desiccant enhanced nocturnal radiative cooling-solar...
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Int. J. of Thermal & Environmental Engineering Volume 5, No. 1 (2013) 71-82
* Corresponding author. Tel and Fax. +20-3-4870596 E-mail: [email protected] © 2013 International Association for Sharing Knowledge and Sustainability DOI: 10.5383/ijtee.05.01.008
71
Desiccant Enhanced Nocturnal Radiative Cooling-Solar Collector System for Air Comfort Application in Hot Arid Areas
Ahmed Hamza H. Ali a*, Wael F. Saleh
b
a Department of Energy Resources and Environmental Engineering, Egypt-Japan University of Science and Technology
(E-JUST), P.O. Box 179, New Borg El-Arab City, Alexandria 21934, Egypt b
Department of Mechanical Engineering, Faculty of Engineering, Assiut University, Assiut 71516, Egypt
Abstract
In this study, the feasibility of implementing desiccant enhanced nocturnal radiative cooling-solar collector system for air comfort application in hot arid areas of Upper Egypt is carried out using the analytical techniques. Investigation of this system is carried out based on a daily cycle one-way airflow direction and the measured weather data. In addition to, a mathematical model for analyzing the heat and mass transfer in the system during adsorption (nighttime mode) and regeneration (daytime mode) is established. The obtained results were verified with correspondent experimental ones and a good agreement existed. Thereafter, the model is used to investigate the system feasibility for air comfort application. It is found that, this system is feasible in use in hot arid areas of Upper Egypt at which the ambient air relative humidity is very low in summer daytime and increased by nighttimes, which makes utilization of the system is effective. Also, the model is used to investigated the effect of the air mass flow rate on this system performance and the results show that, it is preferred to use a low air mass flow rate in order to obtain outlet air having a dry bulb temperature less than the ambient air by values ranging from 5.5 to 7 oC and at the same time having relative humidity not higher than 40%. Keywords: Desiccant Cooling, Nocturnal Radiative Cooling, Solar Cooling, Air Comfort, Hot Arid Areas
1. Introduction
Passive cooling of buildings within the human comfort zone is important subject due to its goal of keeping the environment out of contamination. The upper atmosphere (the sky) is a heat sink for nocturnal emitted long wave radiation by terrestrial units to produce a net cooling. Desiccant based systems process atmospheric air by removing moisture from the air to a desired level and then cooling it using a heat exchanger, followed by some evaporative cooling. However, the desiccant based systems are generally effective in hot humid areas. Rotary desiccant wheels and fixed desiccant beds are the most common desiccant dehumidifier configurations. Pesaran et al. [1] presented a comprehensive desiccant cooling bibliography
containing 1176 pieces of literature available up to the mid-1997. Among the commercially available systems, desiccant wheel rotation allows continuous operation, while a fixed desiccant bed is flexible in positioning but cannot run continuously. Usually, more than one fixed desiccant bed unit is used to compensate for the non-continuous operation drawback. One fixed desiccant bed can be in regeneration while another is in dehumidification process. A different desiccant cooling idea called desiccant enhanced nocturnal radiation cooling and dehumidification has been proposed descriptively by Fairey et al. [2 and 3] and Swami et al. [4]. Swami [5] quantified this idea and clarified its benefits when applied to a residence in hot humid areas.
The idea of the integrated desiccant/enhanced nocturnal radiative cooling-solar regenerated system is summarized as
Ali, and Saleh / Int. J. of Thermal & Environmental Engineering, 5 (2013) 71-82
72
follows. A unit is composed of a plane structure filled with desiccant material and topped with a highly conductive and emissive radiator plate. Ambient air is withdrawn through the desiccant bed during nighttime where dehumidification with probably some cooling occurs. The desiccant regeneration cycle occurs during daytime using ambient air and solar radiation. The dehumidified air at night can be further cooled evaporatively and stored to be utilized later to cool space within building during the warm daytime or it can be used directly to cool and dehumidify the building during the night time. Lu et al. [6 and 7] experimentally investigated a desiccant enhanced nocturnal radiation cooling and dehumidification system. Saito [8] and Techajunta et al. [9] investigated the regeneration in an integrated desiccant/collector system using direct solar energy simulator as a heat source through experiments and mathematical simulations. Their desiccant bed configuration was a layer of adsorbent laying along the diagonal of the collector under glass cover and subjected directly to the radiation flux and their units has no thermal radiation cooling capability. In their model they analyzed the regeneration process only and considered the whole system is one lump in the energy balance equation.
