ABSTRACT

 

A simple solar energy powered intermittent adsorption refrigeration system was
designed,
fabricated and tested. The system uses a Zeolite as the adsorber and water as the
working fluid. The heat source is a parabolic trough concentrator which is to collect and
concentrate solar thermal energy onto a black body coated copper absorber. The
generator drives the refrigerant around the system through a condenser and an
evaporator to complete the refrigeration cycle. Two set of test were carried out and
different times of the year, one in January 2008 a month with the lowest solar
irradiation and the second set in May 2008. The system was evaluated by leaving it
outside under solar radiation and monitoring temperatures at various points on the the
generator, condenser and the evaporator through the use of thermocouple sensors. The
first test carried out revealed that the average highest and lowest temperatures on the
solar collector were 57.2oC and 11.5°C respectively. The average lowest refrigeration
temperature was 18°C. No cooling effect was actually produced due to the period
testing was carried out and imperfection in the fabrication process. The Zeolite was
produced in locally with a pore size of 4􀜣 ̇ and a regenerative temperature of 250oC. The
second test carried out in May 2008 produced a cooling effect by making small changes
on the system. An evaporator temperature of 9oC was attained which linearly increased
to a maximum of 28oC as the day advanced. Maximum absorber temperature of 64oC
was attained over the test period.TGA and DTA confirmed the regenerative temperature
to be at 200oC and to be thermally stable at 600oC.

 

TABLE OF CONTENTS

Cover page i
Declaration iii
Certification iv
Acknowledgement v
Abstract vi
CHAPTER ONE
1.0 Introduction 1
1.1 History of refrigeration 2
1.1.1 The industry today 4
1.2 Solar cooling paths 5
1.3 Refrigeration system 7
1.3.1 Vapor compression system 7
1.3.2 Absorption refrigerator 8
1.3.3 Adsorption refrigeration cycle 9
1.3.4 Gas expansion refrigeration 11
1.3.5 Thermoelectric refrigeration 12
1.3.6 Evaporative cooling 12
1.4 Statement of the problem 14
1.5 Objectives 15
1.6 Justifications 15
CHAPTER TWO
2.0 Literature review 18
8
2.1 Review of past works in the area 18
2.2 Characteristics of the adsorbent- adsorbate pair 21
2.3 Zeolite 22
2.3.1 Commercial and domestic uses 22
CHAPTER THREE
3.0 Design theory 24
3.1 Adsorption cycles for solar cooling 26
3.2 Design of the cooling system 29
3.3. Heat load calculation 30
3.4 Product load 32
3.5.1 Usage load 32
3.5.2 Design of the evaporator 32
3.5.3 Design of the condenser 34
3.5.4 Collector design 36
3.5.5 Design of the parabolic trough 36
3.5.6 Design requirements for concentrators 36
3.5.7 Collector sizing 37
3.8.4 Receiver tube diameter 37
3.9 Energy balance for the parabolic trough concentrator 38
3.9.1 One dimensional energy balance model 39
3.9.2 Convection Heat Transfer between HTF and the Absorber 42
3.9.3 Conduction heat transfer through the absorber wall 43
3.9.4 Convective heat transfer 44
9
3.9.5 Radiation heat transfer 44
3.9.8 Optical properties 44
3.9.7 Solar irradiation absorption in the absorber
3.9.8 No glass envelope case 45
3.10 Design calculations 47
CHAPTER FOUR
4.0 Introduction 60
4.1 Construction 60
4.2 Methodology 61
4.3 Production of Zeolite A 61
4.4 Collector structure fabrication 62
4.5 Construction processes 64
4.6 Assembly 67
4.7 Testing 69
4.8 Results 71
4.9 Discussions 77
4.10 Cost of production of the refrigerator 79
CHAPTER FIVE
5.0 Summary 84
5.1 Conclusion 85
5.2 Recommendations 86

 

 

