mini project
TRANSCRIPT
Table of Contents
ACKNOWLEDGEMENT....................................................................................- 4 -
ABSTRACT..........................................................................................................- 5 -
INTRODUCTION................................................................................................- 6 -
CHAPTER 1.........................................................................................................- 8 -
Air ventilation system...........................................................................................- 8 -
CHAPTER 2.......................................................................................................- 13 -
Air conditioning system......................................................................................- 13 -
What are Lithium Bromide Chillers?..............................................................- 14 -
Is an Absorption Chiller the Best Choice for Us?..........................................- 16 -
Principle of Operation.....................................................................................- 17 -
Chiller Possible Fluid Pairs.............................................................................- 18 -
NH3-H2O system.................................................................................- 18 -
LiBr-H2O system.................................................................................- 18 -
Comparison.................................................................................................- 19 -
Types...............................................................................................................- 20 -
Cooling Cycle.................................................................................................- 20 -
1. Solution Pump and Heat Exchanger...................................................- 21 -
2. Generator.............................................................................................- 21 -
3. Condenser............................................................................................- 22 -
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4. Evaporator...........................................................................................- 23 -
5. Absorber..............................................................................................- 23 -
Possible Configurations..................................................................................- 24 -
First Configuration......................................................................................- 25 -
Second Configuration:................................................................................- 26 -
Third Configuration:...................................................................................- 27 -
Our configuration............................................................................................- 27 -
1. Condenser............................................................................................- 28 -
2. Evaporator...........................................................................................- 30 -
3. Absorber..........................................................................................................................- 30 -
4. Regenerator.........................................................................................- 32 -
5. Solar water heater................................................................................- 32 -
Limitations of our system...............................................................................- 34 -
Conclusions and recommendations....................................................................- 35 -
APPENDIX A.....................................................................................................- 36 -
Mass and Energy Balance...................................................................................- 36 -
Overall mass flow balance..............................................................................- 37 -
Energy balance................................................................................................- 38 -
Rate of heat transfer in generator................................................................- 39 -
Rate of heat transfer in evaporator..............................................................- 39 -
Rate of het transfer in condenser................................................................- 39 -
Rate of heat transfer in absorber.................................................................- 40 -
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Coefficient of performance.............................................................................- 40 -
APPENDIX B.....................................................................................................- 41 -
Surface Area Calculations..............................................................................- 41 -
Condenser...................................................................................................- 41 -
Evaporator...................................................................................................- 42 -
Absorber......................................................................................................- 43 -
Heat exchanger...........................................................................................- 45 -
Generator....................................................................................................- 45 -
Solar collector.............................................................................................- 46 -
APPENDIX C.................................................................................................- 48 -
Thermodynamic Diagrams.............................................................................- 48 -
BIBLIOGRAPHY...............................................................................................- 51 -
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ACKNOWLEDGEMENT
First of all we are thankful to
Almighty Allah
Who enabled us to complete our mini project
We are thankful to
Dr. Naveed Ramazan
For encouraging us to prove our talent
We are thankful to our honorable supervisor
Dr. Shahid Naveed
For believing in us by giving such a tough mini project
We are deeply thankful to
Sir Saad Nazir
For his consistence guidance, encouragement
and affectionate behavior.
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ABSTRACT
Pakistan is a country which has been blessed with all four seasons; summer being
the longest. Due to temperatures as high as 45 ˚C, the amount of heat is significant,
resulting in raising the interior temperature to a level that is uncomfortable for the
passenger. Frequently it can lead to damage the systems and contents within the
interior of the vehicle. The feasibility of a system to cool and to ventilate the
interior space of car while parked as well as in moving has been investigated in this
report. It introduces two systems. First one looks into the practicability of
harnessing the solar energy for possible ventilation of the car cabin. The second
portion is the feasibility of a lithium bromide–water (LiBr-H2O) absorption chiller
for automobiles namely cars with a nominal capacity of 0.3 tons. The various
stages of design are presented including the design of the evaporator, absorber,
solution heat exchanger, generator and condenser. The future trends of research in
this area would be on solar collectors which will be more effective in utilizing the
energy with lesser area.
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INTRODUCTION
Imagine a car parked in sun in summers, what will happen? The temperature of
the car will increase to unbearable level, making it to be felt like an oven. As
human mind has always been trying to discover the means of the comfort, several
researches have been conducted in the field of automobiles to make it more and
more suitable, attractive and comfy for its passengers. Major discomfort in car, as
it was said above, is high temperature in cabin during summers. Since the
invention of first car, several models have been proposed allowing the vehicles
occupants to travel in the comfort of a controlled environment even on the most
hot and humid summer day.
In the history of automobiles the day of November 4,1939 carries very
importance as it was the day when first air conditioned car was displayed, Since
the advent of the automotive air conditioning system, many things have undergone
extensive change to make an air conditioned car affordable and a necessity that car
owners can not live without.
Air conditioning system in today’s modern cars is based on vapor compression
cycle, this system has been working very efficiently for a long time but today when
the world is facing the problems of energy crisis, a need arises in the field of air
conditioning to make it more energy efficient and economical. Besides, this system
is only suitable for running cars and more efficient for long distances.
As solar energy is one of the cheapest and abundant sources of energy in world
and especially in a country like Pakistan, so in our project we have tried to utilize
this energy. In our first proposed model we have used photovoltaic (PV cells).