Normally, in the desiccant enhanced nocturnal radiative cooling-solar collector system, air is used as a working fluid which is the transfer medium for the heat of adsorption and regeneration to and from the absorber/radiator plate. The disadvantage of the air as a heat transfer medium is the low convective heat transfer coefficient due to its low thermophysical properties. In addition, it is a fact that, the desiccant dehumidification cooling techniques are more effective when utilized in hot humid areas.
Therefore, in this research, using the analytical techniques, an evaluation study is carried with a detailed simulation model considering the heat transfer modes within the suggested system, and, mainly implementing the feasibility of this system for air comfort application in hot arid areas of Upper Egypt. In Upper Egypt, the ambient air relative humidity is very low in summer daytime and slightly increases by nighttimes, which makes utilization of this system feasible for air cooling and dehumidification. The suggested configuration is an absorber/radiator plate without upper glazing in order to utilize the convection losses to the ambient to enhance the plate cooling and also to enhance nocturnal radiative cooling. This absorber/radiator plate is design to have a set of offset fins on its backside parallel to the flow direction in order to improve the convected heat transfer process from the plate to the air stream. It is also aimed to examine the effect air mass flow rate on the system performance. Simulation of this suggested system is carried out based on a daily cycle one-way airflow direction and the measured weather data for Assiut, Egypt as the model inputs. The system attempts to combine the moisture removal capabilities of desiccant materials, heat rejection capabilities of nocturnal sky radiation in such a way that each process complements and enhances the other, and at the same time provides isothermal desiccation which, allows for lower regeneration temperature that provide an avenue for using solar driven desiccant system.
2. System Concepts and Description of the Operating Modes
The nocturnal radiative cooling of a surface exposed to sky can be used to lower the temperature of a fluid beneath the radiator plate below that of surrounding ambient air temperature. Detailed explanation of this phenomenon was presented by Ali [10]. As the radiator plate temperature is raised the radiative heat rejection rate to sky increases. Desiccation is a process by which highly hygroscopic materials adsorb moisture from the air. This process is combined by a release of thermal energy roughly equal to the normal heat of water vapor condensation and heat of wetting. A desiccant system can fall into one of two generic operating concepts adiabatic or isothermal. Since the desiccation process in isothermal systems operates at a lower temperature than that in adiabatic systems as reported by Lavan et al. [11], the moisture absorption capacity of isothermal systems is somewhat greater than that of adiabatic systems. However, the concept of the desiccant enhanced nocturnal radiative cooling-solar collector system attempts to take advantage of approaching isothermal desiccation case through nocturnal radiative cooling. The radiator plate is coupled thermally to the desiccant bed during the adsorption process leading to a radiator temperature which tends to remain equal to or less the ambient by few degrees due to heat of adsorption. Thus, a combination of the moisture removal capabilities of desiccant materials and, heat rejection capabilities of nocturnal sky radiation is achieved in such a way that each process complements and enhances the other.
The proposed desiccant enhanced nocturnal radiative cooling-solar collector system for the present analysis is shown in Fig.1. It consists of an absorber/radiator plate made from Aluminum sheet coated with black paint at both sides and has a set of offset fins on its backside parallel to the flow direction. A back plate forms with the absorber/radiator plate a channel having 6 sections 4 of them contain desiccant bed laying along the diagonal (best configuration due to results of Swami et al. [5]) across the air flow path. The absorber/radiator plate is exposed to the ambient while the back plate of the channel is thermally well insulated. Ambient air is flown through the system from left or right sides (locations A or B) based on the operation mode. At nighttime ambient air is flown from location B to location C for adsorption mode and is dehumidified by the desiccant bed. The bed releases heat, which warms the air and consequently the absorber/radiator plate. Thus, the warmer absorber/radiator plate has greater cooling potential with respect to the ambient and from location C to location A the rest of absorber/radiator plate (two sections) serves to cool the air stream without dehumidification. The out-coming air at location A is more dry and cool than at the inlet location B. During the daytime, regeneration mode, the ambient air is flown through the system from location A to location C, where it is heated in the first two sections of absorber/radiator plate by the absorbed solar radiation. Thus, at the beginning of the section having desiccant bed at location C, this air has a relatively high temperature and low relative humidity. This warm air combined with the heat exchange between the absorber/radiator plate and bottom plate with the desiccant bed serve to regenerate the bed from location C to location B. The air stream is then exhausted to the ambient at location B.
