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Thermal analysis and optimization of a system for water harvesting from humid air using thermoelectric coolers

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Condensation of water vapor available in atmospheric air can be considered as a solution for water scarcity problem. In this paper, a comprehensive thermodynamic analysis of water production from humid air using thermoelectric coolers (TECs) is presented. The system consists of a number of thermoelectric coolers, a fan to supply the required air flow circulation, two cold and hot air channels, heat sinks and solar cells for powering the thermoelectric coolers and fan. Effects of various design parameters are investigated and discussed. The proposed design is optimized to get the maximum effectiveness which is defined as the amount of produced water per unit of energy consumption. Sensitivity analysis is used to find the optimum number of TECs, length of the channels and performance of the system at different temperatures. The resulting system is capable of producing 26 ml of water within 1 h from the air with 75% relative humidity and the temperature of 318 K by consuming only 20W of electrical power. In addition, the annual performance and optimization of this device in three southern cities of Iran are presented based on hourly meteorological data. Finally, comparison of the present system with other air water generators indicates that the proposed design is the most energy efficient system among similar devices especially in high relative humidity.
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Energy Conversion and Management
journal homepage: www.elsevier.com/locate/enconman
Thermal analysis and optimization of a system for water harvesting from
humid air using thermoelectric coolers
M. Eslami
a,
, F. Tajeddini
b
, N. Etaati
a
a
School of Mechanical Engineering, Shiraz University, Shiraz, Iran
b
School of Mechanical Engineering, Sharif University of Technology, Tehran, Iran
ARTICLE INFO
Keywords:
Thermoelectric cooler
Atmospheric water generation
Thermodynamic optimization
ABSTRACT
Condensation of water vapor available in atmospheric air can be considered as a solution for water scarcity
problem. In this paper, a comprehensive thermodynamic analysis of water production from humid air using
thermoelectric coolers (TECs) is presented. The system consists of a number of thermoelectric coolers, a fan to
supply the required air ow circulation, two cold and hot air channels, heat sinks and solar cells for powering the
thermoelectric coolers and fan. Eects of various design parameters are investigated and discussed. The pro-
posed design is optimized to get the maximum eectiveness which is dened as the amount of produced water
per unit of energy consumption. Sensitivity analysis is used to nd the optimum number of TECs, length of the
channels and performance of the system at dierent temperatures. The resulting system is capable of producing
26 ml of water within 1 h from the air with 75% relative humidity and the temperature of 318 K by consuming
only 20 W of electrical power. In addition, the annual performance and optimization of this device in three
southern cities of Iran are presented based on hourly meteorological data. Finally, comparison of the present
system with other air water generators indicates that the proposed design is the most energy ecient system
among similar devices especially in high relative humidity.
1. Introduction
Nowadays, water scarcity is one of the most serious issues in the
world. Approximately, around 97.5% of the water content of the earth
is salty seawater which means only 2.5% of the existing water is fresh.
Almost 70% of this amount is frozen at the polar ice caps, and around
30% exists in the form of moisture in the air or underground aquifers.
Therefore, it can be concluded that only less than 1% of the earths fresh
water is accessible for direct human use [1]. Mekonnen et al. [2] no-
tied that as many as four billion people all around the world face the
problem of water scarcity for at least one month per year. All these
factors have brought about the need to study solutions addressing the
water scarcity problem.
Among dierent methods of desalination, atmospheric water gen-
eration (AWG) can be an easy method for fresh water production
especially for places with high relative humidity. In this approach,
ambient air is cooled down below the dew point temperature and the
condensed water is collected. Vapor compression refrigeration, ab-
sorption refrigeration and thermoelectric cooling (TEC) can be used for
this purpose. Thermoelectric coolers are devices which function on the
basis of Peltier eect. By passing an electric current through them, they
produce a temperature dierence resulting in a cooling eect. In
comparison with vapor compression and absorption refrigeration, TEC
devices have no moving parts and require less maintenance. Therefore,
they are suitable for designing simple and portable AWG systems.
However, the designer must be very careful about the performance and
eciency of TECs at various operating conditions.
There are dierent approaches to study properties and modeling the
behavior of thermoelectric coolers [37]. Zhao and Tan [3] presented a
study of material, modeling, and application of thermoelectric coolers.
Fraisse et al. [4] compared dierent methods of modeling TECs. Also,
Mani [5] studied the behavior of thermoelectric coolers numerically
and analytically and revealed that the results of these two approaches
are in good agreement.
The coecient of performance is among the most important topics
related to thermoelectric coolers. For this purpose, Enescu and Virjoghe
[6] provided a review of thermoelectric cooling parameters and per-
formance. In addition, Xuan [7] investigated the eect of thermal and
contact resistance of thermoelectric coolers. Based on his studies, the
amount of COP depends on thermoelectric length. Also by increasing
the thermal contact resistance, this dependence increases signicantly.
The maximum COP of a TEC device in both cooling and heating
https://doi.org/10.1016/j.enconman.2018.08.045
Received 25 April 2018; Received in revised form 2 August 2018; Accepted 12 August 2018
Corresponding author.
E-mail address: meslami@shirazu.ac.ir (M. Eslami).
Energy Conversion and Management 174 (2018) 417–429
0196-8904/ © 2018 Elsevier Ltd. All rights reserved.
T
mode is one of the important issues that should be considered. Cosnier
et al. [8] examined the performance of thermoelectric coolers by ex-
perimental and numerical analysis and revealed that it is possible to
reach the coecient of performances above 1.5 for cooling mode, and 2
for heating mode. Also, Liu et al. [9] used thermoelectric coolers for
various air conditioning applications and showed that it is possible to
reach the COP of 2.59 for cooling mode and 3.01 for heating mode.
These results suggest that TEC devices can be a good choice for water
harvesting if they are used eciently.
Reducing the hot side temperature of a thermoelectric cooler is an
approach to increase the coecient of performance. For example,
Sadighi Dizaji et al. [10] used water ow for cooling the hot side of a
TEC instead of air and showed that it is possible to increase the cold side
performance of TEC signicantly. Seo et al. [11] studied the eect of
dierent heat sink's shapes on the performance of TECs, numerically
and showed the shape of heat sinks can change the operating perfor-
mance of thermoelectric coolers. Also Via'n and Astrain [12] designed a
heat sink for the cold side of a TEC and showed that by using this heat
sink, COP can increase up to 32%. In addition, Zhu et al. [13] studied
the eect of dierent heat exchanger sizes on the performance of TECs
theoretically. According to their studies, the highest amount of COP is
achieved by using the optimal heat sink size.
Another important parameter that signicantly aects the perfor-
mance of a TEC is the electrical current. Tan et al. [14] applied the
second law of thermodynamics and showed that the amount of current
must be precisely determined to achieve the optimal cold side tem-
perature. Also Tan and Fok [15] presented an approach to analyze and
optimize a thermoelectric cooling system.
The application of TECs in water harvesting from air is reported in
several experimental studies [1624]. Vian et al. [16] designed a device
which was able to condense 0.969 L of water from the air in each day.
