solar powered correction - print
TRANSCRIPT
CHAPTER ONE
1.0 INTRODUCTION
1.1 BACKGROUND OF STUDY
In everyday life, we consume a tremendous amount of energy; our lives are styled around
consumption, consumption of natural resources and consumption of energy. The bulk of our
energy consumption goes on space heating (58%) this is something that can easily be provided
for with passive solar design. Next is water heating, which requires 24% of the energy which we
use. So already we have seen that we can meet 82% of our energy needs with solar technologies.
The next 13% of our energy is use to provide electrical power for lights and home. The
remaining 5% is all used for cooking. All this energy consumption are for people who live in the
“developed world”. Therefore, all one energy needs can be met with solar technologies.
Solar energy is clean, green, free, and best of all, isn’t going to be going anywhere for about the
next 5 billion years. Looking at North America for example, we can see that there is a real solar
energy source. while the majority of this is concentrated in the West, there is still enough solar
energy to be economically exploited in the rest of the U.S.
At present, the bulk of our energy comes from fossil fuels- gas, coal and oil. Fossil fuels are
hydrocarbons, that is to say that if we look at them chemically, they are wholly composed of
hydrogen and carbon atoms. Hydrocarbons, when combined with the oxygen in the air and heat,
they react exothermically (they give out heat). This heat is useful, and is used directly as a useful
form of energy in itself, or is converted into other forms of energy like kinetic or electrical
energy that can be used to “do some work”, in other words, perform a useful function. (Gavin D.
J. Harper, 2007).
1
The many of the different types of energy sources around us actually come from the sun and
solar-driven processes. They include:
Fossil fuels – are a result of plant and animal matter from millions of years ago. This plant
matter was formed as a result of solar energy falling on to the earth–so fossil fuels are essentially
sequestered solar power.
Solar energy − direct solar energy is power that we can harness here and now as a result of the
sun falling on the solar powered devices.
Hydro-electric power − the hydrological cycle takes water from the ground and deposit as rain.
Some of this rain ends up at a high ground level. Its fall to lower ground can be used to generate
power. The hydrological cycle is driven by the sun.
Wind energy – wind turbines can be used to pump water or generate electricity. The movement
of air from an area of high pressure to an area of low pressure is a process which is driven by the
sun heating the air and causing it to become less dense.
Wave-power – is created by the wind blowing on the surface of the water. As the wind is a sun
driven process, so is wave power.
Biomass – is the term given to plants which we can burn as fuel. The sun provides the energy to
grow. This plants utilize the energy through process of photosynthesis.
1.1.1 Historical Background
In the developed countries, plug-in refrigerators with backup generators store vaccines safely,
but in developing countries, where electricity supplies can be unreliable, alternative refrigeration
technologies are required. Solar fridges were introduced in the developing world to cut down on
2
the use of kerosene or gas-powered absorption refrigerated coolers which are the most common
alternatives. They are used for both vaccine storage and household applications in areas without
reliable electrical supply because they have poor or no rigid electricity at all. The kerosene
absorption refrigerated coolers burn a liter of kerosene per day therefore requiring a constant
supply of fuel which is costly and smelly, and are responsible for the production of large
amounts of carbon dioxide. They can also be difficult to adjust which can result in the freezing of
the medicine. There are two main types of solar fridges that have been and are currently being
used, one that uses a battery and more recently, one that does not. (Wikipedia, 2010).
As a result of the projected world energy shortage, the use of solar energy for environmental
control is receiving much attention in the engineering sciences literature. Cooling system is a
particularly attractive application for solar energy because of the near coincidence of peak
cooling loads with the available solar power.
Of the cooling system alternatives, the adsorption system appears to be one of the most
promising methods. Many arrangements or cycles are possible: solar collectors can be used to
provide energy for adsorption cooling, desiccant cooling, and Rankine-vapour compression
cycles. Solar refrigeration cooling systems are also possible. Although a large potential market
exists for this technology, existing solar cooling systems are not competitive with electricity-
driven or gas-®red air- conditioning systems because of their high first costs. Lowering the cost
of components and improving their performance could reduce the cost of solar cooling systems.
Improvements such as reduced collector area, because of improved system performance, and
reduced collector cost will lower the cost of solar components. Several solar driven refrigeration
systems have been proposed and are under development such as sorption systems including
liquid/vapor, solid/vapor absorption, adsorption, vapour compression and photovoltaic vapour
3
compression systems. Most of the above mentioned systems have not been economically
justified. (Kum Kin Chun, April 2007).
1.1.2 Environmental Impacts Of Refrigerators
There is major environmental concern regarding conversion refrigerator technology including
contribution to ozone layer depletion and global warming. Refrigerators which contain ozone
depleting and global warming substances such as chlorofluorocarbons (CFC’s), in their
insulation foam or their refrigeration cycle are the most harmful. After CFC’s were banned in
the 1980’s they were replaced with substances such as hydro fluorocarbons –(HCFC’s), which
are ozone depleting substances and hydro fluorocarbons (HFC’s). Both are environmentally
destructive as potential global warming chemicals. If a refrigerator is inefficient it will also
contribute to global warming. The use of solar energy to power refrigerators have on the history
of solar refrigeration. (Solar Powered Refrigerator-Wikipedia, the free encyclopedia.mht).
1.1.3 Solar Collector
The solar collector used in this project is a concentrating type. Concentration of the sun
radiation becomes necessary when higher temperatures are desired with high efficiency. The
concentrating collector is a parabolic trough which consists of a linear parabolic reflector that
concentrate light onto a receiver positioned along the reflector’s focal line. The receiver is tube
positioned directly above the middle of the parabolic mirror and is filled with a working fluid.
The generator is a stainless steel pipe positioned at the focus of a parabolic trough collector. The
generator is positioned so that only daily tracking of the collector is required. During
construction, compound adsorbent of calcium chloride and activated carbon is placed in the
generator, which is then capped closed. Pure (anhydrous) ammonia obtained in a pressurized
4
tank is allowed to evaporate through a valve unto the generator and is absorbed by the salt
molecules, forming calcium chloride-ammonia solution (CaCl2-8NH3).
The generator is connected to a condenser made from a coiled pipe. The coil is immersed in a
water bath for cooling. The condenser pipe descends to the evaporator/connecting tank, situated
in an insulated box where ice is produced. (J. Vanek, M. Green and S. Vanek. 1996).
1.2 PROBLEM STATEMENT
The main technical problem of solar refrigeration is that the system is highly dependent upon
environmental factors such as cooling water temperature, air temperature, solar radiation, wind
speed and others. On the other hand, its energetic conversion efficiency is low, and from an
economical point of view, solar cooling and refrigeration are not competitive with the
conventional systems.
In order to evaluate the potential of the different solar cooling systems, a classification has been
made by Best and Ortega. It is based on two main concepts: solar thermal technologies and
technologies for cold production. The solar technologies relevant are:
a) flat-plate collectors
b) evacuated tube collectors
c) stationary non-imaging concentrating collectors
d) dish type concentrating collectors
e) linear focusing concentrators
f) solar pond
g) photovoltaic and
h) thermoelectric systems.
5
The cooling technologies are:
a) diffusion
b) adsorption
c) absorption
d) desiccant systems
e) thermoelectric cooler.
Among all these technologies, the solar technology used is the linear focusing collector while the
cooling technology is adsorption. Design and construction of a solar powered ice-maker system
also known as solar refrigerator system had been selected for my final year project. A solar
powered ice-maker had been designed and constructed. This system work intermittently to
produce ice and cool perishable product like vegetables, meat, dairies etc. It can also be used to
store vaccines in villages and remote areas where electricity supply is not available.
1.3 JUSTIFICATION
Solar powered icemaker also called solar refrigerator is of great importance most to the
developing countries of the world like Nigeria and particularly in remote villages where supply
of electricity is unreliable or non-existent to keep perishable goods such as vegetables, meat and
dairy cool. This refrigerator can also provide ice cold water and keep much needed vaccines at
their temperature to avoid spoilage.
Electricity consumption used for refrigeration is reduced, hence saving money and reducing the
stress on our electricity generation and distribution network.
The system does not require any moving any moving parts like in the vapour compression
refrigerator system. It has three main components such as the adsorber/generator, condenser and
6
evaporator which greatly reduces the working load of the system. Therefore, cooling is achieved
easily.