Photovoltaic on cars would be very useful in hot climates where the interior of the
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parked car can easily exceed 500C. Powering a fan to ventilate the cabin of a
parked car would considerably reduce the air conditioning requirement.
Second anticipated model is for air conditioning of parked and running car. We
have used lithium bromide cycle instead of vapor absorption cycle according to
availability of energy sources in our environment. In Pakistan during the month of
June avg. temperature is around 450C.Making use of solar collectors this surplus
solar energy is stored and is then utilized in air conditioning cycle .The other
requirement for this cycle is rejection of heat to surrounding i.e. from the
condenser and absorber. We have designed this cycle in such a way that they can
exchange heat with the atmosphere thus eliminating the need for extra cooling
medium. This system will work independently unlike the vapor compression cycle
whose working depends on the engine of the car, making it possible for a parked
car to be air conditioned.
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CHAPTER 1
Air ventilation system
Park your car on the road side, in mid July, under the sun, for an hour and then
observe, what happens when you break an egg on the bonnet of the car. It will start
frying in no time at all. The same will be practiced by the person who dares to sit
in that car. Surely, the latter is experienced by everyone.
The summer season of the Pakistan has the average temperature of about 45
degrees and in few areas, the temperature reaches up to 50.This means that the
temperature inside the car exceeds 50 degrees which is extremely troublesome for
anyone traveling in that car.
The purpose of considering the parked car is lucid to everyone but how this should
be done is a big problem. The simplest way is to ventilate the parked car, so that
the temperature, inside the car remains normal but the extra work that is going to
be done by the air conditioning system of the car increases and also the fuel
requirement which itself is a big issue as the fuel prices are increasing day by day.
The sun emits 1370 +/-3.4% watts per square meter of energy, 51% of it actually
enters the Earth's atmosphere and therefore approximately 700 watts per square
meter of clean energy can be obtained. Solar radiations are commonly used for
diverse heating purposes. In some instances, more sophisticated solar powered
systems have been used for the generation of electricity, and once electricity is
available, it can be used for any desired purpose. However, it must be appreciated
that solar powered systems are usually most practicable where the sun radiations
are the strongest, and this is where cooling, not heating, is commonly the factor of
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greatest interest. Solar ventilation system usually requires that the equipment for
the conversion of solar energy to electrical energy be first employed, and then this
electricity be used for the operation of equipment, such as fan, blowers etc. The
system proposed in this report works on a similar principle, that is, the production
of electricity using solar energy, in order to drive a fan. The use of only a single
small fan in the system is particularly contemplated, which further simplifies the
needed construction.
Our proposed system is comprising of a solar powered exhaust fan configured to
mount to the vehicle and having an inlet that is in communication with the interior
space when mounted on the vehicle; and an array of photovoltaic cells for
generating electrical energy from sunlight, wherein the array may be electrically
connected to the exhaust fan for providing electrical energy to the exhaust fan.
The fan can be mounted on the roof, in the window, in between the mudguard and
wheel of the car. The roof has to be cut for the fan placement or the car need to be
redesigned which is not an option. So by eliminating this configuration, we are left
with the two choices. Configuring the fan in the window is a good option but it
creates the security problems as the window cannot be fully closed and an extra
support is needed by the fan which destroys the outlook of the car leaving us
behind the third configuration that caters for all the disadvantages of the first two
assemblies.
There are many other factors that should be kept in mind while designing a system
like what size electric fan will be adequate to cool your vehicle, distance between
the mudguard and the wheel, photovoltaic cell size & placement and outside air
temperature & density are just a few. Generally speaking, it is best to maximize fan
area coverage and airflow capability when choosing a fan for your vehicle but the
area availability is the biggest constraint. The two fan assembly has a bit better
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airflow potential, but the location of them is more difficult. So, we restrict our
system to single fan assembly.
In this system, an array of photovoltaic cells is used to convert solar energy in to
electrical energy for powering the exhaust fan. A solar array is made up several
hundred modules combine together to generate electricity from solar energy.
The solar array can be mounted in several ways:
Horizontal: This most common arrangement gives most overall power
during most of the day in low latitudes or higher latitude summers and offers
little interaction with the wind. Horizontal arrays can be integrated or be in
the form of a free canopy.
Vertical: This arrangement is sometimes found in free standing or
integrated sails to harness wind energy.[5] Useful solar power is limited to
mornings, evenings, or winters and when the vehicle is pointing in the right
direction.
Adjustable: Free solar arrays can often be tilted around the axis of travel
in order to increase power when the sun is low and well to the side. An
alternative is to tilt the whole vehicle when parked. Two-axis adjustment is
only found on marine vehicles, where the aerodynamic resistance is of less
importance than with road vehicles.
Integrated: Some vehicles cover every available surface with solar cells.
Some of the cells will be at an optimal angle whereas others will be shaded.
Trailer: Solar trailers are especially useful for retrofitting existing
vehicles with little stability, e.g. bicycles. Some trailers also include the
batteries and others also the drive motor.
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Remote: By mounting the solar array at a stationary location instead of
the vehicle, power can be maximized and resistance minimized. The virtual
grid-connection however involves more electrical losses than with true solar
vehicles and the battery must be larger.
The choice of solar array geometry involves an optimization between power
output, aerodynamic resistance and vehicle mass, as well as practical
considerations. For example, a free horizontal canopy gives 2-3 times the surface
area of a vehicle with integrated cells but offers better cooling of the cells and
shading of the riders. There are also thin flexible solar arrays in development.
