector plate and the glazing. During the absorption the temperature at the toP remains at a Ivalue close to 40°C until about 01:00, whereas at the bottom the temperature starts decreasing shortly after 18:00 because onlya minor part of the ammonia was driven off during the day. The results of the t e s t - an ice production of 6 kgm -2 of collector surface, corresponding to an overall coefficient of performance of O . 1 0 - is very satisfactory and agrees well with the predictions. The fact that the ambient temperature was rather low does not invalidate the results because the equilibrium temperature during desorption goes down by the same amount as the condensing temperature so the collector was working under nearly the same conditions as those at a higher ambient temperature. However, although the pre dicted overall COP was reached the collector did not perform quite as well as anticipated. While the COP of the refrigerating cycle was higher than the predicted value, the average collector efficiency was correspondingly lower. The reason for this is found by inspection of the collector temperatures during desorption. Between the top and the bottom of the collector plates there is a considerable temperature difference, up to about 20°C, indicating strong convection between the plate and the glass cover. A small infiltration of ambient air on the back of the collector plate may also have contributed to the rather poor performance. An improvement in the collector efficiency, however, is not too difficult, either by changing the design of the collector plate so that a flat upper surface is obtained and the air gap can be reduced, or by inserting convection-suppressing glass strips in the air gap. With a flat collector surface and a suitable honeycomb layer better performance may be obtained with a single layer of glazing.

Volume 2 Number 2 March 1979

In November 1977 the demonstration plantwas shipped to Khartoum and assembled on the flat roof of one of the engineering buildings at the University of Khartoum. Under the supervision of Dr. Yahia H. Hamid further testing has been carried out b y t w o engineering students, Gamal Shawgi and Osama Ahmed. Adetailed account of the tests will be published later. It can be stated, however, that the plant has performed very satisfactori ly with a gross amount of cooling of more than 9 000 kJ per day. Conclusions From the work up to now two main conclusions may be drawn: it is in fact possible to obtain temperatures down to -10°C by means of an extremely simple solar-powered, solid-absorption refrigerating system with a total COP which compares favourably with that of similar systems of the 'wet' type; and, the present design needs several modifications, in particular with respect to the preparation and containment of the absorbent, but also with respect to cooling of the absorber. If one compares the heat transfer areas of the condenser and the evaporator with the collector area it is evident that on the basis of first cost a solar-powered refrigerating system will never be able to compete with a conventional system (including the prime mover). Even at a very high energy cost the pay-back period will be unrealistically long. The raison d'etre for such systems is rather their inherent simplicity and reliability, that they will be able to operate over long periods in areas where the infrastructure is practically non-existent with only the kind of maintenance that can be provided locally. Future work will be directed towards a design where the solar collector is less expensive. One possibility is to separate the various functions of the present integrated c o l l e c t o r / a b s o r b e r / d e s o r b e r design so that the absorber/desorber is connected thermally

83

5 Venkatesh, A.,Gupta, M.C. Performance of an

(eg by means of heat pipes) to a solar collector of far simpler design and to an air cooler. There may, however, also be certain advantages in having the collector and the absorber/desorber as a single unit which is connected to a separate cooler. In the final analysis this will be determined by the production costs.

6

Messrs P. Bechtoft Nielsen and Bo Stubkier performed the initial investigation for their MSc theses. The former carried out the subsequent theoretical and experimental investigation and made the design for the demonstration plant

8 9

Many valuable discussions with DrYahia H. Hamid, Institute of Solar Energy and Related Environmental Research, Council of Scientific and Technological Research in the Sudan, helped in developing the concepts upon which the project is based. The staff at the workshop of the Refrigeration Laboratory contributed significantly to the final design of the demonstration plant and did an extremely careful job in constructing the plant. The financial support of DANIDA (The Danish International Development Agency)is gratefully acknowledged.