Furthermore, Jradi et al. [17] theoretically and experimentally studied
a system including 5 channels with 20 thermoelectric coolers in each
powered by solar cells. This device is combined with a solar distiller
humidifying ambient air to increase distillate output of water produc-
tion. They showed that it is possible to produce 10 L of water during a
summer day in Beirut. In another study, Yao et al. [18] produced
33.1 g/h of water by using a dehumidication device having more heat
sinks on the two sides of thermoelectric coolers. In addition, Atta [19]
designed a prototype including three TEC elements and a photovoltaic
cell. He applied this system in Yanbu climate conditions and could
produce almost 1 Liter of condensed water per hour. Besides, Joshi
et al. [20] installed 10 TEC in a channel with the length of 70 cm and
tested it in several dierent climate conditions. Based on this design,
they harvested 240 ml of water in 10 h at a relative humidity of 90%
and mass ow rate of 25 g/s. Tan and Fok [21] designed an AWG
system and investigated the eect of input power to TECs and inlet
mass ow rate on the amount of produced water. They revealed that it
is possible to produce 50 ml of water in 3 h in an average relative hu-
midity of 77%. Also, Liu et al. [22] built a portable water generator
with two thermoelectric coolers and investigated the eect of inlet air
relative humidity and air ow rates and showed that the maximum
amount of generated water is 25.1 g per hour with 58.2 W input power.
Munoz-Garcia et al. [23] designed a similar system for irrigation of
young trees. Based on this design, they could harvest 35 ml water per
hour from the air. Moreover, Pontious et al. [24] could harvest 0.21 L of
water in a day with 0.33 kWh of energy consumption.
Recently, Shourideh et al. [25] performed a theoretical and ex-
perimental analysis of a Peltier AWG by optimizing the cold side ex-
tended surface and the cooling system. But they didn't investigate the
Nomenclatures
Ac
cross section area of a n(
m)
2
Afns area (m)
2
A
t
total area (m)
2
C
p
specic heat (kJ/kg K)
COP coecient of performance
D diameter (m)
Eeectiveness (L/J)
ffraction factor
henthalpy (J/kg)
h
conv
convection heat transfer coecient (W/m
2
K)
H height of each channels hole (m)
Icurrent (A)
kfthermal conductivity of base air ow (W/m K)
ksthermal conductivity of base plate material (W/m K)
K
mTEC module thermal conductance (W/K)
l length (m)
l
c
characteristic length (m)
N number
Nu Nusselt number
pperimeter (m)
Ppower (W)
Pr Prandtl number
Qtransferred heat (W)
Rresistance (K/W)
Rthermal resistance (m2K/W)
R
m
TEC module electrical resistance (ohm)
Re Reynolds number
S
m
TEC module Seebeck coecient (V/K)
T thickness (m)
Ttemperature (K)
uair velocity
(
m/s
)
Vvoltage (V)
V
̇
volume ow rate of water (L/s)
w width (m)
Greek symbols
η
0
overall surface eciency
η
feciency of n with an adiabatic tip
kinematic viscosity (m/s
)
2
ΔPpressure drop (Pa)
T
Δ
temperature dierence (K)
ρdensity (kg/m
3
)
ϕ
relative humidity
ω
humidity ratio
Subscripts
a ambient
c cold side
equ equivalent resistance
h hot side
hyd hydraulic
LMTD log mean temperature dierence
max maximum
opt optimum
t
b,
resistance of the un-nned part of the heat sink
t
bas
e
,
thermal resistance of the base surface
t
c,
contact resistance
resistance of the extended surfaces
TEC, m thermoelectric cooler
w water
M. Eslami et al. Energy Conversion and Management 174 (2018) 417–429
418
eect of thermal resistances and other parameters like the number of
TECs in the optimization. They also compared the energy consumption
of their design with some other AWG systems and showed that their
system has a better performance. Besides, Salek et al. [26] performed a
thermodynamic optimization for a solar driven ammonia absorption
refrigeration cycle used for air dehumidication combined with a saline
water desalination cycle.
The above literature review shows that most of the researches on
thermoelectric AWG systems published so far are experimental and no
comprehensive analytical solution and optimization for water har-
vesting by TECs has been provided. Therefore, this article tries to pre-
sent a complete thermodynamic analysis of water production from
humid air using TECs by considering eects of various design para-
meters, including thermal resistances and ns geometry. The resulting
solution provides the necessary information to nd the optimum
number of TECs, length of the channel, electrical current and air mass
ow rate. The objective is to maximize the amount of water production
per unit power consumption of the fan and thermoelectric coolers in
dierent operating conditions. Besides, the possibility of water pro-
duction at dierent atmospheric conditions (relative humidity and
temperature) can be predicted. Hence, the idea of using a controller to
turn the device on and ois also investigated to decrease the power
consumption while producing the same amount of water. As case stu-
dies, the annual performance of the device is investigated for three
southern cities in Iran. These locations are typical examples of places
with high relative humidity but very low annual rainfall.
2. System description
As shown in Figs. 1 and 2, the system considered in this article
consists of a number of thermoelectric coolers placed in series. Air ows
through two channels on the hot and cold sides and heat sinks increase
the surface of heat transfer. A fan supplies the required air ow circu-
lation and solar cells power the thermoelectric coolers and the fan.
The temperature distribution on the surface of the channels on both
sides of the thermoelectric cooler is assumed to be uniform. The dis-
tance between two neighboring thermoelectric coolers is considered to
be 1.5 cm to make this assumption reasonable. The entering air stream
rst passes through the channel on which the cold side of the ther-
moelectric coolers are placed, and after being cooled and dehumidied,
goes through the warm channel and cools the hot side of thermoelectric
cooler. Fig. 2 shows a schematic representation of the TECs inside
channels.
Also, Table 1 presents the specications of the thermoelectric cooler
in this study.
3. Governing equations
Each thermoelectric cooler is identied by four basic characteristics
including
I
max
,
V
ma
x
,T
Δ
max and
Q
ma
x
. Along with the hot side tempera-
ture of a TEC (
T
h
), the parameters required for modeling thermoelectric
coolers are dened, as follows [10,28]:
=SV
T
mmax
h(1)
=
R
TTV
TI
(Δ )
mhmaxmax
hmax
(2)
=
K
TTVI
TT
(Δ )
mh max max max
hmax (3)
where
S
m
is the Seebeck coecient,
R
m
is electrical resistance and
K
mis
thermal conductivity.
By applying energy balance for a thermoelectric cooler, the cooling
power and the heat released from the hot side of the thermoelectric
cooler can be calculated [10,28]:
=−
Q
SIT IR KT
2Δ
cmc mm
2
(4)
=+
Q
SIT IR K
T
2Δ
hmh mm
2
(5)
In these equations, Iis the electric current,
T
c
is the cold side tem-
perature of TEC,
T
h
is the hot side temperature of TEC,
Q
c
is the cooling
power,
Qh
is the amount of heat dissipated from the hot side of TEC and
ΔT is the temperature dierence between hot and cold side of ther-
moelectric cooler:
=−TTT
Δ
hc
(6)
On the other hand, the rst law of thermodynamic gives:
=−
P
QQ
TEC h c (7)
where
P
TEC is thermoelectric power consumption. Combining Eqs.
