Performance of an open-cycle desiccant cooling system using advanced desiccant matrices

0~0-,1332/89 $3.00+ .00 MaxwellPersamon Macmillanpic

Heat Recovery Systemu & ClIP Vol. 9, No. 4, pp. 299-31I, 1989

Printed in Great Britain

P E R F O R M A N C E OF A N OPEN-CYCLE DESICCANT COOLING SYSTEM U S I N G A D V A N C E D DESICCANT MATRICES P. Mxn.qm)AX Department of Mechanical Engineering, Northern Illinois University, DeKalb, IL 60115, U.S.A.

W. M. WOREK* Department of Mechanical Engineering (M/C 251), University of Illinois at Chicago, Chicago, IL 60680, U.S.A. (Received 8 October 1988)

Almract~The desisn and off-design performance of an open-cycle ~ o k d - M desiccant cooling system, ns~ng advanced desiccant matrices, is presented. The performance of the cooled-bed dehmnidifier, the principle component of the cooling system, is simulated ruing a numerical model that includes the heat and mass transfer resistance within the desiccant. Sorption rates predicted by the model are first compared to the result of expmimental tests in order to validate the model. The numerical model is then used to investisate the performance of a cooled-bed cooling system, operating in the recirod_ation mode, at variable indoor and outdoor conditions. The e f f ~ of regeneration temperature on the performance of the cooled-bed system is also documented.

NOMENCLATURE half of protein channel thickness (m) half of cooling channel thickness (m) aluminum sheet thickness (m) thickness of desiccant felt (m) c , , ¢2 constants given by (aY,*/~T*) and (aY~/~Y*) c^,¢, specific heat of air and felt (kJ (kg °C) -') D^ diffusion ~ t of water vapor in air (m2s -.) D. macmpore diffusion coefficient in the pore channels of the felts (m2s - t) D,. ~2, D~. o,.l>,, D. consUmts used in equation (22) g mare fraction of teflon and cotton in desiccant felt [(kli teflon + kg cotton) klg desiccant- ' ] H,,H2,e, enthalpy of process air, cooling air, and purging air, ~ v e l y 001~-') convective heat transfer coefficient in the p r o ~ channel, cooling channel and purging channel, hi, h2,/b respectively (kJ (m2s °C) -l) convective mass transfer coefficient in the p r o c ~ channel times dens/ty of air (kg m-2s) gvc g. effective thermal conductivity of desiccant felt (kW (re°C) - ' ) heat of sorption O0 k s - t H20) RH relative humidity of air 7". process air temperature (°C) cooling air temperature (°C) r2 cooling air temperature at inlet(°C) temperature of desiccantfelt(°C) T* al

a2

r. 7-o ro i". T.

t. TF t^, tar u,,uz, up XA, XAF X, X~

YA,Y~ Y, YF

temperature of aluminum sheet (°C) adsorbing air temperature at inlet (°C) desorbing air temperature at inlet (°C) temperature of purging air (°C) pursing air temperature during preheating (°C) purling air temperature during precooliag (°C) non-dimenmonai time and total n o n ~ o n a l time, respectively time variable, total sorption time, respectively (s) velocity of process air, cooling air and pursing air (m s- t) space variable in the direction of process air flow and length of process channel, respectively (m) non-dimensional space variable in the direction of p r c ~ air flow and non~i ~m~uional channel length, respectively space variable in the direction of cooling air flow and lensth of the cooling air channel, ~ . tively (m) non-dimmmional space variable in the direction of cooling air flow and non-dimansional length of the cooling channel, respectively

*To whom corresl~ndence should be addressed. 299

300 Y

Yo y*

y. ZA Z, Z F

P. MAJUMDARand W. M. WOREK moisture concentration of process air (kg H,O kg dry air- t ) moisture concentration of desorbing air at inlet (kg H20 kg dry air- ~) moisture concentration of process air or adsorbing air at inlet (kg H20 kg dry air -~) moisture concentration of ambient air (kg H20 kg dry air- ~) moisture concentration of air in the pore channels of desiccant felt (kg H : O m -3) moisture sorbed in the solid phase (kg H20 kg of desiccant- ~) space variable in the direction of thickness of desiccant felt (m) non-dimensional space variable in the direction of thickness of felt and non-dimensional thickness of the felt, respectively

