New possibilities for absorption refrigeration machines for energy saving

A. L. Stolk

Nouvelles utilisations possibles des machines frigorifiques absorption en vue des 6conomies d'6nergie

On examine/es rapports sur /e refroidissement par absorption pr6sent#s ~ /a Commission B2 au cours du XVe Congr#s de ~'~IF ~ Venise. Le cyc/e d'absorption #tant directement entraTn# par/a cha/eur, i~ convient des formes sp#ciales de transformation de/~nergie s'appuyant sur /a cha/eur perdue et /a cha/eur so/aire. II existe aussi des d#bouch#s pour les pompes de cha/eur ~ absorption.

Papers on absorption refrigeration, presented to Commission B2 during the XVth IIR congress in Venice, are discussed. Because the absorption cycle is driven directly by heat it is suitable for

The theme of the XVth llR congress in Venice was: 'Refrigeration and the preservation of world resources'. One of the most important resources is energy. The refrigeration machine uses energy to transform a heat flow from low to higher temperatures; the heat is 'pumped upwards'.

Since there is a fast growing interest in absorption cycles some papers dealing with this principle are discussed in this article. The absorption machine is driven directly by heat and is therefore, very suitable for using otherwise rejected heat or solar energy. Absorption cycles form rather simple systems for heat transformation, so there are good possibilities for their use as heat pumps. It is clear enough that the need to save energy has stimulated much research and development.

Spray absorber

The most critical part of the cycle is the absorption process itself. Refrigerants and absorbents as used nowadays, are chosen because of their suitable thermodynamic properties, but all of them show exothermic absorption. The absorption heat must be taken away to keep the solution at the required temperature. For big, continuous systems with a solution pump, the falling film absorber is seen as the best form in which heat and mass transfer are realised simultaneously. The mass transfer can be enhanced by spraying the solution into the vapour chamber, but the droplets absorbing vapour cannot reject the absorption heat.

Bykov et al. 1 described a system with separated heat and mass transfer. The poor solution from the

The author, President of Commission B2 of the IIR, is at the Laboratorium voor Koudetechniek, Techeische Hogeschoot, Mekelweg 2, Delft, The Netherlands. The paper is a review of seven papers presented to Commission B2 at the XV International Congress of Refrigeration in Venice, Italy

78 0140-7007/80/020078-05 $02.00 © 1980 IPC Business Press Ltd. and IIR

specific forms of energy transformation based on waste heat and solar heat. There are also openings for absorption heat pumps.

generator is undercooled to such an extent that the absorption heat can be taken up by this solution without surpassing the required temperature. The absorption itself is an adiabatic isobaric process in this case.

The disadvantage is that a sufficiently low temperature must be available to realise the undercooling and the question can be raised whether or not it is better to use this low temperature in a conventional film absorber. In the described system the undercooled solution is sprayed in a vapour chamber. It is found that the gain in the mass transfer coefficient eliminates the disadvantage of the higher temperature of the solution droplets.

The principle is used in a number of large units with a refrigeration capacity of 6000 kW, producing cold water of 4 - 12°C. Absorbent and refrigerant are lithium bromide and water, respectively. Waste heat from a power plant is used as the driving energy.

The spray absorber gives a pressure loss saving 170 Pa as compared with a film absorber, which is very important, especially for lithium bromide and water cycles. With a hot water temperature of +92°C for the generator and a cooled water temperature of +8.6°C the COP was 0.73. This value has been measured in practice and shows that the system works very efficiently when compared with conventional absorbers.

Heat and cold storage

Using rejected heat for driving refrigeration machines may make it difficult to match the available heat to the cooling demand. Holldorff 2 states that an absorption cycle is well suited to solve this problem. Storage of heat is rather expensive and it may be better to store refrigeration capacity. It is not difficult

Revue Internationale du Froid

to integrate storage facilities for solution and refrigerant in an absorption machine. A flow diagram of such a system is given in Fig. 1.