1.4 AIMS AND OBJECTIVES
The objectives of the project includes the following :
1. To build cost effective solar powered ice-maker using a parabolic trough collector with
no moving parts.
2. To reduce the consumption of electricity used for refrigeration, hence saving money and
reducing the stress on our electricity generation and distribution networks.
1.5 SCOPE AND LIMITATIONS
The scopes of the project are as follows:
1. To study the solar adsorption system.
2. To design the system and test it experimentally.
3. To improve on the efficiency of the solar powered ice-maker using stainless steel pipe for
the generator/adsorber since it is resistant to corrosion, because ammonia react with
certain metals that are corrosive in nature, thereby reducing the system’s efficiency.
The limitations include:
1. Maintaining the system’s pressure in excess of 1,380 kPa without leaks or rupture, that is,
all the piping and fittings used in the system are to withstand a constant minimum
pressure of 1,400 kPa.
2. The collector area is reduced because of improve system performance, and reduced
collector cost which lowers the cost of solar components. Hence, the solar energy
7
collected by the collector was not sufficient enough to heat the entire generator to the
required temperature.
3. Environmental factors such as cloud cover, cooling water temperature, air temperature,
solar radiation, wind speed and others are the main technical problem in the course of the
experiment since the system is highly dependent upon it.
1.6 APPLICATIONS AND UTILIZATIONS
1.6.1 UTILIZATIONS
1. Thermodynamics utilization cycle.
2. Direct conversion to electricity.
1.6.2 APPLICATIONS
1. Water distillation
2. Hot water heating
3. Swimming pool heating
4. Crop drying
5. House heating
6. Gas-absorption cooling/refrigeration
7. Evaporation cooling
8. Irrigation pumping
9. Industrial process heat
10. Solar cooling
11. Electric power generation
12. Photovoltaic power
8
In conclusion, 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. The alternatives for gaining this energy can be found in the sources of renewable energy
such as solar, wind, biomass, wave and tide, etc. In particular, the solar energy alternative is now
being more closely examined in an attempt to utilize this as a source of energy for both domestic
and commercial end users such as refrigerators, air conditioners, hot water heaters, desalination
for water recycling, etc. In this project, the adoption of solar energy as the primary source of
power for an intermittent adsorption refrigeration system is considered.
9
CHAPTER TWO
2.0 LITERATURE REVIEW
2.1 HISTORY OF SOLAR POWERED REFRIGERATION
The solar-powered refrigerators are most commonly used in the developing world to help
mitigate poverty and climate change. By harnessing solar energy, these refrigerators are able to
keep perishable goods such as meat and dairy cool in hot climates, and are also used to keep
much needed vaccines at their appropriate temperature to avoid spoilage. The portable devices
can be constructed with simple components and are perfect for areas of the developing world
where electricity is unreliable or non-existent.
The interest in adsorption systems first started to increase due to the oil crisis in the 1970s, and
then later, in the 1990s, because of ecological problems related to the use of CFCs and HCFCs as
refrigerants. Such refrigerants, when released into the atmosphere, deplete the ozone layer and
contribute to the greenhouse effect. Furthermore, with the increase in energy consumption
worldwide, it is becoming even more urgent to find ways to use the energy resources as
efficiently as possible. Thus, machines that can recover waste heat at low temperature levels such
as adsorption machines can be an interesting alternative for wiser energy management.
(Wikipedia, 2010)
The conventional adsorption cycle may includes mainly two phases:
a) Adsorbent cooling with adsorption process, which results in refrigerant evaporation
inside the evaporator and, thus, in the desired refrigeration effect. At this phase, the
sensible heat and the adsorption heat are consumed by a cooling medium, which is
usually water or air.
10
b) Adsorbent heating with desorption process (also called generation), which results in
refrigerant condensation at the condenser and heat release into the environment. The heat
necessary for the generation process can be supplied by a low grade heat source, such as
solar energy, waste heat, etc.
In comparison with mechanical vapour compression systems, adsorption systems have the
benefit of saving energy, if powered by waste heat or solar energy, simpler control, no vibration
and lower operation costs. In comparison with liquid absorption systems, adsorption systems can
be powered by a large range of heat source temperatures, starting at 50 C and going up to 600⁰
C or even higher. Moreover, the latter system does not need a liquid pump or rectifier for the⁰
refrigerant, does not present corrosion problems due to the working pairs normally used, and it is
less sensitive to shocks and to the installation position. These last two features make it suitable
for applications in locomotives, buses, boats and spacecrafts. Although adsorption systems offer
all the benefits listed above, they usually also have the drawbacks of low coefficient of
performance (COP) and low specific cooling power (SCP). However, these inconveniences can
be overcome by enhancing of the heat and mass transfer properties in the adsorber, by increasing
the adsorption properties of the working pairs and by better heat management during the
adsorption cycle. Thus, most research on this system is related to evaluation of adsorption and
physical-chemical properties of the working pairs, development of predictive models of their
behaviour in different working conditions, and the study of the different kinds of cycles. (Kum
kin chun. April 2007).
11
2.2 SOLAR ADSORPTION REFRIGERATION SYSTEMS FOR ICE-MAKING
Several adsorptive solar cooling units have been successfully tested by various authors in recent
years. One of the very effective forms of solar refrigeration is the production of ice, because, ice
accumulates much latent heat, thus the volume of icemaker can be small.
Solid adsorption refrigeration makes use of the unique features of certain adsorbent-refrigerant
pairs to complete refrigeration cycles for cooling or heat pump purposes. Zeolite and activated
carbon were used as adsorbents in many systems.
(Critoph R.E., 2007) had studied the performance limitations of adsorption cycles and refrigerant
absorbent pairs for Solar cooling and concluded that in general, activated carbon-methanol
combination was preferable for solar cooling, which giving the best COP achievable in a single-
stage cycle. He mentioned a solar vaccine refrigerator studied in his laboratory in the early
1990s. Such machine, could maintain the cold box at 0 C during the daytime, after one⁰
adsorption cycle, performed during the previous night. According to this author, although the
COP and ice production of this machine (which used the pair activated carbon-ammonia), was
less than those produced by a machine with the pair activated carbon-methanol, the former is less
sensitive to small leakages, which makes it more reliable for application in remote areas where
maintenance is not readily available.
(Wang R.Z et al, 2007) performed experiments with a solar powered ice maker that had activated
carbon-methanol as working pair. This icemaker, had a COP ranging from 0.12 to 0.14, and
produced between 5 and 6 kg of ice per m^2 of collector. Analyzing the temperature gradient
within the adsorbent bed, the authors concluded that in order to improve the performance of this
system, the heat transfer properties of the adsorber must be enhanced. This could be achieved by
increasing the number of fins or using consolidated adsorbent.
12
(Wang et al., 2006) developed a consolidated compound adsorbent for adsorption ice maker on
fishing boats , made from a mixture of CaC12 and activated carbon. Experiments performed by
these authors showed that the utilization of this compound could lead to a cooling density 35%
higher than that obtained by the use of powder CaC12.
(Dieng, A.O. et al, 2006) review the fundamental understandings of the solar adsorption systems
and give useful guidelines regarding designs parameters of adsorbent bed reactors, and the
applicability of solar adsorption both in air-conditioning and refrigeration with the improvement
of the coefficient of performance.
(K Sumathy, 2008) presents the description and operation of a solar-powered icemaker with solid
adsorption pair of activated carbon and methanol. A domestic type of charcoal is chosen as the
adsorbent , and a simple flat-plate collector with an exposed area of 0.92 m^2 is employed to
produce ice of about 4-5 kg/day. Also, it is intended to introduce a hybrid system consisting of
solar water heater and icemaker, which can satisfy the requirements of both the solar ice-making
needs as well as good heat collection and heat release in the adsorber.
2.3 TYPES OF REFRIGERATION SYSTEMS
The basic operations of an adsorption refrigeration unit involve freeing the refrigerant from its
bonds with the adsorbent material and then condensing it under pressure. This liquid refrigerant
is then evaporated by reducing its pressure in turn adsorbing heat from its surroundings and
creating cold. This cold is called refrigeration effect (RE) which is achieved in the evaporator.
There are two distinct types of adsorption refrigeration units, these are the intermittent and
continuously operating systems.
