automotive cooling by soalr

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Heat Recovery Systems & ClIP Vol. 13, No. 4, pp. 335-340, 1993 08904332/93 $6.00 + .00Pr in t ed in Gr ea t Br i t a in Per gam on P r es s L td

A P P L I C A T IO N O F A D S O R P T I O N C O O L I N G S Y S T E M S

T O A U T O M O B I L E S

M O T O Y U K I S U Z U K I

Institute of Industrial Science, University of Tokyo, 7-22-1 Ropponsi, Minato-ku, Tokyo 106, Japan

(Received 22 January 1993)

Abstract--Adsorption cooling systems using water as the working fluid could minimizeenvironmentalproblems associated with current automobile air conditioningsystems.The exhaust heat could be usedto provide the thermal energy input to the system.

A number of problems have to be addressed, including adsorbent design and bed configurations.Techniques which might be used to achieve performance argets are discussed.

1. INTRODUCTION

Currently, the total amount of CFC and HCFC used for air conditioning purposes in Japan is

estimated to be 43,000 ton per yr, of which 50% is CFC-12 and 40% is HCFC as of 1986. Air

conditioning apparatus of automobiles, especially those of passenger cars, employ CFC-12 as a

working fluid. The CFCs used for this purpose eventually are emitted to the atmosphere, which

are stable in the troposphere and finally contribute to ozone depletion in the stratosphere. This

fact then encourages activities in the research and development of alternative working fluid such

as HCFCs and HFCs which have shorter lives in the atmosphere. From the standpoint of global

warming, however, almost all the hydrocarbon halides are strongly infrared-active substances.

Therefore the usage of these substances, if any, have to be made under carefully controlled

conditions.Adsorpt ion cooling system which utilizes water as a working fluid is attractive since it minimizes

environmental problems when applied to automobile air conditioning. Also, compared to the

ordinary air conditioning systems, it is expected that adsorption cooling system can utilize the

exhaust heat from the automobiles without losing any mechanical energy output from the engine,

which eventually minimizes gas consumption by automobiles.

In order to achieve this goal, however, there will be some problems to be made clear. The purpose

of this paper is to make a preliminary study to elucidate the technological limits associated with

the application of adsorption systems to passenger car air conditioning.

2. AUTOMOBILE AIR CONDITIONING

2 .1 . Gen era l f ea tu r es

Modern technologies adopted in recent passenger cars are more and more refined. Gas economy

and safety requirements as well as the ease of driving and other principal necessities have

continuously improved the technological grade of the new models.

Main components of the current developments which are of concern here are (1) introduction

of electronic devices in wide varieties of control technologies; and (2) more and more necessities

of good gas mileage from the standpoint of reducing carbon dioxide emissions. The lat ter is related

to (i) minimizat ion of the total weight of a car; and (ii) minimization of the unnecessary dynamic

load to the engine.

2.2. H e a t b a l an c e o f a p a s s e n g e r c a r

In the case of high speed ignition-type engines which are most common in passenger cars, heat

balance is estimated as shown in Table 1 and Fig. 1. Heat losses through radiators and exhaust

gases or radiation heat losses total 65-70% of the combustion energy of the fuel consumed. The

335



336 M. SUZUKI

coolant temperature is usually controlled to 368 K by detecting at the point between the exit of

the engine and the inlet of the radiator and controlling the flow rate of coolant through the radiator.

The coolant after the radiator is about 333 K and returned to the engine. The exhaust gas from

the engine has the temperature of about 700-900 K at the exit of the piston room, which is then

cooled by heat exchanging with the air through the exhaust pipe and muffler walls.

These energy losses might be recovered and utilized as a heat source of an adsorption cooling

system. In the case of a compact passenger car o f 2000 cc class, fuel consumption ranges from 1.2 1h -t (3 x 10-7m-3s 1)at the idling state to 4-5 .51h 1(1.1-1.5 × 10-6m3s t ) at 60k mh ~ 16.7m

s -t) city driving. By considering LHV of gasoline to be 7770 kcal 1 ~ (3.25 x 101°j m-3), the fuel

consumptions correspond to 10,800 and 34,900-49,600 W, respectively. If we consider the city

driving as a standard state to be considered for air conditioning, 23,000-32,500 W, which is 60%

of the fuel consumption, is considered to be the potential energy source to be used for adsorption

cooling system.