Greek letters p

porosity density tortuosity

Subscript A b m al

air felt macro pore aluminum

1. I N T R O D U C T I O N

The utilization of low temperature energy sources to cool buildings has received considerable attention over the last several years. Sorbent cooling systems are activated by solid or liquid sorbents which are able to exchange a transferable component, known as sorbate, from a fluid mixture. In solid desiccant cooling systems, the sorbate is water vapor, which is cycled between the air and the porous desiccant material. Among the different sorbents investigated for use in desiccant cooling systems, the most popular are silica gel and molecular sieve. Molecular sieve requires a higher temperature heat source for regeneration, generaUy greater than 150°C. Silica gel, however, can be regenerated at much lower temperature, typically 50-80°C. Therefore, a desiccant cooling system using silica gel as the sorbent can utilize any low temperature heat source for regeneration. There are two basic configurations of solid desiccant dehumidifiers. The first is a rotary type, presented by Pennigton [1] and Dunide [2], in which moisture from the process air is adsorbed in an impr~nated packing. This configuration is used in a Munters Envirommmtal Control system which bag been developed at the Institute of Gas Technology [3], u~ng molecular sieve as the sorbent. A similar system, which uses a packed drum of silica gel, was ~ by A i ~ h [4]. A complete cooling system has also been developed by American Solar King [5] and is being developed by Tecolen [6]. T h ~ concepts, which consist of a rotating matrix c o n i n g the solid desiccant, are adiabatic. Since the adsorption process in these systems is ~ b a t i c , the temperature of the desiccant and the air stream can be quite hot (i.e. 45°C), which reduces the adsorption capacity of the desiccant. Lunde [7] proposed using stage cooling during the adsorption process to lower the desiccant temperature. A complete cooling system using two cooled, fixed-bed d e h ~ was developed at the Illinois Institute of Technology (HT) [8]. The IIT system consists of a ~ u m i d i t i e r that uses silica gel in the form of a felt that lines both the sides of the process channel. The process stream and the desiccant felts are cooled using a secondary air stream to remove the heat that is generated during adsorption. Several invettigators [9, 10] studied the process of sorption of water vapor by ~ t materials. L i ~ solutions for adiabatic d e h ~ were given by Bank, Close and M ~ [11] using an analogy method, and by Mathiprakautm and Lavlm [12] ~ Laplacetraratforms. Collier [13] numerically studied an adiabatic cooling system and ~ the effect of shape and staged regeneration on cooling system performance. Non-linear solutions for crosscooled dehumidifiers were obtained by Roy and Gidaspow [14] using ~ ' s function for single-blow olx~tion and numerically by Mathipr~asam [15] for periodic, st~mdy-state o ~ t i o n . Worek [16] used the numerical model developed by Ma'dfipmhamm [15] to compare results of numerical predictions with experimental data obtained from a prototype d e h ~ . This comparison gave poor agreement between the numerical model and experimental results and

Performance of an open cycle desiccant cooling system

301

indicated that heat and mass transfer effects within the sorbent must be included. Recently, Majumdar and Worek [17] have developed a model that considers diffusion and sorption of moisture in small interparticle channels within the desiccant felt, and transport of heat in the felt, by conduction. This model is used in this paper to investigate the design-point and off-design performance of an open-cycle, cooled-bed desiccant cooling system using advanced desiccant matrices.

2. SYSTEM CONFIGURATION AND OPERATION The cooled-bed desiccant cooling system consist of two cross-cooled dehumidifiers, two sensible heat exchangers and three direct evaporative coolers. The dehumidifiers are cyclicly switched through four modes of operation; adsorption, preheating, desorption and precooling. In the adsorption process, the first dehumidifier adsorbs moisture from supply air and is cross-cooled. Simultaneously, the second dehumidifier desorbs moisture which was adsorbed in the previous adsorption cycle. Figure 1 shows the arrangement of the components of the system operating in the recirculation mode. Figure 2 illustrates the process, showing the system state points, on a psychrometric chart. Figure l a shows details of the system with a dehumidifier in the adsorption mode. Room air is drawn at the comfort temperature and (state 1) is dried in the adsorbing dehumidifier. The air being dehumidified is heated slightly (due to heat liberated during sorption), even though the dehumidifier is cooled by a secondary air stream. The process air (state 2) is then sensibly cooled in the heat exchanger to state 3 and finally, the air is adiabatically humidified in the direct evaporative cooler until state 4 is reached. This conditioned air is then returned to the room. A cooling stream of

Adsorbing Dehumidifier

T

~

[= I I

Space

E~pcntiv¢

6

C_¢¢I~

Evaporative Cooler

Heat Exchanger

]

(a) Adsorption Mode

Thermal

10

Heat Source S

S., E X ~

2..

