ADSORPTION REFRIGERATION SYSTEM FOR AUTOMOBILES AN EXPERIMENTAL APPROACH

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online), Volume 5, Issue 2, February (2014), pp. 122-132, © IAEME
122
ADSORPTION REFRIGERATION SYSTEM FOR AUTOMOBILES
AN EXPERIMENTAL APPROACH
Peethambaran K M1
, Asok Kumar N2
, John T D3
1, 3
Professor, Department of Mechanical Engg., Govt. College of Engineering – Kannur,
Kerala, India
2
Professor, Department of Mechanical Engg., College of Engineering – Trivandrum,
Kerala, India
ABSTRACT
The use of waste heat for refrigeration and air conditioning purposes have been accepted by
people and various systems have been developed and proven attractive but its implementation in real
applications is still limited. The adsorption system is advantageous in small scale systems if
compared with absorption systems especially for the handling of the system and the cost. Adsorption
refrigeration and heat pump cycles rely on the adsorption of a refrigerant gas into an adsorbent at low
pressure and subsequent desorption by heating the adsorbent. The adsorbent acts as a “chemical
compressor” driven by heat. As it makes use of heat to pressurize the refrigerant, this system can be
used in various situations which enable waste heat recycling like in factories and automobiles.
The objective of this work was to compare various adsorbent-refrigerant pairs and find the
best pair, which would give maximum COP and will be cheap and easily available. The adsorbent-
refrigerant pairs considered for the present study were silica gel-water, silica gel-methanol, zeolite-
methanol, zeolite-water, activated carbon- ammonia, and activated carbon- methanol.
Experiments were carried out to analyse the adsorption nature of these pairs. The variation of
adsorption capacity with temperature was analysed. It is concluded that the silica gel - water is the
best among the pairs compared in terms of coefficient of performance. It is also found that water
attains its saturation point on zeolite quickly followed by water on silica gel and ammonia on carbon.
Key words: Adsorption System, Adsorbent-Refrigerant Pairs, Waste Heat Refrigeration.
1. INTRODUCTION
Technological innovations have lead to various appliances, which have helped us lead lives
that are more comfortable. The implementations of air conditioning systems in automobiles have
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
AND TECHNOLOGY (IJMET)
ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online)
Volume 5, Issue 2, February (2014), pp. 122-132
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123
helped create more comfortable travelling. The expanding population and the energy crisis have
brought serious problems to the world environment and to sustainable development. The electric
driven vapour compression refrigeration system has faced a challenge as CFC s and HCFC s are not
favourable to the environment.
The use of waste heat for refrigeration and air conditioning purposes have been accepted by
people and various systems have been developed and proven attractive but its implementation in real
applications is still limited. Electric driven air conditioning systems have reached a COP of over 4,
while absorption systems are usually in the range of 1.1-1.25. The adsorption system is advantageous
in small scale systems if compared with absorption systems. For exhaust heat utilization, a solid
adsorption system is possibly the best system for refrigeration purposes. A large part of the energy
from the fuel that is burnt gets wasted through the exhaust gases. If a system that uses all the energy
from the exhaust of an engine to run its air conditioning system can be designed, it could be very
well accepted.
Adsorption refrigeration cycles rely on the adsorption of a refrigerant gas into an adsorbent at
low pressure and subsequent desorption by heating the adsorbent. The adsorbent acts as a “chemical
compressor” driven by heat. When the adsorber is cooled, the adsorbate gets adsorbed onto the
adsorbent. While the adsorber is heated in the next cycle, this adsorbate gets desorbed at high
temperature. With the use of a pressure vessel and a check valve its pressure value can be increased.
The rest of the refrigeration system remains the same as that of a vapour compression system.
Exhaust heat recycling is gaining prominence these days because of the increased stress on
fuel consumption and also because it helps to reduce pollution levels to an extent there by making it
environmental friendly. The major disadvantage of general adsorbtion refrigeration system is that the
COP of the system is comparatively lower than the conventional vapour compression system. In
future, there can be scope for improvement of the same so as to be used in vehicles. Objective of this
experimental study is to have a general idea in selection of adsorbent-refrigerant pair, to compare
various adsorbent-refrigerant pairs and find the best pair, which would give maximum COP and will
be cheap and easily available.
2. SCHEME
The first consideration in any refrigeration system is deciding the capacity of the system. In
this work, it is opted for a small capacity system to test its effectiveness. For the desired cooling
effect, the best adsorbent refrigerant pair is to be chosen. For this reason it is decided to compare the
working of following adsorbent-refrigerant pairs namely silica gel-water, silica gel-methanol,
zeolite-methanol, zeolite-water, activated carbon- ammonia, activated carbon- methanol. In the
event of doing estimation, we came to know that adsorption capacity was to be found by conducting
experiments at different temperatures. Then graphs connecting the adsorption capacities and
temperatures are plotted. Then the theoretically best pair is found out.
3. LITERATURE SURVEY
3.1 Adsorption Refrigeration System
An adsorption refrigeration system driven by a heat source is a closed sorption process [1].
There are two main processes inside the system: refrigeration and regeneration. The refrigerant is
vapourised in the generator (or evaporator) and adsorbed by a solid substance with a very high
microscopic porosity. In the regeneration process, the adsorbent is heated until the refrigerant
desorbs and goes back to the evaporator, which now acts as a condenser. There are several pairs of
refrigerant/absorbent such as water/zeolite, methanol/activated carbon. The system is not as widely