A traadsis parthethimafol
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mathematical ansfer, for the sorption (nighttiobtained by apprts. This is donee second is for tird part is for thass balance is apllowing assumpt
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The pressure dthe air stream v
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3. Analytical A
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nvironmental E
73
octurnal radiativ
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3.1 Energy
3.1.1 For P
The energelements as
3.1.1.1 for
αpl,iApl,iqso
σ εpl,iApl,iF
- σ ∑=
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1jpA
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Engineering, 5
e cooling-solar co
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esiccant bed elety system.
es inside the duc
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Plates
gy balance equs follows:
the absorber/rad
olar– hpl,,ambApl,i
Fpl,sky(T4pl,i –T4
s
j-ipl,iiner,-pl (TF
of equation (1)tion absorbed on
adiator plate; to the ambien
d using the formuhird is the net the forth is the cohe channel, and, la presented in diant exchange and all other surfaces it sees.
(2013) 71-82
ollector system
plates and des
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ct are black for lo
es and fins is neg
uations are app
diator plate elem
(Tpl,i –Tamb) -sky) - hpl,airApl-in
j4ipl,4 )T -T =0
) are as followsn the plane of tthe second i
nt air, and, theula presented in hermal radiant enonvection heat tthe value of hpl,Diab [13]. T
between the absolid surfaces (N
siccant bed end
red as a lumped
ongwave radiatio
glected.
plied on the p
ment (i)
ner,i (Tpl –Tair)i
; the first one ithe outer surfacs representing
e value of hpl,amDuffie and Becknergy exchange transferred to th,air is calculated u
The fifth one isbsorber/radiator Noss) including
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(1)
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3.1
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3.1
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3.1
Thbedfol (i)
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p,airAbp,i (Tbp –T
he two terms of e is the heat conthe duct, and,
rrelation equatioe second one is tck plate elemecluding any desi
1.2 For Sections
he energy balanement as follows
1.2.1 for air strel,airApl-iner,i(Tpl –
Tair,in- Tair,out)i =0
he terms of equatnvected heat tsorber/radiator pr stream in the dergy transfer due
1.3 For Sections
he equations govd and air streamllows:
for air stream n
airm (Hair,in - Hair,o
mAbed,air)i(ωs,bed- ω
Ali, and Sa
k plate element (
Tair)i + σ ∑=
Noss
1jbpA
equation (2) arenvected to the athe value of h
on for the data the net thermal renti and all occant surfaces it
s Not Having De
nce equation iss:
am element (i) –Tair)i + hbp,airA0
tion (3) are definto the air streplate; the secondduct from the bae to inflow and o
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ωair)i +hpl,air)iApl,i
aleh / Int. J. of
(i)
bp,4 j-ibp,ip, (TF
e defined as follir stream from t
hbp,air is calculatof Heaton et alradiant exchangeother solid surt sees.
esiccant Beds s applied on th
Abp,i (Tbp –Tair)i
ned as follows; team in the dud is the convecteack plate; the thoutflow.
cant Bed rgy balance for hown in Fig. 2,
air)i(Tbed - Tair)i± q(Tpl –Tair)i+
Fig. 2. Energy
f Thermal & En
j4i, )T - =0
(2)
lows: the first the back plate ted using the l. [14]. While e between the rfaces (Noss)
he air stream
+ airm& Cpair
(3)
the first is the uct from the ed heat to the hird is the net
the desiccant are given as
adsq
and mass transfe
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74
hbp,air)iAbp,i
(ii) for desi
ρbedVbedH∂
∂
hbed,airAbed,a
The first teinflow andthird term from the band the forthe air streof the normfirst term idesiccant bthermal radall other soit sees.