(4)(7):
=+
P
SIT IRΔ
TEC m m
2
(8)
Also, the coecient of performance of the thermoelectric cooler is
dened as:
Fig. 1. A schematic of the system under study.
M. Eslami et al. Energy Conversion and Management 174 (2018) 417–429
419
=Q
P
C
OP
c
TEC (9)
The heat transfer between the air and the TECs in the channels is
clearly related to the enthalpy change as follows:
=−
Q
mh h
̇()
c c ai r in c air out c__ _ _ (10)
=−
Q
mC T T
̇(
)
h h p h air out h air in h___ __ (11)
where C
p
is the specic heat of air across the hot channel and
h
is the
enthalpy of humid air, in J/kg, which itself is a function of specic
humidity
ω
[29]:
=−+ + −hCT ω T( 273) (2501.3 1.86 ( 273)) 100
0
p
(12)
As no water condensation occurs in the hot channel, the specic
humidity of the air does not change so the enthalpy changes, Δh, can be
replaced by C
T
Δ
p
to calculate heat dissipated by the air in Eq. (11).In
addition, since the hot side of TEC is cooled by the air discharge from
the cold channel, then
=TT
air in h air out c__ _ _
.
The heat transfer in the channels can also be related to the tem-
perature dierence between the air stream and the hot and cold sur-
faces by LMTD method [30]:
=
Q
T
R
Δ
cLMTD coldside
c
_
(13)
=
Q
T
R
Δ
hLMTD hotside
h
_
(14)
=−−−
()
TTTTT
Δ
[( ) ( )]
ln
LMTD coldside air out c c a c
TT
TT
___
air out c c
ac
__
(15)
=−−
()
TTT TT
Δ
[( ) ( )]
ln
LMTD hotside h air in h h air out h
TT
TT
___ _ _
hairinh
hairouth
__
__ (16)
In Eqs. (13) and (14), parameters
R
c
and
R
h
are the total thermal
resistance between the TEC surface and air ow in the cold and hot
channels respectively. They can be calculated by adding dierent
thermal resistances as shown in Fig. 3:
In Fig. 3,
R
tc,is the contact resistance between thermoelectric
coolers and the heat sink attached to the cold channel. It can be mod-
eled using the following expression:
=
R
R
Nlw
tc
tc
TEC TEC TEC
,,
(17)
N
TE
C
is the number of thermoelectric coolers and
l
TEC
and
w
TE
C
are
the length and width of each TEC, respectively.
R
tbas
e
,is the thermal
resistance of the base surface on which thermoelectric coolers are in-
stalled and is calculated as follows:
=
R
t
kl w
tbase base
s channel channel
,
(18)
where
t
bas
e
,
l
channe
l
and
w
channe
l
are illustrated in Figs. 1 and 2 and ksis the
thermal conductivity of base plate material.
The convection heat transfer resistance of the un-nned part of the
heat sink
R
t
b
,can be combined with resistance of the extended surfaces
R
tf N,(
)
as an equivalent resistance
R
equ. Assuming the lateral surfaces of
the channel to be insulated [30]:
=
R
ηAh
1
equ
tconv
0(19)
In this equation,
η
0
is the overall eciency of the heat sink, given by
[30]:
=− −
η
NA
Aη1(1
)
fin f
tf0(20)
=Awl2
ffinc (21)
=+ll t
2
cfin
fin
(22)
=+All N N Hl(2 2)
t hole channel hole hole hole channel (23)
where
N
fin,Af,
A
t
,
l
c
, and
N
hol
e
are the number of ns, the n area, the
total area of heat transfer, equivalent n length and the number of air
passages, respectively. Also, Hhol
e
and
l
hol
e
are shown in Fig. 1 and lfi
n
,
Fig. 2. The layout of thermoelectric coolers and the air passages over them.
Table 1
Thermoelectric cooler specications KRYOTHERM TB-127-2,0-1,05(62)
[27].
Parameter Value Unit
I
ma
x
17.6 A
V
ma
x
15.7 V
Q
ma
x
171.0 W
T
Δ
ma
x
69.0 K
l
TEC 62.0 mm
w
TE
C
62.0 mm
Fig. 3. Thermal resistances between the cold side of TEC and air ow in the channel.
M. Eslami et al. Energy Conversion and Management 174 (2018) 417–429
420
w
fin and
t
fin are illustrated in Fig. 4.
Assuming that the n tips are adiabatic,
η
fis given by [30]:
=
η
ml
ml
tanh
f
c
c(24)
in which,
=mhp
kA
conv
sc
(25)
=+
p
wt2(
)
fin fin (26)
=Awt
cfinfi
n
(27)
where pis the perimeter and
Ac
is the cross section area of a rectangular
n.
Also, the convection heat transfer coecient h, in Eqs. (19) and
(25), is calculated from the Nusselt number:
=hNuk
D
conv
f
hyd (28)
In which, kfis the thermal conductivity for air. Assuming turbulent
ow, for both cooling and heating channels [30]:
=
N
u Re Pr for cooling0.023 0.8 0.3 (29)
=
N
u Re Pr for heating0.023 0.8 0.4 (30)
where
Pr
is the Prandtl number and
Re
is the Reynolds number dened
as follows:
=
R
euD
ν
average hyd
(31)
=
u
mN
ρl H
̇/
average ahole
hole hole
(32)
Also, in Eqs. (28) and (31), the characteristic length is hydraulic
diameter Dhy
d
that is given by:
==
+
DA
p
lH
lH
44
2( )
hyd hole
hole
hole hole
hole hole (33)
Finally, the total resistance is obtained:
=+ +
R
RR R
tc tbase equ,,
(34)
With the same procedure for hot channel,
R
h
is calculated. Besides,
the rate of water production in
L/
s
is obtained using the following mass
balance equation for water:
=−
V
mω ω ρ
̇̇()
1000
waairair
average
incoutc(35)
in which,
ρ
average is the density of water at the temperature of
+TT
2
air out c air in c__ __
.
To calculate the required fan power, pressure drop across the air
passage is required [30]:
=pf
ρu
Dl
Δ
average
hyd
channe
l
2
(36)
=−
f
Re(0.790ln( ) 1.64) 2(37)
Dimensions of the channel are chosen to have a turbulent ow for
better heat transfer. Having
pΔ
, the fan power consumption is given by
[30]:
=
P
m
ρ
p
̇Δ
fan (38)
4. Solution procedure
By considering the relative humidity of the outlet air from the cold
channel to be equal to 100% and solving Eqs. (1)(38), the values of
power for the fan and thermoelectric coolers and also the amount of
produced water are obtained. It should be noted that if the amount of
produced water becomes negative, it means that the assumption of
saturated air at the outlet is incorrect. In this case, instead of using this
assumption, the condition =
ω
ω
air air
incoutcis used, in order to obtain
correct results. Fig. 5 shows a ow chart of dierent steps in the solu-
tion.
The geometrical and thermophysical properties of the present
system are given in Table 2.
