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Transcript
2011
THE HYBRID FRIDGE
GREEN REFRIGERATION USING AMMONIA ABSORPTION
NJOROGE ALEX KANYONGA
UNIVERSITY OF NAIROBI
4TH YEAR, DEPARTMENT OF MECHANICAL
AND MANUFATURING ENGINEERING
Email: [email protected]
Phone: 0725 011633
THE HYBRID FRIDGE
2011
Table of Contents
SECTION 1. INTRODUCTION .......................................................................................3
1.1 ABSTRACT ........................................................................................................................................... 3
1.2 OBJECTIVE ........................................................................................................................................... 4
1.3 INTRODUCTION .................................................................................................................................. 5
SECTION 2. OPERATION AND THERMODYNAMICS.............................................6
2.1 PRINCIPLE USED IN AN ABSORPTION REFRIGERATION SYSTEM ......................................................... 6
2.2 A PRACTICAL ABSORPTION REFRIGERATION SYSTEM......................................................................... 7
2.3 THERMODYNAMICS .......................................................................................................................... 10
2.3.1 CONDESER .................................................................................................................................. 10
2.3.2 HEAT EXCHANGER...................................................................................................................... 10
2.3.3 EXPANSION VALVE ..................................................................................................................... 10
2.3.4 EVAPORATOR ............................................................................................................................. 10
2.3.5 ABSORBER .................................................................................................................................. 11
2.3.6 SOLUTION PUMP........................................................................................................................ 11
2.3.7 GENERATOR ............................................................................................................................... 12
2.3.8 CALCULATION OF COP ............................................................................................................... 13
2.4 PHYSICAL CHARACTERISTICS OF THE REFRIGERATION SYSTEM ........................................................ 14
SECTION 3. DESIGN AND CONSTRUCTION ......................................................... 15
3.1 CONSTRUCTION DETAILS .................................................................................................................. 15
3.1.1 REFRIGERANT CYCLE ................................................................................................................... 15
3.1.2 ELECTRICAL WIRING ................................................................................................................... 18
3.1.3 INSTRUMENTATION AND CONRTOL ........................................................................................... 18
3.2 PROJECT COST ESTIMATES ................................................................................................................ 19
SECTION 4. CONCLUSION.......................................................................................... 20
4.1 CONCLISION........................................................................................................................................ 20
4.2 REFERENCES ....................................................................................................................................... 21
Page 2 of 21
THE HYBRID FRIDGE
2011
SECTION 1: INTRODUCTION
1.1 ABSTRACT
The project sets out with an objective to solve refrigeration challenges encountered in the
agricultural sector in Kenya. It presents an easy and sustainable refrigeration system that can be
easily adopted by fishermen, vegetable farmers among others to increase productivity and
reduce waste.
The absorption fridge is unique type of refrigerator that uses thermal compression as compared
to the conventional mechanical compression. It uses low quality heat, such as engine exhaust
heat or solar heat, to provide energy for the refrigeration system. The refrigerant (ammonia in
this case) is alternatively absorbed in the absorber and liberated from an absorbent medium in
the generator.
This project presents a hybrid refrigerator that runs on two sources of heat: solar and electric
heat. In the model presented, solar heat is harvested by heating circulating water that
consequently heats the refrigerant. Electric heating is used in the system to fine tune and act as
a backup, increasing reliability, accuracy and sustainability.
A simple system, comprising of an evaporator, absorber, generator, pump and condenser, is
introduced to explain the principle of operation and the refrigeration cycle. Further, an
advanced model is presented that takes care of refrigerant rectification necessary to make the
system practical and more efficient.
A thermodynamic analysis is carried out for the whole system by taking flow across each
component. Mass and energy balance are considered for the thermodynamic study.
Estimations are made in the system pressures, making it possible for the pump rating needed to
be estimated as 145W. A coefficient of performance calculation is also presented.
In the design and construction consideration, using the Reynolds analogy, fluid flow rate is
estimated at 0.1kg/s and hence used to the refrigeration capacity as 7kW. This is enough
refrigeration muscle to theoretically freeze 100kgs of beef from 20 oC to -5oC in two hours,
assuming perfect insulation.
Construction parameters, both physical and electrical are proposed for optimum performance.
In conclusion, financial considerations are made to estimate the cost of the system at Ksh
17,000.