3.2 Mass BThe equatibed and aifollows:
3.2.1 For S
(i) for air stairm& ( airω
(ii) for desi
ρbedVbed∂ω∂
er for desiccant be
Engineering, 5
(Tbp –Tair)i =0
iccant bed node
ibed
tH
⎟⎠⎞
∂+ σ ∑
=
Noss
k 1
bA
air(Tbed - Tair)i ± q
erm in equation d outflow; the se
of equation (5) ed to the air str
rth term of equatam due to the h
mal heat of condein equation (5) bed; the seconddiant exchange olid surfaces (No
Balance ions governing r stream elemen
Sections Having
tream node (i)out,airin,r ω− )i
iccant bed node
ibed
t ⎟⎠⎞
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ed control volume
(2013) 71-82
(i)
k -ibed,isbed, (TF )
adsq hmAbed,air)i(ω
(4) is the net enecond term in eare the net con
ream; the third tion (5) are the eheat of adsorptioensation plus theis the rate of ed term in equafrom the desiccoss) including an
the mass transfnts i, shown in
g Desiccant Bed
i ± hmAbed,air,i( ω
(i)
ir,i( aibed,s ω−ω
e
k4
ibed,4 )T -T +
ωs,bed - ωair)i = 0
nergy transfer duequation (4) andnvected heat traterm in equationenergy transferron that is summe heat of wettingnergy storage ination (5) is theant bed elementny desiccant sur
fer for the desicFig. 2, are give
airbed,s ω−ω )i =0
ir )i= 0
(4)
(5)
ue to d the
ansfer n (4)
red to mation
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ccant en as
0 (6)
(7)
Ali, and Saleh / Int. J. of Thermal & Environmental Engineering, 5 (2013) 71-82
75
in equation (4) to (7), the positive sings are for the desiccant bed regeneration and negative sings for air dehumidification modes. The first term in equation (6) is the water mass transfer rate due to the inflow and outflow to the control volume. The second term in equations (6) and (7) is the convective mass transfer from the bed to the air stream. The first term in equation (7) is the rate water storage for the desiccant bed. The value of Abed,air,i is the product of the transfer area of unit volume of the desiccant bed multiplied by the desiccant bed element volume; it is a desiccant property.
3.3 Desiccant Equilibrium Properties The set of equations governing the dynamics of sorption process has to be solved along with equilibrium sorption isotherms of the desiccant bed. The sorption isotherms for silica gel bed relating the air stream relative humidity with the desiccant bed humidity ratio and the heat of adsorption of water vapor in silica gel reported by Pesaran and Mills [15 and 16] and used in this study are given by: RH=0.0078 - 0.05759ωbed + 24.1655ω2
bed – 124.478ω3bed
+204.226ω4bed
(9)
hads= (-12400ωbed +3500) ωbed<0.05 (10-a)
hads= (-1400ωbed +2950) ωbed>0.05 (10-b) The ASHRAE [17] psychometrics for moist air is used to calculate ωair in equations (6) and (7). The enthalpy values of both the moisture air Hair and wet desiccant bed Hdes are determined from the following equations:
Hair= Cpair,dryTair +ωair (hg+CpwvTair) (11)
Hdes=(Cdes + ωdesCw)Tdes (12)
3.4 Numerical Method of Solution
Simulation of this system was carried out based on a daily continues operation, one direction airflow from ambient and inlet to the system in direction from location B to location A during nighttime, while, it is in opposite direction from location A to location B during daytime. In practice, for this system, air is in cyclic mode to provide continuous operation. With the system continuing operation, the last condition form the dehumidification process at end of nighttime mode is the initial conditions for the regeneration process during the next daytime mode. The temperatures of the solid boundaries elements were obtained by solving the energy balances Equations (1) to (5) for n-module model which generates (4×n) finite central difference nonlinear equations in (4×n) unknowns. These (4× n) unknowns are the temperatures of the absorber/ radiator plate, air stream bulk temperature, desiccant bed and back plate elements respectively. For solving this set of nonlinear heat balance equations, the multidimensional secant method– Broyden’s method – which is presented in Press et al.[18] was used. While, this set of equations is function of the air and desiccant bed moisture content, therefore the moisture content of the air and desiccant bed
elements are calculated first for a certain time, keeping the temperature constant, from the mass balance equations (6) and (7). These computation procedures are repeated until the change in the temperatures of the solid boundary elements became within the allowable error (below 10-5). The desiccant bed material considered in this study is silica gel grade 01, the properties of this grade and the values of both the convective heat and mass transfer coefficients, which were defined, based on Pseudo-Gas-side Controlled model (PGC) were reported by Helen-Xing [19]. The values of hpl,air and Tsky in equation (1) were determined using the formulae presented by Duffie and Beckman [12]. The calculation starts with all system components having the same initial condition equals to that of the ambient and the bed water content on dry mass basis ωbed(τ=0)=0.25. Simulation should be based on actual hourly values of weather data in order to decide the feasibility of this system. Therefore, in this study, the hourly average values of 30 years measurements of the weather parameters for Assiut, at Upper Egypt (Lat. 27o 12\ and long. 31o 10\) which its method of calculation reported by Degelman [20 and 21] is used.