Two extreme situations are possible. In the first case heat is transferred to the system and no cooling is demanded. Rich solution from storage vessel RSS is pumped to the regenerator or desorber D. In condenser C the vapour is liquefied and subsequently stored in vessel RS. The poor solution goes to vessel PSS. In the second case there is no heat input and the production of cold is at a maximum. Poor solution from PSS is fed into absorber A and absorbs vapour from evaporator E, which is fed with liquid from vessel RS. The rich solution is pumped to RSS. If PSS is full, RSS will be nearly empty and vice versa. It is possible to combine vessels PSS and RSS in one vessel. Because of stratification no mixture will occur between rich and poor solution.

The main features of the system are: all the accumulated fluids have approximately ambient temperature and no insulation is required; there is no thermodynamic degradation during the storage period; and no additional heat transfer surfaces are needed for cooling of the product.

The COP of the system depends greatly on the periods and sequences of heat production and cold demand and on the temperature levels. Holldorff gives some examples and compares the absorption cycle with a Rankine cycle, but with storage facilities. In general the absorption system is 70% better. This applies only for heat driven machines.

The system is very promising, but careful calculation is needed for every specific case. A disadvantage may be that two or three storage vessels are needed instead of one when heat or cold is accumulated. The result of developments in this area may be that, in the near future, rather small accumulators will become available which can be cheaper than the pressure vessels used in the present system.

k 2 J i P i_ . . . . . . i

• ---1- c( RO ® - - -

i

!IT--I Pss

: - - ®

_ ( ~ i . . i . g of

Z ~ [end of heating

z ~ ( ~ [End of cool~ ,~ (~ {l~ginning of

[heoting period RS

RS=NH~ storoge vessel

PSS = storage vessel for poor solution

RSS = storage vessel for rich solution

Fig. 1 An ammonia absorption refrigerating machine incorporating solution storage to give operating flexibility (from B2-66)

Fig. 1 Une machine frigorifique ~ absorption d'ammoniac incorporant un stockage de la so/ution pour assurer/a souplesse de fonctionnement (d'apr#s B2-66)

To domestic woter sucolv,

Cooling woter

Fig. 2 Schematic design of combined domestic absorption refrigerator and water heater (from B2-57)

Fig. 2 Sch#ma d'une combinaison de r#frig#rateur m#nager absorption et de chauffe-eau (d'apr#s B2-57)

H o u s e h o l d r e f r i g e r a t o r s

Statistics of many countries show that half, or even more, of the energy consumed by refrigeration occurs in households. It is, therefore, important to develop more efficient equipment for this area. Mieczynski et al.3 presented an absorption unit for a household refrigerator combined with a heat pump for hot water. The principle of using condensing and absorption heat is not new, but applying this idea to a household refrigerator is interesting.

A schematic drawing is given in Fig. 2. It is a simple and reliable unit with no moving parts. A compression type refrigerator has a better COP than an absorption unit but when the efficiency of transferring heat into electric energy is included, the difference is not significant, even assuming compressor units will be improved in the future. The reason for low efficiency of absorption units is the great amount of heat, due to regeneration and absorption, that has to be rejected to the environment.

In the proposed system a large part of this heat is used for hot water production which gives a very high efficiency. The cooling unit acts as a heat pump for the hot water production. The COP can be defined as:

Qo + Qw /;rh -

Qg where: Qo = amount of refrigeration

Volume 3 Num~ro 2 Mars 1980 79

Ow= amount of heat transferred to the hot water system Qg = heat input in generator. A measured value of this COP is about 1.7.

The extra investment when compared with that for a conventional absorption refrigerator is not large, the reliability is the same and the energy saving is quite substantial. For this reason the proposed unit deserves our full attention. In a pilot unit an evaporation temperature of -15°C has been attained. It seems reasonable to start work on a system for the lower temperatures as required by freezers as well. Perhaps two-stage absorption will be needed in that case, which makes the unit more complicated, but the potential advantage remains.

S o l a r d r i v e n r e f r i g e r a t i o n m a c h i n e s

The development of solar driven refrigeration machines is gaining momentum, particularly in the field of absorption systems.

Keizer 4 reported the development of an extensive mathematical model for liquid absorbent systems. This model is suitable for the analysis of solar driven machines, but can be used for other boundary conditions as well, ie for heat pumps. The model has been tested by experiments on a specially built test rig and the agreement is rather good. When low evaporation temperatures are required, and absorption and condensation temperatures are relatively high due to the environment, the available regenerator temperature is of extreme importance. Fig. 3 shows the results of an analysis of an ammonia-water cycle. The required regeneratior~ temperature is given as a function of the evaporation temperature with the absorption temperature as parameter. Condensation temperature is +30°C. It is a single-stage cycle.