Page 3 of 21
THE HYBRID FRIDGE
2011
1.2 OBJECTIVE
In fabricating a hybrid refrigerator using ammonia absorption, this project seeks to solve the
refrigeration challenges faced by majority of farmers who operate on small scale. These may
include: fishermen, vegetable farmers and vendors, flower famers and small scale butcheries
among others.
Kenya, being primarily an agriculture-based economy, needs a lot of refrigeration to increase
productivity and reduce wastage. This project thus fills in the conspicuous gap in the agriculture
chain of production.
The refrigerator considered in the design is small (about 50×50×100 cm) and is thus considered
sizable for domestic and subsistence use.
Finally, this project seeks to reduce the cost of refrigeration. With little operating costs, and
good sustainability, refrigeration can be considered universal.
Other applications of the hybrid refrigerators may include:
ο‚·
ο‚·
ο‚·
rural refrigeration of medicine in hospitals that operate far away from the national
grid,
use by the UN to refrigerate vaccination medicine in the hotter parts of the country
e.g., northern Kenya,
preservation of laboratory supplies in secondary schools and research institutes in
parts of the country not served by the national grid
Page 4 of 21
THE HYBRID FRIDGE
2011
1.3 INTRODUCTION
Ammonia refrigeration: the hybrid
A refrigerator can be defined as a heat pump that absorbs heat from one part (refrigerated
space) and liberates that heat to the environment.
Ammonia absorption is an old type technique used in refrigeration. It was most famous in the
United States, where it was used to make ice boxes that were used for camping as well as for
domestic use. Its operation was however intermittent, inaccurate and unreliable.
This project presents an innovative and interesting dimension by making the system hybrid.
Thus it can use both electric power as well as solar heat, for more reliability, smooth continuity
and sustainability. Electric power can either be battery and photovoltaic cell powered, or grid
powered, while the solar heating is harvested by circulating water.
Environmental and economical impact:
As the world becomes more self-aware of the changing climatic conditions caused by global
warming, it is vital to reassess our dependence on the burning of fossil fuels to gain energy. This
reassessment, spear headed by international environmental agencies such as UNEP, has to be
complemented by scientists, technologists and engineers. One such way, is in the use of solar
heat for refrigeration.
Kenya, being largely an agricultural country, has high demand for refrigeration, with
applications ranging from fishing, dairy, horticulture, and vegetable farming among others. This
need for refrigeration is indeed a challenge for the rural small scale farmers, who have little
turnover and limited access to the national electricity grid. However, the presence of abundant
solar heat presents a ready source of energy for refrigeration.
This project thus seeks to hit the proverbial two birds with one stone. On one hand, it seeks to
use a non pollutant refrigerant (ammonia), while using the solar heat as the source of energy,
and on the other hand it solves refrigeration problems for farmers, thus impacting positively on
the economy of our country.
Page 5 of 21
THE HYBRID FRIDGE
2011
SECTION 2: OPEATION AND THERMODYNAMICS
2.1 PRINCIPLE USED IN AN ABSORPTION REFRIGERATION SYSTEM
An absorption refrigerator is one that uses a heat source, such as solar heat, to provide energy
to the cooling system. The refrigerant is alternatively absorbed and liberated from an absorbent
medium. The refrigerant used in this project is ammonia while the absorbent is water.
Referring to the system illustrated in figure 1, ammonia leaves the evaporator at low pressure
at state 1 and enters the absorber. The ammonia is rigorously absorbed in the weak solution of
ammonia and water. Heat released during the process is dissipated to the circulated cooling
water and the solution is maintained at a constant temperature.
The liquid which leaves the absorber at stage 2 is high in ammonia concentration. This binary
liquid mixture is compressed by a pump to state 3. At the generator, heat from the sun warms
the liquid to saturation temperature and drives out of the solution the ammonia. The nearly
pure ammonia gas passes to the condenser at stage 6 where it is cooled to liquid. The liquid
ammonia then expands through the throttle valve and flow to the evaporator, where it
evaporates producing the refrigeration effect. Vapor ammonia from the evaporator at stage 1
eventually returns to the absorber. The weak solution, left in the generator after ammonia is
driven out, flows through the regulating valve which drops the pressure to that in the absorber.
It mixes with the solution remaining in the absorber and the cold ammonia vapor coming from
the evaporator.