124
used as the absorption system. However, this application can be integrated with the low temperature
solar collector or the exhaust of automobiles.
The adsorption cycle is illustrated in Fig.1 and proceeds as follows.
Fig 1. Thermodynamic cycle for adsorption
The four basic processes involved in the cycle are:
(i) Heating And Pressurisation
During this period, the adsorber receives heat while being closed. The adsorbent temperature
increases, which induces a pressure increase, from the evaporation pressure up to the condensation
pressure. This period is equivalent to the “compression” in compression cycles.
(ii) Heating, Desorption And Condensation
During this period, the adsorber continues receiving heat while being connected to the condenser,
which now superimposes its pressure. The adsorbent temperature continues increasing, which
induces desorption of vapour. This desorbed vapour is liquefied in the condenser. The condensation
heat is released to the second heat sink at intermediate temperature. This period is equivalent to the
"condensation" in compression cycles.
(iii) Cooling And Depressurisation
During this period, the adsorber releases heat while being closed. The adsorbent temperature
decreases, which induces the pressure decrease from the condensation pressure down to the
evaporation pressure. This period is equivalent to the "expansion" in compression cycles.
(iv) Cooling, Adsorption and Evaporation
During this period, the adsorber continues releasing heat while being connected to the evaporator,
which now superimposes its pressure. The adsorbent temperature continues decreasing, which
induces adsorption of vapour. This adsorbed vapour is vaporised in the evaporator. The evaporation
heat is supplied by the heat source at low temperature. This period is equivalent to the "evaporation"
in compression cycles.

125
3.2 Utilizing Waste Heat
There are three potential uses for waste heat in a vehicle: cabin heating, cabin cooling, and
electricity generation, the last of which could be used for heating and cooling. Heating is already
performed efficiently, compactly, and economically by routing engine coolant through a small finned
tube heat exchanger (HEX) in the cabin air duct. The only drawback is the long delay (5 min or
more) during frigid weather between engine start-up and effective cabin heating and defrosting.
Fig 2. Uniform temperature heat recovery or ‘double effect’ heating
Fig 3. Temperature variation through adsorbers, HTF heater & HTF cooler for ‘thermal wave’
regeneration
Lambert and Jones [2] reviewed the current state of the art in adsorption heat pumps.
Research groups in the United States, Italy, France, China, and Japan have concentrated their efforts
on devising improvements to the all-critical adsorbers, with the primary goal of improving efficiency
(COPC), which requires increasing the percentage of recycled heat. Several investigations agree in
identifying the two most important parameters that must be maximized in order to increase COPC:
the ratio of adsorbent (‘live’) mass to non-adsorbent (‘dead’) mass, and the NTU of the heat
exchanger. According to Lambert and Jones, some previous designs suffer from a low ‘live’–‘dead’
mass ratio, the first of the two critical governing parameters identified above.
3.3 Commonly Used Adsorption Materials
(i) Silica Gel : It is an amorphous form of SiO2, which is chemically inert, nontoxic, polar and
dimensionally stable (< 400°C) (ii) Zeolites : These are natural or synthetic aluminum silicates
which form a regular crystal lattice and release water at high temperature. These are polar in nature.

