AUTOMOBILE A/C BY UTILISING
WASTE HEAT & GASES
1. ABSTRACT
It is the established fact that only about 30% of heat supplied by the
fuel is converted into useful work, in case of internal combustion (I.C)
engines and the rest is going waste to the atmosphere in the form of coolant
losses (35%) and exhaust gas losses (35%). The conventional air conditioning
system which most of the A/C vehicles use is the ‘vapour Compression
refrigeration system ‘, in which the compressor needs mechanical work that is
Higher-grade energy is then taken directly from the engine crankshaft. Thus it
ultimately reduces the brake power (B.P.) available and increasing brake
specific fuel consumption.
The ‘vapour absorption refrigeration system ‘utilizes the waste heat as
it does not involve any compressor and hence not require great mechanical work
instead of that it works directly on the heat energy i.e. .low grade energy.
Thus by making proper use of lost heat (about 60 –70% of total heat). The
conventional air conditioning can be replaced with this system and the same
effect can be experienced. The common vapour absorption refrigeration systems,
which are in practice, are
1.
Aqua
Ammonia system and
2.
Lithium
Bromide water system
2.EXISITING AIR-CONDITIONING SYSTEM
The use of air conditioner for transport purpose may be a luxury in India
but it is commonly used in foreign countries .In comparison to domestic
air-conditioning a very large amount of air-conditioning capacity is required
for a car. This is due to metal construction of the car, the flow of air around
moving car and relatively large glass area in the passenger compartment.
Typically, a car A/C system capacity may be between 1 to 4 tons. The system
works on Vapour Compression Refrigeration System (VCRS) and the compressor
consumes large amount of engine brake power (1 to 10 h.p.) as it is directly
driven by the engine. This affects the fuel economy severely. A loss in economy
level of the order of 1 to 1.5 km/liter can occur due to the use A/C. Maximum power is required when the car is
running at maximum speed under high ambient temperature conditions. Apart far
from this VCRS has got certain drawback, which limits its extensive use among
common car owner.
DRAWBACKS
1. High initial cost.
2.
High
operating cost, since fuel economy is affected, high maintenance cost, costly
refrigerant.
3. CFC’s (Chlorofluorocarbon) if leaks
out of the system causes great damage to the ozone layer.
4. If the car’s reserve power is less,
it can affect its acceleration.
5.
Overloading
and overheating of the engine takes place.
3. THE AUTOMOBILE ENGINE
The prime mover of the automobile (I.C. engine) is a heat engine, which
can convert only a fraction of the total heat of fuel into the useful
work.
20 to30 % for SI
engines
30 to 36% for CI
engines
The remaining heat is lost to the atmosphere through the coolant and
exhaust. Heat balance is given in the below table: -
%AGE OF FUEL ENERGY
S.I. C.I.
To power 26 31
To coolant 30 26
To exhaust 32 30
Radiation 12 13
Also refer
the fig. 1
Thus we have about 60% of heat which is going waste. So, with such a
small efficiency of the heat engine. Obviously it is not worthwhile for a
common man to install such an A/C in his car.
4. AN ALTERNATIVE TO THIS SYTEM
The concept is to use
this otherwise going waste heat, for air-conditioning with the aid of Vapour
Absorption System (VARS) which does not affect the engine power. It need no
maintenance and is environment friendly.
VARS is a ‘heat operated
refrigeration machine ‘ in which the compressor is replaced by the combination
of absorber and generator. A solution known as the absorbent (e.g. water in
case of A qua-ammonia system) which has an affinity for the ‘refrigerant’ used
(i.e. ammonia) is circulated between the absorber and the generator by a pump
(solution pump). I n this system, the low pressure ammonia vapour living the
evaporator, enters the absorber where it is absorbed by the low temperature
water in the absorber .The water has the ability to absorb very large quantity
of ammonia vapour and the solution thus formed, is known as Aqua-ammonia. The
absorption of ammonia vapour lowers the pressure in the absorber, which in turn
draws more ammonia vapour from the evaporator and thus raises the temperature
of solution. Some form of cooling arrangement (usually water-cooling) is
employed in the absorber to remove the heat of solution evolved there. This is
necessary in order to increase the absorption capacity of water. The liquid
pump pumps the strong solution thus formed in the absorber to the generator.
