EXPERIMENTAL INVESTIGATIONS ON HYBRID SOLID DESICCANT – VAPOR COMPRESSION AIR-CONDITIONING SYSTEM FOR INDIAN CLIMATE

Abstract

Hybrid solid desiccant-assisted pre-conditioner and split cooling system, which combines merits of moisture removal by rotary dehumidifier and VCR cooling coil for sensible heat removal, is recently developed as a potential alternative to the traditional vapor compression air conditioners. In the present paper, experiments on a hybrid solid desiccant VCR air-conditioning system were carried out for the typical hot and humid North Indian climate. The overall system performance was evaluated based on variations in ambient conditions i.e. temperature and humidity. The dehumidifier effectiveness is evaluated as a function of various inlet conditions. It is found that the hybrid system can achieve a better performance in hot and humid region due to significant reduction in humidity ratio, because of the introduction of desiccant dehumidifier.
Key words: COP, dehumidification, regeneration, solid desiccant wheel, vapor compression system.

Full text

EXPERIMENTAL INVESTIGATIONS ON HYBRID SOLID DESICCANT –
VAPOR COMPRESSION AIR-CONDITIONING SYSTEM FOR INDIAN
CLIMATE
D.B.JANI , MANISH MISHRA, P.K.SAHOO*
Department of Mechanical & Industrial Engineering,
Indian Institute of Technology, Roorkee, 247667, India
E-mail:dbjani@rediffmail.com,mmishfme@iitr.ernet.in,*sahoofme@iitr.ernet.in
ABSTRACT
Hybrid solid desiccant-assisted pre-conditioner and split cooling system, which combines merits of moisture
removal by rotary dehumidifier and VCR cooling coil for sensible heat removal, is recently developed as a
potential alternative to the traditional vapor compression air conditioners. In the present paper, experiments
on a hybrid solid desiccant VCR air-conditioning system were carried out for the typical hot and humid
North Indian climate. The overall system performance was evaluated based on variations in ambient
conditions i.e. temperature and humidity. The dehumidifier effectiveness is evaluated as a function of various
inlet conditions. It is found that the hybrid system can achieve a better performance in hot and humid region
due to significant reduction in humidity ratio, because of the introduction of desiccant dehumidifier.
Key words: COP, dehumidification, regeneration, solid desiccant wheel, vapor compression system.
1. INTRODUCTION
Traditional vapor compression refrigeration air-conditioning systems employ refrigerants which are
environmentally harmful and require high grade electrical energy for its running. Thus, in space cooling
applications, solid desiccant cooling systems are being developed as an alternative to the conventional vapor
compression refrigeration air-conditioners. Solid desiccant cooling systems are heat driven cooling units in
which heat energy is needed to regenerate the rotary desiccant dehumidifier. Desiccant wheel is the main part
of the system which removes humidity from the supply process air. Hot and dry process air is first sensibly
cooled in heat wheel and then in conventional cooling coil of vapor compression refrigeration air-
conditioner. Latent heat load is removed in rotary desiccant dehumidifier while sensible heat load is removed
later in heat wheel and in conventional cooling coil. Thus, solid desiccant cooling systems can take care of
both sensible and latent load of air conditioned space separately as well as of fresh air requirements. These
systems also have the advantage of being powered by waste heat or solar energy which can easily be
obtained from low grade or renewable energy sources. Thus, these hybrid systems can augment to the
amelioration of comfort, energy and cost saving.
Sheridan and Mitchell (1985) have investigated the energy consumption for air conditioning in hot-
humid and hot-dry climates using hybrid desiccant cooling system. They have concluded that the hybrid
cooling system can save more energy in hot-dry climate than that in hot-humid climate. The performance
evaluation of an integrated hybrid desiccant cooling system in terms of the sensitivity of ambient conditions
has been carried out by Worek and Moon (1988). The performance improvement in hybrid desiccant cooling
system was found 20-80% higher as compared with the traditional vapor compression refrigeration air-
conditioning system at different ambient temperatures. Yadav and Kaushik (1991) have examined a
simulation study of a hybrid solid desiccant air conditioning system consisting of a solid desiccant wheel and
a conventional vapor compression air conditioner unit. It was found that the hybrid system can have