13
Intermittent operating system − The heat is only applied to the generator of the system once
per day. The application of heat separates the refrigerant from the adsorbent, condenses it and
then the liquid refrigerant is stored. These systems operate at a single pressure which self-
regulates the condensation and evaporation rates of the refrigerant. Once the internal pressure of
the system drops below the vapour pressure of the refrigerant it begins to evaporate. This in turn
increases the system pressure until the refrigerant combines again with the adsorbent material.
The stored refrigerant usually produces a cooling effect for approximately 12 to 18 hours at
which stage more heat is applied to the generating unit.
Continuously operating system − They are the same as the intermittent with the exception of
the critical components allowing the system to run on a continuous basis powered by a heat
source such as gas/solar/kerosene, etc. The configuration of a continuous system involves the
generator, condenser and evaporator the same as an intermittent system but also incorporates an
absorber positioned between the evaporator and generator. This additional component allows the
refrigerant to recombine with the adsorbent while the generator continues to operate. A bubble
pump which resembles a coffee percolator is also used in most designs to transport the weak
absorbent from the generator to the absorber to receive refrigerant which has completed the
circuit. This type of system also requires the use of hydrogen. This element is located in the
evaporator and helps the ammonia vaporize increasing the efficiency of the system.
The adsorption refrigeration system has lower COP compared to that of a vapour compression
system. The COP of adsorption refrigeration system can be determined by,
COP abs = refrigeration effect / rate of heat generation
14
2.4 OPERATION OF THE SOLAR POWERED ICE-MAKER
After several studies of the adsorption refrigeration and their working pairs, the solar powered
icemaker designed uses anhydrous ammonia as a refrigerant and calcium chloride as an
adsorbent. The adsorbent used enhances the heat and mass transfer properties in the adsorber, by
increasing the adsorption properties of the working pairs and by better heat management during
the adsorption cycle. The operation of the designed solar powered icemaker involves dissolving
anhydrous ammonia in calcium chloride as the adsorbent, in the generator of the sealed system
where the sun heats the solution. An icemaker like this could be use to refrigerate vaccines, meat,
dairy products, or vegetables. The icemaker uses free solar energy, few moving parts, and no
battery.
The icemaker operates in a day/night cycle, generating distilled ammonia during the daytime and
reabsorbing it at night. Ammonia boils out of the generator as a hot gas at about 200 psi (1,380
kPa) pressure. The gas condenses in the condenser coil and drips down into the storage tank
where, ideally, 3/4 of the absorbed ammonia collects by the end of the day (at 250 degrees
Fahrenheit, six of the eight ammonia molecules bound to each salt molecule are available).
As the generator cools, the night cycle begins. The calcium chloride reabsorbs ammonia gas,
pulling it back through the condenser coil as it evaporates out of the tank in the insulated box.
The evaporation of the ammonia removes large quantities of heat from the collector tank and the
water surrounding it. How much heat a given refrigerant will absorb depends on its “heat of
vaporization,” — the amount of energy required to evaporate a certain amount of that refrigerant.
Few materials come close to the heat of vaporization of water.
15
During the night cycle, all of the liquefied ammonia evaporates from the tank. Water in bags
around the tank turns to ice. In the morning the ice is removed and replaced with new water for
the next cycle.
16
CHAPTER THREE
3.0 SYSTEM DESIGN THEORY AND CALCULATIONS
3.1 INTRODUCTION
This chapter aims at putting together the governing theories surveyed and calculations based on
this theories to bring out the regained designed. Some of the theoretical concepts that are useful
in analyzing the performance of solar refrigerators is being discussed.
3.2 INDICES OF PERFORMANCE
Any solar cooling device essentially consists of two parts: a cooling unit employing a
thermodynamic cycle no different from that employed in conventional refrigerators, and a solar
heat source with a flat-plate or a focusing collector to operate it. In this project a focusing
collector was used, known as the parabolic trough collector. The usual index by which the
performance of a refrigerator is measured is the coefficient of performance which is defined as
the ratio of cooling produced to heat supplied. This same concept may be applied to the
refrigerator component and a cooling ratio may be defined as;
h eat absorbed by refrigerant during refrigerationh eat absorbed by generator contentsduring refrigeration
The performance of the solar collector can be defined by a heating ratio given by;
h eat absorbed by t h econtents of t h e generatorincident solar radiation on t h e collector
17
The overall performance ratio can now be defined as the product of the two above defined ratios,
or explicitly as;
h eat absorbed byrefrigerant during refrigerationincident solar radiation on t h ecollect∨¿¿
The concepts of heating ratio and cooling ratio are especially useful when analyzing systems
where the collector and generator are separate.
Energy is transferred in the form of heat at three temperature levels, i.e., - atmospheric
temperature T a, at which heat is rejected in the condenser and absorber, - the temperature at
which heat is taken from the cold chamber T c, - the temperature at which heat is received in the
generator T g.
It is possible to imagine an arrangement of reversible machines performing a function equivalent
to that of the adsorption plant, Fig. 3.1. Firstly, a reversible heat engine receives a quantity of
heat Q g, at a temperature T g, and rejects heat at a temperature T a while producing a quantity of
work W ga with an efficiency,
W ga
Qg
=T g−T a
T g
(3.1)
where all temperatures are measured on the thermodynamic temperature scale. Secondly, a
reversible refrigerator receives a quantity of heatQc at T c and rejects heat at T a while absorbing a
quantity of work W ca. The coefficient of performance of the refrigerator is
−Qc
W ca
=Tc
T a−T c
(3.2)
18
If W ga is made equal to −W ca this plant will be equivalent to an absorption refrigerator, The
coefficient of performance of the combined plant can be defined asQc /Qg, which on combining
the two previous expressions becomes,
Figure 3.1: Equivalent absorption maching (Wikipedia, 2010)
C . O . P=Qc
Q g
=T c ( T g−T a )T g (T a−T c )
(3.3)
The practical importance of this result is that if a C.O.P. for the cycle under consideration is
known T g may be calculated, since T a is fixed and T c is chosen by the designer.
3.2.1 Generator Pipe/Adsorber
The adsorber is the most important part of the solar powered icemaker and hence the
performance of the solar icemaker depends highly on the characteristics of the adsorber. A good
19
adsorber must have good heat and mass transfer. Recent research showed that the aluminium
alloy, mild steel, zinc, and other alloys have a strong catalytic effect on the decomposition
reaction under the solar refrigerator, therefore modification was made using stainless steel pipe
for the generator and for adsorbent heat transfer metal to the working fluid instead of aluminium
alloy. Since stainless steel is resistance to corrosion and ammonia attack, the efficiency of the
system can be improved. The generator act as a compressor in vapour compressor refrigerator
but being powered by the sun’s radiation. The generator is positioned at the focus of the
parabolic trough solar collector and contains the working pair of a mixture of ammonia and
calcium chloride salt.
3.2.2 Condenser
This is the part which receives hot high refrigerant vapour from the generator, reject the
desorption heat from the refrigerant until it returns to a liquid phase. For the solar icemaker the
condenser can be either air-cooled or water-cooled. But the water-cooled type was used in this
design to ensure quick condensation of the refrigerant vapour into liquid phase. The Condenser
was formed into a coil and placed into a container filled with water for heat dissipation by
condensing the vapour refrigerant.
Unlike the vapour compressor refrigeration cycle, condensation occurs when the vapour
refrigerant gives up its latent heat and is condensed to liquid refrigerant before it flows by gravity
into the evaporator. Pressure and temperature falls to a minimum. The temperature of the
refrigerant is reduced below its saturation temperature ( a phenomenon known as sub-cooling in
vapour compressor refrigerator).
20
3.2.3 Evaporator
The evaporator is located inside the icebox. The evaporator is the part that receives the
condensed refrigerant from the condenser and absorbs heat from the icebox compartment or
surrounding and vaporizes to produce the desired cooling effect. The temperature of the
vaporizing refrigerant in the evaporator must always be less than that of the surrounding or
compartment medium so that it flow to the refrigerant, by this, the evaporator becomes cold due
to the fact that, the temperature of the evaporator coil is low as a result of the low temperature of
the refrigerant in the coil, and the low temperature of the refrigerant remains unchanged since
any heat it absorbs is converted to latent heat as it boils. A copper pipe was shaped into the ice
box compartment as the evaporator.