Solar arrays on cars are mounted and encapsulated very differently from stationary
solar arrays. Solar arrays on cars are usually mounted using industrial grade
double-sided adhesive tape right onto the car's body. The arrays are encapsulated
using thin layers of Tedlar .Any type of photovoltaic cell that full fill the demands
can be used.
After selecting the size and positioning of the solar array, the exhaust fan is
positioned to vent hot air from the interior of the vehicle to the exterior, thereby
creating negative pressure in the interior to draw in cooler outside air into the
interior, i.e. through the vehicle's vents. As in Liana, the Space between the
mudguard and the wheel is 6 inches, so any fan between 2-3 inches can be used.
Also the inlet spacing is provided from the lower end of the car near the wheel so
that long duct system could be avoided.
The electrical connections are provided between the array and the fan by using
suitable cord assembly. The assembly first comprises of the cord that is connected
to the PV cell and that terminate in the form of a female connector. A second cord
is used includes a male connector and a right angle male plug that connects into a
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jack provided in the fan. The wiring can be done along the window edges to the
roof where a small PV cell is installed. In this cord, a thermostat is also installed
which senses the air temperature inside the cabin. When the temperature is below
the predetermined set point, electricity flow can be stopped to the fun by turning
off the switch. In this way, fan only works when required. This prevents the fan
from operating when the air temperature is at a level such that dew and other
moisture may be pulled into the interior as a result of the creation of the negative
pressure inside the cabin. A thermostat can be incorporated at other locations of the
system as desired.
Thus, a solar fan assembly should be used for proper ventilation of the car and to
decreases the air conditioning load which is our main purpose.
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CHAPTER 2
Air conditioning system
After working on the ventilation system, we decided to take cooling of the car
forward to another level. Though the ventilation system was instrumental in
reducing the temperature in the cabin to tolerable value; it was still not enough. On
days of peak temperature of around 50○C of the surroundings, the cabin was still
too hot to be comfortable. Thus, we proposed another, newer, system for the
chilling of the car cabin - the absorption chillers. Through these chillers, we
attempted to remove the need of experiencing any discomfort due to heated cabin
by proposing a system, which will not only reduce the fuel requirement of cooling
the car but also provide a car which is chilled even when parked!
Use of Lithium Bromide Chillers for the purpose of cooling is not an entirely new
phenomenon. The first gas absorption refrigeration system using gaseous ammonia
dissolved in water (referred to as "aqua ammonia") was developed by Ferdinand
Carré of France in 1859 and patented in 1860. Due to the toxicity of ammonia,
such systems were not developed for use in homes, but were used to manufacture
ice for sale.
Since then a lot of research has been done on it to improve the cycle and
determine its possible utilization. Up till now, absorption refrigeration systems
have largely been applied to large cooling loads e.g. that of a house or commercial
building. Due to its large volume, it has not yet been proposed for small systems
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such as the cooling system of a car. We have attempted to apply the same principle
of chilling to automobiles and check the feasibility of this system.
What are Lithium Bromide Chillers?
The electric chiller of today’s car uses an electric motor for operating a compressor
used for raising the pressure of refrigerant vapors, whereas absorption chillers are
refrigerators that use heat instead of mechanical energy to provide cooling. It has a
thermal compressor which consists of an absorber, a generator, a pump, and a
throttling device. This compressor replaces the mechanical vapor compressor
which derives the cooling system. This means that the rejected heat from the
power-generation equipment (e.g. turbines, micro turbines, and engines etc) may
be used with an absorption chiller to provide the cooling. Thus, specifically for our
system, we can use the waste gases from the engine, when the car is in motion, to
drive the thermal compressor or for the case of parked car, we can try to utilize the
solar energy found abundantly during the summers to supply the energy to drive
the cooling system. Therefore, by applying this system, we can strive to achieve a
better utilization of the limited energy resources available to us.
Unlike the vapor compressor air conditioning system, the thermodynamic process
being used in adsorption chillers is not a conventional thermodynamic cooling
process based on Charles' law. Instead, it is based on two very basic principles:
1. Evaporation:
When a liquid evaporates, it carries the heat away, in the form of faster-
moving (hotter) molecules. These same molecules release the heat absorbed
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when they condense. These heats are called latent heat of evaporation and
latent heat of condensation.
2. Pressure effect on Boiling Point:
As the pressure changes the boiling point changes, i.e. it decreases with
decrease in pressure. This means that evaporation can then be applied at
lower temperature by creating small vacuum.
Their application is further explained in the cooling cycle stated latter on.
The chemical process occurring in the absorption cooling works on the affinity of
some pairs of chemical to dissolve in one another. For example, lithium bromide
solution has affinity towards water; water has affinity towards ammonia etc. This
affinity depends on two factors;
1. Temperature,
2. Concentration of the solution.
As the temperature decreases, the affinity of the absorbent increases and it absorbs
larger amount of solute. As the solution becomes diluted, with respect to the
absorber, the affinity starts to diminish and finally it comes to equilibrium
depending on the temperature and pressure. After this point it can no longer absorb
any more solute.
However, there is a small drawback to the absorption chillers. Compared with
mechanical chillers, absorption chillers have a low coefficient of performance
(COP = chiller load/heat input). However, this is not a good basis for comparison
as the sources of energy input are different, with electricity being a more expensive
energy source than the waste heat being used in the chillers.
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Is an Absorption Chiller the Best Choice for Us?