significant energy saving over a traditional vapor compression refrigeration air-conditioning unit. Further,
Jain and Dhar (1995) have investigated four cycles namely the ventilation cycle, the recirculation cycle, the
Dunkle cycle and the wet surface heat exchangers cycle for different climatic conditions of India. The
influence of the effectiveness of heat exchangers and evaporative coolers on overall system performance has
been evaluated. The performance of the Dunkle cycle was found better compared to recirculation and
ventilation cycles in all climatic conditions. Experiments were carried out by Yadav (1995) showing that the
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hybrid vapor compression system can save significant energy as compared to the conventional vapor
compression refrigeration system in hot and humid climatic conditions. A comparative study has also been
done by Dai et al. (2001) for hybrid cooling system showing 20-30% more cooling production than the VCS
alone. Mazzei et al. (2002) have compared the operating costs of the desiccant cooling system and traditional
vapor compression air conditioning systems using simulation. They have predicted savings of about 35% in
operating cost of desiccant cooling system. They have also added that if the desiccant is regenerated by
waste heat, operating costs savings can further increases up to 87%. They evaluated that cost savings and
reduction in cooling power have further improved when indirect evaporative cooling is used in conjunction
with desiccant dehumidification. Subramanyam et al. (2004) have experimentally studied a hybrid system
consisting desiccant wheel and conventional vapor compression air-conditioner for low humidity condition.
They tested the system at various supply airflow rates to evaluate its performance and compared with those
of the traditional and of reheat system. They found that the proposed system could deliver supply air at much
lower dew point temperature compared to the traditional system with a marginal penalty on COP. Jia et al.
(2007) experimented with a solid desiccant cooling system using a novel compound desiccant wheel. It
worked well at lower regeneration temperature and had higher dehumidification capacity due to newer
desiccant materials. Experimental result indicated that novel compound desiccant wheel could remove about
20-40% more moisture over the desiccant wheel employing silica gel. Experimental investigation of a hybrid
air conditioning system consisting of packed bed solid desiccant integrated with an R407C traditional vapor
compression refrigeration system was carried out by Fatouh et al. (2009) for various operating parameters
such as mass flow rate of air, regeneration temperature and evaporator air inlet conditions. They found that
the reactivation temperature and air flow rate had significant effects on the desiccant regeneration process.
Reported results also revealed that solid desiccant based hybrid desiccant air-conditioning system reduces the
compressor power significantly. The performance analysis of a solid desiccant based hybrid air conditioning
system using 100% fresh air was carried out by Hourani et al. (2014) for office building in Beirut. They
found 16.15% reduction in energy consumption for a newly designed hybrid air conditioning system as
compared to conventional evaporative cooling system for the same outdoor conditions.
In the present work, experiments are carried out to evaluate the overall performance of the system and
effectiveness of desiccant wheel during a cooling season from March to mid of October in a hot and humid
climatic condition (Roorkee, India). The experimental investigation has covered a sufficiently wide range of
operating conditions in terms of temperature, relative humidity, flow rate, pressure drop etc. of the process
and regeneration air streams. Analysis has also been carried out to study the variations in regeneration
temperature based on variation in the ambient air condition. Results show that the overall system
performance and the effectiveness of the components change according to varying outdoor conditions.
2. DESCRIPTION OF THE SYSTEM
In the present case, a test room having dimensions 3m × 3m × 3m has been selected for the study.
The total cooling load of the test room is evaluated as 1.76 kW, out of which sensible load is 1.37 kW and
latent load is 0.39 kW (Kulkarni et al., 2011). Sensible heat ratio (SHR) and air flow rate have been
calculated as 0.78 and 360 m3/hr respectively. The comfort conditions are taken as 50% relative humidity
and 26°C dry bulb temperature (ASHRAE Handbook-Fundamentals, 2013). The desiccant material used in