Page 6 of 21
THE HYBRID FRIDGE
Heat removed from system
2011
Solar heat added to the system
6
3
7
4
5
8
1
2
Heat absorbed from surrounding,
producing the refrigeration effect
Cooling water circulating as
heat is rejected from system
Figure 1.
This system is simplified, since an ammonia-water system needs to have a rectification column
and a dephlegmator (reflux condenser) to take care of water that is undesirably evaporated in
the generator.
2.2 A PRACTICAL ABSORPTION REFRIGERATION SYSTEM
This is similar to the previously described absorption refrigeration system with some
rectification to achieve 99.9% ammonia at the entry to the condenser. In addition, two heat
exchangers are introduced to improve overall efficiency and minimize heat loss from the
system. Some observatory instruments are added to the system for study, though these may be
omitted in a commercial version of the refrigeration system.
Figure 2 illustrates the practical version of the absorption refrigeration system.
Page 7 of 21
THE HYBRID FRIDGE
2011
Cooling water cools
dephlegmator
Heat rejected
Solar heating
Cooling water circulates
cooling the absorber
Heat absorbed from the surrounding
produces the refrigeration effect
Figure 2
At stage1, vapor ammonia enters the condenser and is cooled while at high pressure to liquid
ammonia at stage 2. It then passes through the heat exchanger 1, where it is sub cooled as it
exchanges heat with cool ammonia from the evaporator. The liquid ammonia at stage 3 then
Page 8 of 21
THE HYBRID FRIDGE
2011
expands in the expansion valve, where pressure drops to the evaporator pressure at stage 4.
Once in the evaporator, the ammonia evaporates absorbing heat from the surrounding, and
producing the refrigeration effect. This evaporated ammonia, at stage 5 passes through the
heat exchanger 1 where it gets super heated to stage 6 and ends up in the absorber.
In the absorber, the super heated ammonia from the evaporator mixes with the weak solution
from the generator, in an exothermic reaction, to form a high concentration ammonia solution.
Cooling is needed in the absorber to maintain the temperature as the absorption produces
heat.
The strong solution from the absorber at stage 7 is pumped to higher pressure at stage 8 and
enters the heat exchanger 2. In this stage, the strong solution from the pump is pre heated
while the hot weak solution from the generator gets cooled. This is a measure taken to reduce
heat loss from the system, and increase efficiency.
The pre heated solution at stage 9 ends up in the rectification column connected to the
generator. In the generator, ammonia refrigerant is generated and as it rises, it gets in contact
with the descending pre heated strong solution. While in contact, the high concentration
ammonia gets more enriched from strong solution. The vapor ammonia then emerges at stage
13, and enters the second stage of the rectifier. At this stage, the ammonia vapor contains 510% water vapor. In the dephlegmator, the water vapor present is cooled by cooling water and
it descends resulting in high concentration of pure ammonia. Proper design of the rectifier
should achieve 99.9% ammonia. The rectifier is made in the form of a packed bed, a spray
column or a perforated plate column. This pure ammonia then moves to stage 1 and the circuit
is complete.
After evaporation, the weak solution left in the generator flows out at stage 10, through the
heat exchanger 2, to stage 11. The weak solution at high pressure passes through the expansion
valve to drop the pressure to absorber pressure at stage 12.
The [P] and [T] symbols in the figure 2, show the position of pressure and temperature gauges
for the purpose of monitoring and data measurement.
Page 9 of 21
THE HYBRID FRIDGE
2011
2.3 THERMODYNAMIC ANALYSIS
In thermodynamic analysis, the principle of mass and energy balance is taken across each
component. Each component is to be considered as a control volume while heat and work
interaction are taken into account. Further, an assumption that unit mass of ammonia and unit
mass of water are in circulation is to be made. This simplifies calculation.
1. Condenser
In the condenser, only ammonia refrigerant is present.
Mass balance:
π‘šΜ‡1 = π‘šΜ‡2 = π‘šΜ‡
Where π‘šΜ‡= mass flow rate of the refrigerant
Energy balance:
𝑄𝐢 = π‘šΜ‡(β„Ž1 βˆ’ β„Ž2 )
Where 𝑄𝑐 = heat rejected by generator, while
β„Ž1 , β„Ž2 = enthalpy at stages 1 and 2.
