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Journal of Physics: Conference Series
PAPER • OPEN ACCESS
Exergy analysis of a solar regenerated liquid desiccant assisted air
conditioning system for hot and humid climates
To cite this article: Avik Ghosh et al 2019 J. Phys.: Conf. Ser. 1240 012148
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IOP Conf. Series: Journal of Physics: Conf. Series 1240 (2019) 012148
IOP Publishing
doi:10.1088/1742-6596/1240/1/012148
1
Exergy analysis of a solar regenerated liquid desiccant
assisted air conditioning system for hot and humid climates
Avik Ghosh1, A Ganguly1 and J Bhattacharya2
1Department of Mechanical Engineering, Indian Institute of Engineering Science and
Technology, Shibpur, Howrah 711103, India
2Department of Mechanical Engineering, Indian Institute of Technology, Kanpur,
Uttar Pradesh 208016, India
ghoshavik82@gmail.com
Abstract. In this paper, exergy analysis of a novel solar powered liquid desiccant assisted air
conditioning system is presented and simulated. The system aims to provide suitable thermal
comfort conditions inside large office buildings with high internal loads situated in the hot and
humid tropical/subtropical countries of the world. The system consists of process and
regenerating air streams, a liquid desiccant solution loop and a cooling water loop. The primary
objective of this study is to present the exergy of cooling capacity along with the overall
exergy efficiency of the proposed system. The study helps to quantify the optimum operating
and design parameters for system operation based on the second law of thermodynamics. For
the base case, which is representative of a hot and humid climatic condition, the proposed
system is able to maintain the room air conditions within the moderate thermal acceptability
criterion. The exergy of cooling capacity and exergy efficiency for the base case is about 2900
W and 2 % respectively. Parametric analyses show that the system performs the best under
conditions of high ambient insolation and temperature, low ambient humidity and a process air
to desiccant solution mass flow rate of about 3 in the dehumidifier.
1. Introduction
In the modern world, building energy consumption is significantly augmented by the use of air
conditioning systems, thereby placing a huge demand on the electricity supply network. A 6.2 % per
year rise is estimated in the heating, ventilation and air conditioning load of the world [1].
Conventional vapor-compression based air conditioning systems are still the most prevalent
technology in use for providing thermal comfort inside commercial and office buildings situated in the
hot and humid climatic regions. However, they are associated with large primary energy consumption
and global warming. This gives rise to a pressing need to develop air conditioning systems which can
provide suitable thermal comfort in buildings without harming the environment.
Liquid desiccant cooling systems (LDCS) are a viable substitute to conventional vapor
compression based air conditioning systems. They have the ability to treat latent and sensible heat
loads independently, are easy to store and consume much less energy compared to that of vapor
compression based systems. Regeneration of the liquid desiccant solution (LDS) by making use of the
heat energy obtained from solar radiation has gained momentum recently owing to the system’s
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effectual usage of low grade heat and coincidental matching of peak cooling load with maximum solar
insolation.
Previous research on LDCS have been mainly focused on energy analysis, with studies based on
second law of thermodynamics being limited [2]. Of the studies that are available on exergy analysis,
such a novel configuration with the use of direct solar regenerator and dew point indirect evaporative
cooler has not been previously reported in literature. Thus, this paper attempts to bridge that gap by
carrying out both energy and exergy analysis of the proposed solar powered liquid desiccant air
conditioning system for application in large office buildings with high internal loads situated in the hot
and humid climatic regions. The present work focuses exclusively on designing the proposed system
optimally on the basis of the second thermodynamic law.
2. Methodology
2.1. Description of the system
The scheme of the proposed system is shown in figure 1. The system is composed of the process and
regenerating air streams, a LDS loop and cooling water loop. An aqueous solution of CaCl2 is used as
the desiccant.
Figure 1. Scheme of the proposed system.
The process air is a mixture of ambient and conditioned return air from the room, mixed in the air
blender. The process air is dehumidified in a dehumidifier (DEH), cooled sensibly in the air-water heat
exchanger (HEX 3) and dew point indirect evaporative cooler (DPIEC) and finally supplied to the
room. The DPIEC uses a fraction of the intake air as an evaporative sink and supplies the rest to the
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building [3]. This supply air aims to maintain suitable thermal comfort conditions inside the room with
high internal load [4]. Finally, the cycle is completed by bringing the conditioned air inside the room
back to the air blender. It may be noted that the amount of air used as an evaporative sink in the
DPIEC is equal to the amount of ambient air at the air blender inlet. The regenerating air stream
consists of ambient air which is drawn into the regenerator (REG) to absorb moisture from the dilute
LDS in order to re-concentrate it. Finally, this REG outlet air is exhausted to the atmosphere.
The LDS after dehumidifying the process air becomes diluted due to the absorption of moisture and
heats up due to the released latent heat of condensation. It is then preheated in the desiccant-desiccant
heat exchanger (HEX 1) before being supplied to the forced parallel flow direct solar REG [5] where it
loses moisture to the regenerating air stream. Thereafter, the hot concentrated LDS at the REG outlet
is pre-cooled in the HEX 1 and desiccant-water heat exchanger (HEX 2) before being supplied to the
DEH. Cooling water to the HEX 2 and HEX 3 is supplied from a natural draught wet cooling tower.
