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 -

1

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 -

2

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 -

3

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.

4

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.

5

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

6

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.

7

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

8

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

9

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.

10

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

11

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.

12

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

13

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

14

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.

15

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.

16

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

19

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

20

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

21

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.

22

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:

25

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.

26

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

27

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

28

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

32

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

33

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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