In the actual situation, the values of pressure and temperature are measured and used to
determine enthalpy and heat rejected.
2. Heat Exchanger 1
Mass balance:
π‘šΜ‡ = π‘šΜ‡2 = π‘šΜ‡3
π‘šΜ‡ = π‘šΜ‡5 = π‘šΜ‡6
Since the two fluids do not mix, there are two mass balance equations
Energy balance:
π‘„β„Žπ‘₯ = π‘šΜ‡(β„Ž2 βˆ’ β„Ž3 ) = π‘šΜ‡(β„Ž5 βˆ’ β„Ž6 )
I.e. assuming no heat loss in the heat exchanger
3. Expansion valve
Mass balance:
π‘šΜ‡3 = π‘šΜ‡4 = π‘šΜ‡
Energy balance:
β„Ž3 = β„Ž4
4. Evaporator
In the evaporator, only ammonia refrigerant is present.
Mass balance:
π‘šΜ‡4 = π‘šΜ‡5 = π‘šΜ‡
Energy balance:
𝑄𝑒 = π‘šΜ‡(β„Ž4 βˆ’ β„Ž5 )
i.e. only ammonia refrigerant is in the evaporator.
Page 10 of 21
THE HYBRID FRIDGE
2011
Where 𝑄𝑒 is the heat absorbed producing the refrigeration effect. The values of pressure and
temperature are used to determine the enthalpy and heat absorbed.
5. Absorber
In the absorber, ammonia vapor mixes with weak solution from the generator.
π‘šΜ‡ + π‘šΜ‡π‘€π‘  = π‘šΜ‡π‘ π‘ 
Mass balance:
Where π‘šΜ‡π‘€π‘  , is the weak solution mass flow rate (assumed to be water flow rate) π‘šΜ‡π‘ π‘  , is the
strong solution flow rate (summation of the water and ammonia flow rates)
Energy balance:
π‘„π‘Ž = Heat into the absorber – Heat out of absorber
π‘„π‘Ž = π‘šΜ‡β„Ž6 + π‘šΜ‡π‘€π‘  β„Ž12 + π‘šΜ‡π‘ π‘  β„Ž7
Where π‘„π‘Ž = heat rejected by evaporator
6. Solution Pump
Mass balance:
π‘šΜ‡7 = π‘šΜ‡8 = π‘šΜ‡π‘ π‘ 
Energy balance:
π‘Šπ‘ = π‘šΜ‡π‘ π‘  (β„Ž8 βˆ’ β„Ž7 )
Where π‘Šπ‘ = is the work done by the solution pump.
However pump work needs to be calculated before the actual absorption refrigerator is made.
An assumption is made that the solution is incompressible; hence the following alternate
formula is used;
8
π‘Šπ‘ = βˆ’ ∫ 𝑣 𝑑𝑝 = (𝑃𝑐 βˆ’ 𝑃𝑒 )𝑣𝑓
7
It is noted that:
𝑃8 = 𝑃𝑐 = Condenser pressure
𝑃7 = 𝑃𝑒 = Evaporator pressure
Assuming that the flow rate 𝑣𝑝 =100 milliliters/s = 1×10-4 m3/s
The typical maximum pressure in an ammonia absorption refrigerator is 1980kPa and minimum
pressure is 480kPa, therefore the power rating of the solution pump is calculated as follows:
π‘Šπ‘ = (𝑃𝑐 βˆ’ 𝑃𝑒 )𝑣𝑝 = (1930 βˆ’ 480) × 103 × 0.0001 = 145 π‘€π‘Žπ‘‘π‘‘π‘ 
Therefore a pump with an output of 145watts is to be selected.
Page 11 of 21
THE HYBRID FRIDGE
2011
7. Heat Exchanger 2
Mass balance:
π‘šΜ‡ = π‘šΜ‡8 = π‘šΜ‡9
π‘šΜ‡ = π‘šΜ‡10 = π‘šΜ‡11
Since the two fluids do not mix, there are two mass balance equations
Energy balance:
π‘„β„Žπ‘₯ = π‘šΜ‡(β„Ž8 βˆ’ β„Ž9 ) = π‘šΜ‡(β„Ž11 βˆ’ β„Ž10 )
I.e. assuming no heat loss in the heat exchanger
8. Generator , Dephlegmator and Rectification column
A control volume is taken across all these components to simplify analysis.
