Project Abstract

<p> This work investigated the variation of top loss heat transfer coefficient<br>with the emittance of the absorber plate, the collector tilt angle and air<br>gap spacing between the absorber plate and the cover plate. The effects of<br>the emittance of the absorber plate, the collector tilt angle and air gap<br>spacing between the plate and the cover on the collector performance<br>were also considered. Data collected from the thermosyphon solar water<br>heater constructed by the National Centre for Energy Research and<br>Development (NCERD), UNN was used in the analysis. Evaluations of<br>thermal losses by radiative and convective heat transfer coefficient were<br>performed. It was observed that increase in the emittance of the absorber<br>plate resulted in dissipation of more heat to the atmosphere and<br>consequent increase in top loss heat transfer coefficient which led to<br>reduced system performance. The collector tilt angle had little effect on<br>the top loss heat transfer coefficient and consequently had insignificant<br>effect on the performance of the collector. Increase in the air gap spacing<br>between the absorber plate and the cover plate resulted in decrease in the<br>top loss heat transfer coefficient. It was also observed that passive flat<br>plate solar collector had a better performance at a low mean absorber<br>plate temperature. In addition, a correlation that relates the efficiency of<br>the collector with the absorber plate emittance, collector tilt angle and the<br>air gap spacing between the absorber plate and the cover plate was<br>developed for the NCERD thermosyphon water heater. <br></p>