2.3. Weight and load of current air conditioners

The air conditioning system currently employed consists of a compressor which is driven by the

engine, two heat exchangers and a receiver of the condensed working fluid. One of the heatexchangers is the evaporator of the working fluid, which exchanges heat with the indoor air and

the other is the condenser of the working fluid vapor which is cooled by the external air. The total

weight of the three main components is expected to be 15-20 kg. For compact size passenger cars,

air conditioning apparatus of about 200 kcal h- t (2300 W) are used, which compensates radiant

heat input through windows, 970 kcal h-~ (1125 W), heat transmitted through walls, 330 kcal h-t

(380 W), heat input accompanied with natural air ventilation, 2000 kcal h -~ (230 W) and heat

evolution from passengers, 400 kcal h-t (460 W).

3. MODEL CALCULATION

3.1. Conditions

For the conceptual design of adsorption cooling systems, regeneration temperature, Tr,s, ambient

temperature, Ta, and cooling water temperature, T,, should be defined. If a part of the exhaust

gas is utilized as a regeneration gas, T~g can be easily as high as 473 K. Ambient temperature at

summer time is about 303 K, from which Ta is assumed here as 313 K as a safe approximate. The

temperature needed as a target of air conditioning is below 300 K, which will be easily attained

if the evaporator temperature, Tw, is kept around 283 K.

Trac t ion , 30 %

A Cool ing , 35%

3-333K

Frict ion, 5 %

Fue l , 100%

Exhaust Gas ,3 0 % , 7 2 0 - 9 0 0K

F i g . 1 . E n e r g y b a l a n c e o f a p a s s e n g e r c a r .



Ap p l i c a t io n o f a d s o rp t io n c o o l in g s y s t e m s

F b , T ~ -" -

( a ) Ta( b ) T r e g

F e , T ~

( a , b ) T a

A t m o s p h e r e , T aA!

i D F b , T ~ u t

I ~ , °l l O o n ~ i n e r , bW a te r V a p o r 3 E I

~ A . . . t ~ . . s p h e r e T ao i

Fe, T ~ut

( a ) T c o o l ln g

Fig . 2 . Schematic mode l o f adsorp t ion coo l ing sys tem: (a ) adsorp t ion s tep ; (b ) regenera t ion s tep .

33 7

3.2. Ba s ic co n cep t

A mathematical model of mass and heat balances in an adsorption cooling system is found in

Sakoda and Suzuki for the case of solar regeneration system [1]. Similar, but more simple treatment

can be applied for the present case by modifying regeneration temperature to be constant. Theschematic idea of the model is shown in Fig. 2. Adsorption step (a) corresponds to the cooling step

where water evaporation takes place at the water container and at the regeneration step (b), the

adsorbent bed is heated up by the exhaust gas and desorption of water takes place. These two steps

are to be repeated in series and hence at least two units are to be coupled for the purpose of

achieving continuous air conditioning.

3.3. Ad so rp t io n i so th erm

I f the regeneration gas temperature is assumed to be 473 K, water-zeolite system will become

a candidate of the adsorbate-adsorbent system. There must be as many candidates as absorbate

zeolites but as a first approach, NaX zeolite is adopted as an example. Chuikina et a l . [2] measured

the adsorpt ion isotherms of water vapor on NaX at different temperatures, of which data obtainedat 373 K are shown in Fig. 3. The data are replotted against adsorption potential in Fig. 4 where

the adsorption potentials, E(w) and E(reg), which correspond to the adsorption step (cooling step)

and to regeneration step, respectively, are included.

E(reg) = RT ~g ln(Ps(Tr~g)/P~(Ta)) = 21,031 kJ mo1-1

E w) = RT, ln(Ps(T,)/Ps(Tw)) = 4663 kJ tool -z,

where T~g = 473 K, T~ = Tco~ = 313 K, and Tw = 283 K give Ps(T, ,g ) = 15.34 atm = 1.17 x 104mm

Hg (1.55 x 106Pa), P s ( T a ) = 5 5 . 3 2 m m H g (7375Pa), and P~(T,,)=9.21 mm Hg (1228Pa),

respectively.

From the figure, the difference of the equilibrium amounts adsorbed is read as 10.8 mmol g-

102

i 1 0 1 ' =

lO~oq

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I I I l l i l I l l l l i

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, i i i i i ! . . - ' . : : ' , ' , ' . ' , ' , : : ' .