CHAPTER ONE

1.0 INTRODUCTION
Refrigeration is a term used to describe a process which maintains a process space or
material at a temperature less than the ambient temperature. To accomplish this, heat
must be transferred from the materials to be cooled into a lower temperature substance
referred to as a refrigerant. With advancement in science and technology, the role and
function of refrigeration and its application have steadily become indispensable to the
existence of the modern society.
The concept of using solar energy for powering a refrigerator appeared forty years ago
with a prototype using a liquid sorption cycle, Sumathy (1999). The use of sorption
processes to produce refrigeration has been extensively studied in the last twenty years
as a technological alternative to vapour compression systems. Solar-powered
refrigeration can also use solid sorption, with either a chemical reaction, or adsorption.
Several theoretical and experimental studies demonstrated that sorption refrigeration
systems especially those using solid-gas heat cycles are well adapted to simple
technology applications. They can operate without moving parts and with low grade
heat from different sources such as residual heat or solar energy. The main two
technologies concerning the solid – gas sorption concept are the adsorption and the
chemical reaction, including metal hydrides. The similarities and differences between
these systems as well as the advantages and disadvantages of each one are extensively
described by Meunier (1998)
Solar powered refrigeration and air conditioning have been very attractive during the
last twenty years, since the availability of sunshine and the need for refrigeration both
21
reach maximum levels in the same season. One of the most effective forms of solar
refrigeration is in the production of ice, as ice can accumulate much latent heat, thus the
size of the ice-maker can be made 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. Based on his studies Ing. (2004) recommended that Solid
adsorption pair of Zeolite and water is best to produce refrigerating effect while
activated carbon and methanol can serve as a suitable pair for a solar powered, solidadsorption
ice-maker.
.
1.1 HISTORY OF REFRIGERATION
A comparative summary of the historical developments in refrigeration and air
conditioning is presented in Table 1.
Table 1. Historical development in refrigeration and air conditioning. Jordan (1962)
Date Refrigeration Air conditioning
15th c.
B.C.
First mention of making ice, in ancient Egypt, by
night-cooling, for refreshment and fever treatment.
Evaporative cooling used to
cool air in dry climate by
water splashing.
2nd.
A.D.
Galen proposes four degrees of coldness (and four
degrees of heating).
1700 First artificial ice production, by aspirating ether
vapours, for medical purpose.
1800 Natural ice regional and world-wide markets expand. J. Gorrie in Florida made a
22
Ferdinand Carré invented in 1846 the ammonia
absorption cycle.
hospital-ward refrigerated by
blowing air with a fan over
ice, to prevent diseases.
1865 First commercial ice-makers, using Carré’s ammonia
absorption plants.
1873 First commercial refrigerator, by von Linde, using an
ammonia vapour compressor. The first closed-loop
vapour compression refrigerator was patented in
London by J. Perkins.
Linde also built the first
domestic air conditioning (for
an Indian Rajah).
1880 First frozen-meat ocean transport, using air
compression and expansion.
1900 Development of large artificial ice-making firms,
using ammonia compressor driven by a steam engine.
First refrigerated public
building in 1901.
1911 Carrier, in an ASME meeting,
presented the refrigeratordehumidifier
1914 Kelvinator introduces the thermostatic valve.
1918 Frigidaire (assoc. to GM) sells domestic units at
$1000.
1920s GE develops the sealed compressor in 1928.
Frigidaire units at $500 (still bulky: 170 kg).
One million units sold, mostly using SO2.
Carrier units in theatres and
cinemas.
1925 Electrolux developed an absorption refrigerator
23
without moving parts (marketed in USA by Servel).
1928 T. Midgely found a safe refrigerant, CCl2F2,
commercially synthesised in 1929 by DuPont-GM
from CCl4 and HF, trade-named as Freon.
1932 Small window units by GE.
1934 Door-shelves were proposed, but were discarded.
1939 GE develops the two-doors combined frigo-freezer. First car air conditioning unit.
1960 Domestic refrigerators popularise; replacing icechests.
Most American shopping
centres and hotels
conditioned.
1980 Self-defrosting units.
Domestic units with ice-cube and chip-ice dispensers.
Domestic air conditioning
popularised.
The history of refrigeration is nearly the same as the history of making ice, artificial ice,
since the history of natural ice is another story: homo-sapiens era is the quaternary
period in the history of Earth, the last 2 million years, and, although there have been
little climatic changes during the last 10 000 years (Holocene), during the rest of the
quaternary period (Pleistocene) major ice ages occurred, lasting some 100 000 years
each (with intermediate warm periods of some 10 000 years), with polar ice caps
extending to middle latitudes (although the average Earth surface temperature was just 9
ºC below the present 15 ºC). Jordan (1962):
24
1.1.1 The industrial applications
The expansion of the refrigeration industry over the years has been very great