The radiative (optical) properties used in the model are considered as follows: the absorptance (emittance) of the metallic surfaces, which is coated with commercial black paint is 0.94 and the emittance of the silica gel is 0.9 as given by Fairey et al. [3]. The flow (air) mass flow rate varied from 0.0094 to 0.022 kg/s corresponding to air volume flow rate ranging from 0.479 to 1.12 m3/min. The dimensions for Fig.1, which is used in this study, are as follows: total channel flow length L=1.2 m, flow length without desiccant bed E=0.6 m, channel width W=0.75 m, channel height D=50 mm, fins pitch l=148 mm, fin height δ=40 mm and fin length lfin= 90 mm.
4. Results and Discussions
4.1 Assessment of the Theoretical Results
Before reporting and discussing the feasibility of using this system as well as the effect of varying the air mass flow rate on its performance, the results of the model are compared with the correspondent measured data available in the for nearest similar system of Lu et al. [7]. The unit dimensions of [7] were input to the present model while assuming any missed data. Figure 3 shows the input weather data to the model and a comparison between the calculated values of the outlet air dry bulb temperature and absolute humidity ratio with corresponding experimentally measured values of [7]. It is clear from the figure that, the predicted humidity ratio values are in good agreement with the measured data. Also, it is seen from the figure that the predicted air dry bulb during daytime is slightly higher by 1 to 1.5 C than the measured one, while their values during nighttime are in reasonable agreement with the measured data. This slight deviation during daytime can be attributed to using the offset fins on the backside of the absorber leading to higher rate of heat transfer to the air and in the same time decreasing the convection losses from the absorber plate to the surrounding. However, these agreements of the calculated values with the measured data gives confidence in the analysis techniques to investigate the feasibility of using this system in Upper Egypt and the effect of varying the air mass flow rate on its performance.
4.2
Tharesyscolholoothethe4 ahuandFigaretemthewe
2 Performance C
he outlet absolutee the characteristem, desiccant llector that intent summer monthok to the year we maximum air e month of Juneand are the inpumidity ratio andd the statues ofg.1 for the air me shown in Fig. 5mperature duringe relative humieather condition
Ali, and Sa
Fig.
Characteristics
e humidity ratiostics of the perenhanced noct
nded to be used h in hot arid areaweather data fordry bulb tempe. The weather d
ut to the model. d air dry bulb tf the dry bulb tmass flow rate o5. As seen in Fig nighttime varidity reaches ups are applicable
aleh / Int. J. of
3. Verification of
s and System Fe
and air dry bulbrformance for tturnal radiative for air comfort a
as of Upper Egypr Assiut, Egypt erature was recoata of June are sThe predicted otemperature fromtemperature at lof 0.01251kg/s (ig. 4, the ambienied between 30 p to 47%. Hoe for desiccant s
f Thermal & En
f the calculated p
easibility b temperature the suggested cooling-solar application in pt. An overall indicates that
orded through shown in Fig. utlet absolute m the system location C of (0.64 m3/min) nt air dry bulb to 16 oC and
owever, these systems using
nvironmental E
76
parameters and co
silica gel aduring nigsections habulb tempevalues ranrespectivelywhen it pa(from C to result, the tto 7 C. Twithdrawn dehumidifiair at nightto be utilizecan be usedthe nighttim
Engineering, 5
orresponding dat
as a desiccant anghttime when thaving desiccant erature and absonging from 3 y, further decrea
asses through theA of Fig. 1) by total air dry bul
These values inthrough the suged and cooled. Httime can be furted later to cool d directly to coolme.