the desiccant wheel is synthesised metal silicate. Rotational speed of the dehumidifier is kept constant as 20
rph. In the schematic diagram of hybrid solid desiccant vapor compression air-conditioning system
presented in Figure 1, the return room air at state 1 passes through the rotary desiccant dehumidifier. Its
moisture is adsorbed significantly by the desiccant material and the heat of adsorption raises its temperature
up to state 2. The hot and dry air is first cooled sensibly in air to air heat exchanger (2-3) and then in VCR
sensible cooling coil up to state 4. In the regeneration air line, ambient air at state 6 enters air to air sensible
heat exchanger and serves as a heat sink to cool the supply process air. Consequently, its temperature rises
when exiting from sensible heat exchanger at state 7. At this point, it is heated to reach temperature at the
state point 8 which is high enough to regenerate the desiccant material. Moist air at the outlet of dehumidifier
is exhausted to atmosphere at state 9.
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Figure 1. Schematic diagram of hybrid solid desiccant vapor compression air-conditioning system.
Desiccant wheel is one of the most important components of the hybrid solid desiccant vapor
compression air-conditioning system. A desiccant wheel driving system consists of an electromotor, a wheel
disk and a belt used to rotate the desiccant wheel. Physical properties of the desiccant wheel used in the
current study are shown in Table 1. The photographic view of the system is shown in Figure 2.
Table 1. Physical properties of desiccant wheel.
Sr. No.
Parameter
Value
1
Diameter
360 mm
2
Width
100 mm
3
Desiccant material
Synthesized metal silicate
4
Channel shape
Sinusoidal
5
Flow pattern inside the channels
Laminar flow
6
Cross area ratio
(Dehumidification/regeneration)
3:1
7
Rotational speed
20 rph
8
Drive
230V/AC/50 Hz
Figure 2. Photographic view of the experimental system.
3. MEASUREMENTS
Experiments are carried out by simultaneous measurement of temperature, relative humidity,
pressure and flow. Measurement of temperature and relative humidity are carried out by a Retronic
Desiccant wheel
Heat recovery wheel
VCR sensible cooling coil
Electric heater
Test room
1
2
9
5
4
3
7
6
8
1
2
3
4
+
6
7
8
9
Desiccant wheel
Heat recovery wheel
VCR sensible
cooling coil
Test
room
Return air
Exhaust air
5
_
Electrical heater
Ambient air
Condenser
Exhaust fan
Supply
fan
C
EV
Conditioned
air
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Hygroflex HF 320 multifunctional digital transmitter having accuracy of + 0.1 °C and + 0.1% RH
respectively. Pressure drop across desiccant wheel, heat wheel and cooling coil is measured by using a
KIMO CP 100 differential pressure transmitter with + 1.5% accuracy. The air flow rate is measured by using
a KIMO CTV100 velocity transmitter with an accuracy of + 3%. Energy meter made of Indotech ITE-116 is
also used to measure the electrical power consumption of the system with an accuracy of + 2%. The
measurements were carried out once the temperature and humidity of the system attain steady state
condition.
Masibus-85XX micro-controller based scanner along with Control panel is used to control and
operate the system. If either the process air fan or the regeneration air fan stops working, the control system
automatically shuts off the heating process to save the rotary desiccant dehumidifier against risk of
overheating. Tubular type heater of 1.8 kW capacity is used for heating the regeneration air. The
regeneration air is first pre-heated by passing through heat recovery wheel to pre-cool the adsorbed hot
process air coming out from the rotary dehumidifier. The heat required for further heating of the pre-heated
regeneration air is supplied by electric heater to obtain desired regeneration temperature necessary for
desorption of desiccant material.
4. UNCERTAINTY ANALYSIS
Accurate measurement of physical quantities is difficult as uncertainties are always present due to
instrumental, physical and human inadequacies (Holman, 2012). Uncertainty or error analysis is the
procedure employed to assess the uncertainty from measured variables with known values of uncertainties.
The measured variables of hybrid solid desiccant vapor compression air-conditioning system are
temperature, relative humidity, flow rate etc. Each of these quantities has a measurement error. For the
calculation of uncertainty, the root of the sum square (Jia et al., 2006) is used in this study and is expressed
as
1
2
22 2
12
12
.............
Rn
n
R R R
w w w w
x x x