Project Overview

<p> </p><p>INTRODUCTION<br>1.1 Need for Solar Energy<br>Conventional energy resources are not only limited in supply but are<br>exhaustive in nature. In the near future, the present energy conversion<br>systems will change drastically, due to lack of conventional fuels. Thus,<br>there is a need for alternative energy resources, since energy remains central<br>to the existence and survival of mankind. Many studies of world energy<br>supply and demand and of projected national and regional energy<br>requirements suggest that there will be an increasing strain on conventional<br>petroleum and natural gas supplies to the point where substitution for these<br>fuels on a large scale will become necessary towards the end of the century,<br>if not before[1]. The two most significant permanent sources of energy are<br>nuclear and solar energy.</p><p>Nuclear energy is associated with changes in the structure and composition<br>of the nucleus of matter. Nuclear energy requires advanced technology and<br>costly means for its safe and reliable utilization and may have undesirable<br>side effects [2]. On the other hand, although, the engineering design and<br>analysis of solar processes present unique problems, the utilization of solar<br>energy shows promise of becoming a dependable energy source. Solar energy<br>in its manifold forms is potentially the most important energy source that is<br>renewable indefinitely (that is as long as the Sun shines) and, in the very<br>long term when fossil and fission fuels are exhausted, the Sun is the only<br>alternative to those nuclear fusion and fission reactors which might be<br>constructed on Earth [1].</p><p>1.2 The Structure of the Sun<br>The Sun is the source of solar energy. It is a sphere of hot gaseous matter.<br>The Sun generates its energy by fusion reactions. It has a mass of about<br>1.99 x 1030kg with an estimated diameter of 1.39 x 106km. The density of<br>the sun is estimated at 1410kg/m3. The solar interior constitutes the main<br>2<br>mass of the Sun and has gases at pressure of a billion atmospheres and<br>temperature of 8.0 x 106 to 4.0 x 107oK [2].<br>[3] reported that the center to 0.23R (where R = radius of the Sun), which<br>contains 40% of the mass of the Sun is estimated to generate 90% of the<br>interior energy of the Sun. The energy generated in this core part of the Sun<br>is radiated in the form of gamma rays up to a distance of about 0.7R from<br>the centre, where the temperature has dropped to about 130,000oK.</p><p>The zone from 0.7R to 1.0R is known as the convective zone. It contains fluid<br>in which energy transfer is mainly by convection. The temperature at the<br>outer surface of the convective zone is about 6,000oK. This outer layer of the<br>convective zone is called the photosphere. It is essentially opaque, as the<br>gases which it is composed are strongly ionized and are able to absorb and<br>emit continuous spectrum of radiation. The photosphere is the source of<br>most solar radiation.</p><p>Above the photosphere is a layer of cooler gases called the reversing layer.<br>Outside of the reversing layer is the chromosphere. The chromosphere is at<br>about 106oK. The light emitted by the chromosphere is of short wave length<br>because of the high temperature. Still further out is the corona. The corona<br>is made of highly ionized gases of very low density. Its temperature is about<br>106oK.</p><p>1.3 The Energy of the Sun<br>The fusion reactions which have been suggested to supply the energy<br>radiated by the Sun have been several reactions; the one considered the<br>most important is a process in which hydrogen combines to form helium [3].<br>The equation of the reaction is: 41H12He4+26.7MeV (1.1)<br>In the reaction, four protons having a total uncombined mass of 4.0304amu<br>formed helium of mass 4.0027amu. The difference in mass of the reactant<br>and the product is 0.0277atomic mass unit of matter. This mass is converted to energy in accordance with the Einstein relationship (E= mc2).<br>3<br>Several of these chain reactions taking place in the interior region of the Sun<br>generate most of the energy of the Sun.</p><p>1.4 Solar Energy Utilization<br>The direct and indirect uses of solar energy by mankind have been in<br>existence for centuries. Man had used solar energy for drying purposes,<br>warming purposes and so on. The various uses of the solar energy can be<br>categorized into three broad classes; viz; photochemical processes,<br>photovoltaic processes and photothermal processes.</p><p>1.4.1 Photochemical Processes<br>Photochemical processes have been defined as those in which the absorption<br>of solar photons in a molecule produces excited states, or alternatively in a<br>semi-conductor raises electrons from the valence band to the conduction<br>band. As a result of the chemical reactions which may then occur, some of<br>the excited energy may be stored as chemical energy or a useful chemical<br>reaction may be catalyzed. Photochemical is a technology to synthesize<br>valuable chemical materials or fuels by the use of solar energy.</p><p>1.4.2 Photovoltaic Processes<br>Photovoltaic solar system is the process of converting light energy from the<br>Sun into electricity in the absence of mechanical generators. The<br>photovoltaic effect generates electromotive force as a result of the absorption<br>of ionizing radiation. When photons from the Sun are absorbed by some<br>semi-conductors, they create free electrons with higher energies than the<br>electrons which provide the bonding in the base crystal. Once these free<br>electrons are created, there must be an electric field to induce these higher<br>energy electrons to flow out of the semi-conductors to do useful work.</p><p>Photovoltaic solar systems generate direct current. Inverters are required to<br>convert the direct current into alternating current. When the electricity<br>generated is to be used in a later time, a deep cycle motive battery is<br>required to store the electrical energy. Power from photovoltaic solar systems<br>can be used for powering alarm systems, navigational aids, electric bulbs,<br>home appliancies, water pumps, and for grid connection.<br>4</p><p>1.4.3 Photothermal Processes<br>Photothermal processes involve the use of solar plate collectors. Solar plate<br>collectors intercept solar radiation and convert the radiation into heat. Solar<br>plate collectors are either focusing (concentrating) type or flat plate type.<br>Focusing collectors usually have concave reflectors to concentrate the<br>radiation falling on the total area of the reflector onto a heat exchanger of<br>smaller surface area, thereby increasing the energy flux. Focusing collectors<br>use optical system in the form of reflectors. In flat-plate collectors, the area<br>intercepting solar radiation is the same as the area absorbing solar<br>radiation.</p><p>1.5 Need for Solar Plate Collector<br>Solar plate collectors convert solar radiation to heat energy. This heat energy<br>can be utilized in various applications. In Nigeria today, Power Holding<br>Company of Nigeria Plc, a body saddled with supplying power to, and<br>operating, the national grid has failed. Nigerians therefore need an efficient<br>and cost effective means of meeting their energy needs. It is in this respect<br>that the need for solar plate collector systems becomes imperative</p><p>1.6 The Objectives of the Thesis<br>The objectives of this work are<br>a. To consider the theoretical analysis of a passive flat plate solar<br>collector.<br>b. To investigate the effects of collector tilt angle, absorber plate<br>emittance and the air spacing between the absorber plate and the<br>cover plate on the top loss coefficient and consequently on the<br>performance of the collector.<br>c. To develop a correlation that relates the efficiency of the passive flat<br>plate solar collector to the collector tilt angle, absorber plate<br>emittance and the air spacing between the absorber plate and the<br>cover plate.</p> <br><p></p>