5

11

(b) Deaorption Mode

: 13

I

Preheating Dehumidifier

12

5

(c) Purging Mode Fig. H.R.$. 9/4---B

Preeoofing L ~. ~

Delmmidifle~

1.

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P. MAJUMDAR and W. M. WOREK

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20

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.o -

I0

,v

--5 Recirculation Mode Room Condition Outdoor Condition IO

20

30

40

50

I

I

60

70

O 80

Dry Bulb Temperature, °C Fig. 2.

outside air is drawn, at state 5, into another evaporative cooler and is cooled to state 6. A portion of this cool stream is passed through the dehumidifier cooling channels and the remainder is sent to the sensible heat exchanger. Both streams are subsequently discarded to the outdoors. During the desorption mode, the moisture in the desiccant dehumidifier is removed by passing hot air, typically 60-65°C, delivered from a low temperature heat source. A sensible heat exchanger is used to recover the sensible heat from the hot and humid air stream leaving the dehumidifier. Figure I b shows the details of the system in the desorption mode. The outdoor air (state 5) enters the heat exchanger and is heated by the desorbing air. The air leaves the heat exchanger at state 8 and is further heated in an low temperature heat source to the desired regeneration temperature (state 9). The air then enters the desiccant dehumidifier being desorbed, and leaves at state 10. The air then enters the heat exchanger where it exchanges heat with the entering outdoor air before being discarded. The desiccant bed, at the end of the adsorption process, is cool and can be preheated before desorption. This would lower the heat required during desorption. Likewise, the other dehumidifier, which has completed desorption, is hot and should be cooled. This process, of preheating the dehumidifier which has completed adsorption, and precooling the dehumidifier which has completed desorption is called the purging process. During the purging process, the cooling channels of the two units are connected in series. Figure lc shows the dehumidifiers in the purging process. Outside air is taken at state 5 and is sent through the cooling channels of the hot dehumidifier and is heated to state 12. The first dehumidifier is cooled as the air stream removes heat from the bed. This heated air is then passed through the cooling channels of the other dehumidifier, preheating it to state 13. The dehumidifier is said to be operating under periodic, steady-state operation, when the performance is identical after two successive adsorption/desorption cycles. Since the initial temperature and moisture distribution in the bed are not known, it is necessary to assmne these conditions at the beginning of each simulation. The solution of the resorption process then gives outlet conditions of the desorbing air and the final condition of the bed, which is also the initial condition of the bed for the purging (i.e. precooling) process. The solution of the purging process gives outlet conditions of the purging air and also the final condition of the d e h ~ bed. Using this final state of the dehumidifier as the initial state for the adsorption process, the performance in the adsorption mode can be determined and the initial condition of the bed for the purging (i.e. preheating) can he obtained. Finally, the purging process is completed and this gives the initial