. J [ i H L I IF " l [ I I I I. . . I I l l l. . . .

I l l l l i l l l l I l l l lill Ilil i lll l

I I I I I I 1 1l o 0 l o ~ 10

P r e s s u r e , P ( m m H l O

I I I I I

I l l l l lI l l l l l

l l l l l i

] ]I l l I lI I I I I

I I1 1 " 1 , J lI I I I I I

I l l l i

I I

F ig . 3 . A d s o rp t io n i s o th e rm o f wa te r v a p o r o n N a X z e o l it e a t 3 7 3 K [2] .



3 3 8 M . S u z u K ]

e

2 0 . . . . • . . . . • . . . . . . . . . , . . - , . . . .

1 5 •

° s

10 [ • •

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. . . . . . . . . . : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A .

E q ui li br iu m l ev el ~ N ~

0 , , . . . . . . . . . . . . . . . . . . . . . . , i , , , ,

t . e e.s 1.0 l .s z * Ls 3.0

A d s o r p t i o n P o t e n t i a l , e:RTIn(Ps/P ) ( 1 0 ( J / t o o l )

F i g . 4. A d s o r p t i o n i s o t h e r m o f w a t e r v a p o r o n z e o l it e N a X a t 3 7 3 K [ 2] .

w h i c h i s t h e m a x i m u m c a p a c i t y a v a i l a b l e i f e q u i l i b r ia a r e re a c h e d a t b o t h s te p s . A c t u a l o p e r a t i o n ,

h o w e v e r , is e s t i m a t e d t o b e o p e r a t e d i n q u i c k e r c y c l e s w h e n c o m p a r e d w i t h th e t i m e n e e d e d t o r e a c h

e q u i l i b r iu m . T h u s , t h e f r a c t i o n a l a t t a i n m e n t o f th e a d s o r p t i o n e q u i l i b r i u m s h o u l d b e c a l c u l a t e d

a c c o r d i n g t o t h e p r o p e r m o d e l .

3.4. B a s i c e q u a t i o n s

W h e n a d s o r b e n t o f w e i g h t , W s, i s p a c k e d i n a c o n t a i n e r a n d w a t e r o f Ww o i s i n i ti a l ly f e d t o t h e

e v a p o r a t o r , t h e m a s s b a l a n c e o f w a t e r i s g i v e n a s

WsT+dWw=0, (1 )

w h e r e q = 0 a n d W w = W , ,o a t t = 0 . T h e a d s o r p t i o n r a t e i s g iv e n a s

d qd t = k s a v ( q * - q ) (2)

w h e r e q * i s t h e e q u i l i b r i u m a m o u n t a d s o r b e d a t p r e s s u re P ~ ( Tw ) a n d t e m p e r a t u r e , T~ . H e a t b a l a n c e

a t t h e a d s o r b e n t b e d i s d e s c ri b e d i n a s i m p l if i e d m a n n e r a s :

d ( c s W s T , ) Q , , W s ~ t t - ( h o A ) b ( T s Ti~) (3 )d t

wh e re T ib n= Ta c a n b e a s s u m e d f o r t h e a d s o r p t i o n s t ep a n d T~ n = T ~g f o r t h e r e g e n e r a t i o n s t e p,

a n d h e a t b a l a n c e a t t h e e v a p o r a t o r i s g i v e n a s :dd t ( (C w W w + C c W e ) T w ) = L d W w- d t - ( h o A) '(Tw - T , ) + F~C, , (T~, - T~ut). (4)

T h e s e f o u r e q u a t i o n s a r e s o l v e d s i m u l t a n e o u s l y f o r r a t h e r q u i c k c y c l e s o f a d s o r p t i o n

s t e p - r e g e n e r a t i o n s t e p s o f ( 1 ) 6 0 - 6 0 s ; ( 2) 1 2 0 - 1 2 0 s ; ( 3) 1 8 0 - 1 8 0 s ; a n d ( 4 ) 1 8 0 - 6 0 s. A s t h e m o s t

s i g n if i c a n t p a r a m e t e r , U A o = h o A / ( W ~ / p b ) , i .e . o v e r a l l h e a t t r a n s f e r c o e f f i c i e nt b e t w e e n t h e a b -