indeed, with exception of the radio industry, no other field had such a rapid acceptance
and emerging impact upon our lives. Over the years new industrial applications have
opened comparatively new fields in controlled temperature application, Application of
refrigerator in the medical profession are increasing daily not only in the preservation of
certain products, but also in the actual treatment of some physical ailments; also in the
refrigerated food industry development are occurring so rapidly that it is difficult to
keep abreast of them. Increased applications of domestic refrigerators have been
supplemented by the use of low grade energy sources for domestic low temperature
refrigerator.
These are few, but the wide spread application of refrigerator. Present day
refrigeration requirements involve the entire comparative scale, almost down to
absolute zero, with great consideration the challenges facing the energy sector of the
economy.
1.2 SOLAR COOLING PATHS
Solar powered cooling systems can generally be classified into 3 main parts:-
i. Solar energy conversion equipment
ii. The refrigeration system
iii. Cooling loads
Solar driven refrigerator system can further be classified into two main groups
as shown in fig 1.1 ,depending on the mode of energy supplied namely:-
25
a. Thermal/work driven systems:- solar thermal conversion to heat Adsorption.
 Chemical reaction
 Desiccant cooling cycle
 Ejector refrigeration cycle
 Rankine refrigeration cycle
b. Electrical (photovoltaic) driven systems – process to electricity
 Stirling refrigerator cycle
 Thermoelectric Peltier refrigerator cycle
 Vapour compressive refrigerator cycle
Fig 1.1 The possible paths from solar energy to cooling services
26
Each group can be classified according to the type of refrigerator cycle. The appropriate
cycle in each application depends on cooling demand, power and the temperature levels
of the refrigerated object and the environment.
1.3 REFRIGERATION SYSTEMS
Refrigerating effect is produced by the removal of heat from the substance to be cooled.
This phenomenon takes place with the aid of a cooling medium to which the heat flows,
to a lower temperature than the substance being refrigerated.
Before advent of modern refrigeration process, water was kept cool by storing it in
earthen ware jugs so that the water could flow through the pores and evaporate.
Natural ice from lakes and rivers were often cut during the winter and stored in caves
straw-lined pits and later in saw dust –insulated buildings. The early Romans carried
packed trains of snow from the Alps to Rome for cooling the emperor’s drinks. These
methods are all natural ways of refrigeration.
Artificial Refrigeration is produced in many ways which include:-
 Vapour compression
 Absorption refrigeration
 Adsorption
 Thermoelectric
 Gas expansion refrigeration
1.3.2 Vapour compression sy
A common and effective cold producing technology is based on the vacuum
vaporization of volatile liquid.
absorption vapour compression refrigeration cycle base
cycle. Saturated or slightly saturated vapour i
pressure then cooled until the compressed gas condenses to a liquid and the saturated or
slightly saturated cycle flashes to the low pressure vaporized through a
(1962)
Vapour compressor cycles usually work with single component refrigerant, but some
times Mixtures are used. Fig 1.2 main components of a vapour compression system
Fig 1.2 Main components of a vapour
diagrams
27
system
Compression is accomplished either mechanically or by
based on a modified reverse R
e. is pumped by a compressor to a high
n valve
vapour-compression refrigerator, and T-s and p
Rankine
ompressor valve. Jordan
p-h
28
1.3.2 Absorption refrigerator
An absorption refrigeration machine corresponds to vapour-compression refrigerator in
which the compressor is substituted by four elements: vapour absorber, based on
another liquid, a pump for the liquid solution, a generator or boiler to release the vapour
from solution and a valve to recycle the absorbent liquid. Its advantage is that the cycle
requires less work to operate or none at all if the liquid is naturally pumped by gravity
in a thermo-siphon, at the expense of an additional heat source required at the
regenerator Jordan (1962).The basic scheme is presented in Fig. 1.3.
Fig. 1.3. Layout of an absorption refrigeration machine,
There are two working fluids of an absorption refrigerator. The refrigerant and the
carrier (the auxiliary liquid) that absorbs the refrigerant and is pumped up to high
pressure and release the refrigerant vapour at the generator. Ammonia has been
traditionally used as refrigerant in both types of refrigeration.
29
1.3.3 Adsorption refrigeration cycle
An adsorption, also called a solid-sorption cycle, is a preferential partitioning of
substances from a gaseous or liquid phase onto a surface of a solid substrate. This
process involves the separation of a substance from one phase to accumulate or
concentrate on a surface of another substance. An adsorbing phase is called an
‘adsorbent’. Material, which is accumulated, concentrated or adsorbed in another
surface, is called an ‘adsorbate’. The sticking process should not change any
macroscopic of the adsorbent except the changing in adsorbent’s mass.
Both adsorption and absorption can be expressed in term of sorption process. The