Qd heat rejected at the dephlegmator
1
9
10
Qg heat input at the generator
Mass balance:
π‘šΜ‡π‘ π‘  = π‘šΜ‡π‘€π‘  + π‘šΜ‡
Energy balance:
Qg βˆ’ Qd = mΜ‡ws h10 + mΜ‡h1 βˆ’ mΜ‡ss h9
Since 𝑄𝑔 is needed in the calculation of coefficient of performance (COP) it is assumed that 𝑄𝑑
10 - 20% of𝑄𝑔 .
Page 12 of 21
THE HYBRID FRIDGE
2011
9. Calculation of COP
COP =
refrigeration effect
Qe
=
work and heat input to the system Qg + Wp
Therefore, once the heat and work into the system are determined, as well as the refrigeration
effect, the practical coefficient of performance can be calculated.
RESULTS TABULATION
After the construction and testing of the refrigerator is complete, the following table is to filled
for mathematical analysis.
Point
Temperature
℃
Ammonia Concentration
[%𝑁𝐻3 ]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Page 13 of 21
Pressure
(bars)
Enthalpy
[π‘˜π‘—β„π‘˜π‘”]
THE HYBRID FRIDGE
2011
2.4 PHYSICAL CHARACTERISTICS OF THE REFRIGERATION SYSTEM.
As referred from ASHRAE REFRGERATION BOOK 2006, the following are the typical
temperatures achieved in typical absorption refrigerator:
Absorber exit temperature
: 40 oC
Generator exit temperature
: 85 oC
Condenser exit temperature
: 55 oC
Evaporator temperature
: -15oC
1. Flow characteristics:
The Reynolds number is given by the following expression:
𝑅𝑒 =
Where
Re: Reynolds number V: Velocity
𝑉𝐷
𝑣
D: Diameter
v: Kinematic viscosity
For laminar flow, the Reynolds number should be less than 600, and taking the kinematic
viscosity as 1.1×10-6 M2/s, and choosing pipes of 10mm internal diameter, a laminar velocity of
about 100mm/s is achieved.
2. Refrigeration load (capacity):
Using the proposed flow rate of 0.1kg/s, and specific and latent heat of ammonia being 4.448
kJ/Kg K and 1370kJ/kg respectively, the following calculations were made:
Therefore, the proposed refrigeration capacity to absorb heat to 0oC is:
π‘…π‘’π‘“π‘Ÿπ‘–π‘”π‘’π‘Ÿπ‘Žπ‘‘π‘–π‘œπ‘› π‘π‘Žπ‘π‘Žπ‘π‘–π‘‘π‘¦ = {(π‘šπ‘Žπ‘ π‘  π‘“π‘™π‘œπ‘€ π‘Ÿπ‘Žπ‘‘π‘’ × β„Žπ‘’π‘Žπ‘‘ π‘π‘Žπ‘π‘Žπ‘π‘–π‘‘π‘¦ × π‘‘π‘’π‘šπ‘π‘’π‘Ÿπ‘Žπ‘‘π‘’π‘Ÿπ‘’ π‘β„Žπ‘Žπ‘›π‘”π‘’)
+ (π‘™π‘Žπ‘‘π‘’π‘›π‘‘ β„Žπ‘’π‘Žπ‘‘ π‘œπ‘“ π‘£π‘Žπ‘π‘œπ‘Ÿπ‘–π‘ π‘Žπ‘‘π‘–π‘œπ‘› × π‘šπ‘Žπ‘ π‘  π‘“π‘™π‘œπ‘€ π‘Ÿπ‘Žπ‘‘π‘’)}
= {(0.1 × 4448 × 15) + (0.1 × 1370)π‘˜π‘Š
= 7π‘˜π‘Š
To put this refrigeration capacity into perspective, at 7kW refrigeration capacity, it is possible to
theoretically freeze 100 kilos of lean beef from 20oC to -5oC in about two hours.
Page 14 of 21
THE HYBRID FRIDGE
2011
SECTION 3: DESIGN AND CONSTRUCTION
3.1 CONSTRUCTION DETAILS
The pictures below show images of a typical absorption refrigeration system.
(Notice that ice forms on the evaporator)
3.1.1 THE REFRIGERANT CYCLE DESIGN
The refrigerant cycle consists of a generator, a rectifier, a condenser, an ammonia storage tank,
an evaporator, an absorber and a pump.