s o r b e n t b e d a n d c o o l i n g o r h e a t i n g g a s es o n t h e b a s is o f t h e u n i t w e i g h t o f a d s o r b e n t . F o r z e o l i te

b e d s w i t h a h e a t t r a n s f e r d is t a nc e o f 5 m m , U A o i s e s t i m a t e d a s 9 9 0 W m - a K -~ b y a s s u m i n g t h e

e f fe c ti v e t h e r m a l c o n d u c t i v i t y o f t h e p a c k e d b e d t o b e a r o u n d 0 .2 W m - ] [ 3 ]. U A w a s t a k e n a s a

p a r a m e t e r a n d c a l c u l a t io n s w e r e m a d e f o r U A = 1 0 , 2 0 , 5 0 , 1 0 0 an d 2 0 0 t ime s U A o b y c o n s i d e ri n g

f u t u r e i m p r o v e m e n t o f h e a t t r a n sf e r c h a r a c t e ri s ti c s o f a d s o r b e n t b e d s.C y c l i c s t e a d y s t a t e s a r e r e a c h e d a f t e r n u m b e r s o f s uc c e s s iv e c y c l e s w h e r e t h e a m o u n t a d s o r b e d

d u r i n g t h e a d s o r p t i o n s t ep , A q ~ d,, a n d t h e a m o u n t d e s o r b e d d u r i n g t h e n e x t r e g e n e r a t i o n s t ep , A q ~ .,,

b e c o m e e q u a l . T y p i c a l e x a m p l e s a r e s h o w n i n F i g . 5 f o r t h e e a s e o f U A = 5 0 x U A o . A p p a r e n t

d i f f er e n c e s i n c y c l i c s t e a d y s t a t e s f o r d i f f e r e n t c y c l e p e r io d s c o r r e s p o n d m a i n l y t o t h e d i f f e r e n t



Application of adsorption cooling systems

0 .3 5 . . . . , . . . . . . - . . . . , . . . . . . . . . . . , . . . . .

0 .30 ~

i.20

0 . 1 5

i 0.10

0 .0 5 A d s o ~ , ~ F " ~ ¢ n e r a f i o n

0, 00 . . . . . i . . . . . i . . . . . i . . . . . i . . . . . i . . . . .

60 120 180 240 300 360

T i m e ( se c )

Fig. 5. Change of am ount adsorbed o n z eolite during cyclic steady state w ith UA = 49.7 kW m -3 K -[,T~g = 200°C , T~ = 40 °C and T,, = 10°C,

339

d e g r e e s o f a c c u m u l a t i o n o f a d s o r b a t e s i n t h e p a r t i c l e s d u e t o i n c o m p l e t e d e s o r p t i o n d u r i n g

r e g e n e r a t i o n p e r i o d s .

F r o m t h e s e r e su l ts , t h e c o o l i n g c a p a c i t y p e r u n i t m a s s o f a d s o r b e n t , 2Aq ad,/T, ds ( W kg -m

a d s o r b e n t ) , w h e r e 2 i s t h e l a t e n t h e a t o f v a p o r i z a t i o n o f w a t e r a n d :Fads s th e t im e o f t h e a d s o r p t i o n

s t e p , i s c a l c u l a te d . F i g u r e 6 s h o w s t h a t t h e q u i c k e r c y c l e w i th t h e h i g h e r h e a t t r a n s f e r c o e f f ic i e nt

n a t u r a l l y p r o v i d e s a h i g h e r c a p a c i t y f o r c o o l i n g , w h i c h r e s u l ts in s m a l le r a m o u n t o f a d s o r b e n t s f o r

t h e r e q u i r e d c o o l i n g c a p a c i t y .

I n o r d e r t o r e a li z e th e s e q u i c k e r c y c l e s , t h e n e e d f o r th e m o l d e d a d s o r b e n t w h i c h p r o v i d e s g o o d

a d s o r p t i o n c h a r a c t e r is t ic s a n d h e a t t r a n s f e r a b i l it y w ill b e th e m o s t p r o b a b l e w a y w h e n t h i s s y s t e m

i s t o b e c o m m e r c i a l i z e d . T h e m o l d e d a d s o r b e n t s h o u l d h a v e a g o o d r e s i s t a n c e t o v i b r a t i o n s a n d