3.3 WORKING PRINCIPLE OF SOLAR POWERED ICEMAKER
The solar powered icemaker consists of two components: a solar power unit and a refrigeration
unit. The refrigeration unit is an intermittent adsorption system. The intermittent refrigeration
cycle has two major operations.
1. Regeneration
2. Refrigeration
Regeneration is the process of heating the refrigerant-adsorbent fluid to drive off the refrigerant
vapour and condense the vapour in separate container.
Refrigeration takes place when the liquid refrigerant vaporizes, producing a cooling effect
around the evaporator. The refrigerant is re-absorbed by the adsorbent.
The solar powered icemaker has three main components being the combination collector
generator/adsorbent bed for heating the salt-ammonia mixture, condenser coil in water tank and
21
evaporator where distilled ammonia collects during generation. Other components includes the
insulating box as well as connecting pipes.
On a sunny day, the adsorbent bed absorbs solar desorption temperature, the refrigerant begins to
evaporate and desorbs from the bed. The desorbed refrigerant vapor will be condensed into
liquid via the condenser and flows into the evaporator directly; this desorption process lasts until
the temperature of adsorbent reaches the maximum desorption temperature. During night, when
the temperature of the adsorbent bed reduces, the refrigerant vapor from the evaporator gets
adsorbent back in the bed. During this adsorption process, the cooling effect is released from
refrigerant evaporation, and the ice is formed in the water tank placed inside thermal insulated
water box. In general, the performance of solar ice maker are represented in terms of Q radiation
energy, which raises the temperature of adsorbent bed as well as the pressure of refrigerant in the
adsorbent bed. When the temperature the adsorbent reaches the desorption temperature, the
refrigerant begins to evaporate and desorbs from the bed. The desorbed refrigerant vapor will be
condensed into liquid via the condenser and flows into the evaporator directly; this desorption
process lasts until the temperature of adsorbent reaches the maximum desorption temperature.
During night, when the temperature of the adsorbent bed reduces, the refrigerant vapor from the
evaporator gets adsorbent back in the bed. During this adsorption process, the cooling effect is
released from refrigerant evaporation, and the ice is formed in the water tank placed inside
thermal insulated water box.
In general, the performance of solar ice maker are represented in terms of Qref (or ice mass gotten
in water tank) and the performance efficiency Qsolar. They can be expressed as follows:
Qref=∆ x M a Le (3.4)
∆ x=xconc xdil (3.5)
22
Where xconc is the adsorption capacity before desorption, xdil is the adsorption capacity after
desorption, Ma is the mass of adsorbent inside adsorbent bed, Le is the latent heat of vaporization.
COPsolar=¿
Qref Qcc
∫ i ( t )dt(3.6 )¿
where Qcc = ∫T c
T e
M a∆xC pldT is the energy used to cool down the refrigerant liquid from
condensing temperature T c to evaporation temperature T e. ∫ i ( t ) dt is the total radiant energy
absorbed by the collector during one day operation.
3.4 THEORY AND EQUIPMENT SETUP
Figure 3.2 shows the P-T relation of a simple theoretical cycle, which involves two processes:
desorption−condensation and adsorption−evaporation. In the former process corresponding to
the lines from 1−3 in the diagram, the heat applied Q g involves the sensible heat Qsd for heating
adsorbate and adsorbent, respectively, and also the heat of desorption Qde.
Figure 3.2: P−T−X diagram for a basic cycle
At the same time, the desorbed adsorbate is condensed and its liquid is cooled to evaporating
temperature; it would cost Qce cooling load.
Q g = Qde+ Qsd (3.7)
23
Qde= −ma ∫x2
x1
H (x)dx (3.8)
Qsd= ∫T a 2
T g 1
mac va(T)dT+ ∫T a 1
T g 2
ma x2 cvm(T)dT +∫T g 1
T g 2
ma cpa(T)dT +∫T g 1
T g 2
ma x2 c pm(T )dT (3.9)
Qce= ∫T e
T c
ma δ x cva(T)dT (3.10)
In the adsorption−evaporation process, from 3, 4 to 1 in the diagram, the heat Qa is rejected to
the environment, which consists of sensible heat of adsorbent and adsorbate Qsa and adsorption
heat Qad. In addition, heat Qsev is the cooling load provided by the low-temperature evaporated
adsorbate, and must be subtracted from Qa.
Qa = Qad+ Qsa - Qsev (3.11)
Qad= −ma ∫x1
x2
H (x)dx (3.12)
Qsa= ∫T a 1
T g 2
ma cva(T)dT + ∫T a 1
T g 2
ma x1 cvm(T)dT +∫T a1
T a2
mac pa(T)dT + ∫T a1
T a2
ma x1 cpm(T)dT
If cooling capacity Qref is produced in evaporation,
Qref = ∫x1
x2
ma Le δ x (3.14)
the refrigeration COP would be
COPsolar=¿
Qref −Qce
Qg
(3.15)¿
3.5 DESIGN ANALYSIS
3.5.1 Solar Collector
The solar collector used in this project work is a parabolic trough type which is used to
concentrate the incident sun’s radiation to be reflected on the adsorber placed at its focal length.
24
The ideal width and focal length of the parabolic trough has to be determined using the
mathematical characterization of the trough and rays incident upon I, shown in figure 3.3
provided the parabolic equation y= x214 f
, where f is the focal length of the parabola. The rays
incident upon the trough are assumed to be parallel due to the sun's approximately infinite
distance, and at perfect alignment, these rays are parallel to the y-axis. The slope of the trough is
tanθsuch that,
tanθ=dydx
= x2 f
→ θ=tan−1 x2 f
(3.16)
The normal at an arbitrary value of x bisects the incident ray and its reflection, and the incident
ray is shown to be perpendicular to the x-axis. This observation coupled with the definition of y¿
(shown in Figure 3.4 ) yields,
y¿=f ¿− y=x tan β → f ¿=x tan β+ y (3.17)
Figure 3.3: The geometric definition of the simple case
25
Figure 3.4: The geometric definition including misalignment
In Equations 3.17,f ¿ is the y coordinate where the incident light is reflected to, and
mathematically β= x2−2θ. In Figure 3.3, f and f ¿ are equal, while in Figure 3.4, f ¿> f .
Introduce a misalignment, ϕ, and variable ψand re-derive y¿and f ¿according to Figure 3.4.
Algebraically, this is represented by,
ψ=θ−ϕ (3.18)
β=π2−ψ−θ=π
2−2 θ+ϕ (3.19)
y¿=x tan( π2−2θ+ϕ)=x tan [ π
2−2 tan−1( x
2 f )+ϕ](3.20)
With the expanded definition of Equation 3.17 seen in 3.20, Equation 3.17 becomes Equation
3.21.
f ¿ ( x , f , ϕ)=xtan[ π2−2 tan−1( x
2 f )+ϕ ]+ x2
4 f(3.21)
As seen in Equation 3.21,f ¿ , is a function of the x coordinate of the trough, the focal length, f ,
and any misalignment of the trough from perfect alignment, 𝜙.
3.5.2 Modes Of Heat Transfer
26
Heat will always transit from a higher temperature body (hot Sink) to a lower temperature body
(cold sink) on its own accord as explained by the second law of thermodynamics; it is quite
imperative to mention the various modes through which heat interacts in refrigeration systems.
The different modes of heat transfer are as explained.
A) Conduction
Thermal heat conduction can be referred to as method of heat transfer from one part of a body to
another part at a lower temperature and the process involve no motion the particle of the body.
However, this mode of heat transfer occur mostly on solid materials.
Fourier’s Law of Conduction: This law is based on the empirical observations as highlighted
above and the rate of heat transfer by conduction through a solid material is based upon. It states
that;
“the rate of heat flow through a homogenous solid is proportional to the area, A of the solid
which is at right angle to the direction of flow and to the temperature difference across the solid
and inversely proportional to the thickness of the solid through which the heat flows.”
Mathematically,
Q∝ A (T 1−T 2) / x (3.22)
Q=KA (T 1−T 2 )/ x
Q=−KA ( T2−T 1 )/ x (3.23)
Where constant of proportionality, K is a property other material and is called thermal
conductivity (W/mK). In different form, the Fourier’s equation is written a,
27
Q=−KA( dTdx ) (3.24)
q=QA
=−K ( dTdx ) (3.25)
where q is called HEAT FLUX (W/m2). and is the rate of heat transfer in the direction of x per
unit area.