Page 15 of 21
THE HYBRID FRIDGE
I.
2011
The generator
The generator receives heat from two sources, solar and electric. The solar heat gets to the
system via heated water while electric heat is delivered directly using an electric coil heater.
The generator is made of an insulated high pressure aluminum vessel.
II.
The rectifier
The rectification column is made of an indented wide tube that allows evaporated water to
condense and trickle back to the generator. It should have enough length to allow complete
rectification.
III.
The condenser
The condenser is used to facilitate cooling of the generated vapor ammonia back to liquid state.
It is made of long finned tubes that increase the surface area for heat rejection to the
environment.
IV.
The ammonia storage tank
The ammonia storage tank is used to hold up the generated ammonia. Ammonia is then
gradually released (via the regulation valve) to the evaporator for refrigeration. This storage
tank should be well insulated to keep the ammonia in liquid state.
V.
The evaporator
The evaporator is made of thin walled and finned vessel that allows for maximum heat
absorption from the refrigerated space. With the right construction design, is should achieve
sub zero temperatures.
VI.
The absorber
The absorber column is made of a coiled tube and a perforated column that allow for maximum
mixture between ammonia and the weak solution from the generator. The reaction in the
absorber is exothermic; hence it is coiled for heat dissipation.
VII.
Heat exchangers
The system consists of two heat exchangers, one to pre heat and one to sub cool. The two heat
exchangers are designed as shell and tube, both in counter current to maximize the efficiency
of the system.
Page 16 of 21
THE HYBRID FRIDGE
A schematic representation of the ammonia refrigeration cycle.
Page 17 of 21
2011
THE HYBRID FRIDGE
2011
3.1.2 THE ELECTICAL WIRING
Electrical wiring takes care of electrical heating in the generator, pumping of the refrigerant,
circulation of the solar heated water and lighting inside the refrigerator.
I.
Electrical heating.
A small thermostat is used in series with the electrical heating, to maintain the temperature
inside the generator at a specified point. A simple immersion heater rated 500W, 240V should
be sufficient.
II.
Refrigerant Pump
As calculated in the thermodynamic analysis, the pump needed is rated at 145W. This pump
may be selected as one running on solar powered 12V D.C battery or the mains 240V. For this
particular project, the former will be selected for experimental purpose.
III.
Hot water Pump
A small pump is selected to facilitate the circulation of the solar heated water. Rating of water
pump may be varied, depending on the solar intensity. For preliminary design, the pump rating
is selected as one similar to the refrigerant pump, i.e. 145W solar powered by a 12V D.C
battery.
IV.
Internal Lighting
An electrical circuit is designed for the refrigerator cabinet lighting. As is in conventional fridges,
a door operated switch will be used to turn on the light whenever the door is open and vice
versa.
3.1.3 INSTRUMENTATION AND CONTROL
Temperature in the refrigerator is controlled by a regulator knob which controls the control
valve from the ammonia tank (the rate of ammonia flow determines the temperature).
Both the solar heated hot water pump and the electrical heater thermostat are controlled by
the level of ammonia in the storage tank. When ammonia has filled the tank, both the hot
water pump and the thermostat are turned off. When more ammonia needs to be generated,
both the pump and thermostat engage.
The circulation pump (responsible for pumping refrigerant round the loop) is controlled by the
level of ammonia in storage tank. When the tank is full, the pump is turned off to save power.
When the level of ammonia in the tank is low, the pump is turned on to generate some more.
Page 18 of 21
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2011
3.2 PROJECT COST ESTIMATES
The total cost and estimates are tabulated as follows:
ITEM
Body casing material
Insulating material
Ammonia
Valves and Tubes and piping
material
Electrical fitting
DESCRIPTION
ο‚· 2.5 m3 Steel sheets (3mm) for
exterior
ο‚· 2.5m3 plastic for interior
ο‚· Miscellaneous body fittings
ο‚· Approximately 10Kgs of insulating
foam
ο‚· 5 liters of ammonia
ο‚· 5m Ungalvanized steel tubes ( ¼
and ½ inch),
ο‚· 3 stainless steel valves,
ο‚· 3 two liter steel vessels (for
ammonia storage absorber and
generator)
ο‚· miscellaneous plumbing material
ο‚· 150W rated 12V D.C. motor pump
( 2 pieces),
ο‚· heating coil and thermostat (for
electrical heating),
ο‚· 12V solid state battery with
photovoltaic cell charger.