   
  

    
   
  

    

(1)
Where, R is a given function of the independent variables x1,x2 .......,xn and w1,w2,....,wn are the uncertainties
in the corresponding variables. The temperatures, relative humidity and flow rates are measured with
appropriate accuracy of the instruments as explained previously. The total uncertainty associated with
coefficient of performance (COP) is found to be + 14.63%.
5. PERFORMANCE
The following calculations are used to determine overall system performance and effectiveness of the
components. The coefficient of performance of the system (Hurdogan et al., 2010) based on electrical energy
input utilized by vapor compression unit can be defined as follows
cc
th c
Q
COP = Q +W
(2)
In the above equation, Qcc is the cooling capacity and Qth is the regeneration heat input (Yong et al., 2006)
defined as
cc p 1 4
Q = m (h -h )
(3)
th r 8 7
Q = m (h -h )
(4)
The desiccant wheel effectiveness (Nia et al., 2006) can be given as
12
dw
1
ω -ω
ε =
ω
(5)
The effectiveness of heat recovery wheel can be obtained by assuming constant mass flow rate of air streams
flowing across it (Daou et al., 2006). It is given as follows
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23
hrw
26
T -T
ε = T -T
(6)
6. VALIDATION OF RESULT
Figure 3 shows a reasonable agreement of current results as compared to the experimental results of
Sheng et al. (2013). COP of the system slowly rises with successive increment in outdoor air temperature as
observed from the Figure 3. As the ambient air temperature increases, reactivation air is pre-heated to higher
temperature in heat recovery wheel, before it is brought to the regeneration heater. This increases the
temperature of regeneration air and hence leads to increase removal moisture from desiccant surface further.
Thus, the COP of system rises as per equations (2) and (3) described above.
Outdoor air temperature (oC)
27 28 29 30 31 32 33 34 35
COP
0.5
1.0
1.5
2.0
2.5
Sheng et al. (2013)
Current study
Figure 3. Validation of the experimental results.
7. RESULTS AND DISCUSSION
Figure 4 illustrates variation in dehumidifier effectiveness with the change in process air inlet
temperature. It has been observed that slight increase in process air temperature due to increment in ambient
air temperature leads to higher desiccant wheel effectiveness. This is because of the increase in ambient air
temperature the regeneration air temperature increases and hence the effectiveness of the desiccant wheel
also increases (Eq. 5).
Process air inlet temperature of desiccant wheel (oC)
27 28 29 30 31 32 33 34
0.2
0.3
0.4
0.5
0.6
εdw
Figure 4. Effect of variation in process air inlet temperature of desiccant wheel on its effectiveness.
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The corresponding variation in effectiveness of the desiccant wheel with respect to the air inlet
humidity ratio of the process air has been shown in Figure 5. It has been found that because of increment in
ambient air humidity ratio a slight increase in process air humidity ratio raises the desiccant wheel
effectiveness. As the ambient humidity ratio increases with ambient temperature, the regeneration air
temperature also increases and hence the effectiveness of the desiccant wheel increases (Eq. 5).
Process air inlet humidity ratio of desiccant wheel (g/kg)
12 13 14 15 16 17 18 19 20
0.2
0.3
0.4
0.5
0.6
εdw
Figure 5. Effect of variation in process air inlet humidity ratio of desiccant wheel on its effectiveness.
Figure 6 shows the effect of variation in ambient temperature on regeneration temperature. When
ambient temperature increases, there is a successive increase in regeneration air temperature. At constant
humidity ratio, a successive increase in process air inlet temperature will lead to a small difference of vapor
pressure between air and desiccant felt. This could slow down the process of removing the moisture from
process air to desiccant surface. So, reactivation air is heated to a higher temperature to bring out the
moisture extracted by the sorbent in desiccant wheel. Thus, moisture cycled by the system increases at higher