The pump increases the pressure of the solution upto 10bar. The strong solution
of ammonia in generator is heated by heat of coolant and the exhaust gases,
which are waste in atmosphere without any use and the heat, wasted in cooling
of engine. During the heating process, the ammonia vapour is driven of the
solution at high pressure leaving behind the hot weak ammonia solution in the
generator. The weak ammonia solution flows back to the absorber at low pressure
after passing through the reducing valve. But then also the ammonia vapour
contains some particles of water. If these unwanted water particles are not
removed before entering into the condenser, they will enter into the expansion
valve where they freeze and choke the pipeline. In order to remove these
unwanted particles flowing to the condenser, an analyzer is used. The analyzer
may be built as an integral part of the generator or made as a separate piece
of equipment. It consists of a series trays mounted above the generator. The
strong solution from the absorber and the aqua from the rectifier are
introduced at the top of analyzer and flow downward over the trays and into the
generator. In this way, considerable liquid surface area is exposed to the
vapour rising from the generator. The vapour is cooled and most of the water
vapour condenses. So, that mainly ammonia vapour, leaves the top of the
analyzer. Since the aqua is heated by the vapour, less the generator is
condensed in the condenser to high-pressure liquid ammonia. This liquid ammonia
is passed to the expansion valve through a receiver and then to the evaporator.
This evaporator is made up of number of tubes, which is installed in the cabin
of automobile. The function of compressor is performed by the absorbent in the
absorber, and the generator performs the function of compression and discharge.
The complete system is schematically represented in the fig. 2.
5.OPERATING THE
SYSTEM
As we know
that ‘VARS’ is a heat operated refrigerating machine in which heat is supplied
to the generator. So this required heat we will supply from the ‘waste heat’
(coolant loss and exhaust) which is our center of focus. So we have to
distribute the exhaust gases and the coolant to all the system whenever
necessary to satisfy the cold and hot air conditioning and flexibility of
operation in various possible mode.
For this there are two types of circuits.
1)
Coolant
circuit
2)
Exhaust
circuit
1.Coolant Circuit: -
In vapour absorption refrigeration system,
there is necessity of cooling of absorber and condenser, which is achieved by
water-cooling. The water is supplied to this system by radiator and heat gained
by the cooling water from the engine is utilized in generator and heater. The
systematic arrangement is shown in the given fig.
The coolant circuit in various modes of operations is given
below: -
I.
Normal
running with A/C OFF.
Circuit: - (Radiator - V3-Engine – V2 – Radiator)
Valve position: -
a)
V2---0-1
b)
V3---0-1
II.
Normal
running with A/C ON.
i.
For
summer ( or high surrounding temperature)
Circuit :-( Radiator-V3-Condenser –
Absorber-Rectifier-N.R.V.-Engine-V2-Generator-N.R.V-Radiator)
Valve position
a)
V2---0-2
b)
V3---0-2
ii.
For
winter (or low surrounding temperature)
Circuit: - (Radiator –V3
Engine-V2-Heater-N.R.V.-Radiator)
Valve position
a)
V2---0-3
b)
V3---0-1
2.Exhaust Circuit: -
We are using the
waste exhaust gas heat to the generator and heater and then the exhaust gas is
exhausted to atmosphere. Distribution of the gas to the generator, heater and
the atmosphere is maintained by exhaust circuit whenever necessary. The exhaust
gas be either fed to the heater during winter or the generator during the
summer or bypassed to the atmosphere.
Exhaust Circuit: -
A.
Normal
running with A/C OFF.
Circuit: -
(Engine V1 to atm.)
Valve position
V1---0-1
B.
Normal
running with A/C ON
a)
For
summer (or high temperature of surrounding)
Circuit: -
(Engine V1 generator N.R.V. to atm.)
Valve position
V1---0-2
b)
For
winter (or low temperature of surrounding)
Circuit: -
(Engine V1 generator N.R.V. to atm.)