Performance of an open cycle desiccantcoofingsystem

303

condition for the desorption pi'ocess which begins the next cycle. In this manner, the performance of the system can be calculated for a number of cycles until the results of two subsequent cycles are identical (i.e. within 0.1%). 3. MATHEMATICAL MODEL The prindpal components of the desk~ant cooling system are the cooled-bed dehumidifiers. Figure 3 shows a sch~natic of a cooled-bed desic~'~ant dehumidifier. The process air (air to be dehumidified) enters channels of width 2a~, and cooling air is passed through intermittently arranged cooling channels of width 2a2. The wall, of thickness aw, separating the process and cooling channels includes a desiccant felt which is made of micron-sized silica gel particles. Commercially available silica gel particles were fabricated by a specially developed manufacturing procedure into porous, paper-like felts which are bound to aluminum plates of thickness a~. The desiccant felt manufactured in this form has interparticle macropores and intraparticle micropores. This channel design gives good thermal contact between the desiccant and the cooling stream, and reduces the pressure drop in both the process and cooling channels. The design of a cooled-bed desiccant dehumidifier requires a clear understanding of the beat and mass transfer processes that occur within the porous desiccant felt during the sorption processes. For the cooled-bed desiccant dehumidifier geometry considered here, there are gas-side resistances to heat and mass transfer from the air stream to the surfg~ of the desiccant felt and resistances to heat and mass within the desiccant felt. The resistances to mass transfer within the desiccant felt involves a number of physical mechanisms. These are: (1) surface reaction associated with the sorption process; (2) interparticle channel diffusion and sorption within the felt and (3) micropore and macropore diffusion and sorption within the desiccant particles. Often, one or a combination of these transport processes is important during the sorption process. During adsorption, heat is released as the desiccant adsorbs water vapor. The heat generated raises the temperature of the material and is transmitted through the material to the air stream. This reduces the sorption capacity. Therefore, the transport of water vapor and heat within the desiccant are a coupled process and must be considered simultaneously. 3.1. Governing equations The diffusion of mass and the conduction of heat that occur in a desiccant felt can be described by considering conservation of mass and heat on a differential element as shown in Fig. 4. The major assumptions used to derive the equations are: (1) the axial conduction and diffusion in the channels are small compared to the convective flow of heat and mass and are negligible; L

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(2) the heat and mass transfer, from the air in the process and cooling channels to the surface of the desiccant felt, can be approximated by constant transfer co¢fl~ents; (3) the axial conduction and diffusion in the interparticle channels within the desiccant felts are negligible; (4) the sorption process is assumed to be without hysteresis; (5) the porous felt is assumed to be homogeneous and the concentration of material, other than sorbate, is unaffected by the sorption process; (6) the interstices within the porous felt arc considered to be tortuous channels through which the sorbate diffuses; (7) the surface diffusion of sorbed liquid may be neglected for the size of pores considered in this analysis and only the mechanisms of ordinary and Knudsen diffusion are important; (8) rate of diffusion in the pores of the particles is faster compared to the diffusion in the interpa~cle channels; (9) the effective thzrmal conductivity in the porous material is represented by an arithmetic mean corresponding to a parallel arrangement of two phases. A detail description of the mathematical formulation of moisture diffusion and heat conduction in cooled-bed dt,dccant dehumidifier was preseated by Majumdar and Worek [17]. The set of governing equations and boundary conditions for sorption and purging process~, used for this study, is given as: Desiccant felt mass transfer: OY*

02Y * OT* = E's-~-z~ - £'6 ~t

(1)

Desiccant felt heat transfer:

0T*

O2T*

Ot = Es~~

0Y* 4" Em @t

(2)

Process channel mass transfer: OY

0"~ = 2(Y* I , - , / p ^ - Y)

(3)

Process channel heat transfer:

OT,

0x = E2(T* I,- J - T,)

(4)

or: Oy = E~(T,,- T,)

(5)

Cooling channel heat transfer:

Performance of an open cycle deska:antcooling system

305

Boundary conditions, moisturei

og*[ OZ

•O

(6)

• Eio(Y - Y*l.-I/P^)

(7)

z-O

and boundary conditions, temperature:

OT* ~Z

z-

I

OT*

c~z z.o

ffi B E . ( T . . - T:) + F.2~OT~

Ot '

(8)

where B = 1 for adsorption and B ffi 0 for resorption, and

°r*10z z.

I

~ E . ( T i - T*l,.i).

(9)

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Y(0, t) = Y0, 72(0, t) = T~.

(1o)

Inlet conditions, desorption: 71(0, t)---- Tv, Y(0, t)ffi YD.

(ll)

Desiccant felt heat transfer, purging process: aT*

a2T *

at ~

~ z2"

(12)

Cooling channel heat transfer, purging process:

aT,

0x ffi F~(T., - T,)

(13)

Boundary conditions, purging process:

OT*[

= Ea ~t~ + F~(T., _ T, )

(14)

-- O.

(15)

~'X Z--0

and: aT* [ a." ]z.i

Inlet conditions:

r,(x, o) = r.h

(re

T,(x, 0) •* T.~

(17)

during preheating and: during precooling.