adsorption process is caused by the Van der Vaals force between adsorbates and atoms
or molecules at the adsorbent surface. The adsorbent is characterised by the surface and
porosity.
In the adsorption refrigeration cycle, refrigerant vapour is not to be compressed to a
higher temperature and pressure by the compressor but it is adsorbed by a solid with a
very high microscopic porosity. This process requires only thermal energy, no
mechanical energy requirement. The principles of the adsorption process provide two
main processes, adsorption or refrigeration and desorption or regeneration.
The refrigerant (water) is vaporised by the heat from cooling space and the generator
(absorbent tank) is cooled by ambient air. The vapour from the cooling space is lead to
the generator tank and absorbed by adsorbent (Zeolite). The rest of the water is cooled
or frozen.
30
In the regeneration process, the Zeolite is heated at a high temperature until the water
vapour in the Zeolite is desorbed out, goes back and condenses in the water tank, which
is now acting as the condenser.
For an intermittent process, the desorption process can be operated during daytime by
solar energy, and the adsorption or the refrigeration process can be operated during
night-time. The solar energy can be integrated with a generator. The single adsorber is
required for a basic cycle. The number of adsorbers can be increased to enhance the
efficiency, which depends on the cycle. This process can also be adapted to the
continuous process
The adsorption refrigeration cycle relies on the adsorption of a refrigeration gas into an
adsorbent at low pressure and subsequently desorbed by heating. The adsorbent also
acts as a “chemical compressor” driven by heat. In its simplest form an adsorption
refrigerator consists of two linked vessels, one of which contains adsorbent and both of
which contain refrigerant as shown in Figure 1.4 Critoph [1999].
Fig 1.4 The adsorption cycle
In the first step, the whole system is at low pressure and contains refrigerant gas. The
adsorbent contains a large quantity of gas. In the second step, the adsorbent is heated
and rejects the gas which condenses in the second vessel. While it condenses, it rejects
heat. During the third step, the whole system is at high pressure. In the fourth step the
31
gas evaporates and is readsorbed by the adsorbent. During this step, the gas takes heat
for evaporation. In the final step, the system is at the same state than in the first step.
This system produces cold during a half part of the cycle, to produce cold continuously;
two such cycles must be worked out of phase. The adsorbent is made of activated
carbon and the refrigerant gas is ammonia. For increase the performance of the system,
two beds could be used.
1.3.4 Gas expansion refrigeration
An adiabatic expansion of a closed system always reduces its internal energy with
a decrease in temperature i.e. a refrigeration effect proportional to the expansion (that is
why gases are used instead of condensed matter). An adiabatic expansion in a work
producing flow system always reduces the enthalpy with a decrease in temperature, but
an adiabatic expansion in a liquid flow system, maintain the total enthalpy and may
decrease or increase its temperature depending on the relative side of the inversion
temperature.
Gas expansion cycles are only used in special applications as for cryogenic
refrigeration and for special applications where compressed air is already available, as
from gas turbine engines and in cabin –air conditioning on airplane. Gas expansion
cycles basically corresponds to an inverted Brayton cycles. Small stirling cycle
refrigerators have been developed using helium as a working fluid as illustrated in fig
1.5 below. Awoniyi (1980)
Fig. 1.5. Gas expansion refrigeration cycle
1.3.5 Thermoelectric refrigeration
Solid state electrically driven refrigerators (also called thermoelectric
based on the Peltier effect when a direct current flows in a circuit formed by the
dissimilar electrical conductors, some heat is absorbed at one junction and some more
heat is released at the other junction, reversing the effects when revers
the current (joule heating is not reversing, it is always positive). Thermoelectric effects
are due to the free-electron density variation with temperature amongst materials and
the associated fluxes. Jordan
A typical thermoelectri
semiconductor thermo elements
which are connected electrically in series and thermally in parallel.
32
. coolers TEC) are
eltier reversing the sense of
(1980)
thermoelectric module consists of pair of P-type and
is shown in fig 1.6 below forming thermocouples
lers ing N-type
Fig. 1.6 Sketch of a thermo-electric
1.3.6 Evaporative cooling
Mixing water and non-saturated air produces a refrigera
drop below ambient temperature)
cool drinking water in porous earthe
on the floor.
The basic refrigeration effect is due to the energy demanded by evaporating water
(equal to the enthalpy of vaporisation
to evaporative cooling is vaporisation cooling when vacuum is applied to a liquid or
solid (usually aqueous solutions).
Evaporative cooling, however, is not usually covered under Refrigeration because it is
rather limited in practice to slightly cooling the water or the air
system; it’s main limitations
inefficiencies in heat exchangers