4.1. Validation
At rst, the performance of the TEC modeled using Eqs. (1)(9) is
compared with results of the software provided by the thermoelectric
manufacturer (KRYOTHERM) [27]. For
T
h
= 300 K, and T
Δ
= 20 K and
40 K, changes of COP and
Q
c
relative to the electric current is presented
in Figs. 6 and 7. These results approve the accuracy of analytical
equations used for modeling the thermoelectric cooler.
In the next step, the present model is used to simulate the experi-
ments of Jradi et al. [17].Fig. 8 illustrates the amount of water pro-
duced at dierent air ow rates for the electrical current of 2.6 A. Re-
sults show the maximum error of 6% compared to the experimental
data and simulation results of Jradi et al. [17]. Also, eects of changing
the electrical current for the xed air ow rate of 0.0155 kg/s is shown
in Fig. 9 which validates the accuracy of the present simulations.
Fig. 4. A schematic of n sizing.
M. Eslami et al. Energy Conversion and Management 174 (2018) 417–429
421
5. Results and discussion
5.1. Parametric study and optimization
In this section, the eect of dierent parameters on performance of
the AWG system is studied. In order to optimize the design, an objective
function called eectiveness is dened as follows:
=+
E
ff m
PP
̇w
TEC fan (39)
This indicator represents the amount of produced water relative to
the energy consumption of the system. Higher values of this quantity
mean that the system consumes less energy to produce the same
amount of water. This is an important feature for the systems supplied
by solar energy. A system with higher eectiveness needs smaller PV
and battery system.
A sensitivity analysis can provide the path for optimization. For
instance, Fig. 10 shows the eect of changing electrical current on ef-
fectiveness of the system at air ow rate of 0.015 kg/s, ambient tem-
perature of 308 K, relative humidity of 75% and 10 TEC modules placed
in a 77 cm long channel. It is observed that by increasing the current,
Erst rises and then falls. Therefore, it is very important to supply an
Fig. 5. The solution procedure.
Table 2
Fixed parameters needed to solve the problem.
Parameter Value Unit
l
hole 4mm
H
hol
e
30 mm
N
hol
e
8
t
bas
e
1mm
t
fin 3mm
N
fin
7
k
f
c
_
0.0257 W/m K
k
fh_
0.0271 W/m K
k
s229 W/m K
ν
c
15.11E06 m
2
/s
νh16.97E06 m
2
/s
Pr
c0.713
Pr
h
0.703
R0.001 m
2
K/W
Fig. 6. Change of COP vs Current for =
Δ
T20
K
and =
Δ
T40
K
for the con-
sidered TEC (KRYOTHERM) [27].
M. Eslami et al. Energy Conversion and Management 174 (2018) 417–429
422
optimum amount of current to the system. However, the optimum value
of the current is a function of operating conditions and also the number
of thermoelectric coolers.
To nd the optimum number of thermoelectric coolers, 1520 TECs
are considered. The length of the channel is proportional to the number
of thermoelectric coolers. The ambient condition is assumed to be 308 K
with relative humidity of 75%. By changing the electrical currents from
zero to I
max
for ṁ
a
between 0.01 and 0.02 kg/s, the amount of
E
ff
ma
x
is
calculated for all cases. Results reported in Table 3 reveal that the
maximum eectiveness occurs at COP = 1.9 in all cases. The system
with 18 thermoelectric coolers and inlet air ow rate of 0.0117 kg/s has
the maximum value of Ewhich is equal to 1.638E07 L/J. Therefore,
the optimum length of the channel is 1.386 m with 18 TECs placed in
series. This design is used from now on for the study of other para-
meters.
Eects of changing the electricity current for a channel with the
optimum length is shown in Fig. 11. The air owrate of 0.0117 kg/s, the
ambient temperature of 308 K and the relative humidity of 75% are
considered. Results reveal that the maximum amount of water pro-
duction is 42 ml in one hour and it occurs when I= 2.315 A. However,
the maximum Edoesn't occur at this point. In fact,
=E
ff
max
1.638E07L/J is 1.65 times higher than the Eat maximum water
production and occurs at I = 1.349 A.
Clearly, the amount of water production is proportional to the re-
lative humidity of the ambient air. Fig. 12 projects this eect for the air
ow rate of 0.0117 kg/s and I = 1.349 A. Therefore, it is possible to
produce 106 ml of fresh water at high relative humidity within one
hour.
To nd the optimal performance of the system at dierent ambient
temperatures, the relative humidity of the entering air is considered to
be constant and equal to 75%, while the input current to thermoelectric
coolers is varied from 0 to I
max
, also air ow rate changes between
0.001 and 0.02 kg/s. The optimum operating mode of the system is
calculated for each temperature. These results are reported in Table 4.
Based on the above results:
(1) At the optimal performance, with the increase in entering air
temperature, the input electric current to the system decreases.
Because when entering air temperature goes higher, the perfor-
mance range of thermoelectric becomes more limited. If the current
exceeds a specic value, the warm side of thermoelectric cooler gets
too hot which has a negative eect on its performance. So, for an
increase in the entering air temperature, the input current to the
system should be decreased.
(2) With the increase in air temperature, the optimal air ow rate is
Fig. 7. Change of
Qc
vs Current for =
Δ
T20
K
and =
Δ
T40
K
for considered TEC
(KRYOTHERM) [27].
Fig. 8. The amount of produced water as a function of air ow.
Fig. 9. The amount of produced water as a function of electrical current.
Fig. 10. The eect of increasing electric current on the value of eectiveness.
M. Eslami et al. Energy Conversion and Management 174 (2018) 417–429
423
decreased as well, which is justied with the previously discussed
observation: by decreasing input current to the system, and there-
fore decreasing input power to thermoelectric cooler, the generated
cooling power also decreases; thus the optimal air ow rate to the
system decreases as well, since the cooling power wontbesu-
cient for higher amounts of ow rate
(3) By increasing the air temperature,
E
ff
ma
x
and the COP at which
E
ff
ma
x
occurs increase. This is because, with the increase of entering
air temperature, the power consumption of thermoelectric cooler
also decreases. In addition, the optimum air ow rate and fan
power consumption decrease, so the total amount of power con-
sumption decreases, and since warmer air contains more moisture
(at the same relative humidity), water production is easier.
Therefore, with the increase in temperature,
E
ff
ma
x
and COP also
increase.
Figs. 1321 indicate the eect of changing current on water con-
densation, E,
T
h
,
T
c
,T
Δ
,
P
TEC,
Qh
,
Q
c
and
C
O
P
at the optimum air ow
rate of each temperature. The eect of current on water production of
the system with optimum air ow rate at each temperature is reported
in Fig. 13 for the relative humidity of 75%. Based on this gure, the
maximum amount of water produced in an hour occurs at 313 K and is
equal to 44 ml.
Fig. 14 is also presented to investigate eects of the change in
current on the amount of eectiveness. Comparing with Fig. 13,itis
clear that the maximum amount of Edoesn't occur at the current
which maximum amount of water is harvested for all cases. It is in-
teresting to note that 26 ml of water can be harvested from the air with
75% relative humidity and 318 K by using only 20 W electrical power
within 1 h.