(2013) 71-82
ta of [6]
nd works well. Ahe ambient air bed (from B to olute humidity rto 4 C and
ase in its dry bulbe sections not hvalues ranging f
lb temperature dndicate that as ggested system dHowever, this cother cooled evapspace during thel and dehumidify
As seen from Fipasses throughC of Fig. 1) its
ratio are lowere0.13 to 0.75 b temperature oc
having desiccantfrom 2.5 to 3 C. drop ranged from
the ambient aduring nighttimeool and dehumidporatively and ste warm daytimefy the building du
ig. 5, h the s dry
ed by g/kg
ccurs t bed As a
m 5.5 air is e it is dified tored
e or it uring
Fig
In sys(72restemm3nighavbeca slearadairhav1) radradW/
g. 5. Predicted abm3/min)
order to clarify stem performan2 hrs) were inpusults of outlet mperature in cas3/min) are showghttime when thving desiccant cause it has lowsmall amount of ading to a releasdiation loss to skr passes through ve area equal onit is further coo
diation losses tdiation cooling r/m2. These valu
Ali, and Sa
bsolute humidity r
and magnify thece, weather dat
ut to the model. air absolute h
se of air mass flown in Fig. 6. Ahe ambient air bed its temper
w absolute humidf water vapor is ased adsorption hky which causesthe sections not
ne third the totaoled by 3-4 C byto the sky. Drate is carried anues are expected
0
10
20
30
40
50
60
70
80
Dry
Bul
b Te
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T ω
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ratio and air dry
e characteristics ta for three consThe correspond
humidity and aow rate of 0.012As seen from tpasses through
rature is decreadity value, and cadsorbed to the heat less than ths a net cooling et having desiccanal system area asy the effect of thetermination th
nd to be ranging d in hot arid ar
6
Tambientωambient
f Thermal & En
Fig. 4. Weath
bulb temperatur
details of this secutive days
ding predicted air dry bulb
251 kg/s (0.64 the figure, at
h the sections ased. This is consequently, desiccant bed
he net thermal effect. As the nt bed (which s seen in Fig.
he net thermal he night sky from 75 to 85
reas of Upper
Ju12
Toutωout
nvironmental E
77
her data for June
re at outlet and at
Egypt due because ofdaytime, atplace, the anot having gains tempthrough thesurface aresections noof the heat the air in thbed. Also, nighttime dthe regeneprocess depwhich has nighttime.
une 18
t Tloc
Engineering, 5
t location C for ai
to a small amof the low ambt which the desicambient air is dr
desiccant bed. Tperature differee section havingea subject to soot having desicca
of regeneration he section havinit can be notice
dehumidificationeration process pends on the sol
direct consequ
24
cation C
(2013) 71-82
ir mass flow rate
ount of downwabient water vapccant bed regenrawn and passed The air at outlet
ences higher thg desiccant bed (olar insolation ant bed. This is d(desorption hea
ng the desiccant ed from the figun performance ha
during daytimelar radiation fluxuent on the air
0.004
0.006
0.008
0.01
Hid
itR
ti(k
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30
of 0.01251kg/s (0
ard thermal radipor content, at
neration process tthrough the sec
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at) is transferred bed to the desic
ures 5 and 6 thaas close relation e. The regenerx, during the day
dehumidificatio
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Fig
Thdifsho22airandbedits radRHhavits
Fig
g.6. The predicterate.
he psychometricfferent times for own Fig. 7. It :00), (see Fig. 1
r at entrance to td RH=17%. It pd and leaves theabsolute humid
diation losses to H=20.8%. Thenving desiccant babsolute humid
g.7. Psychometric
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s of the temperthe suggested syseen from the
1 for the locatiothe unit is at a dpasses through tese sections at lodity is lowered sky, to a dry bu
n, the air passebed and it is furthity value to a dry
cs of the tempera
aleh / Int. J. of
dity and air dry
rature humidity ystem during thefigure that, at l
ons B, C and A)dry bulb temperthe sections havocation C (at 22and it is coole
lb temperature oes through the her cooled with ny bulb temperatu
ture humidity pa
f Thermal & En
bulb temperatur
paths at two e nighttime is ocation B (at
), the ambient ature of 30oC
ving desiccant 2:00) at which d by thermal
of 25.9 oC and sections not
no changes in ure of 23.9 oC
aths at two differe
nvironmental E
78
re at outlet for th
and RH=23a dry bulb unit and paleaves thesehumidity isto sky, to Then, the aand it is furvalue to a However, asupplied diof air suppl
ent times for the s
Engineering, 5
he presented valu
3.5%. While, at ptemperature of
asses through the sections at pos lowered and ita dry bulb tem
air passes througrther cooled with
dry bulb tempat both times, threctly to the resily conditions req
suggested system
(2013) 71-82
ues of weather da
point B (at 4:00)f 23oC and RH=he sections havin
int C (at 4:00) ats cooled by thermperature of 18gh the sections nh no changes in perature of 16.7he outlet air fromidential space is quired for air con
during the nightt
ata and air mass
) the ambient air=24.8%. It enterng desiccant bedat which it is absrmal radiation lo
8oC and RH=32not having desicits absolute hum
7oC and RH=35m the system ifstill within the r
nditioning.
