Carbon 44 (2006) 747–752
www.elsevier.com/locate/carbon
Performance analysis of an adsorption refrigerator using
activated carbon in a compound adsorbent
Z.S. Lu, R.Z. Wang *, L.W. Wang, C.J. Chen
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200030, China
Received 22 July 2005; accepted 13 September 2005
Available online 2 November 2005
Abstract
To improve the performance of the adsorption refrigeration of CaCl2–ammonia adsorption system, activated carbon has been distributed uniformly in the mass of CaCl2, thereby helping to enhance mass transfer and uplift the cooling power density. A multifunctional
heat pipe adsorption refrigerator, in which activated carbon-CaCl2 is used as compound adsorbent and ammonia as refrigerant, is
designed. Water and acetone are used as working liquids for the heat pipe. This paper presents a study on the adsorption refrigeration
performances of this adsorption refrigerator under two different working conditions, ice-maker for fishing boat driven by the waste heat
from exhaust gases, and solar ice-maker driven by solar water heating. The obtained average SCP (specific cooling power) and the COP
(coefficient of performance) of the refrigerator were measured to be 770.4 W/kg and 0.39 at about 20 °C of evaporating temperature for
the former working condition, and they were 161.2 W/kg and 0.12 at about 15 °C of evaporating temperature for the later working
condition.
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Activated carbon; Carbon/carbon composites; Mixing; Adsorption; Adsorption properties; Thermodynamic properties
1. Introduction
Adsorption refrigeration systems have the advantages of
being environmentally benign, having zero ozone depletion
potential (ODP) as well as zero global warming potential
(GWP) due to the use of natural refrigerants such as
ammonia, water, methanol, etc. It is also attractive for
the efficient use of solar energy and low-grade waste heat.
Less vibration, simple control, low initial investment and
expenditure, and less noise are the advantages of adsorption systems over the existing vapor compression and
absorption systems [1].
Adsorption working pairs for adsorption refrigeration
includes physical adsorption working pairs, chemical
adsorption working pairs and compound adsorption working pairs. The typical physical adsorption working pairs are
*
Corresponding author. Tel.: +86 21 6293 3838; fax: +86 21 6293 2601.
E-mail address: [email protected] (R.Z. Wang).
0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2005.09.016
mainly silica gel–water [2,3], zeolite–water [4,5], activated
carbon–ammonia [6,7], activated carbon–methanol [8]. In
addition, activated carbon-R134a has been also considered
as the physical adsorption working pair [9,10]; A typical
chemical adsorption working pair is CaCl2–ammonia
which has a large adsorption capacity although subject to
the critical problems of expansion and agglomeration
[11]. Compound adsorbents have the advantages of both
porous medium and chemical adsorbents. One typical
example is the mixture of activated carbon and CaCl2,
the well-arranged mixture of which is a good adsorbent
on ammonia, yielding long lasting and stable operational
performances. This paper presents the performances of
an adsorption ice-maker which employs the activated carbon and CaCl2 as compound adsorbent and ammonia as
adsorbate. The results show that the consolidated compound adsorbent can greatly improve the refrigeration performance, due to the fact that its composition and physical
structure contribute to enhancing significantly heat and
mass transfer.
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Z.S. Lu et al. / Carbon 44 (2006) 747–752
In this paper, compound adsorbent of activated carbon
and CaCl2 are used to improve adsorption performance.
Also, in order to enhance heat transfer in the adsorbent
beds, and perform an efficient heat recovery between the
two adsorbers, heat pipes are used in the present system.
2. Compound adsorbent by mixing activated
carbon and CaCl2
In comparison with the activated carbon and CaCl2
adsorbents, the compound adsorbent, obtained by mixing
activated carbon and CaCl2, has a high volume adsorption
capacity while avoiding the problems of agglomeration and
performance attenuation. The high adsorption performance stems from the presence of activated carbon with
its important network of pores cavities which contribute
to enhance significantly the mass transfer. The research
group of Shanghai Jiao Tong University has studied the
compound adsorbent adsorption properties with various
mass ratios of activated carbon and CaCl2 [12]. The result
shows that the best mass ratio of activated carbon and
CaCl2 is 1:4, as shown in Fig. 1. When the compound
adsorbent is consolidated, the adsorption performance var-
Fig. 1. Granular compound adsorbent after adsorption (mass ratio of
activated carbon and CaCl2 is 1:4).