The conclusion is that when watercooled condensers are applicable and the solar driven regenerator has a temperature of +85°C, an evaporation temperature of -15°C will be possible. When an air cooled absorber of +40°C is used, the minimum evaporation temperature will be -5°C.

120 II0 ~,,,,~lerator Tonder, r~r = 3O"C

I00 ~'-,,,,.~~ with mctificmion

| 7o : ~ r,,4o-c k 6 0

50 :065 TA=30=C 40 A 5O

I I I t l I I ;~(3[ --zo -~s 40 -5 o 5 lo

~ . , ° C

Fig. 3 Calculated temperature required at a solar heated generator operating in different absorber outlet temperatures. Ambient temperature is 20°C (from B2-60)

Fig. 3 Ternp#rature exig#e ca/cu/#e au g#n#rateur ~ chauffage so/aire fonctionnant b diff#rentes temperatures de sortie de /'absorbeur. La temperature ambiante est de 20°C (d'apr#s B2-60I

F ,7 I TA =30~C"~A =l ~=3OoC A= =1 o

ss ~ - ~ o , ~ / j j ~ - % =h~ ~ , ~ , ~ ~ , J><'J J SSl=selectNe coot~ and one

1" / I / I I I ~ ff absorb~ efficiency -20 -15 -I0 -5 0 5 I0 15

Tevopora tor '°C

Fig. 4 The effect of di f ferent solar col lectors on the eff iciency of the absorpt ion refr igerator (from B2-60)

Fig. 4 Influence de diff~rents capteurs so/aires sur /e rendement du r#frig#rateur ~ absorption (d'apr~s B2-60)

It is expected that in future, high-performance collectors will be available which give a regenerator temperature of + 110°C.

The efficiency of the solar collectors is important. Keizer analysed the overall situation and an example of his results is shown in Fig. 4. The solar intensity is 600 W m -2. Parameters are the type of solar collectors, the absorption temperature and the absorber efficiency. It is clear from this picture that for a single stage system with evaporating temperatures lower than 0°C, high-performance collectors are needed. For frozen food stores with an evaporation temperature of -25°C, two-stage cycles are unavoidable.

The point must be made that the results were obtained on a test rig in the laboratory with non-ideal processes, as would occur in practice. But there may be a difference between a pilot plant and a real installation in the field. Further calculations and experiments are to be done with variable solar intensity over the day time and with storage of either heat or cold.

An interesting improvement for solar-driven air conditioning units is proposed by Kremnyov et al. 5. Lithium chloride is used as absorbent and water as refrigerant. The solar collector and regenerator are integrated and the main point is that water vapour is directly rejected to the atmosphere. Fig. 5 gives the flow sheet of the system. The rich solution is pumped from absorber 4 to the collector-regenerator 5 and flows in a free film in the air space between collector plate and glass cover. The poor solution goes via heat exchanger 7 and air separator 10 back to the absorber. The evaporated water in evaporator 3 is replaced by fresh make-up water.

The evaporator, absorber and air separator form the low pressure side, ie lower than 1 x 105 N m -2 (1 bar) The regenerator works, of course, at atmospheric pressure. The low and high pressure sides of the system are separated by a pump and valve as usual.

Changes in solar intensity are equalised by a buffer in solution tank 9. The heat from the absorber, air

88 International Journal of Refrigeration

separator and heat exchanger is used for warming up water. It may be a good working system for lower regeneration temperature. The authors claim a difference of 12-14 K as compared to a closed system.

The principle can only be used in hot and dry climates. Other disadvantages are the corrosiveness of the salt solution and pollution of the solution by dust and other material from the environment. The application of corrosion resistant material and special filters to eliminate these disadvantages makes the installation rather expensive.

Wors~e-Schmidt continues his work on solar driven units with solid absorbers e. A good mathematical model is now available which has been tested by experiment. The governing equations are derived from energy balances and heat and mass transfer correlations. Three sets of boundary conditions are considered: the heat flux to the desorption wall related to the net heat from the solar collector; fixed surface temperature of the absorber wall or a heat transfer correlation; and the temperature of the cooling medium.