One of the questions that might arise in the mind is that whether the absorption
chillers a good option for us. Well, this query can be easily satisfied by checking
few pointers, if at least one of the following applies absorption cooling may be
worth considering:
1. You have a combined heat and power (CHP) unit and cannot use all of the
available heat,
2. Waste heat is available,
3. A low-cost source of fuels is available,
4. Your boiler efficiency is low due to a poor load factor
5. Your site has an electrical load limit that will be expensive to upgrade
6. Your site needs more cooling, but has an electrical load limitation that is
expensive to overcome, and you have an adequate supply of heat.
7. Where noise from the compressor is problematic
In short, absorption cooling may fit when a source of free or low-cost heat is
available, electricity is unreliable, costly, or unavailable, or if objections exist to
using conventional refrigeration. Essentially, the low-cost heat source (e.g., from
turbine exhausts or industrial processes) displaces higher-cost electricity in a
conventional chiller.
Applying the mentioned pointers to our system, we can see that more than
sufficient factors can be applied to our system.
First being that we have free waste heat energy easily available to us in the
form of exhaust gases leaving the system during the operation of the engine.
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A second source of energy (low cost) that we can utilize is that of the sun
available to us in more than sufficient amount during those scorching
summer days of June. This energy can even be used to run the cooling
system on a parked car.
Third factor is the raising prices of the fuel these days. By utilizing the waste
heat from the gases or the renewable energy from the sun we can reduce the
fuel requirement of the car and thus also help in preserving the limited
resources that we do have.
Another point to consider is that by separating out the cooling system from
the engine load we increase the efficiency of the car thus leading to a
smoother and faster ride.
Keeping in mind the above points, it becomes clear that the employment of
absorption chillers can be a good alternative for the current chilling systems in the
cars.
Principle of Operation
As stated above, the absorptive refrigeration uses a source of heat to provide the
energy needed to drive the cooling process. The high concentration side of the
cycle absorbs refrigerant vapors (which, of course, dilute that material). Heat is
then used to drive off these refrigerant vapors thereby increasing the concentration
again.
The absorption chiller has to be operated at very low pressures (about l/l00th of
normal atmospheric pressure). At such a low pressure the boiling point of the water
17
is significantly reduced (e.g., at ~ 40°F). This means that the water vaporizes at a
cold enough temperature to produce cooling effect of about 44°F. This allows us a
sufficiently good gradient for cooling the car cabin.
Chiller Possible Fluid Pairs
Just after having a look at the topic, the question that arises in our minds is about
the lithium bromide fluid pair. Why specifically this pair? Are there any explicit
reasons behind it or we can use any available fluid pair. Many pairs have been
proposed over the year through research but only two of them are widely used.
NH3-H2O system
The first chiller to be designed was done with ammonia and water pair. In this
design, liquid ammonia is introduced into hydrogen gas. The liquid ammonia
brought about cooling by evaporating in the presence of hydrogen gas. The now-
gaseous ammonia is next sent into a container holding water, which absorbs the
ammonia. The water-ammonia solution is then directed past a heater, which boils
ammonia gas out of the water-ammonia solution. The ammonia gas is then
condensed into a liquid. The liquid ammonia is then sent back through the
hydrogen gas, completing the cycle.
LiBr-H2O system
A similar system for absorption chillers uses a solution of lithium bromide salt and
water. Water under low pressure is evaporated from the coils that are being chilled.
The water is absorbed by a lithium bromide/water solution. The water is then
driven off the lithium bromide solution using heat. This water is then condensed
18
and returned to the evaporator to complete the cycle. LiBr-H2O has a higher COP
than the NH3-H2O systems.
Comparison
Each system has its own advantages and disadvantages related to its operation. To
determine which one would be best for our system we first need to compare their
pros and cons. The comparison is summarized in the table given below.
NH3-H2O system LiBr-H2O
The COP is between 0.6 to 0.7. The COP is between 0.6 and 0.8
Also applicable for lower temperature
applications, with temperature
achievable as low as -40 F (-40 C).
LiBr-H2O systems cannot operate at
temperatures much below 5°C since
the refrigerant is water vapour.
NH3-H2O system needs a rectifying
column that assures that no water
vapour enters the evaporator where it
could freeze.
Commercially available absorption
chillers for air conditioning
applications usually operate with a
solution of lithium bromide in water
and use steam or hot water as the
heat source.
Ammonia-water systems are more
common for smaller tonnage.
Lithium bromide-water chillers are
used for large tonnage in process
applications.
Considering our system, we can see that though the tonnage required is low but the
available space is also small, whereas ammonia-water chiller requires more units
than the lithium bromide chiller and is more complex in operation. As the cooling
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temperature required is above 5 degrees Celsius in automobiles, therefore, the pair
given to us is the best choice.
Types
Commercially available absorption chillers are available in two types:
1. Single Effect (Stage) Units
They use low pressure (20 psig or less) as the driving force. These units
typically have a COP of 0.7.
2. Double Effect (2-Stage) Units
These are available as gas-fired (either direct gas firing, or hot exhaust gas
from a gas-turbine or engine) or steam-driven with high pressure steam (40
to 140 psig). These units typically have a COP of 1.0 to 1.2. To achieve this
improved performance they have a second generator in the cycle and require
a higher temperature energy source.
Due to the limited space available in the car we decided to use the single stage
adsorption chillers for our system. This way we require less units and consequently
lesser space.
Cooling Cycle
Now, let’s determine the cooling cycle that is to occur during the refrigeration. We
have decided that we are going to use single stage lithium bromide chillers for the
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refrigeration. The energy, material balance and the area calculations are provided
in the appendix.