Fig. 15 indicates that by increasing the electrical current,
T
h
always
rises. Also at a xed current, by increasing the inlet air temperature, the
hot side temperature rises and since it's a limiting factor of the system,
Table 3
Eect of change of number of TECs and length of channel on optimization of the device.
NO TEC I
channel
(m) E
max
(L/J) Current (A) COP ṁa(kg/s)
Re
h
Re
c
15 1.155 1.587E07 1.396 1.9 0.0109 4689 4175
16 1.232 1.615E07 1.375 1.9 0.0111 4776 4252
17 1.309 1.632E07 1.369 1.9 0.0115 4950 4408
18 1.386 1.638E07 1.349 1.9 0.0117 5038 4485
19 1.463 1.637E07 1.344 1.9 0.0120 5212 4641
20 1.54 1.629E07 1.336 1.9 0.0124 5343 4757
Fig. 11. The amount of produced water (in ml/h) as a function of the electric
current, at 308 K and 90% relative humidity and air ow rate of 0.0117 kg/s.
Fig. 12. The amount of produced water (in ml/h) as a function of relative
humidity, air ow rate of 0.0117 kg/s and I = 1.349 A.
Table 4
System optimization results at 75% relative humidity and dierent inlet tem-
peratures.
T
a
(K)
ṁ
a optimu
m
_
(kg/s) Current (A) COP E
max
(L/J)
298 0.0135 1.606 1.7 1.102E07
303 0.0127 1.485 1.8 1.299E07
308 0.0117 1.349 1.9 1.638E07
313 0.0105 1.081 2.4 2.284E07
318 0.0084 0.758 3.2 3.684E07 Fig. 13. Produced water (in ml/h) as a function of the electric current, at 75%
relative humidity for optimum air ow rate at dierent temperatures.
M. Eslami et al. Energy Conversion and Management 174 (2018) 417–429
424
one should be careful about the hot side temperature of TEC in order to
prevent damage.
As shown in Fig. 16, by increasing electric current,
T
c
rst falls then
rises slightly. The reason is that by increasing current,
T
h
increases. At
rst, the optimal ow rate is able to cool the hot side suciently and so
T
h
has a negligible eect on the cold side temperature and
T
c
decreases
as expected. But after a certain current, this optimal ow rate is not able
to cool the hot side enough and an increase in the hot side is observed
which also results in an increase in
T
c
. However, the range of the change
in cold side temperature is not wide, unlike the hot side temperature.
The temperature dierence between the hot and cold side ( T
Δ
) has
an ascending trend with increasing current as shown in Fig. 17. This
behavior is consistent with the previously mentioned change of
T
h
and
T
c
with the current. It should be noted that the air inlet temperature has
little eect on optimum T
Δ
.
Fig. 18 indicates that the input power to thermoelectric cooler is
approximately the same for all inlet air temperatures and only the
performance range of each case is dierent; it shows that optimum
mode of the system occurs at a same thermoelectric cooler's power for
dierent ambient temperatures.
At a xed current, the rate of heat removal from the hot side of
thermoelectric cooler
Qh
decreases by increasing inlet air temperature
as illustrated in Fig. 19. Because with the increase of inlet air tem-
perature, the optimum inlet air ow rate decreases, so the rate of heat
removal from the hot side of thermoelectric cooler decreases.
Also, Fig. 20 demonstrates that the rate of heat transfer from the
cold side has an ascending trend at rst; and at a specic current, the
maximum cooling eect occurs. On the other hand,
Q
c
decreases with
increasing inlet air temperature. The reason is that
Q
c
depends on
Qh
and thermoelectric power and as it was explained before, the optimum
amount of input power to TECs is the same in all cases and
Qh
decreases
with increasing inlet air temperature at a xed current. So
Q
c
must be
reduced with warmer inlet air at a xed current.
Finally, Fig. 21 shows that at a xed current, COP falls with the
increase of inlet air temperature. The reason is that
Q
c
decreases while
the power consumption
P
doesnt change signicantly with increasing
inlet air temperature.
Fig. 14. Eas a function of the electric current, at 75% relative humidity for
optimum air ow rate of system at dierent temperatures.
Fig. 15. Hot side temperature of TECs as a function of the electric current, at
75% relative humidity for optimum air ow rate at dierent temperatures.
Fig. 16. Cold side temperature of TECs as a function of the electric current, at
75% relative humidity for optimum air ow rate of system at dierent tem-
peratures.
Fig. 17. Changes in temperature dierence between hot and cold side of TECs
as a function of the electric current, at 75% relative humidity for optimum air
ow rate of system at dierent temperatures.
M. Eslami et al. Energy Conversion and Management 174 (2018) 417–429
425
5.2. Annual performance in three southern cities of Iran
In this section, the annual amount of water condensation and energy
consumption is calculated for three southern cities of Iran. All these
three cities are faced with the problem of water scarcity. Also, they are
proper candidates to evaluate the proposed system, since they are in the
vicinity of the sea and therefore, have high values of relative humidity.
For this purpose, hourly weather data for Kish island, Bandar-e-Abbas
and Bandar-e-chabahar are obtained via Iran meteorological organiza-
tion [31] for the duration of one year.
At rst, it is assumed that the system is always on, with a constant
electrical current and air ow rate supplied. Although the optimum
values of these two controlling parameters were found in the previous
section for dierent inlet temperatures, the ambient condition is vari-
able during a year. Therefore, now the question is that what electrical
current and air ow rate results in the best yearly performance?
To nd the answer, the eect of changing electrical current on
yearly eectiveness is demonstrated in Fig. 22 for Kish island. The
optimum electrical current at each ow rate is lower than what was
found in the previous section for the corresponding xed inlet
temperature. Again, higher annual Ecan be achieved by reducing the
air ow rate. However, less water is produced if a lower air ow rate is
supplied by the fan as illustrated in Fig. 23. It shows the eect of cur-
rent on water condensation during 1 year for dierent air ow rates.
The interesting point is that the optimum current for maximum pro-
duction in a year is almost the same as the value found in the previous
section for a xed inlet temperature. Therefore, the variable ambient
condition has a little eect on the optimum current at the desired ow
rate for maximum production.
Taking a closer look at the results, it is found that the system with
constant electrical current is not capable of producing water in almost
half of the year. Therefore, a wise decision is to turn othe system in
these times for energy saving purposes. In fact, a fundamental ad-
vantage of the present analysis is that it can predict the possibility of
producing water for dierent values of relative humidity and tem-
perature. If a controller is used to turn the system oand on, then the
same amount of water is gained with higher eectiveness.
To study the behavior of the on/osystem, water condensation,
power consumption (kWh) and eectiveness is calculated during one
Fig. 18. Input power to TECs as a function of the electric current, at 75% re-
lative humidity for optimum air ow at dierent temperatures.
Fig. 19. Heat removal from hot side of TECs as a function of the electric cur-
rent, at 75% relative humidity for optimum air ow rate of system at dierent
temperatures.