2.2. Thermodynamic modelling
The DEH is modeled based on the works of Gandhidasan [6] and Chung [7]. The governing equations
are given in equations (1) through (5).
)1(
,,,, ftftt ambaretadehia
(1)
)1(
,,,, fHfHH ambaretadehia
(2)
dehosdehosdehoadehodadehisdehisdehiadehida hmhmhmhm ,,,,,,,,,,,,,,,,
(3)
dehisdehia
dehoadehia
dehis
dehia
p
dehis
dehia
dehis
dehia
deh pp
pp
t
t
Za
t
t
m
m
,,,,
,,,,
388.3
,,
,,
68.1184.0
,,
,,
174.0
,,
,,
)]686.0exp(152.0[
1
)(
)]985.0exp()(205.0[
1
(4)
dehisdehia
dehoadehia
tt
tt
,,,,
,,,,
(5)
where,
t
,
f
,
H
,
m
,
h
,
deh
,
p
a
,
Z
,
,
a
p
and
s
p
refer to the temperature (°C), fraction of
return air in process air, specific humidity, mass flow rate (kg/s), specific enthalpy (J/kg),
effectiveness of DEH, specific area of packing (m2/m3), packed bed height (m), ratio of vapor pressure
depression of the LDS to the vapor pressure of pure water, partial pressure of water vapor in the air
(Pa) and vapor pressure of the LDS respectively.
The REG is modeled based on the work of Alizadeh and Saman [8]. The governing equations are
given in equations (6) through (12).
)()1(
sambreg II
(6)
0)()()( ,,,,,, fgevapregisspssregiaaparegiaambsregreg hmttCmttCmxttUxI
(7)
0)( ,, fgevapspssaparegiaambsregreg hdmdtCmdtCmdxttUdxI
(8)
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IOP Conf. Series: Journal of Physics: Conf. Series 1240 (2019) 012148
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)(
950
622.0
as
atm
as
evap pp
P
hc
dx
dm
m
(9)
sss Ccbtap /
(10)
dxtthcdtCmdxtthc ambaambaaparegiaasas )()( ,,
(11)
ambggaamba hchchc
111
(12)
where,
I
,
s
,
)(
,
x
,
U
,
p
C
,
fg
h
,
hc
,
s
C
refer to the solar insolation (W/m2), reflectivity of
the LDS, transmissivity-absorptivity product, length (m), overall heat loss coefficient (W/m2°C),
specific heat capacity (J/ kg °C), heat of evaporation of water from LDS (J/kg), convective heat
transfer coefficient (W/m2°C) and mass concentration of the LDS (%) respectively.
The effectiveness of the heat exchangers and the wet bulb effectiveness of the DPIEC are defined
as follows.
dehosregos
dehosregis
HEX tt
tt
,,,,
,,,,
1
(13)
ctowHEXis
dehisHEXis
HEX tt
tt
,,2,,
,,2,,
2
(14)
ctowdehoa
HEXoadehoa
HEX tt
tt
,,,,
3,,,,
3
(15)
3,,,3,,
,,3,,
HEXoawbHEXoa
roomiaHEXoa
DPIEC tt
tt
(16)
The Merkel approach [9] is used to calculate the water temperature at the cooling tower outlet. By
solving equations (1) through (16), the conditions of supply air at the inlet to the room are found out.
The heat load calculation for the office building is done by assuming a sensible heat factor of 0.92
using the approach provided in [4,10].
For the exergy analysis, the saturated state of the ambient atmosphere is selected as the reference
state [2]. The specific exergy of moist air (J/kg of dry air) is expressed as:
)608.11(
)608.11(
ln)608.11[()]ln(1[)( w
w
wTR
T
T
T
T
TwCCe sat
amba
amb
a
amb
a
ambpvpada
)]ln(608.1
sat
w
w
w
(17)
where,
w
,
T
,
a
R
refer to the humidity ratio, temperature (K) and ideal gas constant for dry air
(J/kg K) respectively.
The exergy of cooling capacity (W) is expressed as [11]:
)( ,,,,,, dehidaroomidaroomidaCC eemE
(18)
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doi:10.1088/1742-6596/1240/1/012148
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The system’s exergy efficiency is expressed as the ratio of the exergy of cooling capacity to the
absorbed solar radiation exergy rate by the REG (W) [12]. Analyses based on exergy efficiency take
into account the energy quality and reversible (idealized) version of the system which the first law
analyses miss out on. It is an important tool which can be used to quantify the irreversibilities within
the system.
)1()()1(
,sunapparent
amb
regsamb
CC
ex
T
T
AI
E
(19)
A computer code is written in Java to simulate the proposed thermodynamic model. It takes the
ambient insolation, temperature, humidity, wind speed and radiation tilt factor as inputs and predicts
the room air conditions along with the system’s exergy of cooling capacity and exergy efficiency. The
place under consideration in this study is Kolkata [13], India which is representative of a location
having hot and humid ambient climate.
