Project Abstract

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

Project Overview

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