1. Solution Pump and Heat Exchanger
Figure 1 Solution Pump and Heat Exchanger
The cycle starts from the pump which is placed at the end of the absorber.. A dilute
lithium bromide solution is collected in the bottom of the absorber shell. From
here, a hermetic solution pump moves the solution through a shell and tube heat
exchanger for preheating. In the pre-heater it exchanges heat with the hotter,
concentrated absorbent solution and reaches the temperature of 55 ˚C. This is done
to reduce the amount of heat duty in the regenerator. Simultaneously, the hotter
stream becomes cooler so that the load on the absorber is also reduced and more
absorption takes place. Introduction of heat exchanger increases the efficiency of
the system.
2. Generator
Figure 2 Generator
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After exiting the heat exchanger, the dilute solution moves into the upper shell.
The solution surrounds a bundle of tubes which carries steam at 105 ˚C. The steam
or hot water transfers heat into the pool of dilute lithium bromide solution. The
solution boils, sending refrigerant vapor upward into the condenser and leaving
behind concentrated lithium bromide. The concentrated lithium bromide solution
moves down to the heat exchanger, where it is cooled by the weak solution being
pumped up to the generator. The regeneration of the vapor allows the process to
continue as a cyclic process.
3. Condenser
Figure 3 Condensor
The refrigerant vapor migrates through mist eliminators to the condenser tube
bundle. The refrigerant vapor condenses in the tubes. The heat is removed by the
surrounding air flowing over the tubes and the surrounding temperature is around
40 ˚C. Due to the forced convection it further drops a few degrees. It is here that
the heat initially absorbed by the water to evaporate is released or rejected out of
the system. As the refrigerant condenses, it collects in a trough at the bottom of the
condenser.
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4. Evaporator
Figure 4 Evaporator
The refrigerant liquid moves from the condenser to the evaporator and is sprayed
inside the evaporator tube bundle. Due to extreme vacuum in the shell (6mmHg
(0.8kPa) absolute pressure), the refrigerant liquid boils at around 39°F (3.9°C),
creating the refrigerant effect. The vacuum is maintained by hygroscopic action of
the absorbent - the strong affinity lithium bromide has for water in the absorber
directly below.
5. Absorber
Figure 5 Absorber
As the refrigerant vapor migrates to the absorber from the evaporator, the strong
lithium bromide solution from the generator is sprayed over the top of the absorber
tube bundle. The strong lithium bromide solution actually pulls the refrigerant
vapor into the solution, creating the extreme vacuum in the evaporator. If this
23
vapor is not removed it will continue to increase the pressure till the pressure if the
evaporator increases such that required cooling is no achieved.
The absorption of the refrigerant vapor into the lithium bromide solution also
generates heat which is removed by the surrounding air. The now dilute solution of
lithium bromide collected, from where it flows to the solution pump. The chilling
cycle is now completed and the process begins once again.
Possible Configurations
Now, that we have defined the cooling process that we are going to try out, we
now face the problem of choosing the best configuration, out of many
configurations that we can base our proposition on. Depending upon where we
want to install our system, there can be different types of configurations.
The first thing to define is what class of vehicle we want to install our system in
i.e. whether we are installing it in a car or a bus.
Let’s consider the case of a bus first. Due to larger size of the bus there are many
possible spaces available in it.
1. We can put it near to the engine
If we examine the current cooling system of the buses, we come to know
that the refrigeration systems installed in them already consume a large
volume as there is a separate engine present for the sole purpose of
24
providing energy to the cooling system. Thus if that system is to be
removed, we will be left with a lot of space to install our new system in.
2. On the roof of a bus
Second option available for us is that of the roof of the bus. Again there
would be a large space available on the roof.
Thus we can say that the factor of space availability is not a major factor in the
buses. Besides this, uses generally don’t have to be parked over a long period of
time thus the problem of cabin space being over heated is not a major issue.
Therefore, we will shift our attention to smaller system which faces the problem of
over heating and space availability more frequently, cars.
When working on the different configurations for the cars, we first have to
determine the type or the model of the car i.e.
1. A car without trunk e.g. CULTUS
2. A car with a trunk e.g. LIANA
For cars without trunk we are left with only one option that we install it on the roof
of car. But for a car having trunk, we have more than one options i.e. inside the
trunk (for a car using petrol as a fuel not using CNG cylinders so that some space
is available for our system) or on the roof. Thus, we chose LIANA (a car with
trunk) to check the feasibility of our system.
After choosing the car to analyze the refrigeration on, we then came up with
different possible ways to place our refrigerator so that we could determine the
most suitable configuration. Different possible configurations that are possible are
discussed below with their merits and demerits:
First Configuration:
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We can install our refrigeration system inside the trunk of a car.
But there are various problems associated with this kind of configuration.
1. Nowadays, it is a common practice in our country to put CNG (compressed
natural gas) cylinders inside the trunk of a car as CNG is a cheaper source of
fuel in our country. So it will not be possible for us to fit our refrigeration
system simultaneously with these cylinders because of large volume
occupied by our system.
2. Another problem is that the cooling system of the chiller releases heat
energy into the surrounding which can create hazardous conditions for the
CNG cylinders ( i.e. can cause possible expansion of gas leading to the
bursting of the gas cylinders)
3. Purpose of the trunk to carry luggage from one place to another will lost.
Second Configuration:
We can replace the previous refrigeration system with our system.