126
(iii) Activated Carbon : They are highly porous, amorphous solids consisting of microcrystallites
with a graphite lattice. They are non-polar and cheap. But they are combustible.
3.4 Adsorption Cycle as Applied to an Automobile
With a single adsorber, cooling is intermittent, which is undesirable because it wastes much
of the continuous supply of exhaust heat. Therefore, at least two adsorbers are needed for an
automobile. Multiple adsorbers beyond two can improve COPC by permitting incrementally more
effective ‘thermal wave’ regeneration but add volume and mass, decreasing SCP. Thus, a
compromise must be struck between SCP and COPC to satisfy constraints on both. COPC must be
high enough to ensure adequate cooling even for the worst-case scenario of a subcompact car idling
for an extended duration (i.e. traffic jam), since it has the largest ratio of cooling load to exhaust heat.
Maintaining an already surge-cooled cabin at a comfortable temperature requires 1.7 kW cooling.
Assuming that a realistic 80 per cent of the 3.5 kW available exhaust heat can be extracted (2.8kW),
the required COPC = 1.7 kW÷2.8 kW≈0.60, which can be accomplished with uniform temperature
‘double-effect’ heating. For a given configuration, SCP and COPC are inversely proportional.
However, both SCP and COPC are directly proportional to NTU and inversely proportional to the
fraction of ‘dead’ mass. Thus, the fundamental objectives are to maximize NTU and to minimize
dead mass.
4. FUNCTIONAL REQUIREMENTS
The amount of refrigerant in the full reservoir is mr,reservoir = Qcool,reservoir ÷ ∆hevap
Compact and mid-size cars would require 20 and 40 per cent more refrigerant than the subcompact
(hybrid) car examined above.
4.1 Required Amount of Adsorbent
Three adsorbers, instead of two, are employed to take advantage of the fact that minimum
400 °C exhaust (at idle) can rapidly heat one adsorber, permitting the other two to be cooled for
twice as long at half the rate . A cooling rate that is half the heating rate incurs half the ∆THTF-ads
so that the adsorbent can be cooled closer to ambient and adsorb more refrigerant.
Fig 4. Temperature versus time for the adsorbent in the three adsorbers

127
At any instant, one adsorber is heated while two are cooled. Cycle duration is set at 10 min
and is divided into thirds. Each adsorber is heated for one-third of the cycle (∆t,heating = 3.33 min =
200 s) and cooled for the remaining two-thirds of the cycle (∆t,cooling = 6.67 min = 400 s). Their
phase angles are evenly spaced at 0, 120, and 240, so, at any given instant, one adsorber is being
heated while two are being cooled. The amount of refrigerant that must be expelled from each
adsorber during its heating phase is mr = Q˙cool×∆theating ÷ ∆hevap, the minimum practical adsorption
temperature is Tads,min = 65°C high enough above the foreseeable Tamb = 50°C to permit adequate heat
rejection during cooling phase. At 65°C, the maximum adsorption capacities of various refrigerants
from the corresponding adsorbents are given in the table. In addition, the maximum adsorber
temperature is 200°C at which the adsorbers would be completely depleted of the refrigerants.
therefore the amount of adsorbent required to hold the calculated amount of refrigerant is given by
mads = mr ÷ (MFmax−MFmin)
5. CALCULATION OF ADSORPTION CAPACITY OF VARIOUS REFRIGERANT-
ADSORBENT PAIRS
To find the mass of adsorbent required for adsorbing the given mass of refrigerant, maximum
and minimum adsorption capacity has to be calculated. For this, experiments to analyse the
adsorption nature of various refrigerants on the various adsorbents is conducted. A known mass of
adsorbent is taken in a crucible. The vacuum desiccator is filled with the refrigerant up to its neck
and the crucible containing the adsorbent is placed in it. After placing the compounds in the
desiccator, a vacuum is created inside it using a vacuum pump. Then the setup is kept aside. After 24
hours, the mass of the adsorbent is again measured using the high precision weighing balance. The
increase in mass of the adsorbent gives the mass of refrigerant adsorbed on it. This experiment is
repeated for all adsorbent-refrigerant pairs at different temperatures. The high temperature is
obtained by keeping the desiccator in a water bath.
Weight of the empty crucible = W1 (kg)
Weight of the crucible with adsorbent = W2 (kg)
Weight of the crucible with adsorbent after 24 hours = W3 (kg)
Mass of the adsorbent taken, W4 = W2 - W1 (kg)
Mass of the refrigerant adsorbed in adsorbent, W5 = W3 - W4 (kg)
Adsorption capacity = W5 / (W5 + W4)
From the measured masses, the adsorption capacity is obtained by calculating the mass
fraction of the refrigerant in adsorbent. Then the graphs connecting adsorption capacity and
temperature are plotted and maximum and minimum adsorption capacities are noted from these
graphs.