Fig. 20. Heat removal from the cold side of TECs as a function of the electric
current, at 75% relative humidity for optimum air ow rate of system at dif-
ferent temperatures.
Fig. 21. Changes in coecient of performance of TECs as a function of the
electric current, at 75% relative humidity for optimum air ow rate of system at
dierent temperatures.
M. Eslami et al. Energy Conversion and Management 174 (2018) 417–429
426
year for Kish island at dierent electrical currents and air ow rates.
Fig. 24 shows that the eectiveness of the on/osystem falls as the
supplied electrical current increases. Unlike the always-on case, one
cannot nd an optimum current for maximum eectiveness. The reason
is that during a year, there are times that the ambient relative humidity
is very high. In these periods, it is possible to produce water with very
small values of electrical current. Such a system is shut down most of
the time and produces a small amount of water very eciently.
Therefore, the objective function for optimization of the yearly
performance of the on/osystem can be the total amount of water
production which is the same as the maximum value found in Fig. 23.
As expected, more water can be harvested with higher air ow rates,
although with lower eectiveness. Table 5 summarizes the annual
performance of the system. It is interesting that the on/osystem can
produce larger amounts of fresh water than the always-on device with
approximately the same eectiveness. Also, the same amount of water
can be harvested much more eciently if the system can be switched
owhile there is no water production.
Similarly, the amount of water production, power consumption and
eectiveness for two other cities of Bandar-e-Abbas and Bandar-e-cha-
bahr are investigated for
=ṁ0. 0117 kg/
s
a
and I = 0.9 A and I = 2.1 A.
Results are compared with Kish islands in Figs. 25 and 26.
According to Fig. 25, power consumption for all three cities is ap-
proximately the same if the system is always on. However, the amount
of water condensation in Bandar-e-chabahar is much higher and
thereby the highest Eamong the three cities occurs for this location
which is equal to 0.533 L/kWh. Also, when the system is on/o, the
highest Eis for Kish island. The reason is that the electrical current of
I = 0.9 A is the optimum value obtained based on the annual weather
data of this city. Also, Fig. 26 shows that the maximum amount of water
condensation during 1 year (312 L) is produced in Bandar-e-chabahar
which has higher relative humidity compared to two other cities.
5.3. Comparison between the present design and other AWG systems
Finally, the specic energy consumption (which is the energy con-
sumption (kWh) for condensation of 1 m
3
of water) for dierent AWG
systems proposed in the literature is compared in Table 6.
It is found that the present design is the most energy ecient system
among similar devices proposed in the literature. The reason is that
dierent operating parameters are considered simultaneously in the
present optimization. However, the eectiveness of the present system
is not competitive at lower values of relative humidity. Because the
optimizations are performed for 75% relative humidity, if the system is
intended to work at lower relative humidity, one should optimize the
system again to have better performance. Also, it is important to note
that the ambient relative humidity greatly aects the specic energy
consumption. For example, a 5% increase in relative humidity can de-
crease the specic energy consumption by about 50%. Also by com-
parison between present work and the commercial system using a vapor
compression refrigeration cooling [32], it can be seen that in high re-
lative humidity, the present design is even marginally performing
better.
6. Conclusion
In this study, thermodynamic analysis and optimization of a device
for water harvesting from humid air is presented. Air cooling is
achieved by a number of thermoelectric coolers. All of the necessary
geometrical, meteorological, technical and thermophysical parameters
are taken into account to have an accurate estimate of the actual system
behavior, including thermal resistances and ns geometry.
Optimizations were performed to obtain fresh water with minimum
possible energy consumption. According to the results, 18 TECs in a
channel with the length of 1.386 m has the best Eamong the cases
investigated. Optimal electrical current and air ow rate were
Fig. 22. E(in L/kWh) as a function of the electric current, at dierent mass
ow rate for Kish island during 1 year; The system is always on.
Fig. 23. Water condensation as a function of the electric current, at dierent
mass ow rate for Kish island during 1 year.
Fig. 24. Eas a function of the electric current, at dierent mass ow rate for
Kish island during 1 year. The system is on/o.
M. Eslami et al. Energy Conversion and Management 174 (2018) 417–429
427
calculated for dierent inlet air temperature. It was found that these
two controlling parameters should be decreased at higher air tem-
peratures. The system is capable of harvesting 26 ml of water from the
air with 75% relative humidity and 318 K by using only 20 W power
within 1 h.
Also, the annual performance of this device in three southern cities
in Iran (Kish island, Bandar-e-Abbas and Bandar-e-chabahar) were in-
vestigated for two modes of operation: the always-on and on/o
Table 5
Annual performance of the system in Kish island for dierent air mass ow rates.
The system is always on The system is on/o
ṁa(kg/s) ṁa(kg/s)
0.0135 0.0127 0.0117 0.0105 0.0084 0.0135 0.0127 0.0117 0.0105 0.0084
Maximum water condensation (L) 265.4 241.2 209.7 173.8 112.4 265.4 241.2 209.7 173.8 112.4
Water condensation at E
max
(L) 153.59 147.57 98.03 91.12 46.98 n/a n/a n/a n/a n/a
Iat maximum water condensation (A) 2.4 2.2 2.1 1.9 1.6 2.4 2.2 2.1 1.9 1.6
Iat E
max
(A) 1.2 1.2 0.9 0.9 0.6 0 0 0 0 0
Power consumption at maximum water
condensation (kWh)
1289.91 1086.61 1020.02 793.75 560.83 784.94 652.02 572.72 450.95 281.77
Power consumption at E
max
(kWh) 431.05 409.40 260.52 235.37 114.01 0 0 0 0 0
Power consumption at E
max
of system when is
always on (kWh)
The same as Power consumption at E
max
(kWh) for each air ow rate 201.21 192.97 99.35 92.07 33.61
Eat Maximum water condensation (L/kWh) 0.206 0.222 0.206 0.219 0.200 0.338 0.370 0.366 0.385 0.400
E
max
(L/kWh) 0.356 0.360 0.376 0.387 0.412 ∞∞∞∞∞
Efor Iat
E
ff
max
of the always on (L/kWh) The same as E
max
for each air ow rate 0.763 0.765 0.987 0.990 1.400
Fig. 25. Annual water production, power consumption and eectiveness for three locations for duration of 1 year at air ow rate of 0.0117 kg/s and I = 0.9 A.
Fig. 26. Annual water production, power consumption and eectiveness for three locations for duration of 1 year at air ow rate of 0.0117 kg/s and I = 2.1 A.
M. Eslami et al. Energy Conversion and Management 174 (2018) 417–429
428
system. Results reveal that there is no optimum electrical current for
the eectiveness of the on/osystem. Among the three locations con-
sidered, Bandar-e-chabahar has the highest yearly yield because of
higher ambient air relative humidity. In the end, a comparison between
this device and other air water generators indicates that present design
is the most energy ecient system among similar devices; especially in
high relative humidity.
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Table 6
Specic energy consumption (in kWh/
m
3
) for the present work and other AWG systems.