Initial conditions. Under periodic steady-slam operation, the final condition of the desiccant felt (i.e. temperature and moisture content) will be the initial condition for the next process. Considering the cycle to consist of: (1) resorption; (2) prce~oling; (3) adsorption; (4) preheating, in that order, the initial conditions are: Temperature: T*(x, 0) at the hegining of resorption ,= T*(x, Tn) at the end of preheating; T*(x, 0) at the beginning of precooling ffi T*(x, Tv) at the end of desorption; T*(x, 0) at the heginning of adsorption ffi T*(x, Tin,) at the end of pr~ooling; T*(x,0) at the beginning of preheating ffi T*(x, Tv) at the end of adsorption. Moisture: Y*(x, 0) at the heginning of desorption - Y*(x, Tv) at the end of adsorption; Y*(x, 0) at the beginning of adsorption ffi Y*(x, Tv) at the end of desorption.

306

P. MAJUMDARand W. M. WOgEK

3.2. Desiccant equilibrium properties The system of equations governing the dynamics of sorption has to be solved including the equilibrium sorption properties of the desiccant felt. There is, in general, a single value of moisture content, Y* in the desiccant felt when it is in equilibrium with air of a given dry-bulb T* and humidity ratio, Y*. This statement can be expressed as:

Using the psychrometric relations for moist air, this equation can also be written as: Y~*= Ym*(T*,RH).

(19)

Where relative humidity is given as: Y* I0' RH = 0.622 + Y*/p^'

(20)

and

s -- 4.21429

7.5 T* 237.3 + T*"

(21)

Dini and Worek [18] determined the equilibrium sorption isotherms of the silica gel felt using a Thermogravimetric Analysis (TGA) test set-up. The sorption isotherms for the silica gel felt were determined at temperatures ranging from 22.4 to 79.9°C and for relative humidities ranging from 4 to 95%. These data were fitted by a non-linear polynomial of relative humidity as a function of temperature and moisture content using linear regression analysis. The form of the equation is given as: ] ' m~ = D I

+ D 2 T .2 -I- D3RH 2+ D,,RH 3 + Ds T * 3 R ~ 2 "t- De T*3RH 3,

(22)

where: DI = 0.03298692; D2-- -0.4113481 * 10-s;

D3 = 0.01050466 * 10-3; /)4 --- -0.6586239 • 10-6; D5 -- - 0.7894248 • 10-10; D6 = 0.6747826 • 10 -12. The plot of the curve-fitted polynomial is shown in Fig. 5. Using the equilibrium relationship given by equation (22), Cs and C2 variables used in the analysis are expressed as: Cl = (2D2 T* + 2Ds T * 2 R H 2 + 3/)6 T * 2 R H 3) -Jr-( 2 D 3 R H "F 3 D 4 R H 2 + 2DsT*3RH +3DeT*3RH2Co),

(23)

where: y*

- 1779.75T* log, 10 m Co =

P^

0.622 +

(24)

(237.3 + T*) 2

and: C2 -- (2D3RH + 3D4RH 2 + 2Ds T*3RH) +

T* 0.622 0.622 + Y'Y"

(25)

Performance o f a n open cycle desiccant coofing system

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4. NUMERICAL SOLUTIONS The heat and mass transfer equations governing both sorption processes and the purging/preheating process represent a coupled system of non-linear, first-order, hyperbolic partialdifferential equations for the air in the channels, and coupled, non-linear, second-order parabolic partial differential equations for the desiccant felt. These equations and boundary conditions are written in finite-difference form using an explicit finite-difference scheme. The results of finite difference solutions are checked by making an overall integral balances of heat and mass within the felt and in the channels. 4.1. Comparison with experimental results Work has been ongoing to develop new desiccant felts. A new process of manufacturing desiccant felts for use in a cooled-bed desiccant cooling system is to be tested by the Tennessee Valley Authority. The performance of several newly manufactured felts have been tested by Aharghoui [19] in a dynamic test system. He reported superior sorption capacity of these new felts compared to those developed previously. A comparison was made between the present mathematical model that includes conduction and diffusion in the felts and experimental results obtained by Abarghoui [19] using new desiccant felt. Table 1 gives the physical property data used for the felt and necessary non-dimensional variables needed for the mathematical model. Figure 6 shows the amount of moisture adsorbed per kg of desiccant as a function of adsorption time. 4.2. Performance of the new prototype The performance of the desiccant cooling system consisting of a new prototype dehumidifier, using desiccant felts manufactured by the water-laid technique, was simulated using the model described above. The non-dimensional parameters and inlet properties for all simulations are given in Table 2. Table I. Physical dimemion of the ~ Process channel spacing, 2a~ Desi~.,ant felt thicknen, aw Aluminum sheet thickness, a~ Process channel lenfgth, XAF Length of the channel in y direction, Ym mass flowratc of proce, air, m I mass of desiccant