Q is rate of heat flow through a block of material (W or J/s)
x is thickness of the block
A is the area of block at right angle to the direction of heat flow (m2)
T1 is high temperature side (0C)
T2 is low temperature side (0C)
B) Convection
Convective heat transfer is a result of motion of the molecules of a fluid over a solid surface.
Heat transfer by convection is mostly categorized into free/natural convection, which occur
when the flow of the fluid pass the solid surface is caused as result of density differences within
the molecule due to temperature difference; and forced convection which occurs when the flow
of the fluid past the solid surface is produced by some external means such as fan or gravity
flow.
Convection heat transfer is influence by the velocity of motion of the fluid molecules, the area
and nature of the solid surface and the difference between the temperature of the surface and that
of the moving fluid.
28
The rate of heat transfer between the surface and the fluid is given by the Newton’s Law of
Cooling, which is represented mathematically thus;
Q=hA (t s−t f ) (3.26)
Where, h is called the convective/film heat transfer coefficient (W/m2K). however, standard
values of h for different solid-fluid combination are available in tables and charts.
C) Radiation
Thermal radiation is the third mode of heat transfer. It is the heat energy emitted by a body in the
form of electromagnetic waves due to its temperature. It should be noted that thermal conduction
and convection requires a material medium, and on the contrary, thermal radiation does not
require any medium for transfer of heat, hence takes place in a vacuum. When a radiant energy
falls on a body part of it may be absorbed, part reflected from the body and remainder
transmitted through the body. However, the fractions of the radiant energy absorbed, reflected
and transmitted are respectively called absorbitivity, reflectivity and transmissivity. For a perfect
black surface, the relationship between the amount of radiation emitted, absolute temperature and
type of surface involved is given by the Stefan-Boltzmann equation, given by:Q=σ T 4, where, Q
is the heat radiated (W/m2), σ Stefan-Boltzmann constant ¿5.67 ×10−8 (W/m2K4), T is the
temperature of the body (K).
3.5.3 Heat Transferred On The Generator
29
The generator absorbs the radiant energy transferred from the parabolic trough collector. The rate
of heat loss per unit length pipe is given by,
Qloss=2 π L (T i−T 0 )
1ro h0
+ln( ro
ri)
K+ 1
rihi
(3.27)
ro=¿Outer radius of the generator pipe
ri=¿Inside radius of the generator pipe
L = Length of the pipe
T i=¿Inside surface temperature of pipe
T o=¿Outside or ambient temperature for Minna
h0=¿ convective heat transfer coefficient of still air
hi=¿convective heat transfer coefficient in the pipe i.e for moving air at 3.4 m/s
K = thermal conductivity for stainless steel
3.5.4 Radiant Energy Leaving The Surface
When radiant energy is incident on the mirror, parts are reflected while parts emitted to the
receiver. It is expressed mathematically as:
ρ+ε=1 (3.28)
Where, ρ=Reflectiviy∨fraction of radiant energy reflected
ε=Emissivity∨fraction of radiant energy emitted
The total amount of radiant energy received by the generator (energy leaving the surface) is
given by the equation below;
QR=ρH +εσ T 4 (3.29)
30
Where ,
H = Radiant energy incident on the surface
ε=¿ Emissivity
σ=¿Stephen Boltzmann’s constant
T=¿Ambient temperature
But, H=Ac I c
Where,
Ac = Area of the parabolic trough
Ic = Radiation intensity on collector area
By substitution, we have;
QR=ρ Ac I c+εσ T 4 (3.30)
3.5.5 Heat Generated Through The Generator
The total energy generated Q g(W) being transferred to the pipe to heat the mixture of ammonia
and calcium chloride in the generator is given by,
Q g=QR−Qloss (3.31)
3.5.6 Efficiency Of The Solar Collector
the efficiency of the solar collector is the amount of energy generated in the generator over the amount of incident solar energy on the collector. This can be expressed as;
η=Q g
I c
×100 (3.32)
3.6 DESIGN CALCULATION
3.6.1 Parabolic Trough Solar Collector
31
The design of the parabolic trough is defined by the four main characteristics: the width (W), the
length (L), the focal length (F), and the reflective material.
W = 24 in.
L = 36 in.
¿ Incident area of light hitting the trough on the plane, Ai=cosϕ × L ×W , where cos ϕ≈ 1 for
0<𝜙<40.
Ai=cosϕ × L ×W =1× 36 ×24
¿24¿2=0.5566 m2
¿ Collector area is Given by;
Ac=parabola widthW p× length of the collector L
¿30.25 ×36=1089¿2=0.7026 m2
¿Focal length f, is determined by equation of parabola, y= x2
4 f→ f = x2
4 y (see figure 3.3)
x=width2
=242
=12∈.=0.305 m
y=8∈.=0.203 m
f = 122
4 × 8=4.5∈¿0.114 m(from the collector base)
¿The slope of thetrouph θ , is determined¿equation 3.28 ,
θ=tan−1( x2 f )=¿ tan−1( 12
2×4.5 )=53⁰ ¿
¿The Solar Intensity (Ic) for Minna can be determined from the table below
Table 3.1: Monthly Mean Total Solar Radiation For Minna (MJ/m2/Day)
Months 2004 2005 2006 2007 2008
January 14.6 14.9 15.5 16.4 15.4
32
February 15.7 16.4 16.7 17.5 16.6
March 16.2 17.1 18.6 16.3 17.1
April 15.7 15.6 16.3 16.9 15.9
May 14.2 15.9 16.0 16.0 15.2
June 12.6 15.5 14.8 15.3 14.6
July 12.1 14.6 13.0 12.5 13.1
August 15.1 12.8 12.2 12.8 13.0
September 15.1 15.1 15.1 13.2 14.6
October 15.7 15.3 15.8 14.0 15.2
November 16.6 16.5 16.2 15.5 16.8
December 14.6 14.3 15.4 16.2 15.1
182.6
Mean AverageValue=182.62
=15.22 MJ /m2
I c=15.22 ×106
24 ×60 ×60=176.15W /m2
3.6.2 Determination Of QR
From the equation 3.28,
ρ+ε=1
But emissivity of glass is 0.92
ρ=1−0.92=0.08
Also, from the equation 3.30
QR=ρ Ac I c+εσ T 4
33
¿ (0.08 × 0.7026× 176.15 )+(0.92× 5.67 ×10−8× 3074)
= 473.27 W
3.6.3 Determination Of The Maximum Temperature, T i
The radiant energy reaching the generator is given by,
QR=T i−T o
12 π h iri
+ 12π K
ln( ro
ri)+ 1
2π ho ro
(3.33)
RT=1
2 π h i ri
+ 12 π K
ln( ro
ri)+ 1
2 π ho ro
(3.34)
T i=T o+QR RT (3.35)
RT=1
2 π ×0.024 × 22.71+ 1
2 π × 50ln ( 0.025
0.024 )+ 12 π × 0.025×50
=0.4194
T i=34+ (473.72 ×0.4194 )=233 0 C
3.6.4 Determination Of The Heat Loss
Heat loss by convection is given by,
Qconv=2 π r o L ho (T i−T o )=2 π ×0.025 ×1 ×10 × (233−34 )=312.59 W
Total heat generated by the generator (Q g )
From equation 3.31, this is give by;
Q g=QR−Qconv=473.27−312.59
Q g=160.68 W
3.6.5 Efficiency Of The Collector
34
From equation 3.32,
η=Q g
I c
×100=160.68176.15
×100
η=91 %
3.6.6 Determination Of The Rate Of Heat Transfer Through The Box
A) Determination Of The Icebox Surface Area
The total surface area of the enclosure of the space to be refrigerated is given by the area of the
box. It is important to note that the values computed is the external surface area of the
compartment, and this is because the results will be on the safe side for further heat transfer
calculation.
Icebox Compartment
External Dimensions : 13∈×13∈×13∈( L ×W × H )
Total surface area of the icebox compartment; A=6 (13 ×13 )
A=1014¿2=0.6542m2
B) Determination Of The Overall Heat Transfer Coefficient, U
The icebox is made composed of aluminium for both the outer and inside covering and a foam as
the lagging material to prevent heat loss.