But the problem is:
Previous refrigeration system (vapor compression cycle) occupies very small
volume and has fewer number of units as compared to our system (absorption
refrigeration system). So we cannot put our system inside the bonnet of a car with
the engine.
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Third Configuration:
We can put our system on the roof of a car.
The only problem with this kind of configuration is that car will lose its esthetic
sense. But by ignoring this problem, we can easily install our system on the roof
without any problem of large volume and leakages etc.
Thus, we found third configuration most feasible for our system.
Our configuration
We install our system on the roof of a car.
Our absorption refrigeration system includes the following main components:
Condenser
Evaporator
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Absorber
Heat exchanger
Regenerator
Solar heater
Each of the components is explained below with its position and configuration in
detail.
1. Condenser
In systems involving heat transfer, a condenser is a device or unit
used to condense a substance from its gaseous to its liquid state,
typically by cooling it.
Simply put, condenser is a device or unit used to condense vapor
into liquid.
Condenser in our system
Condenser is a very important component in our system which
takes water vapors and produces saturated liquid. In our
configuration the condenser is placed on the roof of the car.
As water vapors coming from the regenerator enter into the condenser, they
condense by releasing heat into the ambient air. To enhance the exchange of heat
with the surroundings, air is blown over the tubes carrying the vapors. The rate
flow of air over the condenser is being controlled by a fan that is mounted parallel
to the tubes of the condenser.
For condensation we will use a horizontally mounted air cooled condenser in the
shape of a radiator. An air cooled condenser is a heat transfer device for rejecting
heat from a hot fluid directly to fan-blowing ambient air. We are using this kind of
condenser because in our case refrigerant is being cooled by the outside air And
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also because by using this type of system no problem arises for thermal and
chemical pollution of cooling fluids. Besides this the system is flexible for any
plant location and plot plan arrangement like installation over other units. As
installation of a big unit on a small car is a big issue so we preferred this type of
unit which is more flexible in its installation than other types.
Figure 6 Condenser 1 Figure 7 condenser 2
Configuration
Tubes are arranged in such a way that the air is blown by forced convection
over the tubes through a fan mounted on the side of the condenser. This
means both fluids (refrigerant inside the tubes and air blown out side the
tubes) are parallel in flow therefore no correction factor is needed for the
calculation of LMTD (log mean temperature difference). Tubes with fins are used to provide a large surface to volume ratio. Thus
more contact area between air and tubes is provided which make heat
exchange process much better than tubes without fins. The condenser is
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made up of carbon steel through which process fluid at high temperature
flow and heat exchange takes place.
The tubes can be of virtually any material available, such as carbon steel,
stainless steel, Admiralty brass, or more exotic alloys. The minimum
preferred outside diameter is one inch.
2. Evaporator
In evaporators physical process occurs through which a liquid or solid substance is
transformed to the gaseous state; the opposite of condensation. Evaporators are
widely used in refrigeration system and also for the purification purposes.
Evaporator in our system
Evaporator is another important component in which evaporation of refrigerant
occurs. The refrigerant absorbs heat to evaporate thus causing cooling in the
surroundings. Evaporators are usually installed in the location near to the point
which is required to be cooled.
In our system evaporator is installed at such a position inside the bonnet of the car
such that it is exposed to the air from the cabin. The air from the cabin is easily
sucked by the fan that is located on the evaporator and blown over the evaporator
for heat exchange purposes. Pressure inside evaporator is kept very low so that
when the refrigerant from a throttling valve enter into the evaporator, sudden
reduction of pressure occurs causing flashing and thus evaporation occurs that
causes cooling in the surrounding.
Configuration
Evaporator is installed inside the bonnet of car in the form of tubes that are placed
in the form of coils and water is inside the tubes. Air is blown on tubes by fan that
is located on the side of evaporator so that a parallel flow of both fluids occur.
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3. Absorber
Absorber is a unit in which vapors of refrigerant coming from evaporator enter and
get absorbed by the solvent solution inside the evaporator.
Absorber is a critical component since its efficiency directly effects the whole
cycle. Its advantage is that it does not have any moving parts and can operate at
low temperature.
Absorber in our system
In our system absorber is using LiBr-water solution as an absorbent mixture which
requires a corrosion resistant material. Water vapors enter in tubes while absorbent
is sprayed in tubes so that vapors get absorbed in absorbent. Intimate contact
between vapors and absorbent occurs promoting mass and energy balance from
vapors to the absorbent mixture.
Configuration
Absorber is build as horizontal tubes. Tubes are arranged horizontally. Absorber
tubes are mounted on the roof of car.these tubes are made up of corrosion resistant
material such as carbon steel.
Figure 8 Absorber
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4. Regenerator
Regenerator is a unit in which regeneration of absorbent occurs. It requires high
tempearture for seperation of the two fluids. This is the section which requires the
external heat input.
Regenerator in our system
Regenerator is a very important component in absorption refrigeration system. It is
a high temperature component. In our system it is mounted on the roof of car and is
placed below the solar collector. It is a shell and tube heat exchanger in
construction. Heat energy is provided through the hot water collected in the solar
collector. Hot water comes from the solar heater and flows inside the tubes while
water rich solution from the absorber is pumped to the regenerator through the heat
exchanger. Heat is then transferred from hot water to the solution. Due to increase
in temperature water get evaporated and passes to the condenser while the
concentrated absorbent solution is sent to the absorber.