s h o c k s m e t w h e n l o a d e d i n t h e a u t o m o b i l e . A l s o , i n o r d e r f o r t h e q u i c k o p e r a t i o n t o b e r e a l i z e d ,

h e a t t r a n s f e r c h a r a c t e r i s ti c s o f t h e r e s e rv o i r o f a d s o r b e n t s a n d o t h e r a t t a c h e d p a r t s s h o u l d b e

c a r e f u l ly a n a ly z e d . F u r t h e r m o r e , t h e c o m m e r c i a l iz a t i o n o f th i s p ro c e s s , th e t r a n si e n t b e h a v i o r o f

t h e p a s s e n g e r c a r d u r i n g a c o l d s t a r t m u s t b e c l ar if i ed . T h e t r a n si e n t c h a n g e o f t e m p e r a t u r e o f t h e

exhaus t gases , pa r t i cu l a r l y , needs a ca re fu l de f i n i t i on s i n ce t he exhaus t hea t exp lo i t ed f rom t he

e n g i n e i s t h e m o s t i m p o r t a n t e n e r g y s o u r c e f o r t h e a d s o r p t i o n c o o l i n g s y s t e m .

HRS 13/4"~E

|

t

3000

20OO

1001

6 0 s e c - 6 O s e c /

0 0 . . . . . . . . i . . . . . . . . a . . . . . . .

1 0 1 0 1 1 0 2 1 0 3O v e r al l H e a t T r a n s f e r C o e f f l d e n t , U A ( k W / m 3 /s )

Fig. 6. estimated cooling capacity o f short cy cle adsorption cooling system with regeneration temperatureof 200°C. Capacity corresponds to heat removed during adsorption cycle.



340 M . SUZUKI

4 . C O N C L U S I O N

A p p l i c a t i o n o f a d s o r p t i o n c o o l i n g s y s t e m s t o a u t o m o b i l e s b e c o m e s p o s s i b l e i f a p p r o p r i a t e d e s i g n

o f a d s o r b e n t s a n d b e d c o n f i g u r a t i o n s i s m a d e f o r i m p r o v e d h e a t t r a n s f e r c h a r a c t er is t ic s . F o r

i n s t a n c e , a s s h o w n i n F i g . 6 , i f U A o f 1 0 0 k W m - 3 K -~ c o u l d b e a c h i e v e d a n d a d s o r p -

t i o n / r e g e n e r a t io n c y c l e s o f 60 s - 6 0 s c o u l d b e a d o p t e d , a c o o l i n g c a p a c i t y o f 2 8 0 0 W k g Lp e r o n e

u n i t o f a d s o r b a t e b e d i s e x p e c te d . T h e n f o r c o o l in g s y s t e m s o f 2 3 00 W , t h e a d s o r b e n t a m o u n t o fa b o u t 2 k g ( t w o u n i t s o f a d s o r b e n t b e d s , c o n t a i n i n g 1 k g in e a c h ) s e e m s e n o u g h . N a t u r a l l y , t h e

d i f fi c u lt ie s o f a c h i e v i n g t h e s e i d e a l h e a t t r a n s f e r c h a r a c t e r i s t ic s w i l l b e a f o c u s o f f u t u r e s t u d ie s .

A l s o , a n i m p r o v e m e n t in t h e m e c h a n i c a l s t r e n g t h o f a d s o r b e n t i s n e e d e d f o r u t il i z at i on o n

a u t o m o b i l e s a n d t h e a c t u a l d e s i g n o f t h e t o t a l s y s t e m , i n c l u d i n g h e a t e x c h a n g e r s , st il l n e e d s t o b e

c o n s i d e r e d i n o r d e r t o r e a l i z e t h e s e s y s t e m s .

R E F E R E N C E S

1. A. Sako da and M. Suzuki, Simultaneous transport of hea t and adsorbate in c losed typ e adsorption cooling systemutilizing solar heat. J. Solar Energy Engng, 108, 239-245 (1986).

2. V. K. C huikina , A. V . Kiselev, L. V. M ineyeva and G. G . M uttik , Heats of adsorption o f w ater vapour on NaX and

KN aX zcolites at different temperatures. 3".C hem . Soc., Trans. Faraday 1

72, 1345 (1976).3. M. Suzuki, Adsorption Engineering. Kodansha & Elsevier , A msterdam (1991).

automotive cooling by soalr

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