128
0
5
10
15
20
25
30
0 20 40 60 80 100
Adsorptioncapacity(%)
Temperature (oC)
Silica Gel - Methanol
0
5
10
15
20
25
30
35
40
0 20 40 60 80 100
Temperature (oC)
Silica Gel - Water
0
5
10
15
20
0 20 40 60 80 100
Temperature (oC)
Zeolite- Methanol

129
Fig 5. Adsorption capacity versus Temperature plots
6. WORKING OF THE FABRICATED SYSTEM
The exhaust heat driven adsorption refrigeration system is basically a refrigeration system
which contains all the parts of a conventional refrigeration system. In this the main highlight is that it
does not require a compressor which draws power from the engine. Here it is replaced by a chemical
0
5
10
15
20
25
30
35
0 20 40 60 80 100
Temperature (oC)
Zeolite- Water
0
5
10
15
20
25
30
0 20 40 60 80 100
Temperature (oC)
Carbon - Ammonia
0
10
20
30
40
50
60
0 20 40 60 80 100
Temperature (oC)
Carbon - Methanol

130
compressor, which is the adsorption chamber. The compression process is effected by desorption of
refrigerant vapours from the adsorbent. All the parts involved in the system are designed suitably to
attain the proper cooling effect or heat transfer by selecting the proper materials. Detailed design
drawings are made for each component. The machining processes are selected suitably to meet the
requirements and the components are fabricated accordingly. The adsorption chamber is made of
mild steel (AISI 1040) of thickness 2 mm with an outer diameter of 10.16 cm and 30 cm in length.
The chamber is provided with a flange of 6 mm thickness and 15.24 cm diameter. The inlet and exit
plenums are made with mild steel (AISI1040) of thickness 2 mm with an outer diameter of 10.16 cm
and length 4 cm with a flange of thickness 6 mm and a diameter of 15.24 cm. Two asbestos gaskets
discs of 15.24 cm diameter and 4 mm thickness is placed between the plenums and the chamber.
Asbestos cement is chosen as the material of the gasket as it has several of the required properties
which make it ideally suited material for this application. 9 copper tubes of 1.27 cm outer diameter
are placed within the chamber for the exhaust gas to flow through. The chamber is packed and
compacted with adsorbent particles in the space enclosed between the chamber and the copper pipes.
The condenser is used to condense the refrigerant vapours coming out from the adsorption
chamber. The condenser coil is made of copper as it has high thermal conductivity of around 386
W/mK , high thermal diffusivity of around 112.34 x 10-6
,low specific heat of 383 J/kgK which helps
in quick and proper heat transfer and ensures proper condensation. The copper tube of 7.9 mm
diameter is cut out to a length of 4m and then bent at lengths of 50 cm.
The evaporator is used to cool the cabin of the automobile by transferring heat to the
refrigerant, which evaporates and provides the required cooling. The evaporator is made of copper
tubes of 7.9 mm diameter. The overall length of the evaporator is 2m. A blower is placed behind the
evaporator coils so as to cool the cabin.
The exhaust gases are allowed to pass through the adsorber chamber. The temperature of the
adsorber rises slowly. This heating is continued for 200 seconds. As heating is continued the
refrigerant gets desorbed and the pressure inside the adsorber chamber begins to rise. When the
pressure gauge indicates the required pressure (Pcond,in), the outlet valve is opened slowly and the
refrigerant, goes slowly to the condenser. The valve is opened very slowly and when the pressure
falls well below Pcond,in , it is closed.
After 200 seconds, the cooling phase begins. The exhaust gas supply to the adsorber is cut.
Compressed air is blown over the adsorber chamber in order to cool it. When the adsorber chamber