This is given by;
1U icebox
= 1hi
+x1
K1
+x2
K2
+x3
K 3
+ 1ho
(3.36)
Where,
U icebox=(overall heat transfer coefficient ,W /m2 K )
x1 , x2 , x3=0.0004 , 0.035 ,0.0004 m¿)
35
K1=K3=250 W /mK ( thermal conductivity of aluminium)
K2=0.025W /mK (thermal conductivity of polrurethane foam )
h0=¿ 10 W/m2K (convective heat transfer coefficient of still air)
hi=22.71W /m2 K (convective heat transfer coefficient in the pipe)
1U icebox
= 122.71
+ 0.0004250
+ 0.0350.025
+ 0.0004250
+ 110
=1.544
U icebox=0.6477 W /m2 K
C) Heat Gain Through The Icebox, Qicebox
This is given by the equation,
Qicebox=U icebox A ∆ T
Where,
Qicebox=heat gainthrough the icebox , W
∆ T=temperature difference betweenthe condenser∧evaporator , 0C
∆ T=36.9−3.9=33 K
A=0.6542 m2 (area of icebox cube )
U icebox=0.6477 W /m2 K (overall heat transfer coefficient)
Qicebox=¿0.6477× 0.6542× 33=13.98 W
Therefore, Qicebox=Qref=13.98 W
3.6.7 Refrigeration Cycle Analysis
36
Figure 3.5: Pressure-Enthalpy diagram (W. Chekirou, N. Boukheiti, 2007)
The refrigeration process is discuss as follows:
Table 3.2: Thermodynamic properties of refrigerant R-717 (ASME thermodynamics properties
of refrigerants at 40 0F evaporating and 100 0F condensing)
Refrigerant used R-717
Design evaporator pressure 4.961 bar
Design condenser pressure 14.259
Minna ambient temperature 340C
The corresponding temperature values can be obtained from the table of properties of refrigerant
R-717 (see appendix). By interpolating, we have,
Table 3.3: Extract from Appendix
Pressures (bar) Temperature (340C ¿
4.625 2
4.961 T e
4.975 4
37
T e=4+( 4.961−4.9754.625−4.975 ) (2−4 )=4+ (0.04 ) (−2 )=3.9 0 C
Table 3.4: Extract from Appendix
Pressures (bar) Temperature (340C ¿
13.89 36
14.259 T c
14.70 38
T c=38+( 14.259−14.7013.89−14.70 ) (36−38 )=38+( 0.544 ) (−2 )=36.9 0 C
To compute the enthalpies and entropies from the table of properties of refrigerant R-717 (see
appendix). At 4.961 bar, by interpolating, we have,
Table 3.5: Extract from Appendix
Pressures (bar) Enthalpy (kJ/kg) Entropy (kJ/kg K)
4.625 1446.5 5.314
4.961 hg Sg
4.975 1448.5 5.288
h1=hg=1448.5+( 4.961−4.9754.625−4.975 ) (1446.5−1448.5 )=1448.42 kJ /kg
S1=S2=Sg=5.288+( 4.961−4.9754.625−4.975 ) (5.314−5.288 )=5.289 kJ /kgK
At 14.259 bar, the refrigerant is superheated at state 2, by interpolation, we have;
38
Table 3.6: Extract from Appendix (superheat column at 100 k)
Enthalpy , h100 K (kJ /kg) Entropy , S100 K (kJ /kg K ) Pressure , ¿
1745.7 5.692 13.89
h100 K S100 K 14.259
1748.7 5.674 14.70
h100 K=1748.7+( 14.259−14.7013.89−14.70 ) (1745.7−1748.7 )=1744.1kJ /kg
S100 K=5.674+( 14.259−14.7013.89−14.70 ) (5.692−5.654 )=5.684 kJ /kgK
Table 3.7: Extract from Appendix (superheat column at 50 k)
Enthalpy , h50 K(kJ /kg) Entropy , S50 K(kJ /kg K ) Pressure , ¿
1617.8 5.358 13.89
h50 K S50 K 14.259
1620.1 5.340 14.70
h50 K=1620.1+(14.259−14.7013.89−14.70 ) (1617.8−1620.1 )=1618.8 kJ /kg
S50 K=5.340+(14.259−14.7013.89−14.70 ) (5.358−5.340 )=5.350 kJ /kgK
At state 2, the refrigerant is superheated and h2is computed from the superheated values;
h2=1618.8+( 5.289−5.3505.684−5.350 ) (1747.1−1618.8 )=1595.4 kJ /kg
At 14.259, h3=h4=h f is obtained from the table of properties of refrigerant R-717 (see
appendix). By interpolation, we have;
39
Table 3.8: Extract from Appendix
Pressure , ¿ Enthalpy , kJ /kg
13.89 352.3
14.259 h3
14.70 362.1
h f=h3=h4=362.1+(14.259−14.7013.89−14.70 ) (352.1−362.1 )=356.8 kJ /kg
Summary of the refrigeration system analysis;
Refrigerating effect h1−h4=(1448.42−356.8 )=1091.62 kJ /kg
Heat rejected ¿h2−h3=(1595.4−356.8 )=1238.6 kJ /kg
Work input, W ¿h2−h1=(1595.4−1448.42 )=148.98 kJ /kg
Mass flow rate, m (kg /s )
m= refrigeration capacityrefrigerating effect
= 13.98
1091.62× 103=0.00001281 kg/ s
Power input=m×W =0.00001281 ×148.98=0.001908 kW
¿1.908 W
COP=refrigeration capacity , Qref
heat applied ¿the generator ,Q g¿=
13.98160.68
¿0.087
3.7 CONSTRUCTION AND ASSEMBLY
The construction procedure for the system follows the sequence highlighted below;
40
Construction of the frame assembly and support and surface finishing by grinding-off
rough edges of the frame. The welding of the frame joints was done using electric arc
welding method. The frame was constructed using square pipe and angle iron.
Construction of the solar collector frame. A black iron metal sheet was measured,
marked-out and cut to size and tagged to follow the parabolic trough path of the collector.
It was then welded to the frame.
The stand for the condenser was constructed and welded to the frame so that it is high
above the level of the collector.
Measuring, marking-out and cutting of aluminium sheet into require sizes for the ice box
unit and the tank/container for the condenser. For the icebox, the sheets were folded at
the edges into a somewhat rectangular box. While the tank was formed into a cylindrical
shape and folded round at the edges. The cover for the icebox was made to be open from
the top.
Polyurethane foam for insulating the icebox unit was cut to the appropriate sizes; the
forms were attached to the aluminium sheets between the hollow space of the sheet at the
four side, base and cover of the icebox. It was then riveted all through the box to keep it
firm.
The evaporator was formed using copper pipe and the pipe neatly laid on the four side of
the icebox compartment including the base.
The condenser was formed by shaping a copper pipe into coil.
For the frame assembly and the collector, the joints were welded to keep the frame firmly
without vibrating and to withstand the weight and load of the whole assembly. Jockey
wheels were mounted to enable short range movement of the whole assembly.
41
The generator pipe was capped close at the ends with 212
caps and secured firmly.
One of the cap was drilled to accept 14
nipple and a coupling for the rest of the plumbing
(piping connections).
Piping connections were made to connect the three main components together. First, by
connecting the generator to the condenser and then to evaporator. Welding was done
using oxy-acetylene welding method.
Finishing was done by cleaning the frame assembly using emery cloth and the entire
system was polished using a black oil paint and finally the solar powered icemaker was
assembled.