Valve position
V1---0-3
6. AIR CONDITIONING SYSTEM
The outside
air flows through the damper and mixes up with the recirculated air (which is
obtained from the conditioned space.) The mixed air passes through a filter to
remove dirt, dust and other impurities. In summer air conditioning, the cooling
coil operates to cool the air to the desired value. The dehumidification is
obtained by operating the cooling coil at a temp lower than the dew point
temperature (apparatus due point). In winter the cooling coil is made in
operative and the heating coil operates to heat the air. The schematic
arrangement can be shown by fig.6
7.INSTALLATION
For the design of the complete system the requirements are:
1)
Engine
manual (supplied by the manufacture) containing all details about the engine
performance and characteristics, especially cooling and exhaust.
2)
Determining
the cooling capacity required for a particular vehicle in a particular region,
considering the year round meteorological conditions the various parameters of
the air – conditioner can be defined.
The year round air –conditioning can be achieved by
the system which is required in the cities like New Delhi where it is too cold
in winter and quit hot in summer. Thus by knowing the amount of waste heat
available (usable) and the cooling capacity, various component of the system
can be designed. To get rough idea, let us see the heat available (usable) and
the cooling capacity, various components capacity required for a car as 2TR
let’s find the heat requirement for a certain aqua ammonia system.
Case Study of SI Engine
4-Stroke, 6-cylinder (7.5 cm bore and 9-stroke)
Rpm=3300
Fuel consumption = 0.3
kg/min
c.v. =42000
kJ/min
Jacket water
flow rate Q = 65
kg/min
Temperature rise = 12/C
Ventilate air
blown up = 14
kg/min
Enters at 10/C
and leaves at 65/C
(Engine in insulated box)
B.P. = 42.55 kW (100%)
Heat input = 0.3 * 42000
=
12600 KJ/min
i.
Heat equivalent to B.P. =
42.55 * 60
=
2553 KJ/min
ii.
Heat
in cooling water =
(65*4.1868*12)
=
3266 KJ/min (25.9%)
iii.
Heat
in ventilating air =
14*1.055*55
=774
KJ/min. (6.14%)
iv.
Heat
to exhaust and
Other losses = 6007 KJ/min
(47.66%)
So heat available for VARS = Heat in cooling water + Heat in exhaust
=
3266 + 6007
=
9273 KJ/min. (73.59%)
Let us assume that the effectiveness of heat
exchangers be 0.7
Net heat available =
6491.1 KJ/min
Case Study Of An Aqua- Ammonia System-
Now a case study of aqua-ammonia system is as-
In an aqua ammonia vapour absorption system the following data is
available: -
Temperature of weak solution in generator =100degr.C
Temperature of strong solution admitted to generator =80 degr. C
Temperature of condenser = Temperature of absorber =40 degr.C
Temperature rise in evaporator =10
degr.C
Analysis for 2 tonn refrigeration capacity: -
(Mass flow of ammonia through evaporator)
m = 2*3.5/h4-h3 = 7/1600-535 = 0.00657/kg/sec.
i.
Heat
supplied per kg.of ammonia in the generator
=
h12-ha
=1840-(-425)
=2265kj/kg(ammonia)
Q
(kJ/sec) = 0.066*2265
=14.75
kJ/sec
ii.
Heat
rejected in the absorbed per sec.
Qa =mr (h4-ha)
=0.0066(1600+425)
=13.3
kJ/sec
iii.
Degassing
C5-Cw C7-C8 = 0.46-0.4
=0.06
kg/kg of aqua
iv.
Heat rejected
in deflimator (cooler after generator)
=mr
(h12-h1)
=0.0066(1840-1630)
=1.38
v.
Heat
rejected in condenser
Qc =mr (h1-h2)
=0.0066(1630-535)
=7.197
vi.