Configuration
We use a horizontal shell and tube heat exchanger with hot water inside the tubes
and solution which is to be separated flows outside the tubes.As very high
temperature is required and solution is libr-water mixture so we select the
corroision resistant material.e.g.carbon steel. It is installed on the roof of a car
below solar heater so that supply of hot water is easy.
5. Solar water heater
In solar water heater, water is heated by the use of solar energy. Solar heating
systems are generally composed of solar thermal collectors, a fluid system to move
the heat from the collector to its point of usage. The systems may be used to heat
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water for a wide variety of uses, including home, business and industrial uses.
Heating swimming pools, under floor heating or energy input for space heating or
cooling are more specific examples.
Solar water heater in our system
We are using solar water heater to provide heat to the regenerator for the
evaporation of refrigerant. We are providing heat to the regenerator by using solar
energy because it is not only the cheap source of energy but also a valuable amount
of heat can be provided easily to the regenerator.
Configuration
We are using a concentric pipe solar heater. Water is in internal tube and there is a
vacuum between outer and internal tubes to avoid heat losses due to convection.
We are using coating of special selective material on outer surface of internal tubes
making outer surface of internal tube good absorbers and bad radiator so that all
heat get absorbed in water inside the internal tubes that is eventually use to provide
heat in regenerator.
Figure 9 Solar water heater
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Limitations of our system
1. This system requires more pump power than today’s system.
2. Their size and weight are larger and heavier than today’s chiller of the same
capacity.
3. In an absorption machine, if the solution concentration is too high or the
solution temperature is reduced too low, crystallization may occur. This is
most likely to occur in the solution heat exchanger, interrupting the machine
operation. Therefore, solution concentration should always be as given by
the manufacturer.
4. For keeping the required pressure in the absorption chiller, it is necessary to
evacuate the vapor space periodically with a vacuum pump. As at high load
conditions, the control system increases the heat input to the generator,
resulting in increased solution concentrations to the point where
crystallization may occur.
34
Conclusions and recommendations
After scrutinizing the system completely, we can see that the absorption cooling
may be used when a source of free or low-cost heat is available, or if objections
exist to using conventional refrigeration.
Since we are using solar heaters for the heating purposes, their calculated area is
large, making our system infeasible for a car. Secondly, the COP of our designed
system is 0.59, which can be improved by increasing efficiency of heat exchanger
however at high temperature our absorbent gets crystallized.
In addition to that, the solar energy is not freely available all the time and also
there may be weather fluctuations leading to the system malfunctioning. Therefore
we can make our system a hybrid one. We can provide heat to our system not only
through solar collector but also through the waste heat of exhaust gases when solar
energy is not available.
In the light of these conclusions, we recommend:
There should be a continuous experimentation for the efficient use of solar
energy in the day time so that the size of solar collector is reduced.
In addition, to cope with the increasing demands of cooling in Pakistan,
having limited fuel availability, we should find substitutes for more
expensive methods.
As this system is more reliable, less operational and maintenance cost is
required, so this system is better to adopt. Also the low-cost heat source in
35
absorption system displaces higher-cost electricity in a conventional chiller,
making it more economical.
APPENDIX A
Mass and Energy Balance
The following diagram represents the conditions and the cycle of our refrigeration
system.
Figure 10 Block Diagram Of Our System
36
Capacity = 0.3 tons
= 1.03 kW
Refrigeration effect = h5-h4
h5= h of saturated vapor leaving the evaporator at 60C and 0.935kPa=2512.6kJ/kg
h4= h of saturated liquid leaving the condenser at 400C and 7.735kPa=167.5KJ/kg
Hence the mass flow rate of refrigerant is calculated as:
Mass flow rate = capacity/refrigeration effect
Mass flow rate = 1.05kW / 2345.1kJ/kg
= 4.477*10-4 kg/sec
m3 = m4 = m5 = 4.477*10-4 kg/sec
Figure 11 Car For Our Configuration