An example of the result of computed values and experiments is given in Fig. 6. The temperature of the desorber and the degree of desorption are given as functions of time during the desorption part of the intermittent process. There is excellent agreement between model and experiment.

The described mathematical models of systems with liquid absorbents and solid absorbents are suitable tools to compare both principles and to decide which is the best cycle in given circumstances. Most probably, the outcome will be that for each system there is a field of application depending on climate, economic development and infrastructure at the location of the installation.

5 A t ~ air

TO oir condtl io~ I I Isystwn To oooling tower I - ~ 8

Fig. 5 A solar-driven absorption refrigerator using lithium chloride solution in an open type evaporator (from B2-54)

Fig. 5 Un r6frig~rateur solaire 6 absorption utilisant une solution de chlorure de lithium dans un 6vaporateur ouvert (d'apr#s B2-54)

16C

14C

12(

o ,oc

Heat~l sur'face ~ .~: ~ ~ "~

J -

Time, h

,o! 38 "6

3.4

32.

Fig. 6 Computed performance data for a solid absorbent type refrigerator, solar-driven, shown as a function of time in the cooling cycle (from B2-30)

Fig. 6 Performance calcul6e d'un r6frig#rateur ~ adsorbant sol/de, entraFn# par 6nergie so/a/re, en fonction du temps du cycle frigorifique (d'apr#s B2-30)

Hybrid compression absorption cycle

The old idea, described by Osenbr0ck in about 1900, of a combined absorption and compression cycle, has never ceased to draw the attention of engineers. Especially with respect to energy saving is there a renewed interest. The main advantage lies in the use of gliding temperatures by replacing the condenser and evaporator by a resorption cycle. But there are two important disadvantages. The refrigerant must be oil- free to prevent distortion of the absorption process. Oil-free piston compressors are expensive. There is no problem when the installation is big enough for the use of turbo-compressors. The second point is that the absorbent can interfere with the compression part. Absorbents with a very low vapour pressure are needed or special measures must be applied around the compressor. It is worthwhile seeing whether or not the scientific and technical developments since Osenbreck's time give opportunities to eliminate these disadvantages.

Nowotny 7 has made new calculations for the cycle proposed by Altenkirch. Using modern opt/m/sat/on methods he compares the Altenkirch machine with optimised single- and two-stage compression systems. At a -10°C evaporation temperature the Altenkirch machine has a COP of 5.5 compared to 3.7 for the compression cycle. At -40°C the COP is about the same for both principles, viz. 1.5. The condensation temperature in this case is +30°C. The next step to be taken is to build a pilot unit.

It is clear that absorption refrigeration cycles are capable of a new important role, especially aimed at

81 Volume 3 Number 2 March 1980 [J.a. 3 .2--B .,

s a v i n g e n e r g y a n d s i m p l e o p e r a t i o n . T h e r e is m u c h d e v e l o p m e n t a n d e n g i n e e r i n g w o r k t o be d o n e to m a k e t he s y s t e m s r e a d y fo r a b r o a d a n d e c o n o m i c a l

a p p l i c a t i o n .

References Bykov, A.V., Kalnin, I.M., Shmuilov, N.G., Rosenfeld, L.M. Heat utilizing lithium bromide absorption heavy duty refrigeration machine with adiabatic-isobaric absorption process, Paper B2-81

2 Holldorff, G.M. Industrial absorption refrigeration plants with storage facilities suitable for operation with intermittent heating energy, Paper B2-66

3 Mieczynski, M., Patkowski, R. Household absorption refrigerators with systems for water heating, Paper B2-57

4 Keizer, C. Absorption refrigeration machine driven by solar heat, Paper B2-60

5 Kremnyov, O.A., Zhuravlenko, V.Y., Grosman, E.R., Tolstykh, I.P., Shavrin, V.S. Study and development of absorption type solar cooling units, Paper B2-54

6 WorsCe-Schmidt, P. Computer simulation of the solid absorption process, Paper B2-30

7 Nowotny, S. The compression type refrigerating machine with additional liquid cycle, Paper B2 1

The papers were presented at the XV International Congress of Refrigeration in Venice, September 1979

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