time
s flow
r is at rs the d and olute osses 2.9%. ccant
midity 5.7%. f it is range
4.3
In ratbupreflokg0.7Figdesthetembe proandas of i.ecanheaparon flodufol(ni
3 Effect of Air M
order to investite on the prediclb temperature esented in Fig. ow rate is varied/s correspondin
798 and 1.12 m3
gs 8 and 9. Alsosiccant bed watee initial bed wmperature is equ
seen from Figsoduced minimumd the highest ouit is expected. Iwater vapor ads. highest values n be attributed at and mass tranrticles as it is de the desiccant p
ow rate increasering nighttime llowed by higighttime) and de
Ali, and Sa
Mass Flow Rate
igate the effect cted performanc
and absolute 6 are the input d with values o
ng to air volum3/min, and the oo, the effect of ter content is sho
water content onual to that of thes. 8 and 9 that,m outlet temperautlet temperatureIn the same timesorption (nighttimof the outlet air to the lower va
nsfer coefficientepending on the article diameters, the outlet air increases and dgher values oesorption (dayti
Fig. 8.E
aleh / Int. J. of
e on the Perform
of varying the ace parameters (ohumidity) the to the model.
of 0.0093, 0.015me flow rate va
utput results arethe air mass flowown in Fig. 10, n dry basis is e ambient at tha lower air voluature values dure values during e, it has the minme) and desorptabsolute humid
alues of both tht between the aiflow Reynolds n. However, as thdry bulb tempe
during the daytimof water vapoime) i.e. lower
Effect of air volum
f Thermal & En
mance
air mass flow outlet air dry weather data The air mass 54 and 0.022 lues of 0.48, e presented in w rate on the knowing that 0.25 and its
at time. It can ume flow rate ring nighttime
the day time nimum values tion (daytime) ity ratio. This
he convective r and the bed number based he air volume erature values me decreases r adsorption values of the
me flow rate on th
nvironmental E
79
outlet air increases intransfer coebe noticed the air ab(daytime) itime for thperiod (nigday time dduring the content ofexplained bhot arid arinitial bedHowever, equilibriumwill be equbased on thrate, in cassolar collecareas, it is obtain outlambient airwill has Re
he predicted outle
Engineering, 5
absolute humidn the values of efficient betweenthat from Fig. solute humidityis higher in comhe air absolute
ghttime). This isdesorbed the wa
previous nighttf desiccant bedbased on the loreas, and as the d water contethe bed after
m condition i.e. tual the desorbehe above resultse of using the dctor system for preferred to uselet air having a r by values rangelative humidity
et air dry bulb tem
(2013) 71-82
dity at nighttimboth the conve
n the air and the9 that, the integy ratio during mparison with
e humidity ratios because the amater vapor whichtime plus a fewd.The results sw ambient air aair volume flo
ent during dea couple of d
the adsorbed waed water durings of the effect o
desiccant nocturnair comfort app
e a low air mass dry bulb tempe
ging from 5.5 to not higher than
mperature.
me. This is dueective heat and e bed particles. Igration over timregeneration pthe integration o during adsorpmbient air duringh had been adso
w of the initial wseen in Fig. 1absolute humidiw rate increase
esorption decredays reaches toater during nightg daytime. Howof the air mass nal radiative cooplication in hotflow rate in ord
erature less than7 C and of cou40%.