Overall mass flow balance
m1 = m2 + m3
m1 = m2 + 4.77* 10-4 (1)
37
LiBr balance
x1m1 = x2m2
(4.77*10-4 – m2)0.595 = 0.63 m2
2.83*10-4 = 0.035m2
m2 = 8.08*10-3kg/sec (2)
Solving eq. (1) and eq. (2)
m1 = 8.562*10-3 kg/sec
The solutions concentrations are now determined with the help of graph:
x1 = 59.5%
x2 = 63.0%
Energy balance
The enthalpies of the solutions can be read from the graph C1 in appendix C,
h1 = h at 400C and x of 59.5% = -160 kJ/kg
h2 = h at 900C and x of 63.0% = -65 kJ/kg
The temperature of 59.5% solution leaving the heat exchanger is 55 0C and
solution at this point has enthalpy
h6 = -130kJ/kg
The rate of heat absorbed by the solution passing from the absorber to the
generator is
Q6 = m1(h6-h1)
= 8.562*10-3(-130+160)
=0.256 kW
38
Since this same rate of heat transfer must be supplied by the solution that flows
from the generator to the absorber
Q6 = Q7
Q7 = m2 (h2 – h7)
0.256 = 8.08*10-3(-65- h7)
h7 = -96.6kJ/kg
From graph C2 in appendix C, the temperature of the solution leaving the heat
exchanger is,
t7 = 750C
Rate of heat transfer in generator
Qg = m2h2 + m3h3 –m1h1
= (-65) (8.08*10-3) + (4.47*10-4) (2660) – (8.56*10-3) (-130)
= 1.7766 kW
Rate of heat transfer in evaporator
Qe = m3 (h5-h4)
= 4.477*10-4(2512.6 – 167.5)
= 1.05kW
Rate of het transfer in condenser
Qc = m4h4 – m3h3
= 4.477*10-4(167.5 – 2660)
= 1.188kW
39
Rate of heat transfer in absorber
Qa = m5h5 + m2h2 –m1h1
= 4.477*10-4(2512.6) + (8.08*10-3) (-96.6) – (8.562*10-3) (-160)
= 1.78kW
Coefficient of performance
COP = Qe / Qg
= 0.591
40
APPENDIX B
Surface Area Calculations
Condenser
T1 = 90 0 C t1 = 36 0C
T2 = 40 0C t2 = 40 0C
LMTD for counter flow arrangement is calculated as
LMTDcounter = (90-40) – (36-40) / ln (50/4)
=18.20C
In our case the flow arrangement is counter- parallel, so a correction factor Ft is
applied
For this correction factor
K = (T1- T2) / (T1-t1)
=0.925
S = (t2-t1) / (T1-t1)
= 0.074
R = K/S
= 12.40
From kern
Ft = 0.83
41
So,
LMTD* = Ft (LMTDcounter)
= 0.83(18.2)
= 15.2360C
Fourier law of cooling
QC = UA∆T
U =100W/m2k
QC = 1.188Kw
Area = (1.188*103)/ (100) (15.236)
= 0.779m2
Area for finned surface = 0.85(0.779)
= 0.663 m2
Area = 2πrl
0.663m2 = 2π (7.5*10-3) (L)
Lt = 13.8 m
Assuming each length to be 0.30m
Number of tubes = n = 13.8 / 0.3= 46
Evaporator
T1 = 450C t1= 60C
T2 = 250C t2= 60C
LMTD = (45-6) – (25-6) / ln (34/9)
= 27.80C
Fourier law of cooling
42
Qe = UA∆T
U =170W/m2k
Qe= 1.05kW
A = (1.05*103) / (170) (27.8)
= 0.221m2
For finned surface
A = 0.19 m2
Length of coils is calculated as
Area = 2πrl
D= 15mm
L = (0.19)/ (2π) (7.5*10-3)
= 4 m
Absorber
T1 = 480C t1= 360C
T2 = 400C t2= 380C
LMTD = (10-4)/ln (10/4)
= 6.50C
In our case the flow arrangement is counter- parallel, so a correction factor Ft is
applied
For this correction factor
K = (T1- T2) / (T1-t1)
=0.66
S = (t2-t1) / (T1-t1)
43
= 0.166
R = K/S
= 4
From kern
Ft = 0.93
So
LMTD* = Ft (LMTDcounter)
= 0.93(6.5)
= 6.0450C
Fourier law of cooling
Qa = UA∆T
U=600W/m2k
Qa= 1.78kW
A = (1.78*103)/ (600) (6.045)
= 0.494 m2
For finned surface
A = 0.296 m2
Length of coils is calculated as
Area = 2πrl
D= 15mm
L = (0.296)/(2π)(7.5*10-3)
= 6.28 m
44
Assuming each length to be 0.3, then
Number of tubes = n = 6.28 / 0.3 = 21
Heat exchanger
For double pipe heat exchanger
T1 = 900C t1= 400C
T2 = 720C t2= 550C
LMTD = (90-35)-(72-40)/ln (35/32)
= 33.480C
Fourier law of cooling
Qa = UA∆T
U =125W/m2k
Q1x = 0.256kW
A = (0.256*103)/ (125)(33.48)
= 0.06 m2
Diameter of outside pipe =D= 15mm
Diameter of inside pipe = d = 9.5mm
Area = 2πrl
L = (0.06)/(2π)(7.5*10-3)
= 2 m
Generator
T1 = 1050C t1= 550C
T2 = 950C t2= 900C
45
LMTD = (105-90)-(95-55)/ln (15/40)
= 25.50C
Fourier law of cooling
Qg = UA∆T
U =2300W/m2k
Qg = 1.776kW
A = (1.776*103)/ (2300) (25.5)
= 0.03 m2
Area = 2πrl
L = (0.03)/ (2π)(7.5*10-3)
= 1.01 m
Solar collector
For evacuated tube solar collectors
Efficiency of the collector tubes = η= 0.82 -2.19(Tm-Ta)/G
Where,
Mean temperature of collector = Tm = (95+105) / 2
=100
Ambient temperature = Ta = 400C
G = 1100 W/m2
Then,
η= 0.7
For net absorber plate area
46
QCOLLECTOR = η AG
QCOLLECTOR = Qg
1.776*103 = (0.7) (A) (1100)
A = 2.3m2
From the literature:
Net absorber area = 0.6(gross absorber area)
Gross absorber area = (2.7) / (0.6)
= 3.8 m2
Outside diameter of tubes = D = 2 cm = 0.02m
Inside diameter of tubes = d = 1cm = 0.01m
We know that Area = 2πrl
Assuming the length of each tube to be 0.75m
Number of tubes = 3.8/ (2π) (0.01) (0.75)
= 81
47
APPENDIX C
Thermodynamic Diagrams
Figure C1 Enthalpy of LiBr-H2O solution
Figure C2 Temperature- Pressure-Concentration diagram of LiBr
48
Figure C1 Enthalpy of LiBr-H2O solution
49
Figure C2 T-P-Conc diagram of LiBr
50
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