gets cooled the temperature falls and more refrigerant gets adsorbed. Because of this the pressure
inside the chamber falls. When the pressure reaches the evaporator pressure, the refrigerant inlet
valve, which connects the adsorber and evaporator coil, is opened to allow more refrigerant to flow
in at a controlled rate. Because of the increase in flow of the refrigerant, amount of refrigerant
adsorbed also increases. At a particular stage, the adsorbent will become saturated. The pressure
inside the chamber becomes steady and then starts to rise. The inlet valve is then closed stopping the
flow of the refrigerant. The adsorbed refrigerant is then released during the heating phase.
The refrigerant from the adsorber condenses in the condenser at a temperature of 65ᵒC. This
condensed refrigerant is then expanded through the thermostatic expansion valve and evaporated in
the evaporator at a corresponding evaporator pressure and temperature of 5ᵒC. This produces the
cooling effect.
The pressure at which the adsorber is to be opened to the condenser during the heating phase
is known as condenser inlet pressure (Pcond,in). It is almost equal to 25 kPa for water, 2948 kPa for
ammonia, 101 kPa for methanol. at these pressures the condenser valves have to be opened. The
pressure at which the valve connecting the evaporator and adsorber chamber is to be opened is called
evaporator outlet pressure (Pevap,out). It is almost equal to 0.8 kPa for water, 291 kPa for ammonia, 10
kPa for methanol.

131
7. CONCLUSIONS
The theoretical as well as experimental comparison of various adsorbent-refrigerant pairs
namely silica gel-water, silica gel-methanol, carbon-ammonia, carbon-methanol, zeolite-methanol,
zeolite-water was conducted. The most adsorbent zeolite can adsorb 36, 30 wt % of water and
methanol respectively at room temperature and pressure. But the cost of zeolite is very high and it is
scarce in availability. Activated carbon has much greater affinity for methanol compared to zeolite. It
comes to around 55 wt% for methanol and 62 wt% for ammonia. This adsorptivity can be increased
by coating it with CaCl2. Silica gel on the other hand has conductivity similar to that of zeolite,
exhibiting a greater affinity for methanol to about 55wt%.
Water is non-toxic, non-flammable, non-polluting, stable, and has the highest latent heat
among common substances. However, its vapour pressure is very low requiring a large condenser
and evaporator. Moreover, operating at sub atmospheric pressure invites air ‘poisoning’. Operating
the evaporator at just a few degrees above the freezing point requires precise control. Ammonia is
toxic, flammable in some concentrations (16–25 per cent), non-polluting, stable, and has the second
highest latent heat among common substances. Methanol is toxic, highly inflammable, non polluting,
unstable beyond 393 K, and has the third highest latent heat among common substances and
‘poisoning’ by air is a possibility.
From the results, it can be seen that silica gel-water gives the maximum COPc almost equal to
6.39. This is followed by zeolite-water, which has a COPc of 6.21, and carbon-methanol, which has a
value of 6.12. So from COPc point of view it can be seen that silica gel- water is the best among the
pairs compared. Now from the various time study graphs that were plotted, it can be seen that water
attains its saturation point on zeolite quickly followed by water on silica gel and ammonia on carbon.
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ADSORPTION REFRIGERATION SYSTEM FOR AUTOMOBILES AN EXPERIMENTAL APPROACH

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