handling is cumbersome below 0 ºC, and that moist air must be desicca
continuous evaporative-cooling process. However, new developments in desiccant
33
electric-cooler (TEC) with three thermo-elements
refrigerating effect (i.e. a temperature
temperature), is an old technique been used by ancient Egyptians to
earthen pots, and to cool space by splashing some water
vaporisation), a natural process driven by air dryness. Related
ing that are fed to the
are that evaporation is a slow process, that small
rapidly decreases efficiency of the process, water
desiccated to have a
n ), waterted
34
regeneration are showing promise particularly for air-conditioning applications (without
air desiccants, the growing humidity hinders its effectiveness). Jordan (1980)
.
1.4 STATEMENT OF THE PROBLEM
In developing countries there is a growing interest in refrigeration for food and vaccine
preservation. Simple solar refrigerators working without need for electricity supply
would be very valuable in rural areas where there maybe no electricity supply.
Mechanical refrigerators powered by solar cells are available, but are too expensive. In
the last twenty years, adsorption refrigerators using water as a refrigerant and Zeolite as
an adsorber have been successfully developed.
In areas with abundant sunshine, solar radiation is the most easily accessible energy
source. Solar refrigerators can work independently of the electrical network. Extensive
vaccination programmes are in progress throughout the developing world in the fight
against common diseases. To be effective, these programmes must provide
immunization services to rural areas. All vaccines have to be kept within a strict
temperature range throughout transportation and storage. The provision of refrigeration
for this aim is known as the vaccine cold chain.
In Africa about 1800 solar refrigerators are used to store vaccines (WHO). Usually,
refrigeration is produced by a vapour compression cycle, which is driven by electric
power produced or generated by solar cells. However, the investment of about USD
2000 is high and the population cannot afford such systems, in addition, the high-tech
production of solar cells seems to be difficult in developing countries Siegfried (
35
1.5 OBJECTIVE
Everywhere in our world refrigeration is a major energy user. In poor areas “off
guide” refrigerators is actually an important need. Both of these consideration point the
way toward refrigeration using renewable energy as part of a sustainable way of life.
The objective of this project is to develop a suitable grade of Zeolite as the
adsorber (considered as a chemical compressor), design, construct and test a Zeolitewater
solar powered refrigerator with water as the working fluid,
The solar refrigerator to be designed must be simple, cost effective, affordable
and reliable.
1.6 JUSTIFICATION
The technical feasibility of solar cooling has been investigated in many countries
by many researchers, using various refrigeration cycles and design as can be seen from
the above review. Various degrees of successes have been achieved, which
demonstrates that solar adsorption refrigeration is possible. Also the various cooling
system and modifications reviewed have made use of similar solar flat plate collectors
and adsorption materials such as silica gel, activated carbon and Zeolite. However, the
Zeolite system is preferred as it is more cost effective and environmentally friendly
In Nigeria very little has been done in the field of solar cooling where the annual mean
total solar radiation received over 24hours in Nigeria is about 210 W/ m2 which is high
enough to encourage efforts to utilize the abundant energy.
This knowledge will therefore be a basis for further work on solar cooling will be
done in this university and the country as a whole. Because of limited resources, I have
36
placed emphasis on the use of locally available materials; to produce and appropriate
type of Zeolite and the use of a parabolic trough concentrator to assist in obtaining high
temperatures required for high rate of refrigerant generation. . Air cooling is adopted for
the system rather than water cooling due to its availability. Solar energy is adopted
because its.
 A green source of energy
 It abundant and readily available
 Cheap source of energy
Zeolite cooling on the other hand is especially suited and chosen for this solar energy
application for the following reasons:
 The process uses heat during charging, and releases heat when adsorbing,
making it possible to store energy by ‘precharging’ Zeolite for later use.
 Relatively low heating temperatures are involved and only a medium vacuum.
 Zeolite is cheap, safe, light, and re-usable
 Water is environmentally friendly, low cost, and non-toxic.
This project is thus justified by the fact that Adsorption refrigeration systems have the
advantages of being environmentally benign, having zero ozone depletion potential
(ODP) as well as zero global warming potential (GWP) due to the use of natural
refrigerants such as water therefore making it eco-friendly. It is also attractive for the
efficient use of solar energy and low-grade waste heat. Less vibration, simple control,
low initial investment and expenditure, and less noise are the advantages of adsorption
systems over the existing vapor compression and absorption systems.

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