3.8 MATERIAL SELECTION
Selecting materials for the design and construction of any component is very common and
important in engineering practice, in order to give an optimum performance and satisfy some
other properties. The following fact were given high priority when selecting materials for the
design and construction of the solar powered ice maker;
Mechanical properties
Market availability
Fabrication requirements
Service requirements, and
Economic considerations
Table3.9: Material selection Analysis
42
S/
N
Parts Materials Reasons for selections
1 Absorber/generator Stainless steel i. Highly resistance to corrosion
ii. Resistance to oxidation under high
temperature
iii. Good thermal conductivity
2 Condenser Copper i. Soft and ductile and can be coiled into
shape
ii. Resistance to corrosion at ambient
atmosphere and R-717 attack
iii. Good thermal conductivity
3 Evaporator Copper i. Ability to dissipate heat easily
ii. Relatively cheap and available
iii. Ability to retain low temperature
4 Icebox Thin aluminium
sheet
i. Conducts heat and dissipate heat easily
ii. Light weight
iii. Resistant to corrosion
5 Insulator Polyurethane
foam
i. Very low thermal conductivity (0.025
W/mK)
ii. Light weight
iii. High strength during compression
iv. Good resistance to heat
v. Relatively cheap
43
6 Working pair
A Refrigerant/Adsorbent Anhydrous
Ammonia
i. Zero Ozone layer depletion
ii. Environmentally friendly
iii. High latent heat per unit volume
iv. Good thermal stability and vaporizes
easily
v. Suitable for medium-low evaporator
temperature
B Adsorber Granular
Calcium
Chloride salt
i. High adsorption capacity
ii. Better thermal conductivity
iii. High permeability, heat and mass
transfer capability
7 Parabolic trough solar
collector
Mirror glass i. Low coefficient thermal expansion
ii. Relatively high mechanical strength
iii. Reflective capability
iv. Resistance to attack by water and can
be easily cleaned from dust and particle
deposits
8 Frame and support Black iron pipe
and angle iron
i. High strength
ii. Good shock resistance
iii. Ability to withstand weight or load
without bending
3.9 COST ANALYSIS
44
The current market prices for the cost of material used for the fabrication of the solar powered
icemaker are analyzed below;
The cost of fabrication can be divided into three:
i. Material cost
ii. Labour cost
iii. Overhead cost
3.9.1 Material Cost
This is the cost of materials used for the fabrication of the solar powered icemaker based on the
current market prices.
Table 3.10: Material cost
S/N Materials Quantity Cost (₦)
1 Collector construction (including frame assembly) 1 9,400
2 14
" Condenser coil1 1000
3 14
" Evaporator coil1 1000
4 Full sheet 4 ×3ft Plain mirror 1 2,500
5 212
Pressure gauge1 3,000
6 Stainless steel pipe, 1 m length 1 3,500
7 Aluminium sheet 1 1,500
8Miscellaneous
14
" Plumbing 2,000
9 Anhydrous Ammonia, 500 g 1 4,000
10 Granular Calcium Chloride, 500 g 1 2,000
11 RTV Silicone Sealant 1 500
45
12 Black oil paint 1 1,200
TOTAL 31,600
3.9.2 Labour Cost
Labour cost is the cost of various task performed on the icemaker. A sum of ₦10,400 was used
for the fabrication.
3.9.3 Overhead Cost
They are other expenses incurred in the fabrication excluding the material cost and labour cost.
The overhead cost is assumed to be 25% of the total material cost.
Overhead cost= 25100
×total material cost
Overhead cost= 25100
×31600
Overhead cost=₦ 7,900
3.9.4 Total Cost
The cost for the fabrication of the solar powered icemaker using a parabolic trough collector is
the summation of the material cost, labour cost and overhead cost.
Total cost=₦ (31600+10400+7900 )
¿ ₦ 49,900
46
CHAPTER FOUR
4.0 EXPERIMENTAL PERFORMANCE AND TESTING
4.1 EXPERIMENTAL PROCEDURES
The experiment was conducted in order to observe and record temperature of different
components of the unit which allows calculation of RE and COP of the unit. The solar powered
icemaker was tested in two ways. The procedures for the Test 1 involved the system collector
being aligned East-West and facing North. The mirror then was tilted to align with the position
of the suns inclination during the hottest part of the day approximately 10:00 to 14:00hrs. No
other tracking of the sun was done during this testing period. The Test 2 was involved tracking
the sun in 15 minute intervals. This provided greater exposure of sunlight normal to the mirror
surface. This light applied directly to the collector tube and not be deflected away at certain
times of the day as in the case of Test 1. On both the tests the collector tube temperature, the
temperature at the condenser inlet, the condenser water temperature, the temperature at the
condenser outlet, the system pressure and the evaporator temperature were observed and
recorded.
The system in operation during Test 1 begins its cycle during the day when the system mirror is
directed to the sun. All the light striking the parabolic mirror is redirected to the collector tube in
order to heat-up the tube. This heat is applied to the absorbent/refrigerant combination
throughout. The heat releases the refrigerant as a gas which rises and makes its way to the
condenser. For simplicity of construction, the condenser was taken as water cooled meaning that
only a coiled tube was necessary. Due to the increased system pressure the refrigerant can be
condensed at the temperature of the water. The liquid refrigerant travels under gravity to the
47
refrigerant receiver located in a fridge compartment. This process was continued throughout the
day until the heat being applied can no longer release the refrigerant.
After the sun sets, the temperature and the pressure in the collector tube reduces, and the
refrigerant begin to boil. Refrigerants boil at much lower temperatures than most of the other
liquids and therefore draw energy from the surroundings and produce cold. The boiling
refrigerant returns to a gaseous state and can be returned back to the generator to be reabsorbed
ready for the next day. It is this process which gives the intermittent refrigerator its name the
process of heating and cooling occurs in different stages where a continuous cycle requires
heating on a continuous basis in order to maintain a constant cooling effect.
4.2 RESULTS AND DISCUSSION
4.2.1 Observation in Test 1: Non-Sun Tracking
The experiment was initiated at 10:30 am. The mirror was wheeled outside and aligned East–
West and facing North, it was then tilted to align with the point of inclination between the start
time and the midday point. Temperature of the collector tube, at the condenser inlet,
condenser water, at the condenser outlet and evaporator were recorded using mercury-in-glass
thermometer. Figure 4.1 shows these temperatures.
It was at this time the sound of pressure being released was heard. It was also at this point
when the evaporator was observed to decrease in temperature to the point where
condensation formed on the refrigerant receiver and cooling coil. The temperature was
recorded as around 20 0C. It was observed that the condensed refrigerant was not enough to
produce any cooling effect at that temperature since only little amount of condensed
48
refrigerant was received in the evaporator. After sometime the evaporator temperature then
rose to a point which was close to ambient (26 0C). The measurements were taken at intervals
of 15 minutes. The collector temperature reached a maximum at 13:00 hour. The maximum
pressure was 100 kPa and the maximum collector tube temperature was 85 0C.
Clouds played a major part in the reduction of heat to the system. By observing the
temperature readings at times where the sun had been covered by a cloud for some time there
were noticeable drops in the temperature of the collector tube. At around 2pm due to cloud and a
non direct angle being struck on the mirror, less heat was being transferred to the collector tube.
This in turn began to decrease the temperature and pressure of the system. It was at this point
that the temperature in the evaporator also began to decrease. From its maximum point of 26 0C
it dropped to a minimum of 20 0C by 16:00 hours.
Table 4.1: Temperature recordings with time for Test 1
S/N Time (Minutes)(15 mins intervals)
Temperature (Degrees Celsuis)
Collector tube
Condenser in
Condenser water
Condenser out
Evaporator
1 15 34 24 20 24 202 30 36 30 20 20 193 45 38 29 20 20 204 60 46 30 32 22 215 75 50 30 32 22 226 90 53 34 32 32 227 105 56 34 32 30 248 120 58 34 32 30 249 135 64 38 29 32 2410 150 78 40 25 33 2411 165 85 45 30 35 2612 180 79 45 32 35 26
49
13 195 76 38 29 33 2614 210 79 42 32 30 2615 225 75 34 32 30 2516 240 72 38 32 32 2517 255 72 33 31 26 2518 270 58 34 32 27 2619 285 52 35 32 30 2520 300 50 34 32 30 2421 315 48 34 32 30 2322 330 39 29 32 22 2023 345 39 29 32 22 20
Systems Temperatures T 1
0 50 100 150 200 250 300 350 4000
10
20
30
40
50
60
70
80
90
Collector tube
Condenser in
Condenser water
Condenser out
Evaporator
Time (minutes)
Tem
pera
ture
(Deg
rees
Cel
suis
)
50
Figure 4.1: Temperature profiles of different components during Test 1
4.2.2. Observation in Test 2: Sun-Tracking
Figure 4.2 shows the temperature recordings of the components. During this test the sun was
tracked manually on a 15 minute basis by adjusting the stud holding the collector so that it is
positioned to the desired angle of both the mirror and sun’s inclination and was done at the same
time as instrument readings were taken. The amount of direct sunlight again was a big factor in
the operation of the system. Cloud cover was observed from approximately 10:45 in the morning
to after midday with small breaks of approximately one to two minutes. Due to this the collector
tube was not receiving the constant energy input required to sustain high temperatures and
subsequently large temperature drops were recorded during the hottest part of the day. Once the
surface temperature dropped to a level between 51 and 72 oC it was able to be sustained with the
available sunlight input. This shows that this collector despite its imperfections was able to
concentrate the minimal sunlight available and use it to sustain a reasonable temperature in the
collector tube.