dmmnel 2.74mm 1.95 mm 1.27 nun 17.8 cm 14.$ cm 0.000135 ks 17.36 wn

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Performance of this cooling system was determined for varying ambient conditions, regeneration temperatures and for varying room conditions within the ASHRAE comfort zone. Rcsuits of the design-point and off-design simulations are present~l in Figs 7-11. Figure 7 shows the elfect of increasing regeneration temperature on the performance of the cooling system. When the regeneration temperature increasm, the desiccant felts operate at lower moisture levels and therefore, the moisture cycled and cooling capacity increases. However, the heat supplied by the desorbing air, due to the finite effectiveness of the regeneration heat exchanger, increases at a rate faster than the cooling capacity. Therefore, the coefficient of performance of the system decreases as the regeneration temperature increases. Figure 8 shows the effect of outdoor air humidity ratio on the performance of the system. As the outdoor humidity ratio increases, the mass transport in the desorption process becomes less effective and the moisture cycled by the system decreases. Therefore, the overall dehumidification provided by the system decreases. The outdoor humidity ratio also affects the temperature of the stream used to cool the adsorption heat exchanger. As the outdoor humidity ratio increasm, even when an indirect/direct evaporative cooler is used, the temperature of the cooling stream increases. The higher sink temperature in the heat exchanger also leads to a lower sensible cooling capacity of the system. Therefore, the overall cooling capacity of the system (i.e. sensible and latent) decreases dramatically. This decrease also causes a reduction in the coellkient of performance.

Figure 9 shows the performance of the system with varying outdoor air temperature. Increasing the outdoor air temperature causes a slight reduction in the heat input required to meet the specified Tab~....2.~

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4.0 16.0

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63.0°C 0.0142 k4lH=Olqi dry air-* 35.0~C

Performance of an open cycle desiccant cooling system

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air

Fig. 1t.

desorption temperature. Also, an increase in the outdoor air temperature increases the temperature of the stream used to cool the adsorption heat exchanger. This reduces the cooling capacity of the system, which is larger than the reduction of heat input in the desorption process. Therefore, the coefficient of performance of the system decreases with increasing outdoor air temperature. Figure 10 shows the variation of the system performance as the room humidity ratio increases. Increasing the room humidity ratio causes an increase in the moisture cycled and cooling capacity of the system. This is due to the increased capability of the dehumidifier to adsorb more moisture. However, the heat required to desorb the increased moisture adsorbed by the material increases. This causes the coetfufient of performance of the system to increase at a lower rate, as compared to the cooling capacity. Figure 11 shows the effect of changing the room temperature on the system performance. As the room temperature increases, the exit temperature from the adsorbing dehumidifier increases. Therefore, more cooling is done in the adsorption heat exchanger. This gives a higher cooling capacity and coefl~ent of performance. However, since the adsorption process is at a higher temperature, the overall moisture cycled by the system decreases, causing a reduction in the latent load that can be met by the system. 5. CONCLUSIONS The effect of variable indoor and outdoor conditions, including the regeneration temperature, on the performance of an open-cycle, cooled-bed desiccant cooling system, operating in the recirculation mode, is presented. The results show the system coefficient of performance is more sensitive to changes in the regeneration and indoor temperature and the outdoor humidity ratio than to changes in the outdoor temperature and indoor humidity ratio. The results also indicate that the cooled-bed cooling system can effectively utilize low temperature heat (i.e. 50°C) toprovide cooling. This enables the cooled-bed system to be coupled to a variety of low temperature sources of waste heat. The numerical results obtained using the detailed model can now be used to obtain simplified curve-fit equations describing the performance of the system. These equations could then be used to study the performance of the cooled-bed at different locations. 6. R E F E R E N C E S 1. N. A. Pennington, Htmfldity Changer for Air-camtitloni~. U.S. Patent, No. 2.700,537 0955). 2. R. V. Dtmkle. A method of mlar Jdr-eonditioning. Mech. C&nn. F~gag Trans. 73, 73-78 (1965). 3. W. R. Staats, J. Wurm, L. R. Wright and H. A. ~ma~uk, Field tuting of solar-MEC syst,ms final report, hmitute of Gas Tedmology, C ~ , IL, July (1977). 4. J. gouueau and K. C. Hwang, Preliminary d e , In of a solar ~ t w-conditioner, AiRuearch report. The third workshop on the ute of solar energy for the cooling of buildinll~ No. 78-14939 (1978) 5. D. Venhtfizen, Solar King's cooling Igambit, Solar Age 2£, October (1984). 6. B. A. Cohen, private communication. 7. P. J. Lund¢ and It. G. Grelloire, Solar De~icc~t Air.¢o~iomb~ with Sll~¢a Gel. Hart, CT, Aujust (1977). 8. W. M. Worek ~ Z. Lavan, Performance of a cross-cooled desiccant dehum/dif~erprototype, J. Solar Energy Engng 104, 187-196 0982).