ο‚· Electrical wiring, lighting and
miscellaneous
COST ESTIMATE
3000
TOTAL
1000
2000
4000
7000
17,000
The above project cost estimates are grossly approximated and are subject to change.
However, they represent a rough idea on the total cost of the system construction.
The project will be funded partly by my personal sources, and complimented by external
sources.
The time frame from start to completion of the whole project is estimated at two to three
months.
Page 19 of 21
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2011
SECTION 4: CONCLUSION
4.1 CONCLUSION
The Kenyan economy is said to be 60 – 70 % agricultural based. Innovation should thus be
focused on making the agriculture sector more productive and efficient, for faster realization of
vision 2030.
The project has the objective to solve refrigeration challenges faced by farmers in Kenya. It
takes advantage of the abundant solar energy in our country’s climate. Running primarily on
heat from the sun and using ammonia refrigerant that is non pollutant, the hybrid refrigeration
system has developed a solution to solve many refrigeration challenges.
The refrigeration system primarily uses solar heat to supply most of the energy, with electric
heating stepping in to fine tune the system, making it more reliable and accurate. The system
incorporates a rectifier that should theoretically attain 99.9% ammonia. In addition heat
exchangers are used to increase efficiency. Further, calculations are made that estimate the
pump rating at 145W and a refrigeration capacity of 7kW. A financial projection reveals a total
cost of approximately Ksh. 17,000.
As is with any project, this one has both advantages and disadvantages. On the down side, the
project relies on solar heat for refrigeration. This means that in absence of a source of heat, the
refrigeration uses electricity which may not be available or uneconomical. In addition, the
hybrid fridge uses a solar heater making it quite cumbersome for domestic installation. On the
up side, the refrigerant uses ammonia which is ozone friendly. Further, in the presence of
abundant solar energy, as is the case in Kenya, the refrigeration system is cheap and practical.
With almost negligible operating costs, refrigeration is considered universal for higher
productivity and less wastage in the agricultural sector.
In conclusion, the hybrid fridge is recommended for fabrication and testing to study its
performance, and subsequent feasibility for production.
Page 20 of 21
THE HYBRID FRIDGE
2011
4.2 REFERENCES
[1]
Dr. J. Nyang’aya, Department of Mechanical Engineering- University of Nairobi.
Lectures on REFRIGERATION CYCLES
[2]
Indian Institute of Technology; AIR CONDITIONING AND REFRIGERATION Lectures
14,15,16,17. By Prof. M. Ramgopal, Department of Mechanical Engineering.
[3]
Indian Institute of Technology Madras- REFRIGERATION CYCLES. By Prof U.S.P Shet,
Prof T. Sundrarajan and Prof. J.M. Mallikarjuna
[4]
International Sorption Heat Pump Conference June 22-24, 2005; Denver, C.O, USA
ABSORPTION REFRIGERATION- AN EFFICIENT WAY TO USE WASTE HEAT AND SOLAR
HEAT.
[5]
ASHRAE refrigeration Encyclopedia editions on: Fundamentals (2009), HVAC Systems
and Equipment (2008), HVAC Applications (2007), Refrigeration (2006).
[6]
MODERN REFRIGERATION AND AIR CONDITIONING-18TH EDITION-1211
[7]
World academy of Science, Technology and Engineering; HEAT EXCHANGER DESIGN
[8]
THERMOPHYSICAL PROPERTIES OF AMMONIA-WATER SOLUTIONS FOR THE DESIGN OF
ABSORPTION REFRIGERATION EQUIPMENT (formulation for industrial use)
M.Conde Engineering, Zurich, Germany.
[9]
California Energy Commission- OPTIMIZATION OF SOLAR ABSORPTION CHILLERS
[10]
Janilson Arcangelo Rossa and Edson Bazzo: Federal University of Santa Catarina Brazil,
Department of Mechanical Engineering
THERMODYNAMIC MODELING OF AN AMMONIA-WATER ABSORPTION SYSTEM
ASSOCIATED WITH A MICROTURBINE
[11]
DESIGN AND RATING SHELL AND TUBE HEAT EXCAHNGERS; By John E. Edwards
Page 21 of 21