Fig. 2. Consolidated compound adsorbent.
ies with the consolidated compound adsorbent density. At
the optimal density, a significantly improved adsorption
performance is reached. The consolidated adsorbent is
shown in Fig. 2. Here, CaCl2 powder, coconut shell activated carbon (14–28 mesh) and a small quantity of highquality cement are mixed and compacted together in a
consolidated adsorbent in that mixture, the cement acts
as binder. The mass fractions of CaCl2, activated carbon
and cement in the consolidated compound, are in the proportion of 16:4:1, respectively.
3. Description of the system
The heat pipe-type adsorption refrigeration system, used
for the experimental researches, is shown in Fig. 3. It comprises two adsorbers, each of them is heated by a boiler and
cooled by a water heat exchanger via the gravity heat pipe
working liquid (water or acetone). The switching of the
valves, make the adsorber plays alternately the role of condenser or evaporator for the gravity heat pipe.
The main idea of the above design is to solve the corrosion problems. Indeed, when the adsorption refrigerator is
used as ice-maker for fishing boat powered by the waste
heat of the exhaust gases, serious corrosion problems
may occur if the steel adsorber is heated by the exhaust
gases and cooled by seawater directly.
In the laboratory experiment, electric heating was used
to simulate both the exhaust gas heating and the solar heating. A thermostat was used to control the cooling water
temperature, thus various working conditions with different cooling water temperature could be simulated. The
consolidated compound adsorbent was used as the adsorbent, while ammonia was used as the refrigerant. During
the experiments, water was used as heat pipe working fluid
when the system was powered by waste heat, whereas acetone was for the same when the system was driven by solar
energy.
The heat pipe-type adsorption system is composed
mainly of a liquid pumping boiler, a heating boiler, two
coolers, two adsorbers, a condenser, and an evaporator.
In each adsorber, there are 1.88 kg of calcium chloride contained in the compound adsorbent filled in between the fins
of 19 finned tubes. The operation of the system comprises
mainly five working processes: The first working process
consists in the pumping of heat pipe working liquid into
adsorber (PHPL). The heating of the heat pipe (HHP),
which leads to the desorption of ammonia from the desorbing adsorber, constitutes the second working process. As to
the third, it is concerned with the cooling of the heat pipe
(CHP), and causes adsorption in the adsorbing adsorber.
The fourth one is the mass recovery process (MR). It consists in balancing the pressure of ammonia prevailing in the
two adsorbers, thereby contributing to enhance adsorption
as well as desorption. Finally, the fifth process is represented by the heat pipe heat recovery (HPHR), during
which heat is recovered from the high temperature adsorber (end of desorption, about to begin the adsorption) to
Z.S. Lu et al. / Carbon 44 (2006) 747–752
749
Fig. 3. Structure of the refrigeration system. (a) schematic diagram, (b) photo of the system. 1. Liquid pumping boiler; 2. tap-hole; 3. electric heater; 4.
heating boiler; 5. evaporator; 6. vapour pipeline; 7. liquid pipeline; 8. magnetostriction level sensor; 9. adsorbers; 10. condenser; 11. ammonia inlet; 12.
safety valve; 13. vapour pipeline; 14. liquid pipeline; 15. coolers; 16. aspirating hole and heat pipe liquid inlet; 17. cooling water pipeline; 18. water meter;
19. cooling water pump; 20. pressure sensor; 21. temperature sensor; 22. safety valve.
heat the low temperature adsorber (end of adsorption,
about to begin the desorption). This is realized with a
looped heat pipe configuration.
In PHPL process, the hot adsorber is connected to the
cooler. As result, the pressure prevailing therein is lower
than that in the boiler. So, the heat pipe liquid can easily
be pumped from the liquid pumping boiler into the adsorber. During the CHP process, the hot adsorber serves as
the evaporating side of heat pipe, while the cooler serves
as its condensing. The heat pipe liquid evaporates in the
hot adsorber, absorbing the heat from it, and then enters
the cooler via vapor pipeline, wherein it condenses. The
condensed liquid then returns to the adsorber via liquid
pipeline.
In HHP process, the other adsorber is connected to the
boiler. The heating boiler serves as the evaporating side of
heat pipe and the adsorber serves as its condensing side.
The heat pipe liquid is heated to evaporation by the heating
boiler. The generated vapor then enters the adsorber via
the vapor pipeline, and is condensed into liquid when it
touches the cold surface inside the finned tubes in adsorber
to provide desorption heat. At last, the condensed heat
pipe liquid returns to the heating boiler via liquid pipeline
to be heated again.