7

10 11 12

13

14

15

16

intermittent ammonia-water solar refrigerator operating with a flat plate collector, COMPLES/DGS Int Solar Forum, Hamburg, July 12-14 19781"(1978) 4 5 3 - 4 7 2 Swartman, R.K., Swaminathau, C. Comparison of ammonia-water and ammonia-sodiumthiocyanate as the refrigerant-absorbent in a solar refrigeration system Solar Energy17(1975) 123-127 Kutty, M . M . , Arora, C.P. Performance of a solar refrigerator, 5th Nat'l Symp Refrign and Air Cond, Madras, Dec(1976) 113 118 Leibundgut, H.J. personal communication Linge, K. Ueber periodische Absorptionskaltemaschinen, Beiheft Z ges Kalte-lnd.2 (1929) 1 Rosenfeld, L.H. Discussion of ref 4 Muradov, D., Shadiev, O. Intermittent solar refngerator with solid absorbent, Problems of natural sciences (in Russian), (1969) Izd Tash G.V i B G P I , Tashkent Muradov, D., Shadier, O. Experimental inveshgation of the operation of an intermittent solar refrigerator, Proc All-Union Conf on the Utilization of Solar Energy/in Russian)(1969)VNITT, Moscow Muradov, D., Shadiev, O, Testing of a solar refrigerator GehotechnJka7 (1971 ) 3, 33-35 Nielsen, P.Bechtoft, Worsee-Schmidt, P. Development of a solar-powered solid-absorption refrigeration system. Part I: Experimental investigation of the generation and absorption processes, Rept F30-77.01, Refrigeration Laboratory, The Techn Univ of Denmark, August (1977) Worsoe-Schmidt, P. Design of a combined solarcollector/absorber/desorber for a solid-absorption refrigerating system, ASME paper 78-HT-35, presented at the AIAA/ASME Thermophys and Heat Transf Conf, Palo Alto, Calif. May 24-25 (1978) Buffington, R.M. Absorption refrigeration with solid absorbents, Retr, g n E n g n g 2 6 (1933) 137-142, 152 Plank, R., Kuprianoff, J. Die Kleinkaltemaschine (1960) 2.Aufl Springer-Verlag, Berlin/Gottingen/Heidelberg

References

17

1 Swartman, R.F. Review of Solar Refrigeration, Proc COMPLESInt Meeting Heliotechnique and Development, Dharan, SaudiArabia, Nov 2-6 19751T(1976) Development Analysis Associates Inc. Cambridge Mass

Professor P.M. Wors#eSchmidt gained his MSc m

2 Eggers-Lure, A., Nielsen, P. Bechtoft, Stubkier, Bo, Worsee-Schmidt, P. Potential use of solar-powered refrigeration by an intermittent solid absorption system, Proc COMPLES Int Meeting Heliotechnique and Development, Dhahran, Saudi Arabia, Nov. 2-6 1975111" (1 976) 8 3 - 1 0 4 Development Analysis Associates Inc, Cambridge Mass 3 Tabor, H. Cooling with solar energy- absorption machine versus expander plus compressor, Int Conf Exhibit on Solar Building Technology, London, July 25-29 (1977) Provisional P r o c ~ p a p e r 5.2

4

Nielsen, P. Bechtoft, Stubkier, Bo, Worslle-Schmidt, P. A critical survey of intermittent absorption systems for solar refrigeration PaperB2.1 14thlntCongrRefrign (1975) Moscow, Sept. 20-30

84

Mechanical Engineering in 1950 from the Technical University of Denmark and his PhD in 1955 from Stanford University, USA He is currently Professor of Mechanical Engineering and Head of Refrigeration Laboratory at the Technical University of Denmark. Professor Wors~e-Schmidt is a member of the Danish Academy of Technical Sciences, ASME, ASHRAE and the International Institute of Refrigeration and is vice chairman of the Danish

Council for Scientific and Technological Research His recent publications include 'Potential use of solarpowered refrigeration by an intermittent solid absorption system' presented at COMPLES International Meeting, Dhahran, Saudl Arabpa in November 1975 and 'Destgn of a combined solar collector~absorber~ desorber for a sohd absorphon refrigeration system' presented at the AIAA/ASME Therr'nophys=cs andHeatTransfer Conference, Pafo Alto. California. USA, In May 1978

International Journal of Refrigeration