With this temperature drop, it indicates that a large portion of the ammonia was not reabsorbed
into the calcium chloride. The temperature readings did not seem to correspond with that of test
1 and the maximum pressure corresponding to the maximum temperature of 89oC was 120 kPa
51
Table 4.2: Temperature recordings with time for Test 2
S/N Time (Minutes)(15 mins intervals)
Temperature (Degrees Celsuis)
Collector tube
Condenser in
Condenser water
Condenser out
Evaporator
1 15 25 24 24 22 212 30 49 24 25 22 223 45 52 25 25 22 224 60 60 25 26 22 225 75 72 26 28 23 246 90 80 40 30 23 267 105 85 33 31 23 268 120 86 33 31 24 269 135 89 33 33 23 2710 150 70 32 34 23 2511 165 63 30 31 23 2412 180 72 35 29 23 2413 195 74 28 29 24 2314 210 66 29 28 23 2315 225 51 36 30 23 2316 240 58 35 29 23 2317 255 58 35 30 24 2318 270 54 35 32 23 2319 285 60 34 31 23 2220 300 66 35 28 23 2221 315 69 34 23 24 2222 330 50 32 23 24 2223 345 46 30 23 24 22
52
Systems Temperatures T 2
0 50 100 150 200 250 300 350 4000
10
20
30
40
50
60
70
80
90
100
Collector tube
Condenser in
Condenser water
Condenser out
Evaporator
Time (minutes)
Tem
pera
ture
(Deg
rees
Cel
suis
)
Figure 4.2: Temperature profiles of different components during Test 2
4.2.3. Operational Issues
In test 1, the minimum evaporator temperature reached was 20 0C. The collector tube reached a
maximum temperature of 85 oC. In test 2, the minimum temperature reached was 22 oC. This
was due to the re-absorption process or lack of maintaining a high system pressure and therefore
reducing the evaporation of the refrigerant and preventing low temperatures. The temperature of
53
the collector tube however was recorded much higher due to hotter sun in the first 2 hours of day
two with a temperature of 89 oC.
Observations were made during the system’s operation which led to not achieving a reasonable
result in the case of the cooling effect in the fridge compartment. The is because the amount of
heat energy from the sun was not sufficient enough to release all the refrigerant gas as the heat is
been applied to the tube where the gas makes its way to the condenser. This was as a result of
environmental factor such as cloud cover with small breaks of approximate few minutes that
was experienced when undertaking the test.
Some traces of the Calcium Chloride salt combined with the refrigerant gas to flow to the
condenser since there was no sufficient heat to release the refrigerant gas from the salt. As a
result of this, the cooling could not take place as expected and the ammonia was not reabsorbed
into the Calcium Chloride in the case of test 2.
Due to the fact that the system is at its experimental stage, work was done to construct the
system so as to achieve the desired result as experimented. Therefore, certain recommendations
will be made on how to improve on the project so as to achieve the results as expected.
54
CHAPTER FIVE
5.0 CONCLUSION AND RECOMMENDATION
5.1 CONCLUSION
The solar powered icemaker that was design and tested performed fairly in order to achieve the
goals of the project. The COPs were not achieved in both test, although the system was not
operating to the best of its ability in Test 1. In Test 2, the system operated very badly because the
evaporator temperature could not go below 22 oC which has contributed to no refrigeration
effect. Although the results are encouraging, there are rooms for improvements to be made in
order to make the system produce more usable conditions. The system is easy to use and
incorporates no complicated control systems instead relying solely on the changes in system
pressure to control the outputs.
In its current form the device would not be considered applicable for use in a modern home or
office. However, the applications for this particular system could be in remote areas where
adequate refrigeration is not available to stop perishables such as meat and medicines from going
off and where electricity is not readily available from main supply. However, modification and
further testing would be necessary before the system is capable of performing a worthwhile duty.
5.2 RECOMMENDATIONS
A modification would need to be made to the device to give the calcium chloride greater
surface area during the re-absorption or cooling phase of the process to avoid the cooling
55
problem observed in both Test. This could be achieved by introducing a perforated tube
through the centre of the collector tube. The calcium chloride would then be re-packed
around the tube leaving a hollow space the entire length of the collector tube. The surface
area allowed for the ammonia to be reabsorbed into the calcium chloride will then be
greatly increased. By providing extra surface area and hence extra absorption rate, the
rate of refrigerant boiling can be increased, decreasing the evaporator temperature.
A backup sub-system can be made using waste heat to supplement the collector in the
case of environmental factors such as cloudy weather condition.
A continuous operating system can be adopted, since in this design a problem was
encountered in the case of the refrigerant not drawn back into the generator as the mode
of operation is one-way for vaporizing and re-absorption. That is to say, a connection
should be made from the evaporator to the generator for easy re-absorption of the
refrigerant back into the generator so that the cycle is continuous.
A pure (anhydrous) ammonia should be obtained in a pressurized tank and allowed to
evaporate through a valve into the generator. This allows a faster rate for the chemical
reaction to take place between the working pairs and will allow only the refrigerant to
vaporize to the condenser where it is been condensed to liquid refrigerant as the
temperature drops which then flows by gravity into the evaporator to cause the necessary
refrigerating effect.
A larger system should be design and constructed so as to have larger surface area of the
collector concentrating the heat energy from the sun so that this heat is sufficient enough
to heat up the working pair to release the refrigerant gas which allow for higher
efficiency and performance of the system.
56
In order to improve the system it is recommended that the re-absorption process be
studied further by students who might have interest in the project so that a lower
evaporation pressure and therefore temperature may be reached. This will involve
increasing the absorbent surface area and possibly also investigating alternative dry and
wet adsorbent materials.
Further work on this project should consider other working pair such as Activated Carbon
and Methanol, Calcium Chloride + Activated Carbon and Ammonia e.t.c. Further design
should try as much as possible to eliminate joints so as to reduce the rate at which the
refrigerant gas is escaping which reduces system efficiency and performance.
Finally, the University should re-equip the engineering workshop with essential
equipments that were lacking and make available state-of-the-art testing equipment in
order to ensure the smooth running of the work and prevent wasting of time and
resources and also reduce cost for the execution of the work.
57
REFERENCE
Ari R. (1996), “Comparison of Solar Concentrators”, Solar Energy Pg 18-93
Chekirou W. and Boukheiti N. (2007), “Numerical Modelling and Combined Heat and Mass
transfer in a Tubular Adsorber of a Solid Adsorption Solar Refrigerator”.
Derek L. (2008) “Parabolic Trough Solar Collector Analysis”.
Eastop T.D. and McConkey A. (2003), Applied Thermodynamics for Engineering Technologists.
Fifth edition, Longman Publishing Ltd.,Singapore.
http/www.homepower#53.com “A Solar Absorption Icemaker”, Pg 20-23
Jan. F. K. and Frank K. (1981), “Solar Energy Handbook”
Khurmi R.S. & Gupta J.K. (2006), A Textbook of Refrigeration and Air-Conditioning, Third
edition, Chand S. Publisher. London.
Osita U. (2008), “Design and construction of Solar Still for heating Portable Water” Federal
University of technology, Minna
Rasul M.G. and Murphy A. (2006), “Solar Powered Intermittent Absorption refrigeration Unit”.
The Design and Development of a Solar Powered Refrigerator - Appropedia the sustainability
wiki.htm
58
William C.D. and Paul N.C. (1978), “Solar Energy Technology Handbook, Systems Design and
Economics,” Part B; New York, Marcel Dekker Inc
Yusuf A. “Solar Radiation in Abuja”; a correlation with meteorological data.
APPENDIX
Appendix A: Equilibrium Diagrams For Calcium Chloride-Ammonia, and Ammonia
59
60