ABSTRACT
This work investigated the variation of top loss heat transfer coefficient with the emittance of the absorber plate, the collector tilt angle and air gap spacing between the absorber plate and the cover plate. The effects of the emittance of the absorber plate, the collector tilt angle and air gap spacing between the plate and the cover on the collector performance were also considered. Data collected from the thermosyphon solar water heater constructed by the National Centre for Energy Research and Development (NCERD), UNN was used in the analysis. Evaluations of thermal losses by radiative and convective heat transfer coefficient were performed. It was observed that increase in the emittance of the absorber plate resulted in dissipation of more heat to the atmosphere and consequent increase in top loss heat transfer coefficient which led to reduced system performance. The collector tilt angle had little effect on the top loss heat transfer coefficient and consequently had insignificant effect on the performance of the collector. Increase in the air gap spacing between the absorber plate and the cover plate resulted in decrease in the top loss heat transfer coefficient. It was also observed that passive flatplate solar collector had a better performance at a low mean absorber plate temperature. In addition, a correlation that relates the efficiency of the collector with the absorber plate emittance, collector tilt angle and the air gap spacing between the absorber plate and the cover plate was developed for the NCERD thermosyphon water heater.
CHAPTER 1
INTRODUCTION
1.1Need for Solar Energy
Conventional energy resources are not only limited in supply but are exhaustive in nature. In the near future, the present energy conversion systems will change drastically, due to lack of conventional fuels. Thus, there is a need for alternative energy resources, since energy remains central to the existence and survival of mankind. Many studies of world energy supply and demand and of projected national and regional energy requirements suggest that there will be an increasing strain on conventional petroleum and natural gas supplies to the point where substitution for these fuels on a large scale will become necessary towards the end of the century, if not before[1]. The two most significant permanent sources of energy are nuclear and solar energy.
Nuclear energy is associated with changes in the structure and composition of the nucleus of matter. Nuclear energy requires advanced technology and costly means for its safe and reliable utilization and may have undesirable side effects [2]. On the other hand, although, the engineering design and analysis of solar processes present unique problems, the utilization of solar energy shows promise of becoming a dependable energy source. Solar energy in its manifold forms is potentially the most important energy source that is renewable indefinitely (that is as long as the Sun shines) and, in the very long term when fossil and fission fuels are exhausted, the Sun is the only alternative to those nuclear fusion and fission reactors which might be constructed on Earth [1].
1.2 The Structure of the Sun
The Sun is the source of solar energy. It is a sphere of hot gaseous matter. The Sun generates its energy by fusion reactions. It has a mass of about 1.99 x 1030kg with an estimated diameter of 1.39 x 106km. The density of the sun is estimated at 1410kg/m3. The solar interior constitutes the main mass of the Sun and has gases at pressure of a billion atmospheres and temperature of 8.0 x 106 to 4.0 x 107oK [2].
[3] reported that the center to 0.23R (where R = radius of the Sun), which contains 40% of the mass of the Sun is estimated to generate 90% of the interior energy of the Sun. The energy generated in this core part of the Sun is radiated in the form of gamma rays up to a distance of about 0.7R from the centre, where the temperature has dropped to about 130,000oK.
The zone from 0.7R to 1.0R is known as the convective zone. It contains fluid in which energy transfer is mainly by convection. The temperature at the outer surface of the convective zone is about 6,000oK. This outer layer of the convective zone is called the photosphere. It is essentially opaque, as the gases which it is composed are strongly ionized and are able to absorb and emit continuous spectrum of radiation. The photosphere is the source of most solar radiation.
Above the photosphere is a layer of cooler gases called the reversing layer. Outside of the reversing layer is the chromosphere. The chromosphere is at about 106oK. The light emitted by the chromosphere is of short wave length because of the high temperature. Still further out is the corona. The corona is made of highly ionized gases of very low density. Its temperature is about
106oK.
1.3 The Energy of the Sun
The fusion reactions which have been suggested to supply the energy radiated by the Sun have been several reactions; the one considered the most important is a process in which hydrogen combines to form helium [3].
The equation of the reaction is:
41H12He4+26.7MeV (1.1)
In the reaction, four protons having a total uncombined mass of 4.0304amu formed helium of mass 4.0027amu. The difference in mass of the reactant and the product is 0.0277atomic mass unit of matter. This mass is converted to energy in accordance with the Einstein relationship (E= mc2).
Several of these chain reactions taking place in the interior region of the Sun generate most of the energy of the Sun.
1.4 Solar Energy Utilization
The direct and indirect uses of solar energy by mankind have been in existence for centuries. Man had used solar energy for drying purposes, warming purposes and so on. The various uses of the solar energy can be categorized into three broad classes; viz; photochemical processes, photovoltaic processes and photothermal processes.
1.4.1 Photochemical Processes
Photochemical processes have been defined as those in which the absorption of solar photons in a molecule produces excited states, or alternatively in a semi-conductor raises electrons from the valence band to the conduction band. As a result of the chemical reactions which may then occur, some of the excited energy may be stored as chemical energy or a useful chemical reaction may be catalyzed. Photochemical is a technology to synthesize valuable chemical materials or fuels by the use of solar energy.
1.4.2 Photovoltaic Processes
Photovoltaic solar system is the process of converting light energy from the Sun into electricity in the absence of mechanical generators. The photovoltaic effect generates electromotive force as a result of the absorption of ionizing radiation. When photons from the Sun are absorbed by some semi-conductors, they create free electrons with higher energies than the electrons which provide the bonding in the base crystal. Once these free electrons are created, there must be an electric field to induce these higher energy electrons to flow out of the semi-conductors to do useful work.
Photovoltaic solar systems generate direct current. Inverters are required to convert the direct current into alternating current. When the electricity generated is to be used in a later time, a deep cycle motive battery is required to store the electrical energy. Power from photovoltaic solar systems can be used for powering alarm systems, navigational aids, electric bulbs, home appliancies, water pumps, and for grid connection.
1.4.3 Photothermal Processes
Photothermal processes involve the use of solar plate collectors. Solar plate collectors intercept solar radiation and convert the radiation into heat. Solar plate collectors are either focusing (concentrating) type or flat plate type. Focusing collectors usually have concave reflectors to concentrate the radiation falling on the total area of the reflector onto a heat exchanger of smaller surface area, thereby increasing the energy flux. Focusing collectors use optical system in the form of reflectors. In flat-plate collectors, the area intercepting solar radiation is the same as the area absorbing solar radiation.
1.5 Need for Solar Plate Collector
Solar plate collectors convert solar radiation to heat energy. This heat energy can be utilized in various applications. In Nigeria today, Power Holding Company of Nigeria Plc, a body saddled with supplying power to, and operating, the national grid has failed. Nigerians therefore need an efficient and cost effective means of meeting their energy needs. It is in this respect that the need for solar plate collector systems becomes imperative
1.6 The Objectives of the Thesis
The objectives of this work are
a. To consider the theoretical analysis of a passive flat plate solar collector.
b. To investigate the effects of collector tilt angle, absorber plate emittance and the air spacing between the absorber plate and the cover plate on the top loss coefficient and consequently on the performance of the collector.
c. To develop a correlation that relates the efficiency of the passive flat plate solar collector to the collector tilt angle, absorber plate emittance and the air spacing between the absorber plate and the cover plate.