3. Results and discussions
The validation of the proposed thermal model is carried out by comparing the results predicted by our
model with a similar reference model study [14]. Figure 2 shows the comparison between the present
and reference model for a typical day in August (16th of August). We can observe that the proposed
model agrees well with the reference model, having mean absolute errors of 3.2 % and 6.5 % on
comparing the room supply air temperature and humidity ratio respectively, for system operation in
complete ventilation mode.
Figure 2. Comparison of the proposed model with the reference model study for a typical day in
August.
The operating and design conditions for operation of the re-circulating air system for a base case,
which is typical of the climatic condition for a hot and humid environment, is provided in Table 1. It
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can be seen from the simulations that the room temperature and humidity ratio is maintained at 24.3
°C and 9 g/kg respectively, which is suitable for moderate thermal comfort condition. The system’s
exergy of cooling capacity (
CC
E
) and exergy efficiency (
ex
) is about 2900 W and 2 % respectively.
The low value of exergy efficiency is due to the large area of the regenerator along with moderately
high ambient insolation which increases the absorbed solar radiation exergy rate, and low value of
supply air flow rate into the room (or process air flow rate at DEH inlet) which decreases the exergy of
cooling capacity. For the parametric analyses, individual parameters are varied keeping others the
same to estimate the influence of the varied parameters on the second law performance of our system.
Table 1. Operating and design conditions for the base case.
Parameter
Unit
Value
Ambient temperature
°C
34
Ambient relative humidity
%
75
Ambient insolation
Ambient wind speed
Wall radiation tilt factor
Desiccant flow rate
Process air flow rate
W/m2
m/s
kg/s
kg/s
700
2.5
0.5
1.875
3.75
(a)
(b)
(c)
(d)
Figure 3. Parametric analyses of the proposed system.
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Figure 3 presents the variation of the system’s
CC
E
and
ex
with the variation of various design
and operating parameters. Figure 3(a) presents the aforesaid parameters’ variation with the variation of
ambient insolation. A higher ambient insolation increases the evaporation rate of water vapor in the
REG. Thus, the LDS supplied to the DEH is more concentrated. This increases the mass transfer of
water vapor in the DEH, which correspondingly increases the temperature depression of the air in the
DPIEC. Consequently, there is an increase in the system’s
CC
E
. We observe that with the increase of
ambient insolation, the
ex
increases slightly. This is due to the fact that the effect of increase in
CC
E
supersedes the increase in absorbed solar radiation exergy rate in the REG.
Figure 3(b) shows us that both the
CC
E
and
ex
decrease with the corresponding increase of
ambient relative humidity. This occurs because at higher ambient humidity levels, the performance of
the REG shrinks and thus, the LDS supplied to the DEH is more dilute. Thus, the air at the inlet to the
DPIEC has higher humidity ratio. This decreases the temperature depression of the air in the DPIEC
which in turn decreases the system’s
CC
E
. As the absorbed solar radiation exergy rate is constant, the
system’s
ex
correspondingly decreases with the decrease in
CC
E
.
Figure 3(c) shows us that both the
CC
E
and
ex
increase with the increase in ambient temperature.
An ambient air humidity ratio of 25 g/kg is assumed for this analysis. As a result of the increase in
ambient temperature, the air temperature at the REG inlet increases. This correspondingly enhances
the mass transfer of water vapor in the REG. Thus, the LDS supplied to the DEH is more concentrated
which increases the mass transfer of water vapor in the DEH and consequently the temperature drop of
the air in the DPIEC. Correspondingly, the system’s
CC
E
and
ex
is augmented.
Figure 3(d) shows us that both the
CC
E
and
ex
increase with the increase in mass flow rate of
process air. The mass transfer in the DEH increases with increase in the air flow rate which augments
the system’s
CC
E
. The system’s
CC
E
and
ex
show a rapid increase when the process air mass flow
rate is low, but increases at a slower rate at air flow rates more than 5.5 kg/s. This indicates that the
optimum mass flow rate of air to LDS in the DEH is about 3.
4. Conclusions
The paper presents the second thermodynamic law analyses of a novel solar regenerated liquid
desiccant assisted evaporative cooling system. The proposed system has the ability to provide suitable
thermal comfort conditions inside large office buildings with high internals loads situated in the hot
and humid climatic regions. For the base case, which is representative of a hot and humid climate, the
system is able to maintain the room air temperature and humidity ratio at 24.3 °C and 9 g/kg
respectively. The exergy of cooling capacity and exergy efficiency for the base case is about 2900 W
and 2 % respectively. Parametric analyses show that the system performs the best under conditions of
high ambient insolation and temperature, low ambient humidity and a process air to LDS mass flow
rate of about 3 in the DEH.
Acknowledgments
The authors gratefully acknowledge IIT Kanpur’s SURGE 2018 summer research internship program,
during which the fundamentals of this work were carried out.
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