In MR process, the valve between the hot adsorber and
the cold adsorber is opened, and the ammonia vapor in the
hot adsorber quickly enters the cold one, because the pressure in the hot adsorber is much higher compared to that in
the cold adsorber. This process helps to get more desorp-
tion from the hot adsorber and more adsorption in the cold
adsorber.
In HPHR process, the hot adsorber and the cold adsorber are connected. The hot adsorber serves as the evaporating side of heat pipe while the cold adsorber serves as its
condensing side. The heat pipe liquid evaporates in the
hot adsorber, then enters the cold adsorber, condensing
there and transferring heat from the hot adsorber to the
cold adsorber. Afterwards, the heat pipe liquid returns to
hot adsorber again. Thus, a looped heat pipe heat recovery
circuit is performed.
In Fig. 3, the cooler is a coil tube heat exchanger, in
which seawater is circulated in the copper tube and the
vapor of heat pipe working liquid is condensed onto it.
Thus, due to the heat pipe arrangement, the problem of
corrosion caused by seawater used in steel made adsorber
is avoided. The heat exchanger of the evaporator is just a
jacket heat exchanger.
4. Experiments
The adsorption quantity is measured by a magnetostriction level sensor, with relative measuring error of less than
0.05%. It is inserted inside the evaporator. The diameter of
evaporator is 117 mm.
The average SCP is calculated by the formula (1):
SCP ¼
1000 hfg ql V l
m
t
ð1Þ
Z.S. Lu et al. / Carbon 44 (2006) 747–752
1500
with heat and mass recovery
1000
500
with mass recovery
without heat and mass recovery
0
0
5
10
15
20
25
30
35
t /min
Fig. 4. Average SCP variations in half cycle time with mass and heat
recovery operations.
where SCP is in (W/kg), m is the mass of CaCl2 in compound adsorbent for each adsorber (1.88 kg), hfg is the latent heat of vaporization at evaporation temperature of
ammonia (kJ/kg), ql is the density of ammonia liquid at
the evaporation temperature (kg/m3), Vl is the volume of
the ammonia liquid evaporated during the adsorption
phase (m3), and t is the corresponding adsorption time
(s). It is seen that the SCP value is an integrated cooling effect divided by the adsorption time and the adsorbent
mass.
The coefficient of cooling performance (COP) is calculated by the formula (2):
hfg V l ql
COP ¼ R
wh dt
ð2Þ
where COP is the coefficient of performance, wh is heating
power (kW).
4.1. Adsorption ice-maker powered by waste heat from
exhausting gases of fishing boats
As shown in our previously publication before mass and
heat recoveries can improve the adsorption refrigeration
performance significantly [13]. So, the adsorption refrigeration performance of the multifunctional heat pipe-type
refrigerator with mass and heat recoveries is studied, and
the results are compared with those obtained without any
heat or mass recovery. The average SCP comparison
results are shown in Fig. 4. The working condition was
set as follows: The cycle time was 70 min (half cycle time
of 35 min). The heating power was 3.6 kW. Water was
selected as the heat pipe working liquid. The evaporating
temperature was about 20 °C. Fig. 4 shows that the average SCP is improved significantly by mass and heat recovery. When the cycle time is 40 min, the average SCP of the
cycle without any heat or mass recovery, with mass recovery only, and with heat and mass recoveries are 514.3 W/
kg, 797.5 W/kg and 1026.2 W/kg, respectively. The mass
and heat recovery processes have achieved a SCP increase
of 28.7% and 70.8%, respectively, in comparison with cycle
with no recovery process.
The normal fishing period, in China, is from January to
June and from September to November, and the corresponding seawater temperature ranges from 15 °C to
30 °C. So the refrigeration performance variation with
cooling water temperature has been studied. The selected
working conditions was water as heat pipe working liquid,
70 min of cycle time, 40 s of mass recovery time, 2 min of
heat recovery time and approximately 20 °C of evaporating temperature. The performance of adsorption refrigeration is shown in Table 1.
Table 1 shows that the average SCP and COP increase
as the temperature of cooling water decreases. Even at
the highest temperature of cooling water, that is at 30 °C,
the average SCP and COP are still as high as 528 W/kg
and 0.26, respectively.