System Specic energy consumption (kWh/
m
3
)
T
a
(K) Relative humidity % Comments
Present work 252.78 303 90
Present work 489.95 306 85
Present work 911.57 306 80
Present work 769.23 318 75
Present work 22336.38 318 60
Present work 1013.47 Kish island climate Kish island climate The system is on/o
Present work 1876.95 Bandar-e-chabahar
climate
Bandar-e-chabahar
climate
The system is always on
Tan and Fok [21] 7294.11 302 79
Shanshan Liu et al. [22] 2318.72 296.6 92.7
Pontious et al. [24] (Peltier) 1571.43 ––Exact data for climate condition is not available
Shourideh et al. [25] 2002.00 303 60 At the rst hour of test (unsteady)
Shourideh et al. [25] 1870.00 303 60 At the second hour of test (steady)
Shourideh et al. [25] 936.00 306 80 At the rst hour of test (unsteady)
Shourideh et al. [25] 922.00 306 80 At the second hour of test (steady)
Commercial AWG system [32] 257.18 303 90 Ambient water group (vapor compression
refrigeration)
Commercial AWG system [33] 410 303 80 EcoloBlue group (vapor compression
refrigeration)
M. Eslami et al. Energy Conversion and Management 174 (2018) 417–429
429
... This makes them ideal for creating simple and portable AWG systems. Nevertheless, the designer must be mindful of the TEC's performance and efficiency under different working situations [108,109]. ...
... Thermoelectric (TE) devices, when two dissimilar materials create a junction, are a relatively novel technique for cooling hot, humid air and condensing its moisture content to produce water [108,110]. Heat will move from one end of the junction to the other if a voltage is supplied, making one side of the junction colder and warmer the other side, known as the Peltier effect and electron-hole theory. ...
... Peltier effect devices are typically made from solid solutions of bismuth telluride, antimony telluride, or bismuth selenide, as they can provide. The number of thermoelectric coolers employed in a cooling system determines the effectiveness of the system's cooling [108,111,112]. Fig. 23(b) shows the components of the device. ...
Article
Water scarcity is one of the most challenging problems that the world has ever faced. There are numerous methods to remedy the water crises. One is using atmospheric water harvesting (AWH) to provide water. So far, there is much research on the subject of AWH. However, there is still a lack of establishing an extensive comparison between different technologies and methods used to harvest atmospheric water. In this review, we provide details on the thermodynamic performance of the AWH system. The systems are categorized into both active and passive systems. Heat pumps, membranes, thermoelectric solar systems, and adsorption systems are some atmospheric harvesting technologies that will be thoroughly discussed. Based on the comparison that had been made, it was found that TEC systems are the best for small applications. In contrast, systems such as vapour recompression can meet great demands as they can be integrated with different types of energy, such as natural gas and biogas. Solar systems as passive systems can also be coupled with active systems to boost the efficiency of vapour recompres-sion systems and reduce energy consumption. Furthermore, this review will focus on recent development for each category, the utilization of different advanced materials, and the prospect and challenges associated with AWH.
... The desorption process of adsorbents is a key technology in SSAWH. Existing desorption pathways include hot air desorption [40], solar thermal desorption, semiconductor coolers [41], and electrothermal regeneration [42], etc. The hot air desorption, the semiconductor coolers, and the electrothermal regeneration all require additional electricity to power the desorption process [41,42,40]. ...
... Existing desorption pathways include hot air desorption [40], solar thermal desorption, semiconductor coolers [41], and electrothermal regeneration [42], etc. The hot air desorption, the semiconductor coolers, and the electrothermal regeneration all require additional electricity to power the desorption process [41,42,40]. Solar thermal desorption relies on naturally renewable daylight to passively desorb water, which significantly reduces the energy demand on the system devices [21]. ...
... This system was designed considering many parameters such as temperature, humidity, airflow, and pressure. A study of the optimum number of TEC modules and the length of the condensation channel was presented in [11]. In both [12], [13], ON/OFF control systems were developed for dehumidifier systems. ...
... Port B is connected to the ambient air through the thermal resistance of the hot surface fins. The relations between the absorbed heat on the cold side, the emitted heat from the hot side, the temperature difference between cold and hot sides, injected current, and the applied voltage are described by Eq. (9) to Eq. (11). The moist air is fed into the condensation pipe as illustrated in Fig. 7, via a controlled volumetric flow rate source. ...
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In this paper, a Thermo-Electric Cooler (TEC) based ambiently adaptive Atmospheric Water Generator (AWG) system is proposed. Due to the advantages of TEC technology, it has been relied upon in the phase of dehumidifying the air flowing in the proposed system as an alternative to other mechanical methods. An AWG cooling control system has been developed based on the proposed algorithm to determine the temperature that achieves the highest productivity, which depends on the change in thermal load by changing the airflow, which keeps the temperature of the humid air inside the system below the Dew Point (DP) temperature at varying ambient conditions for sustainable water productivity. A novel Maximum Production Tracking (MPT) algorithm is introduced to automatically determine the marginal temperature below the DP for maximum productivity. The proposed system is simulated using MATLAB/SIMULINK under different ambient conditions. The obtained results affirmed the potential and verification of the proposed approach.
... Zhang et al., analyzed temperature and humidity distribution inside a dehumidification runner and the heat and mass transfer coupling relationship [14]. Hao and Eslami analyzed the relationship between dehumidification capacity and energy consumption under different influencing factors [15,16]. In order to improve the efficiency of water intake, research has recently been carried out in the areas of solar air intake and semiconductor refrigeration water intake. ...
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In this paper, by analyzing the heat and mass transfer characteristics of the dehumidification runner microelement channel of a drinking water emergency extraction vehicle, a mathematical model of heat and mass transfer in the water intake process is established, and the influence of the runner parameters (adsorbent thickness, regeneration angle, rotation speed) and air parameters (treatment air temperature/humidity, regenerated air temperature/humidity) on the water intake characteristics is mainly studied. Water extraction experiments are carried out in arid desert areas and humid island environments. The test results showed that compared with the calculated data, the deviations in the temperature and humidity of the treated air outlet were 3.03% and 4.14%, respectively, and the deviation value of the water intake was 8.23% when the moisture content of the inlet air was 2 g/kg.
... They found that maximum produced water is about 400 litre per month in tropical climates with specific energy consumption of 3 kWh/L. AWH by TEC Technology with water productivity and energy consumption were analysed in many studies; 1.6 L/day water capacity and 61 Wh power input was reported [19][20][21][22][23]. Xiaolong Xu et al. used the radiative cooling principle for a house cooling with storage tank and loop heat exchanger [24]. ...
... It is estimated that nearly 52% of the world's population will live in water-stressed regions by 2050. Unfortunately, only about 2.5% of the water on earth is drinkable, and this rate is decreasing day by day due to increasing environmental pollution and climate change [1,2]. Treatment of wastewater for clean drinking water or obtaining freshwater from seawater are some suggested solutions to reduce the gap between the water supply and demand, but wastewater treatment and desalination of seawater are quite costly with today's technology [3]. ...