e the mass
It can me for
eriod over
ption g the orbed water 10 is ity in s the
eases. o the ttime
wever, flow
oling-t arid der to n the
urse it
0
0.05
0.1
0.15
0.2
0.25
0.3
Bed
Wat
er C
onte
nt,
kg/k
g
Ali, and Sa
Fig. 9.Ef
Fig. 10
0 60
5
5
2
5
3
aleh / Int. J. of
ffect of air volum
0.Effect of air volu
12 18
f Thermal & En
me flow rate on the
ume flow rate on
8 24 3
nvironmental E
80
e predicted outlet
the predicted des
30 36Time, h
Engineering, 5
t air absolute hum
siccant bed water
42 48hr
ωb ωb ωb
(2013) 71-82
midity ratio
r contents
54 60
bed,(0.48 bed,(0.798 bed,(1.12
0 66 7
m3/min) m3/min)
m3/min)
72
)
Ali, and Saleh / Int. J. of Thermal & Environmental Engineering, 5 (2013) 71-82
81
5. Conclusions
This study examines the feasibility of implementing desiccant enhanced nocturnal radiative cooling-solar collector system for air comfort application in hot arid areas of Upper Egypt. A mathematical model analyzing the heat and mass transfer, for the proposed system during adsorption (nighttime mode) and regeneration (daytime mode) is established. The actual hourly values of weather data, of the average of 30 years measurements were used as the input to model.
• The model output results were verified with correspondent experimental ones and a good agreement existed.
• The model was used to investigate this system feasibility for air comfort application in hot arid areas of Upper Egypt and it was found that, this system is feasible in use in hot arid areas of Upper Egypt. This is because the ambient air relative humidity is very low in summer daytime and increase by nighttimes, which makes utilization of this system effective.
• The model is used to investigate the effect of the air mass flow rate on this system performance and the results indicate that, it is preferred to use a low air mass flow rate in order to obtain outlet air having a dry bulb temperature less than the ambient air by values ranging from 5.5 to 7 C at the same time it has Relative humidity not higher than 40%.
Nomenclature
A: area, m2
Abed,air: free flow area inside the bed, m2
Abed,s: surface area of the desiccant bed, m2
Apl-iner: plate base surface area &ηfinAfins, m2
Cpair: specific heat of humid air, J/kg K
Cdes: specific heat of the desiccant, J/kg K
Cpair,dry: specific heat of dry air, J/kg K
Cpwv: specific heat of water vapor, J/kg K
Cw: specific heat of liquid water, J/kg K
D: channel height, m
F: configuration factor
adsq : adsorption heat, J /(kg of adsorbed water)
Hair: enthalpy of humid air, J /kg
hbed,air: convective heat transfer coefficient, W/ m2 K
Hdes: enthalpy of wet desiccant (energy content per unit mass of dry desiccant), J /kg
hg: specific enthalpy of water vapor, J /kg
hm: mass transfer coefficient, kg/ m2s
i, j and k: elements numbers
L: length of the unit, m
l: offset fins spacing, m
airm& : mass flow rate of the air stream, kg/s
waterm& : mass flow rate of water from the bed to the air stream (Fig. 2), kg/s
Noss: number of surrounding elements
q: heat transfer flux, W/m2
qsolar: total incident solar radiation flux on the plane of the unit, W/m2
Q: heat transfer rate, W
RH: relative humidity of hypothetical air layer
t: time, s
Tair: temperature of the air stream, oC
Tbed: temperature of desiccants bed, oC
Vbed: volume of desiccant bed element, m3
W: channel width, m
x: length scale m
Greek symbols:
α: absorptance, -
δ: offset fin height, m
ε: emittance, -
σ: Stefan-Boltzmann constant, (5.67051 × 10-8), W/m2 K4
ηfin: fin efficiency
ωair: humidity ratio of air stream
ωbed: desiccant bed water content on dry basis
ωs,bed: humidity ratio of hypothetical air layer
ρbed: bulk density of the desiccant bed, kg/m3
Subscripts:
air: dry air stream
amb: ambient
bed: desiccant
bp: back plate
conv: convection
pl: absorber/radiator plate
rad: thermal radiation
ss: surrounding surfaces
s: surface of the desiccant bed
sky: sky
wv: water vapor
Ali, and Saleh / Int. J. of Thermal & Environmental Engineering, 5 (2013) 71-82
82
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