4.2. Adsorption ice-maker powered by solar energy
In the second experiment, the refrigeration performance
of solar ice-maker with different heating power, simulated
by different electric heating power, was studied. Acetone is
a good choice for the reason of faster heating process and
the working condition was: 40 s of mass recovery, 24 min
of cycle time, approximately 30 °C of cooling water temperature, and approximately 15 °C of evaporating temperature. The adsorber temperature represented versus time
is shown in Fig. 5. As it can be seen, the desorption temperature increases with heating power. Likewise, the performance of adsorption refrigeration performance represented
in Table 2 shows the average SCP as an increasing function
of heating power. The ascending dependency between the
system performance and the heating power can be explained
by the fact that high desorption temperature leads to a good
90
2.42kW
80
2.05kW
70
T / ºC
Average SCP/(W/kg)
750
60
50
1.69kW
40
30
0
6
12
t /min
18
24
Fig. 5. Adsorption bed temperature versus time.
Table 1
Adsorption refrigerator performance variation with different cooling water temperature
Cooling water temp. (°C)
Desorption temp. (°C)
Evaporation temp. (°C)
Heating power (kW)
Average SCP (W/kg)
COP
16
20
25
30
110.1
114.7
114.0
113.7
19.1
21.3
18.1
19.4
3.64
3.64
3.64
3.64
865.8
770.4
676.8
528.0
0.43
0.39
0.34
0.26
Z.S. Lu et al. / Carbon 44 (2006) 747–752
751
Table 2
Adsorption solar ice-maker performance variation with different heating power
Heating power (kW)
Desorption temp. (°C)
Evaporation temp. (°C)
Cooling water temp. (°C)
Average SCP (W/kg)
COP
1.69
2.05
2.42
76.5
79.2
82.4
15.1
13.8
15.2
33.5
33.7
31.2
111.2
113.8
161.2
0.12
0.10
0.12
Table 3
Comparison of several typical studies
Evaporating temp. (°C)
Working pair
Cooling water temp. (°C)
Average SCP (W/kg)
COP
Research status
Ref.
10
25
10
8 (solar ice-maker)
SrCl2–NH3
MnCl2/NiCl2–NH3
Metal hydride–hydrogen
Activated carbon–methanol
40
40
35
32
250
140
50
45.1
0.3
0.4
0.4
0.1
Test
Simulation
Test
Test
[14]
[14]
[15]
[16]
19
21
15 (solar ice-maker)
Activated carbon
and CaCl2–NH3
30
20
30
528.0
770.4
161.2
0.3
0.4
0.12
Test
desorption, which is conducive to improving the adsorption
refrigeration performance. The maximum average SCP is
161.2 W/kg, with the corresponding average COP of 0.12.
The performances could be further improved if the desorption temperature is increased, in which case high temperature solar heating will be required. It should be pointed
that the measured COP is the refrigeration effect divided
by the total heat input, which is not the solar COP, the solar
COP would be about 40% that of the measured COP due to
the solar heating efficiency.
5. Results and discussion
Refrigeration performances are influenced by heat and
mass recovery operations. The average SCP obtained for
the cycle without any heat or mass recovery process, with
mass recovery only, and with heat and mass recoveries
are 514.3 W/kg, 797.5 W/kg and 1026.2 W/kg, respectively,
at the condition of 40 min of cycle time and about 20 °C
of evaporating temperature, about 114 °C of desorption
temperature and about 30 °C of condensing temperature.
In normal fishing period, refrigeration performances of
ice-maker on fishing boats varies with different temperatures of seawater. The average SCP and COP increase as
the temperature of cooling water decreases. At the highest
temperature of cooling water of 30 °C, the lowest average
of SCP and COP are still as high as 528 W/kg and 0.26,
respectively.
For the solar ice-maker refrigeration modes, the average
SCP increases with heating power and the highest average
SCP is 161.2 W/kg, and the corresponding COP is 0.12
when powered by 80 °C hot water. The values could still
be improved if high temperature solar collector was used.
In summary, we have designed a high effective adsorption ice-maker. This work has adopted two new concepts,
mixture of activated carbon and CaCl2 as adsorbent and
heat pipe for heat transfer, which are really very effective
This paper
to improve the performance of adsorption refrigeration
systems. The achieved average SCP and COP are far higher
than those obtained in other experimental systems. A comparison between the study reported in this paper and other
studies is shown in Table 3.
Acknowledgements
This work was supported by National Science Fund for
Distinguished Young Scholars of China under the contract
No. 50225621, Shanghai Shuguang Training Program for
the Talents under the contract No. 02GG03, the State
Key Fundamental Research Program under the contract
No. G2000026309. The authors thank Mr. Z.Z. Xia, Mr.
Y.X. Xu for helping to install the experimental setup.
The contributions from Mr. Daou is also appreciated.
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