Considering
the enthalpy balance across the heat exchanger, we can write,
Heat lost by weak
solution = Heat gain by strong solution
For 1kg ammonia entering into the
absorber mw kg of weak solution is entering then
ms=mw+1
mw
(h8-h9) =(mw+1)(h7-h6);
mw
(350-120) = (mw+1)(260-70)
40 mw = 190
mw = 4.75 kg/kg of ammonia
ms
(strong solution handled by the pump)= mw+1
=4.75+1
=5.75
kg/kg of ammonia
=0.0066
* 5.75
=0.037
kg/sec
vii. c.o.p. Qe/Qg =
h4-h3/Q9=1600-535/2265=0.47
viii.Energetic ne is given by
Ne
= Qe/Qg [Tg/Te (Te-Te/Tq-Te)]
=0.47(100+273/10+273)(40-10/100-40)
=31%
Heat supplied =4.75kj/sec.
Heat rejected in absorber =3.3kj/sec.
Heat rejected =7.197
Heat rejected in deflimator =1.38
Heat supplied =14.75kj/sec.
Heat rejected =13.3+7.197
Heat rejected in condenser =7.197
Heat rejected in deflimator =1.38
Heat supplied =14.75kj/sec.
Heat rejected =13.3+7.197
=20.49kj/sec
Heat supplied =885kj/min
Heat rejected =1229.82kj/min
Heat available =3266+6007
Considering effectiveness =0.7=2286+4204
=6490kj/min
Heat required =885kj/min.
Thus
we see that a large amount of heat is available and our requirement is lesser.
The system here described is simple basic. It can be further improved and made
sophisticated by using various control systems and relays. A basic control
system is shown in fig. 7
Apart
from the new design of vehicles installing (VARS), the existing vehicles can
also be equipped with this system and by studying the make of particular a proper
placed can be found out for erecting the system and tracing various circuits.
8.CONTROLLING THE SYSTEM
The
exhaust coolant circuit is controlled by 3 valves V1, V2 and V3. The valve V1
operates the exhaust circuit and the valves V2 &V3 operate the coolant
circuit where valve V3 is two way valves and other two V1 and V2 are three way
valves. The combination of position of valve for different conditions are as
shown below: -
V1 V2 V3
A/C OFF A/C OFF 1 1 1
A/C ON Summer 2 2 2
A/C ON Winter 3 3 1
9.ERECTION
By studying the manual of the
particular vehicle, an appropriate place can be found out for the erection of
the system for existing vehicles and for newer design, it is to be already
taken into consideration. The condenser, expander, absorber and evaporator
should be kept away from the engine as possible because the engine evolves at
high temp. The conditioned air supply and distribution system remains the same
as in the existing A/C vehicles.
10. ADVANTAGES OF VARS OVER VCRS
1)
No
moving parts so, quiet in operation, subjected to little wear, low maintenance
cost. The pump required quite small power in comparison with compressor.
2)
Large
capacity.
3)
Excellent
part load efficiency and almost constant c.o.p. of the system over a wide range
of load.
4)
Automatic
capacity control is easy.
5)
Smaller
space per unit capacity.
6)
No
harm to the ozone layer.
7)
Inexpensive
refrigerant.
8)
Leakage
can be easily detected in case of aqua ammonia system.
9)
It
can reduce the global warming of atmosphere.
11.CONCLUSION
Thus
we have seen that the VARS is efficient in every respect, and can be
successfully implemented with better designs and sophistication. Now it is the
task of the up coming engineers to overcome the hurdles in the way if any and
make our country’s people enjoy the comfort and luxury of A/C and fuel will
also be saved to a greater extent which would have been consumed in excess by
the (VARS) air conditioner.
12. REFERENCES
·
Basic
Refrigeration and Air conditioning- P.N. Anathnarayan
·
Refrigeration
and air conditioning – C.P. Arora
·
A
course in Refrigeration and Air-conditioning- S.C.Arora, S.Domkundwar
·
Thermodynamics
and Heat Engines- R.Yadav
·
A
course in Internal Combustion Engines – M.L. Mathur, R.P. Sharma
·
Automobile
Engineering –R.B. Gupta
·
A
Text Book of Refrigeration And Air Conditioning –R.S. Khurmi & S.K. Gupta
·
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