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The purpose of this study is to improve the water harvesting capacity of the traditional wire mesh from the fog by modifying its surface using a nature-inspired composite structure consisting of hydrophilic and hydrophobic zones. Hydrophilic zones were obtained by electrospinning or electrospraying of the polyamide 6 (PA6) / chitosan (CH) blend, and similarly hydrophobic zones were attained by electrospraying of polycaprolactone (PCL). The water harvesting capacity of the resulting meshes was tested and compared with each other. The highest water harvesting capacity was achieved with the PA6/CH nanofiber coated wire mesh as 87 mg / cm2/h. This mesh collected twice as much water compared to the uncoated mesh. However, its water collection rate decreased when nanofiber surface reached the saturation level. The addition of hydrophobic PCL particles onto nanofibers reduced the amount of water captured. In this case, the water collection rate of the mesh continued to increase.
... This system depends on either condensation or sorption [6,7] methods. Condensation methods include active cooling condensation [8], a refrigeration technology, such as vapor compression cycle (VCC or VCR) [9,10], adsorption/absorption chiller [11,12] and thermoelectric cooling (TEC) systems [13][14][15]. VCC can be used either for just water harvesting or for both air conditioning and water harvesting. For air conditioning and water harvesting, normally central cooling systems are used and the condensed water is considered the byproduct of air cooling [16]. ...
... AWGs can be further classified as condensation and sorption types [5]. The condensation type AWG uses vapour compression cycle (VCC) [6][7][8][9][10][11] or thermoelectric cooling (TEC) [12][13][14][15][16][17][18][19] to cool down a surface temperature of a heat exchanger below the dew point temperature of the process air. The weak point of condensation AWGs is that the water harvesting process under dry and hot climatic conditions is very energy intensive or even impossible due to very low dew point temperature values (sometimes even negative) [20,21]. ...
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In this paper, a novel concept of an atmospheric water generator (AWG) based on desiccant coated heat exchangers (DCHEs) is presented. Two silica gel coated heat exchangers (SGCHEs) and two zeolite coated heat exchangers (ZCHEs) were prepared by coating a desiccant material on the surface of a conventional air-to-water fin-tube heat exchanger. An experimental set-up was built to investigate the performance characteristics of the prepared DCHEs in terms of the moisture removal capacity MRC, the effective duration of the process te, and the total moisture mass transferred during the dehumidification/regeneration cycle MT. The influence of the regeneration hot-water temperature Tw,in on the performance characteristics was also analysed. The experimental results showed that ZCHEs have better performance characteristics under arid climatic conditions than SGCHEs. It was found that the ZCHEs have approximately four times higher value of the moisture transferred during the dehumidification/generation cycle MT compared to the SGCHEs, 10.15 g for the ZCHEs and 2.60 g for the SGCHEs. Moreover, it can be concluded that the influence of the regeneration water temperature Tw,in for ZCHEs is not as critical as for SGCHEs. The experimental results indicated that when the regeneration hot-water temperature Tw,in decreases from 85 °C to 65 °C, the moisture transferred during the dehumidification/regeneration cycle MT for the ZCHEs decreases from 10.15 g to 7.11 g representing a 30% decrease in the system performance, while for the SGCHEs the same decrease in the regeneration hot-water temperature Tw,in causes a significant decrease in the moisture mass transferred during the cycle MT from 2.60 g to 0.85 g, representing a 67% decrease in the system performance.
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The main objective is to develop and experimentally investigate a thermoelectric fresh water generator (TFWG) based on the fundamental of Thermoelectric Cooling Effect by condensing the moisture from the ambient moist air. It can be made useful to the people in coastal and humid regions with relative humidity above 60% having scarcity of drinking water. A prototype of the generator consisting of a 0.7 m long cooling channel along with ten thermoelectric modules of dimension 0.04 * 0.04 m2 each placed linearly in an array is fabricated and experimented. An internal heat sink of surface area 0.2m² and length 0.65m is placed on the cold side of the modules to enhance heat transfer rate. The observations from the experiments show that with the use of internal heat sink, the quantity of water generated per 10hours increases by 81% as compared without internal heat sink. Electric current, air mass flow rate and humidity of moist air were varied to understand their impact on the quantity of water generated. Based upon the observed results, the quantity of water generated is directly proportional to all the three parameters in the domain of experimentation.
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Freshwater scarcity is increasingly perceived as a global systemic risk. Previous global water scarcity assessments, measuring water scarcity annually, have underestimated experienced water scarcity by failing to capture the seasonal fluctuations in water consumption and availability. We assess blue water scarcity globally at a high spatial resolution on a monthly basis. We find that two-thirds of the global population (4.0 billion people) live under conditions of severe water scarcity at least 1 month of the year. Nearly half of those people live in India and China. Half a billion people in the world face severe water scarcity all year round. Putting caps to water consumption by river basin, increasing water-use efficiencies, and better sharing of the limited freshwater resources will be key in reducing the threat posed by water scarcity on biodiversity and human welfare.
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In this study, an improved novel solar driven atmospheric water generator device has been proposed to generate more water with less power usage. The system consisting of three parts: solar driven ammonia absorption refrigeration cycle, saline water desalination cycle and air dehumidification cycle. The system generates water from two great water sources simultaneously with equal power input to common AWG (Atmospheric Water Generator). Thermodynamic analysis of system has been done by parametric analysis of sub series cycles. The device performance curves have been introduced according to results. These curves can be used to determine the water generation of the improved novel AWG device in every place (or City) with having air relative humidity, and temperature of desired location. The performance of improved novel AWG device was analyzed in four cities of Iran. AWG water generation rate in case cities improved averagely about 165% by improvement process.
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A prototype of a small-scale atmospheric water generator (AWG) that employs the Peltier effect for cooling was designed and constructed. In this study, a systematic design approach was employed, and the AWG system was sized using the cooling capacity and coefficient of performance (COP) behaviors of the thermoelectric cooler (TEC) with respect to the current. Likewise, a mathematical model that uses the surrounding fluid temperature and relative humidity ratio as the driving force for heat and mass transfer was used to optimize and design the rectangular extended surfaces and estimate the water generation rates. The completed AWG system, housed in a 3D printed casing, was used to experimentally investigate the impact of the variation of the airflow velocity, humidity, and TEC current on the water generation rates. The experiments confirm that the inclusion of an intake fan reduces water generation in some cases. The water yield is observed to increase with relative humidity. The tests also suggest that increasing the current of the individual TECs results in an increased water generation rate; however, this increase is coupled with a higher specific energy consumption as a result of the decreased COP. Finally, a comparison between the prototype and several AWGs in literature was carried out.
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Experimental and numerical investigations were carried out for a system including the thermoelectric modules with heat sinks inside a square duct. A thermoelectric module model was developed using the commercial computational fluid dynamics code FLUENT and user-defined functions (UDF). The performance of a thermoelectric module was analyzed by a visualization technique of temperature variation over time. In this study, UDFs are employed to specify the heat flux terms of the cooling capacity and the heat generation of thermoelectric modules. As a result, the data obtained with the numerical method was generally in good agreement with those obtained by the experiments over time at each inlet temperature. Detailed effects of the various parameters of heat sinks attached the thermoelectric module were discussed, and consequently these parameters had a great influence upon the performance of the thermoelectric module.
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