reactivation temperature. This also causes an increase in process air outlet temperature of desiccant wheel as
shown Figure 7.
Ambient temperature (oC)
28 30 32 34 36
Regeneration temperature (oC)
80
100
120
140
Figure 6. Effect of variation in ambient temperature on regeneration temperature
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Process air inlet humidity ratio of desiccant wheel (g/kg)
12 14 16 18 20
Process air outlet temp. of desicant wheel (oC)
35
40
45
50
55
60
Figure 7. Effect of variation in process air inlet humidity ratio of desiccant wheel on its outlet temperature.
Figure 8 shows the variation in COP with respect to the changes in desiccant dehumidifier
effectiveness. With the increase in process air inlet temperature of desiccant wheel, effectiveness of the
dehumidifier increases as discussed earlier. This further increases the amount of moisture exchange in the
dehumidifier without adding any extra energy. It has been found that humidity ratio of the cooling air stream
flowing over evaporator coil also decreases, resulting in an increase in total cooling capacity of the cycle.
Thus, COP of the system increases.
dw
0.30 0.35 0.40 0.45 0.50 0.55
COP
0.8
1.0
1.2
1.4
1.6
Figure 8. Effect of variation in dehumidifier effectiveness on COP.
The experimental results for dry bulb temperature and humidity ratio at each state point in the cycle
have been presented in Table 2 and are depicted in Figure 9. Table 2 indicates the overall behaviour of the
system in terms of the variation in dry bulb temperature and humidity ratio of air at different state points of
the cycle. Figure 9 shows a psychrometric chart, indicating processes between different state points for a
recirculation type hybrid solid desiccant vapor compression air-conditioning system.
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Table 2. Typical experimental measurements at different state points in the cycle
Sr. No.
Temperature (°C)
Sp. Humidity (g/kg)
1
26.20
11.8
2
54.50
7.3
3
43.40
8.1
4
9.0
7.5
5
26.00
10.50
6
34.2
20.20
7
39.2
19.10
8
124.5
19.7
9
47.9
23.90
Figure 9. Psychrometric chart showing important state points in the cycle during experiment.
8. CONCLUSION
The current work reveals the performance of hybrid solid desiccant vapor compression air-
conditioning system working in hot and humid conditions of India by controlling the temperature and
humidity separately. The obtained experimental results show that the overall system performance as well as
dehumidifier effectiveness is sensitive to the variations in outdoor temperature and humidity ratio. Under
given condition, the increase in humidity ratio of process air at inlet, results in obvious increase in moisture
removal as well as in coefficient of performance of the system. It has been concluded that the hybrid solid
desiccant vapor compression air-conditioning system can be a good option over traditional air conditioners to
control humidity in hot and humid climates. Due to large dependence of the performance of the rotary
desiccant dehumidifier on variations in ambient air conditions, further investigations are needed for a
complete assessment of system feasibility in different climatic zones.
9. NOMENCLATURE
COP coefficient of performance
DBT dry bulb temperature (°C)
RH relative humidity (%)
h enthalpy of air (kJ/kg)
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
40
38
36
34
32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
100
90
80
70
60
50
40
30
20
10
0
110 120 130 140 150 160 170 180 190 200 210 220 230 240
250
1%
RH
5%
10
%
20
%
40
%
60
%
80%
100%
Dry bulb temperature (°C)
Humidity ratio (g/kg)
Enthalpy (kJ/kg)
1
2
3
4
6
7
9
8
Process air
Regeneration air
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m air mass flow rate (kg/hr)
Qth regeneration heat addition (kW)
Qcc cooling capacity (kW)
T temperature (°C)
VCR vapor compression refrigeration
Wc compressor work input (kW)
Greek letters
ω humidity ratio of air (g/kg)
ε effectiveness
Subscripts
dw desiccant wheel
hrw heat recovery wheel
p process
r regeneration
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