311

Performance of an open cycle desiccant cooling system

9. M. Epstein and M. A. Grolmes,: Desiccant Cooling System Performmlce: A Simple Approach. Gri-E1/0089, Gas Research Institute, Chicago, IL (1982). 10. D. Schiepp and R. Barlow, Performance of the S E R I Parallel-passage Dehumidifier. SERI/TR-252-1951, Solar Energy Research Institute, Golden, CO (1984). 1I. P. J. Banks, D. J. Clme and I. L. Maclaine-cross, Coupled heat and mass transfer in fluid flow through porous media, J. Heat Transfer 7, 1-10 (1970). 12. B. Mathiprakautm and Z. Lavan, Performance predictions for adiabatic desiccant dehumidifiers using linear solutions, J. Solar Energy Engng. 102, 73-79 (1980). 13. R. K. Collier Jr., Advanced Desiccant Materials Assessment-Phase H GR1-88/0125. Gas Research Institute, Chicago, IL (1988). 14. D. Roy and D. Gidaspow, A. Cross-flow Regenerator--A Green's Matrix Representation. Chem. engng Sci. 27, 779-793 (1972). 15. B. Mathiprakasam, Performance predictions of silica gel desiccant systems. Ph.D. Thesis, Illinois Institute of Technology, Chicago, IL (1979). 16. W. M. Worek, Experimental performance of a cross cooled desiccant dehumidifier prototype. Ph.D. Thesis, Illinois Institute of Technology, Chicago, IL (1980). 17. P. Majumdar and W. M. Worek, Combined heat and mass transfer in porous adsorbent, Energy 14, 161-175 (1989). 18. S. Dini and W. M. Worek, Sorption equilibrium of a solid desiccant felt and the effect of sorption properties on a cooled.bed desiccant cooling system, Heat Recovery Systems 6, 151-167 (1986). 19. I~. A. Abarghoui, Manufacture and dynamic sorption of advanced desiccant felts--design of a cooled desiccant cooling system, M.S. Thesis, Illinois Institute of Technology, Chicago, IL (1985).

. APPENDIX--DEFINITION

OF N O N - D I M E N S I O N A L

AND x^Kyc

PARAMETERS

Y^K~c

z^ ~- p^D^t^

x - - -2plalu, , y ffi -2p~a2u~' z ffi--, a~ 2hl

2h2

K,c ,

X,cN

h,a. E,,=-iT,

PbC2a~ Kvc a.

E =~=D"

a~

C|pA EtsEi4 E~s = "~2 EI~ =

EtgEt-----:~

EIs : El3 E,4, l~p b

CAP

1

E,, = (1 - g)p~----~' E,, = (I - g)ppQG E == PAD/' 'l PbC2l~,

1, E~ = E,,E,-----~,

aaaw F-,'affi a--T-, E~ ffi C A P A L E21E,,

r~

E24 ffi E21E~E'~' E21 ffi CAP E'22 = pbC2/(aAD^),

E25 =

E'~ = a~/a~

2hp ~_H_o, E~ = C A P A L E26E27

Xvc E2e ffi PADA, pbC2K, aY:

a~aw E~,ffi--~,

Z ~ , = .hpaw k

at c =oY:

h2

B ffiT,' C A P A L = p~C~.

K. - ~.K, + (I - ~.)K,

CAP = (I - ~=)ppCp + ~=P^CA, K. =

K,

(1 -~=